EOCENE MAGMATISM IN A REMNANT FOREARC BASIN, MATANUSKA VALLEY, SOUTHERN ALASKA

Basalt-andesite sills that are 100's m thick and km-scale dacite-rhyolite plugs intrude Triassic to Eocene sedimentary and volcanic rocks in a forearc basin setting. The basalts contain olivine and clinopyroxene, are tholeiitic, and some samples show evidence that they are not highly evolved (Cr, Ni, and Mg# up to 346 ppm, 305 ppm, and .59, respectively). Most basalt samples are similar to or are more depleted than mid-ocean ridge basalts in the light rare earth elements (average La/Nd is 0.45 compared to 0.3 to 0.6 for MORB) and among other incompatible (Ba, Rb, K) to compatible (Zr, Ti) elements. Some basalt samples are, however, enriched in Rb, Ba, Th, K, and Pb. The felsic samples are subalkalic and also have low La/Nd ratios (0.25 to 0.74) but are uniformly enriched in the incompatible elements (Rb, Ba, Th, K) when normalized to MORB.

The basalts were derived from a depleted mantle source. The high concentration of incompatible elements in some basalts indicates crustal contamination and inflections in major oxide trends such as MgO and TiO2 with respect to SiO2 indicate the role of fractional crystallization in the evolution of the felsic rocks. The high Ba and Pb in some basalt samples might be indicative of a metasomatised source but several observations show that these intrusions were not formed as part of an arc magmatic system above an actively dewatering slab. First, there is not a uniform decoupling of large ion lithophile elements (Ba) from high field strength elements (Nb, Zr); some basalt and even some rhyolite samples have low Ba/Nb, Ba/Zr, and Pb/Zr ratios in the range of MORB. Also, the basalt-basaltic andesites range to higher TiO2 than is typical for arc rocks (>1.5%). Finally, the intrusions were emplaced in a near-trench setting (within a remnant forearc basin) during a hiatus in nearby arc magmatism.

The Matanuska Valley intrusions are consistent with magmatism above a slab window that formed during Late Paleocene spreading ridge subduction beneath southern Alaska. Parental magmas originated from suboceanic depleted mantle and became contaminated with fluid-mobile elements (Ba, Pb) from forearc crustal rocks. A difference between the timing of ridge subduction (~56 Ma) and available ages of the intrusions (~46 Ma) indicates that the parental magmas resided beneath southern Alaska for ~10 Ma before emplacement.