Abstract:

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 Al₂O₃ 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 in situ 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 in situ microdiamond recognition, described here for the first time. All in situ 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 ).