【作者】 袁野；

【导师】 张成江；

【作者基本信息】 成都理工大学 ， 矿物学、岩石学、矿床学， 2013， 硕士

【摘要】 南澜沧江带火山岩是三江古特提斯构造发展过程中的一个重要组成部分，澄清其形成时代、岩石地球化学特征等问题对揭示南澜沧江带内复杂的古特提斯构造演化历史及其动力学机制至关重要。本文通过对火山岩的岩石学、地球化学及年龄等方面的特征对南澜沧江岩浆岩带的火山岩进行了研究。在南澜沧江岩浆岩带，中晚三叠世火山岩出露齐全，主要可分为中三叠世忙怀组、晚三叠世至早侏罗世小定西组。忙怀组以酸性火山岩为主，为一套流纹岩夹火山碎屑岩组合；小定西组发育为中基性火山熔岩夹火山碎屑岩；忙怀组具有流纹质火山碎屑岩与玄武岩共存的“双峰式”火山岩特征。南澜沧江岩浆岩带火山岩样品SiO₂含量主要集中于48.70%‐54.56%和66.60%‐79.37%的基性与酸性两个区域，缺乏中性火山岩，Al₂O₃含量较高，集中在10.95%‐19.67%，平均值为15.56%。TiO₂含量偏低，为0.07%‐2.57%，平均值为0.94%。K₂O/Na₂O为0.09‐44.94。稀土总量在126.75ppm‐257.56ppm之间，CeN/YbN为1.23‐9.49。球粒陨石标准化稀土配分模式图呈明显的右倾模式，轻稀土中等富集，重稀土相对亏损。具有负Eu异常，δEu介于0.44‐1.07之间，平均0.80。岩石总体上富大离子亲石元素Rb。Sr，Ti相对亏损。岩石地球化学特征及构造环境判别图解指示，南澜沧江带三叠纪火山岩具有弧火山岩与大陆板内火山岩的双重属性，推测其形成环境为火山弧后缘拉张环境。Ar‐Ar年龄测试，得到的坪年龄为236.7±2.2Ma，SHRIMP锆石U-Pb定年研究，给出的²⁰⁶Pb/²³⁸U年龄为231.0±5.0Ma，214±7Ma以及210±22Ma，分别为中三叠世和晚三叠世。结合火山岩年代学测试结果，推测澜沧江洋主碰撞期为早三叠世，中三叠世与晚三叠世早期分别为碰撞后的应力松弛阶段与洋盆继续俯冲期，到早侏罗世，俯冲作用结束，澜沧江洋关闭。更多还原

【Abstract】 The volcanic rocks in southern Lancangjiang zone play an important role in thedevelopment of Sanjiang Tethyan tectonic belt. It is essential to clarify their formationages, geochemical characteristics and other issues in revealing the complex history ofPaleo‐Tethys tectonic evolution and the dynamic mechanism. In this paper, volcanicrocks in southern Lancangjiang magmatic belt were studied based on petrological,geochemical and age characteristics.Middle‐Late Triassic volcanic sequences exposed in full in Yunxian area insouthern Lancangjiang magmatic belt, can be divided into2formations as MiddleTriassic Manghuai Formation, Late Triassic to Early Jurassic Xiaodingxi Formation.Acid volcanics mainly erupted in Manghuai Formation, formed a rhyolite andpyroclastic rock assemblage; In Xiaodingxi Formation developed basic‐intermediatelavas mixed with pyroclastic rocks; Rhyolite pyroclastic rocks and basalt coexist inManghuai Formation, showed “bimodal”volcanic features. The SiO₂of volcanic rocksin southern Lancangjiang magmatic belt range48.70%‐54.56%and66.60%‐79.37%, lack of intermediate ones. The content of Al₂O₃is high, ranging10.95%‐19.67%, averagely15.56%. The TiO₂is relatively low from0.07%to2.57%,averagely0.94%. The K₂O/Na₂O ratio range0.09‐44.94. The REE content isbetween126.75ppm‐257.56ppm, with CeN/YbNratios of1.23‐9.49. The chondrite‐normalized REE patterns show obvious right slanting, moderately enriched LREEand depleted HREE. Negative Eu anomalies are also shown with δEu of0.44‐1.07,averagely0.80. The volcanic rocks generally enrich in Large Ion Lithophile ElementsRb, and depleted in Sr and Ti. Geochemical characteristics and tectonic settingdiscrimination diagrams indicate that Triassic volcanic rocks from southern Lancangjiang zone have dual characteristics of arc volcanic rocks and continentalintraplate volcanic rocks, suggesting that their formation environment is the trailingedge of the volcanic arc extensional environment. Ar‐Ar dating yields the plateauage of236.7±2.2Ma, U-Pb age is231.0±5.0Ma,214±7Ma and210±22Ma, whichbelong to Middle Triassic and Late Middle Triassic. With the dating results, the maincollision stage of Lancang River is inferred in Early Triassic. Middle Triassic andearly Late Triassic are respectively the stress relaxation stage after collision andcontinued subduction stage of oceanic basin. Until the Early Jurassic, subductionended, Lancang River Ocean closed.更多还原