The Lower Main Zone in the Northern Limb of the Bushveld Complex—a >1·3 km Thick Sequence of Intruded and Variably Contaminated Crystal Mushes
The Lower Main Zone in the Northern Limb of the Bushveld Complex—a >1·3 km Thick Sequence of Intruded and Variably Contaminated Crystal Mushes
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DOI：
10.1093/petrology/egs022
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发表时间：
2012-07
影响因子：
3.9
通讯作者：
F. Roelofse;L. Ashwal
中科院分区：
文献类型：
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作者：
F. Roelofse;L. Ashwal
由1563.02 m深的Moordkopje（MO-1）钻孔覆盖的Bushveld杂岩北方边缘的下主带（LMZ）显示出非常小的大规模分异，如斜长石的An%、辉石的Mg#和超过1.3 km的垂直间隔的修正分异指数等参数所示。这些特征，再加上晚进入的褐变岩（现在倒转为斜方辉石），暗示在主带发展的早期阶段，岩浆反复侵入北方翼的岩浆房。重要的是，主带下部（LMZ）某些样品中共存的斜长石（Sri 0·708）和斜方辉石（Sri高达0·711）之间表现出显著的Sr同位素不平衡，这是主带以前没有描述过的特征。下主区的许多特征（例如，LMZ堆积体成分不适合斜长石和辉石的结晶;分层不发达;斜长石和辉石的非共生比例;斜长石和辉石在短的垂直间隔上的分异趋势的解耦）可以解释为，如果它不是作为反复流入的隐晶质岩浆侵入，而是由来自更深的、亚分隔的、地壳暂存室的晶体浆的反复侵入。提出的模型是一种替代以前的模型，建议从布什维尔德复杂，这是为了解释组成的悖论内的布什维尔德复杂的挤压组件的损失。
The lower Main Zone (LMZ) in the Northern Limb of the Bushveld Complex as intersected by the 1563·02 m deep Moordkopje (MO-1) drill hole shows very little large-scale differentiation as exemplified by parameters such as the An% of plagioclase, the Mg# of pyroxene and the modified differentiation index over a vertical interval in excess of 1·3 km. These features, coupled with the late entry of pigeonite (now inverted to orthopyroxene), are suggestive of the repeated intrusion of magma into the magma chamber of the Northern Limb during the early stages of the development of the Main Zone. Importantly, the lower Main Zone (LMZ) exhibits significant Sr isotopic disequilibrium between coexisting plagioclase (Sri 0·708) and orthopyroxene (Sri up to 0·711) in certain samples, a feature that has not previously been described for the Main Zone. Many of the features of the lower Main Zone (e.g. bulk compositions of LMZ cumulates unsuitable for the crystallization of plagioclase þ two pyroxenes; poorly developed layering; non-cotectic proportions of plagioclase and pyroxene; decoupling of the differentiation trends of plagioclase and pyroxene over short vertical intervals) in the Northern Limb may be explained if it was intruded not as repeated influxes of aphyric magma, but instead by the repeated intrusion of crystal mushes from a deeper, sub-compartmentalized, crustal staging chamber. The model proposed is an alternative to previous models suggesting the loss of extrusive components from the Bushveld Complex, which were developed to explain compositional paradoxes within the Bushveld Complex.