四川米易县安宁村含矿层状侵入体的岩石学特征及成因探讨

安宁村含钒钛磁铁矿层侵入体侵位于下元古界变质岩中,厚约500m,岩体层状构造发育,分异程度较高,可分为四个岩相带和若干次级的韵律层。其初始岩浆为富铁钛,贫硅的弱碱性玄武岩浆,源于上地幔。岩浆中的律性成核与重力分异作用的结果,形成层状岩系。成岩温度1250－900℃,氧逸度foz=10^-6.38,平均成岩压力9．55kbar。较高的成岩压力控制了岩浆演化的主要途径和主要矿物组合;而偏高的氧逸度则是较早期大量Ti-Fe氧化物堆积形成岩浆结晶分异型富矿层的主要因素。

PETROGRAPHIC CHARACTERISTICS AND ORIGIN OF THE ANNINGCUN V-TI-MAGNETITE-BEARING LAYERED INTRUSION,MIYI,SICHUAN

Anningcun V-Ti-magnetite-bearing layered intrusion is considered to have been emplaced into the Lower Proterozoic metamorphic rocks. The total thickness is about 500m. Layered structure is well developed in this intrusion,and it might be divided from bottom to top into four petrographic zones. They are lower gabbroic zone.ultrabasic zone,gabbroic zone and pyroxene dioritic zone,as well as several secondary rhythmic layers. The initial magma is characterized by richFe-Ti,poor-Si and high-alkali,and belongs to subalkaline basaltic one which is inferred to be mantle-derived. The emplacement depth of magma is about 25 km. The layered series are formed mainly via rhythmic nucleation and crystal setting, but preliminary liquid-gravity differentiation of the magma is also believed to have happened befor it began to crystalize. The pyroxene diorite face at late stage,however,is a product of the residual liquid by crystalization in situ. The temperature for the formation of all rocks ranges from 1250 C to 900"C,oxygen fugacity equals 10~(-5.38),and average pressure is 9. 55 kbar. It is suggested that the relatively high pressure is an important factor controlling both the evolution path of the magma and the mineral assemblages of rocks. The higher oxygen fugacity is an essential condition under which abundant Ti-Fe-oxides accumulated to form the rich ore bed at early stage.