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首页> 外文期刊>Contributions to Mineralogy and Petrology >A granite-gabbro complex from Madagascar: constraints on melting of the lower crust
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A granite-gabbro complex from Madagascar: constraints on melting of the lower crust

机译：马达加斯加的花岗石复合体：下地壳融化的限制

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The Ranomandry Complex is a Neoproterozoic, nested intrusion from central Madagascar composed of a gabbroic core within a coeval peraluminous granite ring intruding pelitic metasediments. Although xenocryst entrainment and magma mixing have both contributed to marginal phases of the granite, the primary melt is characterised by steep LREE/HREE ratios and negligible, or slightly positive, Eu anomalies. Both isotopic and trace element systematics preclude an origin from either partial melting of the metapelitic country rock or from fractional crystallisation of the gabbroic magma. However, trace-element modelling suggests an origin from the dehydration melting of a plagioclasepoor, garnet-bearing metagreywacke at temperatures of 850-900 ℃ and at lower crustal pressures (> 10 kbar). Melting of an enriched subcontinental mantle generated gabbroic magmas that caused advective heating and anatexis at the base of thickened continental crust during their ascent. Transport of the resulting granite magma was facilitated by the pre-existing plumbing system that overcame thermal and mechanical problems associated with both diapirism and self-propagating dykes as mechanisms for long-distance transport of granite magmas. The nested geometry of the intrusions is an indication of a structurally homogeneous lower crust that contains no pre-existing shear zones or fault systems.
机译：Ranomandry复合体是马达加斯加中部新元古代的嵌套侵入体，由侵入古生的花岗质花岗岩环内的辉长岩芯构成，侵入了泥质变质沉积。虽然异晶夹带和岩浆混合都对花岗岩的边缘相起了作用，但一次熔体的特征是陡峭的LREE / HREE比和可忽略的或略微为正的Eu异常。同位素和微量元素系统学都排除了源于该成岩带乡村岩石的部分融化或辉长岩岩浆的部分结晶的起源。然而，痕量元素模拟表明，斜长石，石榴石的变质灰色藻在850-900℃的温度和较低的地壳压力（> 10 kbar）下的脱水融化作用是起源。富集的次大陆地幔融化产生的辉长岩浆，在上升过程中在增厚的大陆壳的底部引起了对流加热和食盐。预先存在的管道系统促进了最终花岗岩岩浆的运输，该系统克服了作为岩浆岩浆长距离运输机制的透水和自扩散堤坝所引起的热和机械问题。侵入体的嵌套几何结构表明下部地壳结构均匀，不包含先前存在的剪切带或断层系统。

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