14 Lost Oceans, Shifting Continents: Interdisciplinary Records of Paleo-Pacific Plate Histories in the American Cordilleras

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.

Wednesday, 22 April 2026
1:30 PM - 4:00 PM
LMH, Meeting Room

Reappraisal of plagiogranites and mafic rocks of a Late Triassic upper plate ophiolite, Vizcaíno Peninsula, Baja California Sur, Mexico

Paper number: 14-3
Program: Technical Sessions
Time: 2:30 PM – 2:50 PM

Authors

Abstract

The Vizcaíno peninsula, located in the western part of the Baja California Peninsula, western Mexico, exposes a tectonically dismembered Mesozoic ophiolite interpreted as a suprasubduction-zone type. The ophiolite is formed by an ultramafic-mafic plutonic section, including gabbros intruded by plagiogranites with an inferred Late Triassic age, as well as an extrusive section formed by pillow lavas overlain by Late Triassic-Lower Jurassic sedimentary rocks. Here, we focus on the plutonic section of the ophiolite, with particular interest in the plagiogranites, which have been interpreted as evolved melts produced by fractional crystallization of gabbroic magmas and have been used to constrain the timing of the ophiolite formation. However, no geochemical or isotopic data currently support a comagmatic relationships with the mafic rocks of the ophiolite.

We present new geochemical, isotopic, and geochronological (U-Pb and Sm-Nd) data from five outcrops considered part of the ophiolite. The gabbroic rocks show an intra-oceanic arc signature in the NbN vs. ThN diagram and rare earth elements (REE) patterns comparable to cumulate rocks. Whole-rock aliquots define a Sm-Nd isochron age of 219 ± 14 Ma, interpreted as a minimum crystallization age for the plutonic section of the ophiolite. Two felsic intrusives show patterns with enrichment in the light  REEs and large-ion lithophile elements, that we consider as a clear signal of subduction origin. One of the felsic intrusion yielded a U-Pb zircon age of 180 ± 1 Ma, representing the oldest age reported for subduction-related magmatism in the Baja California Peninsula. Additionally, a basaltic andesite pillow lava from the supposedly volcanic section of the ophiolite (San Hipolito locality) yielded a U-Pb zircon age of 90 ± 1 Ma, thereby precluding an origin related to the Late Triassic ophiolite. Gabbros of the plutonic section of the ophiolite show initial epsilon Nd (εNdt) values between +8 and +5, slightly lower than typical depleted mantle values (+8). Based on these results, we proposed that the mafic plutonic rocks formed in a volcanic arc setting (volcanic-type ophiolite) rather than in an oceanic floor environment associated with subduction initiation (suprasubduction-type ophiolite). These interpretations raise questions regarding the Triassic tectonomagmatic evolution of NW Mexico and its relationship to the North American Cordillera.

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

doi: 10.1130/abs/2026CD-410782

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