Miocene Magmatism in the Southwestern Basin and Range Province: Mineralogy, Petrology, and Geochemistry of the Stewart Mountain Basalt Field, Central Arizona

isotopic geochemistry;central Arizona;Basin and Range Province;trace element geochemistry;igneous petrology;Stewart Mountain Basalt Field;Miocene magmatism;basalts

Singer, Kaitlin Iris ; Dr. R.V. Fodor, Committee Chair,Dr. Elana L. Leithold, Committee Member,Dr. James P. Hibbard, Committee Member,Singer, Kaitlin Iris ; Dr. R.V. Fodor ; Committee Chair ; Dr. Elana L. Leithold ; Committee Member ; Dr. James P. Hibbard ; Committee Member

University:North Carolina State University

关键词: isotopic geochemistry;    central Arizona;    Basin and Range Province;    trace element geochemistry;    igneous petrology;    Stewart Mountain Basalt Field;    Miocene magmatism;    basalts;   

Others  :  https://repository.lib.ncsu.edu/bitstream/handle/1840.16/97/etd.pdf?sequence=1&isAllowed=y

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【 摘 要 】

Miocene basaltic magmas were emplaced across southwestern North America contemporaneous with extension in the Basin and Range Province.Over recent decades, regional geochemical studies were conducted to determine the origin of these basalts with respect to tectonic extension, but little has been done to help understand the details of source region compositions, and relationships among the lavas on local scales.The basalts of the Stewart Mountain basalt field (SMBF) in central Arizona, a small (~3.5 km2) area within the southern Basin and Range Province, offers a good opportunity to examine the petrogenesis and mantle sources of extension-related basalts on a local scale.The SMBF lavas represent five petrographic types all with a modal mineralogy containing olivine.Most of the lavas have ~44.5-49 wt. % SiO2 and ~0.85-2.10 wt. % K2O, which characterize them as alkalic basalts, and range from ~4.9-10.7 wt. % MgO.Two quartz-bearing samples with ~54 wt. % SiO2 plot as tholeiitic basaltic andesite.Based on the incompatible element abundances, the SMBF basalts can be divided into three compositional groups: high incompatible element (~0.86-1.37 wt. % P2O5; ~71-88 ppm La), low incompatible element (~0.56-0.65 wt. % P2O5; ~45-58 ppm La), and very low incompatible element group (~0.26 wt. % P2O5; ~33 ppm La) that is confined to the quartz-bearing samples.The 87Sr/86Sr ratios define a high and low Sr ratio group from ~0.7056-0.7060 and from ~0.7049 to 0.7050, respectively, which correspond to the high and low incompatible element groups.In general, the Pb isotopic ratios (206Pb/204Pb~17.7-19.2; 207Pb/204Pb~15.5-15.7; 208Pb/204Pb~37.4-39.1) also correspond to the 87Sr/86Sr ratios and incompatible element high and low groups.Mineral compositions for the SMBF basalts include phenocrysts of olivine that range from ~Fo81 to Fo84 in core values, high-Al2O3 (4.0-6.8 wt. %) clinopyroxene (Mg#s ~78-81; ~Wo47Fs11), and plagioclase of ~An87-69, with exception of xenocrysts of plagioclase with ~An40 cores in the quartz-bearing basalts.Whole-rock, isotopic, and mineral compositions of the SMBF lavas display geochemical and isotopic variations over a relatively small area and lead to the following interpretations.Within compositional groups, the basalts can be related by fractional crystallization of olivine, clinopyroxene, and plagioclase in varying percentages.Among the high and low incompatible element groups, similar trace element ratios, namely La/Ba and Ba/Nb, suggest the basalts are related by different percentages of partial melting of the same source.However, there are some variations among the Sr and Pb isotopic ratios to suggest that despite close temporal and spatial relationships of the lavas, the mantle source for the SMBF lavas appears to have had small scale heterogeneity.Based on the negative Nb-Ta anomalies, enriched light-REE, high La/Nb and Ba/Nb ratios, and isotopic ratios, the SMBF mantle source was an enriched subcontinental lithosphere with subduction zone characteristics and some isotopic heterogeneity.The local occurrences of quartz–bearing basalts suggest localized crustal contamination.This contamination appears to have not affected isotopic ratios, possibly because contamination occurred in the lower crust (e.g., low Pb ratios).SMBF lavas are comparable to other Miocene basalts in the transition zone between the southern Basin and Range and Colorado Plateau basalts, all characterized by Ba/Ta>450, Ba/Nb>28, low 206Pb/204Pb (~17.5-19.2), high 87Sr/86Sr (~0.705-0.707), and Nb depletions suggesting a subduction-related enriched lithospheric source across this tectonic transition zone.The SMBF reveals evidence for varying degrees of partial melting of that lithospheric mantle, and residence in lower crustal reservoirs to differentiate and assimilate continental crust.