Identification and geological significance of the Early Paleozoic Tianjunnanshan remnant ocean basin in the Zongwulong belt, NE Tibetan Plateau

宗务隆构造带夹于中央造山带原特提斯构造域内，发育与古特提斯洋演化相关的天峻南山蛇绿岩，是研究原特提斯向古特提斯转换的关键地质体。该蛇绿岩由超基性岩（蛇纹岩）、辉绿岩、玄武岩和硅质岩组成。蛇纹岩中尖晶石具高Mg#（58.6~64.5）和低Cr#（38.9~43.9）的特征。玄武岩和辉绿岩属于拉斑玄武岩系列，轻稀土元素左倾和重稀土元素平坦，富集Th而亏损Ti，总体上与弧后扩张脊熔岩具有一致的球粒陨石标准化稀土元素以及N-MORB标准化微量元素配分模式，同时这些基性岩具较高的Th/Yb值和εNd（t）值（+7.5~+9.6），显示岩浆来自受洋壳沉积物混染的亏损地幔源区。这些特征均与弧后盆地蛇绿岩类似。最新LA-ICP-MS锆石U-Pb测年结果显示辉绿岩形成于509±4Ma。结合野外接触关系表明，部分天峻南山蛇绿岩形成于寒武纪，早于不整合其上的石炭纪复理石和晚奥陶世花岗岩脉（444.9±4.7Ma）。天峻南山寒武纪蛇绿岩作为早古生代残余洋盆被石炭纪复理石不整合覆盖，并在三叠纪洋盆闭合过程中通过构造方式就位于上覆石炭纪地层中。上述结果表明宗务隆构造带并非一个晚古生代-早中生代构造带，而是原特提斯洋和古特提斯洋相继闭合形成的早古生代-早中生代复合构造带。;The Zongwulong belt is sandwiched in the Proto-Tethyan tectonic domain of Chinese Central Orogenic Belt and contains Tianjunnanshan ophiolite which formed during the evolution of the Paleo-Tethyan Ocean. The Tianjunnanshan ophiolite is the key geological unit to study the transition process from Proto-Tethyan to Paleo-Tethyan. It is composed of ultramafic rocks, doleite, basalt, and chert. Spinels from the serpentinite have high Mg# values (58.6~64.5) and low Cr# values (38.9~43.9). The basalt and dolerite belong to tholeiitic series and are characterized by left-inclined LREE, flat HREE, enrichment in Th, and depletion in Ti. Their chondrite-normalized REE and N-MORB-normalized trace element patterns are consistent with those of back-arc basin lavas. In addition, the mafic rocks have relatively high Th/Yb and low εNd(t) values (+7.5~+9.6), suggesting a depleted mantle source contaminated by subducted sediment. New LA-ICP-MS zircon U-Pb dating of dolerite yields a weighted mean ²⁰⁶Pb/²³⁸U age of 509±4Ma. Combined with field occurrences, it is concluded that some ophiolitic rocks in the Zongwulong belt formed during the Cambrian Period and prior to the unconformably overlying Carboniferous flysch and the granite vein (444.9±4.7Ma). These Cambrian ophiolitic rocks occur as a remnant ocean basin overlain unconfromably by the Carboniferous flysch. They are structurally injected into the overlying strata during closure of the ocean basin in Trassic. These results indicate that the Zongwulong belt is not a Late Paleozoic to Early Messozoic orogenic belt but an Early Paleozoic to Early Messozoic composite orogenic belt formed after the closure of the Proto-Tethyan Ocean and Paleo-Tethyan Ocean.