ABSTRACT

To understand the processes of melt eduction in a ridge subduction zone, we performed U-Pb dating on zircons separated from igneous and sedimentary rocks that were newly dredged from the Chile Triple Junction area and from volcanic rocks collected from the Taitao peninsula, southern Chile. The youngest fraction of the U-Pb age population was used to estimate the age of magmatism or sedimentation. Our new results indicate that the fore-arc region became volcanically active over a period of ~0.4 m.y., after obduction of the Taitao ophiolite (~5.7 to 5.2 Ma) from the west and after granite intrusions related to ridge subduction at ~6 Ma. Fore-arc volcanism produced ejecta of basaltic to dacitic compositions and migrated from offshore (~5.3 Ma) to inland (~4.6 Ma) along the Chile Margin Unit that trends northeast-southwest. The volcanism further extended east to produce the dacitic volcanic plug of Pan de Azucar (~4.3 Ma) and lavas in Fjord San Pedro (~2.9 Ma). The migration took place at a rate of ~2.3 cm/y to ~5.3 cm/y. Another intrusion of a granite pluton, widely distributed offshore of the Taitao ophiolite, took place at ~4.0 Ma. Distributions of old detrital zircon and zircon xenocryst ages were used to evaluate, respectively, the influence of subducted sediments and igneous crustal material. Our results indicate that crustal material influenced only Pan de Azucar and Fjord San Pedro dacites; other acidic magmatism shows moderate evidence for incorporation of subducted sediments. Therefore, melts were formed at shallow depths near the triple junction, due mainly to partial melting of the subducted slab and sediments, and then ejected instantaneously. Depleted oceanic materials became anhydrous, and a volcanic gap was formed along the Andean arc.