Generation of peraluminous granitic magma in a post-collisional setting: A case study from the eastern Qilian orogen, NE Tibetan Plateau

Abstract

Early Paleozoic peraluminous granites are abundant in the eastern part of the Qilian orogen, northeastern margin of the Tibetan Plateau. A combined study involving geochronology, whole-rock geochemical and Sr–Nd–Hf isotopic compositions for three Early Paleozoic peraluminous granitic plutons (Jishishan, Ledu and Shichuan plutons) from the eastern Qilian orogen was carried out to evaluate the causes of chemical variations and generation mechanisms of peraluminous granitic magmas. These granitic plutons have magma crystallization ages of 455–427Ma and are moderately to strongly peraluminous (A/CNK=1.03–1.18). Geochemical and Sr–Nd–Hf isotopic data indicate that they consist substantially of crust-derived melts. The Jishishan and Ledu peraluminous granites were dominantly produced by partial melting of Precambrian orthogneisses. The Shichuan monzogranites, with low HREE contents (e.g., Yb=0.80–1.83ppm) and slightly negative εNd(t) (−5.3 to −2.3) and positive εHf(t) (+1.6 to +3.4), could be derived from immature crustal materials. Relatively high average zircon saturation temperatures (>750°C for each pluton), obvious negative Eu anomalies (Eu/Eu*=0.28–0.80) and low Pb/Ba ratios (0.03–0.16) for the Jishishan, Ledu and Shichuan granites are consistent with crustal melting involving biotite breakdown under fluid-absent conditions. Our results suggest that compositional variations of moderately to strongly peraluminous granitic magmas are mainly controlled by source compositions and melting conditions, while the processes such as mixing with mantle-derived magma, fractional crystallization, restite unmixing and peritectic assemblage entrainment were insignificant (or only play secondary roles) in their genesis. Late Ordovician to Middle Silurian crustal anatexis in the eastern Central Qilian was probably linked with slab break-off which may be an important mechanism in addition to lithospheric delamination for the generation of moderately to strongly peraluminous granites in a post-collisional setting.