Internal structure of the Paleoarchean Mt Edgar dome, Pilbara Craton, Western Australia
Internal structure of the Paleoarchean Mt Edgar dome, Pilbara Craton, Western Australia
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DOI：
10.1016/j.precamres.2021.106163
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发表时间：
2021-06
影响因子：
3.8
通讯作者：
N. Roberts;B. Tikoff
中科院分区：
文献类型：
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作者：
N. Roberts;B. Tikoff
西澳大利亚的古太古代东皮尔巴拉地体是一种圆顶和龙骨地体，经常被强调为记录了地球早期的垂直对流构造体制。在这个被称为“部分对流翻转”的模型中，花岗岩穹隆以顶端形式从一个密集的、下沉的镁铁质表壳层序中升起。部分对流翻转对东Pilbara地体和其他太古宙穹顶-龙骨地体的适用性是广泛争论的，并对早期地球动力学具有重要意义。东皮尔巴拉地体的一个关键数据缺口是花岗岩穹顶的内部结构。我们介绍了在埃德加穹顶内收集的基于场的、微结构和各向异性的磁化率(AMS)数据，以了解其内部结构，并评估其与现有穹顶形成模型的兼容性。野外和显微构造观察表明，大多数组构发育是在亚岩浆和高温固态条件下发生的。AMS的结果揭示了一种连贯的、整个穹顶范围的构造模式：1)次垂直线理径向向内向穹顶中心倾斜，横跨穹顶大部分地区的叶理持续走向西北；2)浅倾的线理定义了一个从穹顶中心延伸到西南边缘的拱形；以及3)代表穹顶最古老的花岗岩成分的混合岩片麻岩在两个不同的瓣片中折叠并压扁在穹顶的边缘。不同时代岩石之间的构造关系表明，不同结晶时代的单元在花岗岩浆作用的最后一次主脉动中同步变形。这些数据与垂直构造模型大体一致，我们综合我们的构造结果提出了埃德加穹隆的三个阶段底辟演化。穹隆发育的关键阶段是3.3-3.2Ga，当时地壳深部广泛的熔体辅助流动导致了陡壁复合穹顶的形成。这些数据表明，底辟过程对古太古代穹顶-龙骨地体的形成具有重要意义。
The Paleoarchean East Pilbara Terrane of Western Australia is a dome-and-keel terrane that is often highlighted as recording a vertically convective tectonic regime in the early Earth. In this model, termed 'partial convective overturn', granitic domes diapirically rose through a dense, foundering mafic supracrustal sequence. The applicability of partial convective overturn to the East Pilbara Terrane and to other Archean dome-and-keel terranes is widely debated and has significant implications for early Earth geodynamics. A critical data gap in the East Pilbara Terrane is the internal structure of the granitic domes. We present field-based, microstructural, and anisotropy of magnetic susceptibility (AMS) data collected within the Mt Edgar dome to understand its internal structure and assess its compatibility with existing dome formation models. Field and microstructural observations suggest that most fabric development occurred under submagmatic and high-temperature solid-state conditions. The AMS results reveal a coherent, dome-wide structural pattern: 1) Sub-vertical lineations plunge radially inward towards the center of the dome and foliations across much of the dome consistently strike northwest; 2) Shallowly plunging lineations define an arch that extends from the center of the dome to the southwest margin; and 3) Migmatitic gneisses, which represent the oldest granitic component of the dome, are folded and flattened against the margin of the dome in two distinct lobes. The structural relationships between rocks of different ages indicate that units of different crystallization ages deformed synchronously during the last major pulse of granitic magmatism. These data are broadly consistent with a vertical tectonics model, and we synthesize our structural results to propose a three-stage diapiric evolution of the Mt Edgar dome. The critical stage of dome development was between 3.3 and 3.2 Ga, when widespread, melt-assisted flow of the deep crust led to the formation of a steep-walled, composite dome. These data suggest that diapiric processes were important for the formation of dome-and-keel terranes in the Paleoarchean.