对南非Bushmanland矿区Black Mountain矿矿石年龄的新限制

IF 1.2 4区地球科学 Q2 GEOLOGY

South African Journal of Geology Pub Date : 2022-12-01 DOI:10.25131/sajg.125.0024

D. Cornell, A. Moses, T. Cawood, M. Richter

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引用次数: 2

摘要

Bushmanland矿区和Namaqua-Natal省的年代地层学一直存在争议，但最近的微束测年解决了几个问题。一个重要方面是沉积呼出矿石的精确年龄及其构造地层背景。已公布的碎屑锆石测年对矿石最大年龄的限制为1285±14Ma（n=4，Gamsberg矿石）、1215±18Ma（n=6，Wortel组）和暂定的1118±33Ma（n=3，黑山的Hotson组）。该矿石比不整合覆盖在其上的1 130±35 Ma Koeris组变质玄武岩更古老。侵入黑山矿床矿石的Aplite岩脉为该矿石提供了另一个潜在的最小年龄限制。通过离子探针U-Pb锆石定年，样品的定年时间为1 175±15 Ma。这表明细晶岩脉属于晚碰撞Springputs花岗岩组，包括1 163±11 Ma Achab和1 149±15 Ma Hoogoor片麻岩，其与矿石的场关系尚未建立。区域M2变质作用记录在1 027±9 Ma的细晶锆石边缘和1 030±6 Ma的独居石和Hotson组主岩片岩中的异长岩中。通过主岩片岩样品中的激光烧蚀ICPMS测年的碎屑锆石反映了一个主要的古元古代物源，主要年龄组为2003±17 Ma，次要年龄组为1847和2105 Ma。仅进行了16次分析，这可能是其他已发表数据集中未发现中元古代小物源成分的原因。黑山矿石的年龄现在限制在1 215±18 Ma和1 175±15 Ma之间。考虑到两个年龄较老的年轻岩浆岩（1 175和1 130 Ma），初步的1 118±33 Ma碎屑锆石最大年龄被证明是不可靠的。因此，SEDEX矿石形成于Namaqua地体约1 210 Ma组合期间或之前，以及约1 150 Ma同构造期Springputs Suite花岗质岩浆作用之前。

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New constraints on the age of ore at Black Mountain mine, Bushmanland Ore District, South Africa

The chronostratigraphy of the Bushmanland Ore District and the Namaqua-Natal Province has long been debated, but recent microbeam dating has resolved several issues. An important aspect is the precise age of the sedimentary-exhalative ores and their tectonostratigraphic context. Published constraints on the maximum age of the ores from detrital zircon dating are 1 285 ± 14 Ma (n=4, Gamsberg ore), 1 215 ± 18 Ma (n=6, Wortel Formation) and a tentative 1 118 ± 33 Ma (n=3, Hotson Formation at Black Mountain). The ore is older than the 1 130 ± 35 Ma Koeris Formation metabasalt which unconformably overlies it. Aplite dykes, which intrude the ore of the Black Mountain deposit, provide another potential minimum age constraint on the ore. A sample was dated at 1 175 ± 15 Ma by ion probe U-Pb zircon dating. This shows that the aplite dykes belong to the late-collisional Springputs Suite of granitoids which includes the 1 163 ± 11 Ma Achab and 1 149 ± 15 Ma Hoogoor Gneisses, for which the field relationship with the ores had not been established. The regional M2 metamorphism was recorded in aplite zircon rims at 1 027 ± 9 Ma and at 1 030 ± 6 Ma in monazite and xenotime in the Hotson Formation host rock schists. Detrital zircons, dated by Laser Ablation ICPMS in a host rock schist sample, reflect a dominant Palaeoproterozoic provenance with major age group at 2 003 ± 17 Ma and minor groups at 1 847 and 2 105 Ma. Only 16 analyses were made, which probably accounts for the absence of minor Mesoproterozoic provenance components found in other published datasets. The age of the Black Mountain ore is now constrained between 1 215 ± 18 Ma and 1 175 ± 15 Ma. The tentative 1 118 ± 33 Ma detrital zircon maximum age is shown to be unreliable in view of two younger magmatic rocks with older dates (1 175 and 1 130 Ma). The SEDEX ores thus formed during or just before the ~1 210 Ma assembly of Namaqua terranes and before the ~1 150 Ma syntectonic Springputs Suite granitoid magmatism.

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来源期刊

South African Journal of Geology 地学-地质学

CiteScore

3.60

自引率

5.60%

发文量

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.