Petrogenesis Of The Linshu Monzonitic Granite In The Southern Part Of The Sulu Orogen

The formation mechanisms of the orogenic belts and related petrogenesis are always the central research focus.The granites in orogenic belt are the important products formed during the tectonic evolution in collisional orogeny,thus,the research on their petrogenesis and magma source is of great significance to reconstruct the crust-mantle interaction and deep dynamic processes during orogeny.The Sulu orogenic belt,located in the binding site between the North China block and the Yangtze block,and characterized by strongly tectonic and magmatic activities,is a composite orogenic belt which experienced a complex tectonic evolution history.This paper focused on the Qingyun monzonitic granite pluton which located in the Linshu County of the Southern Sulu orogenic belt.Combined with systematic studies on the field geology,petrology,geochemistry,geochronology and Hf isotopes,we mainly discussed their formational age,petrogenetic type,source materials,tectonic setting and their petrogenesis model.Based on these studies,combined with the regional geological settings,we got the following achievements and cognitions:Petrology research shows that the monzonitic granites in the study area are mainly composed of alkali-feldspar,plagioclase,quartz and biotite.The rocks are characterized with high Si?SiO₂=71.82%⁷3.71%?,high K?K₂O=4.64%⁵.01%?,riched alkaline?K₂O+Na₂O=8.04%⁸.84%?,poor Mg?MgO=0.29%⁰.37%?,low TFeO/MgO ratios?4.13⁵.24?and weak peraluminous?A/CNK=1.05¹.16?.The rocks also have high contents of rare earth elements(?REE=187×10^-6331.86×10^-6),are characterized by enriched light rare earth elements?LREEs?and depleted hight rare earth elements?HREEs?,with a higher fractionation between LREEs and HREEs[?La/Yb?_N=19.71⁴5.78].In addition,the rocks are depleted in high field strength elements?HFSE?including Nb,Ta,Ti,and P,and relatively enriched in Pb and large ion lithophile element?LILE?including Rb,Th,K,and La,with moderate negative Eu anomalies??Eu=0.62⁰.72?.The lithogeochemical characteristics indicate that the studied monzogranites are I-type granite.Zircon U-Pb dating shows that the age of the monzonitic granite is 126.6±3.8 Ma,which is the result of the Early Cretaceous magmatism in Yanshanian.Moreover,the results of the zircon Hf isotope analysis show that the samples display negative?Hf?t?values ranging from-21.53 to-34.43 with the T_DM2 model ages of 2.55Ga³.35Ga.Both the?Hf?t?and the T_DM2 model ages indicate that the monzonitic granites are derived from the Neo-Archean continental crust,which are the products of the partial melting of the North China Craton lower crust.Therefore,it is believed that the rocks could be mainly derived from the partial melting of the lower crust in granulite facies under the moderate pressure?0.8-1.3 GPa?,corresponding the depth of 35-45 km,within an extensional tectonic environment.In the diagenetic process,the mineral fractional crystallization of clinopyroxene,spinel,plagioclase,apatite,Ti-Fe oxides?such as ilmenite?,hornblende and biotite could be occurred.The delamination of the lithosphere and the underplating of the mantle magma could be the main reason which resulted in the partial melting of the lower crust.