Tectonic evolution of NW Svalbard

Sammanfattning: The Neoproterozoic is a remarkable era in Earth’s history with dramatic variations in climate and biota, possibly resulting in global glaciations events (eg. “snowball earth”, Kirschvink, 1992) and the first appearance of metazoans (Conway Morris, 1993). Its been speculated (Kirschvink, 1992; Hoffman and Schrag, 2002) that snowball events are in part triggered by a predominance of continents at middle to low latitudes. It appears that the rapid increase in diversity of life in the Cambrian followed a Neoproterozoic diversification of evolutionary lineages (Conway Morris, 1993, 2000; Budd and Jensen, 2000), which seems to be related to the break-up of the supercontinents Rodina and Pannotia (Dalziel, 1997). Hence, a reconstruction of the tectonic evolution in the Neoproterozoic is pivotal for understanding the major biological and climatic events occurring during this important era of Earth’s history. The success of this task requires a multidisciplinary approach with accurate geological, geochronological, paleontological, palaeomagnetic and geochemical data. This project focuses on a small piece of the Neoproterozoic tectonic puzzle: Reconstructing the Neoproterozoic to Devonian tectonic evolution of the Northwestern Terrane (NWT) of Svalbard, using zircon U-Pb geochronology, in order to place Svalbard in its global tectonic framework.Svalbard’s Caledonian and older bedrock consists of three main exotic terranes separated by north-south trending strike-slip faults. The Southwestern and the Eastern Terranes has been correlated with the East Greenland Caledonides, whereas the origin of the Northwestern Terrane (NWT) remains enigmatic This is partly due to the complex, polyphase metamorphic history of the pre-Devonian basement, as well as to the lack of fossil-bearing rocks in the NWT. In an attempt to understand the tectonic development of the NWT, a U-Pb ion microprobe study of zircons from gneisses, granitoids and migmatites from the Smerenburgfjorden Complex and Richarddalen Complex has been carried out. Field evidence supported by age dating indicates that an early Neoproterozoic or older metapelitic protolith was intruded by ca. 960 Ma granites. This Neoproterozoic and older basement was later involved in Silurian deformation with subsequent granitoid genesis and migmatization at 420-435 Ma. Inherited zircons in granites and migmatites indicate that two different protoliths were involved in anatexis: one dominantly of Grenvillian-age and another of early Mesoproterozoic to Paleoproterozoic age (1100-1900 Ma). The latter ages are similar to the detrital ages in the Krummedal supracrustal sequence from East Greenland, which is the parent to Grenvillian and Caledonian age granitoids in East Greenland. Lithological and geochronological similarities between the two widely separated areas indicate that the NWT had its origin in the vicinity of East Greenland (Eastern margin of Laurentia) prior to Caledonian orogenesis.

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