UHP metamorphism recorded by phengite eclogite from the Caledonides of northern Sweden: P–T path and tectonic implications

• Published in: Journal of Metamorphic Geology
• Main Authors: Bukała, Michał, Klonowska, Iwona, Barnes, Christopher, Majka, Jarosław, Kośmińska, Karolina, Janák, Marian, Fassmer, Kathrin, Broman, Curt, Luptáková, Jarmila
• Other Authors: Narodowe Centrum Nauki
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2018
• Subjects: Northern Sweden; Norrbotten
• Online Access: http://dx.doi.org/10.1111/jmg.12306; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fjmg.12306; https://onlinelibrary.wiley.com/doi/pdf/10.1111/jmg.12306

Abstract The Seve Nappe Complex ( SNC ) of the Scandinavian Caledonides records a well‐documented history of high pressure ( HP ) and ultra‐high pressure ( UHP ) metamorphism. Eclogites of the SNC occur in two areas in Sweden, namely Jämtland and Norrbotten. The Jämtland eclogites and associated rocks are well‐studied and provide evidence for late Ordovician UHP metamorphism, whereas the Norrbotten eclogites, formed during the late Cambrian (Furongian)/Early Ordovician, have not been studied in such detail, especially in terms of the P–T conditions of their formation. Within the studied eclogite, clinopyroxene contains a high‐Na core and two rims: inner, medium‐Na and outer, low‐Na. Garnet consists of a high‐Ca euhedral core, low‐Ca inner rim and medium‐Ca outer rim. A similar pattern occurs within phengite, where high‐Si cores are enveloped by medium and low‐Si rims. The compositions of the mineral cores, inner rims and outer rims reflect three stages in the metamorphic evolution of the eclogite. Applied Quartz‐in‐Garnet geobarometry, coupled with Zr‐in‐rutile geothermometry reveal that garnet nucleation (E0 stage) took place at 1.5–1.6 GP a and 620–660°C. The eclogite peak‐pressure assemblage developed during the E1 stage, it consists of garnet+omphacite+phengite+rutile+coesite? and yields P–T conditions of 2.8–3.1 GP a and 660–780°C as constrained by conventional geothermobarometry and thermodynamic modelling in the NCKFMM n ASHT system. Later, lower‐pressure stages E2 and E3 record conditions of 2.2–2.8 GP a, 680–780°C and 2.1 GP a, 735°C, respectively. The prograde metamorphic evolution of the eclogite is inferred from inclusions of epidote, amphibole and clinopyroxene within garnet. The presence of amphibole–quartz–plagioclase symplectites, secondary epidote/zoisite and titanite replacing rutile record the later retrograde changes taking place at <1.5 GP a (referred as E4 stage). The obtained P–T conditions indicate that the Norrbotten eclogites underwent a metamorphic evolution characterized by a ...