Petrogenesis and tectonic significance of a Mesozoic granite-syenite-gabbro association from inland South China

摘要

A granite-syenite-gabbro association from inland South China has been studied for zircon U-Pb ages and Hf isotopic compositions as well as whole-rock elemental and Sr-Nd isotopic compositions to constrain their sources and tectonic settings. These rocks have distinctive crystallization ages: the Longyuanba biotite granites have a mean age of~240 Ma, the Longyuanba two-mica granites, Pitou alkali-feldspar granites and amphibole-bearing alkali-feldspar granites, Tabei and Huangbu syenites and Chebu and Chenglong gabbros have a similar age of ~178 Ma, whereas the Dafengnao syenite gives a mean age of ~ 165 Ma. The Longyuanba biotite granites and two-mica granites exhibit S-type characteristics, probably derived from a Neoproterozoic crastal source. The Chenglong gabbro has OIB-like trace element and highly depleted isotope compositions (ε_(Hf)(t) = 10.0 ± 1.3; ε_(Nd)(t) = 5.2), suggesting its parental melt to be of asthenospheric origin with insignificant crustal assimilation. However, crustal contamination is required to explain the isotopic compositions of the Chebu gabbro. The syenites are shoshonitic in composition, and have depleted Sr-Nd-Hf isotopic signatures, which we interpret to have resulted from a mixed source of asthenospheric mantle and metasomatized lithospheric mantle. As expected, the signals of crustal assimilation are conspicuous in the petrogenesis of the more evolved syenites. The Pitou alkali-feldspar granite and amphibole-bearing alkali-feldspar granite exhibit I-type and A-type characteristics, respectively. They are isotopically more depleted than the S-type Longyuanba granites. We suggest that they may have formed through magma mixing of predominantly mantle-derived melts with the Neoproterozoic crust. The Jurassic granite-syenite-gabbro association was the product of asthenosphere-lithosphere-crust interactions, which records the primary role of asthenospheric mantle upwelling in magma generation both in the mantle and in the crust in the Early Yanshanian time in South China. They probably occurred in an intraplate rift-like environment as a tectonic response to far-field stress at plate margins during the early stage of the paleo-Pacific plate subduction. The tectonic transition from the Tethys orogenic regime to the paleo-Pacific regime was accomplished in the Early Jurassic, and the early Yanshanian magmatism should be genetically associated with the paleo-Pacific tectonic regime.