EMPLACEMENT OF THE TUOLUMNE INTRUSIVE SUITE, SIERRA NEVADA BATHOLITH

Numerous emplacement models have been proposed for the ~ 88 to 93 Ma Tuolumne Intrusive Suite (TIS), the classic normally zoned body of the Sierra Nevada batholith. Our studies of the SW and S margins of the TIS indicate that they are marked by an up to 700 m-wide zone of 10 cm to 10s of m thick, steeply dipping, sheets of the outer Kuna Crest tonalite phase and similar-sized screens of older plutonic and metasedimentary host rocks. Contacts with the plutonic host rocks are stepped and ductile deformation resulting from TIS emplacement is generally absent. Magmatic foliation is oriented similarly in most screens, suggesting Kuna Crest sheets wedged aside host rocks, although some inclusions were detached and rotated. Strong magmatic foliation in the Kuna Crest is overprinted by abundant high-T, ductile shear zones with reverse displacements of < 1 m. These zones dip moderately E and less commonly W, recording margin-normal shortening, vertical stretching, and overall TIS-side-up movement. They may result from emplacement of much larger internal phases, which lack appreciable subsolidus deformation; cumulative shortening of < 300 m from the shear zones accounts for only a small percentage of the needed space for the internal phases. The Kuna Crest contact with the more internal Half Dome Granodiorite is commonly sharp, m-scale blocks of Kuna Crest lie within the Half Dome, and in places the Half Dome completely cuts out the Kuna Crest and intrudes TIS host rocks. The Half Dome includes > 10 km2 homogeneous domains and heterogeneous zones recording multiple magmatic pulses. Magmatic foliation cuts internal contacts at high angles, implying that the Half Dome remained mushy for a protracted interval.

Extension by faulting, as proposed by others for the TIS, was not an important emplacement mechanism. Major faults of appropriate age do not extend southward from the end of the TIS, and only local dilation associated with thin aplite dikes has been recognized in the interior of the suite. Our observations support a model of injection of Kuna Crest sheets that wedged aside, detached, and stoped host rocks, followed by much larger granodiorite pulses that stoped parts of the Kuna tonalite and host rocks, but were also accommodated by vertical ductile flow in the Kuna Crest. Vertical material transport probably dominated during emplacement.