Late Permian tectonic evolution at the southeastern margin of the Songnen-Zhangguangcai Range Massif, NE China: Constraints from geochronology and geochemistry of granitoids

摘要

This paper reports the LA-ICP-MS U-Pb zircon dating and geochemical data for the Huangqigou, Xiaobeihu and Lalagou granitic plutons at the southeastern margin of the Songnen-Zhangguangcai Range Massif, which will not only place important constraints on the rock-forming ages, source characteristics and tectonic setting of these granitic plutons, but will also provide insights into understanding the tectonic evolution of the eastern segment of the Central Asian Orogenic Belt. Zircons from three plutons have distinct oscillatory zoning structures in their cathodoluminescene (CL) images, which, combined with the Th/U ratios of 0.28-1.06, indicate their magmatic origins. Zircon U-Pb dating results for these granitic plutons indicate that the granitic magmatism occurred mainly in the late Permian (256-252Ma). The granites have SiO_2 of 58.0-74.3%, Na_2O of 2.74-3.76%, K_2O of 2.75-3.95%, Na_2O/K2O of 0.77-1.13, Mg~# [100(Mg/(Mg+Fe~(2+))] of 21.8-39.9, and A/CNK of 0.88-1.08, with REE patterns characterized by LREE/HREE=7.75-12.82, (La/Yb)N=7.88-14.9 and δEu=0.56-0.87. The trace element spider diagrams indicate that these granites are enriched in large ion lithosphile elements (Rb, Ba, Th, U, etc.) and relatively depleted in high field-strength elements (Ti, Nb, Ta), with Sr and Pb ranging from 205 to 350ppm and from 2.02 to 3.60ppm, respectively. The in-situ Hf isotope analysis reveals that εHf(t) values range from -2.22 to +11.65, and the two-stage Hf model ages (TDM2) range between 641 and 1274Ma. These geochemical characteristics indicate that the late Permian granitic plutons at the SE margin of the Songnen-Zhangguangcai Range Massif can be assigned to metaluminous and weakly peraluminous high potassium calc-alkaline I-type granites with low-Sr and high-Yb features, and that the magmas to have formed these granites were originated from the partial melting of mafic igneous rocks in the lower crust. The source rocks are composed mainly of the juvenile crust with small contributions of ancient continental crust. Considering the existence of coeval mafic intrusive rocks and sedimentary basins, we propose that the Late Permian magmatism in the study area occurred in a post-collisional extensional setting following the collision of the Xingkai Block and the united Jiamusi and Songnen-Zhangguangcai Range Massifs.