SHRIMP U-Pb zircon geochronology and Nd-Sr isotopic study of the Mamianshan Group: implications for the Neoproterozoic tectonic development of southeast China

Northwestern Fujian contains abundant well-studied Precambrian basement, and was a composite terrane in Cathaysia during the Neoproterozoic; however, its magmatic activity, petrogenesis, and tectonic evolution remain controversial. This article focuses on the geochronology and geochemistry of the Neoproterozoic Group in order to resolve the above problems. We provide new SHRIMP U-Pb zircon dating for the Mamianshan Group: 851.9 +/- 9.2 to 825.5 +/- 9.8 Ma for the Longbeixi Formation, 796.5 +/- 9.3 Ma for the Dongyan Formation, and 756.2 +/- 7.2 Ma for the Daling Formation. These ages document the existence of Neoproterozoic magmatism in the northwestern Cathaysia Block. Dongyan Nd-Sr isotopic data show that mafic amphibolite schists, mafic greenschists, and quartzofeldspathic schists were derived from a more depleted mantle (initial epsilon Nd +5.5 and 87Sr/86Sr ratio 0.703409643), a mixture of depleted mantle and crustal components (initial epsilon Nd 1 and 87Sr/86Sr ratio 0.7020452820.704147714), and late Palaeoproterozoic continental crustal materials (initial epsilon Nd < 1 and 87Sr/86Sr ratio 0.71083603), respectively. These new data, together with previous studies, suggest a bi-subduction-collision orogenic model for the Neoproterozoic evolution of the Yangtze and Cathaysia blocks. Our plate tectonic scenario involves earlier NW-dipping subduction during 1.0 Ga860 Ma along the southeastern margin of the Yangtze Block and later NW-dipping subduction near the northwestern margin of the Cathaysia Block starting at ca. 850 Ma. The 796.5 +/- 9.3 Ma age of the volcanic Dongyan Formation suggests that the final assembly of the Yangtze and Cathaysia blocks probably occurred after ca. 800 Ma. The 756.2 +/- 7.2 Ma age of the Daling Formation indicates that post-orogenic extensional magmatism took place after 800 Ma along the northwestern margin of Cathaysia.