Ilmenite as a source for zirconium during high-grade metamorphism? Textural evidence from the Caledonides of western Norway and implications for zircon geochronology

The Proterozoic Lindas Nappe, part of the Caledonides of western Norway, wa s affected by penetrative Sveconorwegian granulite-facies metamorphism,foll owed by a fluid-driven eclogite- and amphibolite-facies Caledonian overprin t, spatially restricted along fractures and shear zones. In mafic granulite s and amphibolites, a luminescent anhedral zircon overgrowth, which gives a n average age of 924 +/- 58 Ma (Th/U = 0(.)52; secondary ion mass spectrome try data), surrounds a magmatic toned core with an age of 952 +/- 32 Ma (Th /U = 1(.)27). In the granulites, a continuous rim of zircon or a discontinu ous corona of similar to 10 mum rounded ta flat zircon crystals is observed at the outer margin of ilmenite grains. Baddeleyite and srilankite (Ti2ZrO 6) blebs are reported around ilmenite included in feldspar or pyroxene. Bad deleyite is interpreted as an exsolution product from magmatic ilmenite, wh ereas srilankite, the zircon corona around ilmenite and the luminescent zir con overgrowth were fanned as reaction products during granulite;facies met amorphism. Textures suggest that magmatic ilmenite war a main source of Zr to form metamorphic zircon. In massive amphibolites, relic ilmenite grains are surrounded by a corona of titanite and a discontinuous corona of micro- zircons. Amphibolite-facies overprint is not associated with any significan t growth or dissolution of zircon. An unsheared eclogite displays a zircon population with a euhedral oscillatory zoned overgrowth giving an age of 45 5 +/- 29 Ma (Th/U less than or equal to 0(.)13). A corona of micro- zircon grains is observed at same distance around rutile, and locally these zircon s show a prismatic overgrowth. A specific low-Th zircon growth event is rel ated to eclogite-facies forming reactions, involving breakdown of a two pyr oxene + garnet + plagioclase + ilmenite assemblage to form a garnet + ompha cite + rutile assemblage in the presence of a fluid. The low Th content of this zircon probably stems from the coeval Precipitation of clinozoisite. T his oscillatory toned zircon records fluid infiltration and coeval eclogiti zation in the dust.