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Geochronology of the Mesozoic volcanic rocks in the Great Xing'an Range,northeastern China:Implications for subduction-induced delamination

Simon A.Wilde  
【摘要】:正Mesozoic volcanic rocks and granitoids are widespread in the Great Xing'an Range,which is part of a large igneous province in the eastern China.However,the ages of the volcanic rocks,especially those in the southern segment of the range,are poorly constrained.Here we present zircon U-Pb and whole rock Ar-Ar ages of 43 volcanic rocks from the four recognized formations(Manketouebo,Manitu,Baiyingaolao and Meiletu)in the southern Great Xing'an Range.The volcanic rocks of the Manketouebo Formation have a large span of ages ranging from 174 to 122 Ma,while those of the Manitu Formation exhibit a smaller age range from 156 to 125 Ma.The Baiyingaolao and Meiletu volcanic rocks both have Early Cretaceous ages between 139 and 124 Ma.These data indicate that the mapped units are not strictly 'formations' and further studies are required to resolve this issue.However,when taken together,these new data define two episodes of magmatism(Late Jurassic and Early Cretaceous)with the Early Cretaceous volcanic rocks being dominant. Combined with previously published data from the northern Great Xing'an Range,and available age data from other parts of northeastern China and surrounding regions,two stages of magmatism,i.e.,Jurassic and Early Cretaceous,can be identified throughout this part of Asia.The Jurassic rocks mainly comprise granites, while volcanic rocks are dominant in the Early Cretaceous.These two stages of magmatism form opposite spatial trends,that is,the Jurassic rocks become younger to the west,whereas the Cretaceous rocks become younger to the east.Between the two stages of magmatism,the 'magma gap' increases eastward in duration from less than 10 Ma in the Great Xing'an Range to more than 40 Ma in Japan.These trends can be explained by westward subduction of the Paleo-Pacific oceanic Plate and its control on subsequent geodynamic processes.Jurassic subduction of the oceanic slab caused crustal shortening and thickening,and formed the westward decrease in age of the granites with characteristics of an active continental margin,while volcanism was rare.By the end of the Jurassic,westward flat-slab subduction of the Paleo-Pacific Oceanic plate changed its direction to the north or northwest.This subsequently caused a transformation in tectonic regime from compression to extension in the Cretaceous and induced large-scale delamination of the thickened lower crust and lithospheric mantle.Delamination was initiated at the western margin of the subducting slab,and migrated eastward.Delamination and consequent upwelling of the asthenosphere triggered extensive volcanic eruption,with only minor granite emplacement.Similar age trends are also observed for other parts of eastern China,suggesting this model can also be applied to explain the geodynamic setting of the Mesozoic large igneous events in China and adjacent regions.

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