Late Carboniferous and Early Permian garnet-bearing granites in the South Tianshan Belt, NW China: Two Late Paleozoic magmatic events and implications for crustal reworking

Abstract

Peraluminous granitoids can provide crucial insights into crustal reworking. Here we report a geochemical study of two Late Paleozoic peraluminous granitic plutons in the South Tianshan Belt, NW China, to constrain their crustal reworking history. Zircon U-Pb analyses yield ages of 311 ± 4 Ma for the garnet-bearing biotite granite and 283 ± 4 Ma for the garnet-bearing muscovite granite. The garnet-bearing biotite granites exhibit peraluminous affinity (ASI = 1.04–1.11), high CaO/Na2O, low Rb/Sr, and negative εNd(t) (−8.7 to –7.5) and zircon εHf(t) (–8.4 to –0.8) values with ancient crustal model ages, which suggest they were derived from dehydration melting of metagraywacke. The garnet-bearing muscovite granites, containing manganese garnet, show high SiO2 (72.3–76.8 wt%) and strongly peraluminous (ASI = 1.14–1.22), high Rb/Sr (37.8–128.9), low Zr/Hf (16.61–31.80) and Nb/Ta (3.05–5.71), and exhibit tetrad REE patterns (TE1,3 = 1.05–1.27), which can be ascribed to high-degree fractional crystallization accompanied by the interaction with fluids. They show negative εNd(t) values (–8.6 to –6.6) and variable zircon εHf(t) values (–4.9 to +2.7), implying ancient crustal with minor juvenile crustal sources. In combination with the occurrence of peraluminous magmatic rocks of the South Tianshan Belt, we propose that the peraluminous magmatic rocks were formed by the partial melting of ancient crustal rocks from the passive continental margin of the Tarim Craton, which represents an important crustal reworking event in the Central Asian Orogenic Belt.