青海沱沱河地区开心岭一带中二叠世-早三叠世基性岩浆事件及其地质意义

摘要

The Middle Permian-Early Triassic tectonic background of Kaixinling zone in Tuotuo River area is an important factor affecting the metallogenic regularity and prospecting direction analysis. The geochemical study of Middle Permian basalt and geochro-nologic data of gabbro from Nuoribagaribao Formation are expected to put constraint on the ore-forming geotectonic background of Middle Permian-Early Triassic in this area. The gabbro with zircon U-Pb age of 247.0±1.7Ma has established an important magmat-ic event and filled the blank of magmatic activity in Early Triassic. This event might have been caused by the subduction and collision during the evolution of the Longmu Co-Shuanghu suture zone and the Kekexili-Jinsha River suture zone. Geochemistry of Middle Permian basalt is consistent with the tholeiitic magma series, which is characterized by high Mg and Fe, low alkali, enrichment of LREE and partial enrichment of HFSE. It has relatively high Zr (>70×10-6 in average) and Zr/Y (5.2~7.0), similar to things of the continental basalt. The depleted Nb, Ta, Zr, Hf, Ti and low Nb/La ratios (0.1~0.3) imply the contamination of crustal components.Various discrimination diagrams show that the basalt was formed in a continental-rift environment and the volcanism was closely re-lated to the regional mineralization. New data from gabbro and basalt are presented in this paper to demonstrate the metallogenic geo-tectonic background of Kaixinling area.%青海省沱沱河地区开心岭一带中二叠世-早三叠世构造背景是影响该区成矿规律和找矿方向分析的重要因素。对开心岭一带中二叠世诺日巴尕日保组玄武岩地球化学及侵入其中的辉长岩进行年代学研究，以期对该区中二叠世-早三叠世成矿构造背景提供一定的约束。采用LA-ICP-MS技术测得的辉长岩中锆石206Pb/238U年龄加权平均值为247.0±1.7Ma，属于早三叠世，该期岩浆事件可能是龙木错-双湖缝合带和可可西里-金沙江缝合带在此阶段发生俯冲的产物。地球化学特征表明，中二叠世玄武岩具有高镁、富铁、贫碱的特征，属于拉斑系列玄武岩；富集LREE和部分HFSE，REE表现为轻、重稀土元素分馏明显的右倾型配分型式。具有相对高的Zr含量（平均含量大于70×10-6）和Zr/Y值（5.2～7.0），Nb、Ta、Zr、Hf和Ti相对亏损， Nb/La值(0.1~0.3)较低，总体具有大陆玄武岩的特征，但受到不同程度陆壳或岩石圈地幔的混染。综合多种判别图解分析认为，该套玄武岩形成于大陆裂谷环境。