The fate of subducted continental margins: Two‐stage exhumation of the high‐pressure to ultrahigh‐pressure Western Gneiss Region, Norway

Abstract

Thermobarometry suggests that ultrahigh-pressure (UHP) to high-pressure (HP) rocks across the Western Gneiss Region ponded at the Moho following as much as 100 km of exhumation through the mantle and before exhumation to the upper crust. Eclogite across the c. 22 000 km 2 study area records minimum pressures of c. 8-18 kbar and temperatures of c. 650-780°C. One orthopyroxene eclogite yields an UHP of c. 28.5 kbar, and evidence of former coesite has been found c. 50 km farther east than previously known. Despite this widespread evidence of UHP to HP, thermobarometry of metapelite and garnet amphibolite samples reveals a surprisingly uniform Ôsupra-BarrovianÕ amphibolite-facies overprint at c. 11 kbar and c. 650-750°C across the entire area. Chemical zoning analysis suggests that garnet in these samples grew during heating and decompression, presumably during the amphibolite-facies event. These data indicate that the Norwegian UHP/HP province was exhumed from mantle depths of c. 150 km to lower crustal depths, where it stalled and underwent a profound high-temperature overprint. The ubiquity of late-stage supra-Barrovian metamorphic overprints suggests that large-scale, collisional UHP terranes routinely stall at the continental Moho where diminishing body forces are exceeded by boundary forces. Significant portions of the middle or lower crust worldwide may be formed from UHP terranes that were arrested at the Moho and never underwent their final stage of exhumation.

Diffusion versus recrystallization processes in Rb–Sr geochronology: Isotopic relics in eclogite facies rocks, Western Gneiss Region, Norway

“…Eclogites occur in a tectonic window nearly 50.000 km 2 in size, making the WGR one of the largest continental crust-dominated HP-UHP occurrences in the world. Amphibolite-facies retrogression is widespread and related to late-to post-orogenic, almost isothermal exhumation and transtensional deformation (Andersen, 1998;Terry and Robinson, 2003;Walsh and Hacker, 2004;Labrousse et al, 2004). Hints to the timing of the amphibolite facies overprint come from widespread 395 ± 2 Ma U-Pb sphene and zircon ages (Tucker et al, 2004, and references therein), consistent with Ar-Ar ages for low-Si muscovite between 399 Ma (eastern parts of the WGR and <390 Ma further west (Hacker, 2007, and references therein).…”

Ultra High Pressure Metamorphism☆

“…of such mafic rocks did transform to eclogite, with the rest remaining unreacted (Straume and Austrheim, 1999;Krabbendam et al, 2000;Walsh and Hacker, 2004). A Barrovian to Buchan amphibolite-facies overprint-with local partial meltingoccurred at 650-800°C during post-UHP decompression from >1.5 GPa to~0.5 GPa, almost completely obliterating the record of (U)HP metamorphism (Krogh, 1980;Chauvet et al, 1992;Dransfield, 1994;Straume and Austrheim, 1999;Hacker et al, 2003b;Terry and Robinson, 2003;Labrousse et al, 2004;Walsh and Hacker, 2004;Root et al, 2005;Engvik et al, 2007). Importantly, this metamorphic history documents that the UHP rocks were exhumed nearly isothermally to depths of 15-20 km.…”

“…A general northwestward increase in metamorphic temperature is indicated by calculated temperatures, U-Pb sphene ages, the distribution of sillimanite, and textural evidence of in situ partial melting ( Fig. 1) (Labrousse et al, 2002;Terry and Robinson, 2003;Tucker et al, 2004;Walsh and Hacker, 2004;Root et al, 2005).…”

“…By far the most common metamorphic facies in the study area is amphibolite-facies (Bryhni, 1966;Mysen and Heier, 1971;Labrousse et al, 2002;Terry and Robinson, 2003;Tucker et al, 2004;Walsh and Hacker, 2004;Root et al, 2005). The main quartzofeldspathic gneisses in the study area contain quartz + plagioclase + biotite ± hornblende ± garnet ± K-feldspar.…”

Arc and Slab-Failure Magmatism in Cordilleran Batholiths II – The Cretaceous Peninsular Ranges Batholith of Southern and Baja California

“…Thus, shallow break-off creates narrow orogens, lower-grade metamorphism, and intense, rapid and higher rates of exhumation, whereas deep break-off creates broad orogens with higher grades of metamorphism and slow, more subdued rebound (Duretz et al 2011). In the case of deep failure the subducted margin might be sufficiently buoyant that it initially rises to the Moho, where it might stall until the over-thickened crust collapses by extension and/or denuded by erosion (Walsh and Hacker 2004).…”