Crust-Mantle Interaction in Dabie Orogenic Belt, Central China: Geochemical Evidence from Late Cretaceous Basalts

摘要

It has been suggested that eclogites in the Dabie orogenic be lt 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 calculated p-T conditions from mineral thermobarometers. The Late Cretaceous basalts studied in the prese nt paper, however, have provided some geochemical evidence for crust-mantle int eraction in the area. These basalts are distributed in Mesozoic faulted basins i n central and southern Dabie orogenic belt. Since little obvious contamination f rom 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 th e same or similar geochemical features with respect to their magma source. In th e spider diagram normalized by the primitive mantle, trace element geochemistry data show that their mantle sources are enriched in certain elements concentrate d in the continental crust, such as Pb, K, Rb and Ba, and slightly depleted in s ome HFSE such as Hf, P and Nb. Pb-Sr-Nd isotopic compositions further suggest the mantle is the mixture of depleted mantle and enriched one . T his 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 deplete d in Hf, P and Nd) with the East China depleted mantle (As compared to the primi tive mantle, it is neither enriched in Pb nor depleted in Hf, P and Nb). It is a lso indicated that the eclogites in the Dabie orogenic belt were surely derived from the exhumation materials, which had delaminated into the deep-seated mantl e. Moreover, the process subsequently resulted in compositional variation of the mantle (especially in trace elements and isotopes), as revealed by the late man tle-derived basalts in the Dabie orogenic belt.