Two stages of zircon crystallization in the Jingshan monzogranite, Bengbu Uplift : implications for the syn-collisional granites of the Dabie-Sulu UHP orogenic belt and the climax of movement on the Tan-Lu fault

The detailed study of zircon can provide vital clues about the petrogenetic environment of granitoid rocks. Morphological and chemical studies of zircon grains from the Jingshan monzogranite in the Bengbu Uplift (Anhui province, eastern China) identify three phases of growth. Zircon I is brown, translucent, ovoid, and occurs as inherited cores. It shows two groups of ²⁰⁶Pb/²³⁸U ages, corresponding to the country rock of the Jingshan monzogranite and the basement of the Bengbu Uplift. Zircon II is colorless, transparent and idiomorphic-hypidiomorphic with Ipr=0.34-0.52, Ipy=0.03-0.24 and Iel=0.26-0.34. It shows a very bright CL due to high contents of trace elements (e.g., Y, U and Th), and the oscillatory zoning associated with sector zoning. It contains 1.40-1.66wt.% HfO₂ with a mean ΕHf (t) of -17.88. These features indicate the igneous crystallization of Zircon II from a peraluminous granite of mainly crustal origin. Based on the field geology, petrography, geochemical analysis, and especially a weighted mean age of 222Ma for Zircon II, we argue that the Jingshan monzogranite is a syn-collisional granite of the Dabie UHP orogenic belt. This provides new evidence for the northward subduction of the South China Block beneath the North China Block just before 222Ma, and allows quantitative estimates of the rate of post-UHP exhumation in the Dabie orogenic belt. Zircon III is also colorless and transparent, but is totally idiomorphic with Ipr=0.41-1.00, Ipy=0.88-1.00 and Iel=0.39-0.83. It occurs only as overgrowths on Zircon II, and shows weak CL due to its depletion in trace elements. Zircon III has widely variable contents of HfO₂ (1.12 to 3.01wt.%) and Hf-isotope compositions very similar to those of Zircon II. These features suggest crystallization of Zircon III from small volumes of leucosome, probably in the beginning stages of migmatisation. Zircon III has a weighted mean age of 156Ma, interpreted as representing the climax of movement on the Tan-Lu fault. This suggests that the compressive movement of the Tan-Lu fault has decreased progressively since its climax at 156Ma. These two major conclusions from the zircon studies contrast with previous interpretations of the Jingshan monzogranite as a late Jurassic post-collisional granite in the Dabie orogenic belt.13 page(s