COMPOSITIONALLY DISTINCT ALKALI BASALT FLOWS AND HYALOCLASTITES IN THE NORTHWESTERN TAHOE BASIN

Young (< 2 Ma) alkali basalts in the NW Tahoe basin comprise at least 5 discrete units erupted from 3 or more vents. Based on new whole-rock, trace, and REE geochemistry, electron microprobe analyses of glass, and field observations, the rocks may be divided into two major groups. An eastern group east of 120^o 08’ W longitude has fairly uniform geochemistry with higher Fe₂O₃ and TiO₂, and lower LREE’s, K₂O, Sr, and Ba compared to a western group. Basalts of the western group can be separated geochemically and spatially into 3 subunits: (1) the most southerly basalts exposed in Ward and Blackwood canyons (excepting Eagle Rock) which have intermediate LREE values. (2) basalts exposed in the Truckee River and Granlibakken Creek canyons and on the ridge between which have the highest LREE values of all samples. (3) trachyandesites forming the high elevations NW of Tahoe City which are the most evolved (highest SiO₂, K₂O and Na₂O, lowest MgO, Fe₂O₃, and TiO₂ of all samples). The basalts of the eastern group as well as the trachyandesites of the western group appear to have erupted from a vent 4 km NW of Tahoe City. Eagle Rock is a distinct unit and most likely forms its own vent (Kortemeier and Schweickert, 2007). Other vents for the western basalts have yet to be identified. A youthful appearing, canyon-filling lava flow overlying a pillow breccia within the Truckee River Canyon near Rampart is more closely related chemically to the eastern Skylandia Beach tuff (Kortemeier et al, 2005) than to any surrounding western units. The chemically unique Rampart flow/Skylandia Beach tuff pair may have erupted from a common eastern vent. Basaltic flows of both groups include a lower section of pillow breccia, pillow lava, and/or hyaloclastite tuff, all indicating interaction with water, overlain by subaerial columnar basalt. The contact at the base of the columns, which marks a former shoreline (Kortemeier and Schweickert, 2007), is 60 m higher in the western group than in the eastern group. Displacement on major N-S normal faults could account for the difference in shoreline elevation. Alternatively, the compositional and shoreline elevation differences may indicate an age disparity between the two groups. Ar-Ar geochronology is currently in progress to clarify these age relationships.