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

Numerous leucogranite dykes, and spatially associated high-grade metamorphic rocks, arewell-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 degrees C) and low eNd(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 generatedmainly by fluid-absent muscovite dehydration melting ofmetasediments. LA-ICPMS zircon and monazite U-Pb dating suggests crystallization ages of 279-267Ma, 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. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

Studies on leucogranite dykes in the Chinese Altai orogenic belt reveal they are peraluminous S-type granites formed in the Early Permian period, with low-temperature characteristics and associated with high-grade metamorphic rocks.