新疆北部花岗岩─水~18O/~16O交换反应动力学──对于造山带岩浆熔融与流体循环的意义  

OXYGEN ISOTOPE EXCHANGE KINETICS BETWEEN COEXISTENT MINERLS AND WATER IN THEGRANITES OF NORTHERN XINJIANG:IMPLICATINS FOR THE ANATEXIS AND FLUID CIRCULATION IN AN OROGEN

作　　者：刘伟[1] 刘建明[2] 储雪蕾[2] 孙世华[2] 

机构地区：[1]中国科学院广州地球化学研究所,广州510640 [2]中国科学院中国矿物资源探查研究中心,北京100101

出　　处：《地球物理学报》1998年第S1期195-205,共11页Chinese Journal of Geophysics

摘　　要：对新疆北部花岗岩的两个典型岩体—阿拉尔和二台北岩体进行了系统的氧同位素组成测定，以探讨造山带的流体循环以及岩浆源区的熔融两个岩体的岩浆源区相对亏损18OZ它们在上部地壳侵位的次团相冷凝过程中共生矿物向富集18O的方向不平衡偏转推覆到阿尔泰山区热板片之下的晚古生代沉积物冷板片，通过进变质过程中的脱水反应提供了大量的含水流体．在倒转地温梯度驱动下，含水流体从下伏冷板片释放到上覆热板片，与富18O的早古生代变质沉积物发生18O／16O交换反应，并且最终导致造山花岗岩源区的熔融，产生相对亏损18O的过铝质花岗岩浆．对于以二台北岩体为代表的非造山花岗岩而言，幔源岩浆的下垫诱导的进变质反应造成流体对流，促进了幔源岩浆与花岗岩源区岩石的18O／16O交换反应，从而降低了后者的δ18O值．流体循环是深熔作用的先驱事件．与两个岩体侵位相联系的流体具有围绕岩体分布的外封皮性质．当岩浆温度下降到某一数值时，它们迅速进入岩体，瞬间流动速率较大．随着18O／16O交换反应的进行，流体来源迅速枯竭.A systematic research on oxygen isotope exchange kinetics is Performed of the two typical orogenic and anorogenic granite plutons, i. e. the Aral and Ertaibei plutons, selected from the Altay Palaeozoic orogen, northern Xinjiang. Source regions of the two plutons,which have been ascribed to mid-crust and lower crust, respectively, are relatively depleted in 18O. However, the two plutons were regressed towards 18O enrichment daring their subolidus cooling when emplaced into the upper crust level. This fact has important implication for the magma melting and fluid circulation in an orogen. Large amount of hydrous fluids were derived throUgh prograde metamorphism of the cold slab of later Palaeozoic sediments and volcanic rocks thrust under the hot terrain of the Altay Mountain region. Driven by the reversed geothermal gradient, hydrous fluids were released from the underlying cold slab and passed into the overlying hot slab where they exchanged 18O with the Early Palaeozoic metasediments enriched in 18O. Eventually the Early Palaeozoic metasediments were melted under the high water fugacity and the peraluminous grnitic magma depleted in 18O were produced. Regarding the anorogenic grantes exemplified by the Ertaibei pluton, underplating of mantle-derived magma induced prograde metamorphism and caused fluid circulation. This gave rise to the 18O/16O exchange reaction between the mantle magma and the source rocks of grAntes and thus lowered the δ8O value of the latter considerably. Therefore, fluid circulation is a precursor of anatexis. The fluid system associated with empacement of the two plutons were shaped like an envelope around the pluton. The fluids entered the pluton rapidly and manifested a relatively large instantaneous infiltration rate when magma temperature droPPed to some degree of hotness. However, fluid sources were rapidly exhausted with progress of the 18O/16O exchange reaction.

关 键 词：18O/16O交换反应动力学 岩浆源区熔融 流体循环 造山带 新疆北部 

分 类 号：P588.11[天文地球—岩石学]