A DETAILED GEOCHEMICAL STUDY OF ISLAND ARC CRUST: THE TALKEETNA ARC SECTION, SOUTH-CENTRAL ALASKA

Abstract

The Early to Middle Jurassic Talkeetna Arc section exposed in the Chugach Mountains of south–central Alaska is 5–18?km wide and extends for over 150?km. This accreted island arc includes exposures of upper mantle to volcanic upper crust. The section comprises six lithological units, in order of decreasing depth: (1) residual upper mantle harzburgite (with lesser proportions of dunite); (2) pyroxenite; (3) basal gabbronorite; (4) lower crustal gabbronorite; (5) mid-crustal plutonic rocks; (6) volcanic rocks. The pyroxenites overlie residual mantle peridotite, with some interfingering of the two along the contact. The basal gabbronorite overlies pyroxenite, again with some interfingering of the two units along their contact. Lower crustal gabbronorite (?10?km thick) includes abundant rocks with well-developed modal layering. The mid-crustal plutonic rocks include a heterogeneous assemblage of gabbroic rocks, dioritic to tonalitic rocks (30–40% area), and concentrations of mafic dikes and chilled mafic inclusions. The volcanic rocks (?7?km thick) range from basalt to rhyolite. Many of the evolved volcanic compositions are a result of fractional crystallization processes whose cumulate products are directly observable in the lower crustal gabbronorites. For example, Ti and Eu enrichments in lower crustal gabbronorites are mirrored by Ti and Eu depletions in evolved volcanic rocks. In addition, calculated parental liquids from ion microprobe analyses of clinopyroxene in lower crustal gabbronorites indicate that the clinopyroxenes crystallized in equilibrium with liquids whose compositions were the same as those of the volcanic rocks. The compositional variation of the main series of volcanic and chilled mafic rocks can be modeled through fractionation of observed phase compositions and phase proportions in lower crustal gabbronorite (i.e. cumulates). Primary, mantle-derived melts in the Talkeetna Arc underwent fractionation of pyroxenite at the base of the crust. Our calculations suggest that more than 25?wt % of the primary melts crystallized as pyroxenites at the base of the crust. The discrepancy between the observed proportion of pyroxenites (less than 5% of the arc section) and the proportion required by crystal fractionation modeling (more than 25%) may be best understood as the result of gravitational instability, with dense ultramafic cumulates, probably together with dense garnet granulites, foundering into the underlying mantle during the time when the Talkeetna Arc was magmatically active, or in the initial phases of slow cooling (and sub-solidus garnet growth) immediately after the cessation of arc activity.