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 SiO 2 (69-75%) contents and K 2 O/Na 2 O (>1.0), low MgO (or Mg # ) values, Ni and V concentrations, low ε Nd (t) (-17.01~-18.13), and high ( 87 Sr/ 86 Sr) i (0.7071~0.7072). The other group (the Yueshan and Zongpu adakitic rocks) is characterized by relatively low SiO 2 (58-67%) contents and K 2 O/Na 2 O (<1.0), high MgO (Mg # ) values, Ni and V concentrations, as well as relatively high ε Nd (t) (-6.63~-9.62) and low ( 87 Sr/ 86 Sr) 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 YueshanHongzhen 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.Keywords: adakite, delamination, lower crust, high potassium, east China geochemical similarity Drummond and Defant, 1990;Martin, 1999). However, some rocks with adakitic compositional features have been argued to be alternative origins, e.g., partial melting of mafic lower crust (Atherton and Petford, 1993;Muir et al., 1995;Petford and Atherton, 1996;Johnson et al., 1997), assimilation and fractional crystallization (AFC) processes involving basaltic magmas (Feeley and Hacker, 1995;Wareham et al., 1997;Castillo et al., 1999). Experimental studies (e.g., Rapp et al., 1991Rapp et al., , 1999Sen and Dunn, 1994;Rapp, 1995; Rapp and Waston, 1995;Springer and Seck, 19...
