Compositions of lavas from seven small to medium-sized seamounts, between lat 34.0°N and 30.5°N offshore southern and Baja California, include low-K2O tholeiitic, transitional, and mildly to moderately alkalic basalt and their differentiates. The low-K2O tholeiites resemble primitive (>9% MgO) mid-oceanic-ridge basalt (MORB) with low incompatible element abundances and very depleted, concave-downward, chondrite-normalized rare-earth-element (REE) patterns and lower 87Sr/86Sr and higher 143Nd/144Nd ratios than typical MORB from the East Pacific Rise. The seamounts with these MORB-like lavas are inferred to have formed at or near the spreading center.

Transitional and mildly to moderately alkalic basalts have higher abundances of incompatible elements and steeper slopes for chondrite-normalized REE patterns with light REE enrichment up to 150 times chondrites. The alkalic compositions indicate more variably enriched mantle sources than those of most seamounts presently located near the East Pacific Rise, but the compositions are within the mantle array defined by other ocean-island basalts. Volcanic rocks from the upper part of Rocas Alijos, a much larger and morphologically more complex edifice than the northern seamounts, located offshore central Baja California at lat ∼25°N, are all highly differentiated trachyte and trachyandesite.

Based on 40Ar/39Ar laser fusion techniques, MORB-like lava from one of the northern edifices is as old as the underlying oceanic crust (>20 Ma), indicating that it originated at a spreading center. Other seamount lava ages are much younger than the oceanic crust on which they reside, ranging from 16.8 ± 0.3 to <7 Ma for some of the northern seamounts to 270 ± 16 ka for the trachyte from Rocas Alijos. Similar highly evolved lavas cap fossil spreading centers like Guadalupe and Socorro Islands, but Rocas Alijos, based on magnetic anomalies, is not an abandoned spreading center but may instead have formed on a leaky transform fault.

Some of the seamounts with transitional and alkalic lavas may have formed as part of a short, age-progressive chain formed by a short-lived mantle plume. Many others, aligned along abandoned spreading centers or faults and fracture zones which are abundant in the tectonically complex region offshore southern and peninsular California, may have resulted from upwelling mantle diapirs in response to localized extension. Some of the episodes of volcanism appear to have been contemporaneous with volcanism in the continental borderland and coastal southern California, suggesting linkage between extension along the continental margin and the seamount province farther offshore. The data available for the abundant volcanic edifices of varying sizes, shapes, and orientations in this region suggest that the seamounts formed from multiple episodes of chemically diverse volcanism, tapping variably enriched, heterogeneous mantle, which occurred sporadically from early Miocene to late Pleistocene.

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