Origins of high δ18O in 3.7–3.6 Ga crust: A zircon and garnet record in Isua clastic metasedimentary rocks

Tectonics of the Isua Supracrustal Belt 1: P‐T‐X‐d Constraints of a Poly‐Metamorphic Terrane

“…The annuli and rim of both garnets also show similar trends (a relative Alm + Py enrichment toward the rim; Figure 6e) suggesting that the different zones recorded similar processes in both samples. However, we argue that the inner zones core-annuli (Grt 1-2 this study and Grt I-III, Gauthiez-Putallaz et al, 2020) represent only one tectono-metamorphic event (M 1 ) instead of two based on our chemical-microstructural analysis (Figure 10a). Note that the inclusion trail in the garnet shows a remarkable continuity from the core to the annuli (Figures 4e, 4f and 6a), suggesting that they grew during a continuous deformation event so that the outline of the crystal shape would likely have followed the end of the inclusion trails.…”

“…Rollinson (2002) and other studies have linked them to up to three different medium-temperature metamorphic events: two in the early Archean and one in the late Archean (Gauthiez-Putallaz, 2020; Rollinson, 2002Rollinson, , 2003. Gauthiez-Putallaz et al (2020) reported four chemical zones using major (Figure 6e) and trace elements and δ 18 O that match microstructural zones. The authors interpret that the core of their garnets (Grt I ) represent an early Archean event at 550°C and the garnet annuli (Grt II&III ) represent later Eoarchean metamorphism at 610°C and 0.60 GPa (Figure 2), whereas the inclusion-poor rim (Grt IV ) recorded final post-tectonic event in the late Archean (Figure 2).…”

“…Additionally, detailed investigations of the textures and chemical composition of garnets across the belt revealed Timeline showing the major geological events interpreted for the ISB (bands) and ages interpreted as metamorphic (motifs with error bars). Interpretation of the garnet growth from Gauthiez-Putallaz et al (2020). Geochronological data from Nutman and Friend (2009).…”

“…the common appearance of three to four distinct zones (core-annuli-rim) within individual grains (Gauthiez-Putallaz et al, 2020;Rollinson, 2002Rollinson, , 2003, leading to the interpretation that the core could represent an earlier medium-temperature metamorphic episode preceding the large syn-tectonic event ( Figure 2) responsible for most of the deformation (Gauthiez-Putallaz et al, 2020;Rollinson, 2002).…”

“…Garnets in sample 714-5E shows very similar major element zoning pattern, with three zones predominantly marked by the change in Ca composition ( Figure 6). Our sample and sample G12/113 of Gauthiez-Putallaz et al (2020) were collected within 50 m of each other and have similar bulk rock composition and mineralogy, and thus can be assumed to have experienced the same tectonic history. When the compositions of the different zones within the garnets are compared, it is clear that they follow a similar pattern: the garnet cores form a separate compositional group in the ternary diagram of Py + Alm-Gro-Sps (Figure 6e), showing that they represent the same metamorphic event in both samples.…”