Monazite petrochronology of polymetamorphic granulite‐facies rocks of the Larsemann Hills, Prydz Bay, East Antarctica

• Published in: Journal of metamorphic geology Vol. 39; no. 9; pp. 1205 - 1228
• Main Authors: Spreitzer, Steven K., Walters, Jesse B., Cruz‐Uribe, Alicia, Williams, Michael L., Yates, Martin G., Jercinovic, Michael J., Grew, Edward S., Carson, Christopher J., Baldwin, Julia
• Format: Journal Article
• Language: English
• Published: Oxford Blackwell Publishing Ltd 01.12.2021
• Subjects: anatectic pegmatites; Bedrock; Biotite; Breakdown; Cambrian; Convergence; Cooling; Crystallization; Decompression; Domains; Electron microprobe; Electron probes; Garnet; Gondwana; Growth; Metamorphism; Metamorphism (geology); Monazite; petrochronology; Phase assemblages; Precambrian; Prydz Bay; pseudosection modelling; Rock; Rocks; Sedimentary facies; Tectonics; Prydz Bay
• ISSN: 0263-4929; 1525-1314
• DOI: 10.1111/jmg.12607

The Prydz Bay coast, including the Larsemann Hills, features relatively extensive bedrock exposures of interest because of the proximity to a hypothesized suture associated with Gondwana assembly. Critical units are the basement Søstrene Orthogneiss (1,126 ± 11 Ma protolith) and cover Brattstrand Paragneiss (maximum depositional age 1,023 ± 19 Ma). The two units share a polymetamorphic history with events at ~900 Ma (D1) and ~530 Ma (D2‐4). Here we present electron microprobe dates of monazite growth zones and Perple_X pseudosection models of granulite‐facies rocks from the Søstrene Orthogneiss, Brattstrand Paragneiss, and D2‐4 pegmatites of the Larsemann Hills. We propose a scenario for Cambrian metamorphism involving a peak stage at 6–7.5 kbar and 800–860°C (D2 convergence), melt crystallization and garnet breakdown during decompression to early retrograde conditions of 3–4.5 kbar and 700–750°C (D2 convergence, D3 extension), and a late retrograde stage with decompression and cooling to 3–3.5 kbar and 550–650°C (D4). We combine monazite chemistry with phase assemblages predicted by pseudosection modelling to link specific monazite growth domains to individual tectonic stages. Monazite domains containing moderate Th and low to moderate Y are interpreted to be preserved from the prograde path when garnet was stable, and constrain the timing of prograde metamorphism at 536 ± 4 Ma. High‐Th, low‐Y domains, dated at 527 ± 2 Ma, represent the earliest stages of post‐peak melt crystallization. Monazite domains with elevated Y and low‐moderate Th are interpreted to represent monazite growth during garnet breakdown at 514 ± 2 Ma. Our monazite ages, combined with published biotite Ar–Ar cooling ages, yield a two‐stage history of cooling at 3–8°C/Myr from ~530 Ma to ~510 Ma followed by cooling at 18–25°C/Myr from ~510 Ma to ~490 Ma, corresponding to 0.2–0.6 mm/yr of exhumation. This duration of granulite‐facies metamorphism in the Larsemann Hills is consistent with estimates for Precambrian granulite facies metamorphic complexes elsewhere.