A PORTION OF THE WESTERN SONORAN PASS INTRUSIVE SUITE AND ASSOCIATED WALL ROCKS: PRELIMINARY GEOLOGIC MAP OF TAMARACK 7.5' QUADRANGLE, CA

This quad is located west of the crest of the Sierra, in an area shaped by glaciation, volcanism, and uplift.

The Kinney Lakes granodiorite forms the outer rim of the Sonoran Pass Intrusive Suite and underlies the eastern portion of this quad. The adajent wall rocks are the Mokelumne Peak orthogneiss (northern edge of this quad) and the Hells Kitchen orthogneiss (western and southern edges of this quad). This study focused on the petrology and structures within these rocks.

Kinney Lakes granodiorite (Kkl). The (Kkl) intrudes two older orthgneiss plutons, the Hells Kithen orthogneiss (Mzhk) and the Mokelumne Peak orthogneiss (Mzmp). Near the contacts the Kkl contains stoped blocks of the orthogneiss rocks. Kkl contains zircons that have a 95.6±1.5 Ma age.

Kkl is a hornblende biotite granodiorite with K-spar megacrysts, titanite and zircon. This rock is white to light gray with an average Color Index of 10%. The Kkl contains mafic enclaves, enclave swarms, schlieren, dikes, and a has planar mineral fabric.

Mokelumne Peak orthogneiss (Mzmp). The Mzmp consists of roughly 85% gneiss and 15% schist/amphibolite, both are foliated. The gneiss contains magmatic intrusive structures (mafic enclaves, mafic enclave swarms, schlieren, and dikes). The schist/amphibolite occur as blocks, sub-meter to map scale, that are considered xenoliths. This rock contains zones of plastic deformation/shears that are a few millimeters to several meters wide.

Three sub-types of Mzmp were identified: (1) hornblende biotite granodiorite/tonalite orthogneiss; (2) K-spar megacrystic biotite hornblende granite orthogneiss; and (3) alkali feldspar granite orthogneiss. Titanite and zircon are the accessory minerals.

Hell’s Kitchen orthogneiss (Mzhk). Mzhk is a hornblende biotite felsic crystalline rock. Outcrops of this rock are very light gray to light gray and has an average Color Index of 23%. Mzhk has a planar mineral foliation and contains intrusive igneous structures (mafic enclaves, mafic enclave swarms, schlieren, and veins and dikes). The Mzhk is granodiorite/tonalite with titanite and zircon accessory minerals.

This geologic map can guide future studies in geochronology, geochemistry, and structural geology of the wallrocks to provide information on the construction Sierra batholith.