New constraints on metamorphism in the Rauer Group, Prydz Bay, east Antarctica

Abstract

Granulite facies metapelites of the Mather and Filla Paragneisses within the Rauer Group, east Antarctica, possess markedly different compositions. The metamorphic evolution of the two metapelite types has been interpreted as temporally distinct, with the Rauer Group preserving at least two distinct granulite facies tectonothermal episodes. Calculated P–T pseudosections and orthopyroxene Al content indicate the revised maximum‐preserved P–T conditions within the Mather Paragneiss to lie in the vicinity of 950–975 °C and 10–10.6 kbar, less extreme than previous estimates. The range of possible P–T paths for the Mather Paragneiss consistent with mineral textural relationships and pseudosections contoured for mineral proportion are significantly shallower (dP/dT) than previous estimates. A near‐isothermal decompression P–T path, and extreme peak metamorphic conditions, are not necessary to explain the development of preserved mineral reaction textures. The Filla Paragneiss contains pelitic assemblages less amenable to rigorous quantitative analysis. Nevertheless, possibilities for the shared or otherwise metamorphic evolution of the Mather and Filla Paragneisses may be postulated on the basis of calculated pseudosections in the context of existing geochronology for the Rauer Group and preserved microstructures. A shared evolution, most likely during Pan‐African granulite facies metamorphism, is plausible and consistent with mineral assemblage development, geochronology and microstructures. A revised interpretation of the Rauer Group's preserved metamorphic evolution may warrant the revision of existing tectonic models, applicable also to the remainder of Prydz Bay. More generally, the employed approach may incite a revision of peak P–T and P–T paths in other granulite facies terranes.

Peak and post–peak development of UHT metamorphism at Mather Peninsula, Rauer Islands: Zircon and monazite U–Th–Pb and REE chemistry constraints

“…Based on their interpretation that 980-580 Ma zircons in their Mather Paragneiss sample were detrital grains of varied metamorphic provenance, Kelsey et al (2008) concluded that the Mather Paragneiss was not older than 580 Ma, citing this as proof that the UHT metamorphism occurred in the Prydz Tectonic Event from 575 through to 510 Ma, in accord with evidence from monazite chemical dating (Kelsey et al 2003b(Kelsey et al , 2007. As argued above, the broad range in Neoproterozoic ages could plausibly be the result of variable resetting of early Neoproterozoic detrital grains by the Prydz Tectonic Event, so that the Mather Paragneiss could have been deposited as early as 1000 Ma but after the Rauer Tectonic Event.…”

“…Kelsey et al (2003b) reported monazite electron microprobe chemical ages for the Mather Paragneiss that ranged from 521 ± 8 to 500 ± 6 Ma. These data were complemented by in-situ results from two further samples that yielded ages of up to 544 ± 11 Ma for monazites hosted in or exhumed from garnet porphyroblasts (Kelsey et al, 2007).…”

“…Zircon and monazite age data on the Filla Paragneiss (Kinny et al, 1993;Kelsey et al, 2007Kelsey et al, , 2008 indicate it to be polymetamorphic, with a 1030-970 Ma high-temperature metamorphic event (Rauer Tectonic Event; Harley and Kelly, 2007b) overprinted by the Cambrian Prydz event metamorphism at 545-510 Ma (Prydz Tectonic Event; Harley and Kelly, 2007b). The Mather Paragneiss, outstanding for its preservation of evidence for ultrahigh temperature (UHT) metamorphism at 1.0-1.2 GPa and 990-1030°C (Harley and Fitzsimons, 1991;Harley, 1998;Kelsey et al, 2003b;Harley and Kelly, 2007a), remains enigmatic as the age and regional significance and extent of this UHT event along with the relationship to the pervasive Prydz event (545-510 Ma) is still not well constrained. Harley et al (2009) recently suggested, on the basis of U-Th-Pb chemical ages of monazite associated with garnet + sapphirine + quartz in a Mather UHT paragneiss from Torckler Island, that UHT metamorphism occurred at >580-590 Ma and was associated with an event separate from the major Prydz event at 545-510 Ma.…”

Extending our understanding of Ultrahigh temperature crustal metamorphism

“…Ouzegane et al, 2003) assemblages. As a result there is considerable uncertainty in the dP/dT slopes of such ITD paths, which may involve considerable cooling contemporaneous with decompression, a point well emphasised recently by Kelsey et al (2003a;2003b).…”

“…2a) but have the advantages of being adjustable in the light of further experiments in adjacent or related systems and usable as frameworks for quantitative pseudosections that are based on the same thermodynamic data. Ouzegane et al (2003) and Kelsey et al (2003a;2003b) have adopted this approach to produce calculated grids in KFMASH and pseudosections in KF-MASH / NCKFMASH in order to interpret Grt + Spr + Crn and Opx + Sil + Qz UHT assemblages respectively. Figure 3a depicts a partial petrogenetic grid in KFMASH based on the experiments of Carrington and Harley (1995a) and contoured for the percentage of Mg Tschermaks component in orthopyroxene following Harley and Thompson (2004).…”