A new record of deeper and colder subduction in the Acatlán complex, Mexico: Evidence from phase equilibrium modelling and Zr-in-rutile thermometry

Abstract

Variable pressure–temperature (P–T) conditions and paths have previously been reported from eclogites and blueschists from the Acatlán complex, Mexico; yet, the large range of P–T estimates, and uncertainties related with conventional thermobarometry suggest that the P–T evolution of the Acatlán complex remains incompletely understood hindering reliable tectonic interpretations. In this paper, we focus on two high-pressure samples: a phengite eclogite and a garnet-epidote blueschist. We couple phase equilibrium modelling and Zr-in-rutile thermometry to re-evaluate the P–T conditions from these rocks and discuss their implications for the Acatlán complex. The phengite eclogite displays a granoblastic texture with a peak mineral assemblage of garnet + omphacite + phengite + rutile + quartz, and an early retrograde mineral assemblage of amphibole + zoisite + plagioclase + titanite. The garnet-epidote blueschist display a nematoblastic texture with a peak mineral assemblage glaucophane + phengite + epidote + quartz + plagioclase + chlorite + rutile + garnet. Our results indicate that the phengite eclogite followed a clockwise P–T path from peak conditions at ~22 kbar and ~690 °C to re-equilibration conditions at ~14.5 kbar and ~660 °C. Similarly, the garnet-epidote blueschist followed a clockwise P–T path. Phengite zoning indicates two different possibilities for the P–T evolution: (1) from peak conditions at ~19 kbar and ~505 °C or (2) from ~13 kbar and ~480 °C, to re-equilibration conditions at ~8.5 kbar and ~487 °C. Epidote + phengite + albite aggregates (pseudomorphs after lawsonite) and phengite zoning constrain the prograde path within the lawsonite stability field, indicating that the complex evolved through a colder subduction zone (gradient of ~6 °C km−1) than pre-existing estimates. Our results indicate that the eclogite and blueschist reached depths higher than previously thought, ~70 and ~50 km respectively, and experienced a near-isothermal exhumation (~0.15–0.60 °C km−1 for the blueschist, and ~0.9 °C km−1 for the eclogite). These new results, combined with published geochronology, result in a burial rate of ~5.4 km Myr−1, and an exhumation rate of ~3.4 km Myr−1.