Cogenetic relationship of the Yangkou gabbro-to-granite unit, Su-Lu terrane, eastern China, and implications for UHP metamorphism

Abstract

The Yangkou gabbro-to-granite complex is a petrotectonic unit of the Su-Lu ultrahigh-pressure (UHP) metamorphic terrane in eastern China. It consists of lherzolites, metagabbro–eclogites, metagranodiorites and deformed granites. Whether or not the felsic members of the complex have undergone UHP metamorphism as is recorded in the metagabbro–eclogites is significant to understanding the relationship between eclogite and host rock. The metagabbro–eclogites have low ε Nd (220 Ma) values (−17 to −12) at the time of UHP metamorphism, indicating that their protoliths are old mafic intrusive rocks emplaced in a continental domain. Geochemical and isotopic studies for the Yangkou unit suggest that the protoliths of the unit (except eclogites) represent a cogenetic magmatic suite, produced by differentiation of a mafic parental magma. The parental magma was derived from melting of a long-term enriched mantle, and was emplaced into a crustal level during continental extension at c . 700–800 Ma. The cogenetic relationship for the protoliths of the unit suggests that the metagranodiorites and deformed granites were also subducted to extreme mantle depths along with the metagabbro–eclogites at c . 220 Ma. The lherzolites, however, are genetically not related to the mafic protoliths; they represent ‘mantle xenoliths’ of tectonic origin.