ISOTOPIC CONSTRAINTS (Pb, Rb-Sr, Sm-Nd) ON THE SOURCES OF EARLY CAMBRIAN PEGMATITES WITH BORON AND BERYLLIUM MINERALS IN THE LARSEMANN HILLS, PRYDZ BAY, ANTARCTICA

Abstract

The Brattstrand Paragneiss, a highly deformed Neoproterozoic granulite-facies metasedimentary sequence, is cut by three generations of ~500 Ma pegmatite. The earliest recognizable pegmatite generation, synchronous with D 2–3 , forms irregular pods and veins up to a meter thick, which are either roughly concordant or crosscut S 2 and S 3 fabrics and are locally folded. Pegmatites of the second generation, D 4 , form planar, discordant veins up to 20–30 cm thick, whereas the youngest generation, post-D 4 , forms discordant veins and pods. Clear associations of pegmatites with broadly coeval granites are lacking. The D 2–3 and D 4 pegmatites are abyssal class (boron-beryllium subclass) characterized by graphic tourmaline + quartz intergrowths and boralsilite (Al 16 B 6 Si 2 O 37 ); the borosilicates prismatine, grandidierite, werdingite, and dumortierite are locally present. In contrast, post-D 4 pegmatites host tourmaline (but not in graphic intergrowths with quartz), beryl, and primary muscovite and are assigned to the muscovite-rare-element class. Spatial correlations between B-bearing pegmatites and B-rich units in the host Brattstrand Paragneiss are strongest for the D 2–3 pegmatites and weakest for the post-D 4 pegmatites, suggesting that the D 2–3 pegmatites may be closer to their source. Strontium-Nd-Pb isotope results for feldspars from nine pegmatites (three from each generation) indicate high and variable initial 87 Sr/ 86 Sr (0.7334–0.7870) and low ɛ Nd (–8.1 to –13.9). ɛ Nd tends to be highest in D 2–3 and lowest in post-D 4 pegmatites while 87 Sr/ 86 Sr shows similar ranges for all three generations. Initial 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb ratios vary considerably (17.71–19.97, 15.67–15.91, 38.63–42.84), forming broadly linear arrays well above global Pb growth curves. The D 2–3 pegmatites contain the most radiogenic Pb, while the post-D 4 pegmatites have the least radiogenic Pb; data for D 4 pegmatites overlap with both groups. Broad positive correlations for Pb and Nd isotopic ratios could reflect source rock compositions controlled by two components. Component 1 ( 206 Pb/ 204 Pb ≥ 20, 208 Pb/ 204 Pb ≥ 43, ɛ Nd ≥ –8) most likely represents old upper crust with high U/Pb and very high Th/Pb. Component 2 ( 206 Pb/ 204 Pb ≤ 18, 208 Pb/ 204 Pb ~ 38.5, ɛ Nd500 –14 to –12) has a distinctive high- 207 Pb/ 206 Pb signature which evolved through dramatic lowering of U/Pb in crustal protoliths during a Neoproterozoic granulite-facies metamorphism. Component 1, represented in the locally derived D 2–3 pegmatites, probably reflects melt sources in biotite and borosilicate gneisses within the Brattstrand Paragneiss, which has a wide range of U/Pb and Th/Pb ratios and an inferred early Proterozoic crustal residence age. The Pb isotope signature of component 2, represented in the “far-from-source” post-D 4 pegmatites, resembles feldspar Pb isotope ratios in Cambrian granites intrusive into the Brattstrand Paragneiss. However, 87 Sr/ 86 Sr ratios in the pegmatites are much higher than in the granites, implying that the pegmatite melts are unlikely to be direct magmatic differentiates of the granites, although they may have broadly similar crustal sources. Temporal shifts in pegmatite source signatures, with a general sense of deeper crustal sources in the younger pegmatite generations, may reflect cooling of the crust after Cambrian metamorphism.