Petrogenesis and geological significance of highly fractionated A-type granites in Kalasayi, East Junggar

Kalasayi granites are located in the westernmost part of the Kalamaili alkaline granite belt in East Junggar and are composed of moyite and adamellite. The LA-ICP-MS zircon U-Pb ages of Kalasayi granites are 307.7±3.2Ma~309.6±2.0Ma. The rocks have high silicon （SiO2 average of 77.25% ）, rich alkali （K2O+Na2O=7.50%~9.23%）, low aluminum （A/CNK=0.922~1.084）, and poor calcium and magnesium. They are also enriched in Rb, K, Th and other LILEs as well as in Zr, Hf and other high field strength elements and rare earth elements, but are depleted in Ba, Sr, Eu. 10000Ga/Al values vary from 7.00 to 10.35. On the whole，Kalasayi granites exhibit characteristics of A-type granites and are highly fractionated products of the Laoyaquan-Huangyangshan A-type granites batholith on the eastern side instead to S-type granites as previously thought. Nd isotopic compositions of the Kalasayi granites are positive（εNd（t）=3.5~6.0）and show young Nd model ages（TDM2=520~630Ma）, with Pb isotope located near the orogenic evolution line, suggesting that the possible magma source might have been the lower part of the young fresh orogenic crust. The results of the 1:50000 regional survey show that Kalamaili ocean basin was closed before the Late Devonian, and then entered into an extensional environment after collision and eventually into the rift stage in the Late Carboniferous. Kalasayi granites should be the products of rift environment rather than the products of post-collisional stage as previously thought.