Continental origin of eclogites in the North Qinling terrane and its tectonic implications

Eclogites in an orogen, serving as a credible marker for deep subduction of oceanic or continental crust, are a key to decipher the process and fate of plate subduction. The nature of eclogite precursors bears important information on orogenic mechanism and pre-subducted geological history. Despite their importance, the nature of ultra-high pressure (UHP) eclogites in the North Qinling (NQ) terrane is still poorly constrained, which leads to conflicting tectonic scenarios for the evolution of the Qinling orogenic belt. In this paper, we provide a combined study of zircon U-Pb ages, major and trace element data, and Sr-Nd-Pb-Hf isotope compositions for eclogites from the NQ terrane. U-Pb age determinations on magmatic zircon cores of two eclogite samples yield similar ages of ca. 800 Ma, indicating that the protoliths of the NQUHP eclogites formed in the Neoproterozoic. The NQ eclogites have tholeiitic compositions and are characterized by high TiO2 (1.40-2.40%), and low P2O5 (0.09-0.22%), Na2O (1.10-2.49%) and K2O (0.17-1.04%) contents. They are moderate fractionation in REEs (La-N/Yb-N = 2.31-4.20) and show negligible Nb, Ta, Ti, and Eu anomalies (Eu/Eu* = 0.82-1.10). The eclogites have high positive whole-rock epsilon(Nd)(t) (2.81-5.53) and zircon epsilon(Hf)(t) (10.5-11.9) values, indicating their derivation from a relatively depleted mantle source. On geochemical discriminant diagrams based on immobile element ratios, the NQ eclogites fall in the field of continental basalts. Moreover, both the geochronological and geochemical features of the eclogite protoliths are quite different from those of the Erlanpging basaltic rocks, but comparable to those of the coeval continental basalts in South Qinling (SQ) orogen and the northern margin of the South China Block (SCB). As the mid-Neoproterozoic basaltic rocks in the SQ orogen and the northern margin of the SCB were well documented to form in a continental rift setting in response to the global-scale breakup of the Rodinia supercontinent, it is inferred that the NQ eclogite precursors might also form in a continental rift setting at ca. 800 Ma, which resulted in the splitting of the NQ terrane from the SCB. As the protoliths of the NQ eclogites have a continental property, we suggest that the ca. 490 Ma UHP metamorphism of the NQ unit might result from its northward subduction beneath the Erlangping arc and thus manifested an arc-continent collision process. (C) 2013 Elsevier B.V. All rights reserved.