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The evolution of the Early Palaeozoic orogen of West Gondwana in the Cambrian to Ordovician basement of the Andes between ~18° and 32° S is investigated for pressure and temperature conditions and age of metamorphism. It is characterized by mid-crust temperatures commonly above the wet granite solidus (~650°C). Widespread felsic migmatite and rare granulite formed at pressures of ca 0.5-0.7 GPa, locally 1.0 GPa. These rocks represent the deepest exhumed sections of the Early Palaeozoic crust. High pressure-low temperature rocks are absent. The crystallization ages, compiled from the literature in combination with new data, for near peak metamorphic conditions of these high-grade metamorphic rocks in NW Argentina and N Chile are ~530-500 Ma and ~470-420 Ma. Both age groups are spatially overlapping. Radiogenic isotope signatures (Sr, Nd, Pb) are used to characterize the Early Palaeozoic basement. The Pb and Sr isotope compositions of the Early Palaeozoic basement indicate mixing arrays between pre-Palaeozoic unradiogenic and radiogenic crust. Crustal residence ages (Sm-Nd T) indicate a prominent event of crust formation around ~2 Ga, which is known continent-wide. This material was recycled during Midproterozoic and Early Palaeozoic orogenies without prominent additions of new crust present in the isotope record, i.e. accretion of compositional exotic material is absent. [ABSTRACT FROM AUTHOR]

New isotopic ages constrain the onset of the Andean Orogeny in central Chile (34°-37°S). They define three pulses of Early Mesozoic magmatism in the Cordillera de la Costa: granitic magmatism at 225-220 Ma, bimodal magmatism at 210-197 Ma, and mafic magmatism at 170-150 Ma. Each pulse has distinct field, petrographic, geochemical, and Sr-Nd-Pb isotopic characteristics and crystallization conditions. Major magmatic sources are the regional Mesozoic subarc-subcontinental mantle and the Paleozoic Chilean continental crust, with the importance of juvenile contributions increasing with time. All three Early Mesozoic magmatic pulses display geochemical signatures that are characteristic of subduction-related magmas of magmatic arcs, such as a Nb-Ta trough and a positive Pb anomaly. The 210-197-Ma magmatism is peculiar for its bimodal nature, the occurrence of transitional A-type granitoids, the presence of fayalite granites, and its association with an episode of crustal extension. A contemporaneous accretionary wedge nevertheless indicates ongoing subduction at that time. The 210-197-Ma magmatic pulse thus was formed in an extensional suprasubduction zone environment. It marks the transition from the Gondwana Orogeny to Andean- style subduction magmatism. We argue that the onset of the Andean Orogeny was related to a reconfiguration of the tectonic conditions of plate convergence during overall near-continuous subduction and arc magmatism since the Paleozoic. [ABSTRACT FROM AUTHOR]

Focuses on a study which documented the mineral chemistry and boron-isotope composition of tourmaline in a suite of Palaeozoic to Mesozoic rock samples from the central Andes of Northwest Argentina. Geological setting and sample description; Techniques in analyzing boron isotopes; Chemical composition of whole-rocks and tourmalines.