Assessment of crustal and mantle processes in the Cerro Galán Ignimbrites: constraints from major and trace element chemistry, oxygen isotopes and 40Ar/39Ar Ages.

MAHLBURG KAY, S., COIRA, B. AND WÖRNER, G.

Jujuy, Argentina

Congreso; XVII Congreso Geológico Argentino.; 2008

AGA

The Cerro Galán complex erupted the largest Neogene ignimbrites in the southern Puna plateau.  These ignimbrites have been suggested to be linked to crustal and mantle melting associated with the latest Miocene to Pliocene gravitational removal of eclogitic lower crust and continental lithospheric mantle (e.g., Kay et al., 1994).  Major questions are the nature and relative percentages of mantle and crustal components in the ignimbrite magmas.  Here, we present new major and trace element data and 18O/16O ratios on quartz to show that the late Miocene Merihuaca, Pliocene Real Grande and Quaternary Galán ignimbrites and intracaldera units could be approximately 50/50 mixtures of mantle and crustal melts.  The mantle melts produced and mixed with deep crustal melts that rose, pooled, and fractionated at mid to upper crustal depths before erupting.  The mantle magmas, which formed in a hot backarc mantle wedge far above the subduction zone, had intraplate-like chemical features as apparently did much of the melted crust. A tendency for slightly more arc-like characteristics in the older ignimbrites gives credence to a diminishing slab influence in a thickening mantle wedge under a delaminating lithosphere and a steepening slab.  Systematic single crystal 40Ar/39Ar age differences between older biotites and younger sanidines in individual flows could reflect incorporation of partially degassed xenocrystic biotite from upper crustal magma chambers just prior to eruption.  The sanidine 40Ar/39Ar ages plateau ages show that the youngest Galán ignimbrite flows could have erupted over a period of 100,000 years.