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Small masses of intraplate mafic plutonic and volcanic rocks are exposed in the Navajo volcanic field (26-24 Ma), Dulce dike swarm (25-20 Ma), and western San Juan Mountains (7-0.6 Ma) along the eastern boundary of the Colorado Plateau. Geochemical and Sr-Nd isotopic data for these rocks were employed to assess the time-space variations in melt compositions and to investigate the contributions from different mantle and crustal sources during magma production. Geochemical data signify that post-26 Ma mantle melts produced in the Four Corners region were heterogeneous in composition, dominantly alkaline, and distinguished by trace-element patterns with elevated LILE and LREE. These magmas were generated by melting of metasomatized subcontinental lithospheric mantle (SCLM) with minor contributions from a mantle source that produced melts with geochemical affinities of oceanic island basalts. Nd and Sr isotopic data reveal that some melts could have experienced minor contamination (<1%) with upper crust, whereas the Dulce magmas formed by contamination of mantle melts with up to 40% lower crust or melting of SCLM with low time-integrated Sm/Nd. Relative to minettes in the Navajo volcanic field, mafic rocks in the western San Juan Mountains have similar Nd and Sr isotopic compositions but are lower in some chemical values (Mg#, K 2 O, Rb, Zr, La/Lu, La/Ta, and Th/Yb) and higher in others (Al 2 O 3 , Na 2 O, and CaO). We postulate that these different chemical traits are evidence of heterogeneity in the lithospheric mantle. Voluminous magma production involving mantle melts from 35 to 26 Ma could have modified the chemistry of the lithospheric mantle beneath the western San Juan Mountains. Alternatively, this region may be underlain by SCLM with distinct chemical attributes imparted by subduction processes during Proterozoic arc accretion. Our work provides a snapshot of broader regional trends wherein heterogeneity in mantle reservoirs was a major contributor to the variable compositions of mafic alkaline rocks produced over the Colorado Plateau and adjacent Southern Rocky Mountains after 26 Ma. REGIONAL GEOLOGIC SETTING Latest Mesozoic to Cenozoic magmatism on the Colorado Plateau and ad jacent Southern Rocky Mountains (Fig. 1) is attributed to shallow subduction of the Farallon plate during the Laramide orogeny, followed by slab rollback and regional Neogene extension (e.g.

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