FOSSIL MAGMA SYSTEMS IN ARC CRUST: THE GEOCHRONOLOGY, GEOCHEMISTRY, AND FIELD PETROLOGY OF THE TUOLUMNE INTRUSIVE SERIES, SIERRA NEVADA BATHOLITH

The Tuolumne Intrusive Series (TIS) is the archetype zoned arc intrusion, and is one of a belt of large (>1000 km²) consanguineous, Cretaceous, composite, calc-alkaline intrusions cresting the Sierra Nevada batholith. Past work on TIS linked the geochemistry with field petrology and modification of magma in situ in large reservoirs, either by fractionation of outer TIS units (Kuna Crest, equigranular Half Dome) to make inner, more felsic units (porphyritic Half Dome, Cathedral Peak), or large-scale mixing between these units. However, recent geochronology of the TIS (Coleman et al 2004) shows that it was intruded from 94-85 Ma. Thus, if the major units solidified at markedly different times then petrologic features that have been inferred to reflect in situ processes in contiguous magma may require alternative or expanded explanations. Aspects of the TIS that must be reexamined within these geochronologic constraints, and which are more or less common to other Cretaceous arc intrusions, include: 1 normal zonation and the presence of mafic magmatic enclaves in outer, more mafic units and rarity in inner, felsic units; 2 large granodiorite interior units with K-spar megacrysts, and late, small leucogranite core units; 3 magmatic structures (complex schlieren zones with variable shape and orientation, megacryst clusters, ladder dikes); 4 variable internal contact relations and magmatic foliations at high angles to contacts; 5 fairly regular major element trends from margins to interior but more scattered trace element trends; 6 correlation of isotopic composition with bulk composition. We suggest that the above result from differentiation processes operating on arc magmas in a time-space-crystallinity continuum. Determining in detail where and by what physical and chemical processes arc magmas are modified, and what constitutes an increment or pulse of magma into a large magmatic system, requires placing an upper bound on the longevity of magma bodies in high and low melt-fraction states at a scale that encompasses areas with distinguishable zircon ages, and also more detailed field work to locate and examine cryptic internal contacts. Ar-Ar geochronology, zircon solubility constraints, crystal studies, thermal models, and comparisons with timescale data from large silicic volcanoes may also help in this regard.