冲绳海槽南部流纹岩中角闪石的化学特征及其对岩石成因的指示

摘要

为了揭示冲绳海槽西南端流纹岩成因,利用电子探针和LA-ICP-MS对该流纹岩中角闪石进行了主量和微量元素测定.所有角闪石主量元素成分变化范围小,晶体化学特征主要表现为:CaB>1.60,CaA=0,(Na+K)A<0.33,NaB介于0.23~0.39之间,均为钙质镁闪石.微量元素以富集Sc、V、Cr、Co、Ni和REE,亏损Rb、Sr、Ba、Zr、Th、U和Pb为特征,这些元素的富集与亏损与复杂的类质同象作用有关.由角闪石温压计得出其结晶时的温度范围为775~839℃,压力为0.12 GPa,大致相当于4 km的深度.角闪石的成因矿物学研究显示该流纹岩中角闪石为壳-幔混合成因,进一步表明冲绳海槽南部流纹岩由幔源玄武质岩浆与壳源长英质岩浆混合形成的中基性岩浆在浅层岩浆房中结晶分异而形成,且壳幔混合源很可能是冲绳海槽西南端火山岩岩浆的主要来源.%In order to reveal the petrogenesis of the rhyolite from the southwestern Okinawa Trough ,the major and trace elements of the amphibole phenocrysts in the rhyolite were measured by electron probe and LA-ICP-MS.The composition variation range of major elements of all the amphiboles is small,the crystal chemistry show CaB >1.60,CaA=0,(Na+K)A<0.33,NaB is between 0.23 and 0.39,that belong to calcic series and classify as mag-nesiohornblendes.Trace elements are characterized by enrichment of Sc,V,Cr,Co,Ni and REE,loss Rb,Sr, Ba,Zr,Th,U and Pb.The enrichment and loss of these elements are related to complex role of isomorphic re-placement.Based on the amphibole geothermobarometer,the temperatures and pressures are 775 to 839℃ and 0.12 GPa,roughly equivalent to the depth of 4 km.Genetic mineralogy studies have shown that amphibolite in the rhyolite is a crust-mantle mixing origin,further indicating that the rhyolite was most likely formed by mixing a mantle-derived basaltic magma with a crustal felsic magma,followed by extensive fractional crystallization in the shallow chamber,and crust-mantle mixed source is likely to be the main source of the volcanic rocks from south-west Okinawa Trough .