Pyroxene exsolution microstructures in garnet from the Almklovdalen peridotite, SW Norway

Abstract

We report the occurrence of oriented pyroxenes in garnets within a garnetite and an orthopyroxenite from the Almklovdalen peridotite body located in the central-western part of the Western Gneiss Region (WGR), SW Norway. The garnetite sample is a non-deformed clot or aggregate that contains optically strongly birefringent purple garnets up to several centimetres in size. The microstructure is defined by millimetre-sized olivine and orthopyroxene grains that partially decorate garnet–garnet grain boundaries, but also occur as inclusions within individual garnets. Additionally, garnet has acicular to lamellar shaped inclusions that are dominantly 40–60 μm thick, up to 600 μm long and seem to have their longest crystal-shape axis oriented in four directions within the garnet host mineral. Individual lamellae are composed of orthopyroxene (dominantly) or clinopyroxene or both. Usually, these early minerals (interstitials, inclusions and host) are severely replaced by kelyphite, amphibole and chlorite. The garnet-orthopyroxenite sample is less altered and shows substantial dynamic recrystallisation. Porphyroclastic orthopyroxenes have abundant chromite and clinopyroxene lamellae while porphyroclastic garnets preserve oriented lamellae of pyroxene that are similar to those of the garnetite sample. These lamellae in cogenetic garnet and pyroxene indicate an origin by exsolution during cooling from temperatures ≥1500 °C at lithospheric mantle depth. Dynamically recrystallised pyroxenes lack oriented inclusions. Garnets of both samples are pyrope-rich (55–79 mol%) and have moderate CaO (4.0–5.7 wt%) and Cr2O3 (0.95–2.90 wt%) contents. Application of classical thermobarometry on the lamellae-bearing mineral assemblages suggests equilibration at metamorphic conditions of circa 2.9 GPa and 730 °C. This P–T estimate is consistent with a cratonic geotherm with 38mWm−2 surface heat flow. In contrast, the dynamically recrystallised mineral assemblage has orthopyroxene with a very low Al2O3 content (0.28 wt%) indicating equilibration at circa 4.1 GPa and 770 °C, i.e. within the stability field of diamond. The microstructures, the chemistry and metamorphic conditions of the minerals in the Almklovdalen garnetite and garnet-orthopyroxenite resemble those of similar rocks with Archaean majoritic garnet precursors that occur in the northwestern part of the WGR. Accordingly, these mantle fragments are interpreted to have been derived from the same depth interval (circa 90-110 km) within the mantle hanging wall of cratonic Laurentia before they have been incorporated into the subducted Baltic crust. Subsequent shared prograde diamond-grade partial recrystallisation implies that the early Scandian tectonic history across the WGR had much more in common than previously recognised. Another implication is that former majoritic garnet was once spatially widespread throughout the E Greenland lithosphere.