CAMBRIAN A-TYPE GRANITES IN PRYDZ BAY, EAST ANTARCTICA: SR-ND-HF-PB-O ISOTOPIC CONSTRAINTS ON MAGMA SOURCES AND THE ORIGIN OF HIGH CONCENTRATIONS OF HEAT-PRODUCING ELEMENTS

Cambrian (519-494 Ma) syn- to post-kinematic granites intrude polymetamorphic granulite-facies gneisses for 170 km along Prydz Bay. The granites range from fine-grained, foliated biotitic veins (<1 m) in the northeast to km-scale unfoliated biotite-hornblende plutons with K-feldspar phenocrysts in the southwest. Accessories include garnet and monazite in the NE; allanite, titanite, chevkinite-perrierite and fayalite in the SW; and fluorite in both. Whole-rock compositions have features reported in A-type granites, e.g., high Fe, Na+K, K/Na, F, HFSE (322-1625 ppm Zr), LREE (107-2101 ppm Ce) and Th (up to 878 ppm Th; Th/U = 28-142). The ratios La/Lu_N (85-976) and Sr/Y (up to 45) are higher toward the NE than in the SW (respectively 10 to 85; <10). High La/Lu_N ratios in the NE are associated with higher initial ⁸⁷Sr/⁸⁶Sr = 0.716-0.756 and lower ε_Nd = −11.4 to −13.1), while the inverse applies in the SW, where ⁸⁷Sr/⁸⁶Sr = 0.7095-0.7300 and ε_Nd = −7.7 to −11.0. Initial ε_Hf (−6.8 to −13.8) correlates strongly with ε_Nd. The Hf-Nd isotopic data plot slightly above the global crustal Hf-Nd array. Magma δ¹⁸O values, estimated from quartz δ¹⁸O, range narrowly (8.5 to 9.8‰) except for an intrusion with ⁸⁷Sr/⁸⁶Sr ≈ 0.752 and δ¹⁸O = 11-11.4‰. Initial Pb isotope compositions from leached feldspars are characterized by high ²⁰⁷Pb/²⁰⁴Pb (15.71-15.77) and low ²⁰⁶Pb/²⁰⁴Pb (17.7-18.16), a signature requiring Pb isotope evolution in an old, high-U/Pb reservoir (≥ 2 Ga), followed by a low-U/Pb (µ<<10) stage for several 100 Ma prior to granite formation. Lowering of U/Pb (avg. 3.6 ppm U) may be related to U removal from older crustal protoliths at ca. 1 Ga during the first of two granulite facies events in Prydz Bay gneisses; the second event at ca. 0.5 Ga had much less impact. By contrast, initial ²⁰⁸Pb/²⁰⁴Pb (ca. 38.6) points to magma sources in early Paleozoic continental crust with typical Th/Pb (ω≈40-50). Pb isotope signatures thus indicate magma sources in Proterozoic rocks with low U and near-average Th concentrations. The high Th concentrations (avg. 172 ppm), which dominate the high heat production of the Cambrian granites (Carson et al. 2014, J Geol Soc London 171, 9-12), thus appear to result from the magmatic processes generating the A-type granites rather than from Th-rich source rocks.