西南印度洋脊龙旂热液区蚀变岩岩石学特征及对热液流体循环的指示

Petrological characteristics of altered rocks and apparent hydrothermal fluid circulation at Longqi hydrothermal fields along the Southwest Indian Ridge

慢速-超慢速扩张洋脊的海底热液活动区多出露类型多样的蚀变岩石,记录了地壳深部的流体与围岩的相互作用,为研究深部热液流体特征以及循环过程提供了样本。本研究选取了中国大洋第30、34和40航次在超慢速扩张西南印度洋脊龙旂热液区(A区、B区和C区)利用电视抓斗采集的蚀变玄武岩、蚀变辉长岩、蚀变辉石岩和蛇纹岩等蚀变岩样品,利用光学显微镜、电子探针开展了岩相学和矿物化学分析。岩相学结果表明,龙旂热液区蚀变岩石样品约95%发生了地壳浅部的脆性变形作用,靠近龙旂1号热液区(A区)约有5%的蚀变岩石混合发育了脆性变形及脆性-塑性变形特征。研究区岩石蚀变属于中-低温变质作用,变质相近似绿片岩相,变质矿物组合为绿泥石-绿帘石-钠长石-阳起石-榍石。其中,A区的蚀变岩中的绿泥石形成温度(201~341℃)以及蛇纹石、阳起石、绿泥石等蚀变矿物的Fe元素含量(17.5%~27.5%)都高于龙旂3号热液区(B区和C区)的绿泥石形成温度(239~303℃)和Fe元素含量(16.8%~26.5%),这也与在该区观测到高温的热液喷口相符合。本研究认为龙旂热液区所在洋脊段发育的拆离断层为热液流体的向上运移提供了通道,洋壳扩张后期轴部的岩浆熔体在轴侧区域的岩浆侵入或喷发活动可能为热液循环提供了热源。

Submarine hydrothermal fields in the vicinity of slow-and ultraslow-spreading oceanic ridges reveal a variety of altered rocks, recording the interaction between the deep crust fluid and the surrounding rock, and providing evidence of the characteristics of the deep hydrothermal fluid and associated cyclic processes. We studied samples of metabasalt, metagabbro, altered pyroxenite, and serpentinite collected by TV-grab during Chinese cruises of DY30, DY34, and DY40 at the Longqi hydrothermal field(areas A, B and C) at the ultraslow-spreading Southwest Indian Ridge. The petrography and mineral chemistry of the rock samples were examined under optical microscope and with an electron microprobe analyzer. The petrographic results show that ~95% of the altered rock samples in the Longqi hydrothermal field exhibited brittle deformation and therefore were probably formed in the upper crust. The remaining 5% of samples, from a deep source close to the Longqi-1 hydrothermal hydrothermal field(Area A) were variable, with brittle and plastic-brittle deformation. The altered rock in the Longqi hydrothermal field exhibited medium-low temperature metamorphism, and was mainly composed of chlorite, epidote, albite,actinolite, and sphene of low green-schist facies. In addition, the chlorite formation temperature(201–341°C) in the altered rocks in Area A, and the Fe content(17.5%–27.5%) of the altered minerals such as serpentine,actinolite and chlorite, were both higher than those for Longqi hydrothermal field areas B and C, where chlorite formation temperature was 239–303°C and the Fe content was 16.8%–26.5%. This is consistent with the high temperature hydrothermal vents observed in Area A. We consider that the detachment fault developed by the ridge section of the Longqi hydrothermal field provides a channel for the upward migration of hydrothermal fluids. The small-scale magmatic intrusion or eruption activity of the magma melt in the axial portion of the shaft during expansion may provide a heat source for the hydrothermal circulation.