Character of the UG2 Chromitite and Host Rocks and Petrogenesis of Its Pegmatoidal Footwall, Northeastern Bushveld Complex
Character of the UG2 Chromitite and Host Rocks and Petrogenesis of Its Pegmatoidal Footwall, Northeastern Bushveld Complex
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东北布什维尔德杂岩UG2铬铁矿和围岩特征及其伟晶下盘岩石成因
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发表时间：
2005
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通讯作者：
J. Mey
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作者：
E. Mathez;J. Mey
UG2铬铁岩及其直接下盘已在Bushveld杂岩东北部Middelpunt矿的一个壁中暴露的特征剖面中以厘米尺度绘制。在该区域，UG2铬铁矿及其粗粒下盘岩石被斜长辉石岩下伏和上覆。UG 2铬铁矿本身是一个巨大的、70厘米厚的层，由75%至90%的铬铁矿组成，斜长石是主要的填隙矿物。紧邻铬铁矿下方是所谓的混合层，这是一个以前未描述但独特的可绘制层，厚度为50至70厘米，由含铬铁矿的伟晶辉石岩和铬铁矿组成，以厘米至分米的尺度密切混合。该混合物是混乱的，没有宏观结构，并含有约15%的铬铁矿散装。混合层和上覆铬铁矿之间的接触是明确的，但在厘米到分米的尺度上是弯曲的。根据对岩芯和露头的观察，混合层似乎存在于东部Bushveld的整个中部，并且可能在整个综合体中无处不在。混合层通常直接位于UG2伟晶岩上，UG2伟晶岩是一种众所周知的广泛分布的伟晶状斜长辉石岩。伟晶岩的粒度比下伏辉石岩粗得多，但两个岩性单元在矿物模式和成分方面相似。它们之间的接触在分米到米的尺度上是不规则的，伟晶状的垂直指状物延伸到辉石岩中。混合层和UG2伟晶岩类包含罕见的块状斜长岩透镜体，被连续的厘米厚铬铁矿边缘包裹。提出了一个模型来解释下盘序列的UG2铬铁矿。在这个模型中，UG2伟晶岩形成时，含水，间隙熔体通过部分熔融的晶体堆，积累下的预先存在的，压实，块状铬铁矿，交代和部分熔融的辉石岩原岩，冷却后重结晶成粗粒岩石。已发表的实验的基础上，说明了水对熔融镁铁质系统的影响，它示出了含水熔体是如何能够渗透的铬铁矿下的辉石轴承组合，但不能部分熔融铬铁矿本身，这在结果仍然相对不可渗透。伟晶岩中的斜长岩透镜体标志着水合程度相对较高的局部区域，斜长岩代表结晶残余物。混合层由UG 2铬铁矿底部逐渐塌陷到密度较低、相对流动的下盘中形成。混合层和UG2伟晶岩是UG2铬铁矿的特征，三个岩性单元应视为一个岩石学单元。
The UG2 chromitite and its immediate footwall have been mapped at the centimeter scale in a characteristic section exposed in a wall in the Middelpunt mine, northeast Bushveld Complex. In this region the UG2 chromitite and its coarse-grained footwall rocks are underlain and overlain by plagioclase pyroxenite. The UG2 chromitite itself is a massive, 70-cm-thick layer composed of 75 to 90 percent chromite with plagioclase as the dominant interstitial mineral. Immediately beneath the chromitite is the so-called mixed layer, a previously undescribed but distinct, mappable layer, 50 to 70 cm thick, consisting of chromite-bearing pegmatoidal pyroxenite and chromitite intimately mixed at scales of centimeters to decimeters. The mixture is chaotic with no megascopic fabric and contains in bulk about 15 percent chromite. The contact between the mixed layer and overlying chromitite is well defined but sinuous at scales of centimeters to decimeters. Based on observations of drill core and outcrop, the mixed layer appears to exist throughout the central sector of the eastern Bushveld and may be ubiquitous throughout the entire Complex. The mixed layer usually rests directly on the UG2 pegmatoid, which is a well known and widespread pegmatoidal plagioclase pyroxenite. The pegmatoid is much coarser grained than the underlying pyroxenite, but the two lithologic units are similar in terms of mineral modes and compositions. The contact between them is irregular at scales of decimeters to meters, with vertical fingers of pegmatoid extending into the pyroxenite. The mixed layer and UG2 pegmatoid contain rare lenses of massive anorthosite encased by continuous, centimeter-thick rims of chromitite. A model is proposed to account for the footwall sequence of the UG2 chromitite. In this model, the UG2 pegmatoid formed when hydrous, interstitial melt percolated up through the partially molten crystal pile, accumulated beneath the preexisting, compacted, massive chromitite, and metasomatized and partially melted the pyroxenite protolith, which upon cooling recrystallized to a coarse-grained rock. On the basis of published experiments illustrating the effect of water on melting in mafic systems, it is shown how the hydrous melt was able to penetrate the pyroxene-bearing assemblages below the chromitite but unable to partially melt the chromitite itself, which in consequence remained relatively impermeable. The anorthosite lenses in the pegmatoid mark local areas of relatively high degrees of hydration partial melting, with the anorthosite representing the crystalline residue. The mixed layer formed by gradual collapse of the base of the UG2 chromitite into the less dense, relatively fluid footwall. The mixed layer and UG2 pegmatoid are diagnostic of the UG2 chromitite, and the three lithologic units should be considered a single petrologic unit.