Abstract
The Permo-Triassic Choiyoi rhyolite province of Chile and Argentina is by its extension, volume and geological significance, the record of one of the most important magmatic events of the southwestern margin of Gondwana. However, aspects dealing with the physical volcanology and tectonic setting are poorly understood. In order to characterize the eruptive activity and style of deposition, a stratigraphic and litofacial study was carried out in the Horcajo Formation, a thick silicic volcanic succession representative of the upper section of the Choiyoi Group cropping out in the Cordillera Frontal of San Juan (western Argentina). A volcanic cycle defined by four eruptive episodes is recognized in the lower and middle sections of the unit: 1. pre-collapse effusive volcanism; 2. phreatomagmatic explosive volcanism; 3. explosive volcanism and volcanotectonic collapse; and 4. post-collapse effusive volcanism and syn-eruptive sedimentation. The emplacement of the eruptive units of the Horcajo Formation was controlled by volcano-tectonic subsidence processes associated with large magnitude ignimbrite forming eruptions. Although this eruptive mechanism would have been dominant, it alternate with important effusive phases which lead to the emplacement of voluminous rhyolite lava flows. Minor phreatomagmatic activity was responsible for the construction of silicic tuff cones or rings also related to the formation of local alluvial and lacustrine environments during brief intervals of syn-eruptive sedimentation. Stratigraphic and structural evidence suggests that the recognized deposits are remnants of an ancient caldera-like eruptive center, although without limits or geometry clearly exposed. The processes involved in the genesis of Horcajo Formation, a widespread unit of Cordillera Frontal, can be considered as some of the most common of the Choiyoi volcanism.
Highlights
Junto con la amalgamación final de Pangea y la terminación del Orógeno Terra Australis, alrededor de los 300-230 Ma (Cawood, 2005), el margen suroccidental de Gondwana fue afectado por un intenso evento magmático que evolucionó en forma discontinua desde asociaciones de arco en un régimen orogénico hasta un régimen extensional posorogénico transicional a intraplaca (Llambías, 1999; Kleiman y Japas, 2009)
The Permo-Triassic Choiyoi rhyolite province of Chile and Argentina is by its extension, volume and geological significance, the record of one of the most important magmatic events of the southwestern margin of Gondwana
A volcanic cycle defined by four eruptive episodes is recognized in the lower and middle sections of the unit: 1. pre-collapse effusive volcanism; 2. phreatomagmatic explosive volcanism; 3. explosive volcanism and volcano-tectonic collapse; and 4. post-collapse effusive volcanism and syn-eruptive sedimentation
Summary
Junto con la amalgamación final de Pangea y la terminación del Orógeno Terra Australis, alrededor de los 300-230 Ma (Cawood, 2005), el margen suroccidental de Gondwana fue afectado por un intenso evento magmático que evolucionó en forma discontinua desde asociaciones de arco en un régimen orogénico hasta un régimen extensional posorogénico transicional a intraplaca (Llambías, 1999; Kleiman y Japas, 2009). El Grupo Choiyoi en la Cordillera Frontal, en donde alcanza su mayor desarrollo, comprende una secuencia dominantemente volcánica de edad Permo-Triásica de composición intermedia a silícica emplazada en ambiente continental en el que se destacan volumétricamente productos de volcanismo explosivo, en general ignimbritas con altos grados de soldamiento (Llambías, 1999). Se llevó a cabo un análisis estratigráfico y litofacial a lo largo de una secuencia volcánica riolítica correspondiente a los tramos basales y medios de la Formación Horcajo (Mirré, 1966), unidad representativa de la sección superior del Grupo Choiyoi aflorante en el sector de Las Caletas, en la Cordillera Frontal de San Juan. Se realiza un breve análisis sobre estructuras de fallamiento y su relación con los depósitos, demostrando por primera vez, y a través de evidencias directas, la acción de procesos de subsidencia volcano-tectónicos durante el emplazamiento de las unidades ignimbríticas del Grupo Choiyoi
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