New constraints from U–Pb, Lu–Hf and Sm–Nd isotopic data on the timing of sedimentation and felsic magmatism in the Larsemann Hills, Prydz Bay, East Antarctica

Abstract

Complexly deformed gneisses in the Larsemann Hills, southern Prydz Bay, are customarily divided into a basement igneous complex (Søstrene Orthogneiss) and overlying metasediments (Brattstrand Paragneiss). New ion microprobe U–Pb zircon ages support this geological model and provide further age constraints for sediment deposition at ca. 1000Ma and high-grade metamorphism at ca. 900Ma. U–Pb zircon data for the Søstrene Orthogneiss, a prominent unit of the basement complex, indicate an igneous protolith age of 1126±11Ma. The igneous protolith of the Blundell Orthogneiss was emplaced at 968±13Ma. The Tassie Tarn Metaquartzite, a unit of the Brattstrand Paragneiss, has an estimated maximum depositional age of 1023±19Ma based on the weighted mean 207Pb*/206Pb* age of 1023±19Ma for the three youngest recognized detrital zircons, whereas the oldest inheritance is 2.54Ga. Metamorphic rims on the detrital zircons define a broad discordia array between ca. 900Ma and ca. 530Ma which is interpreted to reflect metamorphic zircon growth or resetting at these times. Magmatic zircons in the Søstrene orthogneisses have narrow ranges of initial ɛHf (−2.8 to +3.6) and Hf model ages (TDM2 1.53–1.93Ga), compared with much wider ranges (ɛHf=−16.4 to +6.6, TDM2 1.44–2.91Ga) recorded in detrital zircons of similar age in the Tassie Tarn Metaquartzite. This allows a partial provenance link between Søstrene-type basement and overlying metasediments but suggests additional sedimentary contributions to the metaquartzite from older crustal sources. Such contributions may be represented in the metaquartzite by the older detrital zircons, which have significantly older Hf model ages (TDM2 2.45–2.91Ga). A contribution from older crust is consistent with Nd isotope evidence for the Tassie Tarn Metaquartzite which has Nd model ages (TDM2) near 2.0Ga, similar to Brattstrand Paragneiss in general, and older than Nd model ages in Søstrene Orthogneiss (TDM2 ca. 1.7Ga). Relative age relationships between the Blundell Orthogneiss and Tassie Tarn Metaquartzite have been obscured by deformation, but our U–Pb zircon results permit Blundell Orthogneiss to be younger than the Brattstrand Paragneiss. Zircon Hf (ɛHf −1.0 to −3.7, TDM2 1.83–1.95Ga) and whole rock Nd (ɛNd=−3.0 and −5.9; TDM2 1.77–1.98Ga) isotopic data for the former allow the igneous protolith for this orthogneiss to be derived by partial melting or assimilation of both Søstrene Orthogneiss and Brattstrand Paragneiss.The rocks exposed in southern Prydz Bay bear similarities with those exposed the northern Prince Charles Mountains and we argue these regions probably represent a contiguous terrane, albeit with differing degrees of early Cambrian overprinting. We suggest that precursors of the Brattstrand Paragneiss were deposited in a back-arc basin located inboard of a ca. 1000Ma continental arc that was active along the leading edge of the Indo-Antarctic craton. Collision with the Australo-Antarctic craton (ca. 530Ma) merged these rocks into Gondwana and sutured them into their present position in Antarctica.