Hafnium Isotopic Composition of the Bushveld Complex Requires Mantle Melt–Upper Crust Mixing: New Evidence from Zirconology of Mafic, Felsic and Metasedimentary Rocks
Hafnium Isotopic Composition of the Bushveld Complex Requires Mantle Melt–Upper Crust Mixing: New Evidence from Zirconology of Mafic, Felsic and Metasedimentary Rocks
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布什维尔德杂岩的铪同位素组成需要地幔熔化上地壳混合：来自镁铁质、长英质和变沉积岩锆石学的新证据
DOI：
10.1093/petrology/egaa004
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发表时间：
2019
影响因子：
3.9
通讯作者：
Gerdes
中科院分区：
文献类型：
--
作者：
Wilson;Gudelius;Gerdes
尽管经过几十年的密集研究，形成布什维尔德杂岩的岩浆的来源仍然存在很大争议。以前的地球化学-岩石学研究表明，强烈的地幔派生最终导致了经济价值极高的铂族元素和铬矿体。然而，地球化学也指出了一个重要的地壳成分的贡献，它可能是单一来源的，也可能是多种不同来源的组合来源。这包括在布什维尔德岩浆形成之前可能通过俯冲、岩浆上升过程中上下地壳的同化以及包围的德兰士瓦超群的沉积岩内岩浆室吸积过程中的混染，在布什维尔德岩浆形成之前被浓缩的次大陆岩石圈地幔。在这项研究中，将利用镁铁质岩石、长英质岩石和变质沉积岩中具有良好特征的锆石颗粒的Hf同位素数据，以及Zr-Hf块体岩石成分来评价这些不同类型的储集层的贡献。结果表明，镁铁质堆积岩中的岩浆锆石颗粒自下而上地堆积在岩浆岩中。9 km厚的勒斯滕堡层状岩套的εHf2·055 Ga值从−7·5到−10·2，与周围石英岩和变质岩中的变质锆石颗粒和过度生长以及花岗岩熔体、花岗岩和上罗伊堡火山岩中的变化基本相同。通过估算周围马格里斯堡组(εHf2·055 Ga=−6·2至−10·8，6个样品，最大沉积年龄为2080 Ma)和胡登贝克组(εHf2·055 Ga=−7·1至−8·9，3个样品，最大沉积年龄为2070 Ma)中碎屑锆石颗粒的平均Hf同位素组成，以及来自Silverton组的石榴石片岩的六点等时线(εHFT=−6·6 ± 0·7；年龄=2059·4 ± 2·7 Ma)，也获得了相同的值。此外，锆石形态、分带模式、Hf同位素数据和岩石学约束进一步表明，变质锆石是在5 5 0~90 0 °C的水溶液和(或)长英质熔体中沉淀出来的，Hf同位素组成在接触晕中的流体输运过程中变得均一。然而，数值模拟的结果表明，流体的渗入对RLS的Zr-Hf收支和Hf同位素组成的影响很小，这些参数主要受来自三个主要来源的熔体的混合控制：(1)软流圈地幔(>；20 %)；(2)富集次大陆岩石圈地幔(<；80 %)；(3)大量地壳同化(高达40 %)。模拟进一步表明，在B1(高镁安山岩)岩浆形成过程中，下卡瓦瓦克拉通地壳的同化作用很少(<；15 %)，而在B3(拉斑玄武岩)岩浆形成过程中，同化作用高达40 %。锆石εHFT在整个RLS地层中的微小变化是三个主要因素相互作用的结果：(1)相对较低的Zr-Hf含量的热源地幔岩浆的侵入，其εHf2·055 Gaof−8·5 ± 1·9与RLS周围的上地壳岩石相似：(2)高Zr-Hf含量的火山岩和变质沉积岩的显著同化作用；(3)锆石在70 0~90 0 °C的晚期岩浆结晶之前，在壳幔熔体和水流体之间的混合、混合和/或扩散交换。这项研究表明，结合锆石的块状岩石数据和具有良好特征的锆石颗粒的Hf同位素数据，为研究…提供了有力的工具
The origin of magmas that formed the Bushveld Complex remains highly debated in spite of many decades of intense research. Previous geochemical–petrological studies have shown a strong mantle derivation resulting ultimately in highly economic ore bodies of platinum group elements and chromium. However, geochemistry also points to the contribution of a significant crustal component, which may have been derived singly or in combination from a number of different sources. These include subcontinental lithospheric mantle that was enriched prior to Bushveld magma formation, possibly by subduction, assimilation of lower and upper crust during magma ascent, and contamination during magma chamber accretion within sedimentary rocks of the enclosing Transvaal Supergroup. In this study, the contributions of these various reservoirs will be evaluated by employing Hf isotopic data of well-characterized zircon grains in mafic, felsic and metasedimentary rocks, together with Zr–Hf bulk-rock compositions. The results reveal that magmatic zircon grains in mafic cumulate rocks from the floor to the roof of thec. 9 km thick Rustenburg Layered Suite (RLS) show essentially the same variations in εHf2·055 Gafrom −7·5 to −10·2 as those of metamorphic zircon grains and overgrowths in the immediate surrounding quartzite and metapelitic rocks, as well as in granitic melt batches, granophyres, and the upper Rooiberg volcanics. The same values are also obtained by estimating the average Hf isotopic compositions of detrital zircon grains in many quartzite and metapelitic rocks from the surrounding Magaliesberg (εHf2·055 Ga= −6·2 to −10·8, six samples, maximum deposition age at 2080 Ma) and Houtenbeck formations (εHf2·055 Ga= −7·1 to −8·9, three samples, maximum deposition age at 2070 Ma), and by a six-point isochron of a garnet-schist from the Silverton Formation (εHft= −6·6 ± 0·7; age = 2059·4 ± 2·7 Ma). Zircon morphologies, zoning patterns, Hf isotopic data and petrological constraints furthermore reveal that metamorphic zircon was precipitated from aqueous fluids and/or felsic melts at temperatures between 550 and 900 °C, and that the Hf isotopic composition became homogenized during fluid transport in the contact aureole. However, results of numerical modelling indicate that fluid infiltration had only a minor effect on the Zr–Hf budget and Hf isotopic composition of the RLS, and that these parameters were mainly controlled by the mixing of melts derived from three major sources: (1) the asthenospheric mantle (>20 %); (2) enriched subcontinental lithospheric mantle (<80 %); (3) assimilation of significant amounts of crust (up to 40 %). The modelling furthermore suggests that assimilation of lower Kaapvaal Craton crust was minor (<15 %) during B1 (high-Mg andesite) magma formation, but up to 40 % during B3 (tholeiite) magma formation. The minor variation in εHftof zircon throughout the entire stratigraphy of the RLS resulted from the interplay of three dominant contributing factors: (1) intrusion of hot (>1200 °C) mantle-derived magmas with relatively low Zr–Hf concentrations having a similar εHf2·055 Gaof −8·5 ± 1·9 to that of upper crust rocks surrounding the RLS; (2) significant assimilation of volcanic and metasedimentary rocks with high Zr–Hf concentration; (3) mingling, mixing and/or diffusive exchange of Zr and Hf between crust and mantle-derived melts and aqueous fluids prior to late-magmatic crystallization of zircon at temperatures between 700 and 900 °C. This study shows that the combination of Zr–Hf bulk-rock data with Hf isotopic data of well-characterized zircon grains provides a powerful tool to …
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影响因子：
3.5
作者：
BARNES, SJ
通讯作者：
BARNES, SJ
DOI：
10.1007/978-94-017-9652-1_1
发表时间：
2015
期刊：
--
影响因子：
--
作者：
J. Scoates;C. Wall
通讯作者：
J. Scoates;C. Wall
南非共和国、西南非洲/纳米比亚以及博普塔茨瓦纳共和国、特兰斯凯共和国和文达共和国的岩石地层学
DOI：
--
发表时间：
1980
期刊：
影响因子：
--
作者：
L. Kent
通讯作者：
L. Kent
Bushveld 复合体热光环中的熔体生成和流体流动
影响因子：
3.9
作者：
N. Harris;Andy McMillan;M. Holness;R. Uken;M. Watkeys;N. Rogers;A. Fallick
通讯作者：
A. Fallick
南非布什维尔德大火成岩省 Rooiberg 熔岩的地幔来源和岩浆演化
影响因子：
3.5
作者：
Günther;Teschner
通讯作者：
Teschner