An anticlockwise P–T–t path at high‐pressure, high‐temperature conditions for a migmatitic gneiss from the island of Fjørtoft, Western Gneiss Region, Norway, indicates two burial events during the Caledonian orogeny

Abstract

The Blåhø Nappe on the island of Fjørtoft, which represents an isolated portion of the Seve Nappe Complex in the Western Gneiss Region, Norway, has been suggested to have experienced two deep burial cycles during the Caledonian orogeny. However, evidence on this multiple burial process by the derivation of a pressure–temperature–time (P–T–t) path has never been given in the literature. In this study, the ‘diamondiferous’ kyanite–garnet gneiss from the Blåhø Nappe on Fjørtoft was revisited to determine if such a process was correct. Two types of garnet, porphyroblastic garnet‐1 and fine‐grained garnet‐2, were recognized in the gneiss. The core of garnet‐1 is poor in Ca and documents P–T conditions of 1.2–1.3 GPa at c. 880°C based on pseudosection modelling. The inner rims of garnet‐1 and the core of garnet‐2 are both richer in Ca, recording P–T conditions of 1.35–1.45 GPa and 770–820°C. Application of conventional geothermobarometry on the outer rim of garnet‐1 and the rim of garnet‐2 yielded retrograde P–T conditions of 0.75–0.90 GPa and 610–685°C. These estimates define an anticlockwise P–T path at pressures below 1.5 GPa. Accessory monazite was dated with the electron microscope. Relicts of detrital monazite in the gneiss point to Sveconorwegian and possibly also Cryogenian provenance for the detritus of the sedimentary protolith. Metamorphic monazite in the gneiss records a wide age range from 460 to 380 Ma, with a peak c. 435 Ma and a shoulder at 395 Ma. These data suggest that the original (Ediacaran?) Baltica margin sediment (gneiss protolith) was transported to the base of an overlying plate during the early Caledonian (pre‐Scandian) orogeny. A long residence time of the metasedimentary rock at this base caused its heating to 880°C and homogenization of the early garnet chemistry. The late Caledonian (Scandian) collision between Baltica and Laurentia led to further burial, during which the studied gneiss was close to the former surface of the downgoing continental plate and, thus, cooled. The reconstructed P–T–t path confirms the multiple burial history of the Blåhø Nappe but contradicts previous ideas of deep burial of the Fjørtoft gneiss to more than 100 km.

The contribution of elastic geothermobarometry to the debate on HP versus UHP metamorphism

“…The thin lines are the entrapment isomekes of single inclusions while the thick lines and their associated shaded areas are the entrapment isomekes for the averaged P inc showed in Figure 3c,d. The red and blue star symbols in (b) are the P–T estimates by Liu and Massonne (2019) for garnet cores and rims, respectively, obtained with classical geothermobarometric methods and thermodynamic modelling. The difference in slope between the entrapment isomekes of zircon and quartz is due to their difference in thermal expansion and bulk modulus.…”

Pressure-Temperature Evolution of a Mylonitic Gneiss from the Lower Seve Nappe in the Handöl Area, Central Sweden

“…We relate these rocks to the upper portion of the downgoing Baltica Plate. In contrast, the HP-UHP rocks of the SNC recently studied by Fassmer et al (2017), Klonowska et al (2016), Litjens (2002), and Majka et al (2014a) represent deeply subducted rocks, possibly after tectonic erosion (Massonne and Li, 2020). These rocks were exhumed in a subduction channel and became part of the SNC rocks in an exhumation channel located between Baltica and the overlying plate.…”

“…But gneisses of the LSN from northern Jämtland (Fig. 1a) also experienced higher pressures of 16.5 kbar at 650-680 °C (Litjens, 2002) and 25-27 kbar at 680-760 °C (Fassmer et al, 2017; see Fig. 8).…”

“…The associated gneisses yielded HP peak conditions of ca. 16.5 kbar at 650-680 °C (Litjens, 2002) and 25-27 kbar at 680-760 °C (Fassmer et al, 2017). In the LSN, a pervasive amphibolitefacies foliation either overprinted the (U)HP fabric or represents the main metamorphic fabric (Giuntoli et al, 2018).…”

“…Massonne and Li (2020) generalized such a scenario for subduction of an oceanic plate and subsequent continent-continent collision being relevant for the Baltica-Laurentia collision after subduction of the Iapetus as well. The geodynamic scenario is based on earlier observations and geodynamic explanations of significant peak-pressure contrasts between country-rock gneisses and lenses of eclogite (Liu and Massonne, 2019;Li and Massonne, 2016;Massonne, 2012) and different peak pressures of eclogites in a single extended body (Li et al, 2017). The HP-UHP rocks of different type (mainly metasediments and metabasites) were subducted as slices or blocks to great depths and exhumed in a subduction channel (Massonne and Li, 2020).…”