Lead evolution of the Pre-Mesozoic crust in the Central Andes (18-27°):: progressive homogenisation of Pb

Pb-isotope composition of feldspar determined for a suite of granitoid magmatic and metamorphic rocks of Cambrian to Permian age provides a guide to the crustal composition in the Central Andes as it evolved during the Paleozoic. The large range of Pb-isotope composition of magmatic rocks older than 500 Ma contrasts with the homogeneous Pb-isotope composition of magmatic and metamorphic rocks that formed between 500 Ma and the Late Paleozoic, There are four isotopically distinct crustal domains discernible. Three of them, associated with pre-500-Ma-old rocks, are characterised by similar Pb-206/Pb-204 of Pb-207/Pb-204 ca. 16.6-17.0 and are distinguished from each other by contrasting Pb-207/Pb-204 and Pb-208/Pb-204 signatures. We distinguish (i) an unradiogenic source with Pb-207/Pb-204 of ca. 15.4 and Pb-208/Pb-204 of ca. 36.5, which may have been the principal source of crustal lead for Cenozoic magmatic rocks north of 21degreesS, (ii) an old high-mu source with Pb-207/Pb-204 of ca. 15.8 and Pb-208/Pb-204 of ca. 36.5-37.5, and (iii) an Arequipa-type source with Pb-207/Pb-204 of ca. 15.6 and Pb-208/Ph-204 of ca. 38.5. The fourth crustal lead source, a radiogenic source, is characterised by a relatively small range of Pb-isotope compositions of Paleozoic gneisses of the basement (Pb-206/Pb-204 ca. 18.0-18.8, Pb-207/Pb-204 ca. 15.55-15.75, Pb-208/Pb-204 ca. 37.5-38.5) and most intrusions which formed after high-grade metamorphism at ca. 500 Ma. The compositional range given by present day Pb-isotope ratios of radiogenic source (average values: Pb-206/Pb-204 18.38, Pb-207/Pb-204 15.64, Pb-208/Pb-204 38.11) which also represent the present day composition of those parts of the radiogenic source depleted in U at ca. 500 Ma, is clearly reflected in the Pb-isotope ratios of the widespread Cenozoic ignimbrites and andesites from the Central Andes south of 21 degreesS. Rocks of all lead sources have Proterozoic crustal residence ages (t(DM) = ca. 1.6-2.0 Ga). However, the three sources of little radiogenic Pb-206/Pb-204 experienced a dominantly metamorphic redistribution of U, Th, and Pb early in their history. The Early Paleozoic major metamorphic-magmatic cycle incorporated these inherited Pb-isotope domains of minor volume into the radiogenic source south of 21 degreesS. (C) 2002 Elsevier Science B.V All rights reserved.