北大巴山地区斑鸠关组砂岩地球化学特征对物源和构造环境的限定

北大巴山是南秦岭造山带的重要组成部分,保留较完整的地层记录。长期以来,斑鸠关组被认为形成于相对稳定的构造环境。本文通过对斑鸠关组砂岩的沉积序列和地球化学分析,详细分析其物源区和形成构造环境。沉积学特征表明,斑鸠关组砂岩主要为一套深水沉积的细砂岩、粉砂岩和泥岩组合序列,形成于海底扇。砂岩中交错层理恢复的古流向表明,其物源主要来自于北侧。地球化学分析表明,斑鸠关组砂岩SiO_2为61.54%～84.35%,主要为成熟度较低的岩屑砂屑岩。稀土元素标准化配分曲线呈现轻稀土富集、重稀土平坦和Eu负异常的特征。砂岩的风化蚀变指数为51～67,Th/U比值为3.36～7.02,表明砂岩物源区经历了较弱的风化作用。Zr/Sc和Th/Sc比值指示斑鸠关组砂岩没有经历沉积再旋回,为近源沉积。砂岩物源区组成判别图表明,斑鸠关组物源主要为长英质岩石。砂岩源区构造环境判别图以及特征指数表明,斑鸠关组源区形成于大陆岛弧和被动大陆边缘环境。综合区域资料,表明斑鸠关组形成于大陆岛弧相关环境,非大陆裂谷。

Sandstone geochemical constraints on the provenance and tectonic setting of the Banjiuguan Formation in the North Daba Mountain

The North Dabashan Mountain, an important part of the South Qinling orogenic belt, holds well preserved sedimentary records. For a long time, the Banjiuguan Formation was considered to have formed in a relatively stable tectonic setting. In this paper, sedimentary successions and geochemical analyses are used to determine its provenance and tectonic setting. The Banjiuguan Formation consists of fine grained sandstones, siltstones and mudstones deposited in deep waters. Facies assemblages indicate that they were deposited in a submarine fan environment. Paleocurrent data preserved in cross bedding indicates that sediments were proximally derived from north. Chemical classification of the Banjiuguan sandstones shows that the analyzed sediments are mostly litharenite. The REE patterns are characterized by LREE enrichment, flat HREE, and negative Eu anomalies. Chemical Index of Alteration range from 51 to 67, and Th/U ratio is 3. 36 to 7. 02, both of which suggest weak weathering. The rations of Zr/Sc and Th/Sc further show that the sandstone experienced no reworking. Various plots and parameters for sedimentary provenance show that the sediments were sourced predominantly from felsic rocks. Geochemical data suggest that they were deposited in an environment related to continental island arc setting and passive continental margin.