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首页> 外文期刊>Contributions to Mineralogy and Petrology >Kyanite/corundum eclogites from the Kaapvaal Craton: subducted troctolites and layered gabbros from the Mid- to Early Archean
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Kyanite/corundum eclogites from the Kaapvaal Craton: subducted troctolites and layered gabbros from the Mid- to Early Archean

机译：Kaapvaal Craton的蓝晶石/刚玉榴辉岩：太古宙中期至早俯冲的闪粉岩和层状辉长岩

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An oceanic crustal origin is the commonly accepted paradigm for mantle-derived eclogites. However, the significance of the aluminous members of the eclogite suite, containing kyanite and corundum, has long been underrated and their role neglected in genetic models of cratonic evolution. Here, we present a geochemical and petrological study of a suite of kyanite-and corundum-bearing eclogites from the Bellsbank kimberlite, S. Africa, which originate from depths between 150 and 200 km. Although clearly of high-pressure provenance, these rocks had a low-pressure cumulative origin with plagioclase and olivine as major cumulate phases. This is shown by the very pronounced positive Eu anomalies, low REE abundances, and delta O-18 values lower than the Earth's mantle. Many chemical features are identical to modern-day troctolitic cumulates including a light REE depletion akin to MORB, but there are also distinguishing features in that the eclogites are richer in Na, Fe, and Ni. Two of the eclogites have a minimum age of similar to 3.2 Ga, defined by the extremely unradiogenic Sr-87/Sr-86 (0.7007) in clinopyroxene. Phase equilibria indicate that the parent melts were formed by partial melting below an Archean volcanic center that generated (alkali-) picritic to high-alumina tholeiitic melts from a mantle whose oxygen fugacity was lower than today. Fractional crystallization produced troctolites with immiscible sulfide melt droplets within the mafic crust. Instability of the mafic crust led to deep subduction and re-equilibration at 4-6 GPa. Phase relationships plus the presence of a sample with appreciable modal corundum but no Eu anomaly suggest that kyanite-and corundum-bearing eclogites may also originate as plagioclase-free, higher pressure cumulates of highly aluminous clinopyroxene, spinel, and olivine. This is consistent with the crystallizing phase assemblage from an olivine tholeiitic to picritic magma deeper in the Archean oceanic crust or uppermost mantle. We postulate that the magmatic and subduction processes driving modern plate tectonics already existed in the Meso-to Early Archean.
机译：地壳起源的榴辉岩是公认的范式。然而，长久以来低估了包含蓝晶石和刚玉的榴辉岩套件中铝质元素的重要性，并且忽略了它们在克拉通演化的遗传模型中的作用。在这里，我们对一组来自非洲南部贝尔斯班金伯利岩的蓝晶石和刚玉的榴辉岩进行了地球化学和岩石学研究，其深度在150至200公里之间。尽管显然具有高压物源，但这些岩石具有低压累积成因，以斜长石和橄榄石为主要堆积相。这表现为非常明显的正Eu异常，低REE丰度以及比地球地幔低的O-18值。许多化学特征与现代的三辛酸累积物相同，包括类似于MORB的轻稀土元素消耗，但也有明显的特征，即榴辉岩中的Na，Fe和Ni含量较高。其中的两个榴辉岩的最小年龄与3.2 Ga相似，这是由在高辉石中极不放射源的Sr-87 / Sr-86（0.7007）定义的。相平衡表明，母体熔体是通过在太古宙火山中心以下部分熔融而形成的，该熔体从氧逸度低于当今的地幔中产生了（碱）向高铝质可塑熔体。分步结晶产生的菱铁矿在铁镁铁壳内具有不溶混的硫化物熔滴。镁铁质地壳的不稳定性导致深俯冲并在4-6 GPa时重新平衡。相关系加上样品中存在明显的模态刚玉但没有Eu异常，这表明蓝晶石和刚玉的榴辉岩也可能源自无斜长石，高压累积的高度铝的斜ino，尖晶石和橄榄石。这与从太古宙洋壳或最上层地幔中的深部橄榄岩到软岩浆岩的结晶相组合是一致的。我们假设推动现代板块构造的岩浆作用和俯冲作用已经存在于中观至早太古代。

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