Davidson, Guide, Pioneer, Gumdrop, and Rodriguez Seamounts, located at the continental margin offshore California between latitudes 37.5°N and 34.0°N, may represent a previously unrecognized type of intraplate oceanic volcanism. Morphologically unlike typical oceanic-island volcanoes or near-ridge seamounts, they are complex, northeast-trending ridges that reflect the ridge-parallel structure of the underlying oceanic crust. 40Ar/39Ar laser fusion ages of mineral separates indicate at least two episodes of volcanism at ca. 16 and ca. 12 Ma, younger by 7–11 m.y. than the underlying ocean crust. Volcanic rocks are predominantly differentiated alkalic basalt, hawaiite, and mugearite. The lack of coherent liquid- lines-of-descent is consistent with small batches of magma forming, fractionally crystallizing, and erupting in isolation from the previous and subsequent batches. The presence of mantle xenoliths suggests that magmas originated in the upper mantle. Xenoliths of alkalic cumulates and xenocrysts of feldspar, amphibole, and titanomagnetite are consistent with fractionation in the upper mantle as well. Sr, Nd, and Pb isotope ratios of some samples are from depleted-mantle sources like those of MORB (mid-oceanic-ridge basalt); others indicate more variably enriched mantle sources. Magmas formed by small- percentage partial melting of variably enriched MORB-source-type mantle, according to the samples' high abundances of incompatible elements such as Nb, Ta, and Th. Coeval middle Miocene basalts in onshore coastal California have a similar isotopic range, but their trace elements show a subduction-related signature. If the coeval volcanic rocks offshore originated in a slab window, as proposed for those onshore, they were not influenced by a slab component. Although a slab-window origin is plausible for the middle Miocene volcanism, later episodes of small, sporadic eruptions on- and offshore probably resulted from decompression melting of mantle rising along existing zones of weakness undergoing extension related to continued movement along transform-fault systems.

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