Timing of deformation, metamorphism and leucogranite intrusion in the lower part of the Seve Nappe Complex in central Jämtland, Swedish Caledonides

Recent studies in the context of the International Continental Scientific Drilling Project “Collisional Orogeny in the Scandinavian Caledonides” have focused on the importance of the Seve Nappe Complex (SNC) for understanding the subduction history of the Baltoscandian margin during closure of the Iapetus Ocean. In the classical Åre area of western central Jämtland, granulite facies migmatites and leucogranites of the Åreskutan Nappe provide evidence of Early Silurian (c. 440 Ma) high temperature metamorphism and a previous prograde, ultra-high pressure history, with microdiamonds. New LA-ICPMS zircon isotope age investigations of the underlying amphibolite facies Lower Seve Nappes, reported here, have also identified an Early Silurian tectonothermal history with pegmatitic leucogranite (c. 443 Ma) and, at lower structural levels, another felsic intrusion of earliest Middle Ordovician age (c. 469 Ma). The latter intrudes isoclinally folded host rock amphibolites and calcareous psammitic paragneisses and is itself tightly folded. Zircons in an amphibolite proved to be highly discordant but indicate Early Silurian metamorphism during isoclinal folding. Detrital zircons in a paragneiss are dominated by Sveconorwegian populations, but also include a range of younger Neoproterozoic grains down to the Early Ediacaran (c. 600 Ma). This new evidence of early Caledonian deformation and metamorphism indicates that, as farther north in the orogen, the Seve tectonothermal history in central Jämtland probably started early in the Ordovician, or before. Subduction and accretion along the Baltoscandian outer margin occurred prior to Scandian continent-continent collision, with Siluro-Devonian emplacement of the SNC across the foreland basins onto the Baltoscandian platform.

针对国际大陆科学钻探计划“斯堪的纳维亚加里东造山带碰撞造山”（International Continental Scientific Drilling Project “Collisional Orogeny in the Scandinavian Caledonides”）的近期研究，聚焦于塞维推覆体杂岩（Seve Nappe Complex, SNC）对于理解伊阿珀托斯洋闭合期间波罗的斯堪的纳维亚陆缘俯冲历史的重要意义。在耶姆特兰中西部经典的奥勒（Åre）地区，奥勒斯科通推覆体（Åreskutan Nappe）的麻粒岩相混合岩与淡色花岗岩，为早志留世（约4.40亿年）的高温变质作用以及早前的进变质超高压演化历程（含微金刚石）提供了证据。本文报道的针对下伏角闪岩相下塞维推覆体所开展的新型激光剥蚀电感耦合等离子体质谱（LA-ICPMS）锆石同位素定年研究，同样识别出早志留世的构造热演化历史：其中伟晶质淡色花岗岩的形成时代约为4.43亿年，而在更低构造层位中，另一长英质侵入体的形成时代为中奥陶世早期（约4.69亿年）。该晚期侵入体穿切等斜褶皱的围岩角闪岩与钙质砂质副片麻岩，自身亦发生强烈褶皱。一件角闪岩样品中的锆石呈现极强的不谐和性，但指示了等斜褶皱作用期间的早志留世变质事件。一件副片麻岩样品中的碎屑锆石以斯维科诺尔造山期群为主，同时包含一系列新元古代年轻颗粒，最低可至埃迪卡拉纪早期（约6.00亿年）。这项关于加里东早期变形与变质作用的新证据表明，与造山带北部区域一致，耶姆特兰中部的塞维构造热演化史可能始于奥陶纪早期甚至更早。波罗的斯堪的纳维亚外缘的俯冲与增生作用发生于斯堪的大陆碰撞事件之前，塞维推覆体杂岩于志留纪-泥盆纪时期沿前陆盆地侵位至波罗的斯堪的纳维亚地台之上。