NEW VESTIGES OF EARLY CALEDONIAN SUBDUCTION IN THE HIGH ARCTIC

The most recent petrological study from Svalbard’s Southwestern Basement Province (SBP) revealed that the extent of early Caledonian high-pressure (HP) metamorphism is larger than previously thought. The presence of HP rocks is important for interpretation of the tectono-metamorphic history of the Svalbard Caledonides and, in the wider perspective, reconstruction of the early stage of the Caledonian Orogeny when subduction commenced along both margins of the Iapetus Ocean

The type locality for high-pressure low-temperature (HP-LT) rocks within the SBP is the Motalafjella area (the Vestgötabreen Complex). The Vestgötabreen Complex consists mainly of blueschists, eclogites and phengite-garnet schists. Various radiometric dating yielded an age of c. 470-460 Ma for this HP-LT metamorphic event. The pressure-temperature (P-T) estimates were performed for different HP lithologies. Carpholite-bearing schists indicate peak conditions at c. 16 kbar and 330-450°C, whereas eclogites record P-T conditions at 18-24 kbar and 580-640°C. Blueschists yield peak metamorphic conditions at c. 20 kbar and 520°C .

Recent studies, south of the Motalafjella area, in Nordenskiöld Land, revealed a new occurrence of blueschists. The P-T conditions have been estimated to c. 14-18 kbar and 470-490°C. The age of these rocks is unknown, but it is likely that they represent an equivalent to the Motalafjella blueschists. Notably, schists and augen gneisses, which occur south of Nordenskiöld Land, within the northern part of Wedel Jarlsberg Land, have been also subjected to Caledonian HP metamorphism.

All of the P-T estimates for HP rocks from the SBP indicate a low geothermal gradient of 7-8°C/km, which provides evidence for a cold early Palaeozoic subduction zone. More regionally, these HP lithologies may be correlated with the M’Clintock Complex (Pearya Terrane of northern Ellesmere Island), where ophiolitic sequence (465-450 Ma) and island arc lithologies occur. Several lines of evidence suggest that during the early stage of the Caledonian Orogeny SBP and the Pearya Terrane may have formed one composite terrane, which was located on the Laurentian side of the Iapetus Ocean. The Pearya-SBP Terrane may have been dismembered by sinistral strike-slip faults in the late stage of the Caledonian Orogeny.