Structural and thermal evolution of Baltica basement and infolded cover nappes on Nordoyane and their bearing on mechanisms for production and exhumation of high-pressure rocks, Western Gneiss Region, Norway

Detailed mapping, U-Th-Pb monazite geochronology was combined with structural and metamorphic analysis on Nordøyane to date the thermotectonic evolution of UHP and HP units metamorphosed during the Late Silurian-Early Devonian collision between Baltica and Laurentia. The structurally higher unit contains microdiamond-bearing kyanite-garnet-graphite gneiss and associated kyanite eclogites that independently indicate probable diamond-forming conditions of 820°C, 39 kbar. Eclogitized mylonite from the Proterozoic Haram Gabbro, in the structurally lower unit, yields 780°C and 18 kbar. In situ monazite dating with the electron microprobe using a combination of high-resolution element imaging and trace-element analysis of U, Th, Pb, and Y yielded three mean ages of 407.0 ± 2.1, 394.8 ± 2.3, and 374.6 ± 2.7 Main the microdiamond sample and kyanite-garnet mylonite. The oldest two of these have been verified by the SHRIMP II ion probe. These indicate that the UHP unit reached its maximum depth of 125 km at approximately 407 Ma when monazite was included in garnet. This unit then experienced 65 km of exhumation at an average rate of 10.9 mm/yr during top-southeast thrusting that also brought it into contact with the HP unit. Structural features formed during this stage of exhumation include L > S fabrics and tubular folds with axes parallel to the transport direction. Later, both units were exhumed together at an average rate of 3.8 mm/yr to a depth of 37 km at 395 Ma where these rocks experienced extensive re-equilibration, and top-west, left-lateral shearing. During this stage, structural features included development of constrictional L > S fabrics and tubular folds formed at high angles to earlier structural features and evolved from an initial WNW orientation to near orogen parallel. After 395 Ma, these units continued to be exhumed at an average rate of 0.8 to 1.4 mm/yr until 375 Ma, the time of asymmetric monazite porphyroclasts in the late-stage mylonite. The exhumation histories of these units record a change in mechanism from syncollisional exhumation through late- to post-orogenic collapse that was a consequence of plate re-organization.