Origins of Continental Cratons: Evidence from Kaapvaal Xenoliths

粗大的石榴石橄榄岩是南部非洲卡阿瓦尔克拉通的主要岩石类型，其块状和模式成分与大洋橄榄岩的不同之处在于镁和顽辉石的富集度。这种区别是基本的，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不同。大陆橄榄岩和大洋橄榄岩似乎都是残留物，据推测，这种差异反映了克拉通橄榄岩形成的更深层次。相比之下，卡瓦尔克拉通下面的高温橄榄岩的成分类似于大洋岩石圈，可能已经被俯冲。大陆橄榄岩中矿物分布的不均一性在几厘米的尺度上是常见的，被认为是透辉石和石榴石从原生顽辉岩或辉石中出溶造成的。研究起源于火成岩并排除这些固相下特征的散装成分的变化需要0.5-1公斤的分析样品。这一样本量要求限制了所需数据的获取。然而，收集更多的大型捕虏体是可行的。该项目将把这些研究扩展到克拉通内岩石圈顶部的尖晶石橄榄岩和形成岩根的低钙石榴石方辉橄榄岩的火成岩起源的调查。从初步数据来看，克拉通下面的高温橄榄岩似乎比那些(俯冲到南缘以下)向北更富饶。这种可能性将得到调查。对重量至少几公斤的捕虏体进行散装、模式和矿物分析，将与地磁部门的同位素研究结合起来。计划确定卡阿普瓦榴辉岩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.