冀东迁西-遵化地区麻粒岩相变质水活度的研究

WATER ACTIVITIES ON GRANULITE FACIES METAMORPHISM IN THE QIANXI-ZHUNHUA AREA, EASTERN HEBEI PROVINCE

冀东迁西遵化地区广泛分布的麻粒岩相变质岩石的主要矿物组合为:(1)中粗粒斜方辉石+单斜辉石+角闪石+斜长石±石英±石榴石±黑云母;(2)细粒斜方辉石+单斜辉石+斜长石+石英±石榴石。组合(1)的温度为739-860℃,组合(2)的温度为700-750℃。通过角闪石形成二辉石平衡反应计算得出前一组合水活度为0.46—0.62,后一组合水活度为0.11—0.48。水活度的计算结果与实验结果吻合,并得到矿物流体包裹体成分测定结果的证实。研究表明水活度是麻粒岩相变质作用的独立因素,它控制辉石的形成温度、一定 P-T 条件下的变质反应和矿物组合。这一研究结果与角闪麻粒岩向辉石麻粒岩转化温度升高的传统认识不同。

Metamorphic rocks studied show two major mineral associations: the middle-coarse grain association includes orthepyroxene + clinopyroxene + plagioclase + quartz ± biotite and the fine grain association formed by progressive reaction contains orthopyroxene+ clinopyroxene + plagioclase + quartz±garnet. Using two pyroxenes as geotempreture, temperatures for the first are 739-860℃ and for the second 700-750℃. Water activities calculated by equilibrium metamorphic reaction by which amphiboles would form two pyroxenes are 0.46- 0.62 for the first mineral associatin and 0.11-0.48 for the second. The calculated results are consistent with what are modeled by experiments, and are confirmed by the components of fluid inclutions. This is opposite to the traditional idea that amphibole granulite would change into pyroxene granulite through rising tempreture. It follows that water activities are an independent factor for granulite facies metamorphism, which control the forming tempreture of two pyroxenes, the metamorphic reaction and mineral associations under certain P-T conditions.