8 Cenozoic Cordilleran Tectonics, Magmatism, and Crustal Evolution: Perspectives from Mexico and Western North America (Posters)

Tectonic reconstructions of the southwestern North American plate boundary provide an excellent framework for understanding Cenozoic volcanism (arc, rift, slab window, seafloor) in this largely extensional and transtensional setting. Geologic, geochemical and geophysical submissions encouraged.

Wednesday, 22 April 2026
8:00 AM - 5:30 PM
LMH, 5th Floor Chapel

Basaltic Andesite Petrogenesis at the Juan de Fuca Ridge-Cobb Hotspot Intersection

Paper number: 8-3
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Axial Seamount is the youngest volcano in the Cobb hotspot chain and is located on the Juan de Fuca Ridge (JdFR) spreading center in the northeast Pacific Ocean. Axial has a large summit caldera and two ~50-km-long rift zones that extend to the northeast and southwest and accommodate plate spreading. These rift zones are mostly fed by dikes sourced from magma storage bodies beneath the caldera. Axial’s North Rift Zone (NRZ) has a narrow ridge located east of the active spreading axis that rises to ~200 m above the surrounding seafloor and is composed of a series of elongate inflated and hummocky flows.  Although lavas from mid-ocean ridges and oceanic hotspots are typically tholeiitic basalts, 30 samples collected by shipboard wax coring and by the ROV Doc Ricketts from this ridge include unusually high-silica compositions, with whole-rock and glass SiO₂ contents reaching 56.8 wt% and 60.0 wt%, respectively. Similar compositions are rare in mid-ocean-ridge settings and are more commonly associated with regions of restricted magma supply and cold crust, such as overlapping spreading centers. In contrast, the Axial basaltic andesites occur in an area of high magma flux supplied by the Cobb hotspot. Previous geochemical studies indicate that Axial lavas are hybrids produced by variable mixing between subtly enriched Cobb hotspot magmas and more depleted JdFR MORB. The Cobb hotspot has been characterized as HiMu-type, with relatively radiogenic Pb and lower Nd and Hf isotope ratios than MORB. In this study, Sr–Nd–Pb–Hf isotopic compositions were determined for 16 of the high-silica samples, and Magma Chamber Simulator modeling evaluated their petrogenesis. Sr and Hf isotopic ratios show little correlation with major or trace element variations.  Pb and Nd isotopes display weak trends with increasing SiO₂ that possibly reveal the influence of the HiMu component, with Pb ratios decreasing and Nd ratios increasing, although most values overlap within 2σ analytical uncertainty. The elevated silica contents imply prolonged magma storage and extensive fractional crystallization prior to mixing with a more primitive component. The compositional variability along the ridge further suggests that this hybrid magma was poorly-mixed before being laterally injected along the NRZ and erupted.

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

doi: 10.1130/abs/2026CD-411036

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