EARLY MESOZOIC VOLCANICLASTIC ROCKS OF AGNEW MEADOWS, EAST-CENTRAL SIERRA NEVADA, CALIFORNIA
The tuff and tuff breccia clasts are crystal-poor and porphyritic with an average of 11% ß-quartz and plagioclase phenocrysts. The ß-quartz phenocrysts are mostly fragmented with few subhedral to euhedral grains (< 2.6 mm), and ~30% of the intact crystals are embayed. The plagioclase grains are approximately equally fragmented and subhedral to euhedral grains (< 1.8 mm). Harker diagrams show whole-rock compositions are similar, while the breccia clasts are relatively more silica-rich: the tuff samples average 72.4% SiO2 by weight while the breccia clasts average 76.7%. Negative covariation between silica and fluid-immobile elements (Al2O3, TiO2, Zr) indicate the breccia clasts are more fractionated than the tuff. Chondrite-normalized REE patterns are light REE-enriched with small negative europium anomalies and slight depletion in the middle to heavy REE.
Similar physical and chemical characteristics are noted in other Triassic tuffs in the eastern Ritter Range and Mount Morrison pendants as well as in the granite of Lee Vining Canyon, a pluton of the late Triassic Scheelite Intrusive Suite. Observations of the physical, chemical, temporal, and spatial homogeneities between various Sierran intrusive and extrusive rocks suggests that volcaniclastic rocks and plutons were coeval and probably cogenetic. If early Sierran arc plutons and volcanic deposits had similar origins, then at least some of the plutons must be fossil magma chambers.