LA-ICP-MS U-Pb zircon ages of the Qianlishan Complex: Constrains on the evolution of the Khondalite Belt in the Western Block of the North China Craton

摘要

The Qianlishan Complex is located in the westernmost part of the Khondalite Belt, a continent-continent collisional belt along which the Yinshan Block in the north and the Ordos Block in the south amalgamated to form the Western Block,which then collided with the Eastern Block along the Trans-North China Orogen to form the North China Craton. The complex is dominated by high-grade supracrustal rocks and minor S-type granites, of which the supracrustals consist of graphite-bearing sillimanite-garnet gneiss, garnet quartzite, felsic paragneiss, calc-silicate rock and marble. CL images reveal the existence of detrital and metamorphic zircons in major rocks of the Qianlishan Complex. In most cases, detrital zircons occur as either single grains with oscillatory zoning or oscillatory zoning cores, typical of igneous origin, which are surrounded by metamorphic overgrowth rims that are structureless, high bright and low in Th/U ratio. Detrital zircons from the Qianlishan Complex yield nearly concordant Pb-207/Pb-206 ages ranging from 2.3 to 2.0 Ga, suggesting that the protoliths of the high-grade supracrustal rocks in the Qianlishan Complex were deposited at some time after 2.0 Ga. Metamorphic zircons yield two age populations with one at similar to 1.95 Ga and another at similar to 1.92 Ga, of which the former is interpreted as the timing of the collision between the Yinshan and Ordos Blocks to form the Western Block, whereas the later is considered to be the age of subsequent post-orogenic extensional event. Minor S-type granites were emplaced at similar to 1.88 Ga, as a result of partial melting of supracrustals at the stage of the exhumation of the Qianlishan Complex. These new zircon ages, combined with structural and metamorphic considerations, enable resolution of the tectonothermal events involving the collision between the Yinshan and Ordos Blocks to form the Western Block, followed by the post-collisional extension and subsequent exhumation of the Khondalite Belt.