Discovery of Neoproterozoic A-Type Granite in Northern YangtzeCraton and Its Tectonic Significance

Cao Zhengqi; Cai Yitao; Zeng Zuoxun; Hu Zhengxiang; Chen Jing; Jiang Xingfu; Sun Zhengquan; Wu Bo; Liu Chengxin; Guo Pan

doi:10.3799/dqkx.2017.80

Abstract：Researches on the Daleishan gneissic granites can constrain Neoproterozoic Rodinia supercontinent breakup in the northern margin of Yangtze craton.In this paper, zircon U-Pb dating, in-situ Lu-Hf isotope and whole-rock geochemical compositions analyses are presented for the Daleishan gneissic granites in the northern margin of the Yangtze craton.The results show that the Daleishan gneissic granites have high contents of SiO2 (73.18%-77.40%) and alkali (8.07%-8.70%), but low contents of Al2O3 (12.11%-13.92%) and MgO (0.10%-0.34%), and display enrichment in Ga, Rb, Th, Zr, Hf but depletion in Nb, Sr, P, Ti, which indicates the post-orogenic A-type affinity.Zircons selected from two Daleishan gneissic granitic samples show typical oscillatory zone structure with high ratios of Th/U (greater than 0.5), exhibiting the magmatic zircon genesis.LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) zircon U-Pb data from the two samples yield Neoproterozoic crystallization ages of 801.3±3.0Ma (MSWD=0.62) and 796.1±6.3Ma (MSWD=1.70), respectively, which are interpreted to represent the intrusion time.In-situ Hf isotopic compositions show that the two gneissic granitic samples have wide range of εHf(t) values that are indicative of a heterogeneity property.εHf(t) values of these zircon ranges from-7.5 to +8.0, with the positive εHf(t) values, coinciding with the single phase model age (tDM1) of the depleted mantle of (1242-1059Ma), and the negative εHf(t) values coinciding with two-phase model age (tDM2) of the crustal of (1636-1981Ma), suggesting that the oldest existing material in the region belongs to Paleoproterozoic (as old as 1981Ma).These data show that the source materials of the Daleishan A-type granites were usually derived from Palaeoproterozoic ancient crust.The magmatic source of the granites was originated from the crust-mantle migmatization, and the mantle composition could have been generated from the upwelling of the mantle magma in the extensional background.Combining with the pre-existing regional publications, it is proposed that the Daleishan A-type granite was formed at the crustal extensional background in the continental back-arc setting associated with the assembly-break up process of the Rodinia supercontinent.