FELSIC MAGMAS FROM MT. BAKER IN THE NORTHERN CASCADE ARC: ORIGIN AND ROLE IN ANDESITE PRODUCTION

Dacitic magmas in volcanic arcs play a critical role in the growth and development of felsic continental crust through mixing to form andesite, or by directly adding new crustal material through fractionation of mantle derived basalts. This study discusses their importance as felsic endmembers in mixing regimes. A comprehensive data set reveals that the most Si-rich, Mg-poor dacitic compositions analyzed in this study (dacite of Mazama Lake) can be modeled as liquids derived by crystal fractionation from Mt. Baker high-Mg andesites. These Si-rich compositions are in turn back-mixed with mafic magmas to produce more Si-poor dacites (dacite of Cougar Divide) and andesites (andesite of Mazama Lake). The origin of one enigmatic hornblende-bearing dacite unit (dacite of Nooksack Falls) is unconstrained.

The dacite of Mazama Lake represents a near end-member composition with only minor contamination from xenocrystic material. Mineral populations commonly lack disequilibrium textures, and exhibit normal zoning, and mineral chemistry suggests the majority of the crystal population is original to the dacite of Mazama Lake. Textural and chemical evidence indicate a minor addition of xenocrystic material. Fractional crystallization of the andesite of Glacier Creek is one permissible model that can reproduce compositions observed in the dacite of Mazama Lake.

The dacite of Cougar Divide and the andesite of Mazama Lake exhibit mineralogical evidence for magma mixing between more mafic and more felsic magmas. The dacite of Mazama Lake can be used to reasonably reproduce compositions observed in the mixed magmas. Mixing between the andesite of Glacier Creek and dacite of Mazama Lake can reproduce an average composition from the Cougar Divide unit in mixing proportions of ~60% andesite and ~40% dacite. Compositions from the andesite of Mazama Lake can be reproduced by mixing ~70% Tarn Plateau basaltic andesite and ~30% Mazama Lake dacite.

The dacite of Nooksack Falls appears to represent a near-endmember composition, however a distinct set of minerals with compositions expected from a basaltic source suggests the dacite of Nooksack Falls acquired some xenocrystic material. Removal of this contamination does not permit a fractionation origin from known mafic compositions, indicating other hypotheses must be explored.