秦岭造山带武关杂岩石榴黑云片岩的变质作用P-T轨迹与构造意义

Metamorphic P-T path and tectonic implication of garnet-biotite schist of the Wuguan complex,Qinling orogen

秦岭造山带是华北板块与华南板块长期汇聚的产物。该造山带以商南-丹凤缝合带为界分为早古生代的北秦岭造山带和晚古生代-中生代的南秦岭造山带。武关杂岩位于南秦岭造山带的北缘,被认为是一套形成于弧前盆地的杂岩,原岩沉积时代为泥盆纪-早石炭纪,经历了晚石炭纪变质作用。本文对武关杂岩的石榴黑云片岩进行了岩石学研究。该岩石主要由斜长石、石榴石、黑云母和石英组成,含少量白云母、金红石和钛铁矿。变斑晶石榴石具有明显的生长成分环带,其核部以MgO含量增加和MnO降低为特征,幔部FeO含量逐渐增加,MgO、CaO和MnO逐渐减少,而在边部CaO明显升高,MgO、FeO和MnO含量不同程度降低。相平衡模拟和温压计计算结果表明,石榴黑云片岩具有复杂的顺时针P-T轨迹,早期进变质阶段(M1)为升温升压过程,峰期条件为~695℃和~8.3kbar,变质矿物组合为石榴石(核)和所含的黑云母、斜长石、石英及钛铁矿包裹体;中期退变质阶段(M2)是降温降压过程,达到~615℃和~6.2kbar变质条件,变质矿物组合是石榴石(幔)和包含的黑云母、斜长石、石英、白云母和钛铁矿包裹体;晚期是以近等温升压为特征的进变质过程(M3),达到~620℃和~9.1kbar条件,变质矿物组合为石榴石(边)和基质矿物黑云母、斜长石、石英、白云母和金红石。M1和M2阶段的构造了一个"发夹状"的顺时针P-T轨迹,为俯冲带变质作用典型特征,表明岩石经历了早期俯冲和后来的折返过程。而以显著增压为特征的晚期进变质作用(M3)很可能表明岩石经历了又一期俯冲作用。因此,本研究表明武关杂岩中的石榴黑云片岩很可能处于俯冲通道之中,经历了多旋回俯冲与折返过程。

The Qinling orogenic belt,formed by a long term of convergence between the North China and South China plates,consists of the Early Paleozoic North Qinling and Late Paleozoic-Mesozoic South Qinling orogenic belts,which are divided by the ShangDan（Shangnan-Danfeng）suture zone.The Wuguan complex,located in the north of the South Qinling orogen,is a Devonian-Early Carboniferous complex and experienced Late Carboniferous metamorphism.This paper reports a petrological study of garnet-biotite schist from the Wuguan complex.The rock is composed of plagioclase,garnet,biotite and quartz with minor muscovite,rutile and ilmenite.The porphyroblastic garnet shows significant growth zoning.The core of garnet has an increasing Mg O and decreasing Ca O content.Toward to the mantle,Fe O increases while Mg O,Ca O and Mn O decrease gradually.At the rim,Ca O increases abruptly but Mg O,Fe O and Mn O decrease.Phase equilibria modelling and geothermobarometer calculation show that the schist has experienced complex P-T path with three stages of metamorphic processes.An early stage of prograde process（M1）was characterized by heating and burial.The peak P-T condition of-695℃and-8.3kbar was recorded by the peak assemblage of garnet（core）,biotite,plagioclase,quartz and ilmenite.A medium stage of retrograde process（M2）,characterized by cooling and decompression to condition of-615℃and-6.2kbar was recorded by the mineral assemblage garnet（mantle）,biotite,plagioclase,quartz,muscovite and ilmenite.A late prograde process（M3）was characterized by near-isothermal burial to condition of-620℃and-9.1kbar.The related mineral assemblage is garnet（rim）,biotite,plagioclase,quartz,muscovite and rutile.The M1 and M2 stages of metamorphism constitute a＂hairpin＂clockwise P-T path,typical of subduction zone metamorphism,suggesting an early subduction and subsequently exhumation process.By comparison,the M3 stage of metamorphism probably shows that the schist has experienced another stage of subduction.This study suggests that the Wuguan complex witnessed a poly-cycle subduction and exhumation process in the subduction channel.