北山南带音凹峡地区酸性火山岩年代学、地球化学研究：二叠纪裂谷岩浆作用的新证据

摘要

北山造山带位于中亚造山带南缘中段地区,是中亚造山带、塔里木克拉通和华北克拉通的构造结合部。年代学资料表明北山南带晚古生代存在一期重要的岩浆活动,所形成的岩石类型包括了镁铁‒超镁铁质杂岩、花岗岩类以及酸性火山岩。音凹峡地区位于北山南带,该地区广泛分布着厚度巨大的二叠系。这套地层主要是由酸性火山岩及火山碎屑岩组成,流纹岩的锆石LA-ICP-MS U-Pb年龄为273±1 Ma,为早二叠世。针对音凹峡地区酸性火山岩的地球化学研究发现,其具有高 SiO2、K2O+Na2O、Al2O3含量、低 Fe2O3T、MgO、P2O5含量；富集轻稀土元素,重稀土元素无分异,具有明显的Eu负异常；微量元素方面,酸性火山岩富集Cs、Rb、Th、U、Zr和Hf等元素,而相对亏损Ba、Nb、Ta、Sr、P、Ti等元素。酸性火山岩的锆石εHf(t)值为‒6.0~3.9,具有较老的Hf同位素模式年龄,tDM2=1046~1669 Ma。音凹峡酸性火山岩的地球化学证据表明这套火山岩系可能由中‒新元古代壳源岩石部分熔融形成,并与幔源岩浆进行了不同程度的混合,之后发生了分离结晶作用。综合音凹峡地区同时代镁铁质岩石、花岗岩类的研究成果,以及该地区二叠系沉积建造及火山岩特征分析可知,北山南带在早二叠世应处于大陆裂谷的构造背景。%The Beishan orogenic belt, situated in the southernmost Central Asian Orogenic Belt (CAOB), is a conjunction region of the CAOB, North China and Tarim cratons. Geochronological data demonstraed an important magmatic event in the southern Beishan that resulted in the emplacement of the mafic-ultramafic rocks, granitoids and acid rocks. The widely distributed Permian strata in the Yinwaxia area, east of the southern Beishan, consist predominantly of acidic volcanic and pyroclastic rocks with eruption-explosion facies, including rhyolites, dacites, rhyolite breccia lavas, rhyodacites and homogeneous volcanic tuff. The LA-ICP-MS U-Pb age of a rhyolite sample is 273±1 Ma, indicating that they were formed in the early Permian. The Yinwaxia acidic volcanic rocks have high SiO2, K2O+Na2O, Al2O3 contents, and low Fe2O3T, MgO, P2O5 contents. They display light rare earth elements enrichment, without heavy rare earth elements differentiation, and with obviously negative Eu anomalies. They were also enriched in Cs, Rb, Th, U, Zr and Hf, and depleted in Ba, Nb, Ta, Sr, P and Ti. The Yinwaxia acid volcanic rocks have variable zirconεHf(t) values (–6.0 – +3.9) and relative old model ages (tDM2=1046‒ 1669 Ma). Geochemical studies indicate that the Yinwaxia acidic rocks were derived from mixing of ancient continental crust and mantle materials, experienced subsequent fractional crystallization. Evidence of coeval mafic rocks, granitoids, acidic volcanic rocks and the characteristics of the Permian sedimentary formations all support a Late Paleozoic continental rifting in the southern Beishan.