168-5 What’s under the ice? Implications for Precambrian Antarctic terrane architecture from Pb isotope geochemistry of Proterozoic granite till clasts from the Transantarctic Mountains

Session: Laurentia Without Borders: Pre-Pangea Intercontinental Connections

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Pb isotope geochemistry provides a uniquely powerful tool for fingerprinting Precambrian tectonic terranes and reconstructing supercontinents.  In contrast to the Nd system where isotope composition is governed by time and the Sm/Nd ratio, Pb isotope compositions provide a more multidimensional set of parameters, reflecting time, U/Pb, Th/Pb, and Th/U.  While the Precambrian elements of continents such as Laurentia have well characterized Pb systematics, only sparse Pb isotopic data are available for East Antarctica due to its remoteness and the inaccessibility of most of its bedrock.  Here, we present the first Pb isotope data for Precambrian samples from the Transantarctic Mountains (TAM) sector of this continent and compare it to other parts of East Antarctica and other continents that may have been adjacent in the Precambrian. 

Pb isotope analyses were conducted by solution MC-ICP-MS on separated feldspars from granitic erratics with U-Pb ages of 1.2, 1.4-1.9, and 2.0 Ga, taken from moraines sampling different outflow channels of Byrd Glacier.  While the 2.0 Ga samples have highly radiogenic Pb suggesting Phanerozoic disturbance of the Pb system or incomplete separation of feldspar from other minerals, the other samples appear to retain their Precambrian initial Pb isotope compositions and form distinct groups in Pb isotope space.  In particular, samples with ages between 1.8 and 1.9 Ga, all of which are drawn from the most northern outflow channel, have both elevated ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb for a given ²⁰⁶Pb/²⁰⁴Pb, features consistent with derivation from a terrane with high time-integrated Th/U and U/Pb in the Antarctic interior.  These characteristics are similar to bedrock from the small and distant Charcot domain of Austral-Antarctica in Wilkes Land, 60° west of the TAM (Mulder et al., 2019), but they are common features of the Mojave province of southwestern Laurentia, strengthening Proterozoic ties between these two continents.  The other TAM samples have relatively nonunique Pb isotope compositions in that they plot in an area of Pb isotopic space where both the Indian and Australian sectors of Antarctica and terranes of southwestern Laurentia overlap. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5826

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