CRYSTAL COCKTAILS IN THE TUOLUMNE INTRUSIVE COMPLEX

Plutonic rocks may contain multiple populations of a mineral implying that crystals grew in magmas distinct from the one they eventually reside in. This has been especially well documented for zircon in several felsic plutonic systems, where antecrysts from older plutonic units occur in younger ones. If this phenomenon is common for accessory minerals, it begs the question whether rock-forming minerals also preserve evidence for multiple populations, and if so, whether recycling is common among rock-forming minerals.

We examined trace element compositions and zoning patterns in hornblende and K-feldspar from gabbroic to granitic units of the 95-85 Ma Tuolumne Intrusive Complex (TIC), Sierra Nevada, CA. Trace element distributions in hornblende differ in abundances and zoning patterns in different units of the Kuna Crest (KC) lobe, the oldest part of the TIC, implying distinct magmas (Barnes et al. abstract) and no interaction at the emplacement level. At the transition from KC lobe to main TIC reservoir and in KC of the main TIC body, however, at least two distinct hornblende populations can be distinguished. These units and the porphyritic Half Dome lobe furthermore contain two distinct K-feldspar populations, while the Cathedral Peak granite shows evidence for at least three types of K-feldspar. This indicates an increase of mixed crystal populations toward the younger, interior and larger units of the TIC, in agreement with field observations and U-Pb zircon geochronology.

Our data suggest that crystal recycling was an important process during TIC growth and was widespread in the main TIC body. A large enough magma mush zone formed to allow an inward increase in crystal transfers. We conclude that 1) evidence for antecryst recycling is preserved with different rock-forming minerals, 2) mixing of chemically distinct magma mushes may be quite extensive during ascent and at the emplacement level in parts of an intrusion, and 3) large parts of plutons may effectively represent crystal cocktails. This has large implications for interpreting bulk rock geochemistry in plutons and mixed mineral populations in volcanic rocks, which may have tapped pre-mixed magmas. Future studies of magmatism in plutons will benefit from detailed analysis of the origins of these crystal cocktails and their impact on bulk-rock composition.