Luis Aguirre

Metamorphic facies series of the crystalline basement of Chile

ZusammenfassungDie Haupttypen der metamorphen Gesteine Chiles und deren Mineralzusammensetzung werden beschrieben und deren metamorphe Fazies charakterisiert. Das chilenische kristalline Grundgebirge besteht in der Hauptsache aus Graniten und semipelitischen Gesteinen, die unter den Bedingungen der niedriggradigen Metamorphose umgewandelt wurden. Die Gesteine sind in der Regel in der Küstenzone Süd- und Zentral-Chiles aufgeschlossen; in Nord-Chile treten sie isoliert auf. Radiometrische Altersbestimmungen ergaben sowohl für die Tiefenwie für die metamorphen Gesteine ein spätpaläozoisches Alter. In Zentral-Chile wurden drei metamorphe Serien erkannt und für eine erste Klassifizierung der metamorphen Gesteine dieses Gebietes herangezogen. Die Serien entstanden in der Hauptsache durch Metamorphoseprozesse, die unter mittleren bis hohen, mittleren bis tiefen und tiefen Drücken abliefen. Die Bildung dieser Serien, ihre Beziehung zuMiyashiros zirkumpazifischen Gürtelpaaren und die Rolle des Grundgebirges während der Anden-Orogenese werden kurz diskutiert.AbstractThe main rock types and mineral assemblages of the metamorphic rocks of the territory of Chile are described, and the metamorphic facies are identified. The crystalline basement of Chile consists of predominantly low-grade, semipelitic metamorphic and granitic rocks, exposed mostly along the coastal area of Central and Southern Chile, and in isolated areas of Northern Chile. Radiometric ages of both metamorphic and igneous rocks indicate Late Paleozoic. Three metamorphic series were recognized in Central Chile and used for a tentative classification of the metamorphic rocks from the remaining areas. The series correspond to intermediate-high pressure, intermediate-low pressure, and low pressure conditions of metamorphism, respectively. The formation of these series, their relation toMiyashiros circumpacific paired belts, and the role of the basement during the Andean orogeny, are briefly discussed.ResumenSe describen los principales tipos de rocas y asociaciones minerales del basamento cristalino del territorio chileno. Este basamento consiste predominantemente de rocas metamórficas semipelíticas de bajo grado y de rocas graníticas, expuestas de preferencia a lo largo de la región costera de Chile Central y Austral y en áreas aisladas de Chile Septentrional. La edad radiométrica de las rocas metamórficas y graníticas indica Neopaleozoico. En Chile Central se encontraron tres series metamórficas, las que fueron empleadas para la clasificación provisoria de las rocas de otras partes del país. Estas series corresponden a condiciones de metamorfismo a presiones intermedia-alta, intermedia-baja, y baja, respectivamente. Se discuten brevemente la formación de las series, su relación a los arcos circumpacíficos apareados deMiyashiro, y el papel del basamento durante la orogénesis andina.Краткое содержаниеОписаны основные тип ы метаморфных пород, и х состав, а также фаций. Древние криста ллические породы Чил и состоят главным образом из гранитов, semipelitischen Gesteinen, образ овавшихся при низкотемпературном метаморфозме. Эти пор оды располагаются у п обережья южного и центральног о Чили, в северном Чили они встречаются только очень резко. С п омощью радиометриче ских измерений возраст этих пород от несли к позднему пале озою. В центральной части Чили установили три м етаморфоные серии ип ровели их классификацию. Эти серии образовали сь при процессах мета морфизма, протекавших при средних и высоких давлениях. По своему о бразованию они очень сходны с породами, при надлежащими к тихоок еанскому поясу Miyashiro.

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.

Rb/Sr ages of rocks from the Chilean metamorphic basement

Eleven whole rock Rb/Sr age determinations from the Chilean metamorphic basement — formerly considered as Precambrian — define two limiting reference isochrons of 342 and 273 my. Analized rocks are phyllites and schists of sedimentary origin with mineralogical assemblages mainly corresponding to the greenschist facies. Two metamorphic series characterize the Chilean metamorphic basement: an eastern low P/T series, and a western high P/T series. Four of the analyzed samples belong to the former and seven to the latter. The obtained age is interpreted as the age of a main metamorphic episode of the basement. This episode would have taken place in the Upper Paleozoic.

Late Paleozoic K/Ar ages of blueschists from Pichilemu, Central Chile

Two crossite concentrates and one blueschist whole rock were analyzed by the K/Ar method. These samples belong to the high/intermediate pressure Western Series of the Chilean metamorphic basement and, in this area, are intruded by a small monzonite body. Ages obtained were 211 m.y. and 329 m.y. for the mineral concentrates and 211 m.y. for the whole rock. Discussion based on crystal size as a factor for retention of40Ar during localized re-heating of the metamorphic rocks due to the monzonitic intrusion leads to the acceptance of 329 m.y. as the minimum age of crossite crystallization. This age agrees with the whole rock Rb/Sr limiting reference isochrons (273–342 m.y.) previously obtained for the metamorphic basement of Central Chile which did not include samples of the present area. This age provides the first evidence of a Paleozoic blueschist assemblage in the eastern Pacific border and would suggest the existence of a Late Paleozoic subduction zone along the western margin of South America.

Ages and cooling history of the Early Cretaceous Caleu pluton: testimony of a switch from a rifted to a compressional continental margin in central Chile

The Caleu pluton, in the Coastal Range of central Chile, represents the last magmatic event related to the Early Cretaceous rifting along the western margin of South America. The pluton was emplaced into a c. 10 km thick pile of mainly basalts and basaltic andesites deposited in an Early Cretaceous subsiding basin, and affected by very low-grade metamorphism. The cooling history of the pluton is documented on the basis of U–Pb, 40Ar/39Ar step-heating and fission-track dating. The U–Pb date suggests an age of emplacement in the interval 94.2–97.3 Ma. Rapid subsolidus cooling between 550–500 °C and 250 °C is documented by 40Ar/39Ar plateau ages on amphibole, biotite and plagioclase between 94.9 ± 1.8 and 93.2 ± 1.1 Ma. Slower subsolidus cooling to c. 100 °C is identified at the 94–90 Ma interval by the fission-track thermal model. The geochronological data show that the emplacement of the pluton is coeval with the very low-grade metamorphism of the host rocks. Therefore, this metamorphism is probably not the result simply of burial, but also of a regional thermal gradient related to the plutonism. Exhumation of the pluton started coevally with its emplacement and continued to about 90 Ma, being associated with the closure of the Early Cretaceous rifting. The Caleu plutonism represents an asthenospheric-derived event during maximum extension, and marks a turning point between extensional- and compressional-related magmatism.

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.

Contrasting geochemistry and metamorphism of pillow basalts in metamorphic complexes from Aysén, S. Chile

Abstract The geochemistry of pillow basalts from the Chonos Metamorphic Complex (CMC) and the Eastern Andes Metamorphic Complex of Aysen (EAMC) indicates contrasting tectonic environments for these basic lavas. They have E-MORB and continental alkaline affinities, respectively. The MORB-like basalts are metamorphosed in the pumpellyite–actinolite metamorphic facies, with mineral associations indicative of relatively high P/T metamorphism. The continental alkali basalts exhibit pumpellyite–chlorite assemblages developed in a low to intermediate P/T regime. These contrasting eruptive and metamorphic settings agree with recently established age differences between the complexes, and invalidate direct correlation between them.

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.

Geochronology of very low-grade Mesozoic Andean metabasites; an approach through the K–Ar, 40Ar/39Ar and U–Pb LA-MC-ICP-MS methods

Multiple geochronological methods using different metamorphic minerals were combined to date the regional, very low-grade metamorphism affecting Upper Jurassic–Lower Cretaceous volcano-sedimentary successions in the Andes of central Chile. Early Late Cretaceous metamorphic ages (between 82 and 108 Ma) were obtained by the K–Ar and U–Pb methods for celadonite and titanite. A much younger thermal event is responsible for actinolite formation at 8 Ma, most probably related to the intrusion of proximal Miocene granitoids. Previous models for the metamorphism should be reinterpreted taking into account the absence of a greenschist-facies event. The combination of different metamorphic minerals and chronometers is regarded as a powerful analytical tool to date the very low-grade metamorphism associated with the Mesozoic extensional regime developed within the Andes.