T223 Radiogenic Isotopes as Tracers of Geologic Processes: Dates, Rates, and Proxies I

Wednesday, 12 October 2022
8:00 AM - 12:00 PM
502

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

Paper number: T223-14
Program: Technical Sessions
Time: 11:00 AM – 11:15 AM

Authors

Abstract

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/LuN (85-976) and Sr/Y (up to 45) are higher toward the NE than in the SW (respectively 10 to 85; <10). High La/LuN ratios in the NE are associated with higher initial 87Sr/86Sr = 0.716-0.756 and lower εNd = −11.4 to −13.1), while the inverse applies in the SW, where 87Sr/86Sr = 0.7095-0.7300 and εNd = −7.7 to −11.0. Initial εHf (−6.8 to −13.8) correlates strongly with εNd. 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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-380089

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