班公湖-怒江缝合带西段沙木罗组泥质岩地球化学特征及意义

Geochemical characteristics and geological significance of Shamuluo Formation argillaceous rocks in the western part of the Bangong Co-Nujiang suture zone

沙木罗组分布于班公湖-怒江缝合带内部,其沉积背景、源区性质与班-怒特提斯洋的演化息息相关。为了研究班怒带西段沙木罗组泥质岩的物源特征,探讨沉积盆地的构造背景和班-怒特提斯洋的俯冲消减时限,对泥质岩开展了全岩主量元素、微量元素地球化学的研究。地球化学组成特征显示,泥质岩的ICV、CIA、Al2O3/TiO_2、Th/Sc、Rb/Cs、(Gd/Yb)N、La、Ce、∑REE、La/Yb、(La/Yb)N、Eu/Eu*N和LREE/HREE均值分别为0. 80、72、21、0. 88、13. 5、1.58、42. 47×10-6、84. 81×10-6、200. 02×10-6、14. 22、9. 61、0. 65和9. 04,具有中等的Th、U、Hf含量(分别为16. 34×10-6、2. 85×10-6和4. 81×10-6)和Th/U、La/Th值(分别为5. 78和2. 61)。泥质岩的K2O/Na2O-SiO2、La-Th-Sc、ThCo-Zr/10、Th-Sc-Zr/10、TiO_2-Zr、Co/Y-Ti/Zr、La-Th-Sc、Th-Sc、La/Sc-Co/Th、REE-La/Yb、A-CN-K等关系图解具较好的指示意义,稀土元素特征亦具有一定的规律性。上述地球化学特征表明,泥质岩为成熟的泥质岩,源区矿物组成以斜长石、碱性长石为主,源区母岩主要为后太古宙花岗闪长岩、花岗岩、安山岩等中酸性岩浆岩和碎屑岩。泥质岩源区母岩经历了中等程度的化学风化作用,沉积物沉积时气候较温暖、湿润。不同层位的泥质岩地球化学特征指示了班怒带西段沙木罗组沉积于活动大陆边缘的构造背景,从剖面下部到上部可能反映了板块俯冲加剧、岩浆活动加强的地质过程,至少在141 Ma左右班-怒特提斯洋西段正在向北俯冲消减。

The Shamuluo Formation is distributed in Bangong Co-Nujiang suture zone, and its sedimentary background and source characteristic were closely linked with the evolution of Bangong Co-Nujiang Tethys Ocean. In order to discuss sediment provenance, weathering, tectonic setting of sedimentary basin and subduction of Bangong Co-Nujiang Tethys Ocean, the authors made a systematic analysis of petrology and geochemistry of Shamuluo Formation argillaceous rocks in the western part of the Bangong Co-Nujiang suture zone. The mean values of ICV,CIA, Al2O3/TiO2, Th/Sc, Rb/Cs,(Gd/Yb)N, La, Ce, ∑REE, La/Yb,(La/Yb)N, Eu/Eu*N, LREE/HREE are 0. 80, 72, 21, 0. 88, 13. 5, 1. 58, 42. 47 ×10^-6, 84. 81 ×10^-6, 200. 02 × 10^-6, 14. 22, 9. 61, 0. 65 and9. 04 respectively, while the mean values of Th, U, Hf, Th/U, La/Th are 16. 34 ×10^-6, 2. 85 × 10^-6, 4. 81 ×10^-6, 5. 78 and 2. 61 respectively. The diagrams of K2O/Na2O-SiO2, La-Th-Sc, Th-Co-Zr/10, Th-Sc-Zr/10,TiO2-Zr, Co/Y-Ti/Zr, La-Th-Sc, Th-Sc, La/Sc-Co/Th, REE-La/Yb, A-CN-K and REEs also show certain regularity. The argillaceous rocks are mature and the source rock are granodiorite, granite, andesite and sedimentary clastic rock which have much plagioclase and alkali feldspar. The source region of argillaceous rocks has suffered from medium degree of chemical weathering, suggesting a warm and moist climate. The geochemical characteristics of argillaceous rocks indicate that argillaceous rocks tended to occur in active continental margin setting, where its tectonic activity became intensified from the bottom to the top. These results provide sedimentary geochemical evidence for the evolution of Bangong Co-Nujiang Tethys Ocean, which indicates that Bangong Co-Nujiang Tethys Ocean experienced northward subduction about 141 million years ago.