Petrogenesis of the ̃2·77 Ga Monts de Cristal Complex, Gabon: Evidence for Direct Precipitation of Pt-arsenides from Basaltic Magma
Petrogenesis of the ̃2·77 Ga Monts de Cristal Complex, Gabon: Evidence for Direct Precipitation of Pt-arsenides from Basaltic Magma
复制标题
加蓬 ̃2·77 Ga Monts de Cristal 杂岩体的岩石成因：玄武质岩浆直接沉淀铂砷化物的证据
DOI：
10.1093/petrology/egv035
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发表时间：
2015
影响因子：
3.9
通讯作者：
Y. Lahaye
中科院分区：
文献类型：
--
作者：
W. Maier;B. Rasmussen;I. Fletcher;B. Godel;S. Barnes;L. Fisher;Shenghong Yang;H. Huhma;Y. Lahaye
加蓬的蒙茨山杂岩体由几个火成岩体组成，被解释为构造解体的残余物，长>100 km，宽1-3 km，侵位于2765-2775 Ma。它是迄今为止在刚果大陆上发现的最重要的镁铁质-超镁铁质层状杂岩体。杂岩主要由正辉石岩堆晶岩组成，少量的橄榄石正辉石岩和苏长岩，以及罕见的方辉橄榄岩和纯橄榄岩。矿物成分（Fool 84、Mg#Opx 85、Anplag 60-68、Cr/Fe铬铁矿1-1·45）和全岩数据表明，母岩浆是一种低钛玄武岩，含有约10% MgO和0·5% TiO 2。微量元素和Rb-Sr和Sm-Nd同位素数据表明存在富集组分，可能来自于岩石圈下地幔中产生的岩浆的地壳混染。大多数岩石显示出极不寻常的模式，在低浓度的Pd（1-15 ppb），Au（1-2 ppb），Cu（1-20 ppm）和S（<500 ppm）下，Pt富集（10-150 ppb），这表明与全球大多数其他富含PGE的侵入体不同，蒙茨山杂岩中的铂不是岩浆硫化物。同步辐射X射线荧光成像揭示了埋藏的小Pt颗粒的位置，其中大部分与As有关。我们建议，这构成了一些最有力的证据，但在支持岩浆结晶的Pt-As阶段从S-不饱和岩浆。
The Monts de Cristal Complex of Gabon consists of several igneous bodies interpreted to be remnants of a tectonically dismembered, >100 km long and 1–3 km wide, ultramafic–mafic intrusion emplaced at 2765–2775 Ma. It is the most significant mafic–ultramafic layered complex yet identified on the Congo Craton. The complex consists largely of orthopyroxenite cumulates, with less abundant olivine-orthopyroxenite and norite, and rare harzburgite and dunite. Mineral compositions (Fool 84, Mg#Opx 85, Anplag 60–68, Cr/Fe chromite 1–1·45) and whole-rock data suggest that the parent magma was a low-Ti basalt containing approximately 10% MgO and 0·5% TiO2. Trace element and Rb–Sr and Sm–Nd isotope data indicate the presence of an enriched component, possibly derived from crustal contamination of a magma generated in the sub-lithospheric mantle. Most rocks show a highly unusual pattern of strong Pt enrichment (10–150 ppb) at low concentrations of Pd (1–15 ppb), Au (1–2 ppb), Cu (1–20 ppm), and S (<500 ppm), suggesting that unlike in most other PGE-rich intrusions globally, platinum in the Monts de Cristal Complex is not hosted in magmatic sulfides. Synchrotron X-ray fluorescence mapping has revealed the location of buried small Pt particles, most of which are associated with As. We propose that this constitutes some of the strongest evidence yet in support of magmatic crystallization of a Pt–As phase from S-undersaturated magma.