Petrogenesis Of The Early Paleozoic Huichizi Pluton In The East Qinling Tectonic Belt

Several acidic magmatisms at the early Paleozoic occur in the east Qinling tectonic belt,resulting in the formations of a large number of granite plutons and uranium-and rare metal-mineralized granitic pegmatites.These granitic pegmatites are spatially and temporally related to the Huichizi complex pluton,which is the largest I-type granite pluton among those formed in the early Paleozoic in the east Qinling tectonic belt.However,the link of the Huichizi complex pluton and granitic pegmatites is still under debate,due to the limited knowledge about the the classification of rock types,formation age and magmatic evolution processes of the Huichizi pluton.In this study,we clarified the relationship of different rock types of the Huichizi complex pluton,based on the detailed geological survey,and carried out the analyses of in situ zircon U-Pb dating and Hf isotopic composition and mineral and whole-rock compositions,in order to investigate the age,magma evolution processes and tectonic setting of the pluton,as well as its relationship with the early Paleozoic granitic pegmatites.The Huichizi complex pluton is mainly composed of biotite monzonitic granite,tonalite and biotite granodiorite.The tonalite and biotite granodiorite have zircon U-Pb ages of 437.6±4.8 Ma and 431.5±3.7 Ma,respectively,corresponding to the second stage of early Paleozoic magmatism in the region.The biotite syenogranite with a close relationship with the Huichizi pluton has zircon U-Pb age of 417.8±2.9 Ma,corresponding to the third stage of magmatism in the region.Zircons from the tonalite and biotite granodiorite of the Huichizi complex pluton haveε_Hf（t）values from+2.4 to+7.8（with an average of+5.4）and-2.4 to+8.8（with an average of+3.3）,respectively,indicating that they are derived from the partial melting of the thickened juvenile lower crust,with an addition of a small amount of ancient crustal material.Zircons from the biotite syenogranite haveε_Hf（t）values of from+3.0 to+8.3（with an average of+6.0）,indicating that it was derived from the partial melting of the thickened juvenile lower crust as well.Samples from the Huichizi complex pluton and the biotite syenogranite have high silica and total alkali contents,but low magnesium and iron contents,and have A/CNK values from1.01 to 1.10 and from 1.09 to 1.19,respectively,indicating that those of Huichizi are weakly peraluminous calc-alkaline to high-potassium calc-alkaline series rocks,while those of the biotite syenogranite are I-type granite with a feature of strongly peraluminous calc-alkaline.In addition,samples from the Huichizi complex pluton and the biotite syenogranite have low total rare-earth element（REE）contents with an enrichment of light REEs relative to heavy REEs,and are enriched in large ion lithophile elements（LILE）such as Rb,Ba,K,Pb and Sr,and are depleted in high field strength elements（HFSE）such as Nb,Ta,P and Ti.Samples from the Huichizi complex pluton and the biotite syenogranite show an adakite-like geochemistry,with high Sr concentration,but low Yb and Y concentrations.Samples from different rock types of the Huichizi complex pluton have similar pressures in the shallow magma chamber and similar ion substitution patterns of biotite,indicating that they may have a genetic relationship.Moreover,samples from biotite monzonitic granite to tonalite and to biotite granodiorite show gradual increases of oxygen fugacity and Si O₂ contents,but gradual decrease of the formation temperature,Fe₂O₃^T and Mg O contents.In addition,biotite and potassium feldspar from tonalite to biotite granodiorite have gradual increase of Rb and Cs concentrations but a gradual decrease of the Ba concentration,indicating that the biotite granodiorite was formed form a more evolved magma.Therefore,samples of biotite monzonitic granite was formed earlier than those of tonalite,which were followed by those of biotite granodiorite.The Huichizi complex pluton is a pluton with a low-medium degree of fractionation with low and constant rare metal concentrations,indicating that it may not have genetic link with rare-metal mineralized granite pegmatites in the east Qinling tectonic belt.On the other hand,samples from biotite syenogranite have different ion substitution patterns for biotite from those of the Huichizi complex pluton,with higher formation temperature and pressure but lower Si O₂ content than those of the Huichizi complex pluton,indicating that they have may have no genetic relationship.Samples from the biotite granite pegmatites with uranium mineralization have similar ion substitution patterns of biotite to those of biotite syenogranite,and have lower formation temperature but higher oxygen fugacity,possibly suggesting a fractionated product of biotite syenogranite.The subduction of Shangdan oceanic plate occurred at the early Paleozoic（500-470 Ma）in the east Qinling tectonic belt,and ceased during the period of 460-430 Ma,resulting in the collision of the Qinling microcontinent and the North China craton and a thickened crust.The partial melting of the thickened juvenile lower crust and a small amount of ancient crustal materials yielded the parent magma of the Huichizi complex pluton.The crustal compression of the east Qinling weakened after 430 Ma,followed by a post-collision extension stage.The partial melting of the thickened juvenile lower crust at～420 Ma,resulting in the formations of biotite syenogranite and uranium-mineralized granite pegmatites.