Cretaceous high-potassium intrusive rocks in the Yueshan-Hongzhen area of east China: Adakites in an extensional tectonic regime within a continent

Abstract

Adakites are generally associated with subduction zones (i.e., at convergent plate margins). This paper reports geochemical and isotopic data for early Cretaceous high-potassium adakitic intrusive rocks in the Yueshan-Hongzhen area of east China, which occur in an extensional tectonic regime within a continent. Based on petrology and geochemistry, these adakitic intrusive rocks are classified into two groups. One group (the Hongzhen adakitic granites) is characterized by high SiO2 (69-75%) contents and K2O/Na2O (>1.0), low MgO (or Mg#) values, Ni and V concentrations, low eNd(t) (-17.01∼-18.13), and high (87Sr/86Sr)i (0.7071∼0.7072). The other group (the Yueshan and Zongpu adakitic rocks) is characterized by relatively low SiO2 (58-67%) contents and K2O/Na2O ( 1.2 GPa, leaving residual garnet + pyroxene in their sources. Both groups of adakitic rocks have high La/Yb and Sr/Y, indicating that the crustal thickness in the Yueshan-Hongzhen area exceeded 40 km when the adakitic magmas were generated in the early Cretaceous. The present thickness of the crust in the Yueshan-Hongzhen area is only ∼31 km, and therefore the crust appears to have been thinned by at least ∼10 km since the early Cretaceous. The relatively high MgO, Ni, and V values of the Yueshan and Zongpu adakitic rocks suggest that adakitic magmas interacted with mantle rocks, possibly concurrently with delamination of the lower crust.