San Joaquin – Kings Volcanic Field

The San Joaquin-Kings volcanic field comprises more than 100 small lava flow remnants, volcanic necks and dykes, scattered over the predominantly granitic terrain of the southern Sierra Nevada. Apart from minor occurrences of andesite, dacite and rhyolite the rocks are predominantly alkaline varying from alkali olivine basalts, through potassic olivine basalts and trachybasalts, to ultrapotassic rocks including leucite basanite, tephrite and phonolite. The ultrapotassic rocks are confined to areas at the northern and southern edges of the field, the principal localities being at Merced Peak, Bass Lake, Patterson Mountain, Tehipite Dome, Marian Peak and Dunlap (Moore and Dodge, 1980, Table 1). The Deep Spring Valley occurrence of leucite-bearing rocks (No. 50) lies some 70 km to the east. The ultrapotassic rocks (4.1-7% K2O) contain pale green pyroxene, olivine, often as phenocrysts, and sometimes groundmass plagioclase; large biotite or phlogopite phenocrysts are usually present, as are leucite microphenocrysts. Some varieties contain poikilitic alkali feldspar instead of leucite, glass and other groundmass minerals. Apatite and opaques are common and pseudobrookite occurs in some samples. A general description of the field will be found in Moore and Dodge (1980) and of its geochemistry in Dodge and Moore (1981). A detailed petrological, mineralogical and geochemical account of the northern part of the field is available in van Kooten (1980) and of Pb and Sr systematics in van Kooten (1981).