Geochronological and Geochemical Characteristics of Volcanic Rocks in the Permian Gequ Formation in East Kunlun and Their Tectonic Significance
The Paleo-Tethys orogenic belt in East Kunlun has experienced an orogenic process driven by oceanic crust subduction,continent-continent collision,and post-collision extension.To date,the closure time of the Paleo-Tethys ocean has consistently been controversial,with the focus of this debate being the Late Permian Gequ Formation as a sedimentary basin prototype,and the tectonic event represented by the nonconformity between the Middle and Upper Permian.In this study,volcanic rocks from the Gequ Formation located in the Hongshishan area of East Kunlun were studied using zircon U-Pb chronology and petrogeochemistry.These rocks are enriched in large-ion lithophilic elements,such as Rb,Th,and Ba,and depleted in high field-strength elements,such as Nb,Ta,Ti,and P,and have the geochemical characteristics of a continental magmatic arc.The volcanic rocks have high Ba/Th(53.6),Th/Ce(0.23),and Nb/Ta(18.35)ratios,and a low Dy/Yb ratio(1.42),indicating that the Late Permian volcanic rocks were formed by melted pillow basalts,ocean floor sediments,and other ocean crust materials during a subduction to a depth of approximately 60 km.Zircon U-Pb dating results show that the respective weighted average ages based on 206Pb/238U of rhyolite tuff and dacitic tuff in the clastic rocks and volcanic rocks in the lower Gequ Formation are(257.5±2.5)Ma and(256.2±4.8)Ma,respectively.The weighted average age based on 206Pb/238U of the trachyandesitic crystalline tuff in the upper limestone is(251.8±2.3)Ma.Combined with the results of studies on the Gequ Formation and granite,this data suggests that the Paleo-Tethys Ocean began subducting erosion northward in the Late Permian.The Gequ Formation deposited in the forearc basin is a sedimentary response to the initial northward subduction of the Pale-Tethys Ocean,and the angular nonconformity between the Gequ Formation and the underlying strata indicates that the tectonic event wherein the Paleo-Tethys Ocean began subducting northward occurred during the Late Permian.
万方数据
维普
