Cerro Uturuncu SW Bolivia: Preliminary Observations from Field work, Geochemistry and Petrology
Abstract
Cerro Uturuncu is a potentially active composite cone volcano located in the back-arc of the Andean Central Volcanic Zone (CVZ) in SW Bolivia. It is composed primarily of porphyritic andesitic to dacitic lavas ranging in SiO2 contents from 61-67 wt.%. Currently, a minimum of 85 separate flows and domes have been identified with the possibility of up to 30 more. InSAR observations between 1992 and 2006, show current states of unrest with active deformation being most intense at 1-2 cm/yr located around the summit of Uturuncu. This deformation is hypothesized to be attributed to upward movement of magma from the Altiplano-Puna magmatic body located at a depth of 17-20 km. Previously determined 39Ar/40Ar ages place the most active period between 870-271 ka. Lava flows and domes contain phenocrysts of zoned plagioclase, orthopyroxene, biotite and rare quartz. Dacitic flows and domes contain silicic andesite inclusions, mircrogranitic inclusions, crustal xenoliths and norite nodules. Pyroclastic material was not observed suggesting that Uturuncu primarily erupted effusively. Three preliminary observations can be drawn from new major and trace element data. First, two distinct suites of magmas with different trace elements signatures are identified. One suite has low Nb, low Nd and low Sr (12.2-14.8 Nb ppm; 214-253 Sr ppm; 34-38 Nd ppm); the other has high Nb, high Nd and high Sr (16-34 Nb ppm; 46-67 Nd ppm; 308-498 Sr ppm). This second suite is the most dominant containing 60 of the identified flows and domes. Second, the ranges in major element compositions suggest that crustal assimilation and magma mixing were important processes in the evolution of the system. This evidence includes consistent decreases in MgO (3.63-1.18 wt.%) and CaO (5.05-3.27 wt.%) and erratic and variable increases in K2O (3.40-4.05 wt.%) and Na2O (2.03-2.73 wt.%) with increasing SiO2. Third, major and trace element data suggest Uturuncu is atypical compared to other volcanic systems in the Central Volcanic Zone (CVZ) such as Ollagüe and Aucanquilcha located on or near the arc front. K2O contents for Uturuncu rocks are at the high end of average contents for CVZ volcanic rocks with similar SiO2 concentrations. Na2O contents are much lower than the average for CVZ rocks, but andesitic inclusions within Uturuncu flows have Na2O contents that range from the low Uturuncu values to those typical for other CVZ volcanoes.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V23B2441M
- Keywords:
-
- 1020 GEOCHEMISTRY / Composition of the continental crust;
- 1036 GEOCHEMISTRY / Magma chamber processes;
- 8185 TECTONOPHYSICS / Volcanic arcs