Ophiolitic rocks and plagiorhyolites from SW Ecuador (Cerro San José): petrology, geochemistry and tectonic setting

Abstract

This work investigates the petrology and geochemistry of ophiolitic rocks that belong to basalt formations of Cretaceous age and outcrop south of Nobol (Guayas, Ecuador). We studied their petrogenesis and tried to establish the tectonic setting and correlations with the regional geology. These mafic rocks, together with associated felsic dykes, are interpreted to be part of an ophiolite sequence (Cerro de San Jose). Our results show that the mafic rocks are iron rich tholeiitic basalts (Fe2O3t = 13–14.7 wt%), with arc geochemical signature. The associated felsic dykes are trondhjemites (Na2O = 6.5 wt%; K2O = 0.1 wt%) and can be interpreted as plagiorhyolites derived by partial melting of a hydrated mafic oceanic crust. The tectonic setting proposed for these rocks is an arc or back-arc basin where infiltration of melts/fluids derived from a subducted slab and its mantle wedge could have generated the arc signature of these tholeiitic basalts. Furthermore, these melts/fluids could also have induced partial melting of country rock basalts and generation of plagiorhyolite dykes.