西南印度洋洋盆演化和岩浆地球化学印迹

Evolution of the Southwest Indian Ocean and its relative geochemical evidences from magmatism

西南印度洋中脊(SWIR)平均扩张速率约为14 mm/yr,是全球洋中脊系统的重要组成端元,因其具有慢速-超慢速扩张特征,引起全球科学家的广泛关注。基于前人对SWIR的综合研究成果,从构造和岩浆作用两个角度出发,系统地回顾了SWIR的形成和演化历史,探讨了岩浆的分布特征和地幔不均一性成因。SWIR的形成始于冈瓦纳大陆的裂解,中段洋脊区域(26~42°E)是印度洋最早开启的窗口,历经多次洋脊跃迁和扩展作用形成了斜向扩张展布,多分段的构造格局。地幔热点在冈瓦纳大陆裂解过程中扮演了关键角色,并对SWIR的洋底地貌和岩浆作用具有显著影响,其中Bouvet和Marion热点在SWIR的西段和中段岩浆均留下了地球化学印迹。SWIR西段岩浆除却Bouvet热点影响之外表现出与大西洋-太平洋型玄武岩相似的同位素地球化学特征。在SWIR中段,39~41°E附近的岩浆具有显著的DUPAL异常特征,与冈瓦纳大陆的初始形成、裂解紧密相关。受俯冲改造的中-新元古代的造山带岩石圈地幔拆沉是造成SWIR中段地幔不均一性的重要根源。在SWIR东段,46~52°E区域内的局部岩浆组成异常,推测具有大陆地壳物质混染的成因。而在Melville转换断层以东,洋脊形成时间最晚,玄武岩的地幔源区受到了富集组分的交代作用,其同位素组成与相邻的中印度洋中脊(CIR)和东南印度洋中脊(SEIR)地幔源区具有亲缘性。

The Southwest Indian Ridge(SWIR) is an important component of global mid-oceanic ridge system, and it has been a focus for geo-scientists of the world because of its very slow average spreading rate about 14 mm/y.In this paper, we systematically reviewed, from tectonic and magmatic points of view, the formation and evolution history of SWIR and the distribution of magmatic rocks and the origin of mantle heterogeneity.The formation of SWIR began with the break-up of Gondwana and the middle part of SWIR(26°E to 42°E)was formed in the earliest stages. The present-day tectonic patterns of SWIR(e.g., multi-order ridge segments and oblique spreading) are the results of multi-stage ridge jumps and extensions. The mantle hotspots play a key role in the break-up of Gondwana and have a significant impact on geomorphology and magmatism of SWIR. Both the Bouvet and Marion hotspots have left their geochemical traces in the magmatic rocks in the western and central portions of SWIR.In addition, the basalts from the western part of SWIR show isotopic affinities with the Atlantic-Pacific-type basalts. In the middle part of SWIR(39°E to 41°E), the basalts display significant DUPAL anomalies, which are closely related to the initial formation and break-up of Gondwana.The delamination of subduction-modified lithospheric mantle in the Mesoproterozoic to Nesoproterozoic orogenic belts led to the mantle source heterogeneity in the middle part of SWIR.In the east part of SWIR(about 46°E to 52°E), the chemical anomalies are probably caused by the contamination of continental crust. To the east of the Melville transform fault, the ridge was formed in the latest stages, and the mantle source of basalts experienced metasomatism due to enriched mantle components, showing isotopic affinities with the mantle source of the adjacent Central Indian Ridge(CIR) and Southeast Indian Ridge(SEIR).