Geochemical constraints on the origin of Mesozoic alkaline intrusive complexes from the North China Craton and tectonic implications

Mesozoic alkaline intrusive complexes are widespread in the southern portion of the North China Craton and can provide some important constraints on the evolution of the Mesozoic lithosphere beneath the region. Three selected intrusive complexes (Tongshi, Hongshan, and Longbaoshan) are generally high in alkalis (K2O+Na2O=9∼13 wt.%) and Al2O3 (14∼21.6 wt.%) and low in CaO and TiO2 (0; (206Pb/204Pb)i>18). The Hongshan complex has low REE concentrations (∑REE=28.2∼118.7 ppm, (La/Yb)N=4.6∼14.7) and is moderately enriched as demonstrated by their Sr–Nd isotopic ratios ((87Sr/86Sr)i>0.706; εNd0.7078; εNd<−11). We suggest that the Tongshi complex originated from the asthenosphere and the Hongshan complex and the Longbaoshan complex were derived from the partial melting of previously subduction-modified lithospheric mantle, in response to post-collisional lithospheric extension and asthenospheric upwelling. The occurrence of these alkaline intrusive complexes demonstrates that the lithosphere beneath the region must have been considerably thinned at the time of intrusion of these complexes. This study also shed light on the temporal evolution of the Mesozoic lithosphere and the timing of the lithospheric thinning.