Mantle hornblendites of Naein Ophiolite (Central Iran): evidence of deep high temperature hydrothermal metasomatism in an upper mantle section
Mantle hornblendites of Naein Ophiolite (Central Iran): evidence of deep high temperature hydrothermal metasomatism in an upper mantle section
复制标题
Naein蛇绿岩（伊朗中部）的地幔角闪石：上地幔剖面深部高温热液交代作用的证据
DOI：
10.1134/s0869591117010076
复制
发表时间：
2017
期刊：
影响因子：
1.5
通讯作者：
A.
中科院分区：
文献类型：
--
作者：
Torabi;G.;Arai;S.;Morishita;T.;Tamura;A.
Naein蛇绿岩是伊朗中部最完整的蛇绿岩出露，被认为是中生代中东部伊朗微大陆(CEIM)封闭洋壳的残留物。在地幔橄榄岩中蛇绿岩的东北部(Darreh Deh地区)，莫霍面过渡带(MTZ)顶部下方几百米处，角闪岩以岩墙(以前的裂缝和节理)的形式存在，宽度从几毫米到近50厘米。它们与周围的地幔、哈兹伯格岩和硅质岩有着尖锐的边界。样品中角闪石呈淡绿色，颗粒粗大，由镁角闪石(Mg#=0.93)、绿泥石(斜长石和斜长石，Mg#=0.95)、铬尖晶石(铬铁矿，Cr#=0.67和Mg#=0.55)、透闪石、方解石和白云石组成。透闪石是角闪石的逆行变质作用形成的。方解石和白云石以晚期脉状产出。Naein蛇绿岩地幔角闪岩中含有大量的原生水合物相(约94体积%的角闪石和绿泥石)，其独特的矿物学和化学特征与岩浆成因不符。它们可能是地幔橄榄岩与富含硅酸盐成分的海水来源的超临界流体反应的产物。原生水合相(角闪石和绿泥石)的存在可能表明所涉及的溶液中H2O的高活度。角闪岩中铬铁矿的化学成分接近周围方辉橄榄岩和纯橄榄岩的平均铬铁矿成分。这表明铬的主要来源应该是附近橄榄岩中的铬铁矿，它们被热液完全或部分溶解。角闪岩球粒陨石归一化稀土配分模式中Eu的正异常，富钙角闪石的高模式丰度，以及方解石和白云石的存在，表明海水通过辉长岩侵入到最上部的地幔橄榄岩。高铬铬铁矿的存在，角闪石和绿泥石的富铬，表明橄榄岩比辉长岩对流体化学性质的贡献更大。这项研究表明，上地幔中可能来自海水的高温含水流体的循环可以淋滤，并为在最上地幔的节理和裂隙中形成角闪石提供必要的元素。
The Naein ophiolite is the most complete ophiolitic exposure in Cental Iran and considered as a remnant of the Mesozoic Central East Iranian microcontinent (CEIM) confining oceanic crust. In the northeastern part of this ophiolite (Darreh Deh area) within the mantle peridotites, a few hundred meters below the top of the Moho transition zone (MTZ), the hornblendites are present as dykes (former cracks and joints) from a few millimeters to nearly 50 cm wide. They have sharp boundaries with the surrounding mantle harzburgites and dunites. These hornblendites are pale green and coarse-grained in hand specimen and composed of magnesio-hornblende (Mg# = 0.93), chlorite (penninite and clinochlore, Mg# = 0.95), Cr-spinel (chromite, Cr# = 0.67 and Mg# = 0.55), tremolite, calcite and dolomite. Tremolites were formed by retrograde metamorphism of hornblendes. Calcite and dolomite occur as late-stage veins. Very high amount of primary hydrous phases (~94 vol % hornblende and chlorite), as well as peculiar mineralogical and chemical characteristics of the Naein ophiolite mantle hornblendites, do not match a magmatic origin. They are possibly products of the reaction between mantle peridotites and seawater-originated supercritical fluids, rich in silicate components. The presence of primary hydrous phases (hornblende and chlorite) may reveal high activity of H2O in the involved solution. The chemical composition of chromite in the hornblendites is near to the average chromite composition from the surrounding harzburgite and dunite. This suggests that the main source of Cr should be chromites of nearby peridotites, which were totally or partly dissolved by hydrothermal fluids. The positive anomaly of Eu in the chondrite-normalized REE patterns of hornblendes, high modal abundance of Ca-rich hornblende, as well as presence of calcite and dolomite, point to seawater ingression through the gabbros in to the uppermost mantle peridotites. The higher value of MgO than CaO, presence of high-Cr chromite and Cr-enrichment of hornblendes and chlorites indicate a higher contribution of peridotites rather than gabbros to the chemical characteristics of the involved fluids. This study shows that circulation of possibly seawater-derived high temperature hydrous fluids in the upper mantle can leach and provide necessary elements to form hornblendite in joints and cracks of the uppermost mantle.