Origin and Evolution of Late Miocene Backarc Mantle and Crustal Magmas in the Southern Puna Plateau of the Central Andes.

MAHLBURG KAY S. Y B. COIRA,

Pucón, Chile

Congreso; General Assembly IAVCEI 2004.; 2004

IAVCEI

Late Miocene to Recent southern Puna backarc magmatic rocks are mostly small mafic flows and voluminous dacitic ignimbrites. Mafic flows along faults typically have plagioclase and quartz xenocrysts, low FeO/MgO ratios and high Cr and Ni contents. Eruption times and temporal changes from arc to more intraplate La/Ta ratios and flatter REE patterns (lower La/Yb) in Salar de Antofalla region mafic lavas parallel those in Cerro Galán ignimbrites. Mafic lavas (53-55% SiO2) characterized by La/Yb of 10 to 25 have La/Ta = 36-55 at ~ 6.6 to 4.3 and 20-35 at < 3 Ma; glassy ~ 5 to 4.3 Ma andesites and dacites with La/Yb of 24-46 & 75-100 have La/Ta of 45-78 & > 64; and ~ 6.6-4.3 Ma and ~ 2.4-2.0 Ma ignimbrites (67-68% SiO2) show changes in La/Yb from 32-53 to 19-24 and La/Ta from 22-34 to 14-18. For comparison, La/Yb and La/Ta in < 1 Ma near arc “Robledo” ignimbrites (72% SiO2) are ~ 30 & ~ 20 and in CVZ lavas (~ 53% SiO2) are 17-24 & 27-36. The backarc patterns require a decrease in arc-like character of mantle and crustal sources and increasing partial melt percentages. Low Ba/La ratios (most < 15) show little role for slab-derived fluid mobile elements. The mafic magmas fit with rapid mixing of ~ 80% mantle-derived basalt and ~ 20% pooled lower crustal melt fractionating at mid crustal depth. The model is consistent with both seismic evidence for mid crustal melt pools in the northern Puna, and lower crustal delamination in the southern Puna.