Identifying Metasomatized Continental Lithospheric Mantle Involvement in Cenozoic Magmatism From Ta/Th Values, Southwestern North America

Abstract

The accessory minerals rutile and apatite are rare or absent in the convecting upper mantle but occur in shallow, cooler, metasomatized continental lithospheric mantle (CLM) where they serve as carrier phases for the trace elements Ta (in rutile) and Th (in apatite). Because both minerals crystallize near-solidus and are eliminated early during partial mantle melting, the relative abundances of rutile and apatite should control the Ta and Th abundances of mantle melts and provide a means of identifying the involvement of rutile-and/or apatite-bearing metasomatized CLM in mafic continental magmatism. As a test, we investigated published Ta and Th abundances data from~2,000 whole-rock samples of mafic to intermediate composition, Cenozoic volcanic rocks in southwestern North America. Roughly half of the samples have Ta/Th values similar to those of island arc volcanic rocks (<0.2) or ocean island and mid-ocean ridge basalts (>0.6). The remaining samples have intermediate and variable Ta/Th values between 0.2 and 0.6, independent of specific indices of crustal interaction (e.g., wt% P 2 O 5 /wt% K 2 O). We interpret the intermediate Ta/Th rocks as the products of direct melting of, or of extensive melt-rock interaction with, rutile-and/or apatite-bearing CLM. Intermediate Ta/Th rocks also have uniformly high 87 Sr/ 86 Sr (0.706 to 0.708) compared to oceanic basalts that, unlike their Nd isotopic compositions, do not covary with lithospheric age. These observations are consistent with widespread metasomatism of the CLM by Sr-rich, Nd-poor, aqueous fluids generated by dehydration of oceanic lithosphere, and its overlying tectonic mélange during early Cenozoic subduction beneath southwestern North America.Plain Language Summary Volcanism occurred throughout southwestern North America (SWNA) over the past 40 million years, but exactly what melted to produce the resulting volcanic rocks is unclear. We address this issue by investigating the relative amounts of the elements tantalum (Ta) and thorium (Th) in these volcanic rocks. In oceanic settings, volcanic rocks have well-defined Ta/Th values either from~0.6 to~1 for oceanic islands and ocean floor basalts or <0.2 for island-arc volcanoes. But in SWNA, approximately 40% of the~2,000 volcanic rocks for which Ta and Th abundances are available have variable and intermediate Ta/Th values (0.2 to 0.6) that occur independently of chemical indicators used to identify interaction between continental crust and ascending magmas. We suggest that the intermediate Ta/Th values reflect melting of mantle directly beneath the continent that contains minerals that concentrate Ta and Th, such as rutile (a titanium oxide) and apatite (a calcium phosphate). Both minerals can precipitate from water-rich fluids that cool when infiltrating and altering the continental mantle but readily melt upon subsequent heating. We conclude that hydrated, rutile-and/or apatite-bearing continental mantle was widespread beneath SWNA and was an important contributor to the magmas from which the intermediate...

Quaternary basaltic volcanic fields of the American Southwest

“…Mantle source compositions for the mafic volcanoes are heterogeneous on a range of scales, from ~500 m, based upon data from closely spaced volcanoes (Rasoazanamparany et al, 2015), to regional scales with contrasting lithospheric and asthenospheric sources (e.g., Reid et al, 2012;Cousens et al, 2013). The heterogeneities result, at least in part, from fluids emplaced in the upper mantle (metasomatism) during the long early Proterozoic to early Paleogene history of subduction events off what is now the west coast of North America, as well as longer-term recycling of oceanic crust within the mantle (e.g., Reid et al, 2012;Rasoazanamparany et al, 2015;Farmer et al, 2020). A range of geodynamic models has been explored as explanations for basaltic volcanism in the Southwest and, in particular, its concentration along the margins of the Colorado Plateau and Sierra Nevada (Table 3).…”

Squeezing Water From a Stone: H2O In Nominally Anhydrous Minerals From Granulite Xenoliths and Deep, Hydrous Fractional Crystallization

“…Low‐angle subduction induced significant fluid metasomatism and hydration of the deep lithosphere across much of the western United States (Humphreys et al, 2003). The impact of deep lithospheric hydrous metasomatism is manifested by amphibole and biotite‐rich lower crustal xenoliths erupted in 3.0 to 0.89 Ma lamproites (Lange et al, 2000) to the northwest of the Devonian‐age State Line kimberlites (Leucite Hills; Figure 1), as well as distinctive geochemical signatures in Cenozoic volcanic rocks throughout the southwestern United States ascribed to melting of hydrous metasomatized continental lithosphere (Farmer et al, 2020). Importantly, the xenoliths from the State Line kimberlites do not show evidence for hydrous metasomatism (Farmer et al, 2005), and their Devonian eruption ages means they were never impacted by the hydrous overprint of the Laramide Orogeny.…”

Supplemental Material: Quaternary basaltic volcanic fields of the American Southwest

“…1) erupted through older Archean basement, while Quaternary magmas in the northwestern part of the study region erupted through much younger, Mesozoic crust. These different lithospheric/crustal domains contribute to the geochemical signatures of basaltic rocks and aid in interpreting the roles of lithospheric versus asthenospheric partial melting in generating the magmas (e.g., Perry et al, 1988;Farmer et al, 1989Farmer et al, , 2020Crow et al, 2011).…”