Remnant lenses of komatiitic dykes in Kambalda (Western Australia): Occurrences, textural variations, emplacement model, and implications for other komatiite provinces
Remnant lenses of komatiitic dykes in Kambalda (Western Australia): Occurrences, textural variations, emplacement model, and implications for other komatiite provinces
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坎巴尔达（西澳大利亚）科马提岩脉的遗迹透镜体：发生情况、结构变化、就位模型以及对其他科马提岩省份的影响
DOI：
10.1016/j.lithos.2019.05.027
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发表时间：
2019
期刊：
影响因子：
3.5
通讯作者：
Staude
中科院分区：
文献类型：
--
作者：
Staude
在澳大利亚西部的Kambalda（一个典型的含科马铁矿镍矿床），首次发现了科马铁矿岩脉的残余。岩脉由变质透镜组成，排列在三个平面和平行的表面上。它们由角砾岩、具有棘状和漏斗状结构的橄榄石和斜辉石堆积的分异透镜（现在全部由不同数量的透闪石、绿泥石和黑云母组成）和绿泥岩组成。火成岩结构（棘状和堆积状，骨铬铁矿，杏仁核）保存完好，它们与低碳含量（碳酸化在蚀变较多的科马长岩中很常见）的结合意味着等化学变质蚀变。接触体附近的科马提岩岩脉组成与坎巴尔达科马提岩相似（但MgO含量较低），岩脉中心呈斜辉石组成。这被解释为一个斜辉石堆积结晶从一个科马提岩玄武岩。此外，岩心全岩样的微量元素记录了长英质壳的污染。这意味着一个与堤坝相交的钻孔记录了污染到未污染的地幔在几厘米内融化的情况。与世界上所有科马地岩矿床相比，科马地岩岩脉的罕见出现可能是三个因素的结果-科马地岩熔体在其液质之上喷发，熔体的粘度非常低以及层流。在有障碍物的区域（例如，岩性变化），局部涡流会导致局部温度降低，从而导致液相结晶。寄主岩石中局部诱导的孔隙流体对流细胞可能加速温度的降低。然而，在岩脉的层流部分，只有在喷发停止时才能达到熔体的总流质。由于熔体压力的降低，断口闭合以适应压力的变化，在熔体达到液相线之前将熔体挤出。这样的堤坝没有留下它们以前存在的痕迹。这些流动方向变化的区域因此形成了科马铁矿透镜链。
Remnants of komatiitic dykes have been intersected for the first time at Kambalda (Western Australia), a classical locality for komatiite-hosted Ni deposits. The dykes consist of metamorphosed lenses, aligned on three planar and parallel surfaces. They are composed of brecciated komatiitic rocks, differentiated lenses with spinifex- and hopper-textured olivine and clinopyroxene cumulates (all now composed of variable amounts of tremolite, chlorite and biotite), and chloritite.Igneous textures (spinifex and cumulate, skeletal chromite, amygdales) are well preserved and their combination with a low carbon content (carbonation is common in more altered komatiites) implies an isochemical metamorphic alteration. The composition of the komatiite dyke near the contact is similar to the Kambalda Komatiite (but lower in MgO) and changes to a clinopyroxene composition in the dyke centre. This is interpreted to be a clinopyroxene cumulate crystallized from a komatiitic basalt. Furthermore, trace elements in whole rock samples from the centre of the dyke record contamination by felsic crust. This means one drill hole intersecting the dyke has a record of contaminated to uncontaminated mantle melts within a few centimetres.The rare occurrence of komatiite dykes compared to sills in all komatiite occurrences worldwide is probably a consequence of three factors – the komatiite melt erupting above its liquidus, the melt's very low viscosity and its laminar flow. In areas of obstacles (e.g., change in lithology), local eddies can cause a local temperature decrease which leads to crystallization of the liquidus phase. A decrease in temperature might be accelerated by locally induced convection cells of pore fluid in the host rock. In the laminar flowing part of the dykes, however, the overall liquidus of the melt is only reached when the eruption ceases. Due to decreasing melt pressure, the fracture closes to adjust to the changing pressure, squeezing the melt out before the liquidus is reached. Such dykes leave no trace of their former existence. These areas of changes in flow direction therefore form chains of komatiite lenses.
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