Central Andean Plateau Ignimbrites in the Puna Back-arc Cenozoic Volcanic Province (PBCV).

COIRA, B. AND S. MAHLBURG KAY

Pucón, Chile

Congreso; General Assembly IAVCEI; 2004

IAVCEI

Voluminous PBCV ignimbrites from giant calderas dominate Neogene volcanism in the Puna (22º-28ºS) plateau. Geochemical and temporal variations in these ignimbrites are controlled by preexisting basement and Neogene tectonics. Northern PBCV ignimbrites are ~10 to 6 Ma peraluminous high-K dacites (65-69% SiO2) with steep REE patterns (La/Yb=22-40), backarc La/Ta (22-35) and Ba/La (10-18), high 87Sr/86Sr (0.719-0.711), and low εΝd (-7.5 to-10). The melts equilibrated in thickened lower crust and evolved at mid crustal discontinuities before erupting. Basaltic magmas from a hydrated mantle over a steepening slab provided heat for and mixed with the lower crustal melts. K/Na and initial isotopic ratio similarities with nearby Ordovician backarc volcanic rocks reflect the influence of backarc crust last modified during Ordovician. In comparison, southern PBVC ignimbrites have higher Na/K, lower 87Sr/86Sr (0.706-0.711) and higher εΝd (-8 to -3) that reflect the influence of Ordovician and earlier oceanic basement. High La/Yb (30-53) and La/Ta (26-34) in the ~7 to 3 Ma ignimbrites reflect their eruption over an intermediately dipping slab east of a migrating frontal arc at a time of widespread “adakite” (La/Yb >30) eruptions. The ~2 Ma Galán ignimbrite (La/Yb=19-24; La/Ta <18;Ba/La <15) reflects voluminous basement melting due to injection of intraplate mafic magmas after delamination of the thickened eclogitic lower crust. In contrast, the giant metaluminous ~5 to 1.3 Ma ignimbrites of the better known APVC province in the arc (La/Yb=12-17; 87Sr/86Sr=0.7085-0.7117; εNd=-6.7 to -8.1) reflect generation in a region where frontal arc magmatism has dominated since before the Ordovician.