EFFUSIVE SILICIC VOLCANISM IN THE CENTRAL ANDES - THE CHAO DACITE AND OTHER YOUNG LAVAS OF THE ALTIPLANO-PUNA VOLCANIC COMPLEX

The largest known Quaternary silicic lava body in the world is Cerro Chao in north Chile, a 14-km-long coulee with a volume of at least 26 km(3). It is the largest of a group of several closely similar dacitic lavas erupted during a recent (<100,000 year old) magmatic episode in the Altiplano-Puna Volcanic Complex (APVC; 21-24 degrees S) of the Central Andean Volcanic Zone. The eruption of Chao proceeded in three phases. Phase 1 was explosive and produced similar to 1 km(3) of coarse, nonwelded dacitic pumice deposits and later block and ash flows that form an apron in front of the main lava body. Phase 2 was dominantly effusive and erupted similar to 22.5 km(3) of magma in the form of a composite coulee covering similar to 53 km(2) with a 400-m-high flow front and a small cone of poorly expanded pumice around the vent. The lava is homogeneous with rare flow banding and vesicular tops and selvages. Ogives (flow ridges) reaching heights of 30 m form prominent features on its surface. Phase 3 produced a 6-km-long, 3-km-wide flow that emanated from a collapsed dome. Ogives are subdued, and the lava is glassier than that produced in previous phases. All the Chao products are crystal-rich high-K dacites and rhyodacites with phenocrysts of plagioclase, quartz, hornblende, biotite, sphene, rare sanidine, and oxides. Phenocryst contents reach 40-60 vol % (vesicle free) in the main phase 2 lavas but are lower in the phase 1 (20-25%) and phase 3 (similar to 40%) lavas. Ovoid andesitic inclusions with vesicular interiors and chilled margins up to 10 cm are found in the later stages of phase 2 and compose up to 5% of the phase 3 lava. There is little evidence for preeruptive zonation of the magma body in composition, temperature (similar to 840 degrees C), fO(2) (10(-11)), or water content, so we propose that eruption of the Chao complex was driven by intrusion of fresh, hot andesitic magma into a crystallizing and largely homogeneous body of dacitic magma. Morphological measurments suggest that the Chao lavas had internal plastic viscosities of 10(10) to 10(12) Pa s, apparent viscosities of 10(9) Pa s, surface viscosities of 10(15) to 10(24) pa s, and yield strength of 8 x 10(5) Pa. These estimates indicate that Chao would have exhibited largely similar theological properties to other silicic lava extrusions, notwithstanding its high phenocryst content. We suggest that Chao's anomalous size is a function of both the relatively steep local slope (20 degrees to 3 degrees) and the available volume of magma. The eruption duration for Chao's emplacement is thought to have been about 100 to 150 years, with maximum effusion rates of about 25 m(3) s(-1) for short periods. Four other lavas in the vicinity with volumes of similar to 5 km(3) closely resemble Chao and are probably comagmatic. The suite as a whole shares a petrologic and chemical similarity with the voluminous regional Tertiary to Pleistocene ignimbrites of the APVC and may be derived from a zone of silicic magmatism that is thought to have been active since the late Tertiary. Chao and the other young lavas may represent either the waning of this system or a new episode fueled by intrusions of mafic magma.