Two successive phases of ultrahigh temperature metamorphism in Rogaland, S. Norway: Evidence from Y-in-monazite thermometry

In Rogaland, South Norway, a polycyclic granulite facies metamorphic domain surrounds the late-Sveconorwegian anorthosite-mangerite-charnockite (AMC) plutonic complex. Integrated petrology, phase equilibria modelling, monazite microchemistry, Y-in-monazite thermometry, and monazite U-Th-Pb geochronology in eight samples, distributed across the apparent metamorphic field gradient, imply a sequence of two successive phases of ultrahigh temperature (UHT) metamorphism in the time window between 1,050 and 910Ma. A first long-lived metamorphic cycle (M1) between 1,045 +/- 8 and 992 +/- 11Ma is recorded by monazite in all samples. This cycle is interpreted to represent prograde clockwise P-T path involving melt production in fertile protoliths and culminating in UHT conditions of similar to 6kbar and 920 degrees C. Y-in-monazite thermometry, in a residual garnet-absent sapphirine-orthopyroxene granulite, provides critical evidence for average temperature of 931 and 917 degrees C between 1,029 +/- 9 and 1,006 +/- 8Ma. Metamorphism peaked after c. 20Ma of crustal melting and melt extraction, probably supported by a protracted asthenospheric heat source following lithospheric mantle delamination. Between 990 and 940Ma, slow conductive cooling to 750-800 degrees C is characterized by monazite reactivity as opposed to silicate metastability. A second incursion (M2) to UHT conditions of similar to 3.5-5kbar and 900-950 degrees C, is recorded by Y-rich monazite at 930 +/- 6Ma in an orthopyroxene-cordierite-hercynite gneiss and by an osumilite gneiss. This M2 metamorphism, typified by osumilite paragenesis, is related to the intrusion of the AMC plutonic complex at 931 +/- 2Ma. Thermal preconditioning of the crust during the first UHT metamorphism may explain the width of the aureole of contact metamorphism c. 75Ma later, and also the rarity of osumilite-bearing assemblages in general.