| Summary: | Three classic volcanic arcs have developed in western Luzon, the Philippines as a result of subduction associated with an eastward-dipping Benioff zone. A data set for about 200 samples was determined for this and earlier studies. This set includes: (1) major and various trace elements, (2) 30 initial ('87)Sr/('86)SR (0.7038 to 0.7054) and a few ('143)Nd/('144)Nd ratios (0.512921 to 0.512999), (3) 76 conventional K/Ar radiometric dates, and (4) in addition to standard model mineral analyses, over 600 microprobe analyses of 33 thin-sections. === Based on these data, several conclusions can be made: (1) Excellent linear correlations exist between SiO(,2) and the incompatible elements. The slopes of these trends increase progressively from western arc through the central to the eastern arc. Moreover, normalized potassium concentrations (K(,55)) increase with assumed depth to the subducting slab and away from the Manila Trench. (2) Incompatible elements do not appear to have been affected by alteration or weathering. (3) Based on geophysical and geochemical evidence, assimilation and contamination have played an insignificant role in affecting original magma compositions. (4) It can be argued that the sources for these volcanics are enriched (metasomatized) mantle above the Benioff zone. Derivation of the Bataan volcanic rocks by different degrees of partial melting of the mantle can be ruled out. The mantle sources become increasingly more enriched in incompatible elements and radiogenic strontium away from the Manila Trench. (5) All three arcs have apparently undergone early non-selective crystal fractionation of plagioclase, clinopyroxene, magnetitite, and/or olivine based on mass-balance calculations. It appears that very similar early fractionating assemblages and assemblage percents have been removed from original magmas in each of the three arcs. (6) Parallel trends noted in both whole-rock and groundmass Harker diagrams (except the potassium Harker diagrams) among the arcs are probably the result of crystal fractionation. The variations in slopes observed in potassium Harker diagrams between the arcs can be explained by a combination of original primary magma potassium variations between the arcs and the fractionation of similar phases within each arc. === Source: Dissertation Abstracts International, Volume: 47-02, Section: B, page: 0537. === Thesis (Ph.D.)--The Florida State University, 1985.
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