The distribution and main geologic features of discordant bodies of iron-rich ultramafic pegmatite in the Bushveld Complex
The distribution and main geologic features of discordant bodies of iron-rich ultramafic pegmatite in the Bushveld Complex
复制标题
Bushveld杂岩富铁超镁铁质伟晶岩不和谐体的分布及主要地质特征
DOI：
10.2113/gsecongeo.80.4.1109
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发表时间：
1985
期刊：
影响因子：
5.8
通讯作者：
R. N. Scoon
中科院分区：
文献类型：
--
作者：
M. Viljoen;R. N. Scoon
富铁超镁铁质伟晶岩的海侵体，主要由不同比例的hortonite，单斜辉石，钛铁矿和钛磁铁矿组成，大量分布在整个布什维尔德复杂的层状镁铁质序列。新的信息，从外地和地下研究，连同解释详细的磁力调查，提出了沿着与现有文献的概述。一些全岩和矿物化学数据也包括在内。在布什维尔德复杂的一些不同的，但可能在成因上相关的组后积云超镁铁岩被确认。除了主要的富铁超镁铁质伟晶岩群外，还包括镁纯橄榄岩体和著名的含铂超镁铁质（或纯橄榄岩）管。海侵，往往管状机构组成的几乎完全的铁，钛氧化物发生在复杂的主要和上部区域，虽然富铁超镁铁伟晶岩套件的一部分，单独描述。复合体中，大量的铁钛氧化物作为一个核心内的富硅酸盐伟晶岩的外围壳也被确认。富铁超镁铁质伟晶岩的主要产状在一系列的地图中被记录和描述。它们的分布受两个主要的地层控制。富铁超镁铁伟晶岩主要产于下部和下部临界带的超镁铁堆晶岩之上。在斜长石是主要积云相的上临界区和主带中含量丰富，但在上带最高积云Ti磁铁矿层上方很少观察到。同样明显的是，这些岩石在层状序列的顶部变得越来越富铁，因此在上部区域，Fe-Ti氧化伟晶岩是主要的种类。许多岩体，特别是大型岩体，受构造控制，并出现在以断层和后布什费尔德岩脉为特征的扰动区。杂岩中的地层磁性异常可能与含有小斑点的富铁铁镁硅酸盐和Fe-Ti氧化物的斜长岩层有关。这种富含铁的物质被认为是一个循环单元内分离结晶作用产生的累积和捕获的晚期残余和堆间流体。这些伟晶岩流体的侧向迁移和通道进入构造薄弱的地区，导致形成大型的富铁超镁铁质伟晶岩管状体。这些岩体可能是先存堆积岩交代交代的结果，也可能是伟晶岩流体直接结晶的结果。认为伟晶岩流体来源于结晶堆积岩堆，富铁超镁铁伟晶岩的形成是层状层序的一个整体特征。
Transgressive bodies of iron-rich ultramafic pegmatite, composed predominantly of varying proportions of hortonolite, clinopyroxene, ilmenite, and Ti magnetite are abundantly distributed throughout the layered mafic sequence of the Bushveld Complex. New information from field and underground studies, together with interpretations of detailed magnetic surveys, is presented along with an overview of the available literature. Some whole-rock and mineralogical-chemical data are also included.Within the Bushveld Complex a number of distinct but possibly genetically related groups of postcumulus ultramafic rock are recognized. These include, besides the main group of iron-rich ultramafic pegmatite, bodies of magnesian dunite and the well-known platiniferous ultramafic (or dunitic) pipes. Transgressive, often pipelike bodies composed almost entirely of Fe-Ti oxides occur in the main and upper zones of the complex which, although part of the iron-rich ultramafic pegmatite suite, are described separately. Composite bodies in which massive Fe-Ti oxides occur as a core within a peripheral shell of silicate-rich pegmatite are also recognized. The major occurrences of iron-rich ultramafic pegmatite are documented and depicted in a series of maps. Their distribution is subject to two major stratigraphic controls. Iron-rich ultramafic pegmatite occurs largely above the ultramafic cumulates in the lower and lower critical zones. It is abundant in the upper critical and main zones where plagioclase is a major cumulus phase but is rarely observed above the highest cumulus Ti magnetite layer in the upper zone. It is also evident that these rocks become increasingly iron-rich toward the top of the layered sequence, so that in the upper zone, Fe-Ti oxide pegmatite is the dominant variety. Many bodies, particularly the large ones, are structurally controlled and occur in disturbed areas characterized by faulting and post-Bushveld dikes.Stratigraphic magnetic anomalies in the complex may be related to anorthositic layers containing small specks of iron-rich ferromagnesian silicates and Fe-Ti oxides. This iron-rich material is considered to represent the accumulated and trapped, late-stage, residual and intercumulus fluid resulting from fractional crystallization within a cyclic unit. Lateral migration and channeling of these pegmatitic fluids into areas of structural weakness resulted in the formation of large, pipelike bodies of iron-rich ultramafic pegmatite. These bodies may have resulted from metasomatic replacement of preexisting cumulates or they may have crystallized directly from the pegmatitic fluids. The authors believe that the pegmatitic fluids have been derived from within the crystallizing cumulate pile and it is concluded that the formation of iron-rich ultramafic pegmatite is an integral feature of the layered sequence.