Petrographic and geochemical evidence of calc-silicate xenolith – magma interaction in the western Bushveld Complex, South Africa

IF 1.2 4区 地球科学 Q2 GEOLOGY
B. Koovarjee, R. Gibson, P. Nex
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引用次数: 0

Abstract

Underground workings at Rowland Shaft, Lonmin Platinum, near Marikana have exposed a large (>7 x 5 m) calc-silicate xenolith in mottled anorthosite ~20 m below the UG 1 chromitite of the upper Critical Zone. The xenolith comprises a monticellite + forsterite + spinel assemblage that is consistent with a metamorphic Tmax of ≥975°C being reached at P ~1 kbar and XCO2 ~1. Internal variations in the modal proportions of these phases are interpreted as an artefact of sedimentary layering in the xenolith. The xenolith is flanked by a 20 to 30 cm wide magmatic skarn that contains several small, cm- to dm-scale, calc-silicate fragments displaying the same peak assemblage as the main xenolith, as well as a dm-scale chromitite autolith and mm- to dm-scale lenses and stringer-like masses of fine-grained uvarovite. The skarn matrix is mineralogically zoned, with an up to 10 cm wide clinopyroxenite layer proximal to the xenolith contact and calc-silicate fragments grading outwards into a gabbroic, plagioclase-clinopyroxene assemblage in which plagioclase becomes progressively more dominant distally. Notwithstanding its pseudo-ophitic texture, the distal skarn matrix displays a range of features consistent with crystallisation of magma contaminated by Ca from the decomposing xenolith under elevated fO2 conditions, including: (a) the unusual mineral compositions (zoned aluminian-ferrian diopside and anorthite, An>99), (b) the presence of calcite as inclusions in plagioclase and as a minor interstitial phase, (c) corroded wollastonite inclusions in clinopyroxene, and (d) thin grossular reaction rims between clinopyroxene and plagioclase. This chemical evolution is further substantiated by zoned spinel grains comprising Cr-rich cores and Al-enriched rims, which are, in turn, enclosed in garnet aggregates that grade outwards from uvarovite to grossular. Local development of grossular-vesuvianite symplectite indicates limited retrograde hydrous (XCO2 <0.1) fluid influx under subsolidus conditions (T <750°C). Bedding in the xenolith displays a steep westerly dip, approximately orthogonal to the magmatic layering. This, together with evidence in the skarn of both plastic shear strain and out-of-sequence magmatic autoliths, suggests that the xenolith underwent density-driven foundering within the RLS magma from an initial location above the UG 1 chromitite layer.
南非Bushveld杂岩西部钙硅酸盐捕虏体-岩浆相互作用的岩石学和地球化学证据
在Marikana附近的Lonmin Platinum的Rowland竖井的地下工作中,在上临界带ug1铬铁矿下方约20 m的斑驳斜长岩中暴露出一个巨大的(bb70 x 5 m)钙硅酸盐捕虏体。捕虏体由monticellite + forsterite +尖晶石组合组成,在P ~1 kbar和XCO2 ~1条件下达到≥975℃的变质Tmax。这些相的模态比例的内部变化被解释为捕虏体中沉积分层的人工产物。捕虏体两侧为20 - 30厘米宽的岩浆矽卡岩,其中包含若干厘米至厘米尺度的钙硅酸盐小块,与主捕虏体具有相同的峰组合,以及一个厘米尺度的铬铁矿自长体、毫米至厘米尺度的透镜体和细粒uvaroite的细条纹状团块。矽卡岩基质在矿物学上是分带的,在捕虏体接触点附近有一个宽达10厘米的斜辉石岩层,钙硅酸盐碎片向外分级为辉长岩、斜长石-斜辉石组合,其中斜长石在远端逐渐占主导地位。远端矽卡岩基质虽然具有伪蛇绿的结构,但其显示的一系列特征与fO2升高条件下分解捕虏岩中受Ca污染的岩浆结晶一致,包括:(a)不寻常的矿物组成(带状铝铁透辉石和钙长石,An bbb99), (b)方解石作为斜长石包裹体的存在,并作为一个小的间隙相,(c)斜辉石中腐蚀的硅灰石包裹体,(d)斜辉石和斜长石之间的细粗反应环。这种化学演化进一步得到了尖晶石颗粒的证实,尖晶石颗粒由富cr的核心和富al的边缘组成,而尖晶石颗粒又被包裹在石榴石聚集体中,从长云母向外分级到粗晶岩。粗硫-维苏岩共长岩的局部发育表明,在亚固体条件下(T <750℃),有少量逆行含水(XCO2 <0.1)流体流入。捕虏体层理呈陡峭的西风倾角,与岩浆层理大致正交。结合夕卡岩中塑性剪切应变和序外岩浆自岩体的证据,表明该包体从ug1铬铁矿层上方的初始位置开始,在RLS岩浆中经历了密度驱动的沉降过程。
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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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