Late Mesozoic bimodal volcanic rocks in the Jinniu basin, Middle-Lower Yangtze River Belt (YRB), East China: Age, petrogenesis and tectonic implications

Late Mesozoic intrusive and volcanic rocks are widespread in the southeast Hubei Province, Middle-Lower Yangtze River Belt (YRB), East China. Detailed in situ zircon U-Pb and Hf isotope, elemental and Sr-Nd-Pb isotopic data are presented in this paper for late Mesozoic volcanic rocks from the Jinniu Basin, YRB, aiming to constrain their age, petrogenesis, and tectonic implications. The Jinniu volcanic rocks show a bimodal distribution in composition, with dominant rhyolite and dacite, and subordinate basalt and basaltic andesite. New SHRIMP and LA-1CPMS zircon U-Pb ages indicate that the volcanic rocks of three Formations in the Jinniu basin were erupted at quite a short age range of about 5 Ma during the Early Cretaceous (130-125 Ma). The mafic rocks are moderately enriched in large-ion-lithophile-elements (LILE) (e.g., Ba, Th, U, and Pb) and light rare-earth-elements (LREE), and are characterized by negative Nb, Ta, and Ti anomalies, and relatively high TiO2 (0.72-2.06%) and Nb (9.20-26.5 ppm) contents. These analyses indicate that the geochemical characteristics of the mafic rocks in the Jinniu basin are similar to worldwide Phanerozoic Nb-enriched basalt and andesites (NEBA). New in situ zircon U-Pb ages and field geological relationships demonstrate that NEBA in the southeast Hubei Province are not spatially or temporally associated with high-silica adakitic rocks, but were most likely derived from an enriched lithospheric mantle with assimilation of minor crustal materials, and then fractional crystallization during the evolution of the magma. Overall, the felsic rocks in the Jinniu basin have geochemical characteristics, and Sr-Nd-Pb signatures, and in situ zircon Hf isotopic compositions similar to those of the mafic rocks. Compared with the mafic rocks, the felsic rocks are characterized by enriched and variable concentrations of LILE and REE (e.g., Ba = 33.3-1372 ppm, V = 11.4-33.6 ppm, Ybc = 5.07-18.7), and negative Eu anomalies (delta Eu = 0.22-0.98), as well as a wide range of radiogenic Nd-Pb isotopic values with epsilon Nd (t) = -10.2 to -2.4, (Pb-206/Po-204)(i) = 17.659-18.705, (Pb-207/Pb-204)(i) = 15.47-15.663. and Pb-208/Pb-204)(i) = 37.654-38.935, and in situ zircon Hf compositions of epsilon Hf (t) = -12.7 to -1.8. These features indicate that the genesis of felsic magma in the Jinniu basin is consistent with extensive fractional crystallization and large amounts of crustal contamination from an evolved mafic magma (SiO2 = similar to 55%). The bimodal volcanic rocks in this study provide convincing evidence that Early Cretaceous volcanic rocks in the YRB developed in a back-arc extensional tectonic regime. (C) 2011 Elsevier By. All rights reserved.