39 The Mesozoic Convergent Margin of Alta and Baja California

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
1:30 PM - 5:30 PM
LMH, Deli

Late Triassic Paleo-Oceanic Basement Framework of the Peninsular Ranges Batholith and Guerrero Terrane; Evidence from the El Arco-Calmallí District

Paper number: 39-4
Program: Technical Sessions
Time: 2:20 PM – 2:35 PM

Authors

Abstract

It has long been recognized that the western Peninsular Ranges batholith was constructed almost entirely outboard of cratonal North America based on tracer isotope patterns of batholithic rocks comparable to those of intra-oceanic island arcs. Exposure of deep prebatholithic basement is limited due to the shallow level of exposure with extensive tracks of supracrustal volcanic-sedimentary sequences that constitute ~50% or more of the total outcrop. However, at the southern end of the batholith in the El Arco-Calmalli district, deeper basement crops out beneath Alisitos cover. Here we report a Late Triassic zircon U-Pb crystallization age for high level gabbro associated with the 15 km2 Calmalli ophiolite, which in turn is intruded and overlain by Middle Jurassic arc magmatic rocks that include the El Arco porphyry copper deposit. The El Arco-Calmalli geology correlates with Triassic-Jurassic geology of the Vizcaino-Cedros region. The implication is that Vizcaino ophiolite basement extends to the east beneath the Vizcaino desert and comprises at least local basement to the Guerrero composite terrane that covers a major part of western Mexico and Baja California. The Vizcaino and Calmalli ophiolite remnants are both ~3 km thick with fault-bounded psuedostratigraphic mantle, crust and pillow lava components. Zircon U-Pb ages are in good agreement (215.0 ±2.7 Ma for Vizcaino plagiogranite and 215.7 ± 2.2 Ma from Calmalli high level gabbro) with similar enriched €Hf(i) zircon ratios. Lavas in both areas show arc-tholeiite compositions typical of suprasubduction zone ophiolites. Surrounding Middle Jurassic arc volcanics with depleted mantle Sr-Nd hosting the 165 Ma El Arco porphyry Cu (Weber and Lopez, 2006) are interpreted as cover rock to Calmailli ophiolite. This cover bears similarity to Middle Jurassic arc volcanic cover rocks in the Vizcaino-Cedros region.  The Calmalli remnant is atypical of most western Cordilleran Mesozoic ophiolites with a thick section of serpentinized dunite-wehlite at the base we interpret here as mantle transition zone. The MTZ is overlain by a gabbroic sill complex composed of cumulate gabbro and pyroxenite.  Harburgite-dunite tectonite is unrecognized. Detrital zircon ages suggest that Guerrero terrane and Vizcaino-Cedros basement have been in proximity to North America craton throughout their history.  

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

doi: 10.1130/abs/2026CD-411001

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