JURASSIC AMPHIBOLITES OF THE EASTERN ACATLAN COMPLEX (SOUTHERN MEXICO) RELATED TO BOTH BACK-ARC RIFTING AND THE OPENING OF THE GULF OF MEXICO?

The eastern Acatlán Complex in southern Mexico offers the unique opportunity to investigate Early Mesozoic post-collisional tectonics associated with subduction along the periphery of Pangea.

The here studied metasedimentary sequence of the previously mapped Chazumba formation comprises metapelites and metapsammites with interlayered boudinaged amphibolitic rocks and 174±1 mafic to ultramafic tectonic slivers, all of which are cut by ~170 Ma granitic dikes. To the south, the sequence becomes increasingly affected by partial melting and grade into the ~171 Ma Magdalena migmatite (MgdM). Detrital zircon data reveal synchronous deposition of both the ChzF and the paleosome of the MgdM, identifying the migmatite’s protolith as migmatized ChzF, and necessitate nomenclatural revision by merging ChzF and MgdM to the Ayú complex (AC). The maximum depositional age of the AC is provided by the youngest detrital zircon of 214±2 Ma; an upper limit is given by the ~170 Ma San Miguel granitic intrusions and constrains the protolith age of the amphibolites to Upper Triassic–Middle Jurassic. To the north, the AC is tectonically juxtaposed against the 292±8 Ma Tecomate formation along a major shear zone also comprising intercalated slivers of amphibolites.

Preliminary major and trace element geochemistry of the amphibolitic rocks in the AC as well as the slivers in the northern shear zone indicate that most amphibolites are of igneous origin, and display characteristics typical of mid-ocean ridge (MORB) tholeiitic basalts with flat to slightly enriched light REE and flat heavy REE patterns. Some amphibolites are alkalic in composition and show fractionated REE patterns with (La/Yb)n ranging from 4.8 to 17.4. HFS element ratios (Cr/Y, Ti/Y, Zr/Y) of both suites vary widely and are interpreted to reflect heterogeneities in the mantle source indicative of the interaction of different tectonic processes producing melt in both the shallow and deep mantle. Taken together, the amphibolitic rocks are interpreted to represent transitional MORB tholeiites produced in the back-arc region during coeval arc magmatism of the Paleo-Pacific. This Jurassic event is synchronous with the initial rift stages of the opening of the Gulf of Mexico which may have promoted the formation of deeper mantle melt of within plate affinities.