Nature of the Mesozoic lithospheric mantle and tectonic decoupling beneath the Dabie Orogen, Central China: Evidence from 40Ar/39Ar geochronology, elemental and Sr-Nd-Pb isotopic compositions of early Cretaceous mafic igneous rocks

Geochronological, elemental and Sr-Nd-Pb isotopic data of early Cretaceous basic-intermediate rocks from the Dabie Orogen provide new insights into the nature of the late Mesozoic lithospheric mantle beneath the region and its tectonic relationship with neighboring blocks. Basic-intermediate rocks from the North Dabie Complex (NDC) include diabases, lamprophyres and trachyandesites, which have 40Ar/39Ar plateau ages of 127.6-131.8 Ma. Similar rock types from the North Huaiyang Unit (NHY) erupted at nearly the same time (135-116 Ma). Coeval rocks from both tectonic units form a continuous array in Harker diagrams, and exhibit similar geochemical characteristics. Both are significantly enriched in LILEs, and depleted in HFSEs, coupled with very low epsilon(Nd) (t), (Pb-206/Pb-204)(i), and prominent positive Delta 8/4 and Delta 7/4 values, which are similar to those of Mesozoic mafic rocks in the North China Craton (NCC) exterior. These geochemical signatures are inconsistent with crustal contamination during magma ascent, and reflect derivation from an enriched lithospheric mantle source contaminated by the deeply subducted Yangtze crust. The observed geochemical similarities thus suggest that early Cretaceous igneous rocks from the NDC and NHY share a similar continental lithospheric mantle source that is tectonically affiliated to the NCC, although the surface geology of both tectonic units correlates with that of the Yangtze Block. Tectonic decoupling along a suture is proposed to explain the generation of early Cretaceous mafic rocks in the Dabie Orogen. The Wuhe-Shuihou fault likely represents the Mesozoic lithospheric boundary between the Yangtze Block and NCC, despite the fact that the present-day surface suture is situated at the Xiaotian-Mozitan fault or other faults to the north. (c) 2005 Elsevier B.V All rights reserved.