Geological evolution and magmatic models for spatially and temporally variable modes of distributed volcanism, Jeju Island, Republic of Korea : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand

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Dispersed volcanism in intraplate settings produces volcanic fields that may erupt over millions of years to produce hundreds to thousands of scoria and spatter cones, tuff cones and maars, as well as lava flows. Many aspects of this globally ubiquitous process are poorly known, ranging from the tectonic drivers to the mechanisms controlling magma accumulation and ascent. To investigate magma generation processes leading to a variety of individual eruption types at volcanic fields and to understand the spatio-temporal evolution of these whole-systems, knowledge of the geochemical and petrological properties of erupted products must be linked to the geologic and tectonic framework. This study was based on detailed stratigraphic sampling of small- (<0.01 km3) and large-volume (>1 km3) eruptive sequences in the Jeju Island Volcanic Field, Korea at both individual exposed eruption centres and from deep drill cores. This island is the subaerial representation of a volcanic field developed above continental crust over the last 1.8 Ma. Pyroclastic and lava samples were analysed for whole-rock major-, trace-elements and Sr-Nd-Pb isotopes, and for mineral compositions and Sr-Nd-Pb isotopes. The Jeju magmatic system started with small-volume alkali basaltic eruptions sourced at mantle depths equivalent to c. 2.5 GPa in partially hydrous peridotite. These magmas passed through the crust and erupted rapidly, with minor modification. Intrusions and eruptions accommodated regional tectonic strain, and excess melts became stalled to fractionate toward trachyte compositions in both the lower and upper crust. Trachyte erupted sporadically, with the first episode at c. 750 ka. After this, the system started to erupt with volumetric rates two orders of magnitude higher. This accelerated magma production involved alkali basalt melts derived from greater depths/pressures (3.5 GPa) than earlier, along with subalkali basalts derived from c. 2.5 GPa. Despite prevailing extensional tectonics in the Ryukyu Volcanic Arc and strain accommodation at Jeju, further magmas accumulated and evolved to trachyte compositions at lower crustal depths and erupted in a second episode c. 25 ka ago. Small-volume eruptions of rapidly rising primitive alkali basalt also continued throughout the life of the field, and potentially interacted with shallower reservoirs of subalkali magmas to generate bimodal volcanism. Depending on magma volumes, intrusion and plumbing complexities, these generated a range from simple volcanic structures to complex multiple-episode and/or multiple vent eruptive centres at the surface. The geochemical data collected revealed how seemingly simple monogenetic eruptions can be fed by complex and distinct magmatic entities. The same was valid for the entire field, where magma source and evolution conditions vary over time. The variety in volcanic activity is a function of magma types influenced by prior mantle modification events, as well as local and distal tectonic stresses and strain arrangements. This study showed that it is ultimately the site and spatial pattern of melting and melt-production rate that determines the final surface morphology, elevation and spatial distribution of magma types in a volcanic field.
Appendices A-M (the following published articles and their associated data files) removed for copyright reasons: Brenna, M., Cronin, S.J., Smith, I.E.M., Sohn, Y.K., & Németh, K.(2010). Mechanisms driving polymagmatic activity at a monogenetic volcano, Udo, Jeju Island, South Korea. Contributions to Mineralogy and Petrology, 160, 931-950. doi:10.1007/s00410-010-0515-1. Brenna, M., Cronin, S.J., Németh, K., Smith, I.E.M., & Sohn, Y.K.(2010). The influence of magma plumbing complexity on monogenetic eruptions, Jeju Island, Korea. Terra Nova, 23, 70-75. doi:10.1111/j.1365-3121.2010.00985.x. Sohn, Y.K., Cronin, S.J., Brenna, M., Smith, I.E.M., Németh, K., White, J.D.L., Murtagh, R.M.,Jeon, Y.M., & Kwon, C.W. (2012). Ilchulbong tuff cone, Jeju Island, Korea, revisited: A compound monogenetic volcano involving magma pulses, shifting vents, and discrete eruptive phases. Geological Society of America Bulletin, 124, 259-274. doi:10.1130/B30447.1. Brenna, M., Cronin, S.J., Smith, I.E.M., Maas, R., & Sohn, Y.K. (2012). How small-volume basaltic magmatic systems develop: A case study from the Jeju Island Volcanic Field, Korea. Journal of Petrology, 53, 985-1018. doi:10.1093/petrology/egs007. Brenna, M., Cronin, S.J., Smith, I.E.M., Sohn, Y.K., & Maas, R. (2012). Spatio-temporal evolution of a distributed magmatic system and its implications for volcano growth, Jeju Island Volcanic Field, Korea. Lithos, 148, 337-352. doi:10.1016/j.lithos.2012.06.021.
Magmatism, Volcanism, Geology