Geochemical and Re–Os isotope constraints on the origin and age of the Songshugou peridotite massif in the Qinling orogen, central China

Abstract

The Songshugou peridotite massif in the Qinling orogenic belt is one of the largest orogenic spinel peridotite bodies in central China, but its origin remains controversial and its age is poorly constrained. We have carried out an integrated study of major and trace element composition, mineral chemistry, platinum group elements (PGE), as well as Re–Os isotope systematics of 1 harzburgite and 12 dunites from the Songshugou peridotite massif. These samples contain high Mg# olivine (90.0–91.3) and Cr# spinel (83.4–96.0). The harzburgite and dunites are characterized by relatively low whole-rock Al2O3 (0.32–0.60wt.%), CaO (0.26–1.57wt.%), and Na2O (0.07–0.12wt.%) concentrations. The studied samples have very low concentrations of middle and heavy rare earth elements and exhibit enrichments in iridium-group platinum-group elements (IPGE) relative to palladium-group PGE. The Songshugou peridotites exhibit variable enrichments of light rare earth elements, large ion lithophile elements, Re, Zr, and Hf, which resulted from reactions with melt after their isolation from the convecting mantle. Combined with previous results, our data suggest that the Songshugou peridotites are highly refractory mantle residues derived from a forearc mantle wedge. 187Os/188Os values of the studied samples vary from 0.12073 to 0.12390, and 187Re/188Os ratios are 0.005–0.081. The average Re–Os model ages (TMA) and maximum Re depletion model age (TRD) of the Songshugou peridotites are ca. 1.2–1.1Ga, suggesting a tectonic affinity to the South China Block and that the peridotites formed during the assembly of the Rodinia supercontinent. The Songshugou peridotites were sourced from a mantle wedge above a subduction zone, and finally incorporated into the underlying continental lithosphere by exhumation.