Abstract The volcanic Rooiberg Group represents the extrusive phase of the Bushveld Magmatic Province in South Africa, forming the roof as well as the floor of the mafic-ultramafic 2057 Ma Rustenburg Layered Suite. Typically, the volcanic rocks of the Rooiberg Group vary from mafic compositions in the oldest unit (the Dullstroom Formation) to felsic compositions in the younger overlying units (the Damwal, Kwaggasnek and Schrikkloof formations). The lower parts of the Dullstroom Formation (including a basal rhyolitic unit) occur beneath the Rustenburg Layered Suite (RLS) in the southeastern part of the Bushveld Province, whereas the remainder of the Rooiberg Group occurs above the RLS. In this study, petrographic descriptions, whole rock geochemistry and MELTS modeling are used to show that the Dullstroom rhyolite could have evolved from fractional crystallisation of a siliceous and magnesian mafic liquid such as the so-called B1 liquid, parental to the lower parts of the Rustenburg Layered Suite. Due to its unique position at the base of the otherwise andesitic to dacitic Dullstroom Formation, the focus of this contribution is on the Dullstroom rhyolite and a comparison thereof with the rhyolites of the upper formations. Consistent with previous studies, the new data generated in this study show that a clear distinction can be made between the rhyolite in the Dullstroom, and those of the Damwal, Kwaggasnek and Schrikkloof formations. The Dullstroom rhyolite exhibits higher MgO contents (1.41–1.87 wt%) compared to the distinctly ferroan rhyolites of the Damwal, Kwaggasnek and Schrikkloof formations (0.01–0.91 wt% MgO). Similarly, immobile trace elements such as Y and Nb range from 9.72 to 12.7 ppm and 4.43–4.53 ppm, respectively, for the Dullstroom rhyolite, and are significantly different to the upper rhyolites (Y - 12.6–87.2 ppm and Nb - 12.3–24.2 ppm) suggesting likely petrogenetic differences. MELTS modeling shows that the Dullstroom rhyolite could not have evolved from the same liquids that generated the rhyolites of the Damwal, Kwaggasnek and Schrikkloof formations. The modeling suggests that the Dullstroom rhyolite formed through ~20% assimilation of upper continental crustal rocks during fractional crystallisation of the B1 composition, and not from the low-Ti basaltic andesite, as previously proposed for the overlying rhyolites. The modeling aspects of this study provide evidence for different sources and melting-fractionation pathways throughout the evolution of the Bushveld Magmatic Province, consistent with characteristics recorded by the volcanic edifice of this large igneous province.

中文翻译：

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灌木丛大型火成岩省火山岩的不同熔体源区：来自古元古代鲁伊伯格群（南非）的 MELTS 模型的新观察结果

摘要 火山 Rooiberg Group 代表了南非布什维尔德岩浆省的喷出阶段，形成了镁铁-超镁铁 2057 Ma Rustenburg 分层套房的顶板和底板。通常，Rooiberg 群的火山岩从最古老单元（Dullstroom 组）的镁铁质成分到较年轻的上覆单元（Damwal、Kwaggasnek 和 Schrikkloof 地层）的长英质成分不等。Dullstroom 组的下部（包括一个基底流纹岩单元）出现在 Bushveld 省东南部的 Rustenburg 层状岩层 (RLS) 之下，而 Rooiberg 组的其余部分则出现在 RLS 之上。在这项研究中，岩相描述，全岩石地球化学和 MELTS 模型被用来表明，Dullstroom 流纹岩可能是从硅质和镁质基性液体（例如所谓的 B1 液体）的分步结晶演化而来的，它是 Rustenburg 层状套件下部的母体。由于其位于安山质至英安质 Dullstroom 地层底部的独特位置，本文的重点是 Dullstroom 流纹岩及其与上部地层流纹岩的比较。与之前的研究一致，本研究中产生的新数据表明，可以清楚地区分 Dullstroom 中的流纹岩与 Damwal、Kwaggasnek 和 Schrikkloof 地层的流纹岩。与 Damwal 的明显铁质流纹岩相比，Dullstroom 流纹岩表现出更高的 MgO 含量（1.41-1.87 wt%），Kwaggasnek 和 Schrikkloof 地层（0.01–0.91 wt% MgO）。同样，对于 Dullstroom 流纹岩，诸如 Y 和 Nb 等固定微量元素的范围分别为 9.72 至 12.7 ppm 和 4.43-4.53 ppm，并且与上部流纹岩（Y - 12.6-87.2 ppm 和 Nb - 12.3-24.2 ppm）有显着差异ppm) 表明可能的岩石成因差异。MELTS 模型表明，Dullstroom 流纹岩不可能从产生 Damwal、Kwaggasnek 和 Schrikkloof 地层流纹岩的相同液体演化而来。该模型表明，Dullstroom 流纹岩是在 B1 成分的分异结晶过程中通过约 20% 的上陆地壳岩石同化形成的，而不是像先前为上覆流纹岩提出的那样来自低钛玄武质安山岩。