PETROGRAPHY AND GEOCHEMISTRY OF ANORTHOSITES IN THE CHIAPAS MASSIF

The proposal is to formalize a new area of anorthosites in Mexico within the boundaries of the municipality of Mazapa de Madero, in the state of Chiapas, based on petrographic, geochemical investigations and geological mapping in the zone. The representative lithologies of the area studied are anorthosites, monzonites and amphibolites, which are also in tectonic contact with Permo-Triasic intrusions with the Todos Santos Formation and part of the Santa Rosa Group because of a left lateral fault that forms part of the Polochic-Motoagua Fault System. The anorthositic rocks are constituted of plagioclase (oligoclase-andesine) with rutile as an accessory mineral as well as massive ilmenite in bodies with form of lenses that reach metric dimensions. The bodies of ilmenite contain rutlie, clinochlore and apatite. The monzonites, in addition to minerals such as plagioclase and potassic feldspar, contain biotite. The amphibolites are made of hornblende and in minor proportion of interstitial plagioclase. These rocks (amphibolites) are found commonly mix with anorthosites and it is proposed that their origin is a result of the hydration of piroxenites (metagabbros?). All of the rocks studied present evidences of strong deformation and a low grade metamorphism in facies of green schists generated by hydrothermalism.