A Study On The Late Paleozoic Tectonic Setting Of Wangqing Area In Jilin Province

In the paper,the Late Paleozoic tectonic setting in the Wangqing area of Jilin Province has been studied by the methods of petrology,geochronology and geochemistry of the Manhe volcanic rocks and the Kedao Group in the western margin of Khanka Massif.The LA-ICP-MS zircons U-Pb dating shows that the Manhe volcanic rocks formed in the Middle Permian(265±3.8Ma),slightly later than that of the adjacent basaltic andesite(275Ma)from Guanmenzuizi Formation.The minimum ages given by clastic zircons from Shanguqi Formation and Tanqian Formation are 245±3.0Ma,247±3.4Ma,250±6.5Ma and245±4.1Ma respectively,which limit the maximum deposit time of Kedao Group to the Early Triassic.There are three peaks of 266 Ma,513Ma and 925 Ma in the age frequency diagram of detrital zircons.They represent three age groups from 245 to 303 Ma,476 to 549 Ma and575 to 1781 Ma.These age groups are compared with those of the landmark tectonic magmatism events in the Bohai Block and Khanka Massif.The detrital zircons with the ages of(575~1781)Ma are derived from the basement of "Mashan group" and Proterozoic mixed granite(955Ma)in Khanka Massif.The detrital zircons with the ages of(476~549Ma)coincide with the Pan-tectonic magmatic events,which are widely present in Northeast China.The detrital zircons with the ages of(245~303Ma)represent the Early Permian to Early Triassic intrusive rocks related to subduction of the Paleo-Asian Ocean plate and plates collision.In addition,the ages from 1811 Ma to 2840 Ma in the Early Triassic Kedao Group record the landmark tectonic events in the Bohai Block and indicate that the provenance of this area has been supplied from the northern margin of BohaiBlock.Manhe volcanic rocks belonging to medium-K calc-alkaline series with the characteristics of typical island arc volcanic rocks.They concentrate LILEs and LREE,loss HFSEs and HREE as well as have the right handed REE distribution pattern.Clastic rocks of Kedao Group are mainly composed of graywackes.The characteristics of some trace elements are as following: LREE/HREE=5.19~7.80,LaN/YbN=4.72~7.87(reflecting an obvious fractionation between LREE and HREE),a significant negative Eu abnormal(?Eu=0.50~0.70,an average of 0.58).The patterns of the REE distribution and the trace element spider are similar to those of upper crust.The characteristics above mean that the parent rocks are mainly continental crust,which are consistent with the results obtained by discrimination diagrams of source composition.The Kedao Group deposited in the active continental margin and this is supported by different geochemical tectonic discrimination diagram.In summary,the tectonic setting of Wangqing area was under the subduction of the Paleo-Asian Ocean plate to Khanka Massif during Permian.The continuous subduction resulted in the Khanka Massif-Bohai Block collision and the provenance has been supplied from the northern margin of Bohai Block in the early Triassic.