2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 286-15
Presentation Time: 11:45 AM

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


GREW, Edward S., School of Earth and Climate Sciences, University of Maine, 5790 Bryand Global Research Center, Orono, ME 04469, MAAS, Roland, School of Earth Sciences, University of Melbourne, Parkville, 3010, Australia and CARSON, Christopher J., Antarctic Geoscience, Geoscience Australia, PO Box 378, Canberra, 2601, Australia

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 2030 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 (0.7320.789) and εNd500 varies from 8 to 14. Initial Pb ratios measured in leached alkali feldspars, 206Pb/204Pb = 17.7119.97, 207Pb/204Pb = 15.6715.91, 208Pb/204Pb = 38.6342.84, 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. εNd and Pb isotope ratios form positive correlations, i.e. the third generation has the lowest εNd.

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.