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 ε_Nd(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 (<1.0), high MgO (Mg^#) values, Ni and V concentrations, as well as relatively high ε_Nd(t) (-6.63∼-9.62) and low (⁸⁷Sr/⁸⁶Sr)_i (0.7064∼0.7069). 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 ∼1.2 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 ∼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.