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.

赫尔盖兰推覆体杂岩（Helgeland Nappe Complex, HNC）是挪威中北部加里东造山带最高外来岩席（Uppermost Allochthon）的组成部分，其起源于劳伦大陆边缘，在晚志留世—早泥盆世伊阿珀托斯洋（Iapetus）闭合过程中被拼贴至波罗的大陆（Baltica）。隶属于HNC绍伦—托加滕（Sauren–Torghatten, S–T）推覆体的勒岛（Rødøy）、博尔韦尔岛（Bolvær）与莱卡岛（Leka），其基底由超镁铁质岩和镁铁质岩构成，之上不整合覆盖有变质砾岩与细粒变质沉积岩。基底岩石的地球化学与同位素特征表明其形成于俯冲带之上构造环境。上覆变质沉积岩中的陆源碎屑占比随沉积时间逐渐升高，其碎屑物质来自前寒武纪克拉通物源区，砾石及其他碎屑均来源于此。该物源区可能位于现今的东南格陵兰/拉布拉多一带，或是HNC的下推覆体（Lower Nappe），其中后一种假说最能匹配粗粒砾岩所需的短搬运距离条件。沉积作用的最大沉积年龄由定年得到的最年轻锆石颗粒年龄约束为471±8 Ma。最终沉积、推覆冲断与推覆体堆叠作用在约480–475 Ma期间快速相继发生。