Tertiary andean magmatism in Chile and Argentina between 28°S and 33°S: Correlation of magmatic chemistry with a changing Benioff zone

Abstract

The distribution and character of Tertiary magmatism in the non-volcanic region of shallow subduction between 28°S and 33°S in the Andes correlates with other geological and geophysical evidence suggesting that the subduction zone in this region has shallowed over the last 18 Ma with a major change occurring between 11 and 8 Ma. Geologic and geochemical data suggest that the magmatic source region under the Chilean Cordillera (29.0–30.5°S) changed at 16–18 Ma from garnet-poor to garnet-rich coincident with cooling due to shallowing of the subduction zone and thickening of the crust. Magmas older than 16–18 Ma appear to have fractionated at shallower crustal levels and interacted less with the pre-existing crust than have younger magmas. Except for small silicic ignimbrites, volcanism ended in the Cordillera at about 11 Ma. Magmatic activity spread to east to the Calingasta Valley and Precordillera in Argentina between 18 and 7 Ma (exact ages uncertain) as the subduction zone shallowed. The chemistry of the andesitic to rhyolitic composition rocks in this region are consistent with derivation from a thickened crust over a shallowing subduction zone. After 8 Ma, the principal magmatic activity migrated far to the east to the Pocho (7.8-4.9 Ma) and San Luis regions in the Sierras Pampeanas. The volcanic rocks (54–70% SiO 2) in this area are geochemically distinct from recent volcanic rocks in other parts of the Andes, but are consistent with eruption over a deep segment of the seismic zone. Present-day volcanism in the “flat slab” region is lacking in the west due to insufficient heat for melting in the mantle above the slab, and in the east because the slab has already lost the components to flux melting in the mantle above the slab by the time temperatures are high enough to produce melting.