Field setting, mineralogy, chemistry, and genesis of arc picrites, New Georgia, Solomon Islands
Field setting, mineralogy, chemistry, and genesis of arc picrites, New Georgia, Solomon Islands
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DOI：
10.1007/bf00376763
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发表时间：
1984-12
影响因子：
3.5
通讯作者：
W. Ramsay;A. Crawford;J. Foden
中科院分区：
文献类型：
--
作者：
W. Ramsay;A. Crawford;J. Foden
本文介绍了所罗门群岛活动火山科洛破火山岩中的一套苦味玄武岩及其相关的镁橄榄拉斑玄武岩(新佐治亚弧苦皮岩)的野外环境、岩石学、矿物学和地球化学特征。这些熔岩的面积范围约为1002千米，有些地方几乎厚达1千米，位于伍德拉克扩张带与太平洋板块边缘的交汇处。它们含有丰富的橄榄石(FO94-75)和透辉石(Cr2O31.1-0.4%，Al2O31-3%)，以及具有较大范围的铬/(铬+铝)(0.85-0.46)和镁/(镁+铁++)(0.65−0.1)的尖晶石。尖晶石富Fe+，Fe+/(Fe+Cr+Al)为0.06~1.0。最富镁的橄榄石(FO91-94)中含有一组离散的尖晶石，具有最高的铬/(铬+铝)(0.83-0.86)和最低的Fe+含量，两者都可能是原始的包裹体晶体。熔岩在10-28%的镁含量范围内，在氧化物(元素)-氧化镁图上定义了线性趋势，这些趋势被解释为橄榄石(∼0.9)单斜辉石(∼0.1)控制线。这些弧形苦苦岩的重建母岩浆成分中，CaO、Al_2O_3、TiO_2、Zr、V和Sc的比例非常接近球粒陨石。稀土配分模式为轻稀土略富集((La/Sm)N1.3-1.43)，重稀土为平坦。与镁含量相近的MORb相比，所有熔岩均表现出明显的K、Rb、Sr、Ba和LREE的富集性，但提出的母岩浆的TiO2含量接近于推测的原生MORb流体的含量。认为弧母岩浆是洋底上地幔部分熔融的产物，富LILE的水合流体来自俯冲洋壳的脱水和/或部分熔融，可能还有少量的沉积物。
The field setting, petrography, mineralogy, and geochemistry of a suite of picrite basalts and related magnesian olivine tholeiites (New Georgia arc picrites) from the New Georgia Volcanics, Kolo caldera in the active ensimatic Solomon Islands arc are presented. These lavas, with an areal extent in the order of 1002km and almost 1 km thick in places, are located close to the intersection of the Woodlark spreading zone with the Pacific plate margin. They contain abundant olivine (Fo94-75) and diopside (Cr2O31.1-0.4%, Al2O31–3%), and spinels characterised by a large range in Cr/(Cr+Al) (0.85–0.46) and Mg/(Mg+ Fe++) (0.65−0.1). The spinels are Fe+++rich, with Fe+++/ (Fe++++Cr+Al) varying from 0.06 to 1.0. A discrete group of spinels with the highest Cr/(Cr+Al) (0.83–0.86) and lowest Fe+++contents are included in the most Mg-rich olivine (Fo91–94) and both may be xenocrystal in origin.The lavas, which range between 10–28% MgO, define linear trends on oxide (element) — MgO diagrams and these trends are interpreted as olivine (∼0.9) clinopyroxene (∼0.1) control lines. For the reconstructed parent magma composition of these arc picrites, ratios involving CaO, Al2O3, TiO2, Zr, V and Sc are very close to chondritic. REE patterns are slightly LREE — enriched ((La/Sm)N1.3–1.43) and HREE are flat. All lavas show marked enrichments in K, Rb, Sr, Ba, and LREE relative to MORB with similar MgO contents, but the TiO2content of the proposed parent magma is close to those of postulated primary MORB liquids. It is proposed that the arc parent magma was produced by partial melting of sub-oceanic upper mantle induced by the introduction of LILE — enriched hydrous fluids derived by dehydration and/or partial melting of subducted ocean crust and possibly minor sediments.