P-T Conditions for Eclogite / Garnet Amphibolite from Serpentinite Mélanges along the Motagua Fault Zone, Guatemala

The Caribbean plate boundary in Central America lies in Guatemala along three conjugate faults:  the Polochíc (northern), Motagua (central), Jocotán-Chamelecón (south).  Serpentinite mélanges on both sides of the Motagua fault (MF) contain rocks that record subduction, collisions and complex translations.  Interpreting this geology is important for our understanding Caribbean plate evolution.

Serpentinite mélange, north of the MF, hosts eclogite, garnet amphibolite, omphacite-taramite metabasite, jadeitite, albitite distributed over >200 km E-W.  Garnet-clinozoisite amphibolites, in the east, have Cpx and Amph inclusions in garnet in a matrix of amphibole, clinozoisite and late stage albite, quartz, and chlorite.  Rare omphacite in garnet suggests a prior history with eclogite at P > 1.2 GPa.  More abundant Amph and Cpx inclusions and Gln argue reequilibration at ~1 GPa .  Grt-Cpx pairs yield temperature of 500-550ºC whereas Amph-Grt pairs give 400-500ºC.  Altered clinozoisite amphibole-eclogite to the west record a moderate LT eclogite facies of 500-600ºC at 1.5 – 2.0 GPa from Grt-Cpx pairs and paragonite + clinozoisite in the assemblage.  A wide range of matrix alteration assemblages record greenschist-blueschist at lower T and P (300-400ºC at ≤1 GPa)

In contrast, serpentinite mélange, south of the MF, hosts lawsonite eclogite, blueschist, and jadeitite and records P-T conditions that require very deep and cold subduction to ~2.5 GPa at only 470 ºC (Tsujimori et al. 2006).  The P-T-t south of the Motagua fault shows a simple VHP/LT trajectory followed by blueschist alteration during exhumation. A higher T and somewhat lower P maximum was achieved north of the fault, followed by a discrete grt-czo-amphibolite recrystallization event that was more pronounced toward the east.  These interpretations are consistent with a distinct ~130 Ma peak metamorphic age (event) for all eclogites with a second event of 70-55 Ma north of the MF, probably related to a second collision.