富水杂岩体地球化学特征及成因探讨

THE GEOCHEMICAL FEATURES AND THE GENETIC STUDY OF THE FUSHUI COMPLEX IN THE NORTH QINLING

富水杂岩体由拉斑玄武岩(TH)系列和钙碱性岩(CA)系列组成。岩石地球化学成分与岛弧岩系相似。两个系列的地球化学变化取决于不同的地幔源和部分熔融程度及岩浆演化趋势。CA系列在成因上与TH系列无关。综合分析表明岩体的构造模式是岛弧和活动大陆边缘。岩体可能是马里亚纳型俯冲带中两期岩浆活动的产物。岩体属岛弧型橄榄岩—闪长岩亚型的一个特殊类型。

The Fushui complex is dispersed along the northern side of the Shangdan fault in the North Qinling. It consists of the tholeiitic (peridotite-pyroxenite-gab-bro) assemblage A and the calk-alkali (gabbro-monzonite-syenite) assemblage B, The two assemblages show some obvious differences in the major-elemen,t, REE and the trace-element geochemistry.The assemblage A δ<3.5,(Ce/Yb ) n = 1.94~35.76. The assemblage B δ>4, (Ce/Yb)n≈1. The cause of all the differences came from the different varying mantle sources, the partial melting degree and the magma-evolutional mechanism. Generally speaking, the geochemical compositions of rocks are similar to the island-arc rock-series, thus mainly showing the lower Ti content, LILE enriched and higher LILE/HESE ratio. It is further confirmed that the complex body was formed in the island-arc according to some geochemical values (ex. <FeO>/MgO, Ba/Zr,Ba/Ce,Ba/La ratio),discrimnant diagrams of tectonic settings (Cr-Y, Zr-Ti, Logδ-LogT) and TiO2-M' changeable tendencies. Perhaps the complex body was the product of the two-stage magmatic activities in the Mariana types subducted zone.The assemblage A is crystalized from the olivine tholaiitic magma which is produced by partial melting(F<20%) of peridotite in mantle wedge under the condition that a fluid is mixed. The assemblage B is crystallized from the basaltic andesite magma which is produced by partial melting (F = 20-25%) of a transformed mantle mixing with the pre-deplated mantle and the magma generated from oceanic crust. Both the magma TH and the magma CA have no genetic relation, but thay have close relation to the Shangdan fault subduction toward the north.The complex body belongs to a peculiar type of island-arc peridotite-diorite complex.