LEAD AUREOLES IN ALKALI FELDSPAR AROUND THORIAN MONAZITE-(CE) IN LATE ARCHEAN GRANULITE-FACIES PEGMATITES OF THE NAPIER COMPLEX, KHMARA BAY, EAST ANTARCTICA

Pegmatites containing granulite-facies mineral assemblages were emplaced during or soon after ultrahigh-temperature metamorphism of the Napier Complex. U-Pb isotopic data reported in the literature for zircon from such a pegmatite (sample #2233) and for monazite from a paragneiss at “Zircon Point”, Khmara Bay, gave upper and lower intercept ages of 2400-2500 Ma and ~1100 Ma, respectively. Electron microprobe ages of monazite-(Ce) ranging from 1 to 15 mm across in #2233 and in a nearby pegmatite (sample #49) give apparent ages mostly between 700 and 1100 Ma with little evidence remaining of the 2500 Ma event. Apparent age maps calculated from Th-U-Pb data show continuous variations with a few scattered slightly older domains. Monazite-(Ce) contains up to 27% ThSiO4, but only 1-4% CaThP2O8. Y and HREE contents are at the limit of detection or below. PbO contents range 0.7 to 1.3 wt%. Zoning is marked and often patchy; cores are generally enriched in Th. Alkali feldspar in contact with monazite-(Ce) exhibits a Pb enrichment aureole 2 to 4 mm thick, where PbO contents range from 0.2 to 2.0 wt%. Grains completely surrounded by monazite-(Ce) contain up to 9.9 wt% PbO, corresponding to 13% of a Pb-feldspar component PbAl2Si2O8, whereas matrix alkali feldspar has but 0.05 wt% PbO, near the limits of detection. The alkali feldspar is highly perthitic; albite contents range 7 – 26% and celsian is negligible. Plagioclase (An6-23) contains up to 1 wt% PbO. We suggest that the pegmatites were affected by a late Proterozoic event at ~ 1100 Ma under very dry conditions. The ~1100 Ma event was sufficiently hot to reset the chemical U-Th-Pb clock in monazite. Pb was lost from the monazite-(Ce) by diffusion to contiguous alkali feldspar. The order-of-magnitude enrichment of Pb in alkali feldspar relative to monazite-(Ce) reflects the equilibrium Pb distribution between these two minerals. Ages as low as 700Ma indicate partial resetting by a more recent event, e.g., intrusion of Early Cambrian pegmatites. Although traces of a Pb-rich mineral (cerussite?) found in cracks imply some Pb mobility possibly related to these pegmatites, the bulk Pb content of #49 (210 ppm) suggests that little Pb could have been lost from the pegmatite overall, i.e., the intensity of the Early Cambrian event was not sufficient to obliterate the effects of the ~1100 Ma event.