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 (<0.6 wt.%), with high and variable SiO2 contents. Rocks from these complexes are all enriched in LREE and LILE (Cs, Rb, Ba, U, Th), depleted in Nb and Ti, have a highly positive Pb anomaly, and are characterized by lack of a clear Eu anomaly despite trace element abundances and isotopic ratios that vary greatly between complexes. The Tongshi complex has high Cs (2.6∼8.5 ppm) and REE abundances (∑REE=112.6∼297 ppm, (La/Yb)N=13.1∼30.9) and MORB-like Sr-Nd-Pb isotopic ratios ((87Sr/86Sr)i<0.704; εNd>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; εNd<−7). The Longbaoshan complex is extremely REE enriched (∑REE=211.3∼392.6 ppm, (La/Yb)N=32.4∼60.9) and has an EM2-like Sr-Nd isotopic character ((87Sr/86Sr)i>0.7078; 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.