Geology, geochronology, and geochemistry of Isla María Madre, Nayarit, Mexico
Islas Marías archipelago is located 110 km NW of San Blas, Nayarit in the mouth of the Gulf of California. The archipelago is formed by San Juanito, María Madre, María Magdalena, and María Cleofas islands. The position of these islands represents a key point for paleogeographic and paleotectonic reconstructions of northwestern Mexico and of the tectonic evolution of Baja California Peninsula. María Madre is the largest island and covers an area of 145 km2. This study presents the first detailed geological map of the island together with geochemical and geochronological data of its lithological units. Isolated basement rocks are exposed along the western coast in the form of migmatites and orthogneisses of granodioritic to granitic compositions and middle Jurassic ages (163 – 170 Ma). In the west-central part, a metasedimentary sequence, with biotite ± garnet paragneisses and folded calcsilicate rocks with or without garnet bands of unknown ages, is exposed as a roof pendant of the underlying Cretaceous intrusions (80.8 – 83.4 Ma) of tonalitic to granitic compositions. These granitoids are cut by mafic and pegmatitic dikes. We named the overall assemblage of metamorphic and plutonic rocks as “Papelillo Complex” due to the predominant outcrops in the homonymous canyon. The Papelillo Complex is overlain by ignimbrites, volcanic breccias, and lava flows of rhyolitic compositions. 40Ar/39Ar geochronology in sanidines from these rocks yielded Cretaceous (71.6 – 80.6 Ma) and Tertiary (55.4 Ma) ages, suggesting contemporaneous volcanic and plutonic activity. This inference is confirmed by similar REE patterns in both units with typical distributions for a subduction-related magmatic arc environment. Towards the east, the igneous sequence is covered by Late Miocene to Pleistocene? marine and shallow marine deposits and towards the south by an Early Miocene? clastic continental sequence (informally named Isla Magdalena sandstone). Marine and shallow marine sediments were informally named “Ojo de Buey sequence” and subdivided into a lower and an upper member. Detrital zircon ages from both units display major peaks at ca. 83 Ma, indicating sedimentation mainly from plutonic and volcanic rocks of the same age. In the Ojo de Buey sequence, no zircons younger than Late Cretaceous were found. The Isla Magdalena sandstone, instead, shows a prominent peak at ~22 Ma, suggesting detritus from a different igneous source. The lithologies of Isla María Madre are similar to those from the Los Cabos Block in Baja California Sur and from central Sinaloa. The overall area forms part of a Cretaceous plutonic belt, which definitely rules out large latitudinal displacements for the southern Baja California Peninsula.