Sentinel granodiorite

Sentinel granodiorite is a type of granodiorite found Yosemite National Park. It is a poorly understood western "outlier" of the ∼93-85-Ma Tuolumne Intrusive Suite of the Sierra Nevada batholith. It is only slightly older than the undated Yosemite Creek Granodiorite and the Kuna Crest Granodiorite.

Sentinel granodiorite was named after Sentinel Rock in Yosemite National Park, California. The granodiorite forms part of the Tuolumne Intrusive Suite (Tuolumne Batholith), one of the four major intrusive suites within the Sierra Nevada.

Sentinel granodiorite has a less uniform composition than other Yosemite granites and granodiorites. On the west end west end of Yosemite Valley, it is more homogeneous and light-colored, and on the east, it is more streaky, with an uneven composition. On the east, it is also much darker than on the west.

The chemical composition of Sentinel Granodiorite is less uniform than most the other intrusive bodies. In the western part of the zone crossed by the Yosemite Valley Sentinel Granodiorite is homogeneous and lighter colored, though the eastern part it is more streaky with an uneven in composition; the texture and on the whole is considerably darker than the western part.

Sentinel Granodiorite is a light-gray and medium-grained biotite-hornblende granodiorite. The color index is normally about 15, and ranges from 10 to 25. The hornblende is tabular, of crystals which are usually 4 - 6 millimeters long, and helps to define a weak to moderately strong magmatic foliation and lineation. Hornblende makes up from about 3% to 10% of Sentinel Granodiorite. Biotite makes up about 7% to 15%, and the crystals are one to five millimeters long. It is in an aggregate with the hornblende. Plagioclase makes up about 50% of Sentinel Granodiorite, and in size, ranges from one to ten millimeters. Interstitial potassium feldspar makes up ten to thirty percent pf Sentinel Granodiorite, is two to five millimeters across. Sphene is abundant, but rarely more than 1% of Sentinel Granodiorite; it is wedge-shapes, and usually about 0.1 to 1 millimeter long, but can reach 2 millimeters in length. Other accessory minerals are allanite, zircon, and apatite.

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