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 D2–3, forms irregular pods and veins up to a meter thick, which are either roughly concordant or crosscut S2 and S3 fabrics and are locally folded. Pegmatites of the second generation, D4, form planar, discordant veins up to 20–30 cm thick, whereas the youngest generation, post-D4, forms discordant veins and pods. Clear associations of pegmatites with broadly coeval granites are lacking. The D2–3 and D4 pegmatites are abyssal class (boron-beryllium subclass) characterized by graphic tourmaline + quartz intergrowths and boralsilite (Al16B6Si2O37); the borosilicates prismatine, grandidierite, werdingite, and dumortierite are locally present. In contrast, post-D4 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 D2–3 pegmatites and weakest for the post-D4 pegmatites, suggesting that the D2–3 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 87Sr/86Sr (0.7334–0.7870) and low ɛNd (–8.1 to –13.9). ɛNd tends to be highest in D2–3 and lowest in post-D4 pegmatites while 87Sr/86Sr shows similar ranges for all three generations. Initial 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb 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 D2–3 pegmatites contain the most radiogenic Pb, while the post-D4 pegmatites have the least radiogenic Pb; data for D4 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 (206Pb/204Pb ≥ 20, 208Pb/204Pb ≥ 43, ɛNd ≥ –8) most likely represents old upper crust with high U/Pb and very high Th/Pb. Component 2 (206Pb/204Pb ≤ 18, 208Pb/204Pb ~ 38.5, ɛNd500 –14 to –12) has a distinctive high-207Pb/206Pb 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 D2–3 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-D4 pegmatites, resembles feldspar Pb isotope ratios in Cambrian granites intrusive into the Brattstrand Paragneiss. However, 87Sr/86Sr 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.

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