Petrogenesis of early Yanshanian highly evolved granites in the Longyuanba area, southern Jiangxi Province: Evidence from zircon U-Pb dating, Hf-O isotope and whole-rock geochemistry

Abstract

Early Yanshanian (Jurassic) granitoids are widespread in the Nanling Range, South China. Whereas large granitic batholiths commonly crop out in the center of the Nanling Range (corresponding geographically to the central and northern parts of Guangdong Province), many small stocks occur in the southern part of Jiangxi Province. Most of the small stocks are associated closely with economically significant rare-metal deposits (W, Sn, Nb, Ta). Here we report the results for biotite granites and two-mica granites from three Yanshanian stocks of the Longyuanba complex. LA-ICPMS U-Pb dating of zircon yields an age of 156.1±2.1 Ma for Xiaomubei biotite granite, and U-Pb zircon dating using SIMS yields an age of 156.7±1.2 Ma for Longyuanba-Chengjiang biotite granite and 156.4±1.3 Ma for Jiangtoudong two-mica granite. Biotite granites are silica-rich (SiO2=70%–79%), potassic (K2O/Na2O>1.9), and peraluminous (ASI=1.05–1.33). Associated samples are invariably enriched in Rb, Th, Pb and LREE, yet depleted in Ba, Nb, Sr, P and Ti, and their REE pattern shows a large fractionation between LREE and HREE ((La/Yb)N=10.7–13.5) and a pronounced Eu negative anomaly (δEu=0.28–0.41). Two-mica granite samples are also silica-rich (SiO2=75%–79%), potassic (K2O/Na2O>1.2), and peraluminous (ASI=1.09–1.17). However, in contrast to the biotite granites, they are more enriched in Rb, Th, Pb and extremely depleted in Ba, Nb, Sr, P and Ti, and exhibit nearly flat ((La/Yb)N=0.75–1.08) chondrite-normalized REE patterns characterized by strong Eu depletion (δEu=0.02–0.04) and clear tetrad effect (TE1.3=1.10–1.14). Biotite granites and two-mica granties have comparable Nd isotopic signatures, and their ɛNd(t) are concentrated in the −13.0 to −9.6 and −11.5 to −7.7 respectively. Their zircon Hf-O isotopes of both also show similarity (biotite granites: ɛHf(t)=−10.8–−7.9, δ 18O=7.98‰–8.89‰ and ɛHf(t)= −13.8 to −9.1, δ 18O=8.31‰–10.08‰; two-mica granites: ɛHf(t)=−11.3 to −8.0, δ 18O=7.91‰–9.77‰). The results show that both biotite and two-mica granites were derived mainly from sedimentary source rocks with a minor contribution from mantle-derived materials. In spite of some S-type characteristics, the biotite granites were formed by fractional crystallization of I-type magma and assimilation of peraluminous sedimentary rocks during their ascent to the surface. Therefore, they belong to highly fractionated I-type granites. Two-mica granites exhibit a tetrad effect in their REE patterns, but share the same isotopic features with the biotite granites, suggesting that they are highly fractionated I-type granites as well. Their Lanthanide tetrad effects may be attributed to the hydrothermal alteration by magmatic fluids that have suffered degassing at late stages. Granitic magmas undergoing fractional crystallization and wall-rock assilimation can generate highly evolved granites with no REE tetrad effect in the uni-phase system. However, in the late-stage of magmatic evolution in the multi-phase system (i.e., magmatic-hydrothermal system), these magmas also can lead to the highly evolved granites exhibiting mew-shaped REE pattern characterized by tetrad effect as the consequence of melt-fluid and fluid-vapor fractionation, and the resultant autometasomatism. We thus suggest that the REE pattern exhibiting tetrad effect feature is an important indicator of rare metal mineralization in the early Yanshanian time in southern China, implying the metamorphism of the ore fluid.