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首页> 外文期刊>Contributions to Mineralogy and Petrology >Mantle sources and magma evolution of the Rooiberg lavas, Bushveld Large Igneous Province, South Africa
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Mantle sources and magma evolution of the Rooiberg lavas, Bushveld Large Igneous Province, South Africa

机译：地幔来源和岩石山羊山羊的演变，南非Bulsveld大火植物

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摘要

We report a new whole-rock dataset of major and trace element abundances and Sr-87/Sr-86-Nd-143/Nd-144 isotope ratios for basaltic to rhyolitic lavas from the Rooiberg continental large igneous province (LIP). The formation of the Paleoproterozoic Rooiberg Group is contemporaneous with and spatially related to the layered intrusion of the Bushveld Complex, which stratigraphically separates the volcanic succession. Our new data confirm the presence of low- and high-Ti mafic and intermediate lavas (basaltic-andesitic compositions) with 4 wt% MgO, as well as evolved rocks (andesitic-rhyolitic compositions), characterized by MgO contents of 4 wt%. The high- and low-Ti basaltic lavas have different incompatible trace element ratios (e.g. (La/Sm)(N), Nb/Y and Ti/Y), indicating a different petrogenesis. MELTS modelling shows that the evolved lavas are formed by fractional crystallization from the mafic low-Ti lavas at low-to-moderate pressures ( 4 kbar). Primitive mantle-normalized trace element patterns of the Rooiberg rocks show an enrichment of large ion lithophile elements (LILE), rare-earth elements (REE) and pronounced negative anomalies of Nb, Ta, P, Ti and a positive Pb anomaly. Unaltered Rooiberg lavas have negative epsilon Nd-i (- 5.2 to - 9.4) and radiogenic epsilon Sr-i (6.6 to 105) ratios (at 2061 Ma). These data overlap with isotope and trace element compositions of purported parental melts to the Bushveld Complex, especially for the lower zone. We suggest that the Rooiberg suite originated from a source similar to the composition of the B1-magma suggested as parental to the Bushveld Lower Zone, or that the lavas represent eruptive successions of fractional crystallization products related to the ultramafic cumulates that were forming at depth. The Rooiberg magmas may have formed by 10-20% crustal assimilation by the fractionation of a very primitive mantle-derived melt within the upper crust of the Kaapvaal Craton. Alternatively, the magmas represent mixtures of melts from a primitive, sub-lithospheric mantle plume and an enriched sub-continental lithospheric mantle (SCLM) component with harzburgitic composition. Regardless of which of the two scenarios is invoked, the lavas of the Rooiberg Group show geochemical similarities to the Jurassic Karoo flood basalts, implying that the Archean lithosphere strongly affected both of these large-scale melting events.
机译：我们报告了主要和痕量元素丰富的全新岩石数据集和SR-87 / SR-86-ND-143 / ND-144同位素比从罗格伯格大陆大火省（嘴唇）玄武岩上玄武岩瓦斯。古典古代罗叶杆菌组的形成是与丛林型复合物的分层侵入的同时和空间相关，其层次地分离火山速度。我们的新数据确认存在低Ti和高Ti Mafic和中间熔岩（玄武岩和岩组合物）的存在。 4wt％MgO，以及进化的岩石（中塞氏血管晶组合物），其特征在于MgO含量的＆ 4 wt％。高Ti玄武岩熔岩具有不同的不相容痕量元素比（例如（La / sm）（n），nb / y和ti / y），表明不同的纤维化。熔融建模表明，进化的熔岩是通过在低至中等压力（4 kbar）的MAFIC Low-Ti熔岩的分数结晶而形成。原始地幔标准化的罗格伯格岩石的痕量元素图案显示大离子鳞片元素（含柠檬），稀土元素（REE）的富集，并发出Nb，Ta，P，Ti和阳性Pb异常的发音阴性异常。未改变的Rooiberg熔岩具有负εnd-1（ - 5.2至9.4），辐射εsl-1（6.6至105）比率（2061 mA）。这些数据与同位素和痕量的父母融化的同位素和痕量元素组成重叠，尤其是下部区域。我们建议，罗格贝格套件起源于类似于B1-Magma的组成的源，建议为父母对丛林市的下部区域，或者熔岩代表与在深度形成的超微累积相关的分数结晶产物的喷发次数。 Rooiberg Magmas可能通过在Kaapvaal Craton的上外壳内的一个非常原始的披风衍生熔体的分馏来形成10-20％的地壳同化。或者，岩浆代表来自原始，亚岩石型披风羽毛和富含亚欧式岩性岩石罩（SCLM）组分的熔体的混合物，其具有Harzburgitic组合物。无论援引两种情景中哪一个，Rooiberg集团的熔岩都显示了与侏罗纪卡罗洪水沼泽的地球化学相似，这意味着Archean Lithosphers强烈影响这两个大型熔化事件。

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