GEOLOGY AND GEOCHEMISTRY OF TERTIARY VOLCANIC ROCKS IN THE UPPER CARIBOU CREEK AREA, SOUTHERN TALKEETNA MOUNTAINS, ALASKA: EOCENE VOLCANISM IN AN EXTENSIONAL SETTING

New geologic mapping and geochemical sampling of volcanic rocks in the upper Caribou Creek area provides insight into the nature of early Tertiary magmatism that occurred in the southern Alaska orocline, approximately 200 km inboard of the paleo-trench. Volcanic rocks in the upper Caribou Creek area are over 1000 m thick and consist of mafic to felsic lavas, interlayered pyroclastic deposits, extrusive rhyolite domes, and numerous mafic to felsic hypabyssal dikes and plugs. Previous studies reported middle Eocene K-Ar ages (43.5 to 48.4 Ma) for volcanic rocks in the specific vicinity of the map area.

Flows of basalt, basaltic andesite, andesite, and dacite (46.2 to 69.7 %SiO₂) contain phenocrysts of plagioclase and clinopyroxene in fine-grained, commonly trachytic matrices of plagioclase laths, clinopyroxene, and Fe-Ti oxides. Unaltered rhyolite samples (74.1 to 78.1 %SiO₂) contain phenocrysts of plagioclase and quartz, rare clinopyroxene and zircon, and microphenocrysts of aligned plagioclase laths in glassy matrices.

The volcanic rocks form a subalkalic transitional (tholeiitic to calc-alkaline) lava series. Incompatible trace element correlations and sub-parallel trace element spidergram patterns, together indicate that the series is comagmatic. The intermediate and felsic composition rocks probably evolved primarily through fractional crystallization of mafic parent magmas. Flat REE patterns of basalt, basaltic andesite, and andesite samples suggest a depleted mantle source for magma, while variable enrichment of LIL elements (K, Rb, Ba, and Th) indicates some degree of crustal contamination. Nb-Ta depletions in andesite, dacite, and rhyolite samples [no data is available for mafic samples] may either point to a subduction or "arc" affinity, or may represent assimilation of older accreted arc rocks.

Northwest-trending normal faults, some of which are intruded by mafic feeder dikes, and flat-lying lava flows throughout the 1000-m-thick stratigraphic section indicate deposition in a subsiding, northwest oriented graben perpendicular to the trench/subduction zone. Northeast-southwest directed extension in this area during the early Tertiary is consistent with, and possibly related to right-lateral movement along the Castle Mountain-Caribou fault system located 10 km south of the map area.