Geochemistry, 40Ar/39Ar geochronology, and geodynamic implications of Early Cretaceous basalts from the western Qinling orogenic belt, China

The Qinling-Dabie orogenic belt was formed by the collision of the North and South China Cratons during the Early Mesozoic and subsequently developed into an intracontinental tectonic process during late Mesozoic. Field investigations identified the presence of late Mesozoic basalts in the Duofutun and Hongqiang areas in the western Qinling orogenic belt. The petrogenesis of these basalts provides an important constraint on the late Mesozoic geodynamics of the orogen. The representative basaltic samples yield the ⁴⁰Ar/³⁹Ar plateau age of about 112 Ma. These samples belong to the alkaline series and have SiO₂ ranging from 44.98 wt.% to 48.19 wt.%, Na₂O+K₂O from 3.44 wt.% to 5.44 wt.%, and MgO from 7.25 wt.% to 12.19 wt.%. They demonstrate the right-sloping chondrite-normalized REE patterns with negligible Eu anomalies (1.00-1.10) and PM-normalized patterns enriched in light rare earth element, large ion lithophile element and high field strength element, similar to those of OIB rocks. These samples additionally show an OIB-like Sr-Nd isotopic signature with ε_Nd(t) values ranging from +6.13 to +10.15 and initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7028 to 0.7039, respectively. These samples are geochemically subdivided into two groups. Group 1 is characterized by low Al₂O₃ and high TiO₂ and P₂O₅ contents, as well as high La/Yb ratios (>20), being the product of the high-pressure garnet fractionation from the OIB-derived magma. Group 2 shows higher Al₂O₃ but lower P₂O₅ contents and La/Yb ratios (<20) than Group 1, originating from asthenospheric mantle with input of delaminated lithospheric component. In combination with available data, it is proposed for the petrogenetic model of the Early Cretaceous thickened lithospheric delamination in response to the asthenospheric upwelling along the western Qinling orogenic belt.