Age, Geochemical And Tectonic Significance Of Xigongka Pluton And Kaligang Pluton In Western Section Of West Qinling Orogen

The west Qinling orogenic belt exposed more granitic rocks, The formation and emplacement of granitic rock formation evolution process is continental orogenic belts in the extremely important question, Recording the west Qinling mountain orogenic and the original tethys ocean and the ancient tethys ocean is closely related to the subduction of oceanic crust, Continental collision and Post-collision events. The Xigongka granodiorite and The Kaligang Diorite Porphyrite are located in the western part of the western Qinling orogenic belt,and intruded the Lower Triassic Longwuhe Group. We study systematically the petrology, geochemistry and zircon U-Pb isotope chronology in this paper.Study on the element geochemistry, the two pluton are characteristic belong to high-K calc-alkaline I-type granites. In Xigongka pluton, the content of Na2 O and K2 O is 1.98~3.4%, 3.12~4%, respectively. The A/CNK is mainly less than 1.1(0.97~1.2), the average is 1.08. The total of REE is 106.69~136.47×10-6. The LREE and HREE is 97.84~131.03×10-6, 5.44~10.07×10-6, respectively, with the LREE/HREE ratio showing 11~24.1. In Kaligang pluton, the content of Na2 O and K2 O is 2.57%~3.66%, 2.3%~3.95%, respectively. The A/CNK is less than 1.1(0.85~1), the average is 0.92. The total of REE is 117.08~146.42×10-6. The LREE and HREE is 104.89~133.27×10-6, 12.18~13.42×10-6, respectively, with the LREE/HREE ratio showing 8.5~10.1. In the Chondrite-normalized REE diagram, the samples from two plutons show right-slope with weak Eu negative anomaly, LREE enrichment, HREE depletion. Moreover, in primitive mantle normalized trace element spider diagram, the samples show large ion lithophile element(Cs, Rb, Ba, Th and U) significantly enrichment and high field-strength element(Nb, Ta and P) obvious depletion. Therefore, the two plutons drive from the same source.U–Pb analyses of zircon grains from a representative basalt by laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS) yielded crystallization age of 227.2±7.3 Ma in Xigongka granodiorite, suggesting formation during the Carnian of Late Triassic, and 238.7±7 Ma in Kaligang diorite, suggesting formation during the Ladinian of Middle Triassic. The two plutons intrude into the Longwuhe Group, and based on the age of sedimentary detrital zircon dating, suggest that age of the Longwuhe Group should be Olenekain to Ladinian.Through trace elements determination, combining with previous studies, indicate that the two pluton formed in post-collisional setting. Owing to delamination of lithospheric mantle, lead to mafic magma underplating and then result in lower crust metabasic rock partial melting, with degree of crust-mantle magma mixing.