High to ultra-high temperature contact metamorphism and dry partial melting of the Tasiuyak Paragneiss, Northern Labrador: a petrographic, thermobarometic and phase equilibria modeling study

Mitchell, Rhea Katherine (2011) High to ultra-high temperature contact metamorphism and dry partial melting of the Tasiuyak Paragneiss, Northern Labrador: a petrographic, thermobarometic and phase equilibria modeling study. Masters thesis, Memorial University of Newfoundland.

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Abstract

Contact aureoles of the anorthositic to granitic plutons of the Mesoproterozoic Nain Plutonic Suite, Labrador, are particularly well developed in the Paleoproterozoic, regionally metamorphosed, granulite-facies, migmatitic Tasiuyak paragneiss. Regional metamorphism formed the assemblage Qtz-Kfs-PI-Grt-Sil±Bt±Liquid. Phase equilibria modeling yields a minimum peak of 8.3 kbars and 852°C for this event, consistent with the conditions necessary for biotite dehydration melting. These rocks underwent a significant melt loss event(s), removing much of the hydrous material, leaving behind relatively dry compositions. Rocks within the contact aureoles were statically overprinted by lower P, but higher T mineral assemblages and by textures related to a second partial melting event. Samples collected within multiple contact aureoles, near Anaktalik Brook, west of Nain, Labrador are categorized into five main sample types based on textures and mineralogy: A: Spl-Crd domains (Sil pseudomorphs) and Opx-Crd (Grt pseudomorphs) within a quartzofeldspathic domain, B: interwoven PI-rich and Crd-rich domains with Spl-Crd and Opx-Crd regions, similar to type A samples, C: complex arrangements of Spl-Crd domains, rounded Pl-Crd domains, interstitial PI-Opx domains, very fine grained intergrowths of Ksp-Qtz-Crd-Opx (Osm pseudomorphs) and coarse-grained domains of Ksp-Qtz-Crd-Opx (neosome), D: Sil-rich and Fa-rich (Grt pseudomorphs) domains within quartzofeldspathic matrix, and E: garnetiferous domains (Opx or Grt pseudomorphs), amoeboid Spl-Pl domains (Sil pseudomorphs) and coarse-grained Ksp-Qtz-Opx domains enclosed in a quartzofeldspathic matrix. Type A and C samples contain a very fine- to medium-grained geometric intergrowth of Crd-Qtz interpreted to have formed during melt-production during the dissolution of Bt and feldspars in the presence of melt. Evidence for partial melting includes: (i) lobate Ksp and Qtz pseudomorphs after former melt films and pockets (locally enclosing euhedral, idiomorphic cordierite) within and around plagioclase grains exhibiting corrosion along contact with films; (ii) in-filling of fractures in Ksp by PI, PI-Qtz symplectites and Qtz; (iii) thin films of Ab-rich Pion matrix PI; and (iv) Bt-Qtz symplectic rims around Bt coronae around Ilm. The formation of textures and minerals during the contact metamorphism was controlled by the textures and mineralogy of the regional metamorphic assemblage. UHT conditions are supported by: (i) >900°C high PIT gradient solidus estimated for all samples, coupled with extensive textural evidence for contact-related partial melting (ii) pseudomorphs after the UHT mineral osumilite, (iii) Ti-in-Qtz thermobarometry indicating contact metamorphism at temperatures of at least 700-980°C, (iv) high AI orthopyroxenes, (v) Zn- and Cr- bearing spinel and other common UHT textures involving Spl, Qtz, Crd, Opx and/or Crn locally replacing Grt or Sil).

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