U-TH-PB MONAZITE GEOCHRONOLOGY IN KYANITE GNEISSES, NORDØYANE, NORWAY: IMPLICATIONS CONCERNING THRUSTING, EXTENSION AND AGE OF OUTBOARD BASEMENT IN PRE-SCANDIAN BALTICA

Geologic mapping on Nordøyane shows three major tectonic segments separated by extensional faults. Each segment contains Baltica basement and thrusted cover rocks correlated with the Blåhø (Seve) Nappe. The different parts of the Blåhø have the same rock types including garnet ± kyanite gneiss, marble, calcsilicate, and amphibolite or eclogite. Geochronology of monazites in kyanite gneiss in different parts of Blåhø has been studied by in-situ electron probe and ion probe microanalysis. Two samples from the central segment yield preliminary EMP ages of ~397 Ma, ~450 Ma, and ~725-800 Ma. Central and southern segments are juxtaposed on an early tectonic contact. Microstructural analysis indicates migmatization, found in both central and southern segments, predated top-west and sinistral extensional fabrics, dated at 398-395 Ma. Migmatization observed in Blåhø of central and southern segments is tentatively equated with rare 450 Ma age domains in monazite; 725-800 Ma ages are probably detrital. New and previous ages of monazite from Blåhø of the northern segment are Scandian at 375 Ma, 395 Ma, and 407 Ma, and pre-Scandian and possibly detrital or discordant at ~760-800 Ma, probably detrital at 1040-1060 Ma. These results suggest that central and southern Blåhø was deposited farthest outboard near a possible detrital source of 725-800 Ma monazite and underwent pre-Scandian migmatization at 450 Ma. Northern Blåhø was deposited more inboard near detrital sources of 760-800 Ma and 1040-1060 Ma monazite and had no monazite growth at 450 Ma. These units were imbricated and thrust toward the foreland during early Scandian 425-407 Ma. Northern Blåhø experienced UHP metamorphism 407-402 Ma and was brought in contact with HP Baltica basement by thrusting before 395 Ma. During this same time interval the central segment was brought in contact with the eclogite-facies rocks of the southern segment by an extensional fault, and all segments experienced top-west or left lateral shear from 395-375 Ma.