Origin of the Late Jurassic to Early Cretaceous peraluminous granitoids in the northeastern Hunan province (middle Yangtze region), South China: Geodynamic implications for the Paleo-Pacific subduction

Abstract

The Late Mesozoic granitic belt in the northeastern Hunan province (situated in the south of the middle Yangtze region) represents the western front of the large magmatic province of SE China. In order to determine their ages and petrogenesis, we carried out zircon U–Pb dating, Hf isotope and whole-rock geochemical analyses for four granitic plutons, namely Taohuashan, Dayunshan-Mufushan, Wangxiang and Lianyunshan. Our SIMS zircon U–Pb ages, together with previously published data, reveal that the magmatic activities in this area can be roughly subdivided into three phases at 151–146Ma, 132–127Ma and ca. 117Ma, and the Dayunshan-Mufushan batholith therein is a composite pluton. These four plutons are mainly composed of weakly to strongly peraluminous biotite or two-mica monzogranites, with a minor amount of biotite granodiorites. Their geochemical features are similar to S-type as well as fractionated S-type granites, with enrichment in LREEs and negative Ba, Sr, Nb, P and Ti anomalies. All samples show negative zircon εHf(t) values ranging from −12.5 to −3.6, corresponding to crustal Hf model (TDMC) ages of 1.4–2.0Ga. It is inferred that these granitoids were derived from partial melting of metasedimentary rocks analogous to the Neoproterozoic Lengjiaxi Group, predominantly with psammitic component. Fractional crystallization probably played an important role in the magma evolution, while input of mantle-derived magma was insignificant. Combined with other geological evidence, our new data allow us to propose that the Cretaceous (132–127Ma and ca. 117Ma) magmatism might be response to episodic slab rollback of the Paleo-Pacific plate, while the early-stage (151–146Ma) magmatism that overlapped the epilogue of Jurassic magmatic flare-up and subsequent magmatic quiescence probably foreshadowed the transformation from foundering of a subducted flat-slab to slab rollback. Alternatively, slab foundering after a SE-directed intracontinental subduction in the central SCB cannot be ruled out for geodynamic interpretation of the Jurassic magmatism in SE China.