PETROGRAPHIC ANALYSIS AND METAMORPHIC FACIES OF THE ACATLAN COMPLEX

During the field trip of the Metamorphic Petrology curse, at the Earth Science Division of UNAM, rock samples were collected in order to characterize their mineralogy and metamorphic facies.

This report includes the information obtained in field, the data from the laboratory and the integration of the theoretical concepts from the class.

Rocks exposed In the Arroyo de las Vacas belong to the Xayacatlán Formation,which is one of the several phases of the polimetamorphic Acatlán complex.

The rocks are polydeformed and show at least three events of deformation.

NW-SE foliation, related to the youngest event, is the most prominent.

These rocks were formed in conditions of high pressure with retrograde metamorphism. The main lithologies are: phyllites, green schist, chloritoid schist and schist that contain metaeclogites.

Their mineralogy suggests both pelitic and igneous (basic) protoliths. Quartz segregations in both pelitic and basic rocks are folded and show some times in echelon arrangement.

The phyllites are greenish to gray in color, and their main mineral components identified under the microscope are: muscovite, quartz, chlorite and opaque minerals.

The green schist is formed by chlorite, actinolite, albite-tremolite, epidote and quartz with some altered pyrite crystals to goethite limonite. The chloritoid schist is gray color and contains abundant muscovite and quartz.

The metaeclogites are green color and show porfiroblastic textures with an aphanitic matrix. Fenoblasts are garnet with reaction crowns. The matrix has symplectitic texture formed by actinolite tremolite within albite crystals.

Garnet has been chloritized or shows intergrowths with sphene. In some parts remnants of omphacite can be observed. Both textures and mineral assemblages suggest that the rocks reached eclogite facies during the first event of metamorphism. Then, during a second event, the rocks suffer retrogression to green schist facies.