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

Abstract

<div><p>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 <i>in situ</i> 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 <i>ε</i>_Hf(<i>t</i>) values of −0.8 to −9.0, two-stage depleted mantle model ages (<i>T</i>_DM2) of 1.81–1.31 Ga, and weighted mean of <i>δ</i>¹⁸O_Zrc values of 8.53 ± 0.58‰ to 9.12 ± 0.28‰. Combined with U–Pb dating and Hf isotopic data, the elevated and variable <i>δ</i>¹⁸O and <i>ε</i>_Hf(<i>t</i>) 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 <i>ε</i>_Hf(<i>t</i>) values and high <i>δ</i>¹⁸O_Zrc values indicate that the contribution of mantle-derived magmas to these granites may be insignificant.</p></div