Proterozoic-Paleozoic development of the basement of the Central Andes (18-26 degrees S) - a mobile belt of the South American craton

The Late Precambrian-Early Paleozoic metamorphic basement forms a volumetri cally important part of the Andean crust. We investigated its evolution in order to subdivide the area between 18 and 26 degreesS into crustal domains by means of petrological and age data (Sm-Nd isochrons, K-Ar). The metamor phic crystallization ages and t(DM) ages are not consistent with growth of the Pacific margin north of the Argentine Precordillera by accretion of exo tic terranes, but favor a model of a mobile belt of the Pampean Cycle. Peak metamorphic conditions in all scattered outcrop areas between 18 and 26 de greesS are similar and reached the upper amphibolite facies conditions indi cated by mineral paragensis and the occurrence of migmatite. Sm-Nd mineral isochrons yielded 525 +/- 10, 505 +/- 6 and 509 +/- 1 Ma for the Chilean Co ast Range, the Chilean Precordillera and the Argentine Puna, and 442 +/- 9 and 412 +/- 18 Ma for the Sierras Pampeanas. Conventional K-Ar cooling age data of amphibole and mica cluster around 400 Ma, but are frequently reset by Late Paleozoic and Jurassic magmatism. Final exhumation of the Early Pal eozoic orogen is confirmed by Devonian erosional unconformities. Sm-Nd depl eted mantle model ages of felsic rocks from the metamorphic basement range from 1.4 to 2.2 Ga, in northern Chile the average is 1.65 +/- 0.16 Ga(1 sig ma; n = 12), average t(DM) of both gneiss and metabasite in NW Argentina is 1.76 +/- 0.4 Ga (la; n = 22), and the isotopic composition excludes major addition of juvenile mantle derived material during the Early Paleozoic met amorphic and magmatic cycle. These new data indicate a largely similar deve lopment of the metamorphic basement south of the Arequipa Massif at 18 degr eesS and north of the Argentine Precordillera at 28 degreesS. Variations of metamorphic grade and of ages of peak metamorphism are of local importance . The protolith was derived from Early to Middle Proterozoic cratonic areas , similar to the Proterozoic rocks from the Arequipa Massif, which had unde rgone Grenvillian metamorphism at ca. 1.0 Ga. (C) 2000 Elsevier Science Ltd . All rights reserved.