Contact metamorphism of Precambrian gneiss by the Skaergaard intrusion

• Main Authors: Bufe, NA, Holness, MB, Humphreys, MCS
• Format: Article in Journal/Newspaper
• Language: English
• Published: Oxford University Press (OUP) 2014
• Subjects: metamorphic aureole; Skaergaard; cathodoluminescence; titanium-in-quartz thermometry; Greenland; East Greenland
• Online Access: https://www.repository.cam.ac.uk/handle/1810/245395

The Tertiary Skaergaard Intrusion, East Greenland, intruded at the shallow crustal unconformity between Precambrian amphibolite-facies gneisses and overlying Tertiary Plateau Basalts. Maximum contact metamorphic temperatures in quartzo-feldspathic gneisses were determined in two sample traverses across the aureole on the western contact of the intrusion using a combination of microstructural observations (both optical and with cathodoluminescence) and the titanium in quartz (TitaniQ) thermometer. The onset of recrystallisation of the quartz in the gneisses occurred between 390 m and 340 m from the contact while H2O-fluxed melting occurred in gneisses closer than 130 m from the contact (where T > ~ 675˚C). The maximum temperature recorded by quartz at the contact is ~865 ± 70°C. Melt fractions reach 50-60 vol% in some samples although melt is heterogeneously distributed on all scales. Minor bands of amphibolite facies mafic gneiss are extensively reacted to an anhydrous pyroxene-bearing hornfels close to the contact, whereas those further than ~130 m are overprinted by a greenschist facies assemblage. Discrepancies between the expected temperature for the amphibolite to greenschist facies reaction and temperatures obtained from adjacent quartzo-feldspathic gneisses are consistent with the formation of the anhydrous pyroxene hornfels directly from the mafic gneiss, with the lower-grade greenschist facies assemblage forming on the retrograde path after the establishment of limited hydrothermal activity. It is unlikely that devolatilisation reactions in the gneiss produced sufficient H2O to account for the pegmatitic features formed in the Marginal Border Series in the intrusion. A simple one-dimensional thermal model, neglecting any advection of heat by hydrothermal circulation, was fit to the profile of maximum temperature through the aureole. The generally lower temperatures seen in the gneiss compared to those previously reported for the contact metamorphosed basalts higher up the walls of the intrusion are ...