PETROGRAPHY OF THE TULTILAN AMPHIBOLITE, ACATLÁN COMPLEX, SOUTHERN MEXICO: MAFIC INTRUSION IN A LOWER PALEOZOIC ACCRETIONARY COMPLEX?

The Acatlán complex of southern Mexico's Mixteca terrane includes a polydeformed sequence of metasedimentary siliciclastics, the Lower Paleozoic portions of which (Chazumba and Cosoltepec formations) have been interpreted as an accretionary complex developed on an active Gondwanan margin of either the Iapetus or Rheic oceans. This interpretation is based, in part, on the existence within the metasedimentary units of mafic-ultramafic complexes thought to be of ophiolitic origin. The Tultitlan Amphibolite, which represents one of these complexes, is a tabular, NE-trending body entirely enclosed within staurolite-bearing pelitic schists of the Chazumba Fm. The massive amphibolite, which is concordant with the steep main foliation of its host, is intruded by a small metagabbroic body and numerous meta-appinite dikes. The mineralogy indicates a noritic protolith overprinted by at least two phases of metamorphism. The first was an amphibolite facies event that produced cummingtonite-oligoclase at the expense of cumulate bronzite-andesine. The same event in the Chazumba Fm. produced post-kinematic staurolite. The second metamorphism was a retrograde upper greenschist-lower amphibolite facies event, producing talc-tremolite-chlorite at the expense of cummingtonite-oligoclase in the amphibolite, and minor chlorite in the Chazumba Fm. The body is interpreted to have crystallized from a crustally contaminated basaltic magma that underwent differentiation by crystal settling of bronzite to form a norite cumulate. Portions of the remaining melt crystallized to form noncumulate norite with late hydrous differentiates producing veins and dikes of appinite, possibly containing primary cummingtonite. Emplacement of the mafic body was an early event, predating the first metamorphism of the Chazumba Fm that accompanied or postdated development of its principle foliation. It is considered likely that the mafic magma originally intruded the Chazumba Fm, the alumina-rich sediments of which could have provided the necessary contamination. A noritic protolith argues against an ophiolitic origin for the Tultitlan Amphibolite but does not preclude deposition of the Chazumba Fm within an accretionary prism.