Mario Vergara

Andesites and high-alumina basalts from the central-south Chile high Andes: Geochemical evidence bearing on their petrogenesis

High-alumina basalts from seven High-Andean stratovolcanoes (37 °30′S to 41 °S) have major and trace element (including rare earth elements, REE) that are consistent with derivation by partial melting (typically 10–15%) of garnet-free peridotite followed by fractional crystallization of olivine and pyroxene. High-alumina basalts from two stratovolconoes require significantly lower degrees of melting (<5%) or melting of an incompatible, element-enriched source. However, a poorly understood feature of all of these basalts-and calc-alkaline rocks in general-is the mechanism for causing their low TiO2 and heavy REE content relative to oceanic basalts. Further north in Chile (33 °–34 °S and 21 °–22 °S) amphibole-bearing andesites have REE abundances consistent with derivation from a garnet-bearing source such as incompatible, element-rich eclogite (e.g., Franciscan eclogites) or garnet peridotite. The marked petrological and geochemical changes along strike of the Andes are probably related to the varying nature of the subduction process; e.g., dips of the downgoing slab varying from 10 to 30 °.

Jurassic and Early Cretaceous island arc volcanism, extension, and subsidence in the Coast Range of central Chile

More than 2000 km 3 of acid and 9000 km 3 of basic volcanic rocks formed during the Jurassic and Early Cretaceous in the Coast Range of central Chile, between 32°30′S and 34°S. These rocks, which constitute the major part of an •15-km-thick pile of alternately marine and continental deposits, issued from volcanic arcs situated between a land area with Paleozoic basement in the west and a marginal sea in the east. Asthenospheric upwelling led to extension and bimodal volcanism; the volcanic products were deposited in intra-arc basins subsiding at high rates (100–300 m/m.y.). The source of the magmas became more depleted with time due to an increase in degree of partial melting, and their compositions were modified by subduction-related fluids and contamination with a progressively thinner and younger crust. The basic lavas are of high-K calc-alkaline to shoshonitic affinity, chemically resembling the lavas found in some mature island arcs in the western Pacific. The extension and subsidence resulted in a low-relief topography close to sea level, in contrast with the present-day convergent type of Andean volcanism at the same latitude where calc-alkaline intermediate lavas erupt from volcanoes at great height above a thick crust.

Age and Evolution of the Upper Cenozoic Andesitic Volcanism in Central-South Chile

The first ten potassium-argon determinations made on andesitic rocks of the Andes and Coastal ranges of central-south Chile, between latitudes 36° and 42° South, demonstrate the existence of at least two main cycles of volcanic activity; one in the Miocene and the other in the Pliocene to Holocene. The volcanic activity lines seem to have changed eastward from the present Coastal volcanic belt in Miocene times to about 100 km inland in Pliocene and Holocene times. During late Pleistocene to Holocene, a gradual inversion in the direction of migration of volcanism occurred. The geochronological asymmetry of the volcanic chains seems to be a highly important feature in this continental border and plate-contact zone.

Tertiary volcanism during extension in the Andean foothills of central Chile (33°15′–33°45′S)

This lithologic and geochemical study treats two Tertiary volcanic formations in the Andean foothills of central Chile deposited during and after an inferred culmination of crustal attenuation. The Abanico and Farellones Formations, which are described in their type localities just east of Santiago, formed from volcanic arcs in continental basins during the Oligocene and Early Miocene, respectively. Aphyric basic lavas of tholeiitic affinity, acid pyroclastic rocks, and lacustrine deposits constitute the >3100-m-thick Abanico Formation. The overlying >2100-m-thick Farellones Formation consists of calc-alkaline lavas (basalts absent) with thick pyroclastic deposits at the base. Both formations have Nd-Sr isotope signatures within the mantle array; the Abanico rocks (eNd ≈ +5.7) plot closer to N-MORB (normal mid-oceanic-ridge basalt) than the Farellones rocks (eNd = +3.9 to +5.1). The REE (rare earth element) patterns indicate greater depth to the mantle source and a smaller degree of partial melting with time. The Abanico lavas segregated within the stability field of spinel, whereas the lavas of the upper Farellones member show residual garnet in their source. Geochemical changes with time are systematic: the greatest contrast is between the middle and upper Farellones members in 1–2 m.y., e.g., for basaltic andesites, La/Yb increases from 4.3 (Abanico) to 5.6, 6.0, and 11.6 (lower, middle, and upper Farellones members). The bimodal composition of the Abanico Formation and the lower Farellones member indicates that volcanism took place during episodes of extensional conditions. Extension with subsidence is independently shown by the burial metamorphic pattern. Voluminous pyroclastic flows, structural relationships, and other evidence suggest recurrent caldera collapse. The first extensional episode ended with contraction and folding of the Abanico rocks, and the second episode resulted in uplift of the lower and middle Farellones members, followed by a more passive tectonic regime. Sequences showing many similarities with the Abanico and Farellones Formations occur along the Andean foothills of Chile. They decrease in age from Late Cretaceous– Paleocene at 23°S to Early Miocene–late Miocene at 35°S and might be explained by oblique subduction of oceanic ridge.

Time interval between volcanism and burial metamorphism and rate of basin subsidence in a Cretaceous Andean extensional setting

40Ar/39Ar ages were obtained from basaltic flows belonging to a 9-km-thick sequence generated in an extensional ensialic setting of an arc/back-arc basin type during the Early Cretaceous and presently exposed along the Coastal Range of central Chile. The basalts have been affected by very low- to low-grade burial metamorphism, mostly under prehnite–pumpellyite facies. Age values obtained from primary (volcanic) and secondary (metamorphic) minerals permit to quantify the time interval between volcanism and burial metamorphism. A plateau age of 119±1.2 Ma from primary plagioclase represents the best estimation of the age of the volcanism, whereas adularia, in low-variance assemblages contained in amygdules, gave a plateau age of 93.1±0.3 Ma which is interpreted as the age of the metamorphism. Considering the P–T conditions estimated for this metamorphic event, the c. 25 Ma time interval between volcanic emplacement and prehnite–pumpellyite facies metamorphism, the rate of basin subsidence in this extensional geodynamic setting would be comprised in the interval 150–180 m/Ma.

Petrology and geochemistry of lavas from Antuco volcano, a basaltic volcano of the Southern Andes (37° 25′S)

Sierra Velluda (Plio-Pleistocene), Cerro Condor (Pleistocene) and Antuco (Holocene) volcanoes form a volcanic group located in the Southern Andes at latitude 37° 25′S. Lavas from these volcanoes become less silicic with decreasing age. Antuco volcano consists of two units, pre- and post-caldera collapse (Antuco I and Antuco II), and a parasitic cone. Antuco lavas are basalts and those from Antuco I and II are relatively uniform in mineral and chemical composition. Common phenocrysts are plagioclase (An65-An81, 18–33 vol. %), olivine (Fo∼78, 3–18 vol. %) and clinopyroxene (0.5–3 vol.%). The same phenocrysts occur in the parasitic cone lavas, but plagioclase is much more abundant. Key compositional features of Antuco lavas are: ∼ 52% SiO2, > 17% Al2O3, 3.3–4.2% Na2O, < 1.2% TiO2, < 0.9% K2O and Mg/(Mg + σ Fe) ratios < 0.60. These major element features characterize many circumpacific basalts in island-arc and continental-margin environments. The Antuco I and II lavas have light REE abundances equal to 26–33 × chondrites and heavy REE abundances equal to 8–9 × chondrites. Some samples have positive (∼10%) Eu anomalies but no negative Eu anomalies occur in these lavas. Trace element abundances in the Antuco lavas are similar to those in theoretical liquids derived by 5–10% partial melting of garnet-free peridotite followed by 10% fractionation of a clinopyroxene (40%) and olivine (60%) cumulate; thus a plausible model is that the lavas were derived from the mantle by partial melting followed by mafic mineral fractionation.

A low‐grade metamorphic model for the Miocene volcanic sequences in the Andes of central Chile

Abstract Calc‐alkaline basic volcanic rocks in a c. 600 m thick sequence of Miocene age, the Valle Nevado stratified sequence (VNSS), have been affected by very low grade metamorphism characterised by mineral assemblages of the zeolite facies. Metastable conditions prevailed, most of the igneous minerals being wholly or partially preserved. The main metamorphic phases are mafic phyllosilicates and zeolites of calco‐sodic and calcic composition. The intensity of the metamorphism was controlled by depth and by hydrothermal activity related to volcanic centres. From top to bottom of the sequence, the zeolites vary from heulandite‐clinoptilolite through mordenite to laumontite, whereas the phyllosilicates show a compositional transition from tri‐smectite to smectite/chlorite with up to 75% chlorite layers. Strong fluid/rock interaction took place only at flow levels with a high primary porosity and permeability as illustrated by: (a) the similarity of the REE patterns of the basaltic host flows and the secondary zeolites; and (b) the contrast in composition observed between phyllosilicates in vesicular tops as opposed to the massive (central) parts of the same flow. The thermal gradients acting during the metamorphic event were high and are estimated at 150–175°C/km. The metamorphic zonation is interpreted as the result of a rapid accumulation of considerable volumes of rock generated by volcanic centres connected with geothermal field activity and characterised in places by caldera collapse.

Tertiary Andean volcanism in a caldera-graben setting

ZusammenfassungDie miozäne Farellones-Formation im zentralen Chile (32°–35°S) ist eine von mehreren bis zu 3000 m mächtigen, tertiären vulkanischen Abfolgen der zentralen Anden, mit Ignimbriten und Zwischenlagerungen von lakustrinen Sedimenten im unteren, und mit intermediären bis basischen Laven und rhyolitischen Staukuppen im oberen Teil der Abfolgen. Die Gesteine der Farellones-Formation wurden wahrscheinlich in einem vulkano-tektonischen, durch eine Reihe von Caldera-Absenkungen entstandenen Graben abgelagert. Für diese Tatsache sprechen (a) häufige Begrenzung der Formation durch Normalverwerfungen gegen welche die subhorizontalen Schichten ausdünnen und aufgebogen werden, was Ablagerung während der Absenkung andeutet, (b) das große Volumen der sauren Vulkanite und (c) der hohe paläothermale Gradient von geothermalem Charakter. Ähnlich ausgebildete, tertiäre vulkanische Abfolgen im nördlichen Chile und zentralen Peru sind vermutlich ebenfalls durch Eruptionen in Caldera-Gräben gebildet worden. Aufgrund der tektonischen Ausbildung und des geothermalen Umwandlungstyp dürften diese Abfolgen für die Prospektion nach epithermalen Vorkommen von Edelmetallen von besonderem Interesse sein.AbstractThe Miocene Farellones Formation in central Chile (32°-35°S) is one of several up to 3000 mthick Tertiary volcanic sequences in the Central Andes with ash flows and intercalations of lacustrine sediments in their lower part, and intermediate to basic lavas and rhyolitic domes in their upper part. The Farellones rocks were probably deposited in a volcano-tectonic graben formed through a series of caldera collapses. This is suggested by (a) the fact that the formation frequently is delimited by normal faults towards which the subhorizontal strata pinch out and become upraised, indicating deposition during subsidence, (b) the huge volume of erupted acid magma and (c) a high paleothermal gradient of geothermal field type. Similar Tertiary volcanic sequences in northern Chile and central Peru probably also formed by eruptions in a caldera-graben setting. This setting and the geothermal field type of alteration make these sequences good prospecting targets for epithermal preciousmetal deposits.ResumenLa Formación Farellones de Chile central (32°–35°S), deedadmiocena, es una de las muchas secuencias terciarias de más de 3000 m de espesor que existen en la alta cordillera de los Andes Centrales que contiene depósitos de flujos piroclásticos e intercalaciones de sedimentos lacustres en su parte inferior, y lavas intermedias a básicas y domos riolíticos en su parte superior. Las rocas de la Formación Farellones se depositaron, probablemente, en un graben volcano-tectónico formado a través de una serie de colapsos de calderas. Esta interpretación se basa en: (a) la frecuente delimitación de la formación por fallas normales contra las cuales los estratos subhorizontales se adelgazany se levantan, indicando depositación durante subsidencia, (b) el gran volumen de magma ácido extruido y (c) un gradiente paleotermal alto de tipo campo geotérmico. En el norte de Chile y en Perú central existen secuencias terciarias similares depositadas probablemente también en depresiones volcano — tectónicas tipo caldera — graben. Este marco tectónico y la alteración de tipo campo geotérmico, hace que éstas secuencias sean buenos blancos de prospección para depósitos epitermales de metales preciosos.Краткое содержаниеМиоценовые отложени я формации Farellones в центральном Чиле (32° – 35° С) состоят из многочисленных свит третичных вулканито в центральных Анд, мощн остью до 3000 м., с игнимбритами и просл оями береговых седим ентов в нижних свитах и с лав ами от средних до базических, а также риолитовыми энтрузи ями куполов в верхней ее ч асти. Отложения пород этой формации происх одило, вероятнее всего, в вулкано-текто нических грабенах, во зникших в результате прогруж ения кальдер. В пользу этого предпол ожения говорят: а) част о отмеченное отмежева ние сбросами, при кото рых субгоризонтальные с лои утончаются и изгибаются, указывае т на отложения во врем я проседания; б) мощные о тложения кислых вулканитов; в) господс тво высоких палеотер мальных условий в этот период. Третичные свиты вулканитов в се верном Чиле и в центра льной части Перу, сходные в в ыше описанными, образовались, по всей вероятности, так же в результате эрупц ий в грабены кальдер. Т.к. эти свиты имеют тек тоническое происхож дение и геотермально преоб разованы, то с точки зрения их просп екции они представля ют интерес для разведок на эпитермальные руд ные месторождения благо родных металлов.

Relaciones de contacto de unidades volcánicas terciarias de los Andes de Chile central (33°S): una reinterpretación sobre la base de dataciones 40Ar/39Ar

Se dan a conocer ocho edades 40Ar/39Ar de rocas de dos sectores de la Cordillera de los Andes cerca de los 33°S donde se presentan unidades volcanicas separadas por discordancias angulares. Estos sectores corresponden a la Cuesta de Chacabuco en la continuacion norte del Valle Central y al Cerro Las Ollas en la parte oeste de la Cordillera de los Andes. En Cuesta de Chacabuco se reconocio la unidad Chacabuco, formada principalmente por basaltos, andesitas basalticas y rocas sedimentarias clasticas continentales. Esta unidad esta cubierta, con discordancia angular, por flujos piroclasticos de la unidad Algarrobo, un complejo volcanico erosionado. Una edad plateau 40Ar/39Ar (roca total, basalto) de 28,8 ± 0,3 Ma fue interpretada como la mejor estimacion de la edad del volcanismo para la unidad Chacabuco, mientras que las edades plateau obtenidas sobre anfibolas de una lava de andesita basaltica y de un dique andesitico para la unidad Algarrobo, fueron 19,6 ± 0,3 y 18,6±0,4 Ma, respectivamente. Se detecto asi un hiatus del orden de 8,6 Ma entre estas dos unidades. El Cerro Las Ollas ha sido un lugar clasico en la discusion acerca de la naturaleza del contacto entre unidades que han sido atribuidas a las formaciones Abanico y Farellones el que ha sido interpretado como discordancia de plegamiento y erosion o como contacto tectonico. Las nuevas edades obtenidas sobre biotita en tobas muestran una continuidad temporal para rocas situadas bajo y sobre el plano de contacto, evidenciando la pertenencia de estas tobas a un mismo evento volcanico. La mejor estimacion de edad para este evento es la edad media ponderada de 20,1 ± 0,1 Ma. En consecuencia, la relacion discordante observada en el cerro Las Ollas puede ser interpretada como intraformacional

Episodic burial metamorphism in the Andes—A viable model?

Burial metamorphism of regional extent throughout Mesozoic to Cenozoic sequences in the Andean Mountain belt has been attributed previously to a unique model of metamorphic development, involving episodic ∼40 m.y. cycles of extensional basin formation, burial, metamorphism, and then exhumation. A main premise of this model is that breaks in metamorphic grade occur at major stratigraphic unconformities, so marking successive metamorphic cycles. This model is tested in a Mesozoic–Cenozoic sequence east of Santiago, where three metamorphic episodes have been reported on the basis of sharp breaks in metamorphic grade at two main unconformities. In metabasites from this area, reaction progress in mafic phyllosilicates shows a continuum across the sequence without breaks at the unconformities. There are differences in mineral assemblages between the various stratigraphic units, from which contrasting subgreenschist facies can be recognized. However, consideration of the controls on mineral paragenesis at subgreenschist facies conditions demonstrates that these different facies cannot be used as evidence of sharp breaks in metamorphic grade at unconformities, as has been reported in many previous publications. Thus, metamorphic breaks within this Andean section cannot be confirmed. Accordingly, models of Andean burial metamorphism linked to episodic tectonic cycles throughout the Mesozoic and Cenozoic appear unfounded.