Discordant ultramafic pegmatoidal pipes in the Bushveld Complex
Discordant ultramafic pegmatoidal pipes in the Bushveld Complex
复制标题
Bushveld 杂岩中不一致的超镁铁质伟晶管
DOI：
10.1007/s004100000175
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发表时间：
2000
影响因子：
3.5
通讯作者：
F. Kruger
中科院分区：
文献类型：
--
作者：
R. Cawthorn;C. Harris;F. Kruger
不协调的超镁铁管切断了Bushveld杂岩的大部分层状层序。我们详细研究了一条管道，Tweefontein管道，它切断了布什维尔德综合体东部的关键区域，因为它在一个新的道路切割中暴露得很好。现场关系表明，这些管道是在层状岩石非常热且不可能发生脆性破坏的情况下放置的。在这个不协调的岩体中，移位的铬铁矿和斜长岩碎片的存在暗示了侵入岩浆而不是交代的侵位方式。管状斜长石的Sr同位素初始比值在0.7073 ~ 0.7079之间，与临界带的典型比值0.7055 ~ 0.7065、主带的典型比值> ~ 0.708形成鲜明对比。这些数据排除了管道的起源是邻近层状岩石的残余岩浆。管中辉石的组成和广泛的析出表明，它们的形成温度与层状序列本身的温度相当，并且与周围的层状岩石相比，它们经历了缓慢的冷却，因此它们都具有相似的闭合温度。白长石层的优先替换表明侵位温度超过斜长石-辉石共熔温度。管道内的斜长石和辉石（Δplag-px）样品的每毫升δ18O差值在0.4到1.0之间，平均为0.7（9对），而寄主岩石的δ18O差值在Δplag-px之间为0.4到0.6，再次与岩浆形成温度一致。管道和层状寄主岩石中斜长石和辉石的氧同位素比值具有可比性，并且排除了Bushveld杂岩底部变质沉积物在初级矿物学形成中的重要流体贡献。角闪石的存在，以及偶尔高于正常层状层序岩石的Δplag-px值，表明管道中的温度平衡较低，可能是流体的存在。在一些管道和宿主样品中，这两种矿物的绝对δ18O值较高，表明它们与后来的流体发生了反应。这些不一致的超镁铁质管道被认为是由岩浆群侵位形成的，这些岩浆群的sr同位素特征不同于产生Bushveld杂岩相邻层状岩石的岩浆群，但在时间上与主要侵入事件极为密切相关。围岩的溶蚀作用，而不是单纯的机械膨胀作用，为埋管提供了空间。然而，该管道可能最终成为与附近后期断层有关的低温热液流体的通道。
Abstract Discordant ultramafic pipes cut most of the layered sequence of the Bushveld Complex. We have studied one pipe in detail, the Tweefontein pipe, which cuts the Critical Zone, eastern Bushveld Complex, because it is well-exposed in a new road cutting. Field relations suggest that these pipes were emplaced while the layered rocks were extremely hot and incapable of brittle failure. The existence of displaced chromitite and anorthosite fragments in this discordant body is suggestive of an intrusive magmatic, rather than metasomatic, mode of emplacement. Initial Sr isotopic ratios of plagioclase from the pipe are in the range 0.7073 to 0.7079, which contrast with typical ratios of 0.7055 to 0.7065 for the Critical Zone, and >0.708 for Main Zone. These data preclude an origin for the pipe as residual magmas from the adjacent layered rocks. The compositions of, and extensive exsolution in, pyroxenes in the pipe indicate temperatures of formation comparable to those of the layered sequence itself, and that they underwent slow cooling comparable to the surrounding layered rocks, such that they both have similar closure temperatures. Preferential replacement of leuconoritic layers suggests a temperature of emplacement in excess of the plagioclase–pyroxene cotectic temperature. The per mil δ18O difference between plagioclase and pyroxene (Δplag–px) for samples from within the pipes ranges from 0.4 to 1.0, and averages 0.7 (for nine pairs), compared to Δplag–px of 0.4 to 0.6 for host rocks, again consistent with magmatic temperatures of formation. Oxygen isotope ratios for plagioclase and pyroxene in the pipes and layered host rocks are comparable, and preclude a significant fluid contribution from metamorphosed sediments in the floor of the Bushveld Complex in the formation of the primary mineralogy. The presence of hornblende, and occasional higher Δplag–px values than in the normal layered sequence rocks suggest lower temperature equilibration in the pipe, probably in the presence of a fluid. Higher absolute δ18O values for both minerals in a few of the pipe and host samples suggest reaction with a later fluid. These discordant ultramafic pipes are considered to form by emplacement of magma batches, which are Sr-isotopically distinct from those which produced the adjacent layered rocks of the Bushveld Complex, but were nevertheless extremely closely related in time to the main intrusive events. Dissolution of host rocks, rather than purely mechanical dilation, provided the space for pipe emplacement. However, the pipe may have acted ultimately as a channelway for low-temperature hydrothermal fluids related to later faulting in the immediate vicinity.