IGNEOUS ODDBALLS: A CRYPTIC RECORD OF MAINSTREAM MAGMA CHAMBER PROCESSES?

Field observations of pegmatite, aplite, schlieren, and other “oddball” rock types with textures or modal abundances that are distinct from typical granitic and gabbroic rocks that dominate plutons, such as the Mount Sheridan gabbro, Oklahoma and the Deer Isle granite, Maine, may provide insight into a significant fractionation mechanism. Similarities in occurrence and textures suggest a common origin for these oddballs – one of melt extraction from a crystal framework progressively increasing in rigidity during crystallization. Early crystallization, dominated by low rigidity frameworks, favors melt migration as diapirs (pegmatite pods). With increasing rigidity the framework can temporarily support fractures enabling melt migration as self propagating “synmagmatic” dikes. Separation of mafic minerals during melt migration results in melanocratic pegmatite dikes in the gabbro, and is a potential origin for some schlieren in granite as well as “syn-magmatic” leucocratic (aplite/pegmatite) dikes in both pluton types. Dikes of pegmatite/aplite with sharp contacts are the result of brittle fracturing of a rigid framework and filling by residual melt. Diffuse “protopegmatite” in gabbro represent residual melt of insufficient volume to overcome the strength of the crystal framework and migrate. Corresponding situations in granite are likely but more difficult to recognize. A sufficient volume of melt must collect in order to move (diaper/self propagating dike). This requires alternating conditions which promote formation of a loose framework of crystals which trap melt followed by compaction (triggered by replenishments, seismic shaking) to achieve locally the critical volume of interstitial melt necessary to initiate migration. Successful melt migration from the cumulate pile into the active portion of the magma chamber can produce compositional variation during crystallization without leaving a record of the physical process. The low abundance of igneous “oddballs” within plutons may reflect the poor preservation potential for these rock types that are the product of a fractionation mechanism responsible for producing the “mainstream” rock types that define compositionally zoned plutons such as the Mount Sheridan Gabbro and the Deer Isle Granite.