Metamorphic and geochronogical study of the Triassic El Oro metamorphic complex, Ecuador: Implications for high-temperature metamorphism in a forearc zone

Abstract

In the forearc of the Andean active margin in southwest Ecuador, the El Oro metamorphic complex exhibits a well exposed tilted forearc section partially migmatized. We used Raman spectroscopy on carbonaceous matter (RSCM) thermometry and pseudosections coupled with mineralogical and textural studies to constrain the pressure–temperature (P–T) evolution of the El Oro metamorphic complex during Triassic times. Our results show that anatexis of the continental crust occurred by white-mica and biotite dehydration melting along a 10km thick crustal domain (from 4.5 to 8kbar) with increasing temperature from 650 to 700°C. In the biotite dehydration melting zone, temperature was buffered at 750–820°C in a 5km thick layer. The estimated average thermal gradient during peak metamorphism is of 30°C/km within the migmatitic domain can be partitioned into two apparent gradients parts. The upper part from surface to 7km depth records a 40–45°C/km gradient. The lower part records a quasi-adiabatic geotherm with a 10°C/km gradient consistent with an isothermal melting zone. Migmatites U–Th–Pb geochronology yielded zircon and monazite ages of 229.3±2.1Ma and 224.5±2.3Ma, respectively. This thermal event generated S-type magmatism (the Marcabeli granitoid) and was immediately followed by underplating of the high-pressure low-temperature (HP-LT) Arenillas–Panupalí unit at 225.8±1.8Ma. The association of high-temperature low-pressure (HT-LP) migmatites with HP-LT unit constitutes a new example of a paired metamorphic belt along the South American margin. We propose that in addition to crustal thinning, underplating of the Piedras gabbroic unit before 230Ma provided the heat source necessary to foster crustal anatexis. Furthermore, its MORB signature shows that the asthenosphere was involved as the source of the heat anomaly. S-type felsic magmatism is widespread during this time and suggests that a large-scale thermal anomaly affected a large part of the South American margin during the late Triassic. We propose that crustal anatexis is related to an anomaly that arose during subduction of the Panthalassa ocean under the South American margin. Slab verticalization or slab break-off can be invoked as the origin of the upwelling of the asthenosphere.