EXAMINING COUPLED TECTONICS AND PLUTON EMPLACEMENT IN HOST ROCKS ALONG THE NORTHEASTERN MARGIN OF THE TUOLUMNE BATHOLITH, SIERRA NEVADA, CALIFORNIA:

Along its northeast margin, the 86 Ma Cathedral Peak phase of the Tuolumne Intrusive Suite intrudes a diverse set of “host rocks” including a large belt of amphibolite-grade metavolcanics, interspersed with packages of metavolcaniclastic and metasedimentary units, and intruded by the 97.4 +/- 0.4 Ma Soldier Lake Pluton (SLP) and by the 165.2 +/- 0.3 Ma Green Lake Pluton (GLP). The largest package of metasediments near Virginia Peak consists of metamorphosed cross-bedded sandstones and siltstones, limestones, and a large calc-silicate unit. Metavolcanic rocks range in composition from rhyolite to basalt, are dominated by clastic (lapilli to breccia size fragments) and crystal-bearing tuffs and local flows and volcaniclastic units. An ~10 meter wide oblique, dextral slip, ductile shear zone separates the metasediments and eastern metavolcanic package and contains within it lenses of marbles, garnet-bearing calc-silicates, and phyllites.

All units in this region generally display steeply-dipping foliation axial planar to both large-scale (in calc-silicate unit) moderately plunging folds and small-scale, more variably plunging folds in which bedding and sometimes an older mineral fabric are transposed. Mineral lineations are widespread and typically plunge steeply. Fabric intensities and associated strains measured from clastic objects are heterogeneous but increase moderately towards pluton margins.

The GLP and SLP largely consist of medium-grained granodiorite, but with some moderate textural and compositional variations, and are cut by leucogranite dikes. Both plutons display a weakly to moderately developed magmatic foliations, which strike NW, dip steeply northeast, typically are not affected by distance from pluton boundaries and thus are continuous with host rock foliations. Magmatic mineral lineations statistically plunge steeply. Contacts between both of these plutons and their host rocks are usually sharp, discordant to host rock structures, make numerous abrupt changes in orientation and have some host rock blocks preserved near them: all these observations suggest that stoping was an important process, at least during final emplacement. Timing relationships indicate that regional deformation began before 165 Ma and continued to at least 95 Ma and with decreasing intensity to 86 Ma.