ABSTRACT

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.