新疆准噶尔北缘北塔山组火山岩年龄及岩石成因

Geochronology and genesis of volcanic rocks in Beitashan Formation at the northern margin of the Junggar,Xinjiang

对准噶尔北缘北塔山组辉石玄武岩进行了LA-ICP-MS锆石U-Pb年龄测定,获得了玄武岩的喷发年龄380.5±2.2Ma,表明北塔山组火山岩形成于中泥盆世。该地层火山岩中辉石玄武岩和无斑玄武岩的SiO2含量为47.55%～52.97%、Al2O3的含量为8.44%～20.00%、TiO2为0.5%～1.2%,MgO含量为2.8%～15.35%、CaO为3.98%～14.83%、FeOT为9.46%～19.23%,具有亚碱性拉斑玄武岩的特征。其微量元素显示富集大离子亲石元素(LILE)和轻稀土元素(LREE),亏损Nb、Ta和Ti,Eu异常不明显。它们具有极低的初始87Sr/86Sr同位素比值(0.703835～0.704337)和高的εNd(381Ma)值(+6.84～+12.3,t=381Ma)的亏损地幔源区特征。结合区域地质背景,北塔山组火山岩形成于与俯冲作用相关的构造环境,是准噶尔古洋盆于泥盆世时发生的俯冲-消减所引发的岛弧岩浆作用的地质记录。岩浆源区为被流体或沉积物熔体交代改造的地幔楔和软流圈地幔,不同类型的岩石系不同成分的原始岩浆经不同演化过程的产物。

LA-ICP-MS U-Pb zircon ages and geochemical data of whole rocks are reported for mafic volcanic rocks （basahs） from the Beitashan Formation at the northern margin of the eastern Junggar. The zircons with well-defined oscillatory zoning from augitophyre sample yielded mean ^206Pb/^23SU ages of 380. 5 ± 2. 2Ma, which are interpreted as the eruption age of the volcanic rocks of the Beitashan Formation. All basalt samples belong to subalkali tholeiite magma. They display obvious negative anomaly of Nb, Ta and Ti, obvious enrichment of LREE and LILEs （Ba, Sr, K, U and Pb） characteristics. They have positive eNd（t） values （ ＋6. 84 - ＋ 12. 28） and low Sr isotopic compositions （initial STSr/S6 Sr values = 0. 703835 -0. 704337 ）, indicating a deplete mantle source. The regional geology and geochemical evidence indicate that these mafic volcanic rocks were generated at island arc tectonic setting and have similar magma sources. The primary magmas may have been derived from asthenosphere mantle and mantle wedge metasomatized by fluids and melts released from the subducted slab. During the cooling of magmas, they underwent crystallization differentiation of pyroxenes and plagioclase. The different volcanic rocks （eg. basalt, augitophyre, picrite and plagioclase-basalt） were likely resulted from different primary magma contents and differentiated in different magma chamber.