MID-CENOZOIC MAGMATISM OF CENTRAL ARIZONA: PETROLOGY OF BASALTIC LAVAS IN THE GOLDFIELD-SUPERSTITION VOLCANIC PROVINCE

     The Goldfield-Superstition Mountains, southern Basin and Range, Arizona, represent a large silicic province (LSP) with basaltic lavas.  They are comprised of ~18-20 Ma rhyolite-dacite pyroclastics and lavas that were preceded (~20 Ma), attended, and followed by (~14 Ma) basalt.  The oldest basalts, emplaced on granitic basement, have low TiO₂, 1-1.5 wt%, but details reveal varieties.  Examples: (i) Weekes Wash is tholeiitic-alkalic with SiO₂ 48-52 wt%, MgO 9-10.8%, incompatible-element enrichment (La_(n) 150-200; Sr 800-1100 ppm; Ba 850-1050), and Nb-Ta and Ti relative depletions.  Sr-Nd isotopes = 0.70552; 0.512496.  Quartz and feldspar xenocrysts are present.  (ii) Overlying Cottonwood Springs is alkalic with SiO₂ 45-47.5%, MgO 10-11.8%, greater incompatible-element enrichment (La_(n) 200-400; Sr 1300-2200 ppm; Ba 1000-2200) than Weekes Wash, and also with Nb-Ta and Ti relative depletions.  Sr-Nd = 0.70521; 0.512552.  Basaltic lavas interlayered with LSP stratigraphy are: (i) hawaiite (MgO 9-10%; Na₂O+K₂O ~5.9%) with incompatible-element abundances (La_(n) ~125; Sr ~1200 ppm; Ba ~1000) similar to those in Weekes Wash; and (ii) evolved (MgO 5.6-7.5%) alkalic, TiO₂-rich (~2.5%), with low incompatibles (La_(n) 60-100; Sr ~750 ppm; Ba ~400), lacking Nb-Ta, and Ti relative depletions.  Goldfield-Superstition volcanism ended with Black Mesa basanite: SiO₂ ~44%, MgO ~8%, high incompatibles (La_(n) 400, Sr ~1650 ppm, Ba ~2400; Nb ~60), and Nb-Ta, Ti, and K-Rb relative depletions.

Interpretations:  For the oldest basalts, ratios (e.g., Zr/Nb, Zr/Y, Nb/Ta, Nb/P₂O₅) identify an overall common source with some trace element variations (e.g., Zr/Nb 7.5-10.5) and a subduction component.  Some basalt trace-element differences can additionally reflect source-melting percentages, and other variations (e.g., isotopes; SiO₂) may reflect crust input.  Basalts interlayered with LSP shared the oldest basalts' source, based on trace-element ratios, but overall compositions suggest differentiation from oldest basalts.  Youngest mafic lavas, basanite, have trace-element characteristics (e.g., K-Rb depletions; Zr/Nb ~4) consistent with smaller percent melts of yet the same source for all prior basalts.  For ~6 m.y., then, Goldfield-Superstition mafic magmatism tapped a single lithospheric subduction-component source with minor heterogeneities, melting it in varying, generally progressively smaller percentages -- concluding without sign of a Basin-Range asthenospheric source.