The Iglesia Irca intrusion and the role of gas brecciation in the emplacement of the Coastal Batholith of Peru

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Abstract
Summary: In the Iglesia Irca intrusion early undifferentiated pulses of granitic magma were emplaced at pressures of approximately 1 kb as narrow dykes of vesicular felsite. These dykes are associated with penecontemporaneous burst breccias and gas breccias and are themselves sometimes brecciated. Subsequently a large dyke of granophyre, representing a more fractionated composition, was emplaced by subsidence and stoping along the line of earlier microbreccias, explosion breccias and felsites. The dyke cooled relatively slowly at low structural levels and retained its volatiles while at higher levels more rapid cooling accompanied the greater escape of volatiles. This resulted in marked potassium enrichment of the upper levels of the dyke and the country rocks of the hanging wall. It is argued that the Iglesia Irca intrusion provides an example of the association of gas brecciation with vesiculating and volatile-rich magma which is atypical of the Coastal Batholith as a whole.