Mesoproterozoic rifting and Pan-African continental collision in SE India: evidence from the Khariar alkaline complex
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Keywords:
Carbonatite
Amphibole
East African Rift
Carbonatites 是通常与象 REE， Cu，和 P 那样的经济地珍贵的物质的累积有关。在到联系硅酸盐岩石的 carbonatites 和他们的关系的起源上的争论是进行中的大约 45 年了。全球，岩石表示特性地比沉积碳酸盐岩石在 Ba， Sr 和 REE 显示更多的 geochemical 丰富。然而，因为第二等的矿物质效果， carbonatites geochemical 特征被争论。从在在中国的 Qinling， Panxi 区域，和 Bayan Obo 的 carbonatites 的形成岩石的碳酸盐显示出从 LREE 丰富到扁平的模式的 REE 分发模式。他们被超过 10 比第二等的碳酸盐的高预定的一个 Sr 内容描绘。从 Bayan Obo H8 白云石云石的纹理粗糙、有细密纹理的白云石也显示出类似的高 Sr 丰富，显示他们具有火的起源。在中国 carbonatites 的一些碳酸盐显示出 REE (特别 HREE ) 整个岩石的类似于那些的内容和分布模式。这些恼人的 carbonatites 显示相对 high-Si 降低在磅和红外之间的铂组元素和更强壮的分别挤出的 carbonatite。这显示很恼人的 carbonatites 可以是碳酸盐堆积。在 Panxi 区域的 Maoniuping 和 Daluxiang 是大 REE 沉积物。热水的荧石矿石静脉发生在 carbonatite 身体外面并且是在 wallrock 的 emplaced 正长岩。在 Maoniuping 的荧石有类似于 carbonatite 的 Sr 和 Nd 同位素。Daluxiang 荧石在 Maoniuping 显示出与那些不同的 Sr 和 REE 作文。差别被 carbonatites 和形成岩石的碳酸盐反映，显示那个 REE 矿化作用与 carbonatites 有关。碳酸盐矿物质的 cumulate 进程从 LREE 由于为在碳酸盐和 carbonatite 之间的 REE 的系数融化的低分区和增加在 volatiles 和 LREE 使 fractionated 液体富有到 HREE。导出 carbonatite 的液体与 wallrock 交往了形成 REE 矿石静脉。发生在 Bayan Obo orebodies 附近的 carbonatite 女同性恋的数量可以支持一样的矿化作用模型，即液体从 carbonatite 女同性恋演变，这与 H8 白云石大理石反
Carbonatite
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The transition between the lithosphere and the asthenosphere is subject to numerous contemporary studies as its nature is still poorly understood. The thickest lithosphere is associated with old cratons and platforms and it has been shown that seismic investigations may fail to image the lithosphere‐asthenosphere boundary in these areas. Instead, several recent studies have proposed a mid‐lithospheric discontinuity of unknown origin existing under several cratons. In this study we investigate the Tanzania craton in East Africa which is enclosed by the eastern and western branches of the East African Rift System. We present evidence from S receiver functions for two consecutive discontinuities at depths of 50–100 km and 140–200 km, which correspond to significant S wave velocity reductions under the Tanzania craton and the Albert and Edward rift segments. By comparison with synthetic waveforms we show that the lower discontinuity coincides with the LAB exhibiting velocity reductions of 6–9%. The shallower interface reveals a velocity drop that varies from 12% beneath the craton to 24% below the Albert‐Edward rift. It is interpreted as an infiltration front marking the upper boundary of altered lithosphere due to ascending asthenospheric melts. This is corroborated by computing S velocity variations based on xenolith samples which exhibit a dense system of crystallized veins acting as pathways of the infiltrating melt. Mineral assemblages in these veins are rich in phlogopite and pyroxenite which can explain the reduced shear wave velocities. Melt infiltration represents a suitable mechanism to form a mid‐lithospheric discontinuity within cratonic lithosphere that is underlain by anomalously hot mantle.
East African Rift
Asthenosphere
Xenolith
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East African Rift
Indian Shield
Passive margin
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Carbonatite
East African Rift
Isotopic signature
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Mantle plume
East African Rift
Radiogenic nuclide
Snowball Earth
Volcanic Gases
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Carbonatite
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Carbonatites contain some of the highest concentrations of REE in the Earth's crust. Levels of hundreds of ppm REE are characteristic but concentrations can exceed 10 wt.%. Such rare-earth rich carbonatites are common minor components of carbonatite complexes. They are often ferrocarbonatites and, even if only minor components of a carbonatite complex, can still host the majority of the REE. When, more rarely, the rare–earth rich carbonatites occur in larger quantities they are of economic importance. Rare earth minerals form in the ...
Carbonatite
Rare-earth element
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By analysis of publisked and authors data the conclusion was made that primary chemical composition of carbonatites was predo minautly alkali-carbonate similar to ones of carbonatites and tuffs of Oldoinyo Lengai volcano (with high content of Na2CO3, K2CO3, CaCO3 and subordinately MgCO3, FeCO3). Such carbonatite melts reacted on country rocks (more often granitoids) and caused a fenitization of them while residual components СаСО3, MgCO3 and FeCO3 crystallizel as well known types of carbonatites (calcitic, dolomitic, ankeritic). Intrusive carbonatite complexes consist of such ultrabasic and alkaline racks as olivinites (rarely dunites), jacupiragitespyroxenites, melilitites, ijolites-melteigites, nepheline and alkaline syenites, phoscorites and carbonatites wich can be present in different combination (completely alone or two types of mentioned rocks). Silicate alkaline melts (of melilitite, melanephelinite types) related to carbonatite magmas were normatively and modally unsaturated with SiO2 and feldsparless. Basalt melts are not able to generate carbonatites. Carbonatites different from visually similar calciphyres and marbles by geochemical peculiarities and ore potential.
Carbonatite
Ultramafic rock
Nepheline syenite
Nepheline
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Abstract The mantle transition zone (MTZ) discontinuities beneath the Tanzania Craton and the Eastern and Western Branches of the East African Rift System are imaged by stacking over 7,100 receiver functions. The mean thickness of the MTZ beneath the Western Branch and Tanzania Craton is about 252 km, which is comparable to the global average and is inconsistent with the existence of present‐day thermal upwelling originating from the lower mantle. In contrast, beneath the Eastern Branch, an up to 30 km thinning of the MTZ is observed and is attributable to upwelling of higher temperature materials from either the upper MTZ or the lower mantle. The observations are in agreement with the hypothesis that rifting in Africa is primarily driven by gradients of gravitational potential energy and lateral variations of basal traction force along zones of significant changes of lithospheric thickness such as the edges of the Tanzania Craton.
East African Rift
Receiver function
Classification of discontinuities
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