IMPORTANCE OF CRYSTAL MIXING AND FRACTIONATION IN CONTINENTAL ARC MAGMAS: WHERE, WHEN AND HOW?

Studies on continental arc volcanic rocks have long suggested that magma mixing occurs in subsurface magma reservoirs and may trigger volcanic eruptions. Mixed crystal populations in volcanic deposits are more the rule than exception. Modern crystal scale geochemistry has been fundamental in developing this insight, however, it has been mostly applied to volcanic deposits. To better understand if magma mixing is restricted to subvolcanic magma reservoirs or occurs at all crustal levels, and whether fractionation accompanies mixing, application of such analyses in the plutonic realm is crucial.

Extensive, high precision U-Pb zircon geochronology in the Tuolumne Intrusive Complex (TIC, ~8-10 km) has been central in not only providing temporal information, but allowing the use of single zircon ages to detect and track crystal mixing. Mineral-scale geochemistry on major rock-forming minerals yields similar mixing results. Interestingly, the abundance of mixed mineral populations increases with time and toward the interior of the TIC, consistent with an increase in magmatic erosional features and hybridization along internal contacts.

K-feldspar from the innermost TIC granite has been particularly insightful about the timing of mixing and fractionation. K-feldspar is present as cm and mm size phenocrysts and interstitial, anhedral groundmass grains, all of which have different trace element compositions except for a shared outermost, thin rim. Given that the shared rims and interstitial K-feldspar would imply late mixing at crystallinities well above the 50% lock-up threshold, it is more likely that evolved, interstitial melt was drained after mixing and potentially fed high silica rhyolite eruptions, leaving behind a crystal-rich residue. A shallow level example of this is the Organ Mountains caldera in New Mexico that exposes both high silica rhyolites and associated plutonic crystal residues.

Evidence for crystal mixing at deeper arc levels indicates that although mixing may occur at subvolcanic levels and trigger eruptions, it is likely that mixed crystal populations found in volcanic rocks were already pre-mixed at deeper levels and accompanied by fractionation. To further understand when and how deep mixed crystal populations form, crystal scale geochemistry of deep crustal plutons is needed.