23-4 Magmatic Sources and Processes in Southwest Utah across the Basin and Range/Colorado Plateau Transition Zone

Session: Distributed volcanic fields of the southwestern U.S. (Posters)

Poster Booth No.: 11

Presenting Author:

Zhiqing Su

Authors:

Su, Zhiqing¹, Valentine, Greg A.², Widom, Elisabeth³, Murta, Maria Parreira⁴, Kuentz, David C.⁵

(1) Miami University, Oxford, OH, USA, (2) University at Buffalo, Geology Department, Buffalo, NY, , (3) Miami University, Oxford, OH, , (4) University at Buffalo, Buffalo, NY, , (5) Miami University, Oxford, OH, ,

Abstract:

The Southwest Utah Volcanic Field, situated in the southwestern United States, lies at the transition between the Basin and Range and the Colorado Plateau. We have obtained major and trace element data together with Sr, Nd, Hf, Pb and Os isotopic compositions for lava samples spanning much of the transition zone. Although the major-element variations are consistent with fractional crystallization of olivine, pyroxene, plagioclase, and Fe–Ti oxides, isotopic variations cannot be explained by this process. A lack of correlation between Sr–Nd–Pb–Hf isotopic ratios and indices of fractionation (e.g., MgO) or crustal tracers (e.g., Nb/U, Ce/Pb) suggests that crustal assimilation did not play a significant role in the petrogenesis of the samples. Furthermore, assimilation–fractional crystallization (AFC) modeling using Sr and Nd isotopic compositions of both depleted and enriched mantle sources, combined with multiple potential Colorado Plateau crustal assimilants, fails to reproduce the isotopic signatures of most Southwest Utah samples. In addition, some samples exhibit highly unradiogenic Nd isotopic compositions that would require unrealistically large proportions of crust, inconsistent with mafic magmas based on simple mass balance considerations. Highly radiogenic ¹⁸⁷Os/¹⁸⁸Os ratios in many samples are likewise difficult to explain by crustal assimilation, and together with the highly unradiogenic Nd isotope ratios they instead indicate potential inheritance from an ancient, heterogeneous, metasomatized lithospheric mantle source. Although calculated melting depths are below the current lithosphere/asthenosphere boundary, a lithospheric mantle source for the southwest Utah magmas may reflect local lithospheric mantle remobilization. We interpret the Southwest Utah magmas to originate from ancient, metasomatically enriched lithospheric mantle that experienced long-term isotopic evolution, and subsequently foundered, producing present-day heterogeneity at asthenospheric depths beneath the Colorado Plateau. A lack of systematic isotopic variations among magmas sampled across the transition zone indicates that the mantle sources do not change progressively in this region, but rather that the enriched lithospheric source component is widely dispersed.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4, 2026

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