The eastern zone of the Sierras Pampeanas in central and northwestern Argentina is characterized by a Paleozoic granitoid series emplaced at different structural levels in polymetamorphic terranes. Three main plutonic groups have been recognized on the basis of their ages and their relation to the deformational history. G1 (Late Precambrian to Cambrian): Scattered concordant bodies of gabbros, amphibolitized norites, amphibolites, diorites, and tonalites were intruded during an early deformation and regional metamorphism (D1, M1). This group is the only one to show tholeiitic affinities. G2 (Ordovician to Early Devonian): Syn-D2 (Early Ordovician) and late D2 (Middle to Late Ordovician) subgroups consist of small batholiths, plutons, and concordant bodies of tonalitic to granitic composition that were intruded during or shortly after the D2 and M2 (amphibolite to granulite facies) episode. These granitoids, characterized by their peraluminous chemistry, contain primary muscovite, spessartite garnet, and magmatic epidote. Most initial 87 Sr/ 86 Sr ratios are between 0.705 and 0.707. Some Upper Ordovician units have initial 87 Sr/ 86 Sr ratios greater than 0.710, suggesting an important upper-crustal contribution. Minimum crystallization pressures of 4 to 5 kbar are inferred from the granitoid paragenesis. The melts are inferred to be water- and volatile-rich as they produced ubiquitous tourmaline-bearing pegmatite swarms during the final stages of crystallization. This plutonism seems to be related to the Famatinian magmatic arc that was located 400 to 800 km east of the present Peru-Chile trench. These syn-D2 and late D2 granitoids are thought to be part of an inner back-arc zone of this early Paleozoic arc. Post-D2 (Late Ordovician to Early Devonian) granitoids consist of large batholiths (e.g., Achala and Velasco batholiths) and widespread smaller plutons that were emplaced during the later stages of the early Paleozoic (Famatinian) orogeny. Common features of the larger bodies include the presence of muscovite, aluminum silicates, and a dominant porphyritic phase characterized by microcline microperthite megacrysts in a groundmass of medium- to coarse-grained quartz, oligoclase, and biotite. Mineralized pegmatites (Li, Be, Nb, Ta) and U deposits related to post-D2 granites have high levels of LIL (large ion lithophiles: K, Rb, LREE) and HFS (high field strength: Y, Nb) elements. The post-D2 group exhibits compositional features similar to alkali-calcic arc granitoids developed at a great distance from the subduction margin, in the later stages of arc evolution. G3 (Late Devonian to Carboniferous): Hornblende-biotite alkalic-like plutons culminate granitoid activity in the Sierras Pampeanas. These granitoids have high levels of HFS and LIL elements, and some have high initial 87 Sr/ 86 Sr ratios (0.710). The G3 granitoids are interpreted to be within-plate plutons indirectly related to the beginning of late Paleozoic subduction on the Gondwana margin.

ABSTRACT The analysis of the pre-Andean history of the Central Andes shows a complex tectonic evolution. The basement of the Andean continental margin was formed by the accretion of Precambrian blocks during the formation of Rodinia in late Mesoproterozoic times. There are two magmatic arcs of Grenvillian age, one developed on the margin of the craton, known as the Sunsas belt, and another on the accreted terranes. The suture between these blocks with the Amazonian craton has been continuously reactivated by tectonic and magmatic processes. The terranes of Paracas and Arequipa, both of Grenvillian age, have a contrasting Paleozoic evolution. The Arequipa terrane amalgamated to the craton by the end of the Mesoproterozoic, and during the Paleozoic its suture acted as a crustal weakness zone. This zone concentrated the extension and the formation of a large platform in the retro-arc basin, where the Eopaleozoic sediments accumulated. The Famatinian magmatic arc of Ordovician age (475–460 Ma) is preserved in this segment along the continental margin. The Eopaleozoic extension that affected the Paracas terrane reopened the old suture and formed oceanic crust between Amazonia and Paracas. The subduction of this oceanic crust developed a magmatic arc over the cratonic margin, which is preserved in the Eastern Cordillera of Peru as orthogneisses associated with metamorphic rocks of Famatinian age. There are ophiolitic assemblages, paired metamorphic belts, and intense deformation associated with the Paracas collision (~460 Ma)against the Amazonian craton. In northern Eastern Cordillera of Peru the late Paleozoic orogen has within-plate granitic belts and was far away from the active margin. The orogen was deformed and uplifted in two phases (336–285 Ma and 280–235 Ma) known as the early and late Gondwanide orogenies. They are preserved as medium grade metamorphic belts developed along the Paracas segment. Further south along the Arequipa segment in southern Peru and Bolivia, the late Paleozoic–Triassic rocks are represented by granites and acidic volcanic rocks, which are not metamorphosed and are associated with sedimentary rocks. Relics of a magmatic arc are exposed as tonalites and metamorphic rocks (~260 Ma) along the northern continental margin of Peru and in the near offshore platform. The extensional regime that dominated most of the Mesozoic developed rift basins in the hanging-wall of the terrane sutures, which controlled the structural highs and basin margins. The Peruvian Late Cretaceous orogeny produced the emplacement of the Coastal batholith, the beginning of deformation along the coast, and the first foreland basins. The giant Ayabacas submarine syn-tectonic collapse is also controlled by previous sutures. The Cenozoic Andean evolution was dominated by a wave of shallowing of the subducted slab, the migration of the magmatism to the foreland, the steepening of the oceanic plate, and the consequent “inner arc” magmatism. The “inner arc” plutonic and volcanic rocks are the expression of deep crustal melts, associated with crustal delamination and lithospheric mantle removal. The flattening of the oceanic slab is related to ablative subduction and shortening in the Altiplano and Eastern Cordillera. The steepening is associated with rapid removal of mantle lithosphere and crustal delamination, expressed at surface by the “inner arc” magmatism. The suture crustal weakness zones between different terranes partially controlled the location of the delaminated blocks and the “inner arc” magmatism. Both processes triggered the lower crust ductile shortening and subsequent upper crustal brittle development of the sub-Andean fold-and-thrust belt.