Origin of Triassic granites in central Hunan Province, South China: constraints from zircon U–Pb ages and Hf and O isotopes

Abstract

Triassic granites crop out extensively in central Hunan Province, South China. Representative granites include the Baimashan, Weishan, and Ziyunshan plutons. Lithologically, these granites mainly comprise biotite monzogranite, two-mica granites, hornblende-biotite granite, hornblende-biotite monzogranite, and garnet-muscovite granite. These granites have in situ zircon secondary ion mass spectrum U–Pb ages between 223.2 ± 3.3 and 209.3 ± 4.0 Ma, indicating that they likely formed predominantly in the Late Triassic. These granitic plutons have similar zircon Hf and O isotopic compositions, with εHf(t) values of −0.8 to −9.0, two-stage depleted mantle model ages (TDM2) of 1.81–1.31 Ga, and weighted mean of δ18OZrc values of 8.53 ± 0.58‰ to 9.12 ± 0.28‰. Combined with U–Pb dating and Hf isotopic data, the elevated and variable δ18O and εHf(t) values of the individual granites indicate that these Triassic granites were likely produced by partial melting of upper Paleoproterozoic to lower Mesoproterozoic metasedimentary rocks and are S-type granites. The variable proportions of inherited zircons in certain samples with U–Pb ages of 627–992 Ma indicate the involvement of lower–middle Neoproterozoic crustal materials during magma crystallization through wall-rock contamination, which resulted in the wide range of isotopic compositions. Underplating of mantle-derived magma may have provided the thermal energy for partial melting of the upper Paleoproterozoic and lower Mesoproterozoic basements, thereby generating these late Triassic granites. The lack of positive εHf(t) values and high δ18OZrc values indicate that the contribution of mantle-derived magmas to these granites may be insignificant.