A buried Middle Miocene caldera beneath Laguna Pozuelos, Northern Puna?

PREZZI, C., LINCE KLINGER, F., CAFFE, P.J., GIMENEZ, M.

Santiago, Chile

Congreso; GEOSUR 2007. INTERNATIONAL GEOLOGICAL CONGRESS ON THE SOUTHERN HEMISPHERE; 2007

Laguna de Pozuelos is located in the northern Puna. This 100 km long, 20 km wide depression corresponds to an internally drained basin. The Pan de Azúcar volcanic complex crops out in the southern border of the Laguna de Pozuelos Basin and consists of several volcanic dome centers of approximately 12 Ma. Previous interpretation of seismic lines north of the Pan de Azúcar outcrops indicated the presence of buried bodies, probably related to the volcanic complex. A detailed ground magnetic survey of the southern portion of the Laguna de Pozuelos Basin (north of Pan de Azúcar) confirmed the presence of an intrusive body of approximately 1 km in diameter buried at approximate depths of 250 m. The determination of the existence of buried bodies in this area has two major implications. First, the volcanic complexes that outcrop in the Laguna de Pozuelos Basin are closely associated with ore deposits, as part of the Bolivian tin ore polymetallic belts (e.g. Pan de Azúcar Ag?Pb?Zn mining ore deposit, Chinchillas Pb?Zn?Sn mineralized hydrothermal breccia, Cerro Redondo Ag?Zn?Pb?Cu mining prospect). Second, different authors have suggested the existence of a large ancient caldera (Miocene) beneath the central and southern part of the Laguna de Pozuelos Basin, covered by the infilling sediments. It was proposed that the dome volcanic complexes (Cerro Redondo, Chinchillas, Pan de Azúcar) may represent the final stages of such a calderic magmatic system.Trying to confirm the presence of the buried magmatic system and determine the existence, 3D shape, depth, and size of buried dacite bodies, detailed ground gravity and magnetic surveys were carried out in Laguna de Pozuelos Basin. The obtained magnetic map shows the existence of positive (maximum of 210 nT) and negative (minimum of -280 nT) anomalies in the southern sector of the basin. On the other hand, the calculated residual Bouguer anomaly map shows only positive values ranging between 30 and 70 mGal, with the lowest values to the east of Cincel river. The Cincel river is supposed to follow a large-scale ring fracture that would define the eastern limit of the buried calderic system. We applied the curvature technique to analyse the magnetic and gravity signals and used Euler deconvolution to estimate the depth to the sources. We also developed a 3D forward modelling of the gravity and magnetic anomalies. Our preliminary results would support the hypothesis of a large caldera buried beneath the Laguna de Pozuelos Basin. The identification of such a large Middle Miocene caldera would bring new insight into the magmatic evolution of the northern Puna.