Origins of Continental Cratons: Evidence from Kaapvaal Xenoliths

大陆克拉通的起源：来自卡普瓦尔捕虏体的证据

• 批准号: 8708196
• 负责人: Francis Boyd
• 金额: $ 5.94万
• 依托单位: Carnegie Institution of Washington
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1987
• 资助国家: 美国
• 起止时间: 1987-09-01 至 1991-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=8708196&HistoricalAwards=false
• 关键词: Origins; Continental; Cratons; Evidence; Kaapvaal

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.

粗粒石榴石橄榄岩是主要的岩石类型 在南非卡普瓦尔克拉通， 与大洋preidotites不同的是， 镁和顽火辉石。 这种区别的根本在于， 这意味着火成过程的不同， 大陆和大洋岩石圈已经形成。 两 大陆和大洋橄榄岩似乎是残留物， 据推测，差异反映了更大的形成深度 克拉通橄榄岩 相反，高温 卡普瓦尔克拉通下面的橄榄岩 类似于大洋岩石圈，可能已经俯冲。 大陆矿物分布的不均匀性 几厘米尺度的橄榄岩很常见， 据信是由透辉石和石榴石的出溶作用引起的， 原生顽火辉石或褐变辉石。 散装变化研究 组成是火成的起源，并排除这些 亚固相线特征需要重量为0.5 - 1 kg. 该样本量要求限制了 想要的数据。 然而，其他大型捕虏体的收集， 很实用该项目将把这些研究扩展到 尖晶石橄榄岩的火成成因探讨 从克拉通内部岩石圈的顶部， 形成根部的低钙石榴石方辉橄榄岩。 高- 克拉通下面的温度橄榄岩出现在 初步数据显示，北方的土壤比北方的土壤更肥沃。 这些（subducted？0在南部边缘。 这种可能性 将进行调查。 散装，模态和矿物分析 至少重几公斤的捕虏体将与 地磁系的同位素研究。 1998年以来中国大陆海底沉积物的分布和成因深度的测定 Kaapvaal ecligites计划。