The discovery and geological implications of the Neoproterozoic A-type granites in the upper Yangtze craton

Little has long been known for the Niulangba biotite granites in Ebian, Sichuan within the upper Yangtze craton. The purpose of this paper is to present, for the first time, our recent results of SHRIMP zircon U-Pb dating and relevant geochemical data. The Niulangba granites are characterized by higher SiO₂ (SiO₂ >75%) contents, lower Ca (CaO=0.46% to 0.20%) contents, depletion in Mg, enrichment in alkali (Na₂O + K₂O=8.31% to 9.28%) and Al (A/KNC=1.02 to 1.12). The geochemistry of the trace elements suggests the highly depletion in Ba, Sr, Eu (δEu=0.05 to 0.08) and enrichment in Rb, Th and U. The 104* Ga/Al ratios for the bulk-rock samples range between 2.6 and 2.9. The high field strength elements Zr, Y and Ga have higher contents, and no anomaly has been identified. It can be seen from the major and trace element analyses that the Niulangba granites belong to the aluminous A₂-subtype granites. The Y/Nb ratios ranging from 5.0 to 5.9 and Nb/Ta ratios ranging from 4.4 to 5.0 have disclosed that the magmatic source areas were once subjected to highly hybridization of continental compositions. The SHRIMP zircon U-Pb ages are dated at 826±21.4 Ma, in general agreement with the previous data for the Neoproterozoic granites from the southeastern Yangtze craton. This implies that the Niulangba granites should be assigned to part of the highly magmatic activity within the pan-Yangtze craton during this period. These granites may result from the mantle plume-associated crust-mantle interaction against the breakup of the suppercontinent Rodinia during the middle Neoproterozoic.