U-Pb monazite ages in amphibolite- to granulite-facies orthogneiss reflect hydrous mineral breakdown reactions: Sveconorwegian Province of SW Norway

In the Rogaland-Vest Agder terrain of the Sveconorwegian Province of SW Norway, two main Sveconorwegian metamorphic phases are reported: a phase of regional metamorphism linked to orogenic thickening (MI) and a phase of low-pressure thermal metamorphism associated with the intrusion of the 931 +/- 2 Ma anorthosite-charnockite Rogaland igneous complex (M2). Phase M1 reached granulite facies to the west of the terrane and M2 culminated locally at 800-850 degrees C with the formation of dry osumilite-bearing mineral associations. Monazite and titanite U-Pb geochronology was conducted on 17 amphibolite- to granulite-facies orthogneiss samples, mainly from a suite of 1050 (+2)/(-8) Ma calc-alkaline augen gneisses, the Feda suite. In these rocks, prograde negatively discordant monazite crystallized during breakdown of allanite and titanite in upper amphibolite facies at 1012-1006 Ma. In the Feda suite and other charnockitic gneisses, concordant to slightly discordant monazite at 1024-997 Ma probably reflects breakdown of biotite during granulite-facies hll metamorphism. A spread of monazite ages down to 970 Ma in biotite +/- hornblende samples possibly corresponds to the waning stage of this first event. In the Feda suite, a well defined monazite growth episode at 930-925 Ma in the amphibolite-facies domain corresponds to major clinopyroxene formation at the expense of hornblende during M2. Growth or resetting of monazite was extremely limited during this phase in the granulite-facies domain, up to the direct vicinity of the anorthosite complex. The M2 event was shortly followed by cooling through ca. 610 degrees C as indicated by tightly grouped U-Pb ages of accessory titanite and titanite relict inclusions at 918 +/- 2 Ma over the entire region. A last generation of U-poor monazite formed during regional cooling below 610 degrees C, in hornblende-rich samples at 912-904 Ma. This study suggests: (1) that monazite formed during the prograde path of high-grade metamorphism may be preserved: (2) that monazite ages reflect primary or secondary growth of monazite linked to metamorphic reactions involving redistribution of REEs and Th, and/or fluid mobilisation; (3) that the U-Pb system in monazite is not affected by thermal events up to 800-850 degrees C, provided that conditions were dry during metamorphism.