Microdiamond on Åreskutan confirms regional UHP metamorphism in the Seve Nappe Complex of the Scandinavian Caledonides

Abstract

Metamorphic diamond in crustal rocks provides important information on the deep subduction of continental crust. Here, we present a new occurrence of diamond within the Seve Nappe Complex (SNC) of the Scandinavian Caledonides, on Åreskutan in Jämtland County, Sweden. Microdiamond is found in situ as single and composite (diamond+carbonate) inclusions within garnet, in kyanite‐bearing paragneisses. The rocks preserve the primary peak pressure assemblage of Ca,Mg‐rich garnet+phengite+kyanite+rutile, with polycrystalline quartz surrounded by radial cracks indicating breakdown of coesite. Calculated P–T conditions for this stage are 830–840 °C and 4.1–4.2 GPa, in the diamond stability field. The ultrahigh‐pressure (UHP) assemblage has been variably overprinted under granulite facies conditions of 850–860 °C and 1.0–1.1 GPa, leading to formation of Ca,Mg‐poor garnet+biotite+plagioclase+K‐feldspar+sillimanite+ilmenite+quartz. This overprint was the result of nearly isothermal decompression, which is corroborated by Ti‐in‐quartz thermometry. Chemical Th–U–Pb dating of monazite yields ages between 445 and 435 Ma, which are interpreted to record post‐UHP exhumation of the diamond‐bearing rocks. The new discovery of microdiamond on Åreskutan, together with other evidence of ultrahigh‐pressure metamorphism (UHPM) within gneisses, eclogites and peridotites elsewhere in the SNC, provide compelling arguments for regional (at least 200 km along strike of the unit) UHPM of substantial parts of this far‐travelled allochthon. The occurrence of UHPM in both rheologically weak (gneisses) and strong lithologies (eclogites, peridotites) speaks against the presence of large tectonic overpressure during metamorphism.

Textures of eclogites and blueschists from Syros island, Greece: Inferences for elastic anisotropy of subducted oceanic crust

“…Klonowska et al . [], for example, showed that in the Seve Nappe of the Scandinavian Caledonides both strong eclogites and peridotites and the surrounding weak gneissic matrix yield evidence for ultrahigh‐pressure conditions. According to the previously mentioned numerical models, on the other hand, this rheology contrast should have led to enormous tectonic overpressure and/or underpressure, which have not been detected.…”

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

“…Albrecht, ; Kullerud, Stephens, & Zachrisson, ; Nicholson, ; Santallier, ; Stephens & van Roermund, ) it was only recently discovered that the SNC experienced UHP metamorphism. The latter was documented in eclogites and ultramafic rocks of northern Jämtland (Brueckner, van Roermund, & Pearson, ; Gilio, Clos, & van Romermund, ; Janák, van Roermund, Majka, & Gee, ; Klonowska et al., ; Majka, Janák, et al., ) and in metasedimentary rocks of west‐central Jämtland (Klonowska, Majka, Janák, Gee, & Ladenberger, ; Klonowska et al., ; Majka, Rosén, et al., ). UHP metamorphism was constrained in west‐central Jämtland to >445 Ma via Th–U–Pb monazite geochronology (Klonowska et al., ; Majka et al., ).…”

Three-dimensional distribution of primary melt inclusions in garnets by X-ray microtomography

“…A prominent core-rim texture such as that observed in the present case study could be attributed to a 614 multi-stage garnet growth, such as in the examples described by Konrad-Schmolke et al (2008) or 615 Klonowska et al (2017) in which two growth events occur at markedly different P-T conditions. 616…”

“…are not evidence of polymetamorphism (Konrad-Schmolke et al 2008), even though the P-T conditions 96 at which the two stages occur are markedly different (c. 3 GPa difference in Klonowska et al 2017). 97…”