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

Abstract

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 910 Ma. A first long‐lived metamorphic cycle (M1) between 1,045 ± 8 and 992 ± 11 Ma 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 ~6 kbar and 920°C. Y‐in‐monazite thermometry, in a residual garnet‐absent sapphirine–orthopyroxene granulite, provides critical evidence for average temperature of 931 and 917°C between 1,029 ± 9 and 1,006 ± 8 Ma. Metamorphism peaked after c. 20 Ma of crustal melting and melt extraction, probably supported by a protracted asthenospheric heat source following lithospheric mantle delamination. Between 990 and 940 Ma, slow conductive cooling to 750–800°C is characterized by monazite reactivity as opposed to silicate metastability. A second incursion (M2) to UHT conditions of ~3.5–5 kbar and 900–950°C, is recorded by Y‐rich monazite at 930 ± 6 Ma 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 ± 2 Ma. Thermal preconditioning of the crust during the first UHT metamorphism may explain the width of the aureole of contact metamorphism c. 75 Ma later, and also the rarity of osumilite‐bearing assemblages in general.

Closed system behaviour of argon in osumilite records protracted high‐Tmetamorphism within the Rogaland–Vest Agder Sector, Norway

“…Regional metamorphism reached peak conditions of ~850–950°C and 7–8 kbar at c . 1,035 Ma based on U–Pb monazite and zircon geochronology (Blereau et al, ; Degeling, Eggins, & Ellis, ; Drüppel, Elsäßer, Brandt, & Gerdes, ; Laurent, Duchene, Bingen, Bosse, & Seydoux‐Guillaume, ; Laurent et al, ; Möller, O'Brien, Kennedy, & Kröner, ; Slagstad et al, ). Following limited cooling (Blereau et al, ; Laurent, Bingen et al, ; Slagstad et al, ), emplacement of the Rogaland Igneous Complex (RIC) at c .…”

Nanoscale Isotopic Dating of Monazite

“…The light rare earth element phosphate (LREEPO 4 ) monazite is used widely to establish the timing of rock formation, metamorphism, hydrothermal alteration and deformation (Parrish 1990, Seydoux-Guillaume et al 2012, Wawrzenitz et al 2012, Erickson et al 2015, Laurent et al 2018. In particular, monazite may dissolve and regrow in entirety or partially during fluid-rock interaction, providing a high-fidelity temporal record of this process (Poitrasson et al 2000, Janots et al 2008, Kirkland et al 2009, Allaz et al 2013.…”

The Farsund Shear Zone: geochemical evidence for lithological diversity in the wall rock of the Rogaland Anorthosite Province, South Norway

“…In Rogaland, the high-grade gneisses of the basement have suffered two metamorphic cycles (Kars et al, 1980;Tobi et al, 1985). The first one (M1) of regional extension has culminated in UHT conditions in the upper amphibolite to granulite facies (Drüppel et al, 2013) in the time window between c. 1045 Ma and c. 992 Ma (Laurent et al, 2018a), and the second one (M2) has developed in an aureole around the AMC complex at c. 930 Ma and has also reached UHT conditions (Laurent et al, 2018b). An orthopyroxene isograd in quartzofeldspathic rocks (Figs.…”