Polyphase tectonothermal evolution of the Scandinavian Caledonides

Abstract

Summary A polyphase tectonothermal evolution characterizes many of the allochthonous sequences which constitute the Scandinavian Caledonides. Initial Late Cambrian consumption of Iapetus oceanic crust occurred outboard of the Baltoscandian margin with development of an island-arc above a west-dipping subduction complex. Imbrication, polydeformation and variably high-grade metamorphism of Baltoscandian miogeoclinal rocks and underlying Precambrian crystalline basement occurred within an accretionary prism which developed in the subduction complex. This resulted in eastward younging, diachronous metamorphism throughout the Early and Middle Ordovician. Cooling during subsequent uplift was regionally variable, and is reflected by the wide range of radiometric dates reported for the Baltoscandian allochthons (455–535 Ma). This early Caledonian activity has been termed the Finnmarkian orogeny; however, it is best viewed as a complex, diachronous series of tectonothermal events which variably affected the Baltoscandian margin from the Late Cambrian through the Middle Ordovician. Almost complete closure of the Iapetus Ocean appears to have been accomplished by the Late Ordovician when eugeoclinal terranes which had developed along the Laurentian margin were tectonically juxtaposed with eroded remnants of the Late Cambrian-Middle Ordovician arc. Initial imbrication of the Laurentian eugeoclinal sequences with the arc occurred during the Early Silurian (Late Llandovery). These were subsequently imbricated and infolded with predeformed Baltoscandian allochthons. The resultant composite allochthon was subsequently thrust eastward onto Silurian sedimentary successions deposited over previously metamorphosed Caledonian basement. This sequence of Early Silurian to Early Devonian tectonic events has been collectively termed the Scandian orogeny; however, they are regionally diachronous and should be bracketed with the local controls available. Scandian dates of 425–430 Ma are locally recorded in allochthons along eastern portions of the orogen and they probably date initial nappe translation to higher crustal levels along the Baltoscandian margin. In lower structural units Scandian dates range from 410 Ma in central to 385 Ma in western portions of the orogen. These are interpreted to date rapid cooling during the uplift associated with a crustal rebound which followed eastward translation of overriding allochthons.

Tectonics of the Scandian orogeny and the Western Gneiss Region in southern Norway

“…Metamorphic temperatures in outboard-derived nappes between Trondheim and Mer~ker were relatively low (Dallmeyer, 1988;1990), and the reported ages must be close to the metamorphic peak. Data from the same samples referring to different closing temperatures are nevertheless fairly close to one another, indicating rapid cooling.…”

“…Earlier ages belonging to the Finnmarkian phase have also been reported for the southern Caledonides, but from oceanic rocks. Some data (Dallmeyer, 1988;1990) suggest that (a) most Finnmarkian tectonic events occurred outboard, and (b) there was not just a single phase, but multiple events that may or may not be synchronous and/or related to one another genetically or spatially.…”

“…The lowermost nappe in this stack is the Seve Nappe Complex in the east. Parts of it have undergone multiple metamorphic events, and it contains Proterozoic crustal elements (Claesson, 1981;Dallmeyer, 1988;1990) and pre-Scandian eclogites (M6rk etal., 1988). The Seve Nappe Complex is overlain by the K61i Nappe Complex dominated by rocks of oceanic affinity.…”

U/Pb zircon ages of basement gneisses and discordant felsic dykes from Vestranden, westernmost Baltic Shield and central Norwegian Caledonides

“…Since the zircon crystals have metamorphic rather than magmatic habits, the age of 1819 Ma may represent a minimum estimate of the time of crystallization. The lower intercept age corresponds to medium-grade metamorphism in the area (DALLMEYER, 1988;TUCKER &;KROGH, 1988). Within error limits, this age also overlaps with the age of a 401 _+ 3 Ma old pegmatite dyke in the Folda-Vikna area (SCHOUENBORG, 1988) and the Namsos pegmatite age of 404 _+ 2 Ma, sample 83022.…”

“…the compilation by KULLERUD et al, 1986). Only a few ages have been reported from Vestranden (PRIEM et al, 1968;JOHANSSON et al 1986JOHANSSON et al , 1987bTUCKER et al, 1986/87;DALLMEYER, 1988;TUCKER & KROGH, 1988;SCHOUENBORG, 1988). This paper reports four U/Pb-zircon age determinations made in order to date the Precambrian protolith rocks, to constrain the age of the earliest migmafization in the region, and to date cross-cutting, presumably late-Caledonian pegmatites.…”