东秦岭深源麻粒岩包体的性质及其构造意义

CHARACTERISTICS OF LOWER CRUSTAL GRANULITE XENOLITHS FROM THE EAST QINLING OROGENIC BELT AND THEIR TECTONIC IMPLICATIONS

岩相学、岩石化学、矿物温压计和单颗粒锆石 Pb-Pb 年代学综合研究表明,出露于桐柏地区秦岭杂岩中的麻粒岩包体是在早古生代(470Ma)秦岭洋壳消减过程中,俯冲板片在大约30—40km 深度经受麻粒岩相变质(470—435Ma 之间),随后其残片被花岗闪长质岩浆捕虏而上侵就位(435Ma)。古秦岭洋壳俯冲、变质、分熔乃至上侵这一系列构造过程都可以从麻粒岩包体研究结果中得到理想的印证。

It is known that Tongbai of Henan Province is the only area in the whole Qinling Oroge-nic Belt that contains granulites.The granulites in this area are mainly xenoliths found ingranodioritic gneisses.The xenoliths show concordant gneissic structures with the countryrocks(granodioritic gneisses).In addition to the above characteristics,granulites also differin degree of metamorphism from the country rocks which belong to amphibolite facies.Chro-nologically,the granulites are older than the country rocks.Accordingly,the granulites arexenoliths of granodiorite.In other words,the granulites were transported from the lower crust or the upper mantle to the crust along with granodioritic magma.Based on the paragenetic association of minerals the granulite xenoliths can be grouped intotwo categories:(1) two-pyroxene granulites(Opx+Cpx+Plag±Qz)and(2)garnet granuli-tes(Opx+Gar+Plag+Qz+Bi).Two samples of two-pyroxene granulite T_1 and T_4,collec-ted from different outcrops,are geochemically similar to tholeiites.The zircon ages of thesetwo samples are marvelously isochronous(470±20 Ma and 470±14 Ma).Another granulitesample T_(14) represents a metasedimentary rock of intermediate composition and yields a detritalzircon age of 827±5 Ma.However,the country rock of granulite xenoliths;i.e.,granodiori-tic gneiss,has a single zircon age of 435±14 Ma.Geothermometrical and geobarometrical stu-dies based on the coexisting minerals(Opx-Cpx and Opx—Gar)suggest that the granuliteswere formed at 818—840℃ and 9.5×10~8—9.8p_a corresponding to the lower crust or even upperper mantle conditions.Tectonically,the southern boundary of the Qinling Complex is marked by the Qinling col-lisional suture zone which extends from west to east,forming the boundary between the NorthChina plate and the Yangtze plate.The suture zone is characterized by the occurrence ofophiolites in the western segment and by that of granulites in the eastern segment.So the wes-tern segment of the suture zone marks the upper crustal level,while the eastern segment repre-sents the exposure of a deeper crust level.The results of isotopic dating and the geochemicalcharacteristics of the xenoliths are consistent with those of meta-tholeiites in the ophiolites in thewestern segment.Therefore,it is assumed that both ophiolites found in the western segmentand granulite xenoliths found in the eastern segment represent the remnants of the ancientQinling ocean plate.The evolution of granulites is different from that of ophiolites.That isto say,the ophiolites represent obducted fragments from the ocean floor as a result of subduc-tion in the Early Palaeozoic.In the eastern part,when the ocean plate was subducting towardthe lower crust or the upper mantle,it underwent a granulite-facies metamorphism.Sub-sequently,granodioritic magma was formed from partial melting and trapped some fragmentsof granulite while forcing its way upwards.