Pre-accretion metamorphism of the Teloloapan Terrane (southern Mexico): example of burial metamorphism in an island-arc setting

Volcanic and interbedded volcaniclastic rocks of the lower Cretaceous island-arc series of the Teloloapan Terrane in southern Mexico contain metamorphic assemblages characteristic of the zeolite, prehnite–pumpellyite and lower greenschist facies produced by burial metamorphism prior to its accretion to nuclear Mexico. Distribution of secondary assemblages throughout the stratigraphic succession, together with the chemical evolution of metamorphic minerals, reveals a depth-controlled metamorphic zonation characterized by the presence of the diagnostic assemblages laumontite+pumpellyite+epidote and laumontite+celadonite+pumpellyite±epidote (zeolite facies) followed downward by assemblages containing prehnite+pumpellyite±white mica (prehnite–pumpellyite facies) and finally by the presence of the assemblages pumpellyite+actinolite+epidote and epidote+actinolite (greenschist facies). Analysis of assemblages in the Al–Fe3+–FM–K system, reveals that facies boundaries are discontinuous, involving the disappearance of at least one phase and the appearance and/or extension of the field of equilibrium of other diagnostic minerals and assemblages. Application of empirically based thermobarometers, phase equilibria, mineral chemistry, and petrogenetic grids indicates that the P–T conditions of metamorphism ranged from 175 to 342°C and P<3 kbar. The data further indicate high geothermal gradients of about ∼55°C km−1. Seawater-derived fluids were characterized by high aK, high fO2 and low XCO2.