Petrogenesis of Archaean granites in the Barberton region of South Africa as a guide to early crustal evolution

IF 1.2 4区 地球科学 Q2 GEOLOGY
L. Robb, F. Meyer, C. Hawkesworth, N. Gardiner
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引用次数: 4

Abstract

The Barberton region of South Africa is characterized by a broad variety of granite types that range in age from ca. 3.5 Ga to 2.7 Ga and reflect the processes involved in the formation of Archaean continental crust on the Kaapvaal Craton. These granites are subdivided into three groups, as follows: A tonalite-trondhjemite-granodiorite (TTG) suite diapirically emplaced at 3 450 Ma and 3 250 Ma into pre-existing metamorphosed greenstone belt material. TTG melts were derived from melting amphibolite in the lower crust, with individual plutons being emplaced at various crustal levels. The dome-and-keel geometry that characterizes the TTG-greenstone dominated crust at this time is inconsistent with a plate tectonic domain and reworking was likely controlled by gravity inversion or ‘sagduction’; Regionally extensive potassic batholiths (the GMS suite) were emplaced at 3 110 Ma during a period of crustal thickening and melting of a TTG-dominated lower crust. Subsequent to emplacement of the voluminous GMS granites, the thickened continental crust had stabilized sufficiently for large sedimentary basins to form; Late granite plutons were emplaced along two distinct linear and sub-parallel arrays close to what might have been the edge of a Kaapvaal continent at 2 800 to 2 700 Ma. They are subdivided into high-Ca and low-Ca granites that resemble the I- and S-type granites of younger orogenic episodes. The high-Ca granites are consistent with derivation from older granitoids in the lower crust, whereas the low-Ca granites may have been derived by melting metasedimentary precursors in the lower-mid crust. Granites with similar characteristics are associated with a subduction zone in younger terranes, although the recognition of such a feature at Barberton remains unclear. The petrogenesis of granites in the Barberton region between 3.5 Ga and 2.7 Ga provides a record of the processes of Archaean crustal evolution and contributes to discussions related to the onset of plate tectonics.
南非Barberton地区太古宙花岗岩的岩石成因作为早期地壳演化的指南
南非Barberton地区以各种各样的花岗岩类型为特征,其年龄从约3.5 Ga到2.7 Ga不等,反映了在Kaapvaal火山口形成太古代大陆地壳的过程。这些花岗岩可细分为三组,如下:在3450 Ma和3250 Ma底辟侵位到预先存在的变质绿岩带物质中的英云闪长岩-花岗闪长岩(TTG)岩套。TTG熔体来源于下地壳中的角闪岩熔融,个别深成岩体侵位在不同的地壳水平。此时TTG绿岩主导地壳的圆顶和龙骨几何形状与板块构造域不一致,改造可能受到重力反转或“凹陷”的控制;区域广泛的钾岩基(GMS岩套)在3110 Ma时侵位,当时地壳增厚和TTG主导的下地壳熔融。在大量GMS花岗岩侵位之后,增厚的大陆地壳已经足够稳定,可以形成大型沉积盆地;晚花岗岩深成岩体沿两个不同的线性和亚平行阵列侵位,靠近2800至2700 Ma的Kaapvaal大陆边缘。它们被细分为高Ca和低Ca花岗岩,类似于较年轻造山期的I型和S型花岗岩。高钙花岗岩与下地壳中较老的花岗岩一致,而低钙花岗岩可能是由下中地壳中的变质沉积前体熔融而来。具有类似特征的花岗岩与较年轻地体中的俯冲带有关,尽管Barberton对这一特征的认识尚不清楚。Barberton地区3.5 Ga至2.7 Ga之间花岗岩的岩石成因记录了太古代地壳演化过程,并有助于讨论板块构造的起源。
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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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