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Masters Thesis

The geochemistry and petrogenesis of the Conejo volcanics, Santa Monica Mountains, California

Samples of middle Miocene Conejo Volcanics were collected from all of the main stratigraphic units within the western Santa Monica Mountains, Los Angeles and Ventura Counties, California and analyzed for major oxides, trace elements, and isotopic ratios. This study confirmed observations of earlier studies that showed that Conejo lavas range in composition from basalt to dacite, belong to the calc-alkaline magma series, and are characterized by low K20 content. The lavas have a K57.s value of 0.55 and a Peacock index of 64. New results show that basaltic andesite, andesite, and dacite rocks occur throughout the volcanic sequence, implying that primitive magma was repeatedly supplied throughout the eruptive history. Rare-earth element (REE) patterns are smooth and gentle; the light REE are moderately enriched (LaN = 4.0 to 9.3) and are slightly fractionated relative to the heavy REE (LaiLuN = 2.0 to 6.9). No Eu depletion is exhibited. Nearly all samples exhibit a moderate negative Nb-Ta trough on a spider diagram. Initial 87Sr/86Sr values for six samples range from 0. 70325 to 0. 70386 and increase slightly with Si02� Initial 143Nd/144Nd values range from 0.512012 to 0.512997 and show no correlation with Si02� Values of 6180 measured on plagioclase separates range from 6.1%o to 7.9%o and show a correlation with Si02 and 87Sr/86Sri. Values of208Pb/204Pb range from 38.67 to 39.05, 207Pb/204Pb ranges from 15.58 to 15.64, and 206PbJ204Pb ranges from 18.82 to 19.10. Data from this study confirm that the Conejo Volcanics are geochemically very similar to two other nearby coeval volcanic suites, the Zuma Volcanics and volcanic clasts from the Blanca Formation on Santa Cruz Island. Similarities in location, age, and geochemistry imply a similar origin for all three volcanic suites. The calc-alkaline character and Nb-Ta depletion in Conejo Volcanics samples imply a subduction setting for the origin of this volcanic suite. However, a variety of major-oxide and trace-element tectonic discrimination diagrams do not suggest a specific tectonic environment of origin. Furthermore, evidence from sea-floor magnetic anomalies indicates that subduction had ceased in this area well before volcanism began. Accumulating research suggests that magmas with either or both of these geochemical signatures can be generated in areas undergoing extension by the decompression melting of attenuating and rising mantle. It is proposed that volcanism began about 16 Ma along the now-southern edge of the Transverse Ranges block while it was undergoing clockwise rotation. This rotation was caused by the partial coupling of the Transverse Ranges block to the underlying Monterey microplate which had become welded to the northwestward moving Pacific plate at about 20 Ma. Rotation of the Transverse Ranges block caused crustal rifting and attenuation and uplift of the underlying oceanic lithospheric and asthenospheric mantle of the Monterey microplate. Decompression melting of this depleted mantle source would produce magmas with a slightly more primitive isotopic character than seen in the Conejo lavas. For three to four million years, primitive basaltic magmas repeatedly intruded the overlying crust, where they underwent fractional crystallization and assimilated portions of that crust that had an isotopic composition similar to that of Mesozoic mafic and felsic gneisses from the Cucamonga terrane of the southeastern San Gabriel Mountain. These more evolved magmas then ascended through the upper crust and erupted on the Earth's surface.

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