Magmatic activity at Islas Marias Archipelago, Gulf of California: Oceanic lithosphere with gabbroic sills versus Jurassic-Cretaceous arc components.

Abstract:

Islas Marias Archipelago consists of four islands located in the mouth of the Gulf of California. Lithologically three of them (Maria Madre, San Juanito, and Maria Cleofas) are quite similar with a 165-170 Ma metamorphic basement, 75-85 Ma intrusive and extrusive rocks, and a sedimentary sandstone cover, which according to its foraminiferous content recorded multiple uplift and subsidence events related to the opening of the Gulf. However, these units are absent on Maria Magdalena island which is positioned between the other islands. Here, instead, oceanic lithosphere with pillow lavas and gabbroic sills, intercalated with sandstones form the dominant outcrops. Their geochemical and isotopic characteristics are similar to N-MORB with epsilon Nd values around +10 and 87Sr/86Sr of 0.70290. The gabbros are not older than 22 Ma. Magdalena island was obviously uplifted separately from the other islands of the archipelago, probably along a now hidden transform fault system along the East Pacific Rise. Metamorphic and igneous rocks of the other islands can be correlated to lithologically similar units in the Los Cabos Block, Baja California, or to the continental margin units in Sinaloa, Nayarit and Jalisco states when looking at their geochemical and geochronological signatures. Paleomagnetic studies on 35 sampling sites from all 4 islands give evidence for relatively small scale tectonic movements.