Metamorphic record of accretionary processes in the Neoarchaean

该项目与丹麦和格陵兰地质调查局合作，旨在调查西格陵兰岛南部努克地区新太古代地体增生过程的变质记录。该地区是研究太古宙中下陆壳的少数窗口之一，因此为研究早期陆壳的形成过程提供了一个理想的场所。Nuuk地区的地体合并作用与Færingehavn顶部的Tre Brødre和Tasiusarsuaq地体的广泛高变质作用和逆冲作用有关。本项目前期工作表明，这些地体构成了一个新太古代的成对变质带，其中(1)下板块（Færingehavn地体）含相对高压低温变质组合，(2)上板块（Tre Brødre和Tasiusarsuaq地体）以麻粒岩相条件为主，地表地热梯度明显升高。因此，这些资料支持了新太古代大陆地壳的形成是沿着与辐合有关的岩浆弧发生的观点。此外，现有的地质年代学和地球化学资料表明，上板块保留了一个持续约2.5亿年的与俯冲有关的长时间和幕式的火成岩活动记录，表明太古宙俯冲与反复的高温变质事件有关。尽管如此，地壳汇聚显然没有导致下板块的深俯冲，这意味着太古宙的汇聚速度比今天慢得多，或者说，俯冲是间歇性的，与更频繁的板块断裂或回滚有关。本项目以造山带腹地和造山带弧后区域为重点，进一步研究上板块的变质记录和热构造。这将涉及通过伪剖面建模和U-Pb测年重建不同地区的P-T-t-d记录。总体目标是更好地评估Nuuk地区的辐合样式，并澄清上板块的热结构是否符合间歇性俯冲模式。

In collaboration with the Geological Survey of Denmark and Greenland, this project aims to investigate the metamorphic record of the processes of Neoarchaean terrane accretion in the Nuuk region of southern West Greenland. The region is one of a few windows into Archaean mid- to lower continental crust, and therefore provides an ideal site to study the processes that formed the early continental crust. Terrane amalgamation in the Nuuk region was associated with extensive high-grade metamorphism and thrusting of the Tre Brødre and Tasiusarsuaq terranes on top of the Færingehavn. Preliminary work to this project indicates that these terranes constitute a Neoarchaean paired metamorphic belt, including (1) a lower plate (Færingehavn terrane) containing relatively high pressure low temperature metamorphic assemblages, and (2) an upper plate (Tre Brødre and Tasiusarsuaq terranes), dominated by granulite facies conditions along a distinctly hotter apparent geothermal gradient. Thus, the data support the view that the formation of Neoarchaean continental crust occurred along convergence-related magmatic arcs. Moreover, the available geochronology and geochemical data indicate that the upper plate preserves a prolonged and episodic record of subduction-related igneous activity that lasted for ca. 250 million years, suggesting that Archaean subduction was associated with repeated high temperature metamorphic events. Despite this, crustal convergence apparently did not result in the deep subduction of the lower plate, implying that Archaean convergence rates were much slower than today, or that subduction was intermittent and associated with a more frequent slab breakoff or rollback. This project aims to further investigate the metamorphic record and thermal structure of the upper plate, focusing on the orogen hinterland and back-arc region of the orogen. This will involve the reconstruction of the P-T-t-d record of different areas by means of pseudosection modelling and U-Pb dating. The overall goal is to better assess the style of convergence in the Nuuk region, and to clarify whether the thermal structure of the upper plate is consistent with a model of intermittent subduction.