Petrology of Stewart Mountain basalt field in central Arizona, U.S.A.: A lithospheric source with small-scale trace element and isotopic heterogeneities

Abstract

The Stewart Mountain basalt field in central Arizona is composed of three horizons of Miocene lavas over ~4 km 2 . The youngest lava is ~15.5 Ma. The field is in the southern Basin and Range at its transition to the Colorado Plateau. It is also at the northwestern margin of the ~8000 km 2 Goldfield-Superstition volcanic province (G-SVP), where basaltic lavas are ~20–19 Ma. Stewart Mountain basalts are alkalic, and most have from 6–8 weight percent (wt%) MgO, but more primitive and evolved lavas (10.7 and 4 wt% MgO, respectively) are also present. Most incompatible element abundances differ widely for basalts within the 6–8 wt% MgO range, and they distinguish the three horizons (e.g., ranges for P 2 O 5 are 0.5–1.4 wt%; Zr 125–250 ppm; La 40–80 ppm). One lava has quartz and plagioclase xenocrysts and even lower incompatible element abundances (e.g., P 2 O 5 0.25 wt%; La 25 ppm). All Stewart Mountain basalts, however, have Nb-Ta negative anomalies, consistent with a lithospheric mantle source that had subduction characteristics. Isotopic compositions differ across the three basalt horizons (e.g., ranges for 87 Sr/ 86 Sr are 0.7049–0.7061; 206 Pb/ 204 Pb 17.7–19.2; e Nd -3.5 to -6.2), where the xenocrystic lava has the lowest Sr and Pb isotopic ratios. Over its life, the Stewart Mountain field radiogenic isotope ratios decreased to reflect source heterogeneities, and its 206 Pb/ 204 Pb range is as wide as that formed by Oligocene–Miocene basalts collectively across the southern Basin and Range and transition zone. Incompatible-element abundances and ratios also reflect source heterogeneities, whereby the greatest differences are observed as abundances decreasing from middle to upper horizon basalts. Several abundance ratios, such as Zr/Nb, Th/Ta, Th/Nb, and Zr/Hf, record some of the source heterogeneities that are manifested over the short geologic time represented by the successive lava horizons. These temporal compositional changes likely reflect partial melts from a variably metasomatized lithospheric mantle. Compared to the compositions of the older, neighboring G-SVP basalts, Stewart Mountain lavas are generally evolved (MgO <8 wt%). The absence of mantle xenoliths in any Stewart Mountain lava and the xenocrystic lava both point to the compositional evolution having occurred in crustal reservoirs; however, based on the lowest isotopic ratios present in the xenocrystic lava, the upper crust was not a reservoir. Comparing Stewart Mountain basalt incompatible-element abundance ratios to those in the neighboring G-SVP shows enough difference to conclude that these two Miocene basalt localities had lithospheric sources with distinct trace element characteristics. The G-SVP source also had higher, distinguishing e Nd (-1 to -2). All characteristics combined, the Stewart Mountain field shows that lithospheric source heterogeneities can be manifested both temporally and spatially over only a small surface area. Stewart Mountain lithospheric source indicates that magmatism in central Arizona did not have asthenospheric sources by 15 Ma.