Generation, mobilization and crystallization of impact-induced alkali-rich melts in granitic target rocks: Evidence from the Araguainha impact structure, central Brazil
Generation, mobilization and crystallization of impact-induced alkali-rich melts in granitic target rocks: Evidence from the Araguainha impact structure, central Brazil
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花岗岩目标岩中撞击引起的富碱熔体的生成、流动和结晶：来自巴西中部 Araguainha 撞击结构的证据
DOI：
10.1016/j.gca.2009.08.029
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发表时间：
2009
影响因子：
5
通讯作者：
I. McDonald
中科院分区：
文献类型：
--
作者：
R. Machado;C. Lana;G. Stevens;C. R. S. Filho;W. Reimold;I. McDonald
本文提供了重要的见解的产生，提取和结晶的碎屑负载的影响熔体岩石从Araguainha的影响结构，巴西中部。尽管Araguainha目标岩具有混合性质（包括2 km厚的沉积岩序列和下面的花岗岩基底），但暴露的熔体的特征是富含碱的花岗岩基质，嵌入仅来自目标花岗岩的矿物和岩石碎片。熔融岩以块状冲击熔融岩片的形式出现在被侵蚀的中央隆起构造上，并以熔融岩脉的形式出现在中央隆起核部的花岗岩中。大块岩石的主要和微量元素的数据（包括铂族元素）表明，前体熔体产生的本地，主要是由部分熔融的目标花岗岩，没有任何贡献的沉积序列或弹丸。熔体脉的密集网络形成孤立的，通过选择性熔融的斜长石和碱性长石在花岗岩目标。斜长石和碱性长石熔融离散和全等，产生域的熔体静脉，这密切匹配的化学计量这些矿物的基质。组成离散的初始熔体相迁移通过一个密集的网络微裂缝组装成更大的熔体静脉之前。熔体脉的冻结是相当快的，熔体成分淬火的形式中的碱长石和斜长石纹影的熔体脉的矩阵。与此相反，上覆的冲击熔融岩的特征是由钠长石、透长石、石英、黑云母和钠长石组成的花岗斑岩基质。在这种情况下，熔体成分似乎更移动的，并已完全混合，形成花岗岩母体熔体。我们将这些矿物的熔化与地震后超过晶石熔点的温度联系起来。
This paper provides important insights into the generation, extraction and crystallization of clast-laden impact melt rocks from the Araguainha impact structure, central Brazil. Despite the mixed nature of the Araguainha target rocks (comprising a 2km thick sequence of sedimentary rocks and underlying granitic basement), the exposed melt bodies are characterised by an alkali-rich granitic matrix embedding mineral and rock fragments derived only from the target granite. The melt rocks occur in the form of a massive impact melt sheet overlying the eroded central uplift structure, and as melt veins in the granite of the core of the central uplift. Bulk-rock major and trace element data (including platinum group elements) indicate that the precursor melts were generated locally, principally by partial melting of the target granite, without any contribution from the sedimentary sequence or the projectile. The dense network of melt veins was formed in isolation, by selective melting of plagioclase and alkali feldspar within the granite target. Plagioclase and alkali feldspar melted discretely and congruently, producing domains in the matrix of the melt veins, which closely match the stoichiometry of these minerals. The compositionally discrete initial melt phases migrated through a dense network of microfractures before being assembled into larger melt veins. Freezing of the melt veins was substantially fast, and the melt components were quenched in the form of alkali-feldspar and plagioclase schlieren in the matrix of the melt veins. The overlying impact melt rock is, in contrast, characterised by a granophyric matrix consisting of albite, sanidine, quartz, biotite and chlorite. In this case, melt components appear to have been more mobile and to have mixed completely to form a granitic parental melt. We relate the melting of the minerals to post-shock temperatures that exceeded the melting point of feldspars.