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

Petrogenesis of San Marcos dykes (Baja California, Mexico): an insight into magmatic chambers of the Peninsular Range Batholith

Paper number: 35-2
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Northern Baja California hosts significant dyke swarm outcrops, initially mapped at Rancho San Marcos. The San Marcos dyke swarm consists of dense, parallel intrusions oriented NW–SE, which reveal tectonic stress patterns and contribute to understanding the peninsula's paleogeography through paleomagnetic data. These intrusions vary compositionally from gabbro to granite and intrude both the Peninsular Batholith and pre-batholithic metasedimentary rocks. We conducted geochronological, geochemical, and isotopic analyses on samples from these dyke–host rock associations identified in Jamatay Canyon (Ensenada, Baja California). Geochemically, the dykes exhibit calc-alkaline, metaluminous, and I-type granite affinities. Normalized rare earth element (REE) patterns show fractionation of light REE relative to heavy REE. Trace element discrimination diagrams indicate their origin within a magmatic arc. Geochemical modeling supports a common origin for the dykes and rocks of the Peninsular Range Batholith from mantle-derived magmas. Furthermore, geochronological data of the granitic host-rock (Zrn U-Pb age = 126.2 ± 1.6 Ma) and the dykes (Zrn U-Pb age = 124.0 ± 1.4 Ma) suggest relatively rapid cooling (K-Ar ages between 112 and 105 Ma), followed by rheologically complex interactions (flow and fracturing) that allowed emplacement of the dykes within the granite. In terms of detrital zircon age spectra, the pre-batholithic rocks are similar to stratigraphic units in the White-Inyo Range of east-central California. Isotopically, the dykes exhibit an 87Sr/86Sr(t) compositional range from 0.703082 to 0.708305, while 143Nd/144Nd(t) ratios range from 0.512311 to 0.512822 (εNd(t) = -3.12 to +6.85; TDM[Nd] = 0.54 to 1.19 Ga). Based on these data, fractional crystallization (FC) is identified as the primary petrogenetic process of the San Marcos dykes in Jamatay Canyon. We conclude that these dykes act as feeder conduits for coeval surface volcanism associated with the Santiago Peak Formation and represent ascending magmatic pulses within the dynamics of magma chambers derived from the western North American margin during the Cretaceous.

 

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

doi: 10.1130/abs/2026CD-410866

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