Metamorphosed cumulate gabbros from the Støren Group of the Upper Allochthon, northern Western Gneiss Region, Norway: petrology and metamorphic record

Abstract

The Støren Group of the Upper Allochthon, in the Scandinavian Caledonides, contains Late Cambrian to Early Ordovician ophiolitic fragments that developed in a tholeiitic oceanic suprasubduction zone setting. These were obducted onto an epicontinental margin (Gula Complex) in the Early Ordovician, and metamorphosed in most areas to greenschist facies. Later, these rocks were thrust onto Baltica as part of the Upper Allochthon during the Silurian-Devonian Scandian Orogeny. Cumulate gabbros in the Støren Group contain the prograde metamorphic assemblage amphibole-plagioclase-diopside-epidote. They have complex textures that include early metamorphic mineral cores and inclusions, later rims, and late vein assemblages. The textures and mineral compositions in nine samples were interpreted iteratively with Perple_X thermodynamic modeling, to reveal three sequential P-T conditions: ~350°C and ~6 kbars for early prograde mineral cores and inclusions, ~530°C and ~6 kbars for peak prograde conditions, and ~425°C and ~3 kbars for an early retrograde vein assemblage. Additionally, minor but ubiquitous, discontinuous Fe 3+ -rich rims on epidote suggest the rocks underwent isothermal decompression immediately after peak prograde conditions, crossing increasingly Fe-rich epidote isopleths predicted by models. We interpret a single-episode, clockwise P-T path representing an Early Devonian phase of Scandian metamorphism. In the study area, prograde metamorphism produced these assemblages only in rocks close to the contact between the Upper and Middle allochthons, which is interpreted as a late-Scandian detachment fault (Agdenes detachment). Prograde Scandian metamorphism of this part of the Upper Allochthon was a form of contact metamorphism, occurring as the detachment fault juxtaposed cool Upper Allochthon rocks against the hotter, exhuming Middle Allochthon.