Andean Adakite-like high-Mg Andesites on the Northern Margin of the Chilean–Pampean Flat-slab (27–28·5°S) Associated with Frontal Arc Migration and Fore-arc Subduction Erosion

Abstract

Glassy, plagioclase phenocryst-free, ∼7–3 Ma, andesitic lavas erupted at the southern end of the Andean Central Volcanic Zone (CVZ) at 27–28·5°S are argued to contain a component from continental crust that was incorporated into the sub-arc mantle in a major pulse of fore-arc subduction erosion that removed ∼50 km of fore-arc. The 7–3 Ma amphibole-bearing Pircas Negras (54–64% SiO2) and ∼7·7 Ma pyroxene-bearing Dos Hermanos lavas erupted as the frontal arc was displaced some 50 km eastward over a developing bend in the Wadati–Benioff zone at the northern margin of the Chilean–Pampean flat-slab region. Their chemistry differs from that of older Miocene and younger CVZ arc lavas in the region in having more pronounced high field strength element (La/Ta = 40–100; Ba/Ta = 800–2400) and heavy rare earth element (Sm/Yb ∼ 4–9) depletions and an extreme adakitic-like character (∼600–1400 ppm Sr; Sr/Yb ∼ 400–1350). They also differ from older Miocene lavas in having higher 87Sr/86Sr and lower 143Nd/143Nd at the same wt % SiO2, while still showing trends with increasing wt % SiO2 that are best attributed to contamination in the ∼65–70 km thick crust. Overall, the trace element and isotopic features of the Pircas Negras lavas are consistent with modeling that attributes differences from older and younger lavas to enrichment of the arc mantle wedge by partial melts of tectonically eroded fore-arc crust, with the modeled eroded component being an outcrop and geophysically constrained mixture of 80–90% Jurassic mafic magmatic rocks and 10–20% silicic Paleozoic crust. An adakitic-type partial melt of this eroded crust generated at >2 GPa, reacted with mantle peridotite, can explain the high Mg# values (50–61) and Cr (100–350 ppm) and Ni (40–70 ppm) contents in some 5–3 Ma Pircas Negras lavas. Pre-eruption temperatures over 1060°C, calculated from mineral thermometry and the MELTS program, permit these magmas to subsequently melt and react with overlying eclogitic crust. Magma storage in this thick crust then led to amphibole crystallization and suppression of plagioclase fractionation. Quartz and feldspar xenocrysts were acquired from locally pooled mid-crustal magmas and the silicic upper crustal basement during final rapid ascent to the surface. Given a constant arc–trench gap of 300 km over the last 10 Myr, ∼198 km3 Myr–1 km–1 of fore-arc crust needs to be removed by subduction erosion at 8–3 Ma to account for the material balance of the fore-arc crust.