阿尔泰南缘克朗盆地康布铁堡组变质火山岩年龄及岩石成因

Geochronology and genesis of the meta-rhyolites in the Kangbutiebao Formation from the Kelang basin at the southern margin of the Altay, Xinjiang

对阿尔泰南缘克朗盆地康布铁堡组变质流纹岩进行了锆石SHRIMP U-Pb年龄测定,获得了流纹岩的喷发年龄412.6±3.5Ma,408.7±5.3Ma和406.7±4.3Ma,表明盆地内酸性火山岩形成于早泥盆世。岩石地球化学分析结果表明:流纹岩具有高的SiO_2(72.46%～80.07%)、全碱(6.38%～11.34%),低的铝(9.68%～12.19%)、钛(0.17%～0.35%),极低的MgO(0.08%～0.48%)、CaO(0.14%～0.73%)、FeO^T(0.34%～0.55%)等亚碱性流纹岩的一般特征。具有明显的Ti、P、Sr、Ba负异常,Th、U、Pb的正异常,HFSE元素(Nb、Ta、Zr、Hf)和LREE略富集,个别样品表现为LREE负异常的微量及稀土元素特征,δEu=0.51～0.71。综合其地质及地球化学特征,推测流纹岩形成于活动大陆边缘环境,是与俯冲作用有关的玄武质岩浆底侵作用使斜长石稳定的下地壳发生了部分熔融,并且岩浆上升过程中发生了结晶分异和同化围岩。

SHRIMP U-Pb zircon ages and geochemical data of whole rocks are reported for meta-rhyolites from the Kangbutiebao Formation in the Kelang basin at the southern margin of Altay orogenic belt. The zircons with well-defined oscillatory zoning from three meta-rhyolite samples yielded mean (206)Pb/(238)U ages of 412.6 +/- 3.5 Ma, 408.7 +/- 5.3Ma and 406.7 +/- 4.3Ma, which are interpreted as the eruption age of the volcanic rocks of the Kangbutiebao Formation. Meta-rhyolites are high in the concentrations of SiO(2) (72.46% similar to 80.07%) and total alkali (Na(2)O + K(2)O) (6.38% similar to 11.34%), and they have low concentrations of Al(2)O(3) (9.68% similar to 12.19%). TiO(2) (0.17% similar to 0.35%), MgO (0.08% similar to 0.48% CaO (0.14% similar to 0.73%) and FeO(T) ( 0.34% similar to 0.55% which are defined as sub-alkaline volcanic rock series. They display obvious negative anomaly of Ti, P, Sr and Ba, obvious enrichment of LREE and HFSE (Nb, Ta, Zr and Hf), noticeable negative Eu anomalies (delta Eu = 0.51 similar to 0.71) characteristics, resembling those of felsic volcanic rocks formed in an active continental margin. The regional geology and geochemical evidence indicate that these rhyolites were generated by partial melting of lower crust in an active continental margin, and the residues were plagioclase, amphibole, and apatite. The primitive magma was likely resulted from the underplating, and underwent fractional crystallization and minor wall-rocks assimilation during the ascending of magma.