Petrogenesis and tectonic significance of the Baoxing granitic and mafic intrusions, southeastern China: Evidence from zircon U–Pb dating and Lu–Hf isotopes, and whole-rock geochemistry

Abstract

Voluminous Neoproterozoic gabbros, gabbro-diorites, and granites are exposed in the Baoxing area along the western margin of the Yangtze Block. We present here zircon U–Pb and Lu–Hf isotope data, as well as wholerock geochemical data for these rocks, which help constrain the tectonic characteristics and evolution of the western margin of the Yangtze Block. Zircons from these rocks have high Th and U contents and Th/U ratios (N0.2), and show striped absorption patterns, growth striations, or zoning, indicating a typical magmatic origin. Zircon U–Pb dating and Lu–Hf isotope results show that the gabbros formed at ca. 850 Ma with εHf = +5.44 to +11.94, and a single-stage model age (THf) of 1.04 Ga, suggesting a depleted-mantle-derived source. The gabbro-diorites formed at ca. 800 Ma with major εHf = +3.99 to +14.62 and THf = 0.97 Ga, and minor εHf = +0.68 to +5.66 and THf = 1.27 Ga, again suggesting a depleted-mantle-derived source. The granites were emplaced at ca. 770 Ma with εHf = +2.53 to +10.58 and THf = 1.07 Ga, indicating that they were derived from the partial melting of juvenile crust. The geochemical data show that the gabbros and gabbro-diorites are characterized by low SiO2 contents, high Mg# values and Cr, Co, and Ni contents, significant enrichment in light rare-earth elements and large-ion lithophile elements, depletion in high field strength elements (e.g., Nb, Ta, Zr, and Hf), and relatively high Nb/U, Ta/U, and Ce/Pb ratios. These data suggest that the rocks were derived from the partial melting of depleted lithospheric mantle that had been metasomatized by subduction-derived fluids or melts, and which underwent very minimal crustal contamination during magma evolution and ascent. The granites have high SiO2 and K2O, and low MgO contents, exhibit enrichment in Rb, Ba, Th, and Pb and depletion in Nb, Ta, and Ti, and have low Sr/Y and (La/Yb)N ratios and high Y and Yb contents, which together with their Hf isotopic compositions imply a juvenile crust source. Taking into account regional pre-750 Ma arc-related magmatism and the subsequent extensional setting, it is proposed that a long-lived, active continental-margin setting may have evolved along the western margin of the Yangtze Block during the Neoproterozoic, and that the coeval granitic and mafic rocks in the Baoxing area were formed by the subduction of the western Yangtze Block.