10월 26일(목)
․암석학/지구화학/표면동위원소분석
98 • • 2006 대한지질학회 추계학술발표회 초록집
Phlogopite in mantle xenoliths from Jeju Island, South Korea
Bokhyun Nam1,*․Kyounghee Yang1․Károly Hidas1,2․ Young-bo Um1․Csaba Szabó1,2
1Division of Earth Environmental System, Colleage of Natural Sciences, Pusan National University, Busan, South Korea, [email protected]
2Lithosphere Fluid Research Lab, Institute of Geography and Earth Sciences, Eötvös University, Budapest, Hungary
Based on previous geochemical and isotopic studies on the upper mantle xenoliths from Jeju Island (southern Korea), the lithospheric mantle consists of nominally anhydrous miner- als(olivine, orthopyroxene, clinopyroxene, spinel) and went through only slight cryptic meta- somatism after a maximum 30% ancient partial melting. However, detailed geochemical and isotopic studies on Jeju basaltic rocks reveal that the original plume-related melt had rather depleted geochemical features,which, entered into the shallow upper mantle, interacted with metasomatized amphibole- and phlogopite-bearing upper mantle. This interaction was followed by a segregation of low-Al alkalic, moderately enriched high-Al alkalic and highly enriched sub-alkalic magmas. This obvious contradiction can be solved by study of some par- ticular phlogopite-bearing xenoliths from Sangumburi hosted in phlogopite- bearing low-Al alkalic basalt. In this preliminary work, a basic petrographic and major element geochemical study of two xenoliths is presented. The rock type of the xenoliths is spinel-lherzolite showing protogranular texture (SG-7) and spinel-harzburgiteshowing porphyroclastic texture (SG-2).
Although these petrographic features are common for the Jeju Island peridotites, the studied xenoliths are particular, because they have modal composition of mica. The reddish brown mica has euhedral or subhedral shape and always situated at the xenolith/host rock border, surrounded by anhedral olivines and orthopyroxenes.
Although the major element chemical composition of the anhydrous mantle minerals is similar to the recently published data from the Jeju Island by Choi et al. (2002). However, the calculated equilibrium temperatures of our studied peridotites are somewhat lower than the average from the Jeju Island. Based on the EMPA data, the mica is a TiO2- (5.2-6.6 wt%), and F- (3.1-4.5 wt%) rich phlogopite. Our textural observations and the geochemical charac- ter of the phlogopite suggest that the mica is a product of a host rock/peridotite interaction at upper mantle conditions. However the upper mantle beneath the Jeju Island wasregarded to have depleted character, the presence of phlogopite is a direct evidence for K-, Ti-, F- and H2O-bearing fluid/melt percolation in the upper mantle. The source of these mobile elements is the project of the further studies.
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