Contact Metamorphism of Precambrian Gneiss by the Skaergaard Intrusion

Bufe, N. Aaron and Holness, Marian B. and Humphreys, Madeleine C. S. (2014) Contact Metamorphism of Precambrian Gneiss by the Skaergaard Intrusion. Journal of Petrology. ISSN 0022-3530, 1460-2415 DOI

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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 cathodoluminescence) and the titanium-in-quartz (TitaniQ) thermometer. The onset of recrystallization of the quartz in the gneisses occurred between 390 and 340 m from the contact whereas 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 the 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 devolatilization 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 fitted to the profile of maximum temperature through the aureole. The generally lower temperatures seen in the gneiss compared with those previously reported for the contact metamorphosed basalts higher up the walls of the intrusion are consistent with a heterogeneous release of latent heat of crystallization.

Item Type: Article
Additional Information: © The Author 2014
Uncontrolled Keywords: 2014AREP; IA68;
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
06 - Part-III Projects
07 - Gold Open Access
Journal or Publication Title: Journal of Petrology
Identification Number:
Depositing User: Sarah Humbert
Date Deposited: 18 Jul 2014 17:30
Last Modified: 07 Sep 2014 01:41

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