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Original Article |
1 1Department of Geology, Peking University, Beijing 100871, P.R. China (e-mail: bchen@geoms.geo.pku.edu.cn)
2 2Géosciences Rennes, UPR CNRS 4661, Université de Rennes 1, 35042 Rennes Cedex, France
3 3Laboratory of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, PO Box 9825, P.R. China
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, metagabbroeclogites, metagranodiorites and deformed granites. Whether or not the felsic members of the complex have undergone UHP metamorphism as is recorded in the metagabbroeclogites is significant to understanding the relationship between eclogite and host rock. The metagabbroeclogites have low
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. 700800 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 metagabbroeclogites at c. 220 Ma. The lherzolites, however, are genetically not related to the mafic protoliths; they represent mantle xenoliths of tectonic origin.
Keywords: Yangkou unit, eclogites, metagranites, Nd/Sr isotopes, ultrahigh pressure, metamorphism.
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