GEOCHEMISTRY AND GEOCHRONOLOGY OF ISLA MARIA MADRE IGNEOUS ROCKS: PALEOTECTONIC INTERPRETATIONS

Islas Marias Archipelago is located 110 Km NW of San Blas, Nayarit (21°51’N/21°48’N; 106°80’W/106°40’W) in the mouth of the Gulf of California. The archipelago is formed by San Juanito, Maria Madre, Maria Magdalena, and Maria Cleofas islands and its location represents a key point for paleogeographic reconstructions of NW Mexico and the tectonic evolution of Baja California Peninsula. Maria Madre is the largest island and covers an area of 144 Km². We carried out geological mapping and identified the lithological diversity. Oldest rocks are exposed as a plutonic-metamorphic assemblage in the western part of the island, named by us “Papelillo Complex” and include folded and banded orthogneisses of intermediate compositions (66.6–75 wt. % SiO₂) and metasedimentary units as well as garnet-bearing calc-silicates in close association to intermediate-silicic plutonic rocks (56.24-71.73 wt. % SiO₂). We performed U-Pb zircon geochronology in two orthogneiss samples from the west coast yielding ages of 161.6+0.7 Ma (2σ) and 169.7+0.7 Ma (2σ). For a granite from the SW coast and a dioritic body from west coast crystallization ages of 84.16±1.8 Ma (1σ) and 80.08 +0.79/-0.36 Ma (1σ), respectively, were obtained. The crystalline basement is partly covered by volcanic units distributed in the central and northern part of the island. These rocks were observed as strained, fractured, and often hydrothermally altered lava flows, pyroclastic deposits, massive unaltered lava flows, and some hypabyssal bodies. Compositions of the volcanic units range from andesite to rhyolite (52.6-76.2% SiO₂). Most rocks plot in the sub-alkalic field and show quite similar REE patterns consistent with subduction related environments. Geological, geochemical, and geochronological data of Maria Madre igneous rocks suggest a common magmatic evolution and basement history with rocks of the Los Cabos Block in southern Baja California and the Mexican continental margin between Sinaloa and Jalisco. Maria Madre Island forms part of the southward continuation of the Peninsular Ranges batholithic suite that extends from southern California to the Jalisco Block.