Constraining the deep dynamic process beneath the Bangong-Nujiang suture zone: A case study from the early cretaceous trachytic rocks

The geodynamic processes that occurred beneath the Bangong – Nujiang suture zone during the evolution of the Indus – Yarlung Neo-Tethys and Bangong – Nujiang Meso-Tethys are unclear. We conducted a study of Early Cretaceous andesitic rocks from the Nyima area, north – central Tibet, to investigate these processes. The studied rocks have zircon U – Pb ages of 110–104 Ma and negative to positive ε_Hf(<i>t</i>) values (−12.1 to+10.3). These rocks have affinities with trachytes, and have high total alkali contents and Na₂O/K₂O ratios (5–125). They also have high Sr (281–1409 ppm), Y (21.0–28.5 ppm), and Yb (2.00–2.65 ppm) contents, and exhibit depletions in Nb, Ta, and Ti. Such features indicate these rocks are not adakites, but are similar to typical arc rocks. The geochemical and isotopic data suggest the studied rocks were formed by partial melting of amphibolite-facies oceanic lithosphere, with the addition of enriched sedimentary materials. Based on the regional crustal evolution, we suggest that the Nyima andesitic rocks formed by melting of Bangong – Nujiang oceanic lithosphere. As such, the Early Cretaceous igneous rocks along the central Bangong – Nujiang suture zone were not associated with northward subduction in the Indus – Yarlung Neo-Tethys.

班公湖-怒江缝合带（Bangong-Nujiang suture zone）之下在印度河-雅鲁藏布新特提斯洋（Indus-Yarlung Neo-Tethys）与班公湖-怒江中特提斯洋（Bangong-Nujiang Meso-Tethys）演化期间发生的地球动力学过程，目前尚未明确。为厘清上述过程，我们对藏北中部尼玛地区的早白垩世安山岩开展了研究。本次研究的岩石锆石U-Pb年龄（zircon U-Pb ages）为110~104 Ma，ε_Hf(<i>t</i>)值（εHf(t)）介于-12.1至+10.3之间，由负值过渡至正值。该套岩石具有粗面岩（trachytes）的亲缘性，总碱含量与Na₂O/K₂O比值（Na₂O/K₂O ratios）均较高，其中Na₂O/K₂O比值介于5~125之间。同时，岩石具有较高的Sr（281~1409 ppm）、Y（21.0~28.5 ppm）与Yb（2.00~2.65 ppm）含量，并表现出Nb、Ta、Ti元素的亏损特征。上述地球化学特征表明，该套岩石并非埃达克岩（adakites），而是与典型弧岩（arc rocks）相似。地球化学与同位素数据显示，本次研究的岩石形成于角闪岩相洋壳岩石圈（amphibolite-facies oceanic lithosphere）的部分熔融作用，且伴随富集沉积物质的加入。结合区域地壳演化背景，我们认为尼玛地区安山岩形成于班公湖-怒江洋壳岩石圈的熔融过程。据此，班公湖-怒江缝合带中部的早白垩世火成岩，与印度河-雅鲁藏布新特提斯洋的向北俯冲作用并无关联。