GEOCHEMISTRY OF PENGGUAN COMPLEX IN THE LONGMENSHAN REGION,WESTERN SICHUAN PROVINCE,SW CHINA:PETROGENESIS AND TECTONIC IMPLICATIONS

The Pengguan complex,located in the Longmenshan thrust belt,western margin of the Yangtze block,SW China,was formed during the Neoproterozoic.Major and trace element abundances are reported,and the source and tectonic significance of the complex are discussed in this paper.The complex is dominantly composed of acid-intermediate intrusive rocks.12 samples from the Pengguan complex show variable major oxides with SiO2 ranging from 57.70% to 71.90%.The rocks have relative high Na2O contents(K2O/Na2O=0.31~0.85),δ values vary from 1.33 to 2.28 and A/CNK ratios from 0.93 to 1.08,indicating calc-alkaline series metaluminous to weak peraluminous granitoids for the Pengguan complex.The rocks have relative low abundance of REE(∑REE=76.25×10-6~139.80×10-6)with variable differentiation between LREE and HREE(LaN/YbN=3.86~35.23)and weak negative to positive Eu anomalies(δEu=0.70~1.86).The rocks are enriched in Rb,Sr,Ba,K and depleted in Nb,Ta,P,Y.Most samples have relatively high Mg# values(45)and Sr/Y ratios(20.03~111.34)compared to crust-derived calc-alkaline granitoids.Moreover,the Pengguan rocks show the affinity with the adjacent adakitic Xuelongbao plutonic complex.The geochemical features suggest that the Pengguan complex was generated by mixing of the underplated mantle-derived magma and the melting of lower crust.Combined with previous study,we consider the Neoproterozoic extensive igneous rocks along the western margin of the Yangtze block were formed in a subducted-related volcanic arc environment.