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


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Abstract

MgAl-rich sapphirine granulites (bulk XMg 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 Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2 (NCKFMASHT) system, indicates a clockwise PT 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 < 5·5 kbar at 900–1000°C and subsequent near-isobaric cooling to <750–800°C at c. 5 kbar. In situ U–Pb laser ablation inductively coupled plasma mass spectrometry dating of metamorphic zircon within the sapphirine granulites yields concordant ages of 1010 ± 7 Ma and 1006 ± 4 Ma for zircon presumably formed during prograde breakdown of garnet at T > 850–940°C as estimated from Ti-in-zircon thermometry, suggesting that UHT metamorphism and the deduced clockwise PT evolution is linked to regional Sveconorwegian metamorphism at c. 1010 Ma. Most of the metamorphic zircon surrounds largely resorbed inherited oscillatory zoned zircon cores (207Pb/206Pb 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 PT 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.

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