Rubenstone, J.L., 1984, Geology and geochemistry of early Tertiary submarine volcanic rocks of the Aleutian Islands, and their bearing on the development of the Aleutian Island Arc: Ithaca, New York, Cornell University, Ph.D. dissertation, 350 p., illust., maps.
Field mapping, petrologic study, and geochemical analysis of early Tertiary submarine volcanic rocks from the central and western Aleutian Islands gives insight into the early growth of the Aleutian Island Arc, and into its later thermal history as recorded in the low and very-low grade metamorphism of these units. Major contrasts between sequences on Adak Island (central Aleutians) and Attu Island (western Aleutians) reflect fundamental differences in their tectonic environments of formation. The Adak sequence (Finger Bay volcanics; FBV) is dominantly mafic to intermediate pyroclastic rocks. An 40Ar/39Ar age spectrum of a plagioclase separate from the FBV indicates a metamorphic event 34 m.y. ago (due to hydrothermal circulation during cooling of a pluton on southern Adak), and a crystallization age in excess of 50 m.y. FBV igneous rocks are porphyritic, with phenocrysts of (pseudomorphed) olivine, plagioclase, clinopyroxene, and titanomagnetite; hornblende occurs in more-evolved rocks. Phenocryst zoning (e.g., Cr in clinopyroxene) suggests mixing of magmas during differentiation. Major and trace elements in FBV whole-rocks show the same signatures as those of the modern Aleutian volcanic suite: high in Th and light rare-earth elements (LREE) relative to heavy REE, high in Ba and alkalies relative to LREE, and low in Ta, Hf and Zr. Ratios of 143Nd/144Nd (≈ 0.5130) and 87Sr/86Sr (0.7030-0.7033) are within the field defined by the modern Aleutian suite. Fractionation of the observed phenocrysts, along with some mixing of magmas, acconts for differentiation of the FBV from parental magmas derived from a heterogeneous mantle source. The Attu sequence of basaltic (and some spilitic) pillow lava and lesser volcanic breccia is cut by a dike complex of similar composition, overlain by Oligocene bedded sedimentary rocks (including laminated chert). The lavas are aphyric or sparsely porphyritic with phenocrysts of (pseudo-morphed) olivine and plagioclase. Levels of incompatible trace elements (such as LREE and Ba) in the Attu lavas are distinctly lower than in the FBV, and are like those of basalts erupted at ocean ridges or in back-arc basins. In the Attu lavas, 87Sr/86Sr of 0.7028-0.7037 at constant 143Nd/144)Nd (≈ 0.5132) indicates exchange with seawater Sr during metamorphism. Attu may be an uplifted fragment of the Oligocene Bowers Basin, formed in response to south-directed subduction beneath the now-inactive Bowers Ridge island arc.
Theses and Dissertations