The spatiotemporal evolution of the Mazongshan accretionary wedge in the Besihan Orogenic Belt:Constraints from fine field anatomy,detrital U-Pb dating and petrogeochemistry
The Beishan Orogen,located between the Siberia and North China-Tarim cratons,is an important part of the southern margin of the Central Asian Orogenic Belt.There have been different opinions on its tectonic stratigraphic division and evolution for a long time.The Mazongshan accretionary complex in the Jiabuquankouzi area is a natural research object to disclose the tectonic evolution of the Beishan Orogen.In this study,different geological bodies including ophiolites,oceanic island-seamounts,intra-oceanic arc,bathyal-abbysal cherts,active continental-trench turbidites and continental arc within this complex were recognized and analyzed.Furthermore,two tuffaceous siltstone samples(P3-57 and P3-110)from the turbidites and one pebbly litharenite sample(P3-82)yielded maximum depositional ages of 410Ma,430Ma and 286Ma,respectively,which can provide important constraints on the paleogeographic evolution of the Beishan area in the Paleozoic.From the above studies,three points of views can be drawn as follows:(1)The rock assemblages,metamorphic degree and geochronology of different tectonic units have great difference,so these tectonic units should be analyzed individually to ascertain their paleogeographic environments.(2)The detrital zircon age spectra of three samples from the matrix of the Mazongshan accretionary complex in the Jiabuquankouzi area display multiple age peaks at～2.5Ga,～1.5Ga,～0.9Ga,～450Ma,～430Ma,～410Ma,～350Ma,～288Ma,respectively,which represent their sources from the metamorphic crystalline basement of the Beishan area,the assemblages and break-ups of the Columbia and Rodinia supercontinents,the multiple tectonic-magmatic events in the active continental margin,the remnant oceanic basin and syn-collisional orogen that were carried into the accretionary wedge rapidly.(3)The detrital zircon age spectra of the southern and northern components of the Beishan orogen were largely distinctive from each other,indicating that provenances of the Mazongshan accretionary wedge were mainly derived from the north of the suture zone during the Precambrian and the south of the suture zone during the Permian.
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