The Geochemical and Zircon Trace Element Characteristics of A-type Granitoids in Boziguoer, Baicheng County, Xinjiang

Abstract

North-west institute of nonferrous metals geology, Xi an 710054, China 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 SiO 2 varies from 64.55% to 72.29% with a mean value of 67.32%, Na 2 O+K 2 O is high (9.85~11.87%) with a mean of 11.14%, K 2 O is 2.39%~5.47% (mean = 4.73%), the K 2 O/Na 2 O ratios are 0.31~0.96, Al 2 O 3 ranges from 12.58% to 15.44%, and total FeO T is between 2.35% and 5.65%. CaO, MgO, MnO, and TiO 2 are low. The REE content is high and the total ΣREE is (263~1219) × 10 -6 (mean = 776 × 10), showing LREE enrichment 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) ×10-6 (mean = 594×10 . 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 A1-type granites. The average temperature of the granitic magma was estimated at 832~839 o C. The Boziguoer A-type granitoids show crustmantle mixing and may have formed in an anorogenic intraplate tectonic setting under high-temperature, anhydrous, and low oxygen fugacity conditions.