华北南缘舞钢太华杂岩变泥质岩变质作用:变质P-T轨迹与锆石U-Pb年龄

Metamorphism of metapelites in the Taihua Complex,Wugang,southern margin of North China:Metamorphic P-T paths and zircon U-Pb ages

太华杂岩在华北中部造山带南缘舞钢地区存在出露,本文选取该地3个变泥质片麻岩为研究对象,对其进行了岩石学、相平衡模拟和锆石U-Pb年龄分析等综合研究。结果显示其普遍保留了三个变质阶段的矿物组合:进变质阶段(M1)矿物组合主要以矿物包裹体保存在石榴子石变斑晶之中,峰期变质阶段(M2)矿物组合主要保存在基质中,退变质阶段(M3)矿物组合以石榴子石变斑晶周围的后成合晶矿物组合为代表。石榴镁铁闪石黑云二长片麻岩M1矿物组合为Bt1+Pl1+Ilm1+Qz1±Amp1,M2矿物组合为Grt2+Bt2+Pl2+Amp2+Qz2+Mt2±Ilm2,其P-T条件为810~855℃、0.8~1.0GPa,M3矿物组合为:Bt3+Pl3+Qz3+Amp3±Ilm3±Mt3。2个石榴夕线黑云片麻岩M1矿物组合为Bt1+Sil1+Mt1+Ilm1+Qz1±Pl1±Rt1,M2矿物组合为Grt2+Bt2+Sil2+Qz2±Pl2±Ilm2±Mt2,其P-T条件为795~850℃、0.7~0.9GPa,M3矿物组合为Bt3+Sil3+Qz3±Pl3±Ilm3±Mt3±Rt3。结果表明舞钢变泥质片麻岩记录了近等温降压(TTD)的顺时针P-T轨迹,这样的P-T轨迹反映了一个碰撞造山的过程。锆石U-Pb年龄结果指示这些片麻岩源区时代为~2.86Ga,原岩形成年龄为~2.36Ga,在约1.96~1.84Ga期间经历了高级变质作用,而约1.84~1.80Ga可能代表了其退变质作用时间。这表明中部造山带的南部可能经历了一期拉长的构造-热事件(约1.96~1.80Ga),持续时间长达~160Myr之久,可能代表华北克拉通东部陆块和西部陆块在古元古代沿着中部造山带的俯冲-碰撞造山过程。

The Taihua Complex exists in Wugang area on the southern margin of Trans-North China Orogen(TNCO).Three metapelitic gneisses in the region are selected for a study in this paper,including petrology,phase equilibrium modeling and zircon U-Pb age dating analyses.The results show that they generally retain the mineral assemblages of three metamorphic stages:the mineral assemblage of prograde metamorphic stage(M 1)is mainly preserved in the garnet porphyroblasts as mineral inclusions,and the mineral assemblage of the peak metamorphic stage(M 2)is mainly preserved in the matrix,while the mineral assemblage of retrograde metamorphic stage(M 3)is represented by the symplectic minerals surrounding the garnet porphyroblasts.The M 1 metamorphic mineral assemblage in garnet-cummingtonite-biotite-two-feldspar gneiss is Bt 1+Pl 1+Ilm 1+Qz 1±Amp 1,and the M 2 mineral assemblage is Grt 2+Bt 2+Pl 2+Amp 2+Qz 2+Mt 2±Ilm 2,with the P-T conditions of 810~855℃and 0.8~1.0GPa,with the M 3 metamorphic mineral assemblage is:Bt 3+Pl 3+Qz 3+Amp 3±Ilm 3±Mt 3.The M 1 mineral assemblage in two garnet-sillimanite-biotite gneisses is Bt 1+Sil 1+Mt 1+Ilm 1+Qz 1±Pl 1±Rt 1,and the M 2 mineral assemblage is Grt 2+Bt 2+Sil 2+Qz 2±Pl 2±Ilm 2±Mt 2,with the P-T conditions of 795~850℃and 0.7~0.9GPa,while the M 3 mineral assemblage is Bt 3+Sil 3+Qz 3±Pl 3±Ilm 3±Mt 3±Rt 3.The results show that the metapelitic gneisses in Wugang record clockwise P-T paths of an isothermal decompression(ITD),and such P-T paths reflect a collisional orogenic process.The zircon U-Pb age results indicate that the source age of these gneisses was~2.86Ga,with the formation age of the protolith of these gneisses was~2.36Ga and then experienced high-grade metamorphism during ca.1.96~1.84Ga,whereas ca.1.84~1.80Ga likely represents the time of retrograde metamorphism.These indicate that the southern margin of TNCO may have experienced a prolonged tectonic-thermal event(ca.1.96~1.80Ga),and lasted as long as~160Myr,which may represent the subduction-collision orogenic process of the Eastern Block and Western Block of the North China Craton along TNCO in the Paleoproterozoic.