Geochronological and geochemical constraints on the origin of the 304 ± 5 Ma Karamay A-type granites from West Junggar, Northwest China: implications for understanding the Central Asian Orogenic Belt

摘要

U-Pb zircon geochronological, geochemical, and whole-rock Sr-Nd isotopic analyses are reported for a suite of Karamay A-type granites from the Central Asian Orogenic Belt (CAOB) in the western Junggar region of northern Xinjiang, Northwest China, with the aim of investigating the sources and petrogenesis of A-type granites. The Karamay pluton includes monzo-granite and syenogranite. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating yielded a concordant weighted mean ~(206)Pb/~(238) Uage of 304 ± 5 Ma (w = 11), defining a late Carboniferous magmatic event. Geochemically, the rock suite is characterized by high SiO_2, FeOt/MgO, total alkalies (K_2O + Na_2O), Zr, Nb, Y, Ta, Ga/Al, and rare earth elements (REEs) (except for Eu), and low contents of MgO, CaO, and P2O5, with negative Ba, Sr, P, Eu, and Ti anomalies. These features indicate an A-type affinity for the Karamay granitic intrusions. Isotopically, they display consistently depleted Sr-Nd isotopic compositions (initial ~(87)Sr/~(86)Sr = 0.7014-0.7022, e_(Nd)(0 = +5.6-1-7.0). Geochronological, geochemical, and isotopic data suggest that the Karamay A-type granites were derived from remelting juvenile lower crust, followed by fractional crystallization. The Karamay A-type granites as well as widespread late Carboniferous magmatism in the western Junggar region of the southwestern CAOB may have been related to ridge subduction and a resultant slab window. This further demonstrates the importance of the late Palaeozoic granitic magmatism in terms of vertical crustal growth in northern Xinjiang.