Early Cretaceous intermediate-mafic dykes in the Dabie orogen, eastern China: Petrogenesis and implications for crust-mantle interaction

Early Cretaceous intermediate-mafic dykes in the Dabie orogen, eastern China, provide an opportunity to examine the process of crust-mantle interaction caused by foundering of eclogitized lower continental crust (LCC) into underlying mantle. Chronological, geochemical and Sr-Nd-Hf isotopic analyses were carried out on the intermediate-mafic dykes. SHRIMP zircon U-Pb dating yields emplacement ages of 126 +/- 3 Ma and 127 +/- 2 Ma for the intermediate-mafic dykes. The dykes have wide ranges of SiO2 (44.9-61.4 wt.%), MgO (2.6-9.1 wt.%; Mg-#=41.3-61.7) Ni (6.9-135.0 ppm), and Cr (3.9-340.3 ppm) contents. They are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE, e.g., Ba, K, and Pb), and depleted in heavy rare earth elements (HREE) and high field strength elements (HFSE, e.g., Nb, Ta, and Ti) and Sr with weak negative Eu anomalies (Eu/Eu* = 0.74-1.04). Meanwhile, the dykes show relatively high whole-rock initial Sr-87/Sr-86 ratios (0.7056-0.7094) and negative epsilon(Nd)(t) values (-19.9 to -14.2), and display low zircon Hf-176/Hf-177 ratios (0.28193-028208) with negative epsilon(Hf)(t) values (-26.8 to -202). Such typical "continental" geochemical features do not result from crustal contamination, but reflect an enriched mantle source resulting from hybridization with the foundered eclogitized LCC We propose that the foundering of LCC led to the tectonic collapse of the Dabie orogen and the crust-mantle interaction beneath it during the Early Cretaceous. (C) 2012 Elsevier B.V. All rights reserved.