Petrology, Geochenistry, U-Pb Dating And Hf Isotopic Composition Of Zircons In Igneous Rocks From East Kunlun Orogen, Qinghai

Petrology, geochemistry, LA-ICP-MS zircon U-Pb dating and Hf isotopic studieswere carried out on several igneous plutons in the East Kunlun Orogen Belt (EKOB),Qinghai Province. Eight rock samples were collected, including three (GS-1, GI-2,GI-3) from the Xiarihamu area, Utu Meiren County, west EKOB, four samples (GI-5,GI-6, GI-7, GI-8) from the south of Golmud, middle EKOB, and one (DL) from theDulan area, east EKOB. Zircons from gneiss (GS-1) yield a LA-ICP-MS mean age of915.2±2.3Ma (MSWD=0.2), represented as the magma crystallization age, whichindicates that the protolith of this rock was formed during the Early-Neoproterozoic.Major and trace elements of these rocks correspond them to peraluminous S-typegranites.The εHf(t=915Ma) values of these rocks range from-0.11to3.46, withtwo-stage depleted mantle model ages (TCDM) in the range of1.77to2.57Ga. Wepropose that the granites were generated by the partial melting of the JinshuikouGroup, induced by the assemble among different blocks in western China. Zirconsfrom syenogranite (GI-2) yield a LA-ICP-MS mean age of411.7±1.1Ma (MSWD=0.56). Major and trace elements classify the syenogranite into weak metaluminousA-type granite. Since GI-2is the oldest A-type granite reported for the Xiarihamu area,the post collision of EKOB may occur as early as411Ma. The LA-ICP-MS zirconU-Pb analyses of the granitic rocks from the Golmud and Dulan area suggest that theformation ages of granodiorite (GI-5), adamellite (GI-6), syenogranite (GI-8) andgranite porphyry (DL) are249.5±0.95Ma (MSWD=0.33),260.4±1.3Ma (MSWD=0.49),272.5±1.1Ma (MSWD=1.8) and261.2±1.2Ma (MSWD=2.0),respectively. All these granitic rocks show similar chondrite-normalized REE patternsand primitive mantle(PM)-normalized trace element patterns, e.g., REE distributionsshowed a "V" patterns, enrichment of large ion lithophile elements (LILE: Rb, K),and incompatible elements (Th, U) and distinct depletion of Ba, Sr, P, Ti, Eu, etc.,which correspond them to weak metaluminous-weak peraluminous I-S transition typegranites; The εHf(t) values of those four granitic rocks range from-4.9to4.1,two-stage depleted mantle model ages (TCDM) range from1.1to1.62Ga, indicatingthose granites may have similar magma source,and may be generated by ancient crustremelting. Zircons from diorite (GI-3) yield a LA-ICP-MS mean age of260.2±0.98Ma (MSWD=0.44), zircons from lamprophyre (GI-7) yield a LA-ICP-MS mean ageof255.7±1.2Ma(MSWD=0.26), the diorite, lamprophyre and four granitoids fromthe Golmud and Dulan area are the products of Paleo-Tethys northward subduction.The inherited zircons with age of1500~1676Ma might suggest that there was anothermagmatic thermal event in EKOB.