Stop 2. Alpine tonalite

Return to Interstate 8 and proceed cast approximately 18 miles to the Tavern Road exit for Alpine. Proceed east through town approximately 1.7 miles to the first left turn at Victoria Lane. Pass under the freeway and park in the cul de sac opposite the large outcrops.

This rock with abundant dark inclusions is typical of one of the two tonalitic end members found in the western zone of the PRB. It is hornblende- and biotite bearing with lesser amounts of clinopyroxene and/or orthopyroxene. The second type, which is biotite- and pyroxene bearing, more massive, and more or less devoid of dark inclusions, will be seen in the Cibbets Flat pluton at Stop 7. Both types appear to be intimately associated in each pluton and have very similar chemistry (see Figures 3-5). This outcrop yielded a concordant zircon U-Pb age of 108 Ma whereas the Cibbets Flat pluton gave a 105 Ma age (L.T. Silver as reported in Todd and Shaw, 1979). Biotite is abundant in essentially all samples and forms interstitial or oikocrystic patches. Hornblende, when present, nucleated before biotite and also occurs as interstitial or oikocrystic masses within a network of euhedral to subhedral pyroxene and plagioclase grains. Its textural character as well as the geochemical variation within both plutons precludes amphibole fractionation as a viable mechanism for the production of the two end members in this tonalite system. Instead, Lombardi (1991) argued that successive generations of nearly identical parental melts hovered near the minimum water content necessary to nucleate hornblende. These melts then rose as discrete batches within a central conduit system to undergo shallow-level pyroxene and plagioclase fractionation to produce the main lithologic variations within this pluton.

To Stop 3...