MAGNETIC FABRIC AND FLOW DIRECTIONS FROM THE GRANADA IGNIMBRITE, NORTHERN PUNA, ARGENTINA

A study of magnetic fabrics , supported by field evidence, microscopic observations, paleomagnetic data, rock magnetism, AMS and AIRM studies was carried out on Upper Miocene pyroclastic deposits from Granada Ignimbrite in order to determine their flow directions and vents. This study also allowed to evaluate the usefulness of combined different methodologies to resolve problems related to the petrofabric of rocks. The Granada Ignimbrite points out the onset of a large ignimbritic volcanism in the northern Puna and it is formed by subhorizontal dacytic ash flows deposits, often associated with friable surges. These rocks do not show field evidence of tectonic or later processes that might have modified their primary emplacement fabric. These flows contain similar phenocryst assemblages with biotite (main paramagnetic mineral) and titanomagnetites (ferromagnetic mineral) as inclusions in the former.

The analysis of the results permitted to determine that the Granada Ignimbrite has a measurable magnetic fabric. Although susceptibility values are low , results show within site and between sites high consistency and they are not related to the degree of anisotropy (moderate to high). The paramagnetic contribution exceeds the ferromagnetic contribution, suggesting a control of the magnetic fabric by biotite. Petrofabric foliation due to biotite orientation is correlated to the AMS and AIRM magnetic foliation plane .Thus, in this kind of biotitic ignimbrites the pole of magnetic foliation (AMS elipsoid minimum axes) can be a reliable and useful indicator of flow direction and sense.On the other hand, surges and ignimbrites show different magnetic fabrics probably reflecting different flow mechanisms. Finally, results will be confronted with the current models of provenance for the Granada Ignimbrite.