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

Abstract

AbstractThe 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.