The Bahía Laura Volcanic Complex in the Manantial Espejo district

Abstract

We assess the evolution of Jurassic volcanism in the lithological units outcropping in the Manantial Espejo mining district, by combining a detailed geological survey together with petrographic and geochemical analyses of this area of the Deseado Massif (Santa Cruz Province, Argentine Patagonia). Three volcanic cycles were identified, favored by the extensional tectonics occurring in this geological province in Middle to Upper Jurassic times. The first cycle, named Lower Volcanic Cycle, corresponds to the effusion of widespread andesitic lavas (Bajo Pobre Fm.)  interspersed with fluvial reworked pyroclastic sandstones and mass-wasting related volcanic agglomerates. The effusion of these flows overlaid a basement formed by schists and granites. The second one, Intermediate Volcanic Cycle, started with a dacitic ignimbrite followed by two rhyolitic ignimbrites (named #1 and #2). The dacitic ignimbrite and the rhyolitic ignimbrite #2 are high grade, strongly welded flow tuffs with rheomorphic textures, with low proportions of lithoclasts and high amounts of crystalloclasts, all of which suggest high depositional temperatures and its formation from low eruptive columns with relatively low energy, high densities and low gas content (boiling-over type fountains). The rhyolitic ignimbrite #1, interspersed between the previous two, is characterized by the presence of large polymictic lithic fragments, and may be correlated with the extra-caldera deposits formed during the collapse and eruption of the Cerro Primero de Abril Caldera, to the north of the Manantial Espejo district, in the vicinity of Martha Mine. This is followed by a new pause in volcanic activity, during which widespread reworked pyroclastic deposits and travertines were deposited. These hydrothermal deposits evidence the existence of hot-spring activity controlled by structures with NNE and NNW to NW orientation. The heat and CO2 for the geothermal system would have been linked to the degasification of a new rhyolithic magma pulse that marks the onset of the Upper Volcanic Cycle, and which would have ascended through the same extensional system, evidenced by rhyolitic dikes of NNW orientation. The ascent of new rhyolitic magmas though the dikes resulted in the formation of two volcanic centers, the Northern Volcanic Center, composed of an extrusive rhyolitic dome complex with associated block and ash deposits; and the Southern Volcanic Center, characterized by the development of a complex association of facies interpreted as part of a maar-diatreme volcano of hydromagmatic origin. A regionally distributed layer of ash-fall tuffs with accretionary lapilli associated with this explosive event was recognized. After a short period of volcanic calm (inter-eruptive period), the ongoing extensional regime allowed new magma batches to reach the surface resulting in new rhyolithic ignimbrites flow units overlaying the previously deposited units. These younger ash-flow tuffs are also included in the Upper Volcanic Cycle; their characteristics are similar to those of the previous volcanic cycle and they are also interpreted as the result of boiling-over type pyroclastic fountaining. Finally, the andesitic to rhyolitic volcanism of the area is characterized by a calc-alkaline series with moderate to high potassium and a peraluminous signature and shows clear arc affinities. Within this scheme, the Lower Volcanic Cycle is andesitic in composition, the Middle Volcanic Cycle records the evolution of magmas from dacites to rhyolites and, finally, the Upper Volcanic Cycle has a typical rhyolitic composition.