喜马拉雅东构造结高压麻粒岩PT轨迹、锆石U-Pb定年及其地质意义  

High-pressure granulites from Eastern Himalayan Syntaxis: P-T path,zircon U-Pb dating and geological implications

作　　者：刘凤麟[1] 张立飞[1] 

机构地区：[1]北京大学地球与空间科学学院,造山带与地壳演化教育部重点实验室

出　　处：《岩石学报》2014年第10期2808-2820,共13页Acta Petrologica Sinica

基　　金：国家自然科学基金创新群体项目资助(41121062)

摘　　要：喜马拉雅东构造结出露了一套基性高压麻粒岩,其峰期矿物组合为石榴石＋单斜辉石＋石英＋金红石＋斜长石,利用相平衡计算其峰期温压条件为904℃、1.37GPa,利用锆石U-Pb定年方法确定其变质年龄为20.7±2.3Ma。角闪斜方辉石麻粒岩为其第一阶段退变产物,其变质矿物组合为斜方辉石＋角闪石＋斜长石＋石英＋钛铁矿＋磁铁矿,温压条件为压力小于0.6GPa,温度为720～760℃。角闪岩相退变矿物组合为角闪石＋斜长石＋石英＋钛铁矿＋磁铁矿,温度小于745℃,压力小于0.6GPa。在角闪斜方辉石麻粒岩中变质锆石获得的定年结果为9.38±0.22M,根据锆石中角闪石＋斜长石＋石英的矿物包体特征,确定该年龄代表角闪岩相退变质年龄。据此,确定了喜马拉雅东构造结基性高压麻粒岩的PTt轨迹为顺时针2阶段折返过程,即第一阶段发生在20Ma左右的由高压麻粒岩相到角闪岩相退变阶段,第二阶段发生在9Ma左右的从角闪岩相深度折返到地表的阶段,计算得到其折返速率分别为2.4mm/y和2.3mm/y,这2个阶段的折返与目前通常认为的青藏高原2个主要抬升阶段是基本一致的。Along Yalu-Tsangpo River,Eastern Himalayan Syntaxis,expose high-pressure mafic granulites. Peak stage assemblage of high-pressure mafic granulite is garnet ＋ clinopyroxene ＋ quartz ＋ rutile. The reaction plagioclase ＋ orthopyroxene→clinopyroxene ＋garnet ＋ quartz indicates peak pressure exceeded the orthopyroxene-out pressure. The peak conditions of high-pressure mafic granulite are 904℃ and 1. 37 GPa. Zircon U-Pb dating shows that the age of high pressure granulite facies metamorphism is 20. 7 ± 2. 3Ma.Amphibole-orthopyroxene granulite is the product of the first stage retrograde metamorphism,and the assemblage is orthopyroxene ＋amphibole ＋ plagioclase ＋ quartz ＋ ilmenite ＋ magnetite,and the P-T conditions are 〈0. 6GPa and 720 ～ 760℃. The overprinted amphibolite facies assemblage is amphibole ＋ plagioclase ＋ quartz ＋ ilmenite ＋ magnetite and the P-T conditions are〈 0. 6GPa and 〈745℃. The age of metamorphic zircon in amphibole-orthopyroxene granulite is 9. 38 ± 0. 22 Ma. According to the inclusions amp ＋ pl＋ qz in such zircon,this age represents the age of amphibolite facies metamorphism. Therefore,the P-T-t path of the high-pressure granulites from Eastern Himalayan Syntaxis is clockwise and it represents a two-stage exhumation process,i. e. the first stage of exhumation： from the depth of high-pressure granulite facies to the depth of amphibolite facies beginning at about 20Ma; the second stage of exhumation： from the depth of amphibolite facies to the earth surface beginning at about 9Ma. According the P-T conditions and the results of U-Pb zircon dating,the average exhumation rates for these two stages are 2. 4mm /y and 2. 3mm /y,respectively. Our study is consistent with the opinion of two-stage uplifts of Tibetan Plateau and further proves that the uplift of Tibetan Plateau connects tightly with the exhumation of subduction zone.

关 键 词：高压麻粒岩 PT轨迹 锆石U-PB定年 喜马拉雅东构造结 青藏高原隆升 

分 类 号：P588.347[天文地球—岩石学] P597.3[天文地球—地质学]