Melt Extraction Zones in Shallow Arc Plutons: Insights from Fisher Lake Orbicules (Sierra Nevada, Western USA)

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Understanding the processes responsible for complex igneous flow structures and layering along the contacts between homogeneous plutons is fundamental for constraining pluton growth and melt transport mechanisms. Orbicules and comb layers from Fisher Lake, Northern Sierra Nevada, are associated with Cretaceous gabbroic to dioritic intrusions. Orbicule cores range from hornblende-bearing olivine-norites, hornblende-norites and hornblende-gabbros/diorites to broken-off comb layers and occasional metamorphic rocks. Magmatic orbicule cores represent cumulates or crystal mush formed by the fractionation of hydrous basaltic andesite magma at upper crustal pressures. We provide field evidence of dykes containing orbicules and/or comb layers as well as magmatic breccias coexisting with orbicules. Orbicule cores, orbicule matrices and host-rocks have overlapping mineral compositions and ages (110–113 Ma), providing evidence for the formation of comb layers and orbicule bodies directly from rising, hydrous basaltic andesite magma. The hornblende-gabbro cumulate signature of the orbicule matrix indicates that the residual melt expelled after the formation of these orbicule bodies had an andesitic composition. We propose, on the basis of field, petrological and geochemical constraints, that the Fisher Lake orbicule bodies were formed through the continuous tapping of a near-homogeneous magma reservoir remobilizing pre-existing, low-pressure, hydrous cumulates prior to injection as orbicule- and diatreme-like breccia dykes at the margin of cooling upper crustal plutons. The Fisher Lake orbicules and comb layers represent subvertical melt extraction zones forming in the roots of a volcanic plumbing system; they are potentially magmatic features linking deep-seated magmatic systems with shallow volcanic environments.

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