Petrology and U/Pb geochronology of the Santa Maria Ipalapa region in the southeastern part of the Xolapa Complex, Mexico: Constrains of the metamorphic evolution of the Xolapa Terrane

Abstract:

The Xolapa Complex is a metamorphic-plutonic basement that forms a large belt with more than 600 km length and 50-100 km wide along the Pacific coast of southeastern Mexico. This Complex is constituted by a high grade sequence of meta-sedimentary and meta-igneous rocks, locally migmatisized, and which are intruded by strongly deformed plutonic rocks.

Because of their representative characteristics, two samples were analyzed: 1) Para-schist: this rocks present a mineral assemblage composed of biotite, sillimanite, plagioclase, k-feldspar and garnet, and 2) Amphibolite: which are constituted by amphibole, plagioclase, biotite and garnet.

The garnet porphydoblasts in the para-schist are subhedral, presents retrograde compositional zoning, with almandine and pyrope rich core (Alm_74-75Sps_7.-10.1Pyr_12.1-12.5Grs_3.8-3.9) and spessartine rich rim (Alm_69-71Sps_14-19Pyr_7.9-9.6Grs_3.6-3.7).

The garnet in amphibolite, presents a prograde growth zoning with a slight increase in spessartine in the core (Alm_59-60Grs_24-25Pyr_8.0-8.3Sps_7.3-7.6), and low content of spessartine component toward the rim (Alm_60-62Grs_23-24Pyr_8.8-9.6Sps_5.4-5.5).

In order to constrain the P-T evolution of the region, multiequilibria thermobarometry was applied to both samples, the para-schist unit presents P-T data from 706 (ºC) and 7.5 (kbar), in the other hand the garnet amphibolite unit shows P-T data from 734 (ºC) and 7 (kbar).

This study provides new geochronological data (U/Pb in zircons) for the amphibolite facies metamorphism and for the migmatitic event in the region that contributes to the understanding of the tectonic evolution of southeastern Mexico.