华北克拉通孔兹岩带东端孤山剖面石榴石基性麻粒岩的变质作用及年代学研究  

Metamorphic and geochronological study of garnet-bearing basic granulites from Gushan, the eastern end of the Khondalite Belt in the North China Craton

作　　者：王洛娟[1,2] 郭敬辉[1] 彭澎[1] 刘富[1] 

机构地区：[1]岩石圈演化国家重点实验室,中国科学院地质与地球物理研究所,北京100029 [2]大陆动力学国家重点实验室,西北大学,西安710069

出　　处：《岩石学报》2011年第12期3689-3700,共12页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(40730315、41023009);岩石圈演化国家重点实验室和大陆动力学国家重点实验室开放课题基金联合资助

摘　　要：大同孤山石榴石基性麻粒岩出露在华北克拉通孔兹岩带与中部带的构造接触部位,以大小不等的透镜体形式产于孔兹岩带内的夕线石榴片麻岩和紫苏二长片麻岩中。根据岩相学观察、矿物化学研究、P-T视剖面图和传统温压计计算结果,揭示孤山石榴石基性麻粒岩经历了4个阶段的变质演化:早期进变质阶段(M1)的主要矿物组合为石榴石核心及其内部包体矿物单斜辉石+角闪石+斜长石+石英+钛铁矿±金红石。反环带斜长石富钠核部记录了早期压力可达11kbar;峰期变质阶段(M2)的矿物组合是石榴石斑晶和基质中的单斜辉石+斜方辉石+斜长石+钛铁矿+石英,记录的温压条件为850～900℃、9～10kbar;峰期后降压阶段(M3)的标志是石榴石外围发育的后成合晶和冠状环,主要有单斜辉石+斜长石、斜方辉石+斜长石和角闪石+斜长石组合,其形成温压条件为760～820℃、5～8kbar;晚期低角闪岩相角闪石的生长表明岩石又经历了降温冷却的过程(M4),温度降至690℃以下。石榴石基性麻粒岩记录了含有近等温降压(ITD)阶段的顺时针变质作用P-T轨迹,揭示了阴山地块与鄂尔多斯地块之间俯冲碰撞加厚下地壳的折返过程。石榴石基性麻粒岩的变质锆石LA-ICP-MSU-Pb定年得到了两组变质年龄数据,分别为1945±25Ma和1828±36Ma,它们与阴山地块、鄂尔多斯地块碰撞形成孔兹岩带的时代及华北克拉通东、西陆块碰撞形成中部带的时代一致。结合该地区其他研究结果推断,石榴石基性麻粒岩在～1.95Ga鄂尔多斯地块与阴山地块碰撞过程中俯冲进入下地壳底部,经历早期的高压麻粒岩阶段,随后缓慢地抬升到下地壳上部;之后在～1.85Ga东、西部陆块碰撞过程中,石榴石基性麻粒岩折返到中上地壳。Garnet-bearing basic granulites crop out near Gushan Town, the eastern end of the Khondalite Belt and just next to the Trans-North China Orogen. They occur as enclaves within sillimanite garnet gneisses and hypersthene monzonitic gneisses. Four metamorphic stages have been recognized on the basis of petrographic features and mineral compositions. The conventional thermobarometry and the calculated P-T pseudosections are used to determine the P-T conditions of different metamorphic stages. The early pre-peak metamorphic stage （M1） is represented by garnets＇ cores and their mineral inclusions, including clinopyroxene＋plagioclase＋quartz＋ilmenite±rutile. The sodic cores of matrix plagioclases reveal that the highest metamorphic pressure could be over 11kbar. The peak metamorphic stage （M2） is documented by garnet＋clinopyroxene＋orthopyroxene＋plagioclase＋quartz＋ilmenite, which formed at 850~900℃, 9~10kbar. Symplectites and coronas of clinopyroxene＋orthopyroxene＋amphibole＋plagioclase±ilmenite surrounding garnet porphyroblasts imply a near-isothermal decompression retrograde stage （M3）. The P-T conditions of pyroxene＋plagioclase symplectites and coronas were yielded at 760~820℃, 5~8kbar. The occurrences of lower amphibolite-facies amphiboles suggest a late cooling stage （M4）. The metamorphic temperature was lower than 690℃. These data define a clockwise P-T path with near-isothermal decompressional section for garnet-bearing basic granulites.Metamorphic zircons from the garnet-bearing basic granulites yield two groups of U-Pb ages, 1945±25Ma and 1828±36Ma, which are consistent with main metamorphic episodes recorded in the Khondalite Belt and the Trans-North China Orogen, respectively. The combination of metamorphic and geochronological data indicates that garnet-bearing basic granulites have possibly experienced two independent metamorphic events, suggesting that garnet-bearing basic granulites underwent initial crust thickening followed by the exhumation to some exten

关 键 词：石榴石基性麻粒岩 变质作用P-T轨迹 锆石U-PB年龄 古元古代 孔兹岩带 

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