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; G99004Received 20 January 1999; accepted 9 November 1999 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.

Kinematics of the active West Andean fold-and-thrust belt (central Chile): Structure and long-term shortening rate

“…More precisely, at the latitude of our study area, K/Ar and 40 Ar/ 39 Ar ages on plagioclase in ash flows and lavas from this formation range from 30.9 to 20.3 Ma. The Abanico formation is also intruded by porphyric dikes as young as 16.7 Ma as derived from K/Ar age on whole rock [Gana and Wall, 1997;Gana et al, 1999;Aguirre et al, 2000;Nyström et al, 2003;Vergara et al, 2004]. The largest pluton intruding the formation close to Santiago is the La Obra leucogranodiorite, with 40 Ar/ 39 Ar biotite ages of 19.6 ± 0.5 Ma [Kurtz et al, 1997].…”

Nuevos antecedentes estratigráficos y geocronológicos para el Meso-Cenozoico de la Cordillera Principal de Chile entre 32° y 32°30’S: Implicancias estructurales y paleogeográficas

“…1) se ubica en el límite sur de la zona de subducción subhorizontal Pampeana o Chilena que se desarrolla entre ~27° y 33°S (Cahill e Isacks, 1992;Ramos, 1999;Gutscher et al, 2000;Ramos et al, 2002). Al sur de la zona de estudio (al sur de 33°S), en la parte norte del segmento de subducción normal, los depósitos del Eoceno superior al Mioceno inferior han sido estudiados desde la década del 60 y se ha interpretado el ambiente tectónico en que se habrían originado, mediante la caracterización de los depósitos, las estructuras que los afectan, su geocronología y geoquímica, entre otras metodologías (Aguirre, 1960;Thiele, 1980;Aguirre et al, 2000;Charrier et al, 1996Charrier et al, , 2002Godoy et al, 1996Godoy et al, , 1999Sellés, 1999Sellés, , 2000Zurita, 1999;Rivera y Cembrano, 2000;Giambiagi y Ramos, 2002;Nyström et al, 2003;Fock et al, 2005;Fock et al, 2006a, b;Muñoz et al, 2006;Jara et al, 2009a, b;Maksaev et al, 2003Maksaev et al, , 2009Armijo et al, 2009;Farías et al, 2010;Rauld, 2011, entre otros). Estos estudios han permitido asignar las rocas de la Formación Abanico (Fig.…”

Zeolitization of tuffs at Quinamávida, central southern Chile

“…Aguirre et al, 2000;Vergara et al, 1993, Levi, 1969, where the Cenozoic volcanic and volcanoclastic sequences at the latitude of Santiago provide a textbook example of zeolite facies in extensional basin settings (Robinson and Frey, 1999).…”