New insights on the nature of back-arc mafia volcanism in the southern Puna, NW Argentina from 40Ar/39Ar geochronology, mineral chemistry and thermobarometry.

RISSE, A.; TRUMBULL, R.B.; COIRA, B.; KAY, S.M. Y P., VAN DEN BOGAARD

Pucón, Chile

Congreso; General Assembly IAVCEI; 2004

IAVCEI

The distribution and bulk composition of basaltic centers in the southern Puna plateau feature prominently in models of magmatism and crust-mantle interaction in the Central Andes, but geochronologic and petrologic studies from these centers have lagged behind. We carried out a laser incremental-heating 40Ar/39Ar study of groundmass chips from 20 back-arc mafic centers and determined magma intensive parameters based on mineral and melt inclusion compositions. Our geochronology results define two clusters of ages, one at about 5 to 2.5 Ma and a second one at less than 1 Ma. These ages confirm and extend earlier work. Together, the available data demonstrate an overall duration of basaltic andesite eruptions in the region from about 7 Ma to Recent. Current results suggest a gap in activity between about 1 and 2.5 Ma. If this gap is not a sampling artifact, it may be related to the age of the Galán ignimbrite (2.1 Ma) which represents the culminating eruption of >1000 km3 silicic magma. The Galán magmas formed by crustal melting, which may have imposed a rheologic barrier to ascent of mantle magmas. Temperature estimates using mineral equilibria and liquid saturation thermometers are in the range of 1050-1150°C. Pressures based on crystal structure modelling on Cpx phenocrysts are 4-9 kbar, indicating storage and equilibration in magma chambers within the mid to upper crust. We find no systematic differences in magma storage conditions with respect to age or location of the centers.