The geochemical and zircon trace elements characteristics of A-type Granitoids in Boziguoer, Baicheng County, Xinjiang

Abstract

The Boziguoer A-type granitoids in Baicheng County, Xinjiang, belong to the northern margin of the Tarim platform as well as the neighboring EW-oriented alkaline intrusive rocks. The rocks comprise an aegirine or arfvedsonite quartz alkali feldspar syenite, an aegirine or arfvedsonite alkali feldspar granite, and a biotite alkali feldspar syenite. The major rock-forming minerals are albite, K-feldspar, quartz, arfvedsonite, aegirine, and siderophyllite. The accessory minerals are mainly zircon, pyrochlore, thorite, fluorite, monazite, bastnaesite, xenotime, and astrophyllite. The chemical composition of the alkaline granitoids show that SiO2 varies from 64.55% to 72.29% with a mean value of 67.32%, Na2O+K2O is high (9.85%-11.87%) with a mean of 11.14%, K2O is 2.39%-5.47% (mean = 4.73%), the K 2O/Na2O ratios are 0.31-0.96, Al2O3 ranges from 12.58% to 15.44%, and total FeOT is between 2.35% and 5.65%. CaO, MgO, MnO, and TiO2 are low. The REE content is high and the total SREE is (263-1219) ppm (mean = 776 ppm), showing LREE enrichment and HREE depletion with strong negative Eu anomalies. In addition, the chondrite-normalized REE patterns of the alkaline granitoids belong to the "seagull" pattern of the right-type. The Zr content is (113-1246) ppm (mean = 594 ppm), Zr+Nb+Ce+Y is between (478-2203) ppm with a mean of 1362 ppm. Furthermore, the alkaline granitoids have high HFSE (Ga, Nb, Ta, Zr, and Hf) content and low LILE (Ba, K, and Sr) content. The Nb/Ta ratio varies from 7.23 to 32.59 (mean = 16.59) and the Zr/Hf ratio is 16.69-58.04 (mean = 36.80). The zircons are depleted in LREE and enriched in HREE. The chondrite-normalized REE patterns of the zircons are of the "seagull" pattern of the left-inclined type with strong negative Eu anomaly and without a Ce anomaly. The Boziguoer A-type granitoids share similar features with Al-type granites. The average temperature of the granitic magma was estimated at 832-839°C. The Boziguoer A-type granitoids show crust-mantle mixing and may have formed in an anorogenic intraplate tectonic setting under high-temperature, anhydrous, and low oxygen fugacity conditions. © 2013 Geological Society of China.