Sheared Peridotites from northern Lesotho: Metasomatism-induced Deformation and Craton Destabilization

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Catharina Heckel, Alan B Woodland, Jolien Linckens, Sally A Gibson, Hans-Michael Seitz
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引用次数: 0

Abstract

Sheared peridotite xenoliths are snapshots of deformation processes that occur in the cratonic mantle shortly before their entrainment by kimberlites. The process of deformation that caused the shearing has, however, been highly debated since the 1970s and remains uncertain. To investigate the processes involved in the deformation, we have studied twelve sheared peridotites from Late Cretaceous (90 Ma) kimberlites in northern Lesotho, on the southeast margin of the Kaapvaal craton. Various deformation textures are represented, ranging from porphyroclastic to fluidal mosaic. Our sample suite consists of eleven garnet peridotites, with various amounts of clinopyroxene, and one garnet-free spinel peridotite with a small amount of clinopyroxene. All of the peridotites are depleted in Fe, and the Mg# of olivine and orthopyroxene range from 91 – 94. Three groups of sheared peridotites are present and have been identified primarily on the basis of Ca contents of olivine and orthopyroxene. The porphyroclasts preserve pre-deformation P-T conditions of 3.5 – 4.5 GPa and 900 – 1100°C (Group I), 5 – 5.5 GPa and 1200 – 1250°C (Group II) and 6±0.5 GPa and 1400±50°C (Group III). Group III samples lie above the 40mW/m² conductive geothermal gradient, indicating thermal perturbation prior to deformation. The sheared peridotites from Lesotho were affected by various metasomatic events. Pre-deformation metasomatism, involving melts and fluids, is recorded in the porphyroclasts. In Group II and III samples the clinopyroxene porphyroclasts have similar compositions to Cr-rich and Cr-poor clinopyroxene megacrysts, respectively, that have previously described from southern African kimberlites. This suggests a relationship between them. Younger pre-deformation metasomatism is preserved in a zoned garnet from Group II (enrichment in Ti, Zr, Y+HREE) and orthopyroxene in a Group I sample. The latter exhibits a complex zonation, with a highly-enriched (Fe, Ti) inner rim and a less-enriched outer rim. These enrichments must have occurred shortly before deformation. Metasomatism during deformation is revealed by the complex chemical changes recorded in olivine neoblasts with, depending on the sample, increasing or decreasing contents of Ti, Ca, Al, Cr, Mn and Na. Crystallographic preferred orientations of olivine neoblasts are consistent with bimodal, B, C, E, AG-type fabrics and indicate the presence of a hydrous metasomatic agent. We suggest that, akin to the shallower sheared peridotites (Group I), Group II and III were influenced by early (proto-)kimberlite melt pulses and propose the following model: (Proto-)kimberlitic melts invaded the lower lithosphere. These melts followed narrow shear zone networks, produced by deformation at the lithosphere-asthenosphere-boundary, heated and metasomatized the surrounding peridotites and were responsible for megacryst crystallization. Sheared peridotites from close to the melt conduits (Group III) have compositions comparable to Cr-poor megacrysts, while those located at a greater distance (Group II) resemble Cr-rich megacrysts. Reactive infiltration of volatile-rich proto-kimberlite melts caused rheologically weakening of olivine in the lithospheric mantle. The consequence of this positive feedback mechanism of metasomatism, weakening and deformation -- due to the high magmatic and metasomatic activity in the Late Cretaceous -- is the progressive perforation of the lower Kaapvaal lithosphere by rheologically weak zones and the destruction of the protecting dry and depleted layer at its base. This could have caused the observed thinning and destabilization of the lower lithosphere below the southern margin of the Kaapvaal craton.
莱索托北部剪切橄榄岩:交代作用引起的变形和克拉通不稳定
剪切橄榄岩捕虏体是克拉通地幔在被金伯利岩夹带前不久发生的变形过程的快照。然而,自20世纪70年代以来,引起剪切的变形过程一直备受争议,至今仍不确定。为了研究变形过程,我们对莱索托北部Kaapvaal克拉通东南边缘晚白垩世(90 Ma)金伯利岩中的12块剪切橄榄岩进行了研究。不同的变形结构,从斑岩碎屑到流体镶嵌。我们的样品组包括11块石榴石橄榄岩,含有不同数量的斜辉石,和一块无石榴石尖晶石橄榄岩,含有少量的斜辉石。橄榄岩的铁元素均呈贫态,橄榄石和正辉石的mg#在91 ~ 94之间。主要根据橄榄石和正辉石的钙含量鉴定出三组剪切橄榄岩。斑岩碎屑保持变形前的P-T条件为3.5 ~ 4.5 GPa、900 ~ 1100℃(组I)、5 ~ 5.5 GPa、1200 ~ 1250℃(组II)和6±0.5 GPa、1400±50℃(组III)。组III样品位于40 mW/m2导电地温梯度以上,表明变形前存在热扰动。莱索托剪切橄榄岩受各种交代事件的影响。在斑岩碎屑中记录有熔体和流体的变形前交代作用。在第II组和第III组样品中,斜辉石卟啉碎屑分别具有与以前在南非金伯利岩中描述的富cr和贫cr斜辉石巨晶相似的成分。这表明他们之间存在某种关系。II组石榴石(富集Ti, Zr, Y + HREE)和I组正辉石中保存了较早的预变形交代作用。后者呈现出一个复杂的分带,具有高富集(Fe, Ti)的内缘和低富集的外缘。这些富集一定是在变形前不久发生的。变形过程中的交代作用通过橄榄石新生细胞中记录的复杂化学变化来揭示,根据样品的不同,钛、钙、铝、铬、锰和钠的含量会增加或减少。橄榄石新生细胞的结晶学取向与双峰型、B型、C型、E型、ag型织物一致,表明存在含水交代剂。我们认为,与浅层剪切橄榄岩(I组)类似,II组和III组也受到早期(原)金伯利岩熔体脉冲的影响,并提出了以下模型:(原)金伯利岩熔体侵入下岩石圈。这些熔体遵循岩石圈-软流圈边界变形产生的狭窄剪切带网络,加热和交代周围的橄榄岩,并负责巨晶的结晶。靠近熔体导管的剪切橄榄岩(III组)的成分与贫铬巨晶岩相当,而位于较远距离的剪切橄榄岩(II组)类似富铬巨晶岩。富挥发物原金伯利岩熔体的反应性渗透导致岩石圈地幔中橄榄石的流变弱化。由于晚白垩世岩浆和交代活动旺盛,这种交代、弱化和变形的正反馈机制导致了下Kaapvaal岩石圈被流变弱带逐渐穿孔,并破坏了其底部的保护干衰竭层。这可能导致观测到的Kaapvaal克拉通南缘下岩石圈变薄和不稳定。
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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