Crust-mantle interaction in Dabie orogenic belt, central China: Geochemical evidence from Late Cretaceous basalts

Abstract

It has been suggested that eclogites in the Dabie orogenic belt are exhumation products, which had subducted into the deep-seated mantle and undergone ultra-high pressure metamorphism during the Triassic. But no direct evidence supports this process except the calculatedp-T conditions from mineral thermobarometers. The Late Cretaceous basalts studied in the present paper, however, have provided some geochemical evidence for crust-mantle interaction in the area. These basalts are distributed in Mesozoic faulted basins in central and southern Dabie orogenic belt. Since little obvious contamination from continental crust and differentiation-crystallization were observed, it is suggested, based on a study of trace elements, that the basalts are alkaline and resultant from batch partial melting of the regional mantle rocks, and share the same or similar geochemical features with respect to their magma source. In the spider diagram normalized by the primitive mantle, trace element geochemistry data show that their mantle sources are enriched in certain elements concentrated in the continental crust, such as Pb, K, Rb and Ba, and slightly depleted in some HFSE such as Hf, P and Nb. Pb-Sr-Nd isotopic compositions further suggest the mantle is the mixture of depleted mantle (DM) and enriched one (EMI + EMII). This interaction can explain the trace element characteristics of basaltic magmas, i. e., the enrichment of Pb and the depletion of Hf, P and Nb in basalts can be interpreted by the blending of the eclogites in DOB (enriched in Pb and depleted in Hf, P and Nd) with the East China depleted mantle (As compared to the primitive mantle, it is neither enriched in Pb nor depleted in Hf, P and Nb). It is also indicated that the eclogites in the Dabie orogenic belt were surely derived from the exhumation materials, which had delaminated into the deep-seated mantle. Moreover, the process subsequently resulted in compositional variation of the mantle (especially in trace elements and isotopes), as revealed by the late mantle-derived basalts in the Dabie orogenic belt.