The formation of foliated (garnet‐bearing) granites in the Tongbai‐Dabie orogenic belt: partial melting of subducted continental crust during exhumation

Abstract

AbstractFoliated (garnet‐bearing) (FGB) granites are associated closely with and are usually the major wall rocks of the high‐pressure (HP) and ultrahigh‐pressure (UHP) metamorphic rocks in the Tongbai‐Dabie region, the mid segment of the Qinling‐Dabie‐Sulu orogenic belt in central China. These granites appear either as small plutons or as veins, which commonly intrude into or surround the HP and UHP metamorphic eclogites or gneisses. The veins of FGB granites usually penetrate into the retrograded eclogites or gneisses along the foliations. Condensation rims can occasionally be found along the margins of granite veins. These granites are rich in Si and alkali with high Ga/Al ratios, and depleted in Ca, Mg, Al, Ti, Sc, V, Ni, Co, Cr and Sr, which are similar to A‐type granites. In a chondrite normalized diagram, the samples are light rare earth elements enriched with different extent of negative Eu anomaly. Moreover, Rb, Nb, Ta, Sr, P and Ti show different degrees of negative anomalies, whereas Ba, K, La, Zr and Hf show positive anomalies in the primitive mantle normalized diagram. Negative anomalies of Eu and Sr indicate strong influence of plagioclase. In conventional discrimination diagrams, these FGB granites belong to the A‐type granite, with geochemical characteristics affinitive to post‐collisional granites. The εNd (230 Ma) values (−15.80 to −2.52) and TDM values (1.02–2.07 Ga) suggest that magma for the FGB granites were derived from a heterogeneous crustal source. Therefore, the FGB granites may provide clues for deciphering the formation of post‐collisional granites. It is proposed that the magma of the FGB granites both in the HP and UHP units was formed in an extensional tectonic setting slightly post‐dating the HP and UHP metamorphism, most likely as a result of decompressional partial melting of UHP retrograded eclogites during exhumation.