Chemically diverse, sporadic volcanism at seamounts offshore southern and Baja California

Compositions of lavas from seven small to medium-sized seamounts, between lat 34.0N25&N, are all highly differentiated trachyte and trachyandesite. Based on 40 Ar/ 39 Ar laser fusion techniques, MORB-like lava from one of the northernedificesisasoldastheunderlying oceanic crust (>20 Ma), indicating that it originatedataspreadingcenter.Otherseamountlavaagesaremuchyoungerthanthe oceanic crust on which they reside, ranging from 16.8 6 0.3 to <7 Ma for some of the northern seamounts to 270 616 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 orfaultsandfracturezoneswhichareabundant in the tectonically complex region offshore southern and peninsular California, may have resulted from upwelling mantle diapirs in response to localized extension. Someoftheepisodesofvolcanismappearto havebeencontemporaneouswithvolcanism 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, tappingvariablyenriched,heterogeneousmantle, which occurred sporadically from early Miocene to late Pleistocene.

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