Nd-Sr isotopic geochemistry and tectonics of ridge subduction and middle Cenozoic volcanism in western California

Abstract

Episodes of middle Cenozoic near-trench volcanism in California occurred during the transition from convergent to transform plate boundaries as segments of the East Pacific Rise intersected a subduction zone along western North America. Geochemical features of volcanic rocks from the Coast Range Province and Santa Maria Province, which represent two near-trench volcanic episodes, indicate that magmas from each province were derived from depleted mantle and evolved by assimilation-fractional crystallization processes to form predominantly bimodal suites. Basalt and basaltic andesite from both provinces yielded Nd (t) values between ؉9.3 and ؉2.4 and 87 Sr/ 86 Sr(t) ratios of 0.702 58-0.706 72. The observed Nd (t) values that cluster around ؉9 and the 87 Sr/ 86 Sr(t) ratios <0.7029 imply a source of depleted mantle, analogous to mid-ocean-ridge basalt (MORB) sources, for these rocks. Th/Ta and Ba/Ta ratios as low as 0.49 and 35.78, respectively, for the basalt are similar to those of MORB and also suggest a magma source from depleted mantle. Acidic rocks, including rhyolite, dacite, and trachyte samples have Nd (t) values between ؉6.3 and ؊3.2 and 87 Sr/ 86 Sr(t) ratios of 0.703 93 to 0.711 31. The variation among the Coast Range and Santa Maria Provinces volcanic rocks in Nd-Sr isotope ratio space suggests that mixing occurred between the depleted mantle-derived basaltic end-member and an incompatible-element-enriched crustal reservoir through which these rocks erupted. The observed negative correlation of Nd (t) and positive correlation of 87 Sr/ 86 Sr(t) ratios with SiO 2 , respectively, also suggest assimilation of an isotopically distinct crustal component by depleted mantle-derived melts. The ages and paleogeographic distributions of these volcanic rocks indicate that they were erupted during episodes when segments of the East Pacific Rise intersected southern California. Depleted mantle that was emplaced beneath the continental margin during ridge subduction became a source of magma for the episodes of near-trench volcanism as a new strike-slip regime evolved along the continental margin. TABLE 1. AGES OF SOME OLIGOCENE-MIOCENE VOLCANIC UNITS IN WESTERN CALIFORNIA Volcanic unit Age (Ma)* References Coast Range Province (CRP) volcanic centers Halfmoon Bay basalt HMB* Lower Miocene Stanley (1987) Pescadero Beach basalt PB* 22.0 Ϯ 0.7 Taylor (1990) Mindego basalt and related volcanic rocks M † 20.2 Ϯ 1.2 to 23.7 Ϯ 0.7 Turner (1970) Carmel basalt CM † 27.0 Ϯ 0.

Tracing the mobility of individuals using stable isotope signatures in biological tissues: “locals” and “non-locals” in an ancient case of violent death from Central California

“…Unfortunately, 87 Sr/ 86 Sr in bedrock has not been extensively mapped in California. While some regions have been studied by geologists and other scientists (Barnett-Johnson et al, 2008;Hobbs et al, 2005Hobbs et al, , 2010Ingram and Lin, 2002;Ingram and Weber, 1999), and the age of some rock formations has been determined by Sr/Rb dating (Chen and Tilton, 1991;Cole and Basu, 1995;Kistler and Peterman, 1973), a state-wide map of Sr isotope ratios does not, to the best of our knowledge, exist. Existing data suggest there is a NeS gradient in Sr isotopes across the Sierra Nevada, with 87 Sr increasing to the North due to slightly older surface geology.…”

Alkaline intrusions in a near-trench setting, Franciscan Complex, California: Constraints from geochemistry, petrology, and 40Ar/39Ar chronology

“…However, in contrast to the migrating Mendocino triple junction and associated volcanism, the setting of the Leech Lake intrusive rocks would be subduction of a fracture zone without stoppage of subduction. If subduction had ceased and thermal gradients had risen as documented in the Late Cenozoic Coast Ranges (Cole and Basu, 1995), then the facies of metamorphism would be different as noted above (Wakabayashi, 1996).…”

“…A recent example would be the migration of the Mendocino triple junction northward along the California plate margin. Asthenospheric upwelling associated with the passage of the triple junction is considered to have caused the age progressive Late Cenozoic volcanism in the California Coast Ranges (Cole and Basu, 1995). Since seamount volcanism is also frequently associated with fracture zones, this is a very likely mechanism to produce alkaline basalts.…”

“…Subduction of a spreading ridge segment could be a suitable model. For example, Cole and Basu (1995) attribute a large portion of the Miocene bimodal volcanism in coastal southern California to ridge-trench interactions during subduction of segments of the Farallon-Pacific spreading center resulting in the formation of the San Andreas fault system. Trace element and isotope data indicate that depleted suboceanic asthenosphere welled up into the opening slab window after ridge-trench collision and replaced the subcontinental lithospheric mantle.…”

Adakitic rocks associated with the Shilu copper–molybdenum deposit in the Yangchun Basin, South China, and their tectonic implications

“…The low fluorine and high chlorine of the Shilu intrusion also indicate shallow subduction, because chlorine in recycled oceanic crust has high mobility relative to fluorine in the early-stage of subduction (Lassiter et al 2002). The Neo-Tethys ridge was parallel to the subduction zone, similar to the subduction of the northeast Pacific ridge in western North America (Cole and Basu 1995). Subduction of the NeoTethys ridge formed a magmatic line along the subduction zone from the Tibet Plateau to Canton in southern China (Fig.…”