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

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 epsilon(Nd)(t) (-17.01similar or equal to18.13), and high (Sr-87/Sr-86)(i) (0.7071similar to0.7072). The other group (the Yueshan and Zongpu adakitic rocks) is characterized by relatively low SiO2 (58-67%) contents and K2O/Na2O (<1.0), high MgO(Mg-#) values, Ni and V concentrations, as well as relatively high epsilon(Nd)(t) (-6.63similar or equal to9.62) and low (Sr-87/Sr-86)(i) (0.7064similar to0.7069). Both groups are associated with the contemporary Dongling metamorphic core complex, A-type granites, and some per-alkaline igneous rocks that are formed only in an extensional setting. The tectonic setting and chemical compositions suggest that: (1) the adakitic intrusive rocks in the Yueshan-Hongzhen area were formed in an extensional tectonic regime within a continental plate; (2) the Hongzhen adakitic granites were most likely derived directly from partial melting of mafic material with high potassium contents at the base of the continental crust at pressures of similar to1.2 GPa, leaving residual garnet +/- hornblende +/- plagioclase in the source; (3) the Yueshan and Zongpu adakitic rocks were most probably derived from dehydration melting of basaltic materials delaminated into underlying mantle at pressures >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 similar to31 km, and therefore the crust appears to have been thinned by at least similar to10 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.