Geochronology and geochemistry of gneissic metagranites in eastern Dabie Mountains: Implications for the Neoproterozoic tectono-magmatism along the northeastern margin of the Yangtze Block

Abstract

The gneissic metagranites with Neoproterozoic protolith ages are widely exposed along the eastern margin of the Dabie ultrahigh-pressure (UHP) metamorphic belt. In this paper, five representative plutons, including Huangzhen, Daba, Shima, Shuanghe, and Sanzusi, were selected for a detailed chronological and geochemical study aiming to identify the nature of the protoliths and to reveal their implications for the Neoproterozoic tectono-magmatic evolution along the northeastern margin of the Yangtze Block. These gneissic metagranites consist mainly of quartz, albite (oligoclase) and K-feldspar, and minor amphibole and biotite, as well as some metamorphic minerals due to the UHP metamorphism, such as phengite, epidote and minor kyanite and garnet. Mafic alkaline minerals (e.g., aegirine or aegirine-augite) have been observed in some of these plutons. Zircon LA-ICP-MS U-Pb dating results indicate that the protoliths of the gneissic metagranites have been generated in Mid-Neoproterozoic (770–780 Ma), and suffered both ultrahigh-pressure metamorphism in Early Mesozoic and subsequently high-pressure eclogite-facies recrystallization at about 215 Ma. Although the gneissic metagranites generally have high SiO2 contents (70.23%–77.23%) and show metaluminous-weakly peraluminous signatures (ASI=0.90–1.05), there are still some geochemical variances between different plutons. Compared with the Sanzusi pluton, the metagranites from the Huangzhen, Daba, Shima, and Shuanghe have high K2O+Na2O contents (7.76%–9.45%), FeOtotal/(FeOtotal+MgO) ratios (0.82–0.96), HFSEs and Ga concentrations with an average 104×Ga/Al ratio up to 3.07. Combined with the features that the four plutons commonly contain aegirine-augite and have high zircon saturation temperatures (816–918°C), it is suggested that their protoliths belong to peralkaline A-type granites. The Sanzusi pluton generally contains biotite and epidote generated from metamorphic reaction of amphibole, and is chemically enriched in calcium and depleted in potassium with K2O/Na2O ratios ranging from 0.42 to 0.54, suggesting their protoliths should be ascribed to calc-alkaline granitoids (I-type). The gneissic metagranites in the eastern margin of Dabie Mountains show both arc- and rift-like geochemical signatures, and their protoliths likely have been generated by reworking of the preexisting arc-related crust under an extensional setting, suggesting that the northeastern margin of the Yangtze Block during the Middle Neoproterozoic likely have been under the initial stage of a passive rifting rather than in an active rifting setting typically induced by the upwelling of a mantle plume.