Vanesa D. Litvak

Petrogenesis of Miocene volcanic arc rocks over the Chilean-Pampean flat-slab segment of the Central Andes constrained by mineral chemistry

Miocene arc volcanism is manifested widely in the Valle del Cura-El Indio belt region (29°30´–30° South latitude), in the southern Central Andes of Argentina and Chile. The magmas that fed this volcanism are well represented by the Cerro de las Tortolas Formation, which is divided into two volcanic episodes based on petrographic, chemical and age differences: an older basaltic-andesitic event (16–14Ma) and a younger andesitic to dacitic (13–10Ma) one. Representative plagioclase, orthopyroxene, clinopyroxene and amphibole phenocrysts from rock samples already characterized from geochemical and isotopic viewpoints were selected for electron microprobe determination of mineral chemistry. Results indicate an overall homogeneous composition for each of the mineral phases. Equilibrium temperatures were estimated through two-pyroxenes, amphibole-plagioclase and amphibole geothermometers, which show a consistent temperature range between 970 to 850°C. Equilibrium pressure calculated using amphibole composition for volcanic suites produced the most comprehensive results for pressure equilibrium conditions, with results close to 4kb. Changes in the residual mineral assemblages and variations in isotopic signatures indicate that primary magmas were equilibrated at the lower crust with a gradual increase of crustal thickness. These melts evolved towards intermediate magma chambers, where crystallization of phenocrysts occurred at the same temperature and pressure conditions, hence, no increase in depth of intermediate magma chambers is registered although the increase of crustal thickness registered from Lower to Middle Miocene times.

Eruptive centre localization by means of aeromagnetometry in the central area of the Cura Valley, San Juan, Argentina: implications for the evolution of the Cenozoic arc/retroarc

Se genero un mapa de geologia solida para el sector central del Valle del Cura mediante la combinacion de informacion aeromagnetica y mapeo geologico. El analisis geologico-estructural del levantamiento aeromagnetico permitio identificar rasgos circulares interpretados como fracturas anulares asociadas a centros eruptivos. El centro eruptivo de la Brea es de caracter somero, no existiendo evidencias de la ocurrencia de un cuerpo subvolcanico asociado al mismo; esta implantado en un area estructuralmente favorable, dada por la interseccion de un corrimiento submeridiano de primer orden y un sistema de fracturas transversales noreste; su edad se asigna al Eoceno y se encontraria en una posicion de retroarco respecto de los depositos eruptivos de arco de la misma edad. El centro eruptivo del Zancarron se considera como una caldera volcanica, observacion consistente con los indicios geologicos de la presencia de un estratovolcan mioceno en esta region. Hay evidencias geofisicas que indican que volcanitas oligocenas subyacen a las volcanitas miocenas en el nucleo del centro eruptivo, lo cual permite considerar que la actividad magmatica del mismo habria comenzado en el Oligoceno; asi, el centro eruptivo del Zancarron seria un centro adicional al ya reconocido en territorio chileno para el Grupo Dona Ana, delimitandose la extension del frente volcanico oligoceno. Habida cuenta de la asociacion de este grupo con zonas de alteracion hidrotermal mineralizadas en el Valle del Cura, la identificacion de sectores con posible presencia de volcanitas oligocenas no aflorantes en la region resulta de interes para orientar la exploracion minera en la region.

The Late Paleogene to Neogene Volcanic Arc in the Southern Central Andes (28°–37° S)

Evolution of arc magmatism along the Southern Central Andes (~28°–37° S) is strongly controlled by changes in the geometry of the downgoing slab (e.g., slab dip angle). This is particularly evident in the present-day Chilean-Pampean flat-slab and the late Miocene Payenia shallow subduction segments. Typical Andean-type volcanism was established from the late Oligocene to late Miocene in the high Andes (29°30″–30°30″S), with arc-related calc-alkaline volcanism having geochemical signatures that reflect changes in the residual mineral assemblages related to increased crustal thickness (>50 km). The increase in crustal thickness resulted from increased compression along the Southern Central Andean margin due to the subduction of the Juan Fernandez Ridge, and consequent shallowing of the downgoing slab during Mid to Late Miocene. Associated with the decrease in the slab dip angle, the volcanic front migrated to the east. Further south, magmas developed across the present-day Payenia back-arc region (35°–37° S) show an increase in slab-derived components in the middle Miocene to early Pliocene times, which also suggests a progressive shallowing of the subducting slab at these latitudes. However, trace element ratios indicate a low-to-intermediate pressure residual mineral assemblage and no significant increase in crustal thickness is apparent, unlike further north in the Chilean-Pampean flat-slab segment. Although flat-slab geometry still prevails in this latter segment, re-steepening of the slab during early Pliocene times (~5–3 Ma) promoted an increase of arc and back-arc magmatism at these more southerly latitudes of the Southern Central Andes. A dynamic link between slab geometry, geochemistry, and volcanic activity is therefore observed in the Southern Central Andes.

Neogene Growth of the Patagonian Andes

After a Late Cretaceous to Paleocene stage of mountain building, the North Patagonian Andes were extensionally reactivated leading to a period of crustal attenuation. The result was the marine Traiguen Basin characterized by submarine volcanism and deep-marine sedimentation over a quasi-oceanic basement floor that spread between 27 and 22 Ma and closed by 20 Ma, age of syndeformational granitoids that cut the basin infill. As a result of basin closure, accretion of the Upper Triassic metamorphic Chonos Archipelago took place against the Chilean margin, overthrusting a stripe of high-density (mafic) rocks on the upper crust, traced by gravity data through the Chonos Archipielago. After this, contractional deformation had a rapid propagation between 19 and 14.8 Ma rebuilding the Patagonian Andes and producing a wide broken foreland zone. This rapid advance of the deformational front, registered in synorogenic sedimentation, was accompanied at the latitudes of the North Patagonian Andes by an expansion of the arc magmatism between 19 and 14 Ma, suggesting a change in the subduction geometry at that time. Then a sudden retraction of the contractional activity took place around 13.5–11.3 Ma, accompanied by a retraction of magmatism and an extensional reactivation of the Andean zone that controlled retroarc volcanism up to 7.3–(4.6?) Ma. This particular evolution is explained by a shallow subduction regime in the northernmost Patagonian Andes, probably facilitated by the presence of the North Patagonian massif lithospheric anchor that would have blocked drag basal forces creating low-pressure conditions for slab shallowing. Contrastingly, to the south, the accretion of the Chonos Archipelago explains rapid propagation of the deformation across the retroarc zone. These processes occurred at the time of rather orthogonal to the margin convergence between Nazca and South American plates after a long period of high oblique convergence. Finally, convergence deceleration in the last 10 My could have led to extensional relaxation of the orogen.

Paleogene Arc-Related Volcanism in the Southern Central Andes and North Patagonia (39°–41° S)

The influence of tectonic processes in evolution of magmatic suites evaluated through their geochemical signature has always been a subject of debate. Late Paleocene arc volcanism in the Southern Central Andes, particularly in North Patagonia, can be used to infer a direct relationship between magmatic episodes and tectonic changes along the Andean margin. Eocene arc-related volcanism (~44 Ma) in the North Patagonian Andes shows evidence for limited influence of the subducting slab on the composition of arc magmas and they exhibit an alkaline tendency. By Oligocene times (~29 Ma), arc volcanic sequences in the Auca Pan depocenter show predominantly arc-like geochemical signatures and have been derived from a calc-alkaline source. However, a comparison with younger arc sequences (<28 Ma) in the region suggests that the magmatic source turned more tholeiitic in composition with a remarkable increase in the influence of slab-derived fluids, as seen in volcanic rocks from Cura Mallin and Abanico retro and intra-arc basins. It is proposed that the marked geochemical variations between these magmatic periods are related to the tectonic changes associated with the breakup of the Farallon plate at ~28–26 Ma. The geochemical data from Eocene and Oligocene volcanic sequences provide further evidence for the strong link between tectonics and magmatism.