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 206Pb/204Pb of ca. 16.6–17.0 and are distinguished from each other by contrasting 207Pb/204Pb and 208Pb/204Pb signatures. We distinguish (i) an unradiogenic source with 207Pb/204Pb of ca. 15.4 and 208Pb/204Pb of ca. 36.5, which may have been the principal source of crustal lead for Cenozoic magmatic rocks north of 21°S, (ii) an old high-μ source with 207Pb/204Pb of ca. 15.8 and 208Pb/204Pb of ca. 36.5–37.5, and (iii) an Arequipa-type source with 207Pb/204Pb of ca. 15.6 and 208Pb/204Pb 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 (206Pb/204Pb ca. 18.0–18.8, 207Pb/204Pb ca. 15.55–15.75, 208Pb/204Pb 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 the radiogenic source (average values of whole rock Pb-isotope ratios: 206Pb/204Pb 19.10, 207Pb/204Pb 15.70, 208Pb/204Pb 39.02) and the feldspar data (average values: 206Pb/204Pb 18.38, 207Pb/204Pb 15.64, 208Pb/204Pb 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°S. Rocks of all lead sources have Proterozoic crustal residence ages (tDM=ca. 1.6–2.0 Ga). However, the three sources of little radiogenic 206Pb/204Pb 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°S.