PETROGENESIS OF THE NEOPROTEROZOIC STRONGLY PERALUMINOUS GRANITOIDS FROM NORTHERN GUANGXI: CONSTRAINTS FROM ZIRCON GEOCHRONOLOGY AND HF ISOTOPES

Abstract

The western end of the Proterozoic Jiangnan orogen is located at Northern Guangxi, South China. Neoproterozoic strongly peraluminous granitoids are dominant (>90%) in the area, with ca. 8% being the mafic-ultramafic rocks. The granitoids in Northern Guangxi can be geochemically divided into two groups: granodiorites and granites. Their generations were previously considered to be related to a mantle plume (or superplume) event that led to the breakup of the Rodinia supercontinent. In this work, we present new laser ablation (LA)-ICP-MS U-Pb zircon dating and LA-MC-ICP-MS zircon Hf isotopic results for the granitoids from Northern Guangxi. The ages for the Zhaigun, Bendong, Dongma, Sanfang and Tianpeng granitic plutons are 835.8 ± 2.5Ma, 822.7 ± 3.8Ma, 824 ± 13Ma, 804.3 ± 5.2Ma and 794.2 ± 8.1Ma, respectively. These dating results suggest that the two types of the granitoids in the area were formed in two episodes. The granodiorites formed at ca. 835 - 820Ma, whereas the granites emplaced at ca. 810 - 800Ma. The analyses for 8 spots of zircon cores and 2 spots from single zircon xenoliths show early Neoproterozoic ages ranging from ca. 870 Ma to 950 Ma. These ages might record the subduction or collision-related magmatic events during 950 - 870 Ma in Northern Guangxi. Hf isotopes of zircons were also analyzed by La-MC-ICP-MS. The (176Hf/177Hf)i ratios for the zircons from the Bendong, Sanfang and Tianpeng plutons are 0.282176 ± 0.000021, 0.282149 ± 0.000021 and 0.282175 ± 0.000030, respectively. Coupling zircon Hf isotopes and published whole rock Nd isotopes data suggest a basement sedimentary series source for the granitoids and the involvement of relatively more mantle-derived mafic materials in the source of the granodiorites. The dating results also reveals a ca. 35Ma magmatic duration, which is inconsistent with the plume models that predict widespread magmatic eruption and emplacement within period of 1 - 5Ma. The Neoproterozoic granitoids from Northern Guangxi are generally plotted in the collision-related and post-collisional granites areas in the tectonic discrimination diagrams. They have been formed after the collision peak (ca. 870 Ma) between the Yangtze and Cathysia blocks, belonging to the post-collisional granites. The upwelling of deep mantle due to the detachment of subducted slab and the delamination of the lithosphere might cause the partial melting of the basement rocks to generate the strongly peraluminous granitoids.