Triassic granites in South China: A geochemical perspective on their characteristics, petrogenesis, and tectonic significance

Abstract

Granitic rocks of Phanerozoic age are common in South China. They are spatially and temporally closely associated with hydrothermal ore deposits. Therefore, many studies have been devoted to these granites regarding their petrogenesis. This study presents a review on the geochemistry of Triassic granites in South China because the Triassic is an important period for the Indosinian orogeny in Southeast Asia. Petrographic investigations indicate that these granites can be categorized into five groups: cordierite-bearing granites, amphibole-bearing granites, biotite granites, muscovite-bearing granites, and A-type granites. All these granites are distributed in regions where magmatic rocks of Neoproterozoic age predominate. From the distribution of Triassic granites and their associated magmatic rocks, three magmatic zones are discerned: a southwestern zone, a northeastern zone, and a middle zone. Except for cordierite-bearing granites that were only emplaced in the latest Permian to the earliest Triassic, the other four groups were emplaced from ≥240Ma to ~210Ma. Geochemical data do not support the possibility that amphibole-bearing granites have a petrogenetic link to mafic magmas through either magma mixing or assimilation-fractional crystallization. Furthermore, biotite granites do not develop from amphibole-bearing granitic magmas through fractional crystallization, but some muscovite-bearing granites have indeed evolved from biotite granitic magmas. A-type granites are mostly peraluminous, and many of them have metasedimentary sources that previously underwent significant extraction of hydrous felsic melts. On the basis of the major-trace element characteristics, the initial Nd isotope compositions and relict zircon U-Pb ages of the Triassic granites, and their spatial overlap with the Neoproterozoic rocks, it is inferred that the Triassic granites were mainly derived from partial melting of the Neoproterozoic rocks, including both sedimentary and igneous ones. Although there is a tectonic transition from contraction in the Early Triassic to extension in the Late Triassic, granitic magmatism is mainly driven by tectonic extension. The Indosinian magmatism is predominated by reworking of the Neoproterozoic metaigneous and metasedimentary rocks, which are outcropped in the Neoproterozoic orogen.