Petrological characteristics of altered rocks and apparent hydrothermal fluid circulation at Longqi hydrothermal fields along the Southwest Indian Ridge
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摘要: 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。Abstract: 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.
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图 3 龙旂热液区A区块岩石标本照片
a. 样品A11-3,蚀变球颗玄武岩;b. 样品A04-2,蚀变辉长质初糜棱岩;c. 样品A04-4,蛇纹岩;d. 样品A10-5,蛇纹石化辉橄岩
Fig. 3 Photographs of rock samples in the Area A of Longqi hydrothermal field
a. Sample A11-3,altered globular basalt; b. sample A04-2, altered gabbro promylonite; c. sample A04-4, serpentinite; d. sample A10-5, serpentinized pyrolite
图 4 龙旂热液区A区块玄武岩镜下照片
a. 样品A10-1,斑状玄武岩,正交偏光;b. 样品A10-4,碎裂蚀变玄武岩,正交偏光;c. 样品A11-3,蚀变球颗玄武岩,单偏光;d. 样品A11-3,蚀变球颗玄武岩,单偏光。Chl:绿泥石,Pl:斜长石,Qtz:石英,Sph:榍石
Fig. 4 Photomicrographs of thin sections of basalt in the Area A of Longqi hydrothermal field
a. Sample A10-1, porphyritic basalt, cross-polarized light; b. sample A10-4, cataclastic alteration basalt, cross-polarized light; c. sample A11-3, altered globular basalt, plane-polarized light; d. sample A11-3, altered globular basalt, plane-polarized light. Chl: chlorite, Pl: plagioclase, Qtz: quartz, Sph: sphene
图 5 龙旂热液区A区块辉长岩镜下照片
a. 样品A04-2,蚀变辉长质初糜棱岩,糜棱结构,原岩呈碎斑状,可见碎裂的斜长石、辉石定向分布,大部分辉石被次闪石、绿泥石交代,正交偏光;b. 样品A04-2,蚀变辉长质初糜棱岩,受应力作用,斜长石的双晶发生弯曲变形,正交偏光;c. 样品A04-2,蚀变辉长质初糜棱岩,可见岩石后期受应力作用发生脆性变形而形成的裂隙,正交偏光;d. 样品A04-2,蚀变辉长质初糜棱岩,辉石遭受蚀变形成次闪石,并且受构造作用影响弯曲变形,左正交偏光,右单偏光。Chl:绿泥石,Pl:斜长石,Px:辉石
Fig. 5 Photomicrographs of thin sections of gabbros in the Area A of Longqi hydrothermal field
a. Sample A04-2, altered gabbro promylonite, mylonitic texture, the original rock was fragmented and with orientation distribution of fragmented plagioclase, most of the pyroxenes is metasomatized by amphibole and chlorite, cross-polarized light; b. sample A04-2, altered gabbro promylonite, the twin crystals of plagioclase undergo bending deformation under the stressed, cross-polarized light; c. sample A04-2, altered gabbro promylonite, it can be seen that the fractures formed by brittle deformation of the rock under stress in the later stage, plane-polarized light; d. sample A04-2, altered gabbro promylonite. The pyroxene undergoes alteration to form amphibole, and is affected by tectonic deformation and bending. Left picture is under cross-polarized light, right picture is under plane-polarized light. Chl: chlorite, Pl: plagioclase, Px: pyroxene
图 6 龙旂热液区A区块蚀变超基性岩镜下照片
a. 样品A04-1,蚀变辉橄质初糜棱岩,可见辉石大多已经蚀变为次生角闪石及绿泥石,受构造应力作用,碎裂后呈定向分布,正交偏光;b. 样品A12-2,蚀变辉石岩,正交偏光;c. 样品A04-3,次闪石化辉石岩,正交偏光;d. 样品A10-5,蛇纹石化辉橄岩,可见网状结构,单偏光。Act:次闪石,Ep:绿帘石,Serp:蛇纹石,Ol:橄榄石,Chl:绿泥石,Px:辉石
Fig. 6 Photomicrographs of thin sections of altered rocks in the Area A of Longqi hydrothermal field
a. Sample A04-1, altered olivine promylonite. Most of pyroxene has been eclipsed into secondary amphibole and chlorite. Under the influence of tectonic stress, it has been distributed in a directional manner, cross-polarized light; b. sample A12-2, altered pyroxene, cross-polarized light; c. sample A04-3, spheronite pyroxene, cross-polarized light; d. sample A10-5, serpentinized pyrolite. It can be seen mesh structure, plane-polarized light. Act: actinolite, Ep: epidote, Serp: serpentine, Ol: olivine, Chl: chlorite, Px: pyroxene
图 8 龙旂热液区B区蚀变岩石镜下照片
a. 样品B04-1,玄武岩,正交偏光;b. 样品B04-2,蚀变辉长岩,可见辉长结构,大部分辉石已经蚀变,斜长石多发生碎裂,单偏光;c. 样品B04-2,蚀变辉长岩,辉石的次生角闪石反应边,正交偏光;d. 样品B04-4,蛇纹岩中的网状结构,主要为纤蛇纹石,正交偏光。Chl:绿泥石,Pl:斜长石,Px:辉石,Act:次闪石,Ol:橄榄石
Fig. 8 Photomicrographs of thin sections of altered rocks in the Area B of Longqi hydrothermal field
a. Sample B04-1, basalt, cross-polarized light; b. sample B04-2, altered gabbro, gabbro texture. Most of the pyroxene has been altered, plagioclase tends to fracture, plane-polarized light; c. sample B04-2, altered gabbro, secondary amphibole reaction of pyroxene, cross-polarized light; d. sample B04-4, serpentinite, it can be seen mesh structure, cross-polarized light. Chl: chlorite, Pl: plagioclase, Px: pyroxen, Act: actinolite, Ol: olivine
图 9 龙旂热液区C区岩石标本照片
a. 样品C21-4,蚀变玄武岩;b. 样品C20-1,蚀变玄武岩;c. 样品C17-2,蚀变玄武质角砾岩;d. 样品C16-1,蚀变玄武岩
Fig. 9 Photographs of rock samples in the Area C of Longqi hydrothermal field
a. Sample C21-4, altered basalt; b. sample C20-1, altered basalt; c. sample C17-2, altered basaltic breccia; d. sample C16-1, altered basalt
图 10 龙旂热液区C区蚀变玄武岩镜下照片
a. 样品C21-4,脉状构造,早期石英和绿泥石沿岩石裂隙两侧生长,后期被绿帘石充填,形成1.5~3 mm宽的绿帘石脉及石英脉,正交偏光;b. 样品C21-4,气孔中充填两种不同颜色的绿泥石,中间呈深绿色,边缘呈浅绿-黄绿色,单偏光;c. 样品C20-1,弥散状构造,由于强烈的构造作用及热液作用,整块样品被绿泥石交代。原岩遭受蚀变后多保留为50~100 μm的浅褐色玄武质集合体,约占30%,均匀分布于绿泥石之中,可见白色脉状石英,单偏光;d. 样品C16-1,环带状构造,单偏光。Chl:绿泥石,Ep:绿帘石,Sph:榍石,Qtz:石英
Fig. 10 Photomicrographs of thin sections of basalt in the Area C of Longqi hydrothermal field
a. Sample C21-4, vein structure, early quartz and chlorite grew along both sides of the rock fissures, and were later filled with chloridite to form 1.5−3 mm wide chloridite veins and quartz veins, cross-polarized light; b. sample C21-4, stomata are filled with two different colors of chlorite, dark green in the middle, and light green-yellow green at the edges, plane-polarized light; c. sample C20-1, diffuse structure. Due to strong tectonic and hydrothermal effects, the entire sample was replaced by chlorite. After the original rock undergoes alteration, most of the light brown basalt aggregates remaining at 50−100 μm, accounting for about 30%, are evenly distributed in the chlorite, and white veined quartz can be seen, plane-polarized light; d. sample C16-1, annular structure, plane-polarized light. Chl: chlorite, Ep: epidote, Sph: sphene;Qtz: quartz
表 1 研究区岩石样品信息表
Tab. 1 Information of rock samples in the study area
区块 样品编号 岩石名称 手标本特征 显微特征 A A11-3 蚀变球颗玄武岩 块状构造,表面呈灰绿色,切面及表面可见白色的球颗,大小约在1~6 mm,切面可见贯穿岩石的暗绿色裂隙 球颗结构,原岩由针状斜长石及细小辉石颗粒、玄武质玻璃构成,可见显微裂隙及气孔,填充绿泥石及少量石英 A A04-1 蚀变辉橄质初糜棱岩 块状构造,主体呈深绿色−黑绿色,由辉橄岩蛇纹石化形成,原岩结构组成较不清晰 糜棱结构,碎裂的辉石大多呈定向分布,经热液蚀变发生次闪石化、蛇纹石化、绿泥石化等作用 A A04-2 蚀变辉长质初糜棱岩 块状构造,中粒结构,表面通常呈灰色−黑灰色,可观察到弱蚀变现象 糜棱结构,可见碎裂的斜长石、辉石定向分布,大部分辉石被次闪石、绿泥石交代 A A04-3 次闪石化辉石岩 块状构造,表面呈褐黑色,主要由辉石(>90%)组成 碎裂结构,残留柱状结构,主要矿物成分为辉石,均已强烈蚀变为次闪石及蛇纹石,绿泥石及绿帘石填充在裂隙中 A A04-4 蛇纹岩 块状构造,表面颜色呈浅绿色−深绿色,部分表面存在红褐色的铁膜,蜡状光泽,解理发育,切面可见黑色的网格结构 网状结构,由残余的橄榄石颗粒和蛇纹石网脉组成,可见磁铁矿等不透明铁质矿物分布 A A10-5 蛇纹石化辉橄岩 表面呈灰绿色,部分表面具红褐色氧化膜,切面可见网状结构,部分矿物仅保留结构,原矿物成分不可确定 网状结构,蚀变后残余的橄榄石或辉石假象及蛇纹石网脉组成 B B04-1 玄武岩 块状构造,少斑,致密,表面可见黑色玻璃质结壳 基质具间粒结构,斑晶以橄榄石、斜长石为主 B B04-2 蚀变辉长岩 块状构造,中粒结构,表面呈灰色−深绿色,以辉石和斜长石为主 样品具碎裂结构,辉长结构,少量辉石包裹斜长石,构成含长结构 B B04-3 蚀变辉石岩 块状构造,表面呈灰黑色−黑绿色,构造介于块状−板片状之间,原岩矿物的构造不清晰 辉石碎裂程度较高并伴随次闪石化,裂隙中充填着绿泥石及绿帘石等矿物 B B04-4 蛇纹岩 岩石呈板片状,表面呈深绿色−浅绿色,具蜡状光泽 网状结构,蛇纹石主要为纤蛇纹石 C C16-1 蚀变玄武岩 块状构造,样品表面呈灰绿色 角砾结构,显微镜下观察到岩石具有典型的环带状构造,出现钠长石、绿泥石、石英、榍石、绿帘石等蚀变矿物 C C21-4 蚀变玄武岩 样品呈浅绿色−灰绿色,表面见晶洞,填有石英小晶簇。切面见网脉状裂隙,充填绿帘石及石英,可见黄铁矿化 脉状结构,原岩为玻璃质玄武岩,充填物为绿帘石、石英、绿泥石及少量黄铁矿 C C20-1 蚀变玄武岩 样品呈灰绿色,切面可见较多裂隙,可见强烈的绿泥石化现象 样品碎裂程度较重,大量绿泥石充填在玄武质碎屑中 C C17-2 蚀变玄武质角砾岩 表面主要呈黄绿色,色杂,切面呈浅绿色,角砾状结构,切面上还可看到角砾呈环带状,边缘可见白色小球粒 角砾结构,可观察到环带状构造,角砾多已被绿泥石交代,蚀变矿物有榍石、石英、钠长石等 表 2 龙旂热液区各类岩石中绿泥石代表性主量元素及其特征值(%)
Tab. 2 Chlorite electron microprobe analyses and characteristic values of rocks (%) in the Longqi hydrothermal field
主量元素 B04-3 B04-2 A04-2 A04-1 蚀变辉石岩 蚀变辉长岩 蚀变辉长质初糜棱岩 蚀变辉橄质初糜棱岩 SiO2 26.75 28.25 27.60 28.04 27.66 29.78 27.82 29.33 27.67 27.57 28.16 27.94 26.43 28.50 25.72 25.89 25.92 TiO2 0.01 0.08 0.05 0.04 0.01 0.08 0.09 0.00 0.08 0.01 0.02 0.00 0.00 0.00 0.05 0.00 0.00 Al2O3 20.01 19.48 19.10 18.74 19.70 18.45 17.72 19.85 21.24 20.02 19.65 20.78 20.38 19.29 20.35 19.95 20.42 FeO 26.25 17.84 26.41 21.95 13.04 10.90 11.18 11.53 17.48 19.37 19.30 18.07 24.44 20.19 27.41 25.69 27.49 MnO 0.32 0.23 0.37 0.30 0.16 0.11 0.14 0.13 0.26 0.29 0.23 0.29 0.18 0.15 0.23 0.14 0.20 MgO 13.51 20.09 14.52 18.20 22.79 25.90 22.83 25.03 20.42 19.42 19.72 20.74 14.79 19.11 13.08 13.56 13.00 CaO 0.05 0.03 0.06 0.07 0.20 0.13 0.82 0.06 0.11 0.05 0.06 0.03 0.06 0.06 0.01 0.11 0.04 Na2O 0.04 0.04 0.05 0.02 0.02 0.04 0.14 0.03 0.02 0.02 0.04 0.03 0.05 0.07 0.01 0.10 0.04 K2O 0.00 0.01 0.00 0.04 0.01 0.01 0.04 0.02 0.01 0.01 0.01 0.01 0.00 0.00 0.01 0.01 0.00 总量 86.94 86.04 88.14 87.39 83.58 85.40 80.77 85.97 87.28 86.77 87.18 87.89 86.33 87.36 86.87 85.45 87.10 表 3 龙旂热液区各类岩石中绿帘石、阳起石主量元素成分
Tab. 3 Epidote and actinolite electron microprobe analyses of rocks in the Longqi hydrothermal field
主量元素 C21-4 C16-1 A04-2 B04-2 B04-3 玄武岩 玄武岩 辉长岩 辉长岩 辉石岩 绿帘石 绿帘石 绿帘石 绿帘石 阳起石 阳起石 阳起石 绿帘石 绿帘石 阳起石 SiO2 37.44 37.33 39.32 37.58 52.09 52.03 56.39 38.42 38.40 54.21 TiO2 0.66 0.35 0.22 0.54 0.47 0.16 0.00 0.07 0.25 0.36 Al2O3 22.32 22.67 25.00 24.65 4.39 3.13 1.14 25.11 24.88 1.75 FeO 12.40 11.25 9.08 8.36 11.49 3.88 4.65 9.74 9.68 11.64 MnO 0.11 0.05 0.14 0.00 0.22 0.18 0.16 0.07 0.15 0.43 MgO 0.20 0.09 0.04 0.05 15.81 16.01 21.31 0.06 0.05 16.01 CaO 23.15 22.98 22.88 22.25 11.98 22.39 12.54 23.68 23.65 12.30 Na2O 0.03 0.03 0.03 0.01 0.75 0.36 0.24 0.02 0.00 0.26 K2O 0.00 0.01 0.00 0.01 0.02 0.00 0.01 0.01 0.01 0.02 P2O5 0.02 0.03 0.04 0.00 0.01 0.04 0.01 0.04 0.00 0.04 总量 96.33 94.79 96.75 93.45 97.57 98.87 96.74 97.25 97.11 97.06 注:根据样品蚀变程度不同,表中选取电子探针测试结果中具有代表性的绿帘石、阳起石矿物数据。 -
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