Metamorphic evolution of garnet amphibolite in the northern part of the Chuacús Complex, Guatemala: Insights from petrography, mineral chemistry and phase equilibria modeling

Abstract:

The Chuacús Complex, in central Guatemala, is part of the Guatemala Suture Complex, which represents the tectonic limit of the North America and Caribbean plate. The Chuacús Complex is constitute by a polymetamorphic high-pressure sequence of quartzofeldespathic orto and para-gneiss, calcsilicate, marble, pelitic schist, and amphibolite with eclogite relicts.

Two representative garnet amphibolite samples were selected for detailed study. This rocks present a mineral assemblage composed of garnet, calcic amphibole, sodic-calcic amphibole, sodic plagioclase, quartz, minor zoisite and white mica. Both samples contain considerable amounts of titanite and ilmenite. Garnet varies in grain shape, size and show different textural features. The coarse-grained porphydoblasts are mostly subhedral to euhedral and exhibit prograde compositional zoning, with increasing spessartine content to the core (Alm_44-49Grs_32-34Sps_18-20Pyr_1.1-1.5), and increasing almandine and pyrope component to the rim (Alm_64-66 Grs_29-31Pyr_3.3-4.6Sps_0.37-0.96). Amphibole exhibit a constant grain size and shape. Its chemical composition varies along the samples from calcic amphibole (Si=6.57-6.93; X_mg=0.41-0.66) to sodic-calcic amphibole (Si=6.61-6.69; X_mg=0.49-0.54).

In order to constrain the P-T evolution, multiequilibria thermobarometry was combined with thermodynamic modeling. Multiequilibria thermobarometry calculated in the system NCKMnFMASHTO shows a P-T evolution from 15 kbar and 400 °C to 19 kbar and 540 °C. In the other hand, preliminary equilibrium assemblage diagrams calculated in the system NCKFMASHTO shows a P-T evolution from 11 kbar to 13.5 kbar and from 550 °C to 650°C, for the observed mineral assemblage.

This work describes the textural, paragenetic and thermobarometric features from this lithotype and their petrogenetic implications are discussed. This study provides new petrological data that contributes with the understanding of the tectonic evolution of the Guatemala Suture Complex.