石鼓群的划分及变质特征探讨

A PROBE INTO THE DIVISION AND METAMORPHIC FEATURE OF SHIGU GROUP

石鼓群位于云南西北部,面积约1 600km2,为甘孜—理塘板块的结晶基底。据现有资料,分为古元古代黎明岩群、中元古代新主岩群、新元古代巨甸岩群。黎明岩群为云母石英片岩、云母片岩,夹斜长片麻岩、斜长角闪岩。泥质岩石中具铁铝榴石、十字石、蓝晶石组合,基性岩中具铁铝榴石、普通角闪石组合,为中压低角闪岩相。黎明岩群后期抬升减压,叠加低压低角闪岩相变质,新生红柱石,部分原生蓝晶石退变成红柱石。于侵入其中的斜长角闪岩中获得Nd同位素模式年龄1 369.8Ma和1 343.8Ma,绿泥石英片岩中获得1 896.6Ma和2 161MaNd同位素模式年龄,故将黎明岩群时代置于古元古代。新主岩群分哈吉洛岩组和路西岩组,岩性为云母石英片岩、云母片岩、大理岩夹片麻岩及斜长角闪岩,部分岩石具糜棱岩化。哈吉洛岩组为低压低角闪岩相,具红柱石、十字石、铁铝榴石组合,路西岩组为高绿片岩相。于侵入新主岩群中的变质变形花岗质岩脉中获得锆石铅同位素年龄811±2Ma、820±1Ma、823±1Ma,故将新主岩群时代置于中元古代。巨甸岩群分陇巴岩组和塔城岩组,岩性为千枚岩、板岩,夹一层变质杏仁状玄武岩,为低绿片岩相。

Shigu Group is in the NW part of Yunnan, with area about 1 600kin2, which is the crystalline basement of Ganzi--Litang Plate. According to the present material, it can be divided into Palaeoproterozoic Liming rock group, Mesoproterozoie Xinzhu rock group, Neoproterozoic Judian rock group. Liming rock group is composed of mica quartz schist, mica schist with intercalation of plagioclase schist, amphibolite. There is the assemblage of almandine, staurolite and kyanite in argillaceous rock, the assemblage of almandine and homblend in basic rock, which is the middle P amphibolite facies. Liming rock group rise and pressure-decrease in the later period, super- imposed by low P metamorphism of low amphibolite facies, the neogenic andalusite, a part of original kyanite be- comes degradedly andalusite. We have the Nd isotope model age 1 369.8 Ma of amphibolite intruding it, 1 896. 6 Ma and 2 161Ma of chlorite quartz schist, so the age of Liming rock group must be Palaeoproterozoic. Xinzhu rock group can be divided into Hajilo rock formation and Luxi rock formation, composed of mica quartz schist, mica schist, marble with intercalation of gneiss and amphibolite, a part of rock mylonitized. Hajilo rock formation is low P low amphibolite facies, with the assemblage of andalusite, staurolite and almandine, Luxi rock formation is high greenschist facies. We have the zircon Pb isotope age 811 ±2 Ma, 820± 1 ga, 823 ± 1Ma, so the age of Xinzhu rock group must be Mesoproterozoic. Judian rock group can be divided into Longba rock formation and Tacheng rock formation, composed of phyllite, slate with an intercalation bed of metamorphosed amygdaloidal basalt, belonging to low greenschist facies.