滇西“三江”地区高镁钾质火山岩地球化学特征及其地质意义

摘要

本文报导了位于金沙江-哀牢山断裂带中段6个暗色超基性-基性火山岩的岩石学特征与稀土微量及Sr-Nd-Pb同位素地球化学数据,并对其成因和起源条件等进行了讨论.研究表明这6个暗色超基性-基性火山岩属于钾质碱性-钙碱性系列,依SiO2含量包括超基性、基性(或中基性)岩类.岩石的Mg*值较高(54.53 ～79.81),属于高镁火山岩.所有岩石样品具有富集轻稀土和大离子亲石元素,亏损高场强元素的特征.Sr-Nd-Pb同位素显示,源区具有具有较明显的Dupal异常和EMⅡ型富集地幔端元的特征.火山岩的岩石学与地球化学一致表明,本区多数暗色基性火山岩具有原生岩浆性质,其成因与富集地幔的部分熔融有关.源区显著的EMII富集地幔端元可能是扬子古陆和古特提斯洋的双向俯冲及俯冲的下地壳物质和古特提斯洋壳沉积物的再循环的物质记录.这套岩石形成于印度-欧亚大陆碰撞后的剪切和拉张构造环境,软流层地幔的上涌与俯冲的古特提斯洋壳沉积物和扬子古陆壳物质的混合,是诱发源区发生部分熔融的主要原因.金沙江-哀牢山断裂带强烈的剪切和拉张,使这些岩浆直接快速喷发至地表,因此形成具有原生岩浆性质的火山岩.%This paper reports the data of bulk-rock major element, trace element, Sr-Nd-Pb isotopes of the six ultrabasic-basic rocks from the middle area of the Jinsha River-Ailaoshan fault zone. The research showed that all samples are potassic calc-alkiline and alkaline series rocks, and including ultrabasic rocks, basic rocks (or intermediate-basic rocks). These rocks which have high Mg-number (54. 5 ~ 79. 8) are high-Mg volcanic rocks. All samples are enriched in LREE and LILE, and depleted in HFSE. The Sr-Nd-Pb isotopes showed that the source area have obviously Dupal anomaly and the EMU enriched mantle characteristic. The petrological characteristics and geochemistry of the volcanic rocks showed that most of the basic rocks have the primary magma characteristics, and the genesis of the volcanic rocks related to the partial melting of enriched mantel. The obviously EM II enriched mantle characteristics of the source area may be the material records of the paleo-tethys ocean crustal sediments and the subducted lower crust material from the bilateral subducted Yangtze paleo-plate and paleo-Tethys ocean. These volcanic rocks formed in shear-extensional tectonic setting of post India-Eurasia collision, and the upwelling of asthenosphere mixed with the subducted paleo-Tethys ocean crustal sediments and Yangtze paleo-plate materials is the main cause of the partial melting in the source area. Due to the shear-extensional in the Jinsha River-Ailaoshan fault zone, the magma rapidly erupted to the surface and formed the volcanic rocks with primary magma characteristics.