Contrasting Garnet Lherzolite Xenolith Suites from the Letšeng Kimberlite Pipes: Inferences for the Northern Lesotho Geotherm
Contrasting Garnet Lherzolite Xenolith Suites from the Letšeng Kimberlite Pipes: Inferences for the Northern Lesotho Geotherm
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莱森金伯利岩管中对比石榴石辉橄榄石包体套件：对莱索托北部地温的推论
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发表时间：
2013
期刊：
影响因子：
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通讯作者：
J. Dawson
中科院分区：
文献类型：
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作者：
N. P. Lock;J. Dawson
莱索托Letseng-la-Terae的主管金伯利岩和卫星管金伯利岩都含有丰富的地壳和地幔包体。在本文的主题石榴石-辉橄榄岩捕虏体中，两个管道样品的纹理从粗糙到高度变形不等，但最极端的变形仅在主管道样品中可见。所有样品中石榴石周围的反应日冕的发育表明，在寄主金伯利岩夹带之前，尖晶石橄榄岩相条件已经倒退。相对于原始的地幔成分，大多数样品的大块岩石成分已经枯竭。大多数主管道样品在950 - 1000°C和40-45 Kbar的压力下平衡，其他样品的温度范围扩展到820°C和1350°C。在950 - 1000°C的温度范围内，与1305°C平衡的卫星管捕虏体没有重叠。来自两个管道的高t样品位于Rudnick和Nyblade（1999）的白垩纪卡拉哈里地热的高t侧，表明存在扰动地热，与从Thaba Putsoa包体推断的地热相似，但与从其他白垩纪莱索托金伯利岩的地幔包体推断的非扰动地热不同。因此，来自单个金伯利岩侵入体的捕虏岩套件各自具有独特的热特征，莱索托地下没有持续的白垩纪地热梯度扰动。
The Main and Satellite Pipe kimberlites at Letseng-la-Terae, Lesotho, both contain abundant crustal and mantle xenoliths. In garnet lherzolite xenoliths that are the subject of this paper, textures range from coarse to highly deformed in samples from both pipes, but the most extreme deformation is seen only in Main Pipe samples. Development of reaction coronas around the garnets in all samples indicates retrogression to spinel peridotite facies conditions prior to entrainment in the host kimberlites. Bulk rock compositions of most samples are depleted relative to pristine mantle compositions. Most Main Pipe samples equilibrated at 950–1,000 °C and pressures of 40–45 Kbar, with others extending the temperature range to 820 °C and 1,350 °C. In the temperature range 950–1,000 °C, there is no overlap with Satellite Pipe xenoliths which equilibrated at 1,305 °C. High-T samples from both pipes lie on the high-T side of the Cretaceous Kalahari geotherm of Rudnick and Nyblade (1999), indicating a perturbed geotherm, similar to that deduced from Thaba Putsoa xenoliths but unlike the non-perturbed geotherm inferred from mantle xenoliths in other Cretaceous Lesotho kimberlites. Hence, xenolith suites from individual kimberlite intrusions each have distinctive thermal characteristics, and there is no consistently perturbed Cretaceous geothermal gradient beneath Lesotho.