37 Cordilleran Margin Evolution and Supercontinent Tectonics: From Pangea Assembly to North American Orogenesis (Posters)

This session integrates tectonics, petrology, geochronology, stratigraphy, and paleontology to illuminate how Paleo-Pacific plates drove subduction dynamics, crustal evolution, and paleogeographic reconstructions across the complex geological architecture of the American Cordilleras.

Friday, 24 April 2026
8:00 AM - 5:30 PM
LMH, 5th Floor Chapel

Tracing lost Paleo-Pacific lithosphere in Mexico: new insights from ultramafic and mafic rocks from the Vizcaíno Peninsula ophiolite (Baja California Sur, Mexico)

Paper number: 37-1
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

In the northern part of the Vizcaíno Peninsula (Baja California Sur, NW Mexico), outcrops one of the most well-exposed and extensive (ca. 32 km long) suprasubduction-type Mesozoic ophiolitic sequences in the country. This sequence comprises three main tectonically dismembered members, from base to top: i) serpentinized ultramafic rocks, including dunites, harzburgites, lherzolites, and chromitites; ii) intrusive mafic units, composed of gabbronorite, olivine gabbro, norite, and plagiogranite with banded, phaneritic, cumulate, and pegmatitic textures; and iii) extrusive igneous complex with pillow lavas. Here, we use whole-rock geochemical and isotopic data of serpentinized ultramafic and mafic (gabbroic) rocks from the North Vizcaíno ophiolite for the first time, aiming to decipher their petrogenetic connection and geotectonic context within the Mesozoic evolution of the western margin of North America.

Geochemical signatures of both serpentinized ultramafic and intrusive mafic (gabbroic) rocks suggest an abyssal (intra-oceanic) to fore-arc geotectonic setting, consistent with the compositional trend of mantle-derived fractionation: i) a decrease in Al₂O₃/SiO₂ as MgO/SiO₂ increases in serpentinized ultramafic rocks, and ii) a depletion in NbN as ThN decreases in intrusive mafic (gabbroic) rocks. Variations in the geotectonic setting are further reflected in chondrite-normalized REE diagrams, where serpentinized ultramafic rocks display i) U-shaped concave patterns, akin to fore-arc peridotites, and ii) relatively flat patterns slightly depleted in light REE, comparable to average depleted MORB mantle (DMM). The latter REE signature is also observed in intrusive mafic (gabbroic) rocks, suggesting that they might be derived from DMM-like melts. Fractional crystallization modeling based on REE concentrations of the most primitive gabbro (Mg# = 73) from the igneous mafic units suggests a comagmatic origin consistent with progressive fractional crystallization. This is in agreement with the 147Sm/144Nd and εSrt vs. εNdt diagrams, where mafic (gabbroic) rocks plot near the depleted mantle (DM) following the fractional crystallization of the mantle array.

Overall, our new observations suggest that ultramafic and mafic components of the Vizcaíno Norte ophiolite formed within a complex suprasubduction-zone geodynamic setting, involving an intra-oceanic arc developed in an oceanic plate and/or fore-arc zone, similar to the upper Coast Range plate ophiolite located north of the Vizcaino Peninsula.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3

doi: 10.1130/abs/2026CD-410791

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