Origins of Continental Cratons: Evidence from Kaapvaal Xenoliths

粗石榴石橄榄岩是非洲南部Kaapvaal克拉通的主要岩石类型，其体积和模态组成与海洋原始岩的不同之处是镁和顽辉石含量更丰富。这种区别是根本的，因为它意味着大陆岩石圈和海洋岩石圈形成的火成岩过程的不同。大陆和海洋的橄榄岩似乎都是残余物，推测这一差异反映了克拉通橄榄岩的形成深度更大。而Kaapvaal克拉通下伏的高温橄榄岩成分与海洋岩石圈相似，可能发生过俯冲。在几厘米尺度的大陆橄榄岩中，矿物分布的非均质性是常见的，被认为是由透辉石和石榴石从原生顽辉石或鸽岩中析出造成的。研究火成岩起源和排除这些亚固体特征的体积组成变化需要重0.5-1千克的分析样品。这种样本量要求限制了所需数据的获取。然而，收集额外的大型捕虏体是可行的。本项目将把这些研究扩展到克拉通岩石圈顶部尖晶石橄榄岩的火成岩起源和形成克拉通岩石圈根部的低钙石榴石碳酸盐的火成岩起源。从初步资料来看，克拉通下伏的高温橄榄岩向北比俯冲的橄榄岩更肥沃。南缘下0。我们将调查这种可能性。重量至少几公斤的捕虏体的体积、模态和矿物分析将与地磁系的同位素研究相结合。计划确定卡普瓦尔闪星的面积分布和成因深度。

Coarse garnet peridotites that are the principal rock type in the Kaapvaal craton, southern Africa, have bulk and modal compositions that differ from oceanic preidotites in being richer in Mg and in enstatite. This distinction is fundemental in that it implies a difference in the igneous processes through which continental and oceanic lithospheres have formed. Both continental and oceanic peridotites appear to be residues and the difference is speculated to reflect a greater depth of formation for the craton peridotites. In contrast, the high-temperature peridotites underlying the Kaapvaal craton have compositions similar to oceanic lithosphere and may have been subducted. Heterogeneities in mineral distribution in the continental peridotites on a scale of several centimeters are common and are believed to be caused by exsolution of diopside and garnet from primary enstatite or pigeonite. Study of variations in bulk composition that are igneous in origin and that exclude these subsolidus features requires analytical specimens weighing 0.5-1 kg. This sample size requirement has limited acquisition of desired data. Collection of additional large xenoliths, however, is practical. This project will extend these studies to investigations of the igneous origins of the spinel peridotites from the top of the lithosphere within the craton and of the the low-Ca garnet harzburgites that form the root. The high- temperature peridotites underlying the craton appear from preliminary data to be more fertile towards the north than are those (subducted?0 beneath the southern margin. This possibility will be investigated. Bulk, modal and mineral analyses of xenoliths weighing at least several kg will be combined with isotopic studies at the Department of Terrestrial Magnetism. Determinations of the areal distribution and depths of origin of Kaapvaal ecligites are planned.