OLIVINE-PLAGIOCLASE-PYROXENE CUMULATES ASSOCIATED WITH THE HORNBLENDE-RICH SUMMIT GABBRO: EARLY STAGES OF DIFFERENTIATION WITHIN THE LATE JURASSIC SIERRA NEVADA ARC

Geochemical analyses of rare olivine-bearing rocks associated with the ca. 150 Ma Summit gabbro are scrutinized with the goal of revealing initial differentiation trajectories of mafic magmas within the Late Jurassic Sierra Nevada arc. Small plutons of the hornblende-rich Summit gabbro are distributed across the Kern Plateau. This unit displays significant textural variation from pegmatitic to aphanitic-porphyritic textures, the latter suggestive of a transition to shallow emplacement levels. The Summit gabbro typically has a low Mg# (~50), and rarely contains pyroxene, let alone olivine. So while amongst the most mafic rocks exposed in the Sierra Nevada batholith, the Summit Gabbro is not a primitive, mantle-derived magma. Hornblende-bearing anorthosite adcumulates associated with the Summit gabbro contain interlocking, zoned plagioclase having euhedral, calcic cores. Meso- to orthocumulates contain cumulate olivine, orthopyroxene and plagioclase with up to ~35% intercumulus, poikiolitic hornblende. In some cumulates, multi-shell coronas separate adjacent olivine and plagioclase crystals: a talc-oxide assemblage surrounds or completely replaces the olivine; an amphibole-spinel symplectite shell abuts the plagioclase; and an intervening orthopyroxene shell is commonly replaced by amphibole. We interpret the corona textures as having formed by solid-state recrystallization during decompression of olivine-plagioclase autoliths or xenoliths, carried upwards from the deep crust by pulses of the Summit gabbro. The possibility that Summit gabbro magmas traversed the deep arc crust is supported by mantle-like, high-Mg# (~96.5, n=3) olivine compositions found in a single olivine-porphyritic dike. In contrast, we hypothesize that corona-free, olivine-bearing gabbros completed crystallization in the upper crust and represent relatively undifferentiated samples of mantle-derived magma. Geochemical compositions of cumulate rocks and mineral phases will be used to characterize the olivine-plagioclase-pyroxene cumulates and evaluate their origin in the context of the Summit gabbro. Future analysis of olivine-plagioclase-pyroxene-bearing cumulates will be aimed at attempting to characterize the mantle source region of this part of the Sierra Nevada arc.