Crustal melting in a protracted hot setting in the Altai Orogen (NW China): Evidence from Permian leucogranite dykes in the metamorphic belt

Abstract

Numerous leucogranite dykes, and spatially associated high-grade metamorphic rocks, are well-preserved in the Chinese Altai orogenic belt. Integrated petrological, geochronological, and geochemical results of the leucogranite dykes are presented to decipher their petrogenesis and constrain the tectono-thermal setting of the Altai orogen. The leucogranite dykes have high SiO2 (71.9–81.2 wt%) and Al2O3 contents (11.1–15.7 wt%), and low CaO, MgO and TiO2 contents, with a total alkali content (K2O + Na2O) of 4.55–9.88 wt%. All leucogranites show weakly to strongly peraluminous S-type granite affinities with A/CNK ratios of 1.0–2.2. Trace element concentrations indicate a characteristic high Rb content, low Ba and Sr contents, and Rb/Sr ratios of 0.43–35.8. They exhibit nearly flat REE patterns with strong negative Eu anomalies on chondrite-normalized REE patterns. Considering their low zircon saturation temperatures (682–758 °C) and low εNd(t) values (˗12.4 to ˗2.7), as well as the presence of Al-rich minerals (muscovite and garnet), we infer that the leucogranites were low-temperature granites and generated mainly by fluid-absent muscovite dehydration melting of metasediments. LA-ICPMS zircon and monazite U-Pb dating suggests crystallization ages of 279–267 Ma, which places these leucogranite dykes in temporal association with the ultrahigh- to high-grade metamorphic rocks and implies a prolonged heat budget in the region. Together with contemporary magmatic activities and metamorphism in the Chinese Altai orogen, the generation of the leucogranite dykes may have been attributed to long-lasting high heat flow in the early Permian, most likely related to the assembly of the Junggar plate and the Chinese Altai.