PETROGENESIS OF AMPHIBOLITES, GOLDSTEIN PEAK PENDANT, WESTERN SIERRA NEVADA BATHOLITH, CA

A £0.5-km-thick section of amphibolites is sandwiched between metapelites, metaconglomerates and quartzites of the Goldstein Peak pendant. Hornblende-hornfels facies metamorphism by the surrounding Early Cretaceous batholith did not obliterate primary volcanic structures. Mosaic textures of the dominantly hornblende-plagioclase-quartz-biotite rocks indicate textural re-equilibration. Rare, corroded pyroxenes are interpreted as volcanic relicts.

A coarse-grained, massive amphibolite occupies the base of the thickest part of the metavolcanic section. Trace element discrimination diagrams classify this presumed lava as an island arc tholeiitic basalt; its Mg# of 0.77 indicates a mantle derivation. A unique, <1-m-thick zone containing 2-5-cm, star-shaped sillimanite clusters consistently marks the basal amphibolite-schist contact, interpreted as reflecting contact metamorphism of a mudstone protolith. The overlying amphibolites range in composition from basaltic andesite to dacite and are characterized by swarms of typically monolithologic, lenticular inclusions. These rocks are interpreted as hyaloclastites and peperites, and the variably biotite-rich matrix is interpreted as reflecting an original mud component. The amphibolite section thins and ultimately pinches out 3 km to the south where it is directly underlain by quartz pebble metaconglomerate. This thinner section is composed of unsorted, polymictic, 2-5-m-thick layers that include ragged, porphyritic clasts. These presumed ash flow deposits are truncated by channels having apparent widths up to 15 m and are overlain by a section of cross-bedded, laminated amphibolites. The southern section is interpreted as having formed from repeated extrusion of ash flows across a fluvial braid plain, the channels, cross-bedding, and laminae indicating fluvial reworking of the volcanic deposits. Geochemical variations are consistent with overall preservation of igneous compositions during subaqueous extrusion and contact metamorphism, although some mobility of Ti and P is indicated. One sample of laminated amphibolite displays atypically high K₂O and Fe₂O₃ contents and low CaO and Na₂O contents, consistent with subaqueous weathering.