The Beacon Heights “Tinguaite” (Phonolite): A recrystallised agpaitic ignimbrite in the Mesoproterozoic Pilanesberg alkaline complex, South Africa

IF 1.2 4区 地球科学 Q2 GEOLOGY
J. U. Akoh, M. Elburg, T. Andersen
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引用次数: 0

Abstract

An extrusive rock unit in the Mesoproterozoic Pilanesberg alkaline complex that has been referred to as the “Beacon Heights Tinguaite” since the work of Shand (1928) has been reexamined in the field and studied by petrographic microscopy, scanning electron microscopy, and whole-rock major and trace element analysis. The new observations indicate that the unit (here informally renamed the “Beacon Heights Phonolite” to comply with current petrographical nomenclature) is a laminated, volcanic rock in which mafic layers rich in sodic pyroxene alternate on any scale with discontinous, felsic layers consisting of alkali feldspar, nepheline, sodalite and their alteration products natrolite and analcime. Highly strontian apatite, fluorite, a manganiferous pectolite-group mineral, sphalerite and pyrrhotite are minor to accessory minerals. In addition, the rock carries indicator minerals typical of agpaitic rocks (including lamprophyllite and eudialyte-group minerals and their replacement products). The rock contains abundant crystal fragments of felsic minerals (alkali feldspar, nepheline and sodalite), fragments and elongated lenses of nepheline syenite, some of which contain primary magmatic lamprophyllite, and rare armoured lapilli. The overall structure of the rock is that of a welded ashflow tuff (igmimbrite) affected by post-magmatic recrystallisation processes. Whole-rock major and trace element compositions suggest a compositional affinity with members of the agpaitic, intrusive Green Foyaite Suite of the Pilanesberg Complex, and probably also a genetic relationship. The Beacon Heights Phonolite is a rare, possibly unique, example of an ashflow deposit of agpaitic, highly silica-undersaturated composition.
Beacon Heights“Tinguaite”(phonite):产于南非中元古代Pilanesberg碱性杂岩中的一种重结晶无灰质烟灰岩
自Shand(1928)工作以来,中元古代Pilanesberg碱性杂岩中的一个喷出岩单元被称为“Beacon Heights Tinguaite”,已在野外进行了重新检查,并通过岩相显微镜、扫描电子显微镜、全岩主元素和微量元素分析进行了研究。新的观测结果表明,该单元(此处非正式地更名为“Beacon Heights Phonolite”,以符合当前的岩石学命名法)是一种层状火山岩,其中富含钠辉石的镁铁质层与由碱长石、霞石、方钠石及其蚀变产物钠长石和方沸石组成的不连续长英质层在任何规模上交替。高锶磷灰石、萤石、一种含锰的果胶矿族矿物、闪锌矿和磁黄铁矿是次要的副矿物。此外,岩石中还含有典型的指示矿物(包括煌斑岩和透绿石群矿物及其替代品)。岩石中含有丰富的长英质矿物(碱长石、霞石和方钠石)的晶体碎片、霞石正长岩的碎片和细长透镜体,其中一些含有原生岩浆煌斑岩和罕见的装甲蓝柱石。岩石的整体结构是受岩浆后再结晶过程影响的焊接灰流凝灰岩(igmimbrite)。全岩主要元素和微量元素的组成表明,它们与Pilanesberg杂岩的银白色侵入性Green Foyaite岩组的成员具有组成亲和力,可能还有遗传关系。Beacon Heights Phonolite是一个罕见的,可能是独特的,由高度二氧化硅不饱和组成的灰流矿床。
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来源期刊
CiteScore
3.60
自引率
5.60%
发文量
18
审稿时长
>12 weeks
期刊介绍: The South African Journal of Geology publishes scientific papers, notes, stratigraphic descriptions and discussions in the broadly defined fields of geoscience that are related directly or indirectly to the geology of Africa. Contributions relevant to former supercontinental entities such as Gondwana and Rodinia are also welcome as are topical studies on any geoscience-related discipline. Review papers are welcome as long as they represent original, new syntheses. Special issues are also encouraged but terms for these must be negotiated with the Editors.
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