Magnetic characteristics of the Ság-hegy volcanic complex, little Hungarian Plain
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Date
2006-01-01
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Massey University.
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Abstract
The Ság-hegy volcanic complex is located in the little Hungarian Plain Volcanic Field (LHPVF). An 39Ar/ 40Ar geochronolgy gave an isochron age of 5,42 ±0,06 My for the Ság- hegy (Wijbrans et al. 2004). Evolution of the volcano included two clearly distinct events. At first ascending magma entered meteoric water in a fluvio-lacustrine environment. Fuel-coolant interaction (FCI) of water (water saturated sediment) and magma led to the formation of a phreatomagmatic tuff ring. After water supply was used up the interior of the tephra ring was filled by a lava lake. Locally the tuff ring wall collapsed and subsequently lava was able to flow out of the tuff ring. Due to intensive quarrying most of the effusive rocks have been removed, giving excellent insight to emplacement processes of feeder dykes, sills and lava lake remnant (Martin and Németh, 2004). Pyroclastic rocks include massive and bedded units of lapilli stone, lapilli tuff/ tuff as well as pyroclastic breccias. Varying proportions of accidental lithic clasts indicate excavation of basement rocks during the erruption. Juvenile clasts comprise mainly of angular, blocky sideromelane glass shards with nearly equent shapes and a minor proportion of tachylite. A high amount of water within the systeme is evidenced by soft sediment deformation and accretionary lapilli in the pyroclastic bedsets. Dune and antidune bedding, chute and pool structures grading and sorting features suggest that the tuff ring was gradually built up by base surge and intercalated fallout deposits. Subsequent to the phreatomagmatic stage the inner crater has been filled with a lava lake which morphology was determined by the tephra deposits. At contacts to the pyroclastics a chilled margin of several cm thickness is developed which shows platty (onion shaped) jointing. A high number of dykes and sills were injected into adjacent bedsets. These shallow intrusive bodys can be found throughout the whole complex truncating and dissecting the pyroclastic units. In cases where pyroclastic units comprised a high amount of water this included even mingling with the wet tephra, leading to the formation of peperites. The uppermost units were represented by thick lava flows, which covered all underlaying units. These rocks were quarryed out already a century ago except a large strombolian spatter cone which is now exposed at the uppermost level of the quarry as a big sliced remnant including its large multiple feeder dyke. This setting offers a perfect opportunity to study the relationship between dyke and sill enplacement with transitions from vertical to bedding-parallel geometries. Dimensions of the volcanic bodies range from cm thickness of small apophyses from the lava lake into the pyroclastic rocks up to dykes and sills of several m. We performed a detailed study on a section of pyroclastic rocks truncated by dykes and sills and have evaluated the magnetic characteristics. Preliminary results show that magnetic susceptibility of all the pyroclastic units is in the range of ferrimagnetic susceptibility and varies between 2 to 20 x 10-3 SI. (Fig.1). Magnetic fabric anisotropy is generally low (< 5 %) and in the field of oblate fabric geometries, in bedded tuffs a significantly higher (5 to 10 %) but also oblate anisotropy is realized. Magnetic lineations indicate a consistent NE (020) directed material transport for the whole succession. Remanence intensities are quite high with values of 1 to 15 A/m In the pyroclastic units a stable magnetic remanence characterized by a single vector component has been measured, MDF values are in the range of 30 to 160 mT. The field vector has exclusively reversed polarity and steep inclination, which is in agreement with the paleofield direction and therefore is regarded as natural remanent magnetization aquired during deposition of the pyroclastic successions. In the dykes and sills, however, remanence direction scatter significantly and display geometries ranging from steep to flat orientations and show also strong variations in the declination. Coercitivity of magnetic carriers is significanty lower as indicated by the lower MDF values which are in the range of 8 to 30 mT in the dykes and 15 to 30 mT in sills. Beside a minor contribution of a viscose component the remanence vector in the dykes and sills is characterized by a stable single component. However, further investigations are needed to fully understand and interpret the results