FACING UP TO THE COMPLEXITY OF BATHOLITH CONSTRUCTION: USING THE TUOLUMNE BATHOLITH (TB), SIERRA NEVADA, CALIFORNIA AS AN EXAMPLE

A modern view of pluton construction is emerging of incremental growth by addition of a few to many magma pulses, fed from multiple sources, with variable contamination, mixing, and fractionation during ascent and emplacement. Thus pluton construction involves, multiple emplacement events, multiple thermal pulses, many interactions between different geochemical aliquots, and the diachronous preservation of magmatic structures. After 7 years of study, including extensive 1:24,000 to 1:10 scale mapping, microstructural studies, thermal modeling, high precision U/Pb and 40Ar/39Ar geochronology, and elemental and isotopic geochemistry, we are beginning to unravel the complexities of TB construction.

Mapping of metamorphic host rock indicates that beds are steeply dipping prior to emplacement, that margins are often stepped, that only minor ductile emplacement strain is preserved, and that pendants are rotated and associated with extensive stoping. Mapping within the TB has confirmed 4 main units, gradations within these units, complex processes along contacts between units, the presence of 1000s of clustered, late flow instabilities (e.g., crystal accumulations, vertical, compositionally heterogeneous tubes, outward younging troughs), and the presence of 4 magmatic fabrics, two of which overprint all internal structures. Microstructures are mostly magmatic with little to no indication of annealing, except in rare regions of subsolidus deformation along the W and E margins.

U/Pb geochronology supports at least a 10 m.y growth history from margin to center, that lobes are older than the main chamber (Memeti et al., this session), that zircon locally crystallized over an ~1 m.y. time period, and that zircon is recycled from older into younger TB units (Matzel et al., this session). Ti-in-zircon thermometry and low Zr saturation temperatures indicate that zircon grew at near-solidus temperatures. New and published elemental and isotopic studies suggest 4(?) homogeneous source magmas, which fractionated in lobes and in the Cathedral Peak unit, mixed in the main chamber, and were locally contaminated by host rock assimilation. We continue to evaluate a number of emplacement and pluton construction scenarios that are compatible with the mapping, geochronology, geochemical, and isotopic data.