Ebbetts Pass Granodiorite

The Ebbetts Pass Granodiorite is a Late Cretaceous intrusive igneous rock unit located in Alpine County, California. It forms part of the Sierra Nevada batholith and is prominently exposed near Ebbetts Pass. It was first described in 1957 as a distinct granodioritic body within the Sierra Nevada region.[1] Subsequent studies have provided more detailed insights into its age, composition, and structural characteristics.

Geologic Overview

Radiometric dating using the potassium-argon (K-Ar) method has yielded ages 88.3 and 94.6 million years, placing its age in the Late Cretaceous period.[2] The granodiorite is typically fine- to medium-grained and composed predominantly of biotite and hornblende. Textural variations are common, with occurrences of coarse-grained porphyritic types. Accessory minerals include quartz, alkali feldspar, muscovite, and minor amounts of magnetite and ilmenite. Dark hornblende-rich inclusions, dikes composed primarily of quartz and feldspar minerals, and small aplite and pegmatite bodies are found throughout the pluton.[3]

Structural Features

Field studies have identified steeply dipping magmatic foliations within the Ebbetts Pass Granodiorite, generally striking southwest. A secondary northwest-striking magmatic foliation is also commonly observed. These structural features suggest a complex history involving multiple magmatic events and interactions with surrounding rock units.[4]

References

  1. ^ Wilshire, H.G. (1957). "Propylitization of Tertiary volcanic rocks near Ebbetts Pass, Alpine County, California" (PDF). University of California Publications in Geological Sciences. 32 (4). Berkeley: University of California Press: 243–271. Retrieved 3 June 2025.
  2. ^ Robinson, P. T.; McKee, E. H.; Moila, R. J. (1968). "Cenozoic volcanism and sedimentation, Silver Peak region, western Nevada and adjacent California". In Coats, R. R.; Hay, R. L.; Coats, Anderson (eds.). Studies in volcanology: Geological Society of America Memoir 116. Book Publishers. pp. 517–611.
  3. ^ McKee, Edwin H.; Chaffee, Maurice A.; Federspiel, Francis E.; et al. (1982). "Mineral resource potential map of the Mokelumne Wilderness and contiguous roadless areas, central Sierra Nevada, California" (PDF). USGS Numbered Series (1201). Reston, VA: U.S. Geological Survey. doi:10.3133/mf1201D. Retrieved 3 June 2025.
  4. ^ McClure, H.; Miller, R.B (2022). "Construction, Emplacement, and Structure of Two Major Intrusive Suites in the Northern Sierra Nevada Batholith". Geological Society of America Abstracts with Programs. Vol. 54.
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