Ophiolitic chromitites from the Kızılyüksek area of the Pozantı-Karsantı ophiolite (Adana, southern Turkey): Implication for crystallization from a fractionated boninitic melt
Ophiolitic chromitites from the Kızılyüksek area of the Pozantı-Karsantı ophiolite (Adana, southern Turkey): Implication for crystallization from a fractionated boninitic melt
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DOI：
10.1016/j.oregeorev.2016.08.033
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发表时间：
2017-11
影响因子：
3.3
通讯作者：
Erdi Avcı;I. Uysal;R. M. Akmaz;Samet Saka
中科院分区：
文献类型：
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作者：
Erdi Avcı;I. Uysal;R. M. Akmaz;Samet Saka
蛇绿岩广泛分布于土耳其。一种是土耳其南部的Pozantele-Karsantele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-santele-sant铬铁矿中的铬尖晶石晶粒以高Cr# [100 × Cr /（Cr + Al）= 68-81]为代表，其Mg# [100 × Mg /（Mg + Fe 2+）]范围为54 - 71。镓和钴含量分别在18 - 32 ppm和185-266 ppm之间，与Cr#呈负相关。详细的光学研究表明，铬尖晶石颗粒含有硅酸盐，铂族矿物（PGM）和贱金属硫化物（BMS）夹杂物。角闪石单相包裹体是所研究的铬铁矿中唯一的含水硅酸盐相，它们含有低的TiO 2（<0.43wt.%）。橄榄石，具有高Fo（~ 96）和NiO含量（0.48-0.68重量%），和单斜辉石，具有低TiO 2（< 0.1重量%），Al 2 O3（< 2.84重量%）和Na 2 O含量（< 0.4重量%）还观察到初级硅酸盐夹杂物。铬铁矿含有低浓度的总铂族元素（PGE），范围在32至162 ppb之间，平均值为93 ppb。原始地幔标准化铂族元素图解显示从Os到Rh（RhN/OsN= 0.99 ~ 8.5）的斜率几乎为平的，从Rh到Pt和Pd的斜率为负。所有样品都显示出明显的Ru正异常。与地球化学数据一致，Ru，Os和Ir轴承铂族元素硫化物（月桂石-Erichmanite固溶体系列[（Ru，Os）S2-（Os，Ru）S2]相）是最常见的铂族元素检测到的研究铬铁矿样品。它们的Os-Ru置换范围很窄[Ru#; Ru/（Ru + Os）= 0.72-0.97]，表明铂族金属共生体中不存在硅线石。除了最常见的铂族元素的月桂石，几个锇（Os，Ir），铱（Ir，Os），irarsite，和一个单一的颗粒的speryllite（PtAs 2）被检测到作为岩浆包裹体的铬尖晶石。在Cr-尖晶石晶粒中还检测到三种未知的PGE/PGE-BME（贱金属元素）相，其组成分别对应于化学式（Os，Ru，Ir，Rh，Fe，Pd）2S 5、Ir（Rh，Pt，Ni，Cu）S3和（Ir，Rh，Ru）2（Ni，Cu）S3。高Cr#和低Ti含量的铬尖晶石颗粒和角闪石包体中的低Ti含量的含水相的铬尖晶石颗粒需要一个不相容的微量元素的形成所研究的铬铁矿的含水熔体贫化，因此，我们建议的弧前构造环境的生成Kızılyüksek铬铁矿。Os-Ir合金和富含Ru的月桂石的存在意味着铬尖晶石的结晶发生在相对较高的温度（1100-1300 °C）和较低的温度（S2）（-1和-3之间）条件下。铬尖晶石的主要和微量元素组成的变化，铬铁矿的RhN/OsN比值的变化很大，以及与Ir和Ru相比，铬铁矿中Os的贫化可能意味着Kızılyüksek铬铁矿从各种分馏的玻安岩熔体中结晶出来。
Ophiolitic rocks are widely distributed in Turkey. One type, the Pozantı-Karsantı ophiolite from southern Turkey, contains a large number of chromitite deposits located mostly in the mantle peridotites and close to the Moho transition zone dunite and cumulate dunites. Cr-spinel grains from the chromitites are represented by high Cr# [100 × Cr / (Cr + Al) = 68–81], and their Mg# [100 × Mg / (Mg + Fe2 +)] range from 54 to 71. Gallium and Co contents vary between 18 and 32 ppm and 185–266 ppm, respectively, and they show negative correlation with Cr#. A detailed optical investigation reveals that the Cr-spinel grains contain silicate, platinum-group mineral (PGM) and base metal sulfide (BMS) inclusions. Single phase inclusions of amphibole are the only hydrous silicate phases in the investigated chromitites, and they contain low TiO2(< 0.43 wt.%). Olivine, with high Fo (~ 96) and NiO contents (0.48–0.68 wt.%), and clinopyroxene, with low TiO2(< 0.1 wt.%), Al2O3(< 2.84 wt.%) and Na2O contents (< 0.4 wt.%) were also observed as primary silicate inclusions. Chromitites contain low concentrations of total platinum-group elements (PGE) ranging between 32 and 162 ppb, with an average value of 93 ppb. Primitive mantle-normalized PGE diagrams show almost flat to positive slopes from Os to Rh (RhN/OsN= 0.99 to 8.5) and negative slope from Rh to Pt and Pd. All samples show marked positive Ru anomalies. Consistent with the geochemical data, Ru, Os, and Ir bearing PGE sulfide (laurite-erlichmanite solid solution series [(Ru, Os)S2–(Os, Ru)S2] phases) are the most common PGM detected in the investigated chromitite samples. They show a narrow range of Os-Ru substitution [Ru#; Ru/(Ru + Os) = 0.72–0.97], indicating no erlichmanite in the PGM paragenesis. In addition to the most common PGM laurite, several osmium (Os, Ir), iridium (Ir, Os), irarsite, and one single grain of speryllite (PtAs2) were detected as magmatic inclusions in Cr-spinel. Three unknown PGE/PGE–BME (base metal element) phases were also detected in Cr-spinel grains with compositions that correspond to the chemical formulas of (Os, Ru, Ir, Rh, Fe, Pd)2S5,Ir(Rh,Pt,Ni,Cu)S3, and (Ir, Rh, Ru)2(Ni, Cu)S3, respectively. The high Cr# and low Ti content of Cr-spinel grains and amphibole inclusions with low Ti content as hydrous phases in Cr-spinel grains require a hydrous melt depleted in incompatible trace elements for the formation of investigated chromitites; therefore, we suggest a fore arc tectonic environment for the generation of Kızılyüksek chromitites. The presence of Os-Ir alloys and Ru-rich laurites implies that Cr-spinel crystallization took place at relatively high temperature (1100–1300 °C) and low ƒ(S2) (between − 1 and − 3) conditions. Major and trace element compositional variations of Cr-spinel, wide variation of RhN/OsNratios of the chromitites and depletion of Os in the chromitites compared to Ir and Ru may imply that Kızılyüksek chromitites crystallized from a variously fractionated boninitic melt.