Long-term convergence along SW fennoscandia: 330 m.y. of proterozoic crustal growth

摘要

The core of Fennoscandia formed during Svecofennian 1.91-1.86 Ga orogenesis that included amalgamation of Archean micro-continents in the northeast and extensive arc accretionary growth toward the southwest. Post-Svecofennian crustal growth persisted southwestwards and later westwards, mainly by subduction-related magmatism that lasted another similar to 330 m.y. By integrating mapping and geochemistry with new U-Pb geochronology from 31 igneous and 4 metasedimentary rocks in the Idefjorden terrane, we have defined major growth-related crustal units in this southwestern segment of Fennoscandia. Available constraints permit definition of nine lithotectonically distinct, semi-continuous stages of crustal growth between 1850 and 1522 Ma. The first stages included 1.85 Ga continental arc magmatism along the southwestern Svecofennian margin and recurring, 1.83-1.82 Ga growth further south. Voluminous continental arc magmatism at 1.81-1.77 Ga (TIB 1 rocks) and 1.72-1.66 Ga (TIB 2-3 rocks) resulted in large batholithic domains with a high proportion of rejuvenated material. The sixth stage (similar to 1.66 Ga) marks a transition in the south from continental arc magmatism to island arc magmatism that created the Horred supracrustal rocks. After accretion of the Horred sequence, a seventh magmatic stage (1.63-1.59 Ga) represents the return to a fully continental arc system. A short-lived eighth stage (similar to 1.59 Ga) returned to an oceanic arc system represented by an early package of volcano-sedimentary Stora Le-Marstrand formation rocks (SLM 1). Accreted SLM 1 rocks were intruded by stage nine rocks (1.59-1.52 Ga) that represent the last convergent arc magmatism in the Ideflorden terrane. A younger ca. 1.57-1.55 Ga package of volcano-sedimentary Stora Le-Marstrand formation rocks (SLM 2) is recognised in the Koster segment, demonstrating a return to an oceanic arc system in the north, coeval with continued continental arc magmatism in the south. This near-continuous, oceanward-stepping growth of Fermoscandia over this 330 m.y. period represents a remarkably persistent convergent-margin system that permits evaluation of Proterozoic processes of continental growth and pre-Rodinian plate reconstructions. Several stages of arc magmatism were followed by intracratonic episodes of "A-type" magmatism up to 1500 km inboard of the evolving margin. These typically bimodal magmatic episodes also swept westward with time such that the oldest episode in the Idefjorden terrane (1502 Ma) provides a minimum age for this crustal segment to occupy an intracratonic position, inboard of an inferred "proto-SW Norway ". The origin and final fate of such a pre-Sveconorwegian segment are speculative, but juxtaposition alongside the Idefjorden terrane during the 1.50-1.20 Ga interval is supported by independent evidence in SW Fermoscandia.