| The studied area,Wunuer-Toudaoqiao,is located in the central-north segment of Great Xing'an Range,tectonically situated on the eastern segment of Central Asian Orogenic Belt between the North China and Siberia plate.For a long time,there have existed many controversies about the collision and convergence between those continents,micro-plates or island arc terranes.This paper will involve those basic problems,focusing on the petrotectonic assemblages of Wunuer-Toudaoqiao matching belt.According to the construction of petrotectonic assemblages,and the corresponding petrogenesis related with the tectonic setting,the tectonic evolution history about the matching belt has been restoried.Based on our field geological investigations and further studies in recent three years,the petrotectonic assemblages developing with the tectonic evolution of Wunuer-Toudaoqiao matching belt have been almost defined.Those are as fellows:1.The meta-basalts,gabbro,diabase or diabase porphyrite dykes and associated radiolarian cherts,intermittently distributed alone the Wunuer-Toudaoqiao matching belt,were defined as the ophiolitic rock assemblages,which represented the oceanic crust.Based on the petrogenesis analysis of mafic rocks,the ophiolites could be divided into two types.The first type was similar to MORB,which could be derived from the light-depleted asthenosphere mantle.The second type was similar to IAB,derived from a more-depleted and metasomatized mantle undergone MORB-melting extraction.Bose of those two-type ophiolites all contained different degrees of subductive components,indicating they were produced at the back-arc spreading ridge background.Consequently,the two-type ophiolites were SSZ-type ophiolite,probably from Silurian to Early Devonian.2.Additionally,the argillo-arenaceous melanges and blueschists were also redetermined in Toudaoqiao area,associated with the subduction-collision of Wunuer-Toudaoqiao matchting belt.3.The meta-basalts-andesites-dacites-rhyolites in "Daminshan formation",were redefined as the arc volcanic rock assemblages,correlated with the Wunuer-Toudaoqiao ocean subduction.In late Devonian(385~355 Ma)period,the arc attribution belonged to continental margin arc.Evolved to 355 Ma,the arc transformed into island arc,since the north back-arc began to extend with back-arc basin forming.That is,the arc-basin system has been formed since about 355 Ma,while the 350~330Ma arc volcanic rocks were produced under the continuous subduction of Wunuer-Toudaoqiao ocean.4.The high-Mg diorites and adakites were determined from the subductive intrusions distributing in the northwest of Wunuer-Toudaoqiao matching belt.During late Early Carboniferous period(330~315Ma),The high-Mg diorites could be produced from the mantle wedge metasomatized by slab fluids.The adakites with high Mg#values could be generated by partial melting of the slab at the depth of 75-90 km,simultaneously contaminated by the overlain mantle wedge.The high-Mg diorites-adakites suites further demonstrated the existence of Wunuer-Toudaoqiao matching belt.5.To the early period of Late Carboniferous,the Wunuer-Toudaoqiao oceanic subduction basically ended.During the late age of Late Carboniferous to Early Permian(310~270Ma),the tectonic attribution of studied area transformed into the continental collision to post-collision setting.The crust and lithosphere could be thickened rapidly during the early stage of continent to continent collision.After thickened,the lithosphere would be delaminated in post-collision stage,following with the asthenospheric up welling and basaltic magma underplating.The widespread peraluminous granites were mostly produced by partial melting of the crust with basaltic magma underplating in post-collision setting.To the late of post-collision,the lithosphere continued thinning,and induced the mantle-derived magma input indirectly.It would cause the crust-mantle magma mixing and produce the mixing-type granitiods,taking the rock body in east of Mianduhe town as example. |
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