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[SVC44-05] Hornblende fractionation of tholeiitic volcanic rocks from Yoneyama Formation originated in high water content magma
Keywords:Cryptic hornblende fractionation, Tholeiitic rock series, Hydrous magma, Lower crust, Northern Fossa Magna, Ca-amphibole
Whole-rock rare earth elements (REE) content show middle REE depleted normalized patterns. This feature is attributed that Hbl fractionation because Hbls have higher middle REE partition coefficients than other major rock forming minerals. Calculated host rock SiO2 content restored from Hbl gabbroic xenoliths and Hbl megacrysts chemical composition (Putirka, 2016) range between 50 - 59 wt%, which support that Hbls crystallization begin in basaltic magma. It is thought that these xenoliths and megacrysts crystallize at depths of lower crust because calculation results of geobarometer (Ridolfi and Renzulli, 2012) show up to 1857 MPa. These geochemometric and geobarometric results support ‘cryptic Hbl fractionation’. Hbl’s solidus temperature decrease with decreasing pressure, thus Hbls decompose during magma ascent.
Hbl (cryptic) fractionation imply that TH series magmas are rich in H2O contents. The quantitative H2O contents estimated by plagioclase-melt hygrometer are up to ~5 wt%.