Phase equilibrium modelling of the amphibolite facies metamorphism in the Yelapa-Chimo Metamorphic Complex, Mexico

Abstract

The Yelapa-Chimo Metamorphic Complex forms part of the Jalisco Block in western Mexico and exposes a wide range of Early Cretaceous metamorphic rocks; such as paragneiss, orthogneiss, amphibolites, and migmatites. However, the pressure–temperature (P–T) conditions of metamorphism and partial melting remain poorly studied in the region. To elucidate metamorphic P–T conditions, phase equilibrium modelling was applied to two sillimanite–garnet paragneisses, one amphibole–orthogneiss, and one amphibolite. Sillimanite–garnet paragneisses exhibit a lepidoblastic texture with a biotite ​+ ​sillimanite ​+ ​kyanite ​+ ​garnet ​+ ​quartz ​+ ​plagioclase ​+ ​K-feldspar mineral assemblage. Amphibole–orthogneiss and amphibolite display a nematoblastic texture with an amphibole ​+ ​(1) plagioclase ​+ ​quartz ​+ ​(1) titanite assemblage and an amphibole ​+ ​(2) plagioclase ​+ ​(2) titanite ​+ ​ilmenite retrograde mineral assemblage. Pseudosections calculated for the two sillimanite–garnet paragneiss samples show P–T peak conditions at ~6–7.5 ​kbar and ~725–740 ​°C. The results for amphibole–orthogneiss and the amphibolite yield P–T peak conditions at ~8.5–10 ​kbar and ~690–710 ​°C. The mode models imply that metasedimentary and metaigneous units can produce up to ~20 ​vol% and ~10 ​vol% of melt, respectively. Modelling within a closed system during isobaric heating suggests that melt compositions of metasedimentary and metaigneous units are likely to have direct implications for the petrogenesis of the Puerto Vallarta Batholith. Our new data indicate that the Yelapa-Chimo Metamorphic Complex evolved through a metamorphic gradient between ~23–33 ​°C km−1 and the metamorphic rocks formed at depths between ~22 km and ~30 ​km with a burial rate of ~2.0 ​km ​Ma−1. Finally, the P–T data for both metasedimentary and metaigneous rocks provide new constraints on an accretionary framework, which is responsible for generating metamorphism and partial melting in the Yelapa-Chimo Metamorphic Complex during the Early Cretaceous.