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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.
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.
The Lospe Formation is an 830-m-thick sequence of sedimentary and minor volcanic rocks at the base of the onshore Neogene Santa Maria basin of central California. Eighteen outcrop samples (14 from lacustrine and shallow-marine mudstones of the Lospe Formation, and 4 from bathyal marine shales of the overlying Point Sal Formation) were collected from a measured stratigraphic section at North Beach (informal name) near Point Sal, and analyzed using Rock-Eval pyrolysis and vitrinite reflectance. The Rock-Eval data indicate that mudstones of the Lospe are low in organic carbon (range 0.18 to 0.80 weight percent, mean about 0.35 percent), and therefore are generally poor potential source rocks of petroleum. In contrast, shales of the Point Sal Formation exhibit much higher total organic carbon (range 1.47 to 3.63 weight percent, mean about 2.4 percent), and therefore are good to very good potential source rocks. These results should be regarded as preliminary because only a small number of samples were analyzed, and because interpretation of the data is complicated by weathering effects, relatively high thermal maturity, and evidence of migrated bitumen in some samples.Vitrinite reflectance values (range 0.68 to 1.56 percent RQ, mean 1.29 percent RO) and calculated maximum burial temperatures (range 106 °C to 192 °C, mean 172 °C) of the Lospe and Point Sal Formations in the North Beach section are the highest ever reported for Neogene rocks of the onshore Santa Maria basin. These high values can be explained by a combination of burial heating plus a local heat source such as a nearby gabbro sill and (or) a high-temperature hydrothermal system. The local heating event is poorly dated but probably late early or early middle Miocene, and may have stimulated thermal generation of oil and gas from organic-rich strata of the Point Sal Formation.1 Above base of Lospe Formation in the measured stratigraphic section at North Beach. Numerous normal(?) faults of unknown displacement probably have removed parts of the section (Johnson and Stanley, 1994).
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