35 The Mesozoic Convergent Margin of Alta and Baja California (Posters)

This session will cover aspects including the structure, petrology, geochronology, and stratigraphy of the Mesozoic convergent margin of North America from Alta California to Baja California Sur.

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

Petrology of the Early Cretaceous San Julián batholith: implications for the evolution of the southern Peninsular Ranges Batholith, Baja California

Paper number: 35-5
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The Peninsular Ranges batholith (PRb) is a belt of Cretaceous plutons ranging from ca. 132 to 80 Ma that crops out continuously from Riverside, California (~34°N), to the Sierra El Arco (~28°N), Baja California. This geological province exhibits a distinctive petrologic zonation parallel to the axis of the peninsula, reflecting changes in the crustal composition and defining a western zone associated with an oceanic arc, and an eastern zone, akin to a continental arc. The western side of the PRb is related to the Jurassic-Cretaceous Alisitos oceanic arc, with exposure levels varying latitudinally from upper- to middle-crustal levels in the El Rosario area (~30°N) to middle-crustal levels in the southern PRb (~29°N). The western zone of the PRb is also characterized by magnetic highs, attributed primarily to the magnetite-bearing mafic plutons.  The San Julián batholith was emplaced near the juxtaposition zone between the Mid-Cretaceous Alisitos volcanic and related sedimentary intra-arc rocks and the volcanic, volcano-sedimentary and plutonic oceanic-arc assemblages of the Jurassic-Early Cretaceous Nuevo Rosarito-El Arco segment. This batholith correlates with a magnetic high (> 400 nT) that spans over 1000 km2. The exposed portion of the batholith covers 260 km2 and is composed of calcic, metaluminous, tholeiitic to calc-alkaline gabbroic rocks. The mafic units are intruded by mylonitized, calc-alkaline, granodioritic to granitic dikes and surrounded by mylonitized granitic rocks. The felsic units yield U-Pb in zircon ages ranging from 138 to 132 Ma.  Neodymium isotope compositions show that the gabbroic rocks originated from juvenile melts (εNdt > +5), whereas the felsic units record minor contamination from continentally derived material (εNdt > +1.8). Overall, the results indicate that the juvenile Alisitos oceanic arc evolved into a mature arc, with continental signatures by ca. 132 Ma, and that the Jurassic-Early Cretaceous Nuevo Rosarito-El Arco segment represents the basement of the Alisitos arc. The deformed felsic units of the San Julián batholith were mylonitized by a contractional event during its emplacement, possibly associated with the closure of a back arc basin or marginal sea that had previously separated the oceanic arc from the continental margin. 

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

doi: 10.1130/abs/2026CD-411006

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