Paper No. 28-8
Presentation Time: 8:30 AM-5:00 PM
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 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 typical hornblende-plagioclase Summit Gabbro is not a primitive, mantle-derived magma. Found within the Summit gabbro, however, are hornblende-bearing anorthosite adcumulates along with meso- to orthocumulates of cumulate olivine, orthopyroxene and plagioclase containing up to ~35% intercumulus, poikiolitic hornblende. In some cumulate rocks, multi-shell coronas separate adjacent olivine and plagioclase crystals: an amphibole-spinel symplectite shell abuts the plagioclase; ortho- and clinopyroxene shells surround olivine, and are commonly partially to completely replaced by amphibole. We interpret the coronas as having formed by solid-state recrystallization during slow subsolidus cooling at high (>6 kbar) pressures, consistent with deep crustal emplacement. In contrast, we hypothesize that small bodies of corona-free olivine gabbros completed crystallization in the upper crust and represent relatively undifferentiated magmas. Coronitic bodies are interpreted as olivine-plagioclase autoliths carried upwards from the deep crust by pulses of the Summit gabbro. SEM analyses show that olivine in both corona and non-corona samples have Mg-numbers (71-78, n=36) consistent with crystallization from mantle-derived magmas. 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.