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Two series of Ediacaran collision-related granites in the Guéra Massif, South-Central Chad: tectonomagmatic constraints on the terminal collision of the eastern Central African Orogenic Belt
Precambrian Research ( IF 3.2 ) Pub Date : 2020-09-01 , DOI: 10.1016/j.precamres.2020.105823
J. Gregory Shellnutt , Ngoc Ha T. Pham , Meng-Wan Yeh , Tung-Yi Lee

Abstract The Guera Massif of South-Central Chad is one of the least studied regions of the Saharan Metacraton. The oldest known rocks were emplaced during the Ediacaran and have geochemical characteristics of collision-related granites. The granites can be subdivided into two series on the basis of their compositions: 1) ferroan and alkalic to alkali calcic, and 2) magnesian and alkali calcic to calc-alkalic. The ferroan rocks (589 ± 6 Ma, 589 ± 6 Ma, 593 ± 7 Ma, 590 ± 8 Ma) tend to be younger than the magnesian rocks (595 ± 8 Ma) indicating there was a secular compositional change. The chemical variability within each series can be explained by fractional crystallization, but the ferroan series requires lower water content (~1 wt%) and relatively reducing conditions (ΔFMQ −1) compared to the magnesian series (H2O = ~2 wt%; ΔFMQ + 0.7). The older magnesian series is isotopically chondritic to moderately enriched (ISr = 0.7037 to 0.7073; eNd(t) = −3.2 to + 0.2) and was likely derived from a subduction-modified mantle source at an ‘Andean-type’ margin. In contrast, the ferroan series is isotopically enriched (ISr = 0.7048 to 0.7176; eNd(t) = −9.4 to −14.1) and has a significant amount of inherited Mesoproterozoic zircons (206Pb/238U = 1039 ± 24 Ma), indicating that it was likely derived by partial melting of an older crustal source. The changing nature of the granites likely records the transition from an active margin to a collisional zone between the Congo- Sao Francisco Craton and the Saharan Metacraton.

中文翻译:

乍得中南部 Guéra 地块中与埃迪卡拉纪碰撞相关的两组花岗岩:构造岩浆对中非造山带东部东部碰撞的限制

摘要 乍得中南部的 Guera 地块是撒哈拉 Metacraton 研究最少的地区之一。已知最古老的岩石是在埃迪卡拉纪时期形成的,具有与碰撞相关的花岗岩的地球化学特征。花岗岩根据其组成可细分为两个系列:1) 铁质和碱性至碱性钙质,以及 2) 镁质和碱性钙质至碱性钙质。铁质岩(589±6 Ma、589±6 Ma、593±7 Ma、590±8 Ma)比镁质岩(595±8 Ma)更年轻,表明存在长期成分变化。每个系列内的化学变异性可以通过分步结晶来解释,但与镁系列(H2O = ~2 wt%;ΔFMQ)相比,铁质系列需要较低的水含量(~1 wt%)和相对还原的条件(ΔFMQ -1) + 0.7)。较旧的镁质系列是同位素球粒状到中等富集(ISr = 0.7037 至 0.7073;eNd(t) = -3.2 至 + 0.2),并且可能来自“安第斯型”边缘的俯冲改造地幔源。相比之下,铁质系列是同位素富集的(ISr = 0.7048 至 0.7176;eNd(t) = -9.4 至 -14.1)并且具有大量继承的中元古代锆石(206Pb/238U = 1039 ± 24 Ma),表明它很可能是由较旧的地壳源的部分熔融产生的。花岗岩的变化性质可能记录了刚果-圣弗朗西斯科克拉通和撒哈拉梅克拉通之间从活动边缘到碰撞带的转变。2) 并且很可能来自“安第斯型”边缘的俯冲改造地幔源。相比之下,铁质系列是同位素富集的(ISr = 0.7048 至 0.7176;eNd(t) = -9.4 至 -14.1)并且具有大量继承的中元古代锆石(206Pb/238U = 1039 ± 24 Ma),表明它很可能是由较旧的地壳源的部分熔融产生的。花岗岩的变化性质可能记录了刚果-圣弗朗西斯科克拉通和撒哈拉梅克拉通之间从活动边缘到碰撞带的转变。2) 并且很可能来自“安第斯型”边缘的俯冲改造地幔源。相比之下,铁质系列是同位素富集的(ISr = 0.7048 至 0.7176;eNd(t) = -9.4 至 -14.1)并且具有大量继承的中元古代锆石(206Pb/238U = 1039 ± 24 Ma),表明它很可能是由较旧的地壳源的部分熔融产生的。花岗岩的变化性质可能记录了刚果-圣弗朗西斯科克拉通和撒哈拉梅克拉通之间从活动边缘到碰撞带的转变。表明它很可能是由较旧的地壳源的部分熔融产生的。花岗岩的变化性质可能记录了刚果-圣弗朗西斯科克拉通和撒哈拉梅克拉通之间从活动边缘到碰撞带的转变。表明它很可能是由较旧的地壳源的部分熔融产生的。花岗岩的变化性质可能记录了刚果-圣弗朗西斯科克拉通和撒哈拉梅克拉通之间从活动边缘到碰撞带的转变。
更新日期:2020-09-01
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