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Alvarez J.,University of Chile | Mpodozis C.,Antofagasta Minerals | Arriagada C.,University of Chile | Astini R.,National University of Cordoba | And 4 more authors.
Journal of South American Earth Sciences

During the Paleozoic the Andean basement of central Chile and Argentina grew westwards by the amalgamation of diverse tectonostratigraphic terranes some of them derived from Laurentia. The last to be accreted, in the Devonian, corresponds to the hypothetical Chilenia terrane. However, direct evidences about the nature of its basement are scarce because volcanics and intrusives associated to a Late Paleozoic arc and the Choiyoi Large Igneous Province concealed almost all older geological units. Indirect evidences about the nature of Chilenia can be obtained from the examination of the detrital zircon age populations in late Paleozoic accretionary prisms formed after its collision along the Pacific margin of Gondwana which may have incorporated sediments derived from the erosion of the Chilenia basement. Zircon populations from three of these accretionary complexes, El Tránsito, Huasco and Choapa (north-central Chile, 28-32°S) include Ordovician (Famatinian), Cambrian (Pampean), Neoproterozoic (Brasiliano) and Mesoproterozoic (Grenvillian) zircons whose sources can be tracked to Gondwana. Nevertheless, the three complexes also include a very large subpopulation of zircons that cannot easily be traced to well-known Gondwana sources and that are derived from the erosion of late Neoproterozoic to Early Cambrian (580-530 Ma) magmatic/metamorphic sources, that possibly form a significant component of the Chilenia microcontinental basement. © 2011 Elsevier Ltd. Source

Martinez F.,University of Chile | Arriagada C.,University of Chile | Mpodozis C.,Antofagasta Minerals | Pena M.,University of Chile
Andean Geology

The Triassic and Jurassic tectonic history of northern Chile has been dominated by extension, although clear evidence about the nature and geometry of the extensional basins and subsequent inversion structures has been adequately illustrated in only a few cases. In this contribution we present a structural study of the Lautaro Basin located at the western edge of the Frontal Cordillera in the Atacama region of northern Chile. The Lautaro Basin is a Jurassic half-graben, filled by at least 2,600 m of marine deposits of the Lautaro Formation and developed on top of, at least 2,000 m of Triassic volcanic successions of the La Ternera Formation, also accumulated during an earlier period of extensional deformation. Detailed field mapping and construction of a regional balanced cross-section, supported by good exposures along the Copiapó River valley, allow reconstruction of the structural style of both the Jurassic and Triassic extensional depocenters. New structural data have shown that the Lautaro Basin has a complex structural framework reflected in two major Mesozoic extensional periods, overprinted by Cenozoic inversion involving thin- and thick-skinned tectonics. Shortening was accommodated by a combination of inversion of pre-existing normal faults, buttresses, development of footwall short-cuts, and both thin and thick-skinned thrusting. New estimates of shortening are up to 13.1 km (30%), while Mesozoic extension is estimated to be 3 km (7%). Source

Kay S.M.,Cornell University | Mpodozis C.,Antofagasta Minerals | Gardeweg M.,AURUM Consultores
Geological Society Special Publication

The Central Andean margin, where the name andesite originated, is the type locality for arc andesites erupted through thick continental crust. The <25 Ma mid to high K2O andesites erupted from 25.58S to 28.28S exhibit a large variation in trace element and isotopic ratios, reflecting formation over an evolving slab, a crust thickening to 65-75 kmand a frontal arc that migrated c. 45 km eastward at 8-3 Ma. Andesites at 28-26.88S have the most variable and extreme heavy rare earth element (REE), high field strength element (HFSE) and Ba/La ratios and wt% Na2O, with the highest values in those erupted as the frontal arc migrated and the slab shallowed to the south. The required garnet-bearing, feldspar-free residue is generated in both the thick crust and the mantle wedge, into which crust was injected in a peak of forearc subduction erosion as the arc migrated. Andesites at 25.5-26.88S, east of the Puna plateau under which the slab shallowed at 18-7 Ma and then steepened as lithospheric delamination occurred, generally lack extreme REE and HFSE ratios. Their upper crust-like features reflect eruption in a mixed stress regime and incorporation of westward-flowing radiogenic crust from a region of extensive deep crustal melting to the east. Source

Alvarez J.,University of Chile | Mpodozis C.,Antofagasta Minerals | Blanco-Quintero I.,University of Los Andes, Colombia | Blanco-Quintero I.,University of Granada | And 3 more authors.
Journal of South American Earth Sciences

The La Pampa Gneisses are an enclave of orthogneisses emplaced within late Paleozoic to Triassic granitoids of the Chollay Batholith, in the Cordillera Frontal, to the east of Vallenar. Previous geochronological data (a Rb/Sr "errorchron" of 415±4Ma) allowed to some authors to suggest that these rocks were part of the Chilenia Terrane accreted to Gondwana during the Middle Devonian (. ca. 390Ma). New petrographic, chemical and geothermobarometric studies, together with U-Pb geochronological data show that the protolith of the La Pampa Gneisses derives from peraluminous tonalites emplaced during the Pennsylvanian at 306.5±1.8Ma, ruling out the hypothesis considering these rocks as remnant of the pre-collisional Chilenia basement. The tonalites were metamorphosed between 5.06 and 5.58kbar and 709-779°C during the middle Permian (267.6±2.1Ma), possibly in conjunction with the San Rafael tectonic event and the emplacement of the oldest granitoids of the Chollay Batholith. A new intrusive episode occurred at ca. 240Ma, followed by exhumation and cooling during a regional Triassic extensional episode. © 2013 Elsevier Ltd. Source

Goss A.R.,University of Florida | Kay S.M.,Cornell University | Mpodozis C.,Antofagasta Minerals
Contributions to Mineralogy and Petrology

The Pleistocene Incapillo Caldera and Dome Complex (5,570 m) marks the southernmost siliceous center of the Andean Central Volcanic Zone (~28°S), where the steeply dipping (~30°) segment of the subducting Nazca plate transitions into the Chilean "flatslab" to the south. The eruption of the Incapillo Caldera and Dome Complex began with a 3-1 Ma effusive phase characterized by ~40 rhyodacitic dome eruptions. This effusive phase was terminated by an explosive "caldera-forming" event at 0.51 Ma that produced the 14 km3 Incapillo ignimbrite. Distinctive and virtually identical chemical signatures of the domes and ignimbrites (SiO2 = 67-72 wt%; La/Yb = 37-56; Ba/La = 16-28; La/Ta = 30-50; 87Sr/86Sr = 0.70638-0. 70669; εNd = -4.2 to -4.6) indicate that all erupted lavas originated from the same magma chamber and that differentiation effects between units were minor. The strong HREE depletion (Sm/Yb = 6-8) that distinguishes Incapillo magmas from most of the large ignimbrites of the Altiplano-Puna plateau can be explained by the extent and degree of partial melting at lower crustal depths (>40 km) in the presence of garnet. At upper crustal depths, this high-pressure residual geochemical signature, also common to adjacent late Miocene/Pliocene Pircas Negras andesites, was partially overprinted by shallow-level assimilation and fractional crystallization processes. Energy-constrained AFC modeling suggests that incorporation of anatectic upper crustal melts into a fractionated "adakite-like" dacitic host best explains the petrogenesis of Incapillo magmas. The diminution of the sub-arc asthenospheric wedge during Nazca plate shallowing left the Incapillo magma chamber unreplenished by both mafic mantle-derived and lower crustal melts and thus stranded at shallow depths within the Andean crust. Based on its small size and distinctive high-pressure chemical signature, the Incapillo Caldera and Dome Complex provides an endmember model for an Andean caldera erupting within a waning magmatic arc over a shallowing subduction zone. © 2010 Springer-Verlag. Source

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