THE REAL GUERRERO TERRANE, SOUTHERN MEXICO: NEW INSIGHTS FROM RECENT STUDIES

In the southernmost part of the Cordilleran region of North America, the composite Guerrero terrane (GT) in western Mexico has been currently defined as an Upper Jurassic-Lower Cretaceous intraoceanic volcanic arc complex accreted onto the continental margin of Mexico during the Late Cretaceous. However, new data indicate that the tectonic evolution of this part of the Cordilleran region is more intricate that previously thought. In the southeastern GT three lithotectonic units has been recognized: 1) a strongly evolved, probably pre-Mesozoic high-grade metamorphic crust (eNd ₍₀₎=- 6.4, T_DM=1.36 Ga, and ⁸⁷Sr/⁸⁶Sr₍₀₎=0.7203) beneath Mesozoic arc sequences (xenolith evidences); 2) an intensely deformed and metamorphosed Triassic-Lower Jurassic volcanogenic assemblage (Tejupilco Schist) of calc-alkaline arc affinity, and syntectonic peraluminous granites (186.5 ± 7.4 Ma U-Pb zircon age, eNd _(i)=- 3.5, T_DM=1.27 Ga, and ⁸⁷Sr/⁸⁶Sr_(i)=0.7078); and 3) a mildly deformed Cretaceous submarine volcanogenic sequence (Arcélia-Palmar Chico Group), with small ultramafic bodies and within-plate basalt or E-type MORB signatures. Although the Arcélia-Palmar Chico Group is thrust eastward over the Tejupilco Schist, a former angular unconformity is indicated by clasts of this basement in the overlying back-arc basin sequence.

Accordingly, we envisage an offshore east-facing Triassic-Lower Jurassic arc (Esquisto Tejupilco) with old sialic roots, colliding by Toarcian time against the continental margin accompanied by penetrative ductile deformation, regional metamorphism and magmatism during the here defined Náhuatl orogeny. The Triassic-Lower Jurassic Arteaga complex in the southern GT, is inferred to be the corresponding back-arc basin. The Cretaceous arc assemblages unconformably overlying to pre-Upper Jurassic basement units are therefore post-collision sequences of a west-facing continental arc-back arc basin that were folded and faulted in Late Cretaceous time.