ABSTRACT

Two distinct NW- to WNW-trending mafic dike swarms of the previously dated ca. 2.21 Ga Anantapur-Kunigal and ca. 2.18 Ga Mahbubnagar-Dandeli swarms, emplaced in the Dharwar craton, are studied in greater petrological detail. Petrography classifies these as dolerite, olivine-dolerite, and meta-dolerite. The bulk rock geochemistry of 39 new and 11 published samples of both dike swarms are compared to those of three ca. 2.21 Ga and two ca. 2.18 Ga samples previously dated through baddeleyite U-Pb geochronology. All 50 samples are sub-alkaline tholeiite basalt to basaltic andesite in compositions and both swarms have similar differentiation/crystallization trends. Nonetheless, based on the slightly enriched REE patterns of ca. 2.21 Ga samples, as opposed to practically flat ca. 2.18 Ga REE patterns; we propose that 41 samples constitute the ca. 2.21 Ga Anantapur-Kunigal dike swarm and 9 samples belong to the ca. 2.18 Ga Mahbubnagar-Dandeli swarm. Petrogenetic modelling suggests that each swarm was derived through different degrees and proportions of partial melting of both an ambient asthenospheric and metasomatized sub-continental lithospheric mantle (SCLM) and further differentiated through similar fractional crystallization processes. Both mantle melts segregated within the garnet to garnet-spinel transition zone. Geodynamic conditions suggest that dikes of both swarms were emplaced in a similar tectonic system within a single LIP, which was likely triggered through the heating by the same (pulsating?) mantle plume. Such an indirect influence from a mantle plume, not recorded in geochemical signatures, is supported by ~1576°C potential mantle temperature (T_p) estimates for more magnesian parents within both swarms. Available geochronological data on mafic dikes and Palaeopositions of the Dharwar and North American cratons during ca. 2.21–2.18 Ga are well supportive of their link to the Superia supercraton reconstruction.

摘要

两个不同的 NW 到 WNW 趋势的镁铁质堤坝群，它们是先前约会的约。2.21 Ga Anantapur-Kunigal 和约。2.18 位于 Dharwar 克拉通的 Ga Mahbubnagar-Dandeli 群得到了更详细的岩石学研究。岩石学将这些分类为辉绿岩、橄榄石辉绿岩和偏辉绿岩。将两个堤坝群的 39 个新样本和 11 个已发表样本的大块岩石地球化学与三个约 2.21 Ga 和两个 ca。2.18 Ga 样品以前通过斜斜锆石 U-Pb 年代学确定年代。所有 50 个样品在成分上都是亚碱性拉斑玄武岩到玄武安山岩，两个群具有相似的分化/结晶趋势。尽管如此，基于约略富集的 REE 模式。2.21 Ga 样品，而不是几乎平坦的约。2.18 Ga REE 模式；我们建议 41 个样本构成 ca。2. 21 Ga Anantapur-Kunigal 堤防群和 9 个样本属于约。2.18 Ga Mahbubnagar-Dandeli 群。岩石成因模型表明，每个群都是通过周围软流圈和交代化的次大陆岩石圈地幔 (SCLM) 的不同程度和比例的部分熔融衍生出来的，并通过类似的分步结晶过程进一步分化。两种地幔熔体都在石榴石到石榴石-尖晶石过渡带内分离。地球动力学条件表明，两个群的堤防都位于单个 LIP 内的类似构造系统中，这很可能是由相同（脉动？）地幔柱加热触发的。这种来自地幔柱的间接影响（未记录在地球化学特征中）得到了约 1576°C 的潜在地幔温度 (T_p ) 估计两个群体中更多的镁质父母。大约在 20 年期间，关于 Dharwar 和北美克拉通的基性岩脉和古位置的可用地质年代学数据。2.21-2.18 Ga 非常支持他们与 Superia 超级克拉通重建的联系。