Galena lead isotopic variations in a Mesozoic-Cenozoic Andean arc, Antarctic Peninsula
The Antarctic Peninsula magmatic arc was the site of Andean-type subduction from at least Silurian times to the late Cenozoic. Hydrothermal galenas from eight localities in accretionary complex, arc volcanic sequences and plutonic rocks along a strike length of 1100 km have restricted 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb compositions (18.628–18.734, 15.599–15.673 and 38.441–38.689 respectively) plotting on crustal average growth curves. The local geological setting, and comparison with Andean leads, suggest that the major source reservoir was isotopically homogeneous sedimentary rocks forming the late Palaeozoic–Triassic accretionary complex. The lead data suggest that old continental basement is absent at least as far south as 65°S. Galenas from a large (13 × 2 km) epithermal vein system on Hurd Peninsula fall in three populations: a main isotopically homogenous array (18.643, 15.647 and 38.503, 2σ = 0.020, 0.020 and 0.050, n = 33) interpreted as a large hydrothermal system that extracted homogenous leads from early Triassic turbidites. Two smaller sub-populations, with more radiogenic 206Pb/204Pb, are associated with mid- to late Cretaceous intrusive rocks, and may represent either local contributions of magmatic lead or younger hydrothermal episodes. In western Graham Land, two localities associated with mantle-derived mafic intrusions of Cenozoic age have low 207Pb/204Pb and 208Pb/204Pb ratios indicating addition of some mantle-type lead.