Sveconorwegian Mid-crustal Ultrahigh-temperature Metamorphism in Rogaland, Norway: U-Pb LA-ICP-MS Geochronology and Pseudosections of Sapphirine Granulites and Associated Paragneisses

Abstract

MgAl-rich sapphirine granulites (bulk X Mg 0·71–0·75) occur as boudinaged layers in migmatitic garnet–orthopyroxene–cordierite–spinel gneisses and migmatitic garnet–sillimanite metapelites in the vicinity of the c. 930–920 Ma Rogaland anorthosite–mangerite–charnockite complex, SW Norway. Investigation of the mineral reaction history of the sapphirine granulites and the surrounding paragneisses, combined with geothermobarometric calculations and constraints from pseudosections calculated in the Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 (NCKFMASHT) system, indicates a clockwise P–T path that reached peak-metamorphic ultrahigh-temperature (UHT) conditions of c. 1000°C at c. 7·5 kbar by prograde heating. UHT peak metamorphism is followed by near-isothermal (ultra)high-temperature decompression to P 850–940°C as estimated from Ti-in-zircon thermometry, suggesting that UHT metamorphism and the deduced clockwise P–T evolution is linked to regional Sveconorwegian metamorphism at c. 1010 Ma. Most of the metamorphic zircon surrounds largely resorbed inherited oscillatory zoned zircon cores ( 207 Pb/ 206 Pb apparent ages 1220–1841 Ma), testifying to the sedimentary origin of the sapphirine granulites. Epitactic growth of xenotime on metamorphic zircon at 933 ± 5 Ma is suggested to be related to crystallization of anatectic melt during post-decompressional cooling. The clockwise P–T path culminating at mid-crustal UHT conditions at c. 1010 Ma followed by (U)HT decompression is interpreted to result from collisional tectonics during the early stages of the Sveconorwegian Orogeny, followed by gravitational collapse of the mountain plateau.