FASCICULI INSTITUTI GEOLOGICI HUNGARIAE AD ILLUSTRANDAM NOTIONEM GEOLOGICAM ET PALAEONTOLOGICAM GEOLOGICA HUNGARICA SERIES GEOLOGICA TOMUS 26 Mio/Pliocene Phreatomagmatic Volcanism in the Western Pannonian Basin by Ulrike Martin and Károly Németh BUDAPEST, 2004 © Copyright Geological Institute of Hungary (Magyar Állami Földtani Intézet), 2004 All rights reserved! Minden jog fenntartva! Serial editor: TAMÁS BUDAI Revised by Chapter 1 — GÁBOR BADA (Vrieje Universität, Amsterdam, Netherland) and CSABA SZABÓ (Eötvös University, Budapest, Hungary) Chapter 2 — FERENC MOLNÁR (Eötvös University, Budapest, Hungary) Chapter 3 — MURRAY M. MCCLINTOCK (Otago University, Dunedin, New Zealand) and KURT GOTH (Landesamt für Umwelt und Geologie, Freiberg, Germany) Chapter 4 — JAMES D. L. WHITE (Otago University, Dunedin, New Zealand) Chapter 5 — IAN SKILLING (Pittsburgh University, Pittsburgh, Massachusetts) General revision by: VOLKER LORENZ (Würzburg University, Germany) TAMÁS BUDAI (Geological Institute of Hungary, Budapest) Technical editor: OLGA PIROS DEZSÕ SIMONYI DTP: OLGA PIROS, DEZSÕ SIMONYI Cover design: DEZSÕ SIMONYI Digital Map Production Assistance: GÁBOR CSILLAG (Geological Institute of Hungary, Budapest) and ÁKOS NÉMETH (Hungarian Meterological Service, Budapest) Published by the Geological Institute of Hungary — Kiadja a Magyar Állami Földtani Intézet Responsible editor: KÁROLY BREZSNYÁNSZKY Director HU ISSN 0367–4150 ISBN 963 671 238 7 Geologica Hungarica series Geologica Tomus 26. 3 Contents Preface of Volker Lorenz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Volcanology and its importance in Hungary (Károly Brezsnyánszky) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Chapter 1 Phreatomagmatic volcanic fields in a Mio/Pliocene fluvio-lacustrine basin, western Pannonian Basin, . . . Hungary: a review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Colour Plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Chapter 2 Late Miocene to Pliocene palaeogeomorphology of the western Pannonian Basin based on studies of volcanic erosion remnants of small-volume intraplate volcanoes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Colour plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Chapter 3 Mio/Pliocene phreatomagmatic volcanism in the Bakony – Balaton Highland Volcanic Field, Hungary . . . 73 Colour plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 Chapter 4 Shallow sub-surface intrusive processes associated with phreatomagmatic volcanism north of the Keszthely Mountains, Hungary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Colour plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 Chapter 5 Mio/Pliocene phreatomagmatic volcanism in the Little Hungarian Plain Volcanic Field (Hungary) and at the western margin of the Pannonian Basin (Austria, Slovenia) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 Colour plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 Chapter 6 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 Colour plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Preface On the occasion of the Second International Maar Conference held in Hungary in September 2004, the Geologica Hungarica, series Geologica presents this monograph on the Mio/Pliocene (8- 2 My) small volume intraplate alkaline volcanism in the western Pannonian Basin. The volcanic activ- ity occurred in the Bakony – Balaton Highland Volcanic Field (BBHVF) just north of Lake Balaton, in the smaller Little Hungarian Plain Volcanic Field (LHPVF) just to the north, and in the Styrian Basin Volcanic Field barely reaching into westernmost Hungary. The western Pannonian Basin is underlain by Neogene siliciclastic sediments which overlie Mesozoic karstic limestones which in turn overlie crystalline basement rocks. As volcanism was active during and after deposition of the Neogene sili- ciclastic sediments, volcanicity was largely synsedimentary and consequently effected, more or less, by the unconsolidated water saturated sediments. The volcanic fields of the western Pannonian Basin will be visited during two identical volcanological field trips run prior and after the conference. In this monograph the present state of physical volcanological research (over the last 10 years) on the volcanism of the western Pannonian Basin is presented. The authors of the several papers present the relevant details and interpretations of the regional geology, of the volcanic fields and also of the many individual volcanoes and their various phreatomagmatic and magmatic eruption styles. In addition, the authors compare the western Pannonian volcanic fields with other volcanic fields in the world many of which they know from personal aquaintance and studies. The list of ref- erences contains not only very informative Hungarian publications but also a wealth of internation- al publications relevant to the understanding of volcanological processes generating maars, tuff- rings, diatremes, scoria cones, lava lakes and their tephra and volcanic rocks, but also relevant to the understanding of volcanic fields. The overview publications and detailed descriptions of the individual volcanoes of the three vol- canic fields provide the reader with a state of the art view on the volcanicity of the Neogene volcan- ism of the western Pannonian Basin which for many decades has been almost unknown to volca- nologists from many countries. The monograph has been organized in such a way that it contains both the overview publications but also the publications of the individual volcanoes and their out- crops so that the publications can easily be used as field guides for volcanological field trips of groups but also for individuals. The various levels of erosion of the many individual volcanoes make the volcanic fields of the western Pannonian Basin very informative in respect to research of maars, diatremes and lava lakes, and complimentary to other volcanic fields displaying monogenetic volca- noes exposed at different levels of erosion, as, e.g. the maar volcanoes visited on the occasion of the First International Maar Conference in the West Eifel Volcanic Field in 2000. This monograph is also complimentary to the field guide published on the volcanic field in southern Slovakia, also vis- ited prior and after the Second International Maar Conference. Würzburg, 15th of June 2004. PROF. DR. VOLKER LORENZ Geologica Hungarica series Geologica Tomus 26. Institute for Geology University of Würzburg, Würzburg, Germany Volcanology and its importance in Hungary Magmatism and volcanism played an important role in the geological development of Hungary. The formation of acidic rocks due to volcanic activity during the Variscan cycle can be found in the Velence–Balaton and the Mecsek area south-west of Hungary. Typical “Pietra verde” type pyroclas- tic rocks, displaying a distinctively alkaline-trachytic character, are intercalated in the Middle Triassic carbonat sequence in the South Bakony Mts north of Lake Balaton. Middle Triassic to Jurassic basic and ultrabasic suites in different tectonic units of NE Hungary are commonly intercalated with deep- sea sediments. The Tisza Tectonic Megaunite south-east of the Hungary is related to basic-alkaline volcanism in the Early Cretaceous. Volcanic rocks are best exposed in the Mecsek Mts and they extend as far as to the basement of the Great Plain. In the Eocene andesitic-type volcanism occurred both in the Transdanubian Range and the Mecsek Mts. Intermediate volcanism during the Miocene is of particular importance. In Hungary occurrences of Miocene volcanic rocks are distributed in a broad belt from Dunazug–Börzsöny Mts to the Zemplén Mts, basically from Budapest to the north-east tip of Hungary. Furthermore volcanic rocks are covered under thick Neogene to Quaternary sediments in the northern part of the Great Plain, which form all together the innermost zone of the volcanic belt of the Inner Carpathians. Particularly important are metallic ore deposits of Hungary that are associated with this Miocene volcanic belt. The latest volcanic activity, occurred in several regions of Hungary, and has been dated to be of Mio/Pliocene age. One of the most important volcanic regions in relation to the Second International Maar Meeting 2004 is the western Pannonian Basin, also because of the scenic impression of indi- vidual volcanic mountains in the Tapolca Basin, west of the Bakony – Balaton Highland area. Volcanological research in Hungary has of long tradition, first of all because of the old mining of metallic ore deposits related with the Neogene volcanism. Professor József Szabó (1822–1894) is know well in the scientific community as “father of Hungarian geology”, the first teaching geology in Hungarian language at the university. He established an internationally recognized petrographic system of trachytic rocks. For long time after Szabó’s activity the descriptive petrology prevailed in the Hungarian geology. In addition, the contribution of Lajos Lóczy sen. has a great importance. He studied the morpholog- ical development of the Balaton Highland, in particular, the basaltic volcanoes of the Tihany Peninsula. Since the late 1970’s, early 1980’s new trends of volcanological studies are appeared in Hungary, focusing on the Neogene intraplate basalt volcanism. On one hand petrogenetic studies, based on detailed mineralogical, chemical and isotope geochemical investigations, have been car- ried out. On the other hand investigations and new theories tried to support interpretation of tecton- ic, geodynamic position and significance of the volcanism in the geological evolution of the area. A new detailed morphogenetic approach and the theory of phreatomagmatic volcanism con- ducted by Ulrike Martin and Károly Németh gave way to a new view in interpreting basaltic volcan- ism of the western Pannonian Basin. We strongly believe that the IAVCEI–CVS–IAS Second International Maar Conference held in Hungary jointly organised with Slovakia and Germany will pro- mote both the scientific and the educational activity in the field of volcanology in the region and worldwide. Budapest, 14th of July 2004. DR. KÁROLY BREZSNYÁNSZKY Geologica Hungarica series Geologica Tomus 26. Director Geological Institute of Hungary, Budapest, Hungary Introduction In the western Pannonian Basin there are three distinct areas where during Mio/Pliocene time alkaline basaltic volcanism took place (front inner cover). Volumetrically the largest volcanic field, the Bakony – Balaton Highland Volcanic Field (BBHVF) is located adjacent to the north shore of the Lake Balaton (front inner cover). There, the vent remnants are advanced in erosion down to their crater/vent zone and give a good opportunity to study the feeding systems of predominantly phreatomagmatic volcanoes. North of the BBHVF, in a more dispersed setting, less vents form the Little Hungarian Plain Volcanic Field (LHPVF — front inner cover). The erosion level of these volca- noes allows studying the crater zone of predominantly phreatomagmatic volcanoes. In the western margin of the Pannonian Basin, a cluster of strongly eroded alkaline basaltic volcanoes form the Styrian Basin Volcanic Field (SBVF), a term which is generally used for volcano clusters located in Burgenland, Styria (both in Austria and in Slovenia). In this book the present state of research in the field of physical volcanology will be presented predominantly focusing on the BBHVF and LHPVF. The book reflects a summary of the results and volcanological view of the authors on the basis of their past ten years research on understanding the eruptive mechanism of the Mio/Pliocene volca- noes in the western Pannonian Basin. The book starts with a general review of the Neogene small-volume intraplate volcanism in the western Pannonian Basin, with respect to other fields in the Pannonian Basin system of the same age and to other fields world-wide. The general review put the volcanism in a tectonic and geochem- ical framework based on the relevant literature. This information allows to create a general vol- canological model, which is based on the authors own results. The general review of the volcanism is followed by a summary of the geomorphological aspects of the study of the volcanic fields in this region. This is a “stand alone” summary of the authors view of the syn-volcanic morphology and the possible volcanic landforms on the basis of studies of the preserved volcanic rocks. The chapter is followed by systematic descriptions and interpretations of the BBHVF, the Keszthely Mts. region and the LHPVF with a brief description of a few sites from the SBVF. The reason to present the BBHVF and the Keszthely Mts region separately is, that the latter is predominantly characterised by pre- served intrusive complexes. Their description therefore thematically could be well separated from the BBHVF. The specific chapters of the volcanic fields are followed by concluding remarks. Alongside a monographic style of the book, the chapters and the listed sites are arranged in a way that they are in concert with the relevant field trips organised before and after the Second International Maar Conference (2IMC) 2004. The 2IMC an international volcanological meeting under the auspices of the International Association of Volcanology and Chemistry of the Earth Interior (IAVCEI) and the International Association of Sedimentologists (IAS) and also supported by other scientific organisations such as the Society for Economy Geology (SEG), Geologische Vereinung (GV), Deutsche Geologische Geselschaft (DGG), Magyarhoni Földtani Társulat (MFT) hosted in Hungary in 2004 was a joint meet- ing of Hungary, Slovakia and Germany. There is no direct reference to the certain field trip stops in the main text of the chapters to keep the scientific information of certain volcanic fields confined and relevant. The field trip programmes are presented in separate hard page (as a page marker plate) with references of the single stops to the related chapters. Enclosed at the back inner cover are 3D digital terrain models which highlight the field trip routes. Numbers on the maps correspond to page numbers of the related pages in the chapters of the monograph. Geologica Hungarica series Geologica Tomus 26 ULRIKE MARTIN and KÁROLY NÉMETH Heidelberg–Balatonlelle Phreatomagmatic volcanic fields in a Mio/Pliocene fluvio-lacustrine basin, western Pannonian Basin, Hungary: a review Abstract 12 Introduction 12 Tectonic framework of the western Pannonian Basin and its relevance to the Neogene intraplate volcanism 16 Lithospheric structure and magma ascent of Neogene alkaline basaltic volcanic fields 18 Geochemistry and petrogenesis of eruptive products of the Miocene to Pliocene volcanism in the western Pannonian Basin 19 Isotope geochemistry 20 Petrography of pyroclastic rocks 21 Major element variations in monogenetic volcanoes 23 Age of the Neogene intraplate volcanoes in the western Pannonian Basin and their relationship to the immediate pre- and syn-volcanic sedimentation in the region 25 Distribution of volcanic erosion remnants 26 Lavaflows, scoria and spatter cones 27 Hyaloclastite and peperite 29 Phreatic and phreatomagmatic eruptive centres 30 Phreatomagmatic activity 30 Maars and tuff rings 32 Types of phreatomagmatic volcanoes 33 Maar crater-fill sediments, Gilbert-type deltas 34 Maar lake carbonates 34 General features of the western Pannonian Basin 35 References 37 Colour plates 49 Contents ULRIKE MARTIN and KÁROLY NÉMETH12 Abstract The volcanic erosion remnants in the western Pannonian Basin are grouped into volcanic fields such as the Bakony – Balaton Highland Volcanic Field (BBHVF), Little Hungarian Plain Volcanic Field (LHPVF), and Styrian Basin Volcanic Field (SBVF). These volcanic fields were active in Late Miocene to Late Pliocene times and are located in the territories of Hungary, Austria and Slovenia. They are formed by at least 100 alkaline basaltic eruptive centres such as variably eroded scoria cones, tuff rings, maars, maar volcanic complexes, shield volcanoes, mesa flows and shallow subsurface intrusive complexes (dykes and sills). In this paper the basic volcanologic characteristics of the volcanic fields in the western Pannonian Basin are present- ed as a review of past research activities as well as the result of ongoing physical volcanological research in the region. It is demonstrated that the distribution of volcanoes in the western Pannonian Basin is structurally controlled by old structural ele- ments in the pre-volcanic rock units. The geographical distribution of different vent types, such as phreatomagmatic versus mag- matic vents, are in relationship with the general hydrogeological characteristics of the pre-volcanic rock units. In areas where thick Pannonian sandstone beds build up the underlying strata the so called “normal maar volcanic centres” have usually a thick late magmatic infill in the maar basins. The eruptive mechanism of these volcanoes was controlled by the unconsolidated, water- saturated porous media aquifer, that lead to the formation of “champagne glass” shaped volcanoes and flat tuff rings. In areas where relatively thin Pannonian sandstone beds cover the thick Mesozoic or Palaeozoic fracture controlled karstwater bearing aquifer, large maar volcanic sequences are common, classified as Tihany-type maar volcanoes. These maar volcanic centres are commonly filled with thick maar lake deposits, building up Gilbert-type gravely, scoria rich deltas in the northern side of the maar basins, suggesting mostly north to south oriented palaeo-fluvial systems. In the elevated, northern part of the field erosion- al remnants of scoria cones and associated shield volcanoes indicate a minor impact of the ground and surface water that may have led to phreatic and phreatomagmatic explosive activity. Keywords: phreatomagmatic, volcanic glass, maar, tuff ring, scoria, hydrogeology, erosion, Pannonian Basin, vent, aquifer, dyke, sill, monogenetic Introduction The Bakony – Balaton Highland (BBHVF), the Little Hungarian Plain (LHPVF) and the Styrian Basin Volcanic Fields (SBVF) are located in the western Pannonian Basin, Hungary (Figure 1.1). The largest number of Neogene volcanic ero- sional remnants is in the BBHVF, which includes the Keszthely Mts., where shallow subsurface sills and dykes associated with monogenetic volcanoes are exposed (Plate 1.1). The BBHVF is close to the Lake Balaton north shore. The volcanic centres of the BBHVF were active between 7.54 My and 2.8 My (BALOGH et al. 1982, BORSY et al. 1986, BALOGH and PÉCSKAY 2001, BALOGH and NÉMETH 2004 — Figure 1.2) and produced mostly alkaline basaltic volcanic products (SZABÓ et al. 1992, EMBEY-ISZTIN 1993). The volcanoes in the LHPVF are in the same age and compositional range as the BBHVF (BALOGH, et al. 1982, 1983, 1986, HARANGI et al. 1994, 1995, PÉCSKAY et al. 1995, SZABÓ et al. 1995, BALOGH and PÉCSKAY 2001). The eruptive centres of the BBHVF have a close relationship with the eruption centres of the LHPVF erup- tive centres according to their composition, age and general eruption mechanisms (JUGOVICS 1969b, 1972, HARANGI et al. 1994, NÉMETH and MARTIN 1999c, MARTIN et al. 2003). The two volcanic fields operated simultaneously (BALOGH and PÉCSKAY 2001, WIJBRANS et al. 2004) but the general palaeoenvironment (KÁZMÉR 1990) and their hydrogeology could have caused different styles of eruptive mechanism mainly in the explosive volcanic activity (NÉMETH and MARTIN 1999c). The BBHVF itself alone has approximately 50 basaltic volcanoes in a relatively small (around 3500 km2) area (JUGOVICS 1969a), however, the number of vents maybe far more than 50 due to the existence of volcanic complexes and nested volcanoes (MARTIN et al. 2003). The volcanic erosional remnants form a more scattered distribution in the LHPVF, and the number of volcanoes is less (~10) than in the BBHVF (JUGOVICS 1915, 1916, 1972), however, volcanic erosional remnants buried under thick Quaternary deposits are known from the region (TÓTH 1994). Individual Neogene alkaline basaltic volcanic erosional remnants are located close to the triple border of Hungary, Austria and Slovenia as well as close to the eastern metamorphic core complexes in the Eastern Alps (JUGOVICS 1916, 1939, KRALJ 2000) together often referred as SBVF. The volumetrically largest field of all is the BBHVF, often mentioned together with the LHPVF due to similarities in the age, timing and eruption mechanism. The BBHVF belongs to the Transdanubian Range unit, which is correlated with the Upper Austroalpine nappes of the east Alpine orogen (MAJOROS 1983, KÁZMÉR and KOVÁCS 1985, TARI 1991). The underlying basement of the volcanic fields consists of Palaeozoic rocks (Silurian schist, Permian red sandstone — CSÁSZÁR and LELKESNÉ-FELVÁRI 1999) and a thick Mesozoic carbonate sequence (BUDAI and VÖRÖS 1992, BUDAI and HAAS 1997, HAAS and BUDAI 1999, HAAS et al. 1999). This basement forms a large-scale anticline structure of Eoalpine origin (Figure 1.3) in the Transdanubian Range area and is locally covered by Tertiary sediments (TARI et al. 1992, 1999, HORVÁTH 1993, SACCHI and HORVÁTH 2002). Tertiary sediments were deposited in local sedimentary basins on a regional erosional unconformity (MÜLLER and MAGYAR 1992, MÜLLER 1998, TARI and PAMIC 1998, JUHÁSZ et al. 1999, MÜLLER et al. 1999). In the Neogene, just shortly before the volcanism started, a large lake, the Pannonian Lake, occupied main parts of the Pannonian Basin (Figure 1.4.), which had a very colourful sedimentary environment as reflected in the irregular basin morphology Phreatomagmatic volcanic fields in a Mio/Pliocene fluvio-lacustrine basin, western Pannonian Basin, Hungary: a review 13 Figure 1.1. Structural elements of the Carpatho-Pannon region (A) with respect to the Mio/Pliocene alkaline basaltic volcanic fields. The location of the volcanic erosional remnants of the Little Hungarian Plain and Styrian Basin Volcanic Fields (B) and the Bakony – Balaton Highland Volcanic Field (C) are shown in relevance to other rock rock units. The western Pannonian Basin was occupied by the Pannonian Lake around 9 My (D); that lake gradually vanished and had only small basins in the southern part of the Pannonian Basin 4.5 My ago (D). During onset of the volcanism, the region was occupied by an alluvial plain with a flat morpholohy, and potentially with large, but shallow water masses (D). Please note: The model on "D" is modified after MAGYAR et al. (1999) ULRIKE MARTIN and KÁROLY NÉMETH14 Figure 1.2. K/Ar age distribution map of the Little Hungarian Plain (A) and the Bakony – Balaton Highland (B) Volcanic Fields (after BALOGH et al. 1982, 1986, BORSY et al. 1986, and BALOGH and NÉMETH 2004) Phreatomagmatic volcanic fields in a Mio/Pliocene fluvio-lacustrine basin, western Pannonian Basin, Hungary: a review 15 (KÁZMÉR 1990). The lacustrine sandstones, mudstones, marls of the brackish Pannonian Lake are widespread in the Pannonian Basin (JÁMBOR 1980, 1989, KÁZMÉR 1990, MÜLLER and MAGYAR 1992, MÜLLER 1998, GULYÁS 2001). Just before volcan- ism started the area in the western Pannonian Basin formed an alluvial plain with unconsolidated water- saturated sediments in significant spatial and temporal variation (KÁZMÉR 1990). The alkaline basaltic volcanism in the western Pannonian Basin was of a predominantly subaerial, intracontinental type. However, large shallow water bodies may have been present during erup- tions, which most likely led to the formation of emergent volcanoes (KOKELAAR 1983, 1986, WHITE 1996a, 2001, WHITE and HOUGHTON 2000). These volca- noes quickly breached the water table of these lakes (MARTIN and NÉMETH 2002a, 2004b). The dis- tribution of volcanoes in the west- ern Pannonian Basin is related to the distribution of palaeo-valleys, which formerly were occupied by streams with good water supply (MARTIN et al. 2003). These “wet” valleys are most likely related to the reactivation of pre-Neogene fracture zones similar to the zones of structural weakness in the Eifel Volcanic Fields, Germany (LORENZ and BÜCHEL 1980a, b, BÜCHEL and LORENZ 1982, HUCKENHOLZ and BÜCHEL 1988, 1993, BÜCHEL et al. 2000). After volcanism ceased, fluvial/alluvial sedimentation was widespread in the western Pannonian Basin. Major ero- sion affected the region well after volcanism ceased (CSILLAG et al. 1994, JORDÁN et al. 2003, NÉMETH et al. 2003b). Lake Balaton, as one of the major landmarks of western Pannonia, is a recent landform and its history dates back only 17,000 to 15,000 years (CSERNY and CORRADA 1989, CSERNY 1993, CSERNY and NAGY-BODOR 2000, TULLNER and CSERNY 2003). However, pre-Lake Balaton lacustrine systems very likely existed in the region also throughout the Quaternary (TULLNER and CSERNY 2003). All types of eroded volcanoes can be found in the western Pannonian Basin (Plate 1.2) which show similar char- acteristics as most other monogenetic intracontinental volcanic fields such as Hopi Buttes, Arizona (WHITE 1991b, ORT et al. 1998), Western Snake River, Idaho (GREELEY 1982, GODCHAUX et al. 1992, BRAND 2004, WOODS and CLEMENS 2004), Waipiata Volcanic Field, New Zealand (NÉMETH and WHITE 2003), or West Eifel, Germany (LORENZ 1984). The most prominent geomorphologic formations are the circular, lava capped buttes. These centres are usually related to underlying phreatomagmatic volcanoes such as maar structures and tuff rings. Individual maar structures without lava infill are less common and difficult to identify. Such volcanic structures are locally filled by post-maar lava flows that subsequently have been buried under thick lacustrine units. In the northern part of the BBHVF Strombolian scoria cone remnants and Hawaiian spatter cone deposits are common. However, they com- monly consist of scoria beds, which are inter-bedded with phreatomagmatic tuffs and lapilli tuffs, suggesting simul- taneous Stromblian and phreatomagmatic activity. Large lava flow fields in the western Pannonian Basin only exist Figure 1.3. A general stratigraphy of the southern part of the Transdanubian Range (after BUDAI et al. 1999). The section shows all the identified rock formations and also gives a hint of the approximate location of certain rock for- mations ULRIKE MARTIN and KÁROLY NÉMETH16 in the northern part of the BBHVF and they form the areas of highest elevation today (Plate 1.1). The lava flows are eroded and their type is often hard to reconstruct due to the advanced erosion. Smaller lava flows are inferred to have filled valleys. The largest lava fields are in the Bakony Mountains and consist of eroded shield volcanoes such as Kab-hegy and Agár-tető (Plate 1.1). Lava plugs intruded into small vents and are preserved to their higher resist- ance to erosion (Hegyes-tű). The BBHVF is of great volcanological and palaeo-geomorphological interest. The relatively long volcanic history of the area (7.54–2.8 My — BALOGH and PÉCSKAY 2001) and the adjacent lacustrine to fluviatile environment make the western Pannonian Basin an ideal area for studying phreatomagmatic volcanoes in relation to a changing lacustrine environment and the palaeogeomorphological evolution of the Late Miocene to Pliocene landscape. There is a great potential in developing our knowledge about: 1. eruption mechanisms resulting from magma/water interactions with different magma/water-ratio, 2. the relationship of volvanic activity and the confining palaeo-environment, 3. the related palaeohydrology and petrophysical characteristics of the pre-volcanic units, and 4. interrelationships between tectonics and lithospheric rheology that control the magma ascent in various tectonic regimes. Tectonic framework of the western Pannonian Basin and its relevance to the Neogene intraplate volcanism The Pannonian region has been considered to be part of the Alpine belt, and it reveals the complexity of orogenic evolution (HORVÁTH and TARI 1999). Continental to oceanic rifting of the Tethyan realm followed the Variscan conver- gence, subduction and continental collision, all shaping the Palaeozoic to Mesozoic substrata of the region. Subsequently, two periods of basin formation and development occurred in a compressional-transpressional regime during the Late Cretaceous and Palaeogene (TARI et al. 1993, TARI 1994). From the earliest Miocene large-scale lat- eral displacement and block rotation took place in the internal domain of the orogen, simultaneously with the forma- tion of the Pannonian Basin (HORVÁTH 1993, CSONTOS 1995, FODOR et al. 1999, BADA and HORVÁTH 2001). This has been characterised by lithospheric extension, interrupted by compressional events (HORVÁTH 1995). Gravitational collapse of the Intra-Carpathian domain, combined with subduction zone roll-back are thought to have been the driv- ing mechanism of the Neogene back-arc extension (RATSCHBACHER et al. 1991, FODOR et al. 1999, BADA and HORVÁTH 2001, HORVÁTH et al. 2004), which gave way to widespread volcanism in the basin. The modern Pannonian Basin is in an initial phase of positive structural inversion, the related structural features are not yet fully developed (HORVÁTH and CLOETINGH 1996, GERNER et al. 1999, BADA et al. 2001). The structure of the basin system is the result of distinct modes of Miocene through Pliocene extension exerting a profound effect on the lithospheric configura- tion. In summary, the Miocene through Quaternary evolution of the Pannonian Basin was characterised by consider- able depth-dependent lithospheric stretching (TARI et al. 1999) as a consequence of the collapse of former orogenic terrains and the subduction rollback of the Carpathian arc (CSONTOS et al. 1992, KOVAČ et al. 1994, FODOR et al. 1999, BADA and HORVÁTH 2001, HORVÁTH et al. 2004). Therefore, the Pannonian basin was classified as a typical Neogene back-arc basin in the Mediterranean system (HORVÁTH and BERCKHEMER 1982). These plate-scale process- es led to the formation of the early Inner Carpathian and the late East Carpathian volcanic arcs (SZABÓ et al. 1992, LEXA 1999). There is a large number of models describing the evolution of the Pannonian Basin which is summarized in BADA and HORVÁTH (2001), such as: 1. asthenospheric dome-triggered active rifting (STEGENA 1967), 2. active rifting that has been initiated by a subduction generated mantle updoming (HORVÁTH et al. 1975, STEGENA et al. 1975), 3. a hinge retreat of the subduction of the European margin driven by the negative buoyancy of the slab that induces trench suction forces and hence, passive rifting in the overriding plate (ROYDEN et al. 1983a, 1983b, ROYDEN and KARNER 1984), and 4. a similar hinge retreat is inferred to be sustained by an eastward mantle flow pushing against the downgoing slab (DOGLIONI et al. 1999) similarly to the inferred present situation in the Etna region (DOGLIONI et al. 2001). In summary, these basin evolution models generally reflect two major views, i.e. active versus passive rifting. The volcanism in the Pannonian Basin is marked by submarine pyroclastic and coherent trachyandesitic lava of Karpathian age (~17.5–16.2 My), Badenian (~16.2–3 My) pyroclastic and coherent basaltic, andesitic, dacitic and rhy- olitic, and Late Miocene to Pliocene (~12–2 My in the western Pannonian Basin) alkali basaltic rocks, which are inter- layered with coeval sedimentary rocks (SZABÓ et al. 1992). Structural interpretation of reflection seismic profiles reveals distinct modes of upper crustal extension in the Middle Miocene – Recent Pannonian Basin (TARI et al. 1992). Phreatomagmatic volcanic fields in a Mio/Pliocene fluvio-lacustrine basin, western Pannonian Basin, Hungary: a review 17 While some sub-basins in the system show little extension (planar rotational normal faults), others are characterised by a large magnitude of extension (detachment faults, metamorphic core complexes — TARI et al. 1992). Seismic strati- graphic interpretations indicate that the non-marine post-rift sedimentary fill of the Pannonian Basin can be described in terms of sequence stratigraphy (VAKARCS et al. 1994). Starting in the latest Miocene (Figure 1.4.), a considerable amount of basaltic magma erupted in the Transdanubian Range (TR) and Balaton Highland area in the Pliocene (SZABÓ et al. 1992, NÉMETH and MARTIN 1999c, NÉMETH et al. 2000). A total magma output has been estimated for the BBHVF on the basis of volume esti- mates of coherent lavas and dense rock equivalents of juvenile pyroclasts from phreatomagmatic and magmatic explosive eruptive products. This gave a minimum estimate of ~2.7 km3 and a maximum estimate of ~6.5 km3 total magma output (NÉMETH et al. 2000). A realistic estimate of 4±0.5 km3 of total magma output (NÉMETH et al. 2000) Figure 1.4. (A) Outline of the Paratethys–Mediterranean region during Late Miocene (after MÜLLER et al. 1999). Diagram is by the authors' permission. (B) Correlation between the Late Neogene (a) Mediterranean and (b) Central Paratethys stages. Column (c) illustrates four possible redrawings of the Late Miocene chrono-stratigraph- ic framework of the Pannonian Basin (Central Paratethys — from SACCHI et al. 1997). Numbers on "c" represent possible solutions of correlation. Diagram is after SACCHI and HORVÁTH 2002 with the authors' permission. (C) Mediterranean and Paratethys stages for the last 15 My (after RÖGL 1998, and MAGYAR et al. (1999). Transdanubian is referred after (SACCHI et al. 1999a) as an intermediate stage (or substage) between the Pannonian s.str. (s. STEVANOVIC1951) and the Pontian s.str. (ANDRUSSOV 1887). The Transdanubian substitutes the lower part of Pontian s. STEVANOVIC (1951). The interval of Neogene volcanism in the western Pannonian Basin is marked by two thick black lines on the left side of the diagram. The diagram is after SACCHI and HORVÁTH 2002, with the authors' permission A G E (M y) (Pannonian Basin) ULRIKE MARTIN and KÁROLY NÉMETH18 over ca. 5.7 My time indicates that the BBHVF rather belongs to a volcanic field with low magma output ratio, which is typical for a strike-slip tectonic regime, or regions of moderate lithospheric extension such as the San Francisco Volcanic Field, Arizona. Approximately 90% of the total magma output is estimated to be basanitic in composition, leaving 10% for more differentiated rock types such as tephrite, phonotephrite, that are the major constituents of the phreatomagmatic pyroclastic deposits (NÉMETH et al. 2000). Estimating of the erupted volume of juvenile material from pyroclasts is more complicated due to the uncertainty of the eroded volumes. The tectonic role and style of magmatism is still under debate (SZABÓ et al. 1992, EMBEY-ISZTIN et al. 1993, HARANGI et al. 1995, KEMPTON et al. 1997). In general most of the workers agree that the Neogene alkali basalts from western and central Europe dom- inantly derived from asthenospheric partial melting (EMBEY-ISZTIN et al. 1993). However, there are growing evidences that in most cases they were modified by melt components from the enriched lithospheric mantle through which they have ascended (EMBEY-ISZTIN et al. 1993). A debate on the role of crustal contamination of the magmas still exists. Most of the models deal with some contamination from a subducted slab from former subduction in the region and/or some degree of metasomatozism (BALI et al. 2002). In general, the incompatible-element patterns of the lavas of the volcanic fields of the western Pannonian Basin show that these lavas are relatively homogeneous (EMBEY-ISZTIN et al. 1993) and are enriched in K, Rb, Ba, Sr, and Pb with respect to average ocean island basalt such as Hawaii, and resemble alkali basalts of Gough Island. Heat flow values in the Pannonian Basin greatly vary (DÖVÉNYI and HORVÁTH 1988), and are up to 300 mW/m2 in areas of volcanic fields that are inferred to be located above shallow magma chambers (SACHSENHOFER et al. 1997, 1998, 2001). Elevated heat flow values from the Austrian basins were obtained from areas related to the Eastern Alps, a consequence of rapid uplift of the whole orogen (SACHSENHOFER et al. 1997, 2001, SACHSENHOFER 2001). In spite of the large number of data available from tectonic and geochemical studies, the question of the rela- tionship between volcanism and the Neogene tectonic processes in the Pannonian Basin has not been addressed adequately yet. One possible explanation, which is in agreement with other field-based observations such as geomorphological evolution and facies relationships, could be that magmatism is related to the latest stage of post-rift faulting (FODOR et al. 1999). In this course, basaltic volcanism may have occurred after the ces- sation of post-rift sedimentation and preserved incipient denudation surfaces (NÉMETH and MARTIN 1999d, NÉMETH, et al. 2003b). The Pliocene basaltic volcanism may also belong to the late-stage inversion of the Pannonian Basin, which was generally associated with uplift and denudation (HORVÁTH 1995, HORVÁTH and CLOETINGH 1996). Lithospheric structure and magma ascent of Neogene alkaline basaltic volcanic fields Continental monogenetic volcanic fields are subject to the same physical constraints as other volcanic systems. Dense mantle-derived magmas are prone to pond near their levels of neutral buoyancy, at depths of 25–30 km, in the upper mantle/continental crust boundary and/or in rheological and density contrast zones between the brittle/ductile transition in mid-crustal levels (RYAN 1987a, b, WALKER 1989, LISTER 1991, LISTER and KERR 1991, WATANABE et al. 1999, 2002). Eruption of such magmas in small volumes requires substantial injected volumes of which only a small proportion reaches the surface, and/or specific stress conditions within the transsected lithosphere (LISTER 1991, WATANABE et al. 1999). Various mechanical considerations of fluid-filled crack propagation (e.g. LISTER 1991) through a lithosphere that is under tectonic stress conclude that either extension (e.g. ascend of mantle material and then upflow of magma along “open” (extensional) fractures) or tectonic inversion (e.g. mantle material ponds at the Moho and other density and/or rheology contrast zones in the lithosphere and then magma is expelled by tectonic forces) seem to be a viable mechanism for volcanic activity of the Neogene alkaline basaltic volcanism in the western Pannonian Basin. During pure extension, when the vector of the maximum compressional stress is vertical and the minimum com- pressional stress is in horizontal position (predominantly normal faulting) vertical dyke propagation is favoured (LISTER 1991, WATANABE et al. 1999). Magma can reach the surface and predominantly form monogenetic, deep-rooted vol- canoes (WATANABE et al. 1999). When the maximum compressional stress is in horizontal and the minimum is in verti- cal position, melt intrusion is only possible when this configuration temporarily switches into either pure extension or to a period when the maximum and minimum compressional stress orientation change place (WATANABE et al. 1999). If the switching period is short, magma can be trapped and form sill-like reservoirs. Further movement toward the sur- face is possible during a new switching period, which process must lead to a multiple level magma “pocket” build up through a series of so called “failed eruptions”. When the maximum compressional stress is in vertical and the mini- mum in horizontal position, but their differential stress is small (e.g. strike slip system) magma gradually can reach the Phreatomagmatic volcanic fields in a Mio/Pliocene fluvio-lacustrine basin, western Pannonian Basin, Hungary: a review 19 surface through multiple “failed eruptions” (WATANABE et al. 1999). In this situation a large magma supply rate is nec- essary in general for the magma to reach the surface. In this condition the generation of polygenetic volcanic systems is favoured if the differential stress is generally small in absolute value and does not change significantly with depth. In conditions when the differential stress is variable according to the depth (e.g. depending on the changeable physical conditions of the crust — temperature, lithology distribution etc.) both polygenetic volcanoes and monogenetic volcanic fields can form. Locations of highly differentiated silicic igneous bodies such as in the axis of the Little Hungarian Plain sub- basin, which is filled by about 3000 m thick low density Miocene to recent siliciclastic sediments (TARI 1994) and cap a metamorphic complex, suggest that these thick, low-density rock-filled basins may have functioned as a den- sity trap (WALKER 1989) and led to magma chamber formation where alkaline basaltic magma fractionated to tra- chyte, trachyandesite in the otherwise predominantly alkaline basaltic volcanic products. Similar density traps have been reported in the Yucca Mts area (Nevada, USA), where low density thick (up to 6 km) ignimbrites that filled a basin functioned as density trap against the ascent of the otherwise buoyant, hot basaltic melt (CONNOR et al. 2000). The inferred timing (~12–9 My) of the formation of such otherwise bimodal (trachyte – basalt — SCHLEDER and HARANGI 2000) polygenetic volcano in the axis Little Hungarian Plain region is in good concert with the pre- dominantly strike-slip controlled tectonic regime in the region in this time (TARI et al. 1992, TARI 1994). Perhaps the alkaline basaltic volcanism in the western Pannonian Basin post-dates this complex volcano and is coincident with the general tectonic inversion that most of the workers accept but the reason of this volcanism is not fully under- stood. From a tectonic point of view, magma may reach the surface in the general compressional regime when it may switch for various time length to be pure extensional or strike-slip dominated. The general complexity of the Neogene “so called” monogenetic volcanoes of the western Pannonian Basin from both a geochemical (see later) and volcanological point of view, and the newly identified dyke and sill complexes associated with these volcanoes suggest that a temporal switch from a compressional tectonic regime to a more strike-slip dominated regime may be a sensible reason for the melt to reach the surface. However, the general agreement on fast uprise of the basaltoid melt that carry large mantle nodules somehow indicate more pure extensional periods temporaly during the general compressional regime. Because up to now there is no clear sign of temporal and/or spatial distribution of volcanic features indicative for these two major scenarios, we suggest that in a generally “unstable” compres- sional regime, the tectonic stress field may have switched to either pure extension or strike-slip regimes according to other controlling factors such as changes of heat flow or position of the mantle anomaly. For a working hypoth- esis, if there is any temporal change in tectonic regime (e.g. gradual transition from pure extension to pure com- pression as a result of the tectonic inversion), 1. monogenetic volcanoes (e.g. short-lived volcanoes with simple architecture) that issued lavas and/or pyro- clastic rocks carried large mantle nodules are expected to be older and 2. younger volcanic edifices may be more complex, often associated with sill complexes. There are sill and dyke complexes that are relatively young (~3 My) and volcanoes erupted large mantle nodules that are old, but it is too early to draw any broader conclusion on the basis of the very limited research that has been carried out in this respect. In summary, it can be concluded that the lithosphere in the Pannonian Basin has a very complex structure. The crust as well as the lithosphere is strongly attenuated and high heat flow prevails (hottest basin in continental Europe). Rock units with variable density, rheology and heat conductivity could have facilitated magma ponding that might have led to further magma evolution via fractionation in these magma storage places (chambers). Such lithos- pheric architecture could temporarily trap otherwise “fast” uprising basaltoid melt of mantle origin when the region- al stress field is switching to a more compressive regime for a short time interval. Such a scenario is very likely dur- ing basin evolution dominated by strike slip faulting, such as is inferred for the western Pannonian Basin during the onset of volcanism in the Late Miocene through Pliocene. Geochemistry and petrogenesis of eruptive products of the Miocene to Pliocene volcanism in the western Pannonian Basin Alkaline basaltic volcanism throughout the Neogene was widespread in the Pannonian Basin, leading to the forma- tion of distinct volcanic fields. Volcanism, in general, since the Miocene is associated with the tectonic development of the Carpatho-Pannonian region and is connected with the formation of the Pannonian Basin (SZABÓ et al. 1992). Volcanic activity has previously been divided into three main genetic types (SZABÓ et al. 1992) according to the com- mon composition, eruption style, and location of the volcanic centres: 1. Early Miocene mainly acidic explosive volcanism that led to the accumulation of extensive ignimbrite sheets (welded and non-welded type, predominantly rhyolitic in composition), block-and-ash flow deposits as well as their ULRIKE MARTIN and KÁROLY NÉMETH20 reworked volcano-sedimentary units, often intercalated with normal marine to terrestrial sediments (LIFFA 1940, PANTÓ 1963, 1966, HÁMOR et al. 1980, SZÉKY-FUX and KOZÁK 1984, PÓKA 1988, CAPACCIONI et al. 1995, SZAKÁCS et al. 1998, LEXA 1999). The recent spatial distribution of this volcanic province exhibits a great separation which is inferred to be the result of a large right lateral displacement along the Mid-Hungarian shear zone (Figure 1.1) during the Early Miocene (STEGENA et al. 1975, BALLA 1980, 1981, ROYDEN et al. 1983a). 2. Middle Miocene – Pliocene calc-alkaline, mainly intermediate stratovolcanic complexes in the Inner Western Carpathians and in the East Carpathians, related to a subducted oceanic slab (KONEČNÝ et al. 1999b). Their geochem- ical compositions show a transitional character between active continental margin and island arc type magmatic rocks (DOWNES et al. 1995a, b). The thickness of the crust increased with time and from west to east beneath this volcanic arc. The small-to-medium sized block-and-ash flow dominated lava dome fields (ZELENKA 1960, BALLA and KORPÁS 1980, KORPÁS and LANG 1993, KARÁTSON et al. 2000) and their associated reworked volcaniclastic successions often formed thick accumulations of volcaniclastic mass flows (KARÁTSON and NÉMETH 2001). Such volcaniclastic succes- sions can be traced in the entire Carpathian Volcanic Chain and often bear significant information on basin evolution. 3. Pliocene–Pleistocene alkali basaltic volcanism in the Pannonian Basin is considered to have been related to an up-welled, then cooled asthenospheric dome (SZABÓ et al. 1992). This is thought to have induced the thermal regime from which magmatic melts ascended (SZABÓ et al. 1992). Various geochemical studies of the alkaline basalts sug- gest mantle up-welling as a major driving force of the alkaline basaltic Neogene volcanism (SZABÓ et al. 1992). The compositional difference in space and time are inferred to reflect the existence of local individual small-sized diapiric bodies as well as several processes (e.g. fractional crystallisation, mixing), which modified the original magma (SZABÓ et al. 1992). Such diapiric bodies are often referred to as common Central European mantle up-welling feeding vol- canic fields across Europe in the Neogene (DUDA and SCHMINCKE 1978, WILSON and BIANCHINI 1999). The eruption of basaltic melts was temporally associated with the final phase of the development of the Pannonian Basin, however, it often has been considered to post-date the cessation of the post-rift sedimentation. The accumulation of volcanic debris on an erosional surface and the volcanism itself is coeval with the start of the basin inversion as was pointed out earlier (HORVÁTH 1995, FODOR et al. 1999, NÉMETH and MARTIN 1999d, MARTIN et al. 2003). There is a general agreement regarding the petrogenesis of Neogene alkali basaltic rocks in the Pannonian Basin. The basaltic rocks were formed during the Late Cenozoic post-orogenic phase and their eruption was related to the evolution of the extensional Pannonian Basin following Eocene–Miocene subduction and its related calc-alkaline vol- canism (SZABÓ et al. 1992, EMBEY-ISZTIN et al. 1993). The alkaline volcanic centres, dated by K/Ar methods are between ~12 and 1 My in age (PÉCSKAY et al. 1995), forming well-distinguishable volcanic fields. Some fields are near the western (Graz Basin, Burgenland, Slovenia), northern (Nógrád–Gömör/Gemer), and eastern (Eastern Transylvanian Volcanic Field) margins of the basin, but the majority are concentrated near the Transdanubian Range (Bakony – Balaton Highland and Little Hungarian Plain Volcanic Field). Coherent lavas range from slightly hy-norma- tive transitional basalts through alkali basalts and basanites to olivine nephelinites. No highly evolved coherent lava (extrusive nor intrusive) compositions have been identified from any of the locations yet (SZABÓ et al. 1992, EMBEY- ISZTIN et al. 1993, HARANGI et al. 1995). This makes the Pannonian Neogene basaltic volcanic fields different from other European volcanic fields such as the Eifel, where e.g. phonolitic lava flows as well as ignimbrites are common (BOGAARD and SCHMINCKE 1985, FREUNDT and SCHMINCKE 1986, BEDNARZ and SCHMINCKE 1990, HARMS and SCHMINCKE 2000). The presence of mantle peridotite xenoliths, xenocrysts, and high-pressure megacrysts in coher- ent lavas, even in the slightly more evolved ones and in pyroclastic rocks, is inferred to indicate that differentiation took place within the upper mantle (DOWNES et al. 1992). However, the mantle source often is referred to be hetero- geneous (DOBOSI 1989, DOBOSI et al. 1991, DOBOSI and FODOR 1992, SZABÓ and BODNAR 1995, 1998, DOBOSI et al. 2003). The study of peridotite xenoliths revealed a strong relationship between deformation and temperatures of peridotites, in as much as coarse-grained protogranular and poikilitic xenoliths had high temperatures (up to 1175 °C), whereas fine-grained equigranular and mosaic xenoliths had low temperatures (800–900 °C — EMBEY-ISZTIN et al. 2001). This picture suggests that diapiric uplift of hot mantle material into a cooler uppermost mantle has proba- bly taken place (EMBEY-ISZTIN et al. 2001). Isotope geochemistry The Sr and Nd isotope ratios from the Neogene coherent lava flows of the Pannonian Basin span the range of Neogene alkali basalts from Western and Central Europe (DUDA and SCHMINCKE 1978, 1985, MERTES and SCHMINCKE 1985, BEDNARZ and SCHMINCKE 1990), and suggest that the magmas of the Pannonian Basin dominantly derived from asthenospheric partial melting. Pb isotope studies, however, indicate that in most cases the astenospheric melt com- position was modified by melt components from the enriched lithospheric mantle through which the magma ascend- ed (EMBEY-ISZTIN et al. 1993). Various metasomatic processes may have interacted with the uprising melts (BALI et al. 2002), similarly to other alkaline volcanic provinces in Central Europe (WITTEICKSCHEN et al. 1993, SHAW 1997, SACHS Phreatomagmatic volcanic fields in a Mio/Pliocene fluvio-lacustrine basin, western Pannonian Basin, Hungary: a review 21 and HANSTEEN 2000, SHAW and EYZAGUIRRE 2000). Delta 18O values indicate that the magmas have not been signifi- cantly contaminated with crustal material during ascent and isotopic and trace-element ratios therefore reflect mantle source characteristics (EMBEY-ISZTIN et al. 1993). The uniform oxygen isotope ratio in the phenocrysts suggests that the mantle source of the alkali basalts was also homogeneous with respect to its oxygen isotope composition, which is in contrast to the relatively wide variation of Sr, Nd and Pb isotope ratios in the source (DOBOSI et al. 1998). Variations in radiogenic isotope compositions in the basalts have been interpreted as result of the interaction of subduction-relat- ed fluids with the mantle source of the basalts. If this was the case, then the fluids, which caused significant changes in the Sr and Pb isotope ratios of the mantle source, did not noticeably modify its oxygen isotope composition. Incompatible-element patterns show that the basic lavas, which erupted in the Balaton area and Little Hungarian Plain, are relatively homogeneous and are enriched in K, Rb, Ba, Sr, and Pb with respect to average ocean island basalt, and resemble alkali basalts of Gough Island type (EMBEY-ISZTIN et al. 1993). In addition, 207Pb/204Pb is enriched rela- tive to 206Pb/204Pb. In these respects, the lavas of the Balaton area and the Little Hungarian Plain differ from those of other regions of Neogene alkaline magmatism of Europe (EMBEY-ISZTIN et al. 1993). This may be due to the introduc- tion of marine sediments into the mantle during the earlier period of subduction and metasomatism of the lithosphere by slab-derived fluids rich in K, Rb, Ba, Pb, and Sr. Lavas erupted in the peripheral areas have incompatible-element patterns and isotopic characteristics different from those of the central areas of the basin, and more closely resemble Neogene alkaline lavas from areas of western Europe where recent subduction has not occurred (EMBEY-ISZTIN et al. 1993). However, in this respect there is no agreement yet. The alkaline volcanic activity that occurred in the Persani Mountains (eastern Transylvanian Basin) and Banat (eastern Pannonian Basin) regions of Romania between 2.5 My and 0.7 My (DOWNES et al. 1995b) produced coherent alkaline basaltic lavas that are primitive, silica-undersaturated alkali basalts and trachybasalts (7.8–12.3 wt.% MgO; 119–207 ppm Ni; 210–488 ppm Cr), which are LREE-enriched (DOWNES et al. 1995b). Mantle-normalised trace-element diagrams revealed an overall similarity to continental intraplate alkali basalts, but when compared to a global average of ocean island basalts (OIB), the Banat lavas are sim- ilar to average OIB, whereas the Persani Mts. basalts have higher Rb, Ba, K and Pb and lower Nb, Zr and Ti. These features slightly resemble those of subduction-related magmas, particularly those of a basaltic andesite related to the nearby older arc magmas (DOWNES et al. 1995b). With 87Sr/86Sr varying from 0.7035–0.7045 and 143Nd/144Nd from 0.51273–0.51289, the Romanian alkali basalts are indistinguishable (DOWNES et al. 1995b) from those of the west- ern Pannonian Basin (Hungary and Austria — HARANGI et al. 1994, 1995, EMBEY-ISZTIN and KURAT 1997) and Neogene alkali basalts throughout Europe. It is inferred that, although the Romanian alkali basalts have a strong asthenospher- ic (i.e. OIB-type mantle source) component, their Pb isotopic characteristics were derived from mantle, which was affected by the earlier subduction (DOWNES et al. 1995b). It is in general agreement that Neogene alkaline basaltic rocks in the Pannonian Basin have some characteristics that represent some influence by former subduction in the region and associated metasomatic processes in the mantle. Petrography of pyroclastic rocks The western Pannonian volcanic fields also consistently comprise basal vitric pyroclastic units overlain by lavas (NÉMETH and MARTIN 1999c, MARTIN et al. 2003). The pyroclastic rocks of the volcanic fields contain various propor- tions of country rock clasts, which apparently represent vent-filling assemblages. Locally there are well-bedded tuff ring deposits preserved (Figure 1.5). Dykes and lava flows have sub-planar to highly irregular, locally peperitic (MARTIN and NÉMETH 2000), contacts with pyroclastic rocks, suggesting intrusion shortly after emplacement of the tuffs and tuff breccias while they were still unconsolidated. The pyro- clastic rocks typically have aphyric or sparse- ly feldspar-phyric juvenile clasts (siderome- lane glass shards), whereas the slightly younger dykes and lavas are characterized by abundant pyroxene±kaersutite phenocrysts. The volcanic glass shards are variously shaped from blocky to strongly stretched being microvesicular and/or containing abun- dant microlites and/or microphenocryst (Plate 1.3). The vesicle morphology of the glass shards exhibits features characteristic for both magmatic degassing and sudden col- lapse due to cooling of the melt by magma Figure 1.5. Typical phreatomagmatic accidental lithic rich (white angular clasts from the Triassic carbonates) lapilli tuff from Pula, BBHVF The shorter side of the photo is 10 cm water interaction (Figure 1.6). There are abundant deep-seated xenoliths, 1 to 15 cm in size, in the uppermost beds of pyroclastic deposits at some of the volcanoes (Plate 1.4). ULRIKE MARTIN and KÁROLY NÉMETH22 Figure 1.6. Scanning Electron Microscope photos of volcanic glass shards from the western Pannonian Basin made on JEOL 6740 Superprobe housed at the Geological Institute, TU-Bergakademie, Freiberg, Germany A — Ság-hegy; phreatomagmatic lapilli tuff. Note the smooth surface and blocky shape of the glass shard. [on rock fragment] B — Ság-hegy; phreatomagmatic lapilli tuff, a close-up of a vesicle. Note the angular limit of the vesicle, as well as the fracture, blocky shape of the shard. [on rock fragment] C — Hajagos; phreatomagmatic lapilli tuff. Glass shards are moderately vesicular, with angular shape vesicles. [on rock fragment] D — Pula; phreatomagmatic lapilli tuff with moderately vesicular phonotephritic lapilli. [on rock fragment] E — Kis-Hegyes-tű; tephritic glass shard with low vesiculaity of a phreatomagmatic lapilli tuff. [on polished thin section] F — Kis-Hegyes-tű; phreatomagmatic lapilli tuff with blocky, non-vesicular tephritic glass shards. Microlites shown on this image are slightly darker. [on polished thin section] Major element variations in monogenetic volcanoes Compositional variations among eruptive products of individual volcanoes just recently have been studied in detail from the Pannonian region (MARTIN et al. 2003, NÉMETH et al. 2003c). In former studies it is assumed that monogenet- ic volcanoes are small to very small volcanoes such as scoria cones, tuff cones and rings, and maars, which formed by single, typically brief eruptions (WALKER 1993). Monogenetic volcanoes might form in 2 distinct settings: 1. as isolated fields of volcanoes on continental lithosphere, ranging from thinned lithosphere (<30 km) resulting from stretching and extension (e.g. Ethiopia, Basin and Range — BARBERI and VARET 1970, ARANDA-GOMEZ et al. 1992) to normal or thick lithosphere (e.g. San Francisco field, Hopi Buttes etc. — CONWAY et al. 1997, 1998), and 2. as “parasitic” vents along the rift zones or flanks of large polygenetic central volcanoes (e.g. Tolbachik (Russia), La Palma (Spain), Mauna Loa (Hawaii, Usa), Tavenui (Fiji), Sawaii (Western Samoa) etc. — FLEROV and BOGOYAVLENSKAYA 1983, DOUBIK and HILL 1999).Some single eruptions forming monogenetic volcanoes atop large central volcanoes are known to have produced petrologically variable magmas (KLÜGEL et al. 1999, 2000) that reflect the presence of magma reservoirs within the large volcano. Such a variation has not been demonstrated in detail in single (small volume) mono- genetic volcanoes of continental fields, which are thought to lack stable magma-storage zones. However, a general trend of compositionally more evolved eruption products in higher stratigraphic level in the volcanic units of complex phreatomagmatic-to-magmatic volcanoes from the Eifel (Germany) region have been reported (DUDA and SCHMINCKE 1978, HOUGHTON and SCHMINCKE 1989). Compositional variations among scoria cones in volcanic fields in a single cone Phreatomagmatic volcanic fields in a Mio/Pliocene fluvio-lacustrine basin, western Pannonian Basin, Hungary: a review 23 Table 1.1. Composition of volcanic glass shards from pyroclastic rocks of erosion remnants of Neogene alkaline basaltic volcanoes of the western Pannonian Basin Sample abbreviations (first number refers to the sample number, the numbers in brackets refer to the location of the sample and correspond to the numbers shown on Figure 1.2: 53 (20) — Horog-hegy, Sz3 (8) — Szigliget, SzV3 (8) — Szigliget, 77 (24) — Pula, 48 (5) — Fekete-hegy south, 43 (5) — Fekete-hegy south, 64 (14) — Hajagos, 65 (14) — Hajagos, VD (10) — Boglár, 52 (19) — Öreg-hegy, K27 (15) — Kopasz-hegy , K28 (15) — Kopasz-hegy, K29 (18) — Kereki-domb, KH30 (22) — Kis-Hegyes-tű, HJ27 (14) — Hajagos, HJ22 (14) — Hajagos, TIH85 (33) — Tihany, BlB (33) — Tihany, II/11 (33) — Tihany, SzK31 (21) — Szentbékkálla, SzK19 (21) — Szentbékkálla, SzK12 (21) — Szentbékkálla, SzK7 (21) — Szentbékkálla ULRIKE MARTIN and KÁROLY NÉMETH24 have been recently described from the Transmexican Volcanic Belt (STRONG and WOLF 2003, SIEBE et al. 2004). However, scoria cones from the Transmexican Volcanic Field often form transition between monogenetic and composite volcanoes (MCKNIGHT and WILLIAMS 1997). In contrast, monogenetic volcanoes are formed by more or less direct erup- tion of magma from the mantle, with each volcano resulting from successful propagation of a small batch of magma to the surface along a new pathway (SPERA 1984, HASENAKA and CARMICHAEL 1985, HASENAKA and CARMICHAEL 1987, HASENAKA 1994, CONNOR and CONWAY 2000). Volcanic rocks from the western Pannonian Basin, were subject of whole-rock analyses that gave systematically basaltic composition (EMBEY-ISZTIN 1993). In spite this, electron microprobe analyses on volcanic glass shards from associated, phreatomagmatic pyroclastic rocks (JEOL 8600 Superprobe, housed in the University of Otago, Geology Department, 15 kV acceleration voltage, 5–20 µm electron beam diameter, OXIDE9 standard, and ZAF correction method) systematically gave a more evolved tephritic, phono-tephritic composition (MARTIN et al. 2003, NÉMETH et al. 2003c — Table 1.1). The com- position of the erupted magmas in the studied areas falls to the alkali basalt field, with the dominant magma type being basanitic (Table 1.1 and Plate 1.4). The pyroclastic rocks are commonly more evolved than the lava flows from the same sites. Volcanic glass shards from all sites are predominantly tephritic, phonotephritic in composition with a minor proportion of teph- riphonolitic or trachybasaltic glass shards (Table 1.1 and Figure 1.7). Compositional vari- ations of the initial pyroclastic sequences and subsequent lava flows and/or dykes suggest a complex magma evolution within a relatively short period of time (hours to weeks). This compositional bimodality of tuff ring formation and lava flow sequences can be explained in two different ways: 1. by the presence of “readily” evolved tephritic–phonotephritic melt at upper crustal level, which — after a short period of residence (days to weeks) — continued its way to the surface and interacted explosive- ly with external water or water-saturated sed- iments. Shortly after emptying these shal- low-level “micro” magma storage places, a deep-sourced basanitic melt reached the surface and generated scoria cones and/or subsequent lava flows and lava lakes, which were commonly involved in peperite-forming processes at each locality (MARTIN and NÉMETH 2000, 2004c). This model is similar to that described from the Canary Islands (KLÜGEL et al. 2000). Alternatively, 2. the ascending melt evolved during its way to the surface, producing individual chemically zoned magma batches with evolved top levels and less evolved bottom parts, as suggested for the Rothenberg vol- cano in the German Eifel (HOUGHTON and SCHMINCKE 1989). The top level of each ini- tial magma batch interacted with external water causing phreatomagmatic explosions. After exhausting the external water supply, a lower magma batch which was less evolved (basanite) managed to reach the surface without phreatomagmatic interaction, filling the craters and experiencing intensive inter- action with the unconsolidated water-rich slurry that occupied the vent zones leading to peperite-forming processes (MARTIN and NÉMETH 2002a, b, 2004b). Figure 1.7. TAS diagrams showing the composition of volcanic glass shards and subsequent lava flows from the BBHVF Sample abbreviations (first number refers to the sample number, the numbers in brackets refer to the loca- tion of the sample and correspond to the numbers shown on Plate 1.1): 53 — Horog-hegy (20), K28 — Kopasz- hegy (15), K29 — Kereki-domb (18), 43 — Fekete-hegy south (5), 52 — Öreg-hegy (19), KH30 — Kis-Hegyes- tű (22), 64 — Hajagos (14), 65 — Hajagos (14), Sz3 — Szigliget, Kamon-kő (8), K27 — Kopasz-hegy (15), 48 — Fekete-hegy south (5), 77 — Pula (25), VD — Boglár, Vár-domb (10), HJ27 — Hajagos (14), SzV3 — Szigliget, Vár-hegy (8), HJ22 — Hajagos (14), SzK31 — Szentbékkálla mafic pyroclastic flow (21), SzK19 and SzK12 — Szentbékkálla (21), II/11, BlB and TIH88 — Tihany Maar Volcanic Complex (33) Phreatomagmatic volcanic fields in a Mio/Pliocene fluvio-lacustrine basin, western Pannonian Basin, Hungary: a review 25 Age of the Neogene intraplate volcanoes in the western Pannonian Basin and their relationship to the immediate pre- and syn-volcanic sedimentation in the region Intracontinental Mio/Pliocene volcanic fields of the western Pannonian Basin developed between 7.56 and 2.3 My (BALOGH et al. 1986, PÉCSKAY et al. 1995, BALOGH and NÉMETH 2004) across an area in size of about 50,000 km2 (Figure 1.1). In the western Pannonian Basin, there are two closely related volcanic fields, that contain the largest num- ber of volcanoes, 1. Bakony – Balaton Highland Volcanic Field (BBHVF) and 2. Little Hungarian Plain Volcanic Field (LHPVF — Plate 1.5). Phreatomagmatic volcanoes in the northern LHPVF tend to comprise broader, lensoid landforms and in their crater/vent volcanic facies peperites are common (MARTIN and NÉMETH 2004a, c). The depth of magma-water inter- action in these volcanoes is inferred to have been shallow (MARTIN and NÉMETH 2004b). The presence of peperites indicates that the host sediment (both siliciclastic and pyroclastic) into which the magma intruded or on which the lava erupted was water saturated (MARTIN and NÉMETH 2000). In the BBHVF, especially in the central and eastern part, a large number of volcanic remnants exhibit features that are characteristic for magma-water interaction in deeper zones (e.g. karst water) of the pre-volcanic sedimentary sequence (NÉMETH et al. 2001). Shallow lakes may have existed in an alluvial plain during onset of volcanism (MAGYAR et al. 1999), which may have led to shallow subaqueous-to-emergent volcanism (Figures 1.1 and 1.4). Textures of pyroclastic rock units as well as the com- mon occurrence of peperites prove this (MARTIN and NÉMETH 2004b). Shallow lacustrine siliciclastic sedimentary units that deposited in these shallow lakes represent the immediate pre-volcanic rock units of the volcanic facies in the western Pannonian Basin. On the basis of unconformity-bounded sedimentary units in the Neogene sequence of the continental sedimentary record of the western Pannonian Basin, three major maximum flooding surfaces have been identified and dated by magnetostratigraphic correlation to be 9.0 My, 7.3 My and around 5.8 My (LANTOS et al. 1992, SACCHI et al. 1999, SACCHI and HORVÁTH 2002). The first maximum flooding event correlates with the Congeria czjzeki open lacustrine beds (LŐRENTHEY 1900, MÜLLER and MAGYAR 1992, MAGYAR 1995, MAGYAR et al. 1999), which marks the Lower Pannonian stage of LŐRENTHEY (1900). After the flooding event, a significant base level drop and subaerial erosion took place around 8.7 My (MÜLLER and MAGYAR 1992, SACCHI et al. 1999). The second maximum flooding event took place around 7.3 My ago and is represented by the appearance of Congeria rhomboidea beds (MÜLLER and MAGYAR 1992, MAGYAR et al. 1999, SACCHI et al. 1999). General low-stand and subaeri- al conditions in the marginal areas are estimated to have occurred around 6 My ago (SACCHI et al. 1999), which was followed by the last known flooding around 5.3 My ago. However, this flooding event has not affected the region of the western Pannonian vol- canic fields. It has reached only the southern margin of the basin (MAGYAR et al. 1999). The intensive geochronological research car- ried out in the past decades on young alkaline basaltic rocks from the Pannonian Basin has con- firmed that K/Ar data on these rocks give the rea- sonable geological ages and the most frequent error is caused by the presence of excess Ar (BALOGH et al. 1996). In spite of the presence of excess Ar detected from the Neogene basaltic rocks of the Pannonian Basin the geological age of these rocks has been obtained by applying the isochron methods (MCDOUGALL et al. 1969, HARPER 1970, MCDOUGALL and COOMBS 1973, SHAFIQULLAH and DAMON 1974, HAYATSU and CARMICHAEL 1977, MCDOUGALL and DUNCAN 1980, MCDOUGALL et al. 2001). Although there are analytical and sampling diffi- culties a great number of K/Ar age data are avail- able from the Neogene basaltic volcanic rocks from the western Pannonian Basin. There is no apparent spatial distribution pattern among major age groups of volcanic rocks (Figure 1.2). The age Figure 1.8. Aerial photograph of the Füzes-tó region, a 2.64 My (39Ar/40Ar — WIJBRANS et al. 2004) old erosion remnant of a scoria cone Note the tuff ring (line) in which the scoria cone formed and which is still well-preserved, (crater rim marked with dashed line). Note that the scoria cone must have been either open, or subsequently partly collapsed toward the north-east ULRIKE MARTIN and KÁROLY NÉMETH26 dates between 8 to 2.3 My seem to be randomly scattered in the area (Figure 1.2). There is a general centre point of ages at around 3.5–4 My BP, derived from volcanic remnants in the western part of the BBHVF (Figure 1.2). On the basis of new, laser induced step heating 39Ar/40Ar high precision ages, it seems, that one of the major part of the alka- line basaltic volcanism in the region, falls into the 3.8–4 My old period, which is in good concert with the previous K/Ar dates (Halom-hegy/3.78, 3.82, Hajagos/3.81, Hegyesd/3.9, Fekete-hegy lava field/3.81, Szigliget diatreme Várhegy pyroclastic sequence/4.08 — BALOGH et al. 1986, BORSY et al. 1986, WIJBRANS et al. 2004). An older age group of volcanoes can be identified on the basis of the 39Ar/40Ar ages at around 4.2 to 4.8 My (Szent György- hegy/4.22, Szigliget lava/4.53, Kis-Somlyó/4.63 and Tóti-hegy/4.74), however dates from Szigliget is likely to be in error, and they rather belong to the previous age group (WIJBRANS et al. 2004). These numbers also represent similar values than previous K/Ar dates from the same volcanoes (BALOGH et al. 1986, BORSY et al. 1986). The oldest known volcanic remnants are in Tihany, and their ages are fixed at around 8 My by repeated attempt to obtain isochron dates by the K/Ar method (BALOGH and NÉMETH 2004). The youngest volcanic edifices are erosion remnants of scoria cones topping the Haláp (3.08 39Ar/40Ar — WIJBRANS et al. 2004), Agár-tető (2.9 K/Ar — BALOGH et al. 1986), Füzes-tó (2.64 39Ar/40Ar — WIJBRANS et al. 2004) and Bondoró (2.3 K/Ar — BALOGH and PÉCSKAY 2001). Among these locations are the most well-preserved scoria cones in the western Pannonian Basin, which are still holding some primary morphol- ogy as well-defined crater rim (Figure 1.8). Overall it can be summarised that the Neogene basaltic volcanism was active in the western Pannonian Basin between ~ 8 and 2.3 My, having a total duration of 5.3 My. Distribution of volcanic erosion remnants Studies of vent distribution in a volcanic field are very useful to establish the relationship between volcanism and tectonism and give some conclusions on the relationship between structural elements and the location of volcanic edi- fices (CONNOR and CONWAY 2000). Such studies have been successfully applied on various volcanic fields. Major ten- dencies of vent migration and tectonic events have been found elsewhere (CONNOR 1990, TOPRAK 1998, CONNOR et al. 2000, ROWLAND and SIBSON 2001). Volcanic erosion remnants in the western Pannonian Basin are clustered into three major well-distinguished vol- canic fields. The Styrian area (in Austria and Slovenia) is well-separated from the Bakony – Balaton Highland and Little Hungarian Plain Volcanic Fields (both are in Hungary), and features only largely separated vents. Volcanic ero- sional remnants from the BBHVF and LHPVF are not clear- ly separated from each other and the transition between these two fields is more continuous (Plate 1.5). The signif- icant difference between these two fields is that the appar- ent vent density in the BBHVF is larger, and vent clustering is more prominent. Vent alignment is more characteristic in the LHPVF, where vents seemingly follow the Rába Fault Zone (Plate 1.5). The distribution of identified volcanic erosion remnants in the BBHVF is represented by contouring vent density on the basis of a rectangular grid with uniform spacing of 2 km and a search radius of 5 km (Figure 1.9, A). On this map, the BBHVF is characterised by one major vent cluster in the geometrical centre of the field and by two additional clus- ters, one in the east and one in the west, all together form- ing a more or less east–west-trending alignment (Figure 1.9, A). The highest vent density reaches 20 vents in an area of 80 km2 (0.25 vents/km2), centred around a nested Figure 1.9. Vent density maps of the Bakony – Balaton Highland Volcanic Field (BBHVF — MARTIN et al. 2003) The distribution of vents is analysed by the density contours drawn manually on the basis of rectangular grids with the spacing of 2 km and search radius of 5 km on figure A, and with a spacing of 1 km and search radius of 2.5 km on figure B Phreatomagmatic volcanic fields in a Mio/Pliocene fluvio-lacustrine basin, western Pannonian Basin, Hungary: a review 27 maar system (Fekete-hegy — MARTIN et al. 2003). This location represents mafic volcaniclastic flow deposits (referred also as hydroclastic flow — NÉMETH and MARTIN 1999b), with large amounts of dm-size lherzolite xenoliths, mantle and/or deep crustal nodules (TÖRÖK and DE VIVO 1995, TÖRÖK et al. 2003), and pyroclastic deposits indica- tive of high-energy phreatomagmatic explosive eruptions (MARTIN et al. 2003). Further individual vent clusters are shown on a vent density map on the basis of a 1-km rec- tangular grid and 2.5 km search radius (Figure 1.9, B), mimicking major known crustal structural zones orientated mainly NE–SW and NW–SE (TARI 1991, BUDAI et al. 1999, BUDAI and CSILLAG 1999, DUDKO 1999). In general, using larger search radius and larger steps on grid, it is inferred that vent-distribution features are related to deep subsur- face features such as the geometry of the melting anomaly (CONNOR 1990). In contrast, smaller search radius and steps on the grid give information on the surface structure of the pre-volcanic system (CONNOR 1990). It has been thus inferred that the Mio-Pliocene volcanism in the BBHVF was related to a characteristic melting anomaly from where the magma intruded into shallow subsurface crustal inho- mogeneities, such as fault lines. Along with the vent clus- tering and alignments the north–south elongation of indi- vidual vents is likely to be related to valley pattern and inher- ited structural elements of the basement rocks (Figure 1.10). Analysis of the present morphology of the central part of BBHVF has revealed that north-south oriented tex- tural pattern exist in this region, either representing palaeo- valleys and/or surface expressions of old structural ele- ments (JORDÁN et al. 2003). In the LHPVF, however, the vents are scattered. It is also noteworthy that major vol- canic complexes such as Ság-hegy, Somló, Kab-hegy and Tihany (Plate 1.5) fall on a straight line that has no obvious surface expression in the form of faults or other structural elements (JUGOVICS 1969b, JÁMBOR et al. 1981). Lavaflows, scoria and spatter cones There is a clear evidence for the presence of preserved fissure-vent systems in the western Pannonian Basin. In a small area around the northern part of the Keszthely Mts (e.g. Sümegprága — Plates 1.1 and 1.5) NE/EW elongated coherent lava rock outcrops occur. The extent of the lava rocks is strongly related to shallow subsurface intrusions such as sill and dyke systems and adjacent small lava flows, plugs. The linear alignment, of the lava rock outcrops sug- gests a fissure related origin, however, their surface exposures either have been eroded already, or never existed. The age distribution of the different eruptive centres also shows an alignment which probably is related to older structural zones in the basement, probably performed by the fluvial systems during the volcanism. Elongated structures of indi- vidual centres, or eruption complexes, especially in the middle part of the BBHVF (Hajagos-hegy, Sátorma-hegy, Fekete-hegy) also suggest longitudinal orientation of individual vents (Figure 1.10). The principal source of the lava flows appears to have been elongated, north to south, north-east to south-west trending former lava lakes. The best example for this is the Hajagos-hegy with a north to south trending lava lake, which overflowed southwards (Kő-hegy — Plate 1.6). The original lava lake was probably 700–1000 m long and 800 m wide. The buttes of Badacsony, Szent György-hegy, Csobánc, Haláp (Plate 1.1) show slightly elongated north–south shape. In the middle part of the BBHVF, there is a large volcanic complex, called Fekete-hegy volcano (Plates 1.1 and 1.5), which consists of smaller eruptive centres with different intercalated lava layers (MARTIN et al. 2002). The lava filled individual centres that also show a NE-SW-trend. The Fekete-hegy is interpreted as a complex lava channel, spatter cone and scoria cone system with several large intercalated lava lakes as well as lava flows fed by scoria cones in the surrounding phreatomagmatic tuff Figure 1.10. North-south elongated volcanic erosion remnants from the BBHVF on digital terrain model The elongated volcanic remnants are named on the map ULRIKE MARTIN and KÁROLY NÉMETH28 ring(s — MARTIN et al. 2002). The southernmost outcrops clearly show irregular shapes of the former lava lake and pyroclastic beds of a former tuff ring (MARTIN et al. 2002). Shield volcanoes are common and give the major sources of lavas in intraplate provinces (WALKER 1993, 2000, CONNOR and CONWAY 2000). Eruptions of large Hawaiian-type volcanic centres are commonly related to fissure-vent systems, but in a small plain-basalt province eruptions are related to central vent systems. However, there are sev- eral examples where shield volcanoes developed along basement fissure systems (JOHNSON 1989). In the BBHVF there are two shield volcanic complexes probably associated with a large number of eruption vents (KORPÁS 1983, NÉMETH and MARTIN 1999d). The larger one (Kab-hegy — Plate 1.1) represents the highest topographic point in the Transdanubian Range. Individual lava flows tend to be around 5 to 8 km long and cover around 50 km2 in area (JUGOVICS 1971, KORPÁS 1983, KORPÁS and SZALAY MÁRTON 1985). The total thickness of the lava cover reaches several tens of metres (JUGOVICS 1971, KORPÁS 1983, KORPÁS and SZALAY MÁRTON 1985). The lava field around Kab- hegy is a thick accumulation of various lavas that have been recognised already in the year 1934 by VITÁLIS. The lava flow units are often separated by thick palaeosoil layers as well as intensive alteration horizons of the basalt itself, indicating time gaps between effusion of lava flows and suggesting a complex eruptive history of this region (VITÁLIS 1934, VÖRÖS 1962, 1966, 1967). The top of the lava field is inferred to be an eroded scoria cone preserved as a plug. Adjacent to the lava fields of Kab-hegy small Strombolian scoria cone remnants and Hawaiian spatter deposits are common. The other large shield volcanic complex is the Agár-tető south-west of the Kab-hegy with a present elevation of 499 m. There is a small remnant of a scoria cone sitting over the lava plateau of the Agár-tető. The wide range of measured K\Ar age (5.25–2.8 My — BALOGH et al. 1986), the different lava flow units, the slightly different petrographic characteristics of the lava flow(s) and the lava inter-beds on the flank of the topping scoria cone sug- gest a long-lived volcanic activity, which might be related to stabile melt sources over structural weakness zone. Such time sequence is well known from several small to medium size shield volcanoes such as the Rangitoto Island (Auckland Volcanic Field, New Zealand — JOHNSON 1989, ENDBROOKE 2001) or large scoria cone examples from the Eifel (Germany) region (HOUGHTON and SCHMINCKE 1989). Moreover, Rangitototo (New Zealand) has a well- developed capping scoria cone as well as eroded satellite vents on the flank of the main edifice of the shield. The small spatter deposits on the top zone of the Agár-tető represent former summit craters of small (50–100 m) scoria and associated spatter cones. The preserved deposits represent small vent zones of this former explosion centres. Small remnants of lava cone structures are traceable everywhere where large lava lakes preserved. These areas usually are small (few tens of metres) irregularities in the large lava fields. They are inter-bedded with lava, and con- sist of spatter deposits (Badacsony, Szent György-hegy, Fekete-hegy, Sátorma-hegy — Figure 1.10). The magmatic explosive and effusive volcanic activity which produced large volumes of eruptive products as well as the pres- ence of the elevated Mesozoic basement under this volcanic zone suggest that there was no magma-water interaction during the eruptive history, thus this area is inferred to have been already a higher elevated area in the Pliocene. These shield vol- canoes in comparison with eastern-Australian examples, are rel- atively small with less than 1 km3 volume of lava products (7 km3 in eastern-Australia, JOHNSON 1989). Large spatter and scoria cones (Figure 1.11) are strongly eroded in the BBHVF. They remained only as erosional rem- nants. Only the summit craters of the Agár-tető, Bondoró and Füzes-tó (Plate 1.5) are preserved in a recognisable morpholo- gy. Strombolian scoria cone remnants are however often pre- served only as scoria mounds on top of larger volcanic erosion remnants such as the Boncsos-tető on top of the Fekete-hegy (MARTIN et al. 2002). Highly vesicular scoriaceous deposits often accumulated between lapilli tuffs rich in accidental lithic clasts indicating that eruption styles may have changed especial- ly in the late stage of the eruptions of individual vents according to the available water to sustain magma–water interaction (HOUGHTON and HACKETT 1984, HOUGHTON and SCHMINCKE 1986, HOUGHTON et al. 1999). Scoriaceous deposits are often rich in irregular shaped mud chunks which are preserved between scoriaceous lava spatters often in significant thickness (tens of metres: e.g. Ság-hegy — Plate 1.1), indicating an active quarrying of an unstable volcanic conduit and/or presence ofFigure 1.11. Welded lava spatter-rich deposit from Ság-hegy Phreatomagmatic volcanic fields in a Mio/Pliocene fluvio-lacustrine basin, western Pannonian Basin, Hungary: a review 29 water-rich slurry in the vent zones during more magmatic fragmentation of magma (KOKELAAR 1986, WHITE 1991b, ORT et al. 1998, MCCLINTOCK and WHITE 2000). Scoria cone remnants are usually preserved in western Pannonian Basin as near vent strongly baked, red, slightly bedded sequences with large spindle or highly vesicular fluidal bombs. Welding of scoriaceous lapilli is common (Plate 1.6). Strombolian scoria and spatter deposits are common in relation with maar volcanoes. Such beds often reflect irregularities in the magma/water interaction having clear magmatic and phreatomagmatic fragmentation styles such as it was observed during the eruption of the Ukinrek Maar, Alaska (KIENLE et al. 1980, SELF et al. 1980, ORT et al. 2000). Scoria cones often develop in maar basins such as the La Breńa Maar in Mexico (Plate 1.7, A), which is a recent analogy how e.g. the Pula maar (Plate 1.1) may have looked like prior to the water filled its basin. Scoria cones are inferred to have grown in maar basins in the BBHVF such as inferred for Uzsa maar (Plates 1.1 and 1.5). Such scoria cones (e.g. Uzsa) may have collapsed into the maar basin, feeding extensive volcanic debris avalanches and associated debris flows, which tend to accumulate in the maar basin. A remnant of a Strombolian scoria cone in the Füzes-tó region preserves near vent scoriaceous volcaniclastic breccia with muddy matrix that is inferred to represent remnants of the water saturated slurry in the vent during the Strombolian activity. The uprise of magma at Füzes-tó is inferred to have involved turbulent jets that did not generate shockwaves as sug- gested in theoretical considerations (MASTIN 2004). Pyroclastic cone growth modelling focuses on the role of ballistic (no-drag) ejection that often are referred as Strombolian activity as a result of weak-intensity, strongly intermittent activity observed to be associated with bursting of large gas bubbles extending across much of the vent, and producing ballistic emplacement of clasts larger than 10 cm (e.g. MCGETCHIN et al. 1974, JAUPART and VERGNIOLLE 1997, VERGNIOLLE and MANGAN 2000). Particles with a size less than 10 cm are normally unable to follow ballistic trajectories instead depositing from eruption clouds with char- acteristic jet dynamics (e.g. subplinian — e.g. SPARKS et al. 1997). The grain size pattern observed from scoria cone remnants of western Hungary suggests other than the classic ballistic (no-drag) model of cone growth (e.g. subplin- ian) and indicates not only Strombolian dynamics of the eruptions as it suggested on the basis of experimental studies (e.g. RIEDEL et al. 2003). No pahoehoe or aa lava formations in the western Pannonian volcanic fields have been preserved. The blocky appearance of the preserved lava flow remnants indicate that they were originally predominantly aa type flows. Direct surface remnants of tumuli, hornitos, pressure ridges, lava tubes, caves or channels are not known from the BBHVF, but several surface irregularities from the Kab-hegy lava field suggest their existence, however, further research needs to constrain this conclusion. On the top of the Fekete-hegy, micro-pahoehoe surfaces on red, rugged lava flows, which are covered by vegetation, are inferred to be the youngest lava flow surfaces in the west- ern Pannonian Basin. Columnar jointing, that is a product of the progressive cooling of lavas or intrusions (SPRY 1962, DEGRAFF and AYDIN 1987, BUDKEWITSCH and ROBIN 1994, LYLE 2000), is widespread in the coherent lavas from the western Pannonian Region. Usually the simple thin sheet-like lava bodies produce simple, upright joints. Thicker lava bodies have two or multiple-tiered layering with lower, well developed upright joints (colonnade) and irregular, semi or wholly radial small joint systems in the top (entablature — LYLE 2000), among which the best exposed is in the Hajagos (Plate 1.7, B). In thick (~10 m) lava flow remnants in the BBHVF. The joint package reaches several metre thicknesses. Other examples comprise Badacsony and Hegyes-tű. Radiating, rosette-like joints in thick lava flows may represent former individual lava channels or feeder dykes that are especially common in the Sümegprága shallow subsurface sill and dyke complex (Plate 1.7, C). Rosette-like joints related to lava tubes are present in the upper level of the Hajagos-hegy basalt quarry, and the Badacsony basalt quarry (MARTIN and NÉMETH 2002b). Hyaloclastite and peperite Hyaloclastite forms by quench fragmentation of magma in contact with water. It can form when lava flows erupt into water or flow from land into water, or where magma intrudes wet unconsolidated sediments (RITTMANN 1958, 1962, 1973, MCPHIE et al. 1993). Such hyaloclastite deposits were found in several places in the western Pannonian region and are inferred to represent lava flows entering a maar lake such as the Uzsa maar. At Hajagos, there are volcaniclastic breccias from in situ debris flanks that consist of large, strongly chilled, micro-vesicular, black, angular basaltic fragments in micro- crystalline carbonate cemented matrix. The fragments in several places contain fine-grained, brown, silty, muddy frag- ments in the vesicles. Another locality close to the Hegyes-tű columnar jointed basalt outcrop shows hyaloclastitic volcani- clastic sediment that contains large pillowed lava fragments indicating lava and water interaction in a water-rich vent. Peperite forms when lava intrudes into wet, unconsolidated sediment. Peperite can be described on the basis of juvenile clast morphology being blocky or fluidal (globular — BUSBY-SPERA and WHITE 1987) but other shapes occur and mixtures of different clast shapes are also found (SKILLING et al. 2002). Magma is dominantly fragmented by quenching, phreatomagmatic explosions, magma-sediment density contrasts, and mechanical stress as a consequence of inflation or movement of magma or lava (DOYLE 2000, SKILLING et al. 2002, ZIMANOWSKI and BÜTTNER 2002, WOHLETZ 2002). According to the observations of BUSBY-SPERA and WHITE (1987) blocky peperite forms usually during interaction of coarse grained, water saturated sediment and melt. In contrast globular peperite forms when melt intrudes into fine grained sediment (BUSBY-SPERA and WHITE 1987). This tight relationship between host sediment grain size distribution and the peperite type seems not always to be the case (DOYLE 2000, DADD and VAN WAGONER 2002, HOOTEN and ORT 2002, MARTIN 2002). Variations are clearly demonstrated from the western Pannonian Basin (MARTIN and NÉMETH 2000). Peperites are common in the western part of the BBHVF and on the LHPVF. Blocky peperite identified from the Hajagos-hegy (Plate 1.1) basalt quarry (lower level — MARTIN and NÉMETH 2000) is related to the feeder dykes that invad- ed fine grained host sediment and lava lake margin that developed in the volcanic depression caused by the phreatomagmatic eruptions (MARTIN and NÉMETH 2000). In near vent position of the lower part of the lava flow at Hajagos there is a lava foot breccia, where small, pillowed lava fragments mix with yellowish sandstone fragments. The peperite formed when a lava flow entered water-saturated sediment, probably in a swampy area. In several localities large (2–3 m wide, 3–4 m high) peperitic bubble structures formed in the lava flow units. Inside the bubbles highly vesicular, close- ly packed pillowed lava formed in sandy matrix. The lava flows are inferred to have formed as tumuli by the vaporisation of the swamp water, during the flow movement. This kind of tumuli structure is common in the lower level of the Badacsony (Plate 1.1) lava flows and in the Hajagos-hegy southern region. Peperites are also described from Balatonboglár, Temető-domb (Plate 1.1 — NÉMETH et al. 1999b). Large black, red scoria fragments in a fluidised sandy matrix represent magma and water-saturated sediment interaction in near vent position. Peperitic lava lake margins have been described from the Ság-hegy, where a lava lake fed small sills (Plate 1.7, E) that intruded into the wet irregular shape tuff ring (MARTIN and NÉMETH 2004c). A crater lake of a small tuff ring at Kis-Somlyó has been flooded by a basan- ite lava flow and developed pillow lava, as well as delicate mixture of basanite melt and silt forming peperite (MARTIN and NÉMETH 2004b). Phreatic and phreatomagmatic eruptive centres Explosive volcanic eruption could be a result of elevated heat of pore water due to dyke (phreatic explosion) or cryptodome emplacement or direct contact between hot magma and various aquifers or standing water body (phreatomagmatic explosion — CAS and WRIGHT 1988). Phreatic explosions are steam generated and do not involve the ejection of fresh magma (CAS and WRIGHT 1988). Phreatic explosions resulting in steep, deep and often wide craters such as formed by the USU 2000 (Hokkaido) erup- tion due to the heat of emplaced shallow subsurface magma body (OHBA et al. 2002). Clear phreatic explosion cen- tres and their products are not described yet from the western Pannonian Basin, however, a large amount of the pyro- clastic beds associated with volcanic remnants of the region is commonly very rich (90 vol.%) in accidental lithic rock fragments derived from various pre-volcanic rock units (Plate 1.8, A). This very high percentage of non-volcanic coun- try rock fragments in the accumulated deposits around vents led to the conclusion that volcanism in the western Pannonian region might be synsedimentary with the siliciclastic sedimentation in the Pannonian Lake (JUGOVICS 1937, KULCSÁR and GUCYZNÉ SOMOGYI 1962, JÁMBOR and SOLTI 1975, JÁMBOR et al. 1981). Phreatomagmatic activity Phreatomagmatic explosions involve dynamic explosive interaction between magma and external water source such as groundwater, or a surface body of water such as a lake or the sea, and the ejection of a significant juve- nile magmatic component (WOHLETZ 1983, FISHER and SCHMINCKE 1984, WOHLETZ and MCQUEEN 1984, WOHLETZ 1986, CAS and WRIGHT 1988, ZIMANOWSKI et al. 1991, WHITE and HOUGHTON 2000). The term “phreatomagmatic eruption” is predominantly used for terrestrial magma-water interaction driven processes. Eruptions initiated in standing water bodies are often referred to as Surtseyan style eruptions (KOKELAAR 1983, KOKELAAR and DURANT 1983, KOKELAAR 1986, WHITE and HOUGHTON 2000). They are characterised by eruption cloud that breach the water surface in advance of the eruption (emergent volcanism). In case the eruption is fully subaqueous with no subsequent water surface breach, the magma water interaction lead to explosive eruption which fed subaqueous pyroclastic density currents mantling the sea/lake floor and move radially, leading to an accumulation of a pyroclas- tic mound (WHITE 1996a, 2000, 2001, WHITE and HOUGHTON 2000, MARTIN and WHITE 2001). Volcanic edifices resulted from both of these volcanic eruptions (emergent and fully subaqueous) often have a similar basal setting, exhibiting pyroclastic rocks rich in juvenile chilled fragments and only a few accidental lithics or minerals derived from the synsedimentary non-volcanic units (e.g. sea floor sediments — WHITE 1996a, BELOUSOV and BELOUSOVA 2001, MARTIN and WHITE 2001). In case of emergence, the vent temporally could be blocked from the open water, and more or less subaerial conditions may be reached, resulting in similar eruption styles than in other terrestrial ULRIKE MARTIN and KÁROLY NÉMETH30 Phreatomagmatic volcanic fields in a Mio/Pliocene fluvio-lacustrine basin, western Pannonian Basin, Hungary: a review 31 phreatomagmatic explosions or even lava fountaining as it has been reported from Surtsey (KOKELAAR 1983, HOUGHTON and NAIRN 1991). During emergent volcanism, tuff cones often build up to levels above the water sur- face, which consists of steeply dipping juvenile clast-rich pyroclastic units (SOHN and CHOUGH 1992, 1993). In case of magma-water interaction in terrestrial setting, the volcanic landform and the erupted products largely depend on the depth of explosion locus and the type of bed rocks (hard rock versus soft rock — LORENZ 1986, 2002, 2003). In case of an unstable volcanic conduit wall, the recycling of erupted pyroclasts as well as the sed- iment laden slurry in the vent could play an important role in the course of the eruption and determine the type of deposit that may accumulate around the vent (HOUGHTON and SMITH 1993, WHITE 1996b). The majority of the eruptive centres of the volcanic fields in the western Pannonian Basin have a phreatomagmat- ic history. Subaerial phreatomagmatic explosions usually produce low rimed tuff rings (WATERS and FISHER 1970, HEIKEN 1971, KELLER 1973, WOHLETZ and SHERIDAN 1983, SOHN and CHOUGH 1989, GODCHAUX et al. 1992, ALLEN et al. 1996, MASTROLORENZO 1994, SOHN 1996, VESPERMANN and SCHMINCKE 2000). The water source of the phreatomagmatic centres in the Western Pannonian region is predominantly a combination of ground water and some water from shallow lakes and/or fluvial systems (NÉMETH and MARTIN 1999c, NÉMETH et al. 1999b, 2001, MARTIN et al. 2003). The identification of magma-water interaction is based on the common presence of 1. chilled juvenile lithic clasts, 2. the angular, blocky, moderately vesicular volcanic glass shards and the 3. variable amount of fragments from the disrupted pre-volcanic rock units. The blocky shape of the volcanic glass shards and a low vesicularity attest to the sudden chilling, as well as the high confining pressure during the magma-water interaction (NÉMETH and MARTIN 1999a) as it has been concluded from other similar volcanic fields (HEIKEN 1972, 1974, HEIKEN and WOHLETZ 1986, ZIMANOWSKI 1986, 1997, 1995, 1998, DELLINO and LAVOLPE 1995, BÜTTNER and ZIMANOWSKI 1998, DELLINO 2000, DELLINO et al. 2001, DELLINO and LIOTINO 2002). Generally the phreatomagmatic products from the western Pannonian Basin are rich in chilled semi-angular juvenile vol- canic lithic fragments and fresh to moderately palagonitized volcanic glass shards typical for fragmentation driven by magma/water interaction (e.g. FRÖCHLICH et al. 1993, BÜTTNER et al. 1999, 2002 — Plate 1.3). The sideromelane glass shards are light brown to yellow (Plate 1.3). The glass shards are slightly to strongly palagonitizated, having a palagonite rim and/or palagonite bands along microfractures (Plate 1.8). The glass shards commonly contain a few elongated microvesicules, which are filled with secondary minerals, especially if the glass shard itself shows advanced stage of palagonitization (Plate 1.3 and 1.8). The vesicles are slightly stretched (Plate 1.3 and 1.8). The phreatomagmatic rock units are characteristically rich in mud, silt and sand derived from the immediate pre-volcanic Neogene shallow marine to fluvio-lacustrine sedimentary sequences (NÉMETH a