Distinguishing East and West Antarctic sediment sources using the Pb isotope composition of detrital K-feldspar

Abstract

New Pb analyses of K feldspars (Kfs) from Archaean–Mesozoic crystalline rocks from across the Weddell Sea region of Antarctica show Pb isotopic compositions that vary geographically. Five distinct basement provinces are defined, each with characteristic Kfs Pb compositions, and indicate that the comparison of the Pb isotopic composition of individual detrital Kfs can be used as a sedimentary provenance tool in this region. This tool is tested on Permian sandstones deposited in a retro-arc foreland basin because the potential source regions for these sandstones, the active Palaeopacific margin arc of West Antarctica and uplifted regions of now ice-covered East Antarctica, comprise rocks with markedly different Pb isotope compositions. Pb compositions of detrital Kfs from sandstone samples collected from Dronning Maud Land suggest that their provenance was entirely from within East Antarctica, while those collected from the Theron Mountains, Coats Land, were wholly derived from the active margin of West Antarctica. Kfs Pb composition of bedrock samples is largely dependent on age and because Kfs tends to only survive a single cycle of erosion, transport, deposition and diagenesis, its provenance when compared with that determined by detrital zircon geochronology, may qualify what proportion of the more robust mineral zircon has been recycled through several sedimentary cycles. Detrital zircon analyses from a Permian sandstone from the Theron Mountains suggest that 45% of its detrital zircon was recycled from pre-existing (meta)sedimentary rocks. The detrital feldspar data also indicate that Archaean and 2.1 Ga crust, with a very different Pb isotope composition to any exposed in the Weddell Sea region, is likely to exist beneath the East Antarctic Ice sheet.