ISOTOPIC CONSTRAINTS (PB, RB−SR, SM−ND) ON THE SOURCES OF EARLY CAMBRIAN PEGMATITES IN THE LARSEMANN HILLS, PRYDZ BAY, ANTARCTICA: A TWO-COMPONENT MODEL

The Neoproterozoic granulite-facies Brattstrand Paragneiss is cut by 3 generations of ~500 Ma pegmatite. The first forms concordant and discordant pods and veins up to 1 m thick, locally folded, whereas the second forms planar, discordant veins up to 20−30 cm thick, and the third, discordant bodies. Generations 1 & 2 contain tourmaline + quartz intergrowths and 5 other borosilicates, whereas the third carries tourmaline, beryl and primary muscovite. Spatial correlations between B-bearing pegmatites and B-rich units in the host Brattstrand Paragneiss are strongest for generation 1 and weakest for generation 3, suggesting the former may be closer to its source.

The Rb-Sr isotopic compositions of 9 pegmatite samples, 3 from each generation, form a scattered array with a nominal age of 514±20 Ma (MSWD 3892). ⁸⁷Sr/⁸⁶Sr₅₀₀ ratios are high (0.732−0.789) and ε_Nd500 varies from −8 to −14. Initial Pb ratios measured in leached alkali feldspars, ²⁰⁶Pb/²⁰⁴Pb = 17.71−19.97, ²⁰⁷Pb/²⁰⁴Pb = 15.67−15.91, ²⁰⁸Pb/²⁰⁴Pb = 38.63−42.84, form broadly linear arrays well above global Pb growth curves. Generations 1 & 3 pegmatites have the highest and lowest Pb isotope ratios, respectively; data for generation 2 overlap with both. ε_Nd and Pb isotope ratios form positive correlations, i.e. the third generation has the lowest ε_Nd.

The Pb isotope trends are interpreted in terms of two source components. Component 1 contains very radiogenic Pb and most likely represents old upper crust with high U/Pb and very high Th/Pb. Component 2 has a distinctive high-²⁰⁷Pb/²⁰⁶Pb signature that evolved through dramatic lowering of U/Pb in crustal protoliths during the Neoproterozoic granulite-facies metamorphism. Component 1 is strongest in the locally-derived generation 1 pegmatites and could reside in the Brattstrand Paragneiss, which contains ancient detritus (detrital zircons to 2.1 Ga) and includes fertile lithologies with very high U/Pb and Th/Pb. Component 2 is strongest in the distally derived generation 3 pegmatites and isotopically (Pb) resembles the Cambrian granites intrusive into the Brattstrand Paragneiss. Generation 3 pegmatites could be highly evolved differentiates of the granites, but, given their much higher ⁸⁷Sr/⁸⁶Sr, more likely are small degree disequilibrium melts of the same or similar sources to those for the granites.