喜马拉雅结晶岩系中的石榴子石

高喜马拉雅地区广泛分布着一套结晶岩系,它是由前寒武系铁铝榴石-闪岩相的巴罗型区域变质岩系、混合岩和第三纪花岗岩组成。在我国境内聂拉木一带出露宽度达50公里。应思淮(1973)把这套岩系命名为珠穆朗玛群,张旗(1979)把它叫做聂拉木群。对于其中区域变质岩的变质带划分,存在不同的看法。

GARNETS OF CRYSTALLINE ROCKS IN THE HIMALAYAS

Garnets of metamorphic rocks,migmatite and granite in the high Himalaya were studied and their physical constant,such as unit cell,refractive index and specific gravity,and chemical composition were determined in this paper.The results are briefly summarized as follows:1.The garnets in Himalayan metamorphic rocks can be divided into the following five classes:A) garnet associated with sillimanite,B) garnet associated with kyanite,C) garnet associated with staurolite,D) garnet associated with biotite and muscovite and E) garnet occurring in the metamorphic basic rocks.The garnets of classes A and B consist mainly of almandite and pyrope components,displaying the smallest unit cell (11.53-11.55 A) for the Barrovian metamorphic fades.The compositions calculated from two monomineralic analysis of garnets are Alm 72.53%,Pyr 20.15%,Spe 1.18% And 2.33% and Aim 82.88%,Pyr 13.65%,Spe 1.50%,Gro 1.90%,respectively.The class D garnets,displaying intermediate unit cell,consist mainly of almandite component and some grossular components i.e.Alm 78.92%,Gro 11.61%,Pyr 3.44%,And 5.70% and Spe 0.32%.The garnets of class E occur in metamorphic amphibolite that were derived from basic volcanic rocks.They have high Ca content,large unit cell and low refractive index.Two monomineralic analyses are Gro 41.09%,Alm 54.54%,Pyr 3.83%,,Spe 0.55% and Alm 37.37%,Gro 33.52%,Spe 9.06%,Pyr 14.27%,And 5.78%,respectively.2.The unit cell of migmatitic garnets varies in a wide range from 11.55 A to 11.66 A,which can be distinguished into two groups,i.e.11.55-11.57 A and over 11.60 A.Therefore,we speculated that Himalaya migmatites might be formed by more than one process.In other words,they are not only related to Pre-Cambrian metamorphism,but also to Tertiary granitization.3.Garnets of the Himalayan Tertiary granites have 11.54-11.57A unit cell,some of them are Mn-rich (MnO 7.05%) garnets.For example,the composition of one sample is Alm 75.90%,Spe 17.34%,Pyr 2.11% and And 4.55%.4.The unit cell of garnet in pelitic metamorphic rocks may be regarded as an indication of the grade of regional metamorphism.The demarcation of Barrovian metamorphic zones is as follows. 5.The Pre-Cambrian Nyalam group in high Himalaya is a dismembered progressive regional metamorphic belt.