Origin of volcanic seamounts at the continental margin of California related to changes in plate margins

Abstract

[1] Volcanic samples collected with the Monterey Bay Aquarium Research Institute's ROV Tiburon from eight seamounts at the continental margin offshore central to southern California comprise a diverse suite of mainly alkalic basalt to trachyte but also include rare tholeiitic basalt and basanite. All samples experienced complex crystal fractionation probably near the crust/mantle boundary, based on the presence in some of mantle xenoliths. Incompatible trace elements, poorly correlated with isotopic compositions, suggest variable degrees of partial melting of compositionally heterogeneous mantle sources, ranging from MORB-like to relatively enriched OIB. High-precision 40 Ar/ 39 Ar ages indicate episodes of volcanic activity mainly from 16 to 7 Ma but document one eruption as recent as 2.8 Ma at San Juan Seamount. Synchronous episodes of volcanism occurred at geographically widely separated locations offshore and within the continental borderland. Collectively, the samples from these seamounts have age ranges and chemical compositions similar to those from Davidson Seamount, identified as being located atop an abandoned spreading center. These seamounts appear to have a common origin ultimately related to abandonment and partial subduction of spreading center segments when the plate boundary changed from subduction-dominated to a transform margin. They differ in composition, age, and origin from other more widespread near-ridge seamounts, which commonly have circular plans with nested calderas, and from age progressive volcanoes in linear arrays, such as the Fieberling-Guadalupe chain, that occur in the same region. Each volcanic episode represents decompression melting of discrete enriched material in the suboceanic mantle with melts rising along zones of weakness in the oceanic crust fabric. The process may be aided by transtensional tectonics related to continued faulting along the continental margin.

Petrogenesis of Davidson Seamount lavas and its implications for fossil spreading center and intraplate magmatism in the eastern Pacific

“…The samples analyzed for Sr isotopes were subjected to a harsh HCl‐leaching procedure to remove seawater alteration effects, and thus, alteration is also not a likely cause of the spread in Sr isotope values. Davis et al [2010] observed variable Sr isotope ratios from other southern California seamount lavas as well and thus the spread of 87 Sr/ 86 Sr values is most likely a source feature of Davidson seamount lavas. Both highly incompatible trace element and Sr, Nd and Pb isotope ratios are not affected by normal partial melting and fractional crystallization processes and thus these ratios suggest that both alkalic and transitional lavas came from a common, though moderately heterogeneous mantle source or sources.…”

“…This combined tectonic and mantle‐melting process produces voluminous mid‐ocean ridge basalt (MORB) with a fairly uniform major element composition that is distinct from magmatism at subduction zones and intraplate hot spots. The majority of fossil spreading centers in the eastern Pacific (Figure 1) are occupied by volcanic ridges that were built after the cessation of seafloor spreading activities [e.g., Batiza et al , 1982; Batiza and Vanko , 1985; Lonsdale and Castillo , 1998; Davis et al , 1995, 2002; Clague et al , 2009; Davis et al , 2010]. The composition of these ridges is distinct from that of normal MORB.…”

Underplated seamount in the Narooma accretionary complex, NSW, Australia

“…Fig. 12. [Ce/Yb] N -Ce plot of Davis et al (2010). Note that samples show varying degrees of partial melting and that only one (P920A) is a product of crystal fractionation.…”

Mantle updrafts and mechanisms of oceanic volcanism

“…Further out on the Pacific plate, distinctive magmatic and isotope lineages develop on oceanic islands that are derived from low-SiO 2 mafic parents. Magma from depths of 100 km or more produces rocks with even lower SiO 2 (62)(63)(64). With great distance and lithospheric age, basalts from oceanic islands are even more enriched and diverse (48,49), indications of still lower fractions of partial melting.…”