Contrasting styles of peraluminous S-type and I-type granitic magmatism: Identification and implications for the accretionary history of the Chinese South Tianshan

Abstract

Peraluminous granitoids have aluminum saturation indices (A/CNK) higher than 1.0, which overlap to some extent between S- and I-type granitoids. However, their source and petrogenesis are still disputed. For example, whole-rock compositions alone are not always a valid way to discriminate the sources of peraluminous granitoids. To identify the geochemical affinities, source and petrogenesis of the peraluminous granitoids, we present new geochemical data, in situ zircon U-Pb ages and Hf-O isotopic data, and whole-rock Sr-Nd isotopic data for the peraluminous granitoids in the South Tianshan Orogen Belt (STOB), Northwesten China. Zircon U-Pb ages suggest that these peraluminous granitoids were emplaced in the latest Carboniferous (ca. 299 Ma). They contain the diagnostic mineral muscovite and have high δ¹⁸O_Zrn values (>8.0 ‰) demonstrating a close affinity with S-type granitoids. Their low εNd(t) values (−5.3 to −7.6), combined with variable zircon εHf(t) values (−0.35 to −10.18), indicate that these S-type granitoids were likely derived from partial melting of metasedimentary rocks. In addition, inherited zircon cores from the S-type granitoids have variable δ¹⁸O values (6.34–10.5 ‰) and zircon εHf(t) values (−4.3 to +6.3), with age populations (ca. 400 to 500 Ma) similar to those of detrital zircons from late Carboniferous metasedimentary rocks in the region. These data show that the S-type granitoids were dominantly derived from late Carboniferous metasedimentary rocks rather than Precambrian crustal materials. The studied granitoids have a transitional composition between I- and S-type granitoids, which could be related to low compositional maturity of the late Carboniferous metasedimentary source. According to the spatial and temporal distribution and petrogenesis of the Carboniferous intrusive rocks in the STOB, we propose that a slab roll-back model can account for the generation of late Carboniferous S-type granitoids in the STOB.