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

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 epsilon(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.