EVIDENCE FOR REPEATED INTRUSION, REMOBILIZATION AND PARTIAL MIXING DURING PLUTON CONSTRUCTION IN THE TUOLUMNE INTRUSIVE SUITE, SIERRA NEVADA BATHOLITH, CALIFORNIA

The Tuolumne Intrusive Suite (TIS) of the Sierra Nevada Batholith is the archetype example of a large, composite, zoned granitic intrusion. However, important aspects of the physical and chemical history of the TIS magma chamber(s) remain poorly understood. We have begun a multifaceted field and laboratory investigation of the TIS to better understand the internal physical and chemical processes recorded in the TIS. Preliminary field work has been conducted between the outermost contact of the TIS and the inner Half Dome granodiorite-Cathedral Peak granite contact. There are several important observations from the field work that bear on models for the development of the TIS: 1) Small (cm-dm) ductile shear zones are present in the host rocks (El Capitan granite) and outer TIS units (Kuna Crest and Sentinel) but not in the inner TIS units (Half Dome and Cathedral Peak); 2) the Half Dome is remarkably heterogeneous (grain size and modal variation, mafic enclaves of variable composition and composite enclaves, sheets of felsic and mafic material showing mingling, 'ladder dikes', schlieren and mineralogic layering); 3) layering in Half Dome is especially common near the outer and inner contacts, is variably oriented, and displays evidence for magma flow (crystal imbrication, size grading, and 'cross-bedding'); 4) magmatic layering and schlieren fabrics are cross-cut by a later steep magmatic fabric that also crosses the internal contacts between inner and outer Half Dome and inner Half Dome and outer Cathedral Peak; 5) meter scale, K-feldspar megacryst pipes and crystal accumulations are common near the Half Dome/Cathedral Peak contact and are associated with schlieren and magmatic layering. Previous work has suggested that the TIS formed from successive emplacement of several large pulses of magma into an inwardly solidifying (and fractionating) magma chamber. These field observations and recently published geochronology suggest a more complicated history of intrusion, reintrusion, partial mixing, disruption and remobilization of slightly older pulses (mush?), and perhaps collapse of internal contacts.