Mesozoic crustal thickening of the eastern North China craton: Evidence from eclogite xenoliths and petrologic implications

摘要

A suite of xenoliths of eclogite, garnet clinopyroxenite, and felsic gneiss is found in Early Cretaceous high-Mg [Mg# >45, where Mg# = molar 100 x Mg/(Mg + Fe_total)] adakitic intrusions from the Xuzhou-Huaibei (Xu-Huai) region along the southeastern margin of the North China craton. The primary mineral assemblage of garnet + omphacite/augite + quartz + rutile ± pargasite of the eclogite and garnet clinopyroxenite xenoliths defines a minimum pressure of >1.5 GPa, while the estimated peak metamorphic temperatures range from 800 to 1060 °C. An Sm-Nd whole-rock–garnet isochron and zircon U-Pb dates show that timing of the eclogite facies metamorphism took place ca. 220 Ma. This Triassic age agrees with the age of eclogites from the Dabie-Sulu ultrahigh-pressure metamorphic (UHPM) belt. The ages of abundant Late Archean to early Paleoproterozoic (2.3–2.6Ga) inherited zircons correspond to the most prominent crustal growth event in the North China craton. In addition, these xenoliths and their host high-Mg adakitic intrusions have complementary major and trace element compositions, suggesting that the adakites formed by partial melting of Archean metabasalts that were the protoliths of the Xu-Huai eclogite and garnet clinopyroxenite xenoliths. Trace element and Sr-Nd isotopic modeling shows that the high-Mg adakitic intrusions can be modeled as melts from 40% partial melting of the metabasalts in the eclogite facies, followed by interaction with the convecting mantle and variable degrees of crustal assimilation. Together with the similar zircon age populations between the xenoliths and the host rocks, these lines of evidence strongly suggest their genetic link via thickening, foundering, and partial melting of the Archean North China craton mafic lower crust, followed by adakitic melt-mantle interaction. The crustal thickening resulted from Triassic collision between the Yangtze craton and the North China craton, which produced the Dabie-Sulu UHPM belt in the subducting Yangtze plate and eclogitization of the basaltic crustal root of the overriding North China craton plate. Such processes may have played an important role in generating the high-Mg character of the continental crust.