Geochemical,Geochronology,and Tectonic Significance Of Triassic Meiwu Pluton In The Western Qinling Orogen

There are many granitic rocks in the west Qinling orogenic belt,which records the tectonic events,such as the subduction of oceanic crust.,continent-continent collision and post collision.In this paper,we present the petrography,geochemistry,and zircon LA-ICP-MS U-Pb geochronology of the Triassic,the Meiwu,the Riduoma and the Gengzhaxi plutions in the middle part of the western Qinling Orogenic belt,in an attempt to discuss the petrogenesis and tectonic settings,and to constrain the tectonic-magmatic evolution and geodynamic process.The obtained conclusions can be summarized as follows: The Triassic intrusive rocks in the Meiwu area are alkali-rich and high-K calc-alkaline,belonging to I-type granites.Moreover,the samples show enrichment in LILE and LREE,depletion in HFSE.The Meiwu pluton is composed of granodiorite(with LA-ICP-MS zircon U-Pb age of 235±3.1Ma)and biotite granite(with LA-ICP-MS zircon U-Pb age of 238±3.1Ma).The LA-ICP-MS zircon U-Pb age of Riduoma diorite is 236.8±3.6 Ma,and the Gengzhaxi diorite is 244.1±5.4 Ma.The new ages obtained in this paper shows that these three plutons are the product of the magma activity formed during the same period(as the Early Indosinian).The Triassic intrusive rocks in the Meiwu area are formed in post-collisional setting,resulting from partial melting of the lower crust,with degree of crust-mantle magma mixing.The Triassic intrusive rocks in Meiwu area have similar geochemical characteristics,which are consistent with those of the Dewulu and the Xiahe plutons in the region.It is indicated that there are extensively magmatic activities during the early Indosinian in the middle part of the western Qinling Orogenic belt.They are formed in the tectonic setting of the early post-collision and related to the subduction of the oceanic crust.