Large-scale early cretaceous lower-crust melting derived adakitic rocks in NE China: implications for convergent bidirectional subduction and slab rollback

Abstract

ABSTRACT Large volumes of Early Cretaceous adakitic rocks crop out in northeast China. In this paper, we summarize their spatial-temporal distribution and geochemical characteristics. These adakitic rocks exhibit variable MgO, Cr and Ni contents, as well as potassium enrichments, in contrasting with those of the slab melting derived adakites. Positive εNd(t) and εHf(t) values and low 87Sr/86Sr(i) indicate significant Phanerozoic crust growth in NE China. Based on their geochemistry, these adakitic rocks are mainly derived from partial melting of juvenile lower crust. We interpret the melting to have resulted from extension and associated delamination. The ages of adakitic rocks are 141–120 Ma in the Xing’an region, 129–113 Ma in the Songliao basin, and 113–90 Ma in the Jiamusi region. Based on the lower-crustal-melting geochemical characteristics and spatial distribution of ages, we propose a new two-stage convergent bidirectional subduction model: Mongol-Okhotsk oceanic plate subduction and associated slab rollback in the west from 141–114 Ma and the Paleo-Pacific oceanic plate subduction and associated slab rollback in the east from 113–90 Ma.