新疆阿尔泰阿拉尔花岗岩多维矿物相-水氧同位素交换反应动力学  

OXYGEN ISOTOPE EXCHANGE KINETICS BETWEEN COEXISTING MINERALS AND WATER IN THE ARAL GRANITE PLUTON, ALTAY MOUNTAINS, NORTHERN XINJIANG

作　　者：刘伟[1] 贺伯初 陈振胜[2] 

机构地区：[1]中国科学院长沙大地构造研究所 [2]中国地质科学院宜昌地质矿产研究所

出　　处：《地质学报》1996年第2期129-141,T001,共14页Acta Geologica Sinica

基　　金：国家自然科学基金(编号 49373164);中国科学院院长基金

摘　　要：新疆阿尔泰阿拉尔花岗岩中共生石英、长石、黑云母的δ^(18)O值具有宽广的变化范围,3者表现出显著的^(18)O/^(16)O不平衡关系,尤其是石英、长石具有倒转△^(18)O_(石英-长石)关系(△^(18)O_(石英-长石)<0)。上述关系清楚地表明在花岗岩-水之间发生了^(18)O/^(16)O交换反应。根据质量平衡约束关系对岩石与外来流体的初始δ^(18)O值作了估计。由于花岗岩的初始δ^(18)O值被大大地改变了,简单运用全岩或者单矿物样品的δ^(18)O值判断岩浆来源的方法是不可靠的。水-岩之间的^(18)O/^(16)O交换反应并不伴随着岩石矿物学上的强烈蚀变效应,这种脱藕关系暗示着交换反应发生于较高温度即岩浆次固相-岩浆期后冷凝过程中。交换机制主要为扩散控制。定量模拟表明:阿拉尔岩体具有高流体流动速率(3×10^(-14)mol/s)、长水-岩反应时限(0.8-6Ma)、高水/岩值(0.79-6.14)。在造山带岩浆弧环境的中部地壳层次下,变质流体流动和环流是非常强烈的。Coexisting minerals quartz, feldspar and biotite vary widely in δ18O values and display a conspicuous 18O/16O disequilibrium relation; especially, a quartz-feldspar reversal (△18OQ-F<0) exists in the Aral granite pluton, Altay Mountains, northern Xinjiang. This relation explicitly shows that the 18O/16O exchange reaction definitely occurred between granite and water. Initial δ18O values of the granite and exotic water were evaluated by the mass balance consideration. The conventional method of discrimination between various magma sources simply with δ18O values of either whole rock or separate minerals is misleading and unreliable. Experiments carried out by the authors show that the 18O/16O exchange reaction is not necessarily accompanied by what geologists describe as petrological and mineralogical alteration effects. The decoupling relation suggests that the exchange reaction occurs at a relatively high temperature during subsolidus-postmagmatic cooling of magmas. The exchange mechanism is mainly diffusion-controlled. It is demonstrated through quantitative modelling that the hydrothermal system associated with the Aral pluton is long-lived (0. 8 -6 Ma), with a relatively high fluid flow rate (3 ×10-14mol/s) and a high W/R ratio (0. 79 - 6. 14). The above results imply that an intense flow and convection may exist in the midcrust level of orogenic magmatic arcs.

关 键 词：花岗岩 多维矿物相 氧同位素 水-岩 动力学 

分 类 号：P588.121[天文地球—岩石学]