Recent progress in Scandian ultrahigh-pressure metamorphism in the northernmost domain of the Western Gneiss Complex, SW Norway:
continental subduction down to 180–200 km depth
<p>Earlier workers established 20 kbar and 750–800 °C as being the maximum Scandian metamorphic grade for the Caledonized basement
units in the northernmost ultrahigh-pressure metamorphic (UHPM) domain of the Western Gneiss Region, SW Norway. These early
pressure estimates were entirely based on jadeite isopleths applied to bimineralic eclogites; low Al values in orthopyroxene
from opx-eclogites were interpreted to be due to ’disequilibrium effects'. After the initial discovery of microdiamond and
majoritic garnet in the area, the low Al<sub>2</sub>O<sub>3</sub> isopleths in opx (cores), ranging from 0.15 to 1.50 wt% in newly discovered and already known occurrences of ’internal' and
’external' opx eclogites–websterites, were reinvestigated and taken as the first indicator that metamorphic grade, on a regional
scale, occurred at much higher pressures than previously thought. Regional-scale diamond-grade UHPM conditions were subsequently
demonstrated by <em>in situ</em> microdiamond discoveries in two widely separated orthopyroxene eclogite–websterite localities included in Mg–Cr (mantle-derived)
and Fe–Ti type (crust-derived) peridotite–websterite bodies, leading to a simple strategy for <em>in situ</em> microdiamond recognition, described here for the first time. All <em>in situ</em> microdiamonds are associated with multiphase solid-inclusion assemblages interpreted to be crystallized from Scandian supercritical,
dense, COH-rich subduction zone fluids. In addition, the recent discovery at Fjørtoft of a second, Scandian, majoritic garnet
generation (in garnet websterites enclosed in garnet peridotite), having enriched light REE (LREE) patterns (in clinopyroxene
and garnet), contrasts strongly to the Archaean–mid-Proterozoic LREE-depleted majoritic garnet patterns (and exsolved pyroxenes)
discovered in garnet peridotites from Otrøy. The latter is strong evidence that Scandian continental subduction (and eduction)
occurred down to 180–200 km depths, making the northernmost UHPM domain by far the deepest subducted (and educted) part of
the Western Gneiss Complex. Such a conclusion is also consistent with (some) geothermobarometric results from internal and
external orthopyroxene eclogites and garnet websterites showing results indicative of pressure–temperature estimates deep
in the diamond-eclogite facies (850–950 °C and 5.5–6.5 GPa ).
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