Ordovician marginal basin development in the central Norwegian Caledonides

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Abstract
Summary: In the Caledonides of central Norway, volcano-sedimentary successions, of the Lower and Upper Hovin and Horg Groups (Ordovician to ?Lower Silurian), unconformably overlie a variable substrate which includes fragmented oceanic crust and ensimatic immature arc assemblages initially deformed and metamorphosed in earliest Ordovician times. In western areas, remnants of a mid-Ordovician, evolved island-arc complex comprise a volcanic-plutonic suite of clear calc-alkaline affinity. To the E, in the Trondheim district, continental margin-type basic to andesitic volcanics, and sediments of comparable age, denote accumulation in a fault-dissected, back-arc marginal basin characterized by marked facies variations and bipolar sediment dispersal patterns. Sediment composition reflects the nature of the contemporaneous volcanism, although an input of metamorphic detritus from a continental margin source can also be determined. Thick, massive to pillowed tholeiitic greenstones, mainly of OFB chemistry, occur in several areas and some contain thick sheeted-dyke units, gabbro complexes and subordinate plagiogranites. These ophiolitic assemblages appear to occur at two stratigraphic levels within the Lower Hovin Group, reflecting separate phases of crustal thinning and oceanic lithosphere accretion. Mega-breccias and an olistostrome may relate to abortive ophiolite displacement or intra-basinal movements in late Arenig times. During the Caradoc and Ashgill, and possibly in earliest Silurian times, continental margin-type acidic volcanism developed widely, particularly in the S, while in central and northern parts of the basin, turbidites and resedimented conglomerates, derived from the basin margins, show SW transport in the deeper-water axial trough. Although there is strong evidence that the marginal basin and arc lay at the edge of the continental plate (Baltoscandia) on the SE side of the Iapetus, with SE subduction, this is not supported by the faunas in the lowest parts of the Ordovician successions, which are largely of North American affinity. A compromise model involving an elongate microcontinental plate within the Iapetus is invoked.