Origin and tectonic implications of Late Jurassic high-Mg diorites along the Bangong-Nujiang suture zone, Tibet

The Shiquanhe-Num Co ophiolitic mélange belt (SNOB) is one of two such belts along the Bangong-Nujiang suture zone (BNSZ) separating the Lhasa and Qiantang terranes of central Tibet. This study reports new geochronological, petrological and geochemical data for high-Mg dioritic dikes intruding the Shiquanhe ophiolitic peridotite. Based on LA-ICP-MS zircon U–Pb dating, dioritic dikes yield a Late Jurassic age of 162.8 ± 1.3 Ma. These are high-Mg diorites (53.0%–56.8% SiO2, 7.07%–8.82% MgO) with moderate TiO2 (0.95%–1.01%), high K2O+Na2O (6.02%–8.01%), and high contents of Cr (150–182 ppm), Ni (91–145 ppm) and Th (2.8–5.9 ppm). They are very similar to sanukites of the Setouchi volcanic rock belt (SW Japan). Shiquanhe diorites have high Sr/Y ratios (up to 41.3), distinct positive Pb anomalies, negative Nb anomalies and continental crust-like trace element patterns without negative Eu anomalies (average Eu/Eu* = 0.96). These geochemical characteristics indicate that Shiquanhe diorities were derived by partial melting of mantle peridotite as a result of interaction with hydrous melts of subducted sediments. Similar high-Mg magmas are common in the early evolution of a new subduction zone. Integrated with field observations and results from other studies of Late Jurassic high-Mg andesites and diorites along the SNOB, we argue that the Shiquanhe high-Mg diorites formed at an early stage of northward subduction of the Bangong-Nujiang Tethys ocean (BNO) beneath the Qiangtang terrane. Therefore, these results provide critical constraints on the Jurassic tectonic evolution of the Shiquanhe-Num Co ophiolitic mélange belt and are significant for understanding the evolution of the BNO.狮泉河-纳木错蛇绿混杂岩带（Shiquanhe-Num Co ophiolitic mélange belt，简称SNOB）是班公湖-怒江缝合带（Bangong-Nujiang suture zone，简称BNSZ）内的两条同类岩带之一，分隔了西藏中部的拉萨地体与羌塘地体。本研究报道了侵入于狮泉河蛇绿岩橄榄岩中的高镁闪长岩脉的全新年代学、岩石学与地球化学数据。通过激光剥蚀电感耦合等离子体质谱（LA-ICP-MS）锆石U-Pb测年，获得闪长岩脉的形成时代为晚侏罗世，年龄为162.8 ± 1.3 Ma。该类高镁闪长岩的SiO₂含量介于53.0%~56.8%之间，MgO含量为7.07%~8.82%；TiO₂含量中等（0.95%~1.01%），K₂O+Na₂O含量较高（6.02%~8.01%），且Cr（150~182 ppm）、Ni（91~145 ppm）与Th（2.8~5.9 ppm）含量均较高。它们与日本西南部濑户内火山岩带（Setouchi volcanic rock belt，SW Japan）的萨努基岩（sanukite）极为相似。狮泉河闪长岩具有较高的Sr/Y比值（最高可达41.3），呈现显著的正Pb异常、负Nb异常，微量元素配分模式类似大陆地壳，且无负Eu异常（平均Eu/Eu*=0.96）。上述地球化学特征表明，狮泉河闪长岩系由俯冲沉积物的含水熔体与地幔橄榄岩发生相互作用，进而引发地幔橄榄岩部分熔融所形成。类似的高镁岩浆在新生俯冲带的早期演化阶段较为普遍。结合野外地质观察以及沿SNOB分布的晚侏罗世高镁安山岩与闪长岩的其他研究成果，我们提出：狮泉河高镁闪长岩形成于班公湖-怒江特提斯洋（Bangong-Nujiang Tethys ocean，简称BNO）向北俯冲到羌塘地体之下的早期阶段。综上，本研究结果为狮泉河-纳木错蛇绿混杂岩带的侏罗纪构造演化提供了关键约束，对理解班公湖-怒江特提斯洋的演化历程具有重要科学意义。