Prehnite-pumpellyite facies metamorphism in the Cenozoic Abanico Formation, Andes of central Chile (33°50'S): chemical and scale controls on mineral assemblages, reaction progress and the equilibrium state

Abstract

ABSTRACT. In the El Volcán and Rodeo de los Bueyes areas, Andean Principal Cordillera (east of Santiago; 33º50'S), an Upper Oligocene-Lower Miocene volcanic series belonging to the Abanico Formation (Late Eocene-Early Miocene) is exposed. The rock successions outcropping in both areas, ca. 3,300 m total thickness, have been affected by very low-grade, non-deformative metamorphism in the prehnite-pumpellyite facies. This is represented by the widespread development of secondary mineral assemblages composed of epidote, mixed-layer chlorite-smectite, albite, quartz, white mica, and titanite. These mineral assemblages also contain pumpellyite, prehnite or prehnite+actinolite in a few samples. Chemical characteristics, such as low compositional variability of mixed-layer chlorite-smectite and actinolite independent from the metadomain where these phases are hosted, along with a high proportion of chlorite layers in the former, suggest that these phases closely represent the whole rock effective bulk composition. On the contrary, pumpellyite compositions show a higher variability and a strong metadomain control evidencing its lower equilibration kinetics and leaving only those that grow in the rock matrix as the more closely representative of a whole rock effective bulk compositional control. Mineral assemblages from both areas show evidence of having been formed during the same metamorphic event where reactions have ocurred equivalently between them. However, local differences in chemical variables controlling this process are recognizable. A higher CO 2 concentration in the fluids during the metamorphism in the El Volcán area than in the Rodeo de los Bueyes area is suggested by the scarce development of prehnite and the ubiquitous presence of calcite in the metamorphic assemblages of the former. Pressure and temperature of this metamorphic event have been roughly estimated to be lower than 2-3 kbar and 320ºC, respectively. Esto está representado en el desarrollo extensivo de arreglos de minerales secundarios formados por epidota, interestratificados de clorita-esmectita, albita, cuarzo, mica blanca, y titanita. Estos arreglos minerales contienen además pumpellyita, prehnita o prehnita+actinolita en algunas muestras. Las características químicas tales como una baja variabilidad composicional de los interestratificados de clorita-esmectita y actinolita independientemente del dominio donde se hospedan estas fases, junto con una alta proporción de capas de clorita en las primeras, sugieren que estas fases representan cercanamente la composición efectiva de la roca total. Por el contrario, la composición de las pumpellyitas muestra una mayor variabilidad y un fuerte control del metadominio donde éstas se desarrollan evidenciando una baja cinética de equilibrio y dejando sólo a aquellas desarrolladas en la matriz como formadas bajo un control composicional efectivo de la roca total. Los arreglos minerales de ambas áreas muestran evidencias de haberse formado bajo un mismo evento metamórfico bajo el cual las reacciones se ...

Chemical and isotopic features of cold and thermal fluids discharged in the Southern Volcanic Zone between 32.5°S and 36°S: Insights into the physical and chemical processes controlling fluid geochemistry in geothermal systems of Central Chile

“…The Ca 2+ -SO 4 2 − waters (samples 4 and 6) likely derived by dissolution of gypsum/anhydriterich layers ( Fig. 2a and c), which, coupled with Ca (Hardie, 1983;Mariner et al, 2003), since basaltic to andesitic hydrothermally altered volcanic outcrops, characterized by a secondary mineral assemblage rich in chlorite, chalcedony/quartz and albite formed by partial or complete replacement of primary An-rich plagioclase, were recognized in this area (Vergara et al, 1993;Muñoz et al, 2010). A similar process was invoked to explain the Ca…”

Tectono-stratigraphic evolution of the Andean Orogen between 31 and 37°S (Chile and Western Argentina)

“…The dominantly volcanic, middle-late Eocene to Oligocene Abanico Formation, and the Miocene Farellones Formation make up the pre-Pliocene Cenozoic deposits in the Principal Cordillera of Central Chile (318 -368S) (Aguirre 1960;Klohn 1960;Gonza ´lez & Vergara 1962;Charrier 1973Charrier , 1981aThiele 1980;Charrier et al 2002;Godoy 2011). The Abanico Formation consists of a locally strongly folded, c. 3000 m-thick succession of volcanic, pyroclastic volcaniclastic and sedimentary deposits including abundant subvolcanic intrusions of the same age (Vergara et al 2004), with a well-developed paragenesis of low-grade metamorphic minerals (Padilla & Vergara 1985;Levi et al 1989;Aguirre et al 2000;Fuentes et al 2001Fuentes et al , 2005Bevins et al 2003;Fuentes 2004;Mun ˜oz et al 2006Mun ˜oz et al , 2010. The outcrops of this formation form two north-south orientated swaths separated by the Farellones Formation (Fig.…”

Metamorphism and linked deformation in understanding tectonic processes at varied scales

“…At shallow levels of active continental magmatic arcs, as exemplified by the Andean Cordillera, low-grade metamorphism (i.e. zeolite and/or prehnite-pumpellyite facies up to greenschist facies) is well documented [Aguirre et al, 2000, Muñoz et al, 2010. Because this metamorphism affects sediments and interlayered volcanic rocks, it is frequently mentioned as "burial" metamorphism, which requires a temporal evolution from extensional versus compressional tectonic regimes.…”