SUGARLOAF MOUNTAIN, CENTRAL ARIZONA: AN EXAMPLE OF MIOCENE ANDESITIC HYBRID MAGMA FORMED FROM ALKALIC BASALT AND CRUSTAL COMPONENTS

Sugarloaf Mountain is a 200-m high landform in central Arizona within the transition from the southern Basin and Range to the Colorado Plateau. It is composed of Miocene lavas that are alkalic basalt overlain by andesite and dacite. Sugarloaf offers an opportunity to evaluate the origin of andesite with respect to the underlying basalt. Accordingly, sixteen samples were examined for petrography, major- and trace-element abundances, and mineral compositions.

Results: Basalts have 47.2-49.1 wt.% SiO₂ and 6.7-7.7 wt.% MgO, and they contain olivine (Fo_73–86) and clinopyroxene (Fs_8-13Wo_45-49; Mg# 78-84) phenocrysts. Andesites (and dacite) have 61.4-63.9 wt.% SiO₂ and 3.5-4.7 wt.% MgO. They also have olivine (Fo_77–86) and clinopyroxene (Fs_8-18Wo_35-45; Mg# 73-86) phenocrysts, and additionally have quartz (0.2-3 vol. %), Na-plagioclase (An_28-47; 3-6 vol. %), amphibole (Mg# 60-65), ± orthopyroxene (Fs_22-35Wo_2-3; Mg# 65-77), ± biotite (Mg# 60), ± Na,K feldspar (Ab_33-42Or_58-66). Notable for the andesites are (i) resorbed quartz, Na-plagioclase, and amphibole, and spongy zones and margins in and on Na-plagioclase cores, some with thin Na,Ca-plagioclase rims (An_48-53), and (ii) some incompatible element abundances lower than those in the basalts (e.g., Ce 77-105 vs 114-166 ppm; Zr 149-173 vs 183-237; Nb 21-25 vs 34-42).

Conclusions: Sugarloaf basalt fractional crystallization alone cannot yield the andesites because (a) 61.4-63.9 wt.% SiO₂ cannot be achieved at 3.5-4.7 wt.% MgO under any P or fO₂ conditions (e.g., MELTS; mass-balancing), and (b) incompatible element abundances decrease with MgO evolving from ~7 to ~4 wt.%. However, the mineral assemblages, compositions, and textures are all consistent with basalt having mixed with rhyolitic magma or having assimilated crustal granitoid rock. Binary mixing (assimilation) calculations yield good results (R² >0.99 for calculated vs actual major elements). For example, a hybrid mix of 73% average upper crust and 27% Sugarloaf basalt has an R² ~0.999 fit for matching the major elements in a Sugarloaf andesite. Results suggest that Sugarloaf andesites formed by basalt incorporating SiO₂-rich crustal material, but with the requirement that the SiO₂-rich component had incompatible element concentrations lower than those observed in the hybrid andesite and dacite.