Peak to post-peak thermal history of the Saglek Block of Labrador: A multiphase and multi-instrumental approach to geochronology

Abstract

A B S T R A C TThe Saglek Block of coastal Labrador forms the western margin of the North Atlantic Craton, where Archean gneisses and granulites have been reworked during the Paleoproterozoic. Previous work has established that the block is a composite of Eoarchean to Mesoarchean protoliths metamorphosed to upper amphibolite and granulite facies at around 2.8-2.7 Ga. New in-situ microbeam dating of accessory minerals in granoblastic gneisses reveals a complex peak to post-peak thermal history. Zircon growth at ca. 3.7-3.6 Ga provides the age of formation of the tonalitic protoliths to the gneisses. Further zircon growth in syn-tectonic granitic gneiss and monazite growth in a variety of orthogneisses confirm peak metamorphic conditions at ca. 2.7 Ga, but also reveal high-temperature conditions at ca. 2.6 Ga and 2.5 Ga. The former is interpreted as the waning stages of the 2.7 Ga granulite event, whereas the latter is associated with a younger phase of granitic magmatism. In addition, apatite ages of ca. 2.2 Ga may represent either cooling associated with the 2.5 Ga event or a previously unrecognized greenschist-facies metamorphism event that predates the Torngat Orogeny.

Gneiss-forming events in the Saglek Block, Labrador; a reappraisal of the Uivak gneiss

“…3.3 Ga. The former are all grey orthogneisses and match published ages and descriptions of Uivak I gneiss (Kusiak et al 2018;Sałacińska et al 2018 and references therein). The two younger samples are of homogeneous grey gneiss (L1410) and porphyroblastic augen gneiss (L1467), but their geochemistry and zircon internal structures are similar and the equivalent ages (3318 ± 5 Ma and 3325 ± 3 Ma, respectively) suggest that they were co-genetic.…”

“…The first precise age of the formation of Uivak I gneiss (3732 ± 6 Ma) was determined on zircon by Secondary Ion Mass Spectrometry (SIMS) by Schiøtte et al (1989a). Further high-precision SIMS zircon ages were provided by Kusiak et al (2018), Sałacińska et al (2018) and Vezinet et al (2018). Vezinet et al (2018) have identified a felsic gneiss with a protolith age of 3.86 ± 0.01 Ga. Isotopic U-Pb dating of zircon grains also identified a significant amount of older zircon, up to ca.…”

“…Early mapping (Bridgwater et al 1975;Hurst et al 1975;Collerson 1976, 1977) followed by geochronological studies (Collerson and Bridgwater 1979;Shiotte et al 1989a, b;Bridgwater and Schiøtte 1991) defined and dated major lithologies. More recently, a resurgence of interest in the Saglek Block has led to several detailed studies (Krogh and Kamo 2006;Shimojo et al 2016;Komiya et al 2017;Kusiak et al 2018;Sałacińska et al 2018) that revised earlier interpretations and produced new controversies (e.g. Tashiro et al 2017;Whitehouse et al 2019), especially regarding the oldest components of the block.…”

Two Neoarchean tectonothermal events on the western edge of the North Atlantic Craton, as revealed by SIMS dating of the Saglek Block, Nain Province, Labrador

“…A section of the Saglek Block investigated in recent studies by the same group (Kusiak et al 2018;Sałacinśka et al 2018Sałacinśka et al , 2019Whitehouse et al 2019) extends for 80 km along the Labrador coast, from Ramah Bay in the north to Hebron Fjord in the south (Fig. 1b).…”

“…1b). Gneisses in the southern part of the section formed in several stages throughout the Archean, with the oldest and most abundant TTG-type protoliths (the Uivak gneiss) formed between c. 3850 and 3600 Ma (Schiøtte et al 1989;Komiya et al 2017;Kusiak et al 2018;Sałacinśka et al 2018Sałacinśka et al , 2019, with lesser episodes of felsic plutonism at c. 3.3-3.0, 2.7 and 2.5 Ga (Schiøtte et al 1990(Schiøtte et al , 1992Krogh & Kamo 2006;Komiya et al 2015;Sałacinśka et al 2019). Krogh & Kamo (2006) suggested that TTG gneisses from outcrops around Saglek Bay differ in age across the Handy Fault (Fig.…”