- UW Department of Geology and Geophysics
Basaltic lavas form part of the Miocene (∼20.5 to 18 Ma) Goldfield-Superstition silicic large igneous province in central Arizona. Most of the basalt erupted early in the development of this southern Basin and Range volcanic province, and only small amounts of basalt co-erupted with the silicic volcanism. We examined 50 samples of basalt from lower, middle, and upper stratigraphic positions of the province to establish basalt petrogeneses, the characteristics of basalt sources, relationships among compositionally different lavas, and the igneous processes that relate various basalt types. We base our study largely on major and trace elements, mineral compositions, and a small data set for Sr, Nd, and Hf isotopes.
Lower-section basalts include the Weekes Wash basalts, which are transitional between alkalic and tholeiitic (SiO2 49–51 wt. %) and with MgO 8.1–10.2 wt. %. They are associated with lavas that are andesitic (SiO2 ∼59–62 wt. %; MgO 8–2.5 wt. %) or are seemingly andesitic due to alteration and felsic xenocrysts. Weekes Wash basalts have incompatible-element abundances that correlate positively with MgO. Their87Sr/86Sr ratios are ∼0.705. Olivines, rarely fresh, have Fo86–89 cores, and clinopyroxenes have Mg#s 86–89. Overlying the Weekes Wash are the Cottonwood Spring basalts, which are alkalic (SiO2 ∼45.5–47.5 wt. %), and can be categorized into subgroups defined by lower and higher incompatible-element abundances, or low Ti-P and high Ti-P (e.g., Ba ∼1100 versus 1800 ppm; La ∼75 versus 110 ppm). The Cottonwood Spring basalts are the closest to primary lavas we observed (MgO 10.1–11.8 wt. %), and their87Sr/86Sr ratios are ∼0.705–0.706. Their olivines are Fo86–89, and their clinopyroxenes have Mg#s 86–90. Overlying the Cottonwood Spring basalts is the Apache Gap Fe-Ti-enriched basalt (TiO2 ∼2.6 wt. %), which has the lowest MgO (∼5.6–7.5 wt. %) and incompatible-element abundances observed for any basalts in the province (e.g., Ba ∼400 ppm; La ∼25 ppm). All lower-section basalts have primitive-mantle normalizations showing Nb-Ta negative anomalies.
Middle-section basalts erupted among silicic lava and pyroclastic flows from ∼19 to 18.5 Ma, and they compositionally resemble Weekes Wash basalts. Upper-section (post-18.5 Ma) lavas are the Willow Springs hawaiite (∼9.5 wt. % MgO;87Sr/86Sr ∼0.705) and Black Mesa basanite (SiO2 ∼44 wt. %; MgO ∼8 wt. %; CaO 14.7 wt. %;87Sr/86Sr ∼0.706). A Nb-Ta anomaly is clear in the hawaiite but weak in the basanite, as the basanite has the highest Nb and Ta observed (∼60 and ∼3 ppm; lowest Zr/Nb, ∼4 versus all others >7).
Relevant interpretations are the following. Absence of ultramafic mantle xenoliths in Goldfield-Superstition basalts suggests that magmas occupied crustal reservoirs. The two subgroups of the Cottonwood Spring basalts attest to small-scale trace element and isotopic heterogeneities in lithospheric mantle sources that, based on Nb-Ta and Ce versus Ce/Yb modeling, had subduction zone characteristics and were garnet bearing. For Weekes Wash basalts, the decreasing incompatible-element abundances with decreasing MgO is consistent with a hybrid origin by the low-Ti-P subgroup of Cottonwood Spring basalts having assimilated lower crust. Trace-element modeling based on assimilation fractional crystallization (AFC) demonstrates that mixing Cottonwood Spring basalt with ∼50 percent (partial) melts of lower-crust pyroxenite in proportions from 80:20 to 50:50 yields Weekes Wash basalt compositions.
Apache Gap lava represents AFC processes of high-Ti-P Cottonwood Spring basalts in a crustal reservoir, enabling Fe-Ti enrichment after ∼60 percent crystallization, as estimated by mass balancing, of mainly clinopyroxene and plagioclase. Upper-section hawaiite models as a differentiate after ∼50 percent crystallization of mainly clinopyroxene and plagioclase from a composition resembling low-Ti-P Cottonwood Spring basalt. Black Mesa basanite originated in a garnet-bearing lithospheric mantle source generally similar to those for Cottonwood Spring basalts, but additionally carbonitized (e.g., basanite has SiO2-undersaturation; high CaO) to yield magma relatively enriched in Nb and Ta. Basanite undersaturation is consistent with a smaller percent of source melting at the close of Goldfield-Superstition basaltic magmatism compared to initial source melting.
This study of the Goldfield-Superstition volcanic province demonstrates that magmas produced from Miocene lithospheric mantle spanning ∼2.5 million years took various paths, from erupting as nearly primitive lavas from their lithospheric sources, to interacting with lower crust where they assimilated, differentiated, and provided heat to create a silicic large igneous province. Finally, the basalts reflect some changes in source and melting characteristics over the time of their emplacements, but they sustained lithospheric-source and ancient subduction characteristics throughout.