Volcanological and petrological evolution of Volcan Tata Sabaya, SW Bolivia

Abstract

Tata Sabaya (19°08'S 68° 32'W; 5430 m a.s.l) is located on the northern shores of Salar de Coipasa on the Altiplano of Bolivia. It is a 1700-m-high symmetrical composite cone; one of two Holocene centers in a chain of edifices trending E-W from the main N-S-trending volcanic arc of the Andean Central Volcanic Zone (CVZ). Four main stages are envisaged for the evolution of the edifice. Stage 1: formation of a pyroclastic “shield” possibly around a central dome complex. Stage 2: formation of an andesitic composite cone on top of the early pyroclastic sequence. Stage 3: collapse of a 120° sector of the southern flank of the volcano which produced a prominent debris avalanche deposit covering ∼300 km 2 of the Salar de Coipasa to the south. Collapse was induced by inflation of the edifice by an input of fresh magma into the sub-volcanic magma chamber. Stage 4: flows and domes which have since healed the edifice. Tata Sabaya shows a restricted range of whole-rock compositions, most units fall in the range 60–62.5% SiO 2 with a general evolution to the more silicic stage 4 lavas through time. The typical phenocryst assemblage is plagioclase > hornblende > hypersthene > augite > oxide. Biotite is found in the stage 4 lavas and is dominant over pyroxene. Hornblende and biotite are thought to be a late-stage sub-liquidus products. By far the most mafic compositions at Tata Sabaya are the hornblende and plagioclase inclusions in two of the stage 4 domes (∼ 52% SiO 2). Macroscopic, microscopic, and chemical evidence for magma mixing (mixed phenocryst and matrix populations, resorption features, mixed glass compositions, contraindicative temperature indicators etc.,) are found throughout Tata Sabaya. The whole-rock chemistry bears out that magma mixing was a dominant process and that it was responsible for buffering the composition of the magma in an open magmatic system. Despite being located 35km to the east of the main arc front the rocks of Tata Sabaya are compositionally similar to those from other centers in the northern (17–19°S) segment of the CVZ. The high K 2O, calc-alkaline suite is characterised by LREE and extreme LILE enrichments (up to 1800 ppm Ba and Sr). 87Sr/ 86Sr (0.7056–0.7068) and 143Nd/ 144Nd (0.51217–0.51227) are similar to other CVZ lavas; this is regarded as a baseline signature derived during deep crustal contamination. This is corroborated by Pb-isotope data which indicates influence of the low 206Pb/ 204Pb Precambrian basement that is thought to underlie the northern portion of the CVZ. Modification of the isotope ratios during open system processes at shallow level is also indicated. The most extreme incompatible element enrichments are found in the quenched mafic inclusions. These characteristics have been reported from other Holocene mafic lavas elsewhere in the northern CVZ, and may represent a distinct recent, magmatic episode.