The timing of successive fluid events in the Southern Marginal Zone of the Limpopo Complex, South Africa: Constraints from 40Ar–39Ar geochronology
The timing of successive fluid events in the Southern Marginal Zone of the Limpopo Complex, South Africa: Constraints from 40Ar–39Ar geochronology
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南非林波波杂岩南缘带连续流体事件发生的时间：来自 40Ar-39Ar 地质年代学的限制
DOI：
10.1016/j.precamres.2014.08.014
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发表时间：
2014
影响因子：
3.8
通讯作者：
M. Knoper
中科院分区：
文献类型：
--
作者：
G. Belyanin;J. Kramers;C. Vorster;M. Knoper
我们在南非林波波杂岩的高变质南缘带进行了地质年代学研究，旨在提供流体活动时期的时间限制。本工作主要包括角闪洞和云母的40ar /39Ar测年，以及伴随的岩石学，并进一步包括峰后变质闪长岩侵入的锆石U-Pb年代学。特别研究了侯特河剪切带（南边缘带位于Kaapvaal克拉通之上）北向30公里内的所谓复水化带，但采样还包括麻粒岩带更北的斑片状交代区域，在区域尺度上没有发生峰后变质复水化。基性麻粒岩中的角闪孔和特隆杰米岩中较老一代的锆石测年在2700 ~ 2730 Ma之间，与南缘区麻粒岩变质高峰的公布年龄相对应。复水化带角闪岩的40ar /39Ar测年表明，该事件的年龄在2660 ~ 2670 Ma之间，与之前对侯特河剪切带的研究结果一致。U-Pb锆石的主要种群可以追溯到特隆杰米特，确定其侵入的年代，也在这个年龄范围内，Matok岩体的公布年龄也是如此。这些数据表明，2660-2670 Ma的再水化是一个离散的事件。另一个40ar /39Ar年龄群，由角闪石和云母的年代确定，在2010-2040 Ma，这是林波波杂岩中央区一个主要的构造变质事件的年龄，以前在南缘带报道过Rb-Sr黑云母年代。我们在2010-2040 Ma区间发现了走滑剪切作用（由角闪洞构造确定年代）和局部钾交代作用（由角闪洞和云母确定年代）。后者并不局限于复水化带，在一个案例中似乎与金矿化有关。得到了一些介于2600 ~ 2000 Ma之间的40ar /39Ar日期，但这些日期是不同的，可以通过混合，不完全重置和/或过量的40ar来解释。它们并不表示发生在这个中间年龄段的事件。
We have carried out a geochronological study in the high grade metamorphic Southern Marginal Zone of the Limpopo Complex, South Africa, aimed at providing time constraints on episodes of fluid activity. This work chiefly comprises40Ar/39Ar dating, with accompanying petrography, of amphibole and mica, and further incudes zircon U–Pb geochronology on a post peak metamorphic trondhjemite intrusion. The so-called Zone of Rehydration within 30 km N of the Hout River Shear Zone (where the Southern Marginal Zone is thrust over the Kaapvaal Craton) was studied in particular, but sampling also included areas of patchy metasomatism in the Granulite Zone further north, where no post-peak metamorphic rehydration on a regional scale occurred.Amphibole in a mafic granulite and an older generation of zircons in the Petronella trondhjemite yielded dates in the 2700–2730 Ma range, corresponding to the published age of peak granulite metamorphism in the Southern Marginal Zone. A cluster of40Ar/39Ar dates on amphiboles from the Zone of Rehydration indicate an age between 2660 and 2670 Ma for this event, in accord with results from previous work on the Hout River Shear Zone. The main population of U–Pb zircon dates from the trondhjemite, dating its intrusion, also falls in this age range, as does the published age of the Matok pluton. These data suggest that rehydration at 2660–2670 Ma was a discrete episode. A further40Ar/39Ar age cluster, defined by dates on both amphiboles and micas, is seen at 2010–2040 Ma, which is the age of a major tectonometamorphic event in the Central Zone of the Limpopo Complex, and previously reported from the Southern Marginal Zone as Rb–Sr biotite dates. Events we found in the 2010–2040 Ma bracket are strike-slip shearing (dated by fabric forming amphibole), as well as localized potassic metasomatism (dated by both amphiboles and micas). The latter was not confined to the Zone of Rehydration, and appears in one case to be associated with gold mineralization. A number of40Ar/39Ar dates intermediate between 2600 and 2000 Ma were obtained, but these are diverse and can be accounted for by mixing, incomplete resetting and/or excess40Ar. They do not indicate events in this intermediate age bracket.