229-15 Retrieving metamorphic conditions of felsic granulite from Lac Dumoine terrane, western Grenville Province, Canada

Session: Crustal Petrology (Posters)

Presenting Author:

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Abstract:

The Lac Dumoine terrane in western Quebec is a critical portion to address the tectonic processes of the Grenville Province (e.g. Gervais et al., 2023; Slagstad et al., 2024). Empirical thermobarometry has determined the pressure-temperature (P-T) conditions to be 1.35 GPa, 720°C for metabasites (Indares & Dunning, 1997). However, metamorphic conditions of the host felsic rocks are poorly understood, which has hampered our understanding of the Grenville tectonics. In this study, we examined a felsic garnet granulite, 19-61, from the Grenville Province to constrain the metamorphic P-T conditions. The sample consists of garnet, quartz, K-feldspar, plagioclase, biotite, clinopyroxene, along with accessory ilmenite, titanite, and apatite. Garnet is subhedral to anhedral in shape, variable sizes, and commonly adjacent to ferromagnesium minerals such as amphibole and biotite. Garnet shows relatively homogeneous composition and mainly consists of almandine, with a core composition of Alm67Py3Grs27Sps3 and Alm69Py2Grs24Sps4 at the rim. Plagioclase is the dominant mineral in the specimen, with an oligoclase composition of Ab83-86An13-16Kfs1. Amphibole is  ferro-pargasite, a composition of 1.86p.f.u.) Ca in B-site and 0.42 p.f.u. Na in A-site. Biotite is dominantly composed of annite, with a Mg# of 0.16 to 0.17. Phase equilibrium modeling was conducted using Perple_X ver. 6.9.0 with the sample-specific composition and suitable solid solution models for felsic composition. The intersecting P-T conditions for the garnet core composition (XCa = 0.28, Mg# = 0.05) are at ~1.05 GPa and 800 °C, in the field with a mineral assemblage of plagioclase, K-feldspar, garnet, melt, biotite, clinopyroxene, ilmenite, titanite, and quartz. The garnet rim composition (XCa = 0.24, Mg# = 0.03) was equilibrated at 1.0 GPa and 780 °C in the field with a mineral assemblage of plagioclase, K-feldspar, garnet, melt, biotite, clinopyroxene, ilmenite, and quartz. Our results show that the peak metamorphic pressure was slightly lower than previous estimates and that of a migmatized metabasite (~1.5 GPa,) using pseudosection modeling, but temperatures were higher and more consistent with the migmatized metabasite (850 °C). We interpret that the felsic granulite was equilibrated at the determined conditions, erasing prograde information. Ongoing geochronological study will be integrated to address the timing of high-pressure metamorphism in the conflicting tectonic models.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11060

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