Source area and emplacement conditions of Riscos Bayos Ignimbrites, Caviahue-Copahue Volcanic Complex (Argentina)

HAAG, MAURÍCIO B; MONCINHATTO, THIAGO R; SOMMER, CARLOS A; SAVIAN, JAIRO F; - CASELLI, A.T.

Viena

Congreso; EGU General Assembly 2020; 2020

T he Caviahue-Copahue Volcanic Complex (CCVC, Argentina) composes one of the most activevolcanic centers in the Southern Volcanic Zone (SVZ) of the Andes, characterized by the presenceof voluminous explosive and effusive deposits. Despite its young age (< 5 Ma), CVCC deposits werestrongly affected by two glaciations, leading to the removal of a considerable volume of theoriginal deposits, requiring alternative techniques for the reconstruction of this volcanic center.The Riscos Bayos Ignimbrites (RBI) consist of a sequence of non-welded ignimbrites, locatedapproximately 15 km southeast of the CVCC. This unit is commonly associated with the putativecollapse of Caviahue caldera (15 x 20 km, 1 km deep) during the Pleistocene, although the sourcearea and emplacement conditions of RBI still poorly constrained. In this work, we combinefieldwork, anisotropy of magnetic susceptibility (AMS, 23 sites) and rheological analyses (17samples) in order to trace RBI source region and constrain its emplacement conditions, addressingits relevance to CVCC evolution. Rheological parameters, including viscosity, glass transitiontemperature, and liquidus temperatures were obtained using numerical models available from theliterature, while AMS samples were measured using a Kappabridge MFK1-A (Agico) and the dataprocessed using Anisoft5 (Agico). The magnetic mineralogy was characterized using severalexperiments, including isothermal remanet magnetization, thermomagnetic curves, hysteresisloops, first-order reversal curves and scanning electron microscopy. Our data indicate liquidustemperatures ranging from 969 to 1100 ºC, glass transition temperatures from 653 to 721 ºC, andviscosity (at liquidus temperature) from 3.4 to 7.3 log Pa.s. The absence of welding features in thesamples implies RBI emplacement at temperatures below the glass transition temperature,suggesting a fast and effective cooling of the pyroclasts before their settling. The low crystalcontent of the samples suggests eruption temperatures close to the calculated liquidustemperature of the melt. AMS directional analyses indicate a consistent transport sense to SSE (Azof approximately 100º), implying the southern rim of the CVCC as the main source region of RBI.Magnetic experiments show primary, multi-domain, high curie temperature (580 ºC)titanomagnetites as the main carriers of the AMS signal. Most ellipsoids display oblate to triaxialgeometry, with a low degree of anisotropy (< 5%) and magnetic susceptibility (1.0 x 10-2 SI). The low