U–Pb zircon age dating of diamond-bearing gneiss from Fjørtoft reveals repeated burial of the Baltoscandian margin during the Caledonian Orogeny

Abstract

The first find of microdiamond in the Nordøyane ultra-high-pressure (UHP) domain of the Western Gneiss Region (WGR) of the Scandinavian Caledonides reshaped tectonic models for the region. Nevertheless, in spite of much progress regarding the meaning and significance of this find, the history of rock that the diamonds were found in is complex and still largely ambiguous. To investigate this, we report U–Pb zircon ages obtained from the exact crushed sample material in which metamorphic diamond was first found. The grains exhibit complicated internal zoning with distinct detrital cores overgrown by metamorphic rims. The cores yielded a range of ages from the Archaean to the late Neoproterozoic / early Cambrian. This detrital zircon age spectrum is broadly similar to detrital signatures recorded by metasedimentary rocks of the Lower and Middle allochthons elsewhere within the orogen. Thus, our dating results support the previously proposed affinity of the studied gneiss to the Seve–Blåhø Nappe of the Middle Allochthon. Metamorphic rims yielded a well-defined peak at 447 ± 2 Ma and a broad population that ranges between c. 437 and 423 Ma. The data reveal a prolonged metamorphic history of the Fjørtoft gneiss that is far more complex then would be expected for a UHP rock that has seen a single burial and exhumation cycle. The data are consistent with a model involving multiple such cycles, which would provide renewed support for the dunk tectonics model that has been postulated for the region.

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

“…The new P–T estimates presented here confirm the complexity of the metamorphic evolution experienced by the rocks in the island of Fjørtoft (Walczak et al., 2019). The garnet–kyanite gneiss presented here lacks major mineralogical evidence for an UHP equilibration before its current, lower pressure, mineral assemblage.…”

“…The samples studied (Figure 1a) are a quartz+garnet vein (FJ1704) from the Ulla Gneiss (Terry et al., 2000) and a garnet–kyanite gneiss (FJ1702) from the Blåhø nappe, where microdiamond formation has been documented (Dobrzhinetskaya et al., 1995; Larsen et al., 1998; Liu & Massonne, 2019; Walczak et al., 2019). The vein sample FJ1704 consists of quartz and garnet in equal amounts (Figure 1b,d).…”

“…It records a Scandian (430–390 Ma) overprint in amphibolite, granulite, and eclogite facies over Proterozoic (>3.1–1.5 Ga) igneous and metamorphic rocks (Gorbatschev, 1985). The complex metamorphic history of the WGR is currently thought to be the result of multiple burial and exhumation cycles in the period between 450 and 390 Ma (Walczak et al., 2019). Several lenses of HP and UHP eclogite and garnet peridotite occur at different levels of the tectono‐stratigraphy (Cuthbert et al., 2000; van Roermund, 2009).…”

Halogens in Eclogite Facies Minerals from the Western Gneiss Region, Norway

“…The large, combined size of the UHP domain outcrop within the WGR and their correspondence with ultra-coarse, decussate, metasomatic garnet websterites (Figure 1) suggest that subduction of giant UHP terrains might have a significant geochemical influence on the SCLM in collisional orogens, even though subduction was transient. This is reinforced by emerging evidence for earlier, extensive diamond-facies UHP metamorphism in the Seve Nappe Complex of the Scandinavian Caledonides [35] thought to record one or more arc-continent collisions with Baltica during early to middle Ordovician, pre-Scandian closure of Iapetus.…”

“…The overlying allochthons locally occupy linear inliers within the WGR, where they have been infolded and/or imbricated into the basement [34]. Those derived from Baltica basement or cover have undergone Scandian eclogite-facies metamorphism and possibly earlier, Ordovician high-pressure metamorphism [35] though the infolded oceanic allochthons only show greenschist to amphibolite metamorphism [36]. In the main nappe pile outside the basement windows, HP and UHP eclogite-facies metamorphism was widespread within allochthons derived from Baltica [35,37] including the Lindås Nappe near Bergen, in which mid-to-late Proterozoic granulite-facies anorthosite underwent early Scandian eclogite-facies transformation related to fluid ingress [37].…”