MAGMATIC PROCESSES INSIDE AND OUTSIDE THE TANK: QUESTIONS AND LESSONS LEARNED FROM MID-CRUSTAL PLUTONS

Magma mixing, recharge, fractionation, and assimilation are proposed to explain evolution of plutonic magmas; all are applied to the Sierra Nevada batholith (SNB). One theme of SNB studies is that most magma differentiation (s.l.) occurred deeper than the level of emplacement. Mid-crustal intrusions in Norway (Bindal Batholith) and tilted plutons in the Klamath Mountains (KM) may provide insight into the diverse processes that control magma evolution and pluton construction.

Assimilation. Many Bindal plutons contain diverse xenolith suites. In the Sausfjellet pluton, solubility (or “meltability” or “reactability”) of xenolith minerals was the principal control of assimilation. Soluble pelitic contaminants disappeared entirely into the host magma, but insoluble (marble) contaminants had no measurable effect on composition. In the Horta pluton, the soluble component varied as a function of T and magma composition: in mafic magmas, high-T assimilation of calc-silicates led to silica undersaturation, but in more evolved magmas only siliceous material could assimilate, leading to silica oversaturation. In the Jackass Lakes pluton, central SNB, abundant rhyolitic xenoliths, compositionally similar to their host, were non-reactive, and therefore preserved.

Crustal anatexis and mixing. In Bindal, andesitic magmas emplaced as 5–7-km diameter bodies at ~25 km formed anatectic aureoles in host metapelites. This magmatic intraplating was followed by hybridization of anatectic and intraplated magmas, yielding enclave-bearing peraluminous granites, whose collection and migration were localized along pluton margins. Locally, hybridization resulted in metaluminous garnet hornblende diorite. Does this latter process provide clues to the “garnet REE signature” of many arc magmas?

Magma recharge and mixing. The tilted Wooley Creek batholith (KM) exposes ~9 km of structural relief. The lower (dioritic) half of the pluton shows evidence for intense magma mixing, but the upper (granodioritic) half does not. We suggest that the lower half of the system trapped multiple basalt pulses, with episodic, possibly catastrophic convection. It represents the thermal engine for crustal assimilation and maintenance of a long-lived (evolved) upper part of these system.