ABSTRACT

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.

摘要

在国际大陆科学钻探项目“斯堪的纳维亚喀里多尼群岛的碰撞造山运动”的背景下，最近的研究集中在塞夫纳普综合体（SNC）对于了解伊阿佩图斯海域封闭期间巴尔托斯堪迪亚边缘的俯冲历史的重要性。在耶姆特兰中西部的古典Åre地区，ÅreskutanNappe的花岗石相移辉岩和白云母岩提供了志留纪早期（约440 Ma）高温变质的证据，以及以前的微金刚石演化，超高压的历史。此处报道的新的LA-ICPMS锆石下层闪粉岩相下的锆石同位素年龄研究还确定了早志留世构造热史，其发育有成岩白云母（约443 Ma），并且在较低的结构水平下，另一个最早的奥陶纪中期（约469 Ma）的长生入侵。后者侵入等轴折叠的宿主岩闪石和钙质的半成石的帕拉尼戈斯，并且本身被紧密地折叠。闪石中的锆石被证明是高度不协调的，但表明等斜线折叠过程中的早期志留纪变质作用。石器时代的碎屑锆石受Sveconorwegian族群的支配，但也包括一系列直至Ediacaran早期（约600 Ma）的新元古代新生代。喀里多尼亚早期变形和变质作用的这一新证据表明，在造山带的更北端，耶姆特兰中部的塞夫构造热史可能始于奥陶纪或更早。在斯堪的纳维亚大陆与大陆碰撞之前，沿巴尔托斯堪的亚外缘发生了俯冲和增生，