Lead isotopes and Andean magmagenesis.

Abstract

The isotopic composition of lead is usually reported as the ratio of a radiogenic isotope to the non-radiogenic 204Pb. Pb isotope compositions of Late Cainozoic volcanic rocks from the NVZ and SVZ (preceding abstract) have the values 206Pb/204Pb = 18.72-18.99 and 18.48-18.66 respectively. This small variation within each volcanic zone, regardless of basement rock type or age, implies that NVZ and SVZ magmas are derived from homogeneous and probably subcrustal sources which have regional differences in 206Pb/204Pb. Apart from one Andean volcanic rock analysed to date (from the Cook Islands) NVZ and SVZ volcanic rocks contain Pb with a distinct imprint of a moderately high 207Pb and 208Pb source, the ultimate cause of which is recycled continental material, most likely in the form of subducted sediments and slab-derived fluids which enrich the mantle wedge. Igneous rocks of Mesozoic and Cainozoic age from the CVZ have a far greater range of Pb isotope composition than both the NVZ and SVZ combined, indicating varying degrees of interaction with both U- and Th-rich upper crustal rocks and with Th-rich but U-poor high-grade gneisses exposed in the Arequipa massif.-J.M.H.