Southern Sierra high-K volcanics
Southern Sierra high-K volcanics
Pliocene to middle Miocene
Variable but generally high-K to ultrapotassic mafic to intermediate low-volume eruptive and hypabyssal (sub-eruptive) units including dikes, volcanic necks, and partially eroded (or glaciated) lava flow remnants distributed sporadically across the central and (primarily) southern Sierra (e.g., Moore and Dodge, 1980; Feldstein and Lange, 1999). Includes the San Joaquin, Kings River, and Kern volcanic fields. Also includes ~3.5 to ~2.5 Ma precaldera dacite field and older Miocene alkalic volcanic units surrounding Long Valley caldera (Hildreth, 2004; Bailey, 1989). Compositions include alkali olivine basalt, potassic olivine basalt (trachybasalt), ultrapotassic basalt (leucite basanite, tephrite, phonolite), basaltic andesite, trachybasaltic andesite, and trachyandesite (e.g., Moore and Dodge, 1980; Van Kooten, 1980; Farmer and others, 2002). Variable phenocryst content includes phlogopite ± olivine ± clinopyroxene ± biotite ± sanidine ± plagioclase ± augite (Van Kooten, 1980; Bergquist and others, 1982; Farmer and others, 2002). Radiometric dates range from ~12.3 Ma (e.g. Bergquist and others, 1982) to ~2.5 Ma (e.g. Farmer and others, 2002; Hildreth, 2004). K2O content ranges from moderate to >8 wt% (Dodge and Moore, 1981; Farmer and others, 2002). Per Farmer and others (2002), includes pulses of magmatism in Miocene (~12-8 Ma) and Pliocene (~4-3 Ma) and a third (Quaternary) pulse represented by the Golden Trout volcanic field (map unit gtv). Various workers reported systematic spatial changes in K content of eruptive products; Van Kooten (1980) reported east to west increasing K in the central Sierra; Putirka and others (2012) reported southward increasing K across the range; Farmer and others (2002) reported higher K content in the northern San Joaquin and Kings regions and lower in the southern Kern volcanic field. In addition to generally distinctive high-K content, these rocks also include compositions with the lowest SiO2 and highest MgO concentrations of any Sierran volcanic rocks (Farmer and others, 2002). Predominantly erupted through and overlying granitic terrain; glacially eroded.