Role of H₂O in the formation of garnet coronas during near‐isobaric cooling of mafic granulites: the Tasiusarsuaq terrane, southern West Greenland

Abstract

The Mesoarchaean Tasiusarsuaq terrane of southern West Greenland consists of Tonalite-trondhjemite-granodiorite gneisses and, locally, polymetamorphic mafic and ultramafic rocks. The terrane experienced medium-pressure granulite facies conditions during M 1A in the Neoarchean, resulting in the development of two-pyroxene melanosome assemblages in mafic granulites containing garnet-bearing leucosome. Reworking of these rocks during retrogression introduced garnet to the melanosome in the form of overgrowths, coronas and grain necklaces that separate the mafic minerals from plagioclase. NCFMASHTO pseudosection modelling constrains the peak metamorphism during M 1A to 850°C and 7.5 kbar at fluid-saturated conditions. Following M 1A , the rocks retained their M 1A H 2 O content and became fluid-undersaturated as they underwent near-isobaric cooling to 700°C and 6.5-7 kbar, prior to reworking during M 1B . These low H 2 O contents allowed for the formation of garnet overgrowths and coronas during M 1B . The stability of garnet is greatly increased to lower pressure and temperature in fluid-absent, fluid-undersaturated mafic rocks, indicating that fluid and melt loss during initial granulite facies metamorphism is essential for the introduction of garnet, and the formation of garnet coronas, during retrogression. The occurrence of garnet coronas is consistent with, but not unique to, near-isobaric cooling paths.

The Majorqaq Belt: A record of Neoarchaean orogenesis during final assembly of the North Atlantic Craton, southern West Greenland

“…The suturing of the southern five NAC blocks is suggested to have formed by collisional tectonics controlled by SE-dipping subduction zones, and possibly involving the closure of back-arc basins by subduction (Friend and Nutman, 1991;Nutman and Friend 2007;Windley and Garde, 2009). The currently accepted model for the formation of the NAC can be summarised by three overlapping phases: 1) Two protracted TTGforming events at 3.85-3.70 Ga and 3.20-2.82 Ga, attributed to the partial melting of basaltic lithologies such as amphibolite and eclogite (Tappe et al, 2011), which generates proto-blocks of continental crust; 2) a geographically wide-spread and protracted period of metamorphic zircon growth at 3.0-2.7 Ga (Garde et al, 2000) that records the timing of granulite facies metamorphism and the extensive emplacement of granitic rocks with a strong thermal overprint of 850°C at~0.75 GPa, indicating a Mesoarchaean thermal gradient of 1100°C/GPa (Dziggel et al, 2012); and, 3) accretion of island-arc type crustal blocks from the Fiskefjord block and southward, which culminated in the frequently cited (e.g., Baadsgaard and McGregor, 1981) 'Qaarusuk dyke' injection at ca. 2.72 Ga, demarking the end of penetrative Archaean deformation (Friend et al, 1996).…”

Mid-crustal shear zone development under retrograde conditions: pressure–temperature–fluid constraints from the Kuckaus Mylonite Zone, Namibia

“…6; cf. Dziggel et al, 2012). For the rocks to have replaced their high-T assemblages with the minerals observed in the samples described here, they must have experienced an addition of H 2 O (Fig.…”

Prograde and retrograde evolution of eclogite from Adrar Izzilatène (Egéré-Aleksod terrane, Hoggar, Algeria) determined from chemical zoning and pseudosections, with geodynamic implications

“…Previous studies on eclogite have demonstrated that the phase relations in these rocks, and the occurrence of overprinting amphibolite or granulite facies assemblages, is a function of not only pressure and temperature but also components such as Fe 2 O 3 (Ague et al, 2001;Chinner, 1960;Clarke et al, 1989;Diener and Powell, 2010;Rebay et al, 2010) and H 2 O (Baldwin et al, 2007;Dziggel et al, 2012;Rebay et al, 2010;Śtĭpskà and Powell, 2005). Rebay et al (2010) quantitatively investigated the role of H 2 O on the stability of eclogite assemblages, and found that these rocks might not form 'true' eclogite facies assemblages under fluid-saturated conditions.…”