Pulses of Plagioclase-laden Magmas and Stratigraphic Evolution in the Upper Zone of the Bushveld Complex, South Africa
Pulses of Plagioclase-laden Magmas and Stratigraphic Evolution in the Upper Zone of the Bushveld Complex, South Africa
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DOI：
10.1093/petrology/egx067
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发表时间：
2017-08
影响因子：
3.9
通讯作者：
Q. Yuan;O. Namur;L. Fischer;R. Roberts;X. Lü;B. Charlier
中科院分区：
文献类型：
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作者：
Q. Yuan;O. Namur;L. Fischer;R. Roberts;X. Lü;B. Charlier
南非布什维尔德杂岩的上主带和上带(UUMZ)的堆积岩含有世界上主要的Fe-Ti-V±资源，赋存于钛磁铁矿和磷灰石中，通常被认为是从最后一次重大岩浆注入岩浆室中结晶出来的。在这项研究中，我们提供了岩石学、模式比例、全岩主要元素化学(2 60个样品)、电子探针数据(斜长石、橄榄石和辉石的∼10000分析)，以及对位于该杂岩西翼超过2.1公里的Bierkraal钻芯进行的磁铁矿中铬的汇编分析(2 39个样品)。UUMZ剖面呈现广泛的正向上分馏趋势，但斜长石中更原始的斜长石含量、辉石和橄榄石中的镁#含量、全岩中的铬含量和磁铁矿分离中的铬含量发生了一系列逆转，并伴随着各种矿物的出现或消失。斜长岩层或浅辉长岩层与这些反转密切相关；斜长岩中的反转被用作边界，将UUMZ划分为18个旋回。这些旋回被解释为岩浆室由斜长石岩浆(高达20卷)补充的迹象。%斜长石)，也以铬含量的尖峰为标志。此外，在UUMZ的下半部还发现了丰富的富含Fe-Ti氧化物的斜长岩。它们是由一批新的含有斜长石的岩浆和残留的岩浆混合而成的混合熔体结晶而成。这种混合液体的进一步结晶可能导致在UUMZ下部形成磁铁矿层。因此，布什维尔德UUMZ是由来自深部洞穴的富含晶体的岩浆多次侵位而形成的。浅室中的缓慢冷却解释了堆积堆积在单个周期内自下而上的系统成分演化。残余熔体在磷灰石饱和后不久即达到硅酸盐液体不混溶。此后，富Fe和富Si共轭熔体的分离和成对熔体的结晶产生堆积物，Fe-Ti氧化物平稳向上减少，而斜长石模式在每个含磷灰石旋回中增加。系统的地球化学分析和详细的岩性地层学对比表明，西部和东部上带之间可能存在连通，但与北翼的贝尔维尤段有显着差异。
Cumulate rocks of the Upper Main Zone and Upper Zone (UUMZ) of the Bushveld Complex, South Africa, contain the world's major resources of Fe-Ti-V ±, hosted in Ti-magnetite and apatite, and are commonly considered as having crystallized from the last major injection of magma into the magma chamber. In this study, we present the petrography, modal proportions, whole-rock major element chemistry (260 samples), electron microprobe data (∼10 000 analyses for plagioclase, olivine, and pyroxene), and compiled analyses of Cr in magnetite (239 samples) for the UUMZ sampled over 2.1 km of the Bierkraal drill cores in the western limb of the Complex. The UUMZ section exhibits a broad normal fractionation trend upwards, but a series of reversals to more primitive anorthite contents in plagioclase, Mg# in pyroxenes and olivine, Cr in whole-rocks and Cr in magnetite separates are observed, accompanied by the appearance or disappearance of various minerals. Anorthosite or leucogabbro layers are closely linked to these reversals; the reversals in An % of plagioclase are used as boundaries to divide the UUMZ into 18 cycles. These cycles are interpreted as indications of magma chamber replenishment by plagioclase-laden magmas (up to 20 vol. % plagioclase) and are also marked by spikes in Cr content. In addition, abundant Fe-Ti oxide-bearing plagioclase-rich rocks are identified in the lower half of the UUMZ. These have crystallized from a hybrid melt produced by the mixing of a new plagioclase-bearing magma batch and the resident magma. Further crystallization of this hybrid liquid may lead to the formation of magnetite layers in the lower part of the UUMZ. The Bushveld UUMZ therefore grew by multiple emplacements of crystal-laden magmas coming from deep-seated chambers. Slow cooling in a shallow chamber explains the systematic bottom-up compositional evolution in the cumulate pile within individual cycles. The residual melt reached silicate liquid immiscibility soon after the saturation of apatite. Thereafter, segregation of conjugate Fe-rich and Si-rich melts and crystallization of the paired melts produces cumulates with a smooth upward decrease in Fe-Ti oxides, whereas plagioclase mode increases in each apatite-bearing cycle. A comparison of systematic geochemical analyses and a detailed lithological stratigraphy between the Bushveld limbs demonstrates the possible connectivity between the western and eastern Upper Zone but indicates notable differences from the Bellevue section of the northern limb.