摘要
“三江”特提斯巨型造山带由４条板块缝合带和夹于其间的微陆块和弧地体构成。在古特提斯洋闭合而成的澜沧江和金沙江缝合带内，均发育典型的蛇绿岩－蛇绿混杂岩和洋岛玄武岩（ＯＩＢ）组合。ＯＩＢ集中产出于大洋的特定区段（２２０－２４０Ｎ──澜沧江洋；２４０－２７０Ｎ──金沙江洋），形成于洋壳（ＭＯＲＢ）顶部，并被浅水碳酸盐岩覆盖，ＯＩＢ主要岩石类型为苦橄玄武岩、夏威夷岩和钾质粗面玄武岩，三者具特定的空间配置与时间演变规律。根据三江地区ＯＩＢ时空展布规律、岩石组合特点、岩石地球化学特征、源岩化学估算及热幔柱实验成果，提出早古生代时期的澜沧江洋和金沙江洋岩石围下方分别出现地幔热拉，其直径约２００－２５０ｋｍ，具特定的不同于周围地幔的热－化学结构。地幔热柱的尾柱区高热物质熔融，产生苦橄质或苦橄玄武质熔浆，地幔热拉上涌，其冠形头部与周围地幔“混杂”而具化学分带，该区物质熔融，产生富Ｎａ和富Ｋ质ＯＩＢ。
he Lancangjiang and Jinshajiang-Ailaoshan suture zones in Southwestern China are products of the subduction and closure of both southern and northern branches of the Paleo-Tethys Ocean, located in the original triangular gap between the Gondwana and Larauisa Lands. The ocean-island basalts associated with ophiolite have been investigated along the two suture zones.OIBs occurred in the region of range from 220 - 240N along the Lancangjiang suture zone and in 240-270N along the Jinshajiang-Ailaoshan suture zone, respectively, and were overlapped by shallow marine limestones. Three OIB types were determined to be erupting along both oceanic ridges based On their major, trace and REE compositions, i. e., trachytic basalt, hawaiite and picritic basalt. The trachytic basalt occurred in the cerntral segment of OIB region (e. g., 220 - 240N in the Lancangjiang suture zone ), and closely associated with picritic basalt, whereas hawaiite distributed at the margins of the OIB region, and overlain on the top of MORBs and radiolarian chens. These OIBs show enrichment in the incompatible (LILE and HFSE) and LRE elements and can be characterized as typical hot-spot basalts. The quantitative calculation for incompatible element abundance and ratios in the mantle source suggests that the source that produced picritic basalt highly enriched in incompatible elements comparing with MORB source, and sources for trachytic basalt and hawaiite probably are mixing sources of an end-member, characteristic of a picritic basalt source with an end-member of normal depleted MORB source.Based on the spatial-tempral distribution, rock association and geochemical characteristics of OIBs in the region, a model is suggested, in which two ridge-centered mantle plumes with diameter of about 200-300km occurred in the Lancangjiang and Jinshajiang oceanic basins in DevonianCarboniferous period, respectively. Both the mantle Plumes have chemical-thermal structure being different from surrounding mantle. The melting of narrow trial region and large head in which the plume mixed with surrounding mantle can explain the development of picritic basalt, trachytic basalt and hawaiite as well as geochemical variation seen along the Jinshajiang and Lancangjiang suture zones.
