Tertiary Volcanic Rocks of the Central Talkeetna Mountains, Alaska

Volcanic rocks of the central Talkeetna Mountains consist of more than 400 m of shallow-dipping basaltic and andesitic lavas with subordinate dacitic and rhyolitic lavas and tuffs. All of the lavas are intruded by rhyolite and dacite dikes, sills, and small domes and by minor basaltic dikes. The eruption of these rocks began with the outpouring of mafic lavas, followed by alternating mafic, intermediatecomposition, and felsic lava flows. The youngest phase of magmatism is recorded by the shallow rhyolite and dacite intrusions that crosscut the lavas. Geochemically, the basalt samples are relatively depleted in light-rare-earth elements (chondrite-normalized La/Yb ratios of 0.9–2.3, with La 9.5–31.3 times chondrite) and have compatible-element contents similar to those of midocean-ridge basalts, indicating a depleted mantle source for the basalts. The felsic lavas and intrusions are the most evolved rocks in the study area (chondrite-normalized La/Yb ratios of 3.7–7.6, with La 32.7–64.0 times chondrite and highest Ba, Rb, Th, and K contents) and evolved from the mafic magmas by a combination of fractional crystallization and crustal assimilation. All of the samples (basalts through rhyolites) show enrichment in Cs, Ba, Th, and Pb, which is consistent with metasomatism of the depleted mantle source.