THE MIDDLE JURASSIC MAGDALENA MIGMATITE OF THE AYÚ COMPLEX, SOUTHERN MEXICO: A CLASSICAL EXAMPLE OF ADVANCED PARTIAL MELTING OF THE PRE-MESOZOIC MIDDLE CRUST OF SOUTHERN MEXICO

The Magdalena Migmatite (MM) constitutes the structurally deepest unit of the Acatlán Complex and it is considered the partially melted equivalent of the overlying siliciclastic Chazumba lithodeme (CL) separated from the Paleozoic Acatlán Complex by a composite normal/thrust shear zone of Mesozoic age. Together these units define the Ayú Complex characterized by a Middle Jurassic high-grade metamorphism (171 Ma) and their Triassic-Jurassic protoliths (190-171 Ma). However, the MM, in contrast to the purely siliceous CL, shows abundant calcic silicates (Ca-pyroxene-amphiboles-titanite) and some carbonates as the dominant assemblages.

A detailed field and petrographic study conducted in the total area of exposure (8x5 km) between Ayú and Magdalena in the state of Oaxaca, permitted us to establish fundamental aspects of the MM petrogenesis and to propose the unit as a classical example for the study of partial melting occurring in the middle crust in the presence of sub saturated aqueous fluid conditions.

The process of partial melting took place at peak temperatures estimated at 800 ºC on the basis of the break down reaction of clinochlore to enstatite-forsterite-spinel and Na-rich neosomes. On the other hand the absence of high pressure phases such as kyanite and garnet or the low pressure andalucite and cordierite in the accompanying metapelites allowed pressure estimates in the range 5- kbar for the anatectic event. A singular aspect of the process observed in the field is the abundance of syntectonic to postectonic S-type granites characterized by Al-rich phases such as muscovite, garnet and some sillimanite. Some of these bodies contain orthopyroxene, indicating an origin at lower levels in the crust and melting under granulite facies.

The origin of the MM and the heat of metamorphism are attributed to a Jurassic plume, or to the emplacement of a metamorphic core complex, but none of these models have been sufficiently supported, except that the accompanying tectonic regime occurred under extensional conditions and the cooling process was rather fast (21-100 ºC/Ma).

The coexistence in time and place of a gabbroic-ultramafic layered body in the upper part of the migmatitic structure is considered an important advective heat source for the high degree of melting that affected considerable parts of the local lower and middle crust.