Tectonic reconstruction and sediment provenance of a far-travelled oceanic nappe, Helgeland Nappe Complex, west-central Norway

The Helgeland Nappe Complex (HNC), part of the Uppermost Allochthon of the north-central Norwegian Caledonides, originated near the Laurentian margin and was transferred to Baltica during the closure of Iapetus in Late Silurian–Early Devonian time. The islands of Rødøy, Bolvær and Leka, located in the Sauren–Torghatten (S–T) nappe of the HNC, are composed of ultramafic and mafic basement rocks unconformably overlain by metaconglomerates and fine-grained metasedimentary rocks. Geochemical and isotopic characteristics of the basement rocks are consistent with formation in a supra-subduction zone setting. Overlying metasedimentary rocks record an increasing proportion of continental detritus supplied to the basins through time. Precambrian cratonic source regions supplied cobbles and other detritus. This source area may have been located in modern SE Greenland/Labrador or in the Lower Nappe of the HNC. The second alternative best accounts for the short transport distances required by the coarse-grained conglomerates. The maximum age of deposition is constrained by the age of the youngest zircon grain dated at 471±8 Ma. Final sedimentation, nappe thrusting and nappe stacking occurred in rapid succession during <em>c.</em> 480–475 Ma.

海尔盖兰推覆体杂岩（Helgeland Nappe Complex, HNC）是挪威中北部加里东造山带（Caledonides）最高外来岩系（Uppermost Allochthon）的组成部分，其起源于劳伦大陆边缘（Laurentian margin），并在晚志留世至早泥盆世伊阿珀托斯洋（Iapetus）闭合时期迁移至波罗的大陆（Baltica）。坐落于HNC绍伦—托加滕（Sauren–Torghatten, S-T）推覆体中的勒岛（Rødøy）、博尔韦尔岛（Bolvær）与莱卡岛（Leka），其基底由超镁铁质与镁铁质岩石构成，其上不整合覆盖有变砾岩及细粒变沉积岩。基底岩的地球化学与同位素特征，与俯冲带之上构造环境（supra-subduction zone）下的形成过程相符。上覆变沉积岩记录了随地质时间推移，供给沉积盆地的大陆碎屑占比逐渐升高的演化特征。前寒武纪克拉通物源区为该套沉积岩提供了砾石及其他碎屑物质，该物源区可能位于现代东南格陵兰/拉布拉多区域，或是HNC下推覆体（Lower Nappe of the HNC）。第二种物源假说最能解释粗粒砾岩所需的短搬运距离条件。沉积作用的最大年龄由测年得到的最年轻锆石颗粒年龄（471±8 Ma）限定。最终沉积作用、推覆体逆冲与推覆体堆叠作用于约480–475 Ma期间快速相继发生。