Tectonic evolution of the Guerrero terrane, western Mexico.

Abstract

The Guerrero terrane of western Mexico is characterized by an Upper Jurassic-Lower Cretaceous volcanic-sedimentary sequence of arc affinity. The arc assemblage rests unconformably on partially metamorphosed rocks of possible Triassic-Jurassic age. These "basement units," the Arteaga and Placeres Complexes and the Zacatecas Formation, are composed of deformed turbidites, basalts, volcanic-derived graywackes, and blocks of chert and limestone. Sandstones from the basement units are mostly quartzitic and have a recycled orogen-subduction complex provenance. They have negative ᵋNdi (-5 to -7), model Nd ages of 1.3 Ga., and enrichment in light REE, indicating that they were supplied from an evolved continental crust. The volcanic graywackes are derived from juvenile sources (depleted in LREE and ᵋNd = +6), though they represent a small volume of sediments. Primary sources for these turbidites might be the Grenville belt or NW South America. Basement rocks in western North America are not suitable sources because they are more isotopically evolved. Igneous rocks from the basement units are of MORB affinity (depleted LREE and ᵋNdi = +10 to +6). The Jurassic(?)-Cretaceous arc volcanic rocks have ᵋNdi (+7.9 to +3.9) and REE patterns similar to those of evolved intraoceanic island arcs. Sandstones related to the arc assemblage are predominantly volcaniclastic. These sediments have positive ᵋNdi values (+3 to +6) and REE with IAV-affinity. The Guerrero terrane seems to be characterized by two major tectonic assemblages. The Triassic-Middle Jurassic "basement assemblage" that corresponds to an ocean-floor assemblage with sediments derived from continental sources, and the Late Jurassic-Cretaceous arc assemblage formed in an oceanic island arc setting. During the Laramide orogeny the arc was placed against nuclear Mexico. Then, the polarity of the sedimentation changed from westward to eastward, and sediments derived from the arc-assemblage flooded nuclear Mexico. This process marks the "continentalization" of the Guerrero terrane, which on average represents a large addition of juvenile crust to the western North American Cordillera during Mesozoic time.