Geochronological and geochemical study of the Darbut Ophiolitic Complex in the West Junggar (NW China): Implications for petrogenesis and tectonic evolution

The Darbut Ophiolitic Complex consists of ultramafic rocks, cumulates, pillow lavas and abyssal radiolarian cherts, and occurs in the Sartuohai–Suluqiaoke area which is situated in the eastern part of the West Junggar (NW China), Southern Altaids. In the present study, we report new geochronological and geochemical data obtained for gabbro and basalt blocks embedded in the Darbut Ophiolitic Complex. Zircon U–Pb analyses from one representative gabbro sample by LA-ICP-MS yielded a weighted mean age of 391 ± 6 Ma, showing that it was emplaced in the Middle Devonian. The samples bear N-MORB, E-MORB, and arc-like geochemical signatures. They are tholeiitic in composition with low concentrations of Na2O + K2O varying from 1.36 to 4.09 wt.%, Mg and Ti depleted, large ion lithophile elements (LILEs) enriched, with insignificant negative Eu. They also show negative Nb–Ta anomalies and high positive εNd(t) values, with relative high La/Nb, Ba/Nb, Ba/La, Ba/Th and Zr/Nb ratios. We interpret that these volcanic rocks were derived from a slightly metasomatized depleted mantle source in a back-arc basin setting, which corresponds to 10–30% partial melting of a depleted spinel lherzolite mantle. The data presented here in combination with previously published data suggest a northwestward subduction model for the West Junggar. During the period of the Early Devonian to Early Carboniferous, the West Junggar was dominated by normal northwestward subduction, and the ridge subduction commenced in the Late Carboniferous.