Mesozoic adakites in the Lingqiu Basin of the central North China Craton: Partial melting of underplated basaltic lower crust

抄録

Intermediate to felsic volcanic rocks of the Baiqi formation from the Lingqiu basin in the central part of North China Craton were studied. Single zircon U-Pb dating indicates that these volcanics formed at 125.8±3.0 Ma. Their Sr and Nd isotopic compositions (¹⁴³Nd/¹⁴⁴Nd = 0.51180-0.51182, ⁸⁷Sr/⁸⁶Sr = 0.7062-0.7063) fall in the range of the nearby late-Mesozoic basaltic rocks. These volcanics share geochemical affinities to the adakites formed in the modern arcs, e.g., high Na₂O (>4.06%), Al₂O₃ (>15.4%) and Sr (645-1389 ppm) contents and Sr/Y ratios (55∼103), and thus being termed as adakitic rocks. However, the Baiqi adakitic rocks were not temporospatially associated with active subduction. Furthermore, their low Cr (2.19-47.4 ppm, with average of 25) and Ni (1.57-20.7 ppm, with average of 12) contents and Mg# (22-47, with average of 32) argue against interaction with the lithospheric mantle. Combined with the geological setting, we suggest that the Baiqi adakitic rocks resulted from partial melting of a thickened lower continental crust associated two episodes of basaltic underplating events. We propose that enormous conductive heating from 80-140 Ma basaltic underplating resulted in partial melting of pre-existing mafic lower crust formed by ∼150-160 Ma basaltic underplating. This study provides a case for partial melting of the thickened lower continental crust in association with basaltic underplating events.