Geochronological and geochemical constraints on petrogenesis of the Hongyuan intrusion in central West Junggar, Xinjiang, NW China

Late Paleozoic intermediate-acidic intrusions, which are widespread in central West Junggar (NW China), provide critical information for regional tectonic evolution. The Hongyuan intrusion, distributed in the southeast of central West Junggar, is selected as a representative for discussion in this study. Petrological observation shows that the Hongyuan intrusion is dominated by syenogranite and monzonitic granite, with a small amount of quartz syenite. Zircons separated from the monzonitic granite yield a weighted mean Pb-206/U-238 age of 287 +/- 2Ma (n = 19, MSWD = 1.3). The Hongyuan intrusion displays a pronounced I-type affinity. It belongs to high-K calc-alkaline suit and has high concentrations of Na2O + K2O (7.00-8.09 wt%). It is peraluminous in composition and the FeOT/MgO ratios are low (2.55-4.43). The large ion lithophile elements (LILE, such as Th, U and Cs) and light rare earth elements (LREE) are enriched with significant Eu anomalies (Eu-N/Eu-N* = 0.26-0.51), and the high field strength elements (HFSE, such as Ti and Nb) are depleted. Initial Hf isotopic ratio of zircons separated from the monzonitic granite show small range of variation ((Hf-176/Hf-177)(i) = 0.282951-0.283025), the epsilon Hf(t) values are relatively high (+12.8 to +15.3) and the Hf model ages are very young (T-DM1 = 315-425 Ma). The comprehensive petrological and geochemical features indicate that the Hongyuan intrusion was formed by decompression melting of a juvenile lower crust, and has undergone a remarkable process of fractional crystallization.