Neoproterozoic intrusions along the northern margin of South Qinling, central China: Geochemistry, zircon ages, and tectonic implications

Abstract

We report whole-rock Sr–Nd isotopic compositions and U–Pb ages and Hf isotopic compositions of zircon grains from Neoproterozoic intrusive rocks exposed in northern South Qinling, central China. The rocks have diverse compositions and were emplaced at ca 751–714 Ma. On the basis of their distinct geochemical signatures, four compositional groups are recognized. High-Ti gabbros are tholeiitic, with low SiO2 and MgO, and high FeOT and TiO2 contents, and exhibit enrichment in large-ion-lithophile elements and absence of Nb and Ta negative anomalies. These gabbros have high εNd(t) values of +1.9 to +5.3 that are similar to the compositions of E-MORB or OIB, implying an origin from asthenospheric mantle. Low-Ti gabbros are calc-alkaline and have high SiO2 and MgO, and low FeOT and TiO2 contents. These gabbros have typical arc-like trace-element signatures and εNd(t) values that vary from −3.2 to +6.0, consistent with derivation from the partial melting of heterogeneous lithospheric mantle that was modified by subduction. Elemental and Nd–Hf isotopic compositions of I-type granitoids imply significant contributions from both mantle and crustal rocks. S-type granites originated from the partial melting of metasedimentary rocks with isotopic compositions similar to paragneisses of the basement in South Qinling. The extensive melting of both mantle and crustal rocks under relatively low pressures and high temperatures suggests an extensional environment during the Neoproterozoic. Our data, in combination with existing data from basement rocks and Neoproterozoic igneous rocks, constrain the crustal evolution of the northern Yangtze Block and South Qinling, in which subduction along the northern margin of the Yangtze Block prior to ca 800 Ma led to arc–continent collision, inducing high-grade metamorphism of the basement rocks of the Douling Complex. Subsequent protracted continental rifting was related to breakup of the supercontinent Rodinia.