OUR UNDERSTANDING OF TEMPORAL AND STRUCTURAL EVOLUTION OF LOWER-CRUSTAL CONTINENTAL BASEMENT OF LOFOTEN-VESTRÅLEN, NORTH NORWAY: A WORK IN PROGRESS

We present field/structural and 40Ar/39Ar and U-Pb isotopic analyses that bear on the tectonic evolution of deep-crustal, continental basement rocks exposed in Lofoten-Vesterålen, north Norway. An east-to-west transect at this latitude (68.5^oN) progressively traces the Precambrian basement-cover contact from the unmetamorphosed/undeformed nonconformity in the Swedish foreland, to greenschist-facies mylonites framing an external window, to upper amphibolite-facies mylonites against an internal window, across the amphibolite/granulite isograd in Vesterålen, and finally to limited occurrences of rare eclogite-facies shear zones in the westernmost coastal islands of Lofoten. The eastern half of this transect already is very well characterized and accepted as the mid-to-upper crustal, Caledonian (Silurian), continental-continent subduction zone boundary. Evolution of the structurally underlying, lower-crustal rocks to the west, however, is far less clear. The eclogite shear zones cross cut the gneissic banding of Proterozoic (1.8-1.7 Ga) granulites that form the bulk of the Lofoten basement, and our 40Ar/39Ar dates indicate eclogitization occurred prior to ca. 433 Ma. This timing, thus, appears to predate Scandian eclogitization of the Western Gneiss Region of west Norway at 425–400 Ma but is compatible with “early” Caledonian eclogitization (i.e., ca. 500–450 Ma) as interpreted for other terranes in Scandinavia (e.g., the Bergen Arcs, the Seve nappe, and the Tromsø nappe complex). We report new 40Ar/39Ar dates for muscovite separated from rocks defining the granulite-amphibolite-facies transition in Vesterålen that range from ca. 450-550 Ma. As of this writing we are analyzing more samples to improve on our understanding of this timing. We also report our structural analyses (geometric and kinematic) on rocks of the Lofoten eclogite shear zones and for rocks defining the amphibolite-granulite transition in the context of existing timing constraints and speculate on their significance for how these lower-crustal manifestations fit into the framework of the overlying, mid-to-upper-crustal levels of the Caledonian crustal column.