Paper No. 179-4
Presentation Time: 8:45 AM
THERMOCHRONOLOGIC CONSTRAINTS ON THE LATE PALEOZOIC AND EARLY MESOZOIC TECTONIC EVOLUTION OF COASTAL CENTRAL CHILE (33.5 S)
Basement outcrops of the central coast of Chile (~33.5 S) record a complex Paleozoic-Mesozoic tectonic history of arc magmatism, accretion, and deformation, the structural architecture of which modulated subsequent phases of deformation and magmatism, including the modern plate boundary. We present argon thermochronology data that, when integrated with field and structural observations, yield new insight into three major tectonothermal events. Late Carboniferous-Early Permian ages were obtained from the Santo Domingo igneous complex near Isla Negra, where an amphibole plateau age of 303 Ma approximates the timing of intrusion in the Punta de Tralca tonalite (PdT) and fabric development by a combination of magmatic and solid-state deformation. A 296 Ma biotite age from a dike that cuts the PdT places a minimum age constraint on the intrusion of the Estero Córdoba unit. NNW-striking foliations and ENE-plunging mineral lineations observed in the igneous rocks are also documented in the country rock of the Valparaiso metamorphic complex (VMC). At Las Cruces, amphibole from garnet-amphibolite gneiss of the VMC yielded Early Permian maximum ages. Here, the VMC is intruded by garnet-bearing leucocratic intrusions that are synkinematic with respect to a suite of NW- to WNW-striking shear zones associated with L>S tectonites. A leucocratic intrusion yielded a biotite plateau age of 189 Ma, and biotite from an associated shear zone margin yielded Late Triassic-Early Jurassic maximum ages. At Agua Salada, migmatitic tonalitic and amphibolite gneisses are crosscut by leucocratic veins/dikes that are folded and crosscut by Middle Jurassic shear zones. The dominant structural trend is associated with subvertical W- to WSW-striking foliations, an orientation similar to that documented for the Cartagena dike swarm; both lineations and fold hinges plunge steeply to the SW. Amphiboles from two gneissic samples from Agua Salada yielded Late Triassic to Early Jurassic plateau or plateau-like segments with evidence for Jurassic resetting, and biotite yielded Middle Jurassic plateau ages (159-157 Ma). Together, these new data yield further insight into the punctuated history of subduction, arc magmatism, accretion, and segmentation along the proto-Pacific margin of Gondwana and development of the Andean margin.