PLUTON EMPLACEMENT AND MAGMA CHAMBER PROCESSES – INSIGHTS FROM THE TUOLUMNE INTRUSIVE SUITE

The Tuolumne Intrusive Suite (TIS) is the type zoned intrusion of the Sierra Nevada batholith and illustrates processes operating during construction and emplacement of composite, long-lived, large-volume, mid- to upper crustal arc plutons. Our work on this suite and other plutons in the Sierras and Cascades indicates that most intrusions do not fit simple end-member models of magma ascent (e.g., diking, hot-Stokes diapirs) and emplacement. In the TIS, contacts with host rocks and between major internal units are steep, extending for > 2 km of topographic relief, as are magmatic foliation and lineation. Field evidence does not support a sill or laccolith-like geometry in contrast to popular models for plutons. Multiple material transfer processes operated during emplacement. Narrow ductile aureoles record modest vertical flow, but are discontinuous along strike, and segments of the TIS margin truncate host rock contacts and foliation at a high angle. Meter-scale sheets invade host rocks, forming rafts in places, whereas rotated xenoliths are prominent in parts of the margin of the TIS. Mixed block types occur in some xenolith-rich domains and small xenoliths lie > 700 m inward from the TIS contact. These xenolith relationships, and the discordant contacts, indicate that magmatic wedging and stoping were important emplacement processes.

Contacts between major units of the TIS change markedly along strike and range from gradational over many 10s of m to sharp reflecting varying rheology and magmatic erosion of outer pulses by inner ones. Schlieren and other flow features are prominent near major contacts. Sheet-like and more common irregular blob-like bodies, ranging from < 1 m to 10s of meters across, are distinguished by small textural and modal difference in parts of the TIS. Other domains display minimal heterogeneity and apparently represent much larger (10s to 100s of cubic km) magmatic pulses. Internal contacts and flow features are cut by magmatic foliation, which in part records regional strain; its development throughout the suite supports the existence of at least a moderate-sized magma chamber. We emphasize that the along-strike variability of internal and external contacts, and host rock response to emplacement, reflect temporally and spatially variable rheology and processes during intrusion.