Abstract

The 1.07 Ga Granite Mountain exfoliation dome is a subalkalic, metaluminous to marginally peraluminous granite, which is classified as an anorogenic or A-type granite. This study on the Granite Mountain exfoliation dome shows the following: 1) the pluton is biotite-hornblende granite with accessory titanite, magnetite, zircon, apatite, allanite, pyrite, and fluorite with sporadically occurring rapakivi; 2) modal composition is syeno- to monzogranite or true granites; 3) seven whole rock chemical analyses of the granites vary continuously from 72-77 wt% SiO₂ with high alkalies, K₂O/Na₂O = 1.37-1.67, but low in CaO, MgO, TiO₂, Cr₂O₃, and Al₂O₃; 4) A/CNK = 0.92-1.00; 5) trace element enrichments in Rb, Sr, Y, Zr, Nb, and Ba; and 6) major element tectonic discrimination diagrams suggest that Granite Mountain has continental epeirogenic uplift granitoid affinities, while trace element tectonic discrimination diagrams suggest that Granite Mountain formed in a within-plate tectonic setting.

Contact metamorphic mineral assemblages suggest T-P ranges of 750° to 850° C and, .2-.5 GPa respectively. The assemblage of titanite + magnetite + quartz suggests crystallization occurred at fO₂ ranging from 10⁻⁷ to 10⁻¹⁰ Pa under low fO₂ of less than 1.5 wt.% H₂O (Wones, 1989).

Based on the mineralogy, major and trace element geochemistry, and tectonic setting, the Granite Mountain exfoliation dome fits the description of A-type granites, but with I-type affinites. Granite Mountain probably formed from the partial fusion of older sub-ducted-related tonalitic crust. However, Nd data for Granite Mountain, Nd = +3.4 at 1.06 Ga (Patchett and Ruiz, 1989), suggests there may have been a significant mantle component as well in the origin of Granite Mountain.