Compositionally heterogeneous podiform chromitite in the Shetland Ophiolite Complex (Scotland): Implications for chromitite petrogenesis and late-stage alteration in the upper mantle portion of a supra-subduction zone ophiolite
Compositionally heterogeneous podiform chromitite in the Shetland Ophiolite Complex (Scotland): Implications for chromitite petrogenesis and late-stage alteration in the upper mantle portion of a supra-subduction zone ophiolite
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DOI：
10.1016/j.lithos.2012.11.013
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发表时间：
2013-03
期刊：
影响因子：
3.5
通讯作者：
E. Derbyshire;B. O’Driscoll;D. Lenaz;R. Gertisser;A. Kronz
中科院分区：
文献类型：
--
作者：
E. Derbyshire;B. O’Driscoll;D. Lenaz;R. Gertisser;A. Kronz
~492 Ma设得兰蛇绿岩杂岩(SOC；苏格兰)的地幔序列中含有丰富的成分不均一的豆荚状铬铁矿，包裹在岩石莫霍面附近的拉长纯橄榄岩透镜中。利用岩相学、矿物化学和晶体结构数据研究了这些煤层中记录的铬铁矿成因和晚期蚀变事件。铬尖晶石对蛇纹石化和其他后期蚀变的抵抗性质意味着原始火成岩成分保存在未改变的晶体核心中。来自Viels、Hagdale、Harold‘s Grave、Nikka Vord和Cliff五个采样点的铬铁矿矿物学和结构显示出显著的豆荚间化学不均一性。铬尖晶石矿物化学特征与SOC橄榄岩上俯冲带熔体提取一致。纯橄榄岩透镜中心的铬铁矿矿层与不同铬铁矿矿缝位置的不同铬尖晶石成分相结合，支持了一种从空间(和时间？)上形成铬铁矿的模型。通过沟道化和/或多孔熔体流动的熔体-岩石相互作用的波动量。SOC的普遍蛇纹岩作用导致原生(地幔)硅酸盐矿物组合几乎完全被蛇纹石(含少量温石棉和反蛇纹石的利蛇纹石)所取代。纯铀矿和铬铁矿中的岩浆硫化物(例如，镍黄铁矿)在当地转化为还原的镍硫化物种类(例如，黑兹伍德矿和千里石)。在研究区西部广泛蚀变的悬崖铬铁矿矿层中记录了蛇纹岩化(进积)氧化事件，铬尖晶石广泛地蚀变为铁铬矿。铁铬尖晶石可能含有悬崖铬尖晶石体积的50%，并含有相当数量的1-2μm的尖晶石(PtAs2)和砷镍包裹体，表明这些矿物与铁铬矿在高达~500°C的温度下形成共生，因此不仅保存了对上地幔楔体中发生的复杂熔融过程的关键见解，而且还可以用来构建这类超俯冲带蛇绿岩的下地幔部分的全面蚀变历史。
The mantle sequence of the ~492Ma Shetland Ophiolite Complex (SOC; Scotland) contains abundant compositionally heterogeneous podiform chromitite bodies enclosed in elongate dunite lenses in the vicinity of the petrological Moho. Chromitite petrogenesis and late-stage alteration events recorded in these seams are examined here using petrography, mineral chemistry and crystal structural data. The resistant nature of Cr-spinel to serpentinisation and other late-stage alteration means that primary igneous compositions are preserved in unaltered crystal cores. Chromitite mineralogy and texture from five sampled localities at The Viels, Hagdale, Harold's Grave, Nikka Vord and Cliff reveal significant inter-pod chemical heterogeneity. The Cr-spinel mineral chemistry is consistent with supra-subduction zone melt extraction from the SOC peridotites. The occurrence of chromitite seams in the centres of the dunite lenses combined with variable Cr-spinel compositions at different chromitite seam localities supports a model of chromitite formation from spatially (and temporally?) fluctuating amounts of melt–rock interaction through channelised and/or porous melt flow. Pervasive serpentinisation of the SOC has led to the almost complete replacement of the primary (mantle) silicate mineral assemblages with serpentine (lizardite with minor chrysotile and antigorite). Magmatic sulphide (e.g., pentlandite) in dunite and chromitite is locally converted to reduced Ni-sulphide varieties (e.g., heazlewoodite and millerite). A post-serpentinisation (prograde) oxidisation event is recorded in the extensively altered Cliff chromitite seams in the west of the studied area, where chromitite Cr-spinel is extensively altered to ferritchromit. The ferritchromit may comprise >50% of the volume of the Cliff Cr-spinels and contain appreciable quantities of 1–2μm inclusions of sperrylite (PtAs2) and Ni-arsenide, signifying the coeval formation of these minerals with ferritchromit at temperatures of up to ~500°C. The SOC chromitite Cr-spinels thus not only preserve key insights into the complex melting processes occurring in the upper mantle wedge but can also be utilised to construct a comprehensive alteration history of the lower mantle portions of such supra-subduction zone ophiolites.