PETROLOGY AND MAGMA GENESIS OF A POST-MIOCENE BASALT FLOW AT SPECIE MESA, SOUTHWESTERN COLORADO

The Cenozoic magmatic record in the Four Corners region was dominated by emplacement of felsic to intermediate plutons, and large-scale caldera eruptions from 28 to 26 Ma. Previous studies documented a record of mantle magmatism from 26 to 4 Ma, manifested as diatreme-dike complexes and dike swarms across the region. At Specie Mesa, near Placerville, the eroded remnant of a Quaternary basaltic lava flow marks the last pulse of mafic magmatism in the western San Juan Mountains. This flow is not widely known, and had not been studies in any detail, yet it represents an important part of the post-Pliocene magmatic history. Our research was focused on characterizing the petrology and geochemistry of this flow in order to understand how it fit into the regional mantle magmatic record after 26 Ma.

The Specie Mesa flow is composed of alkaline olivine-pyroxene basalt that is classified as tephrite on the TAS diagram. Bulk-rock chemical analyses reveal that these rocks have notably lower Mg#, CaO, and K₂O and higher Na₂O relative to 26 to 24 Ma mafic rocks in the region. Specie Mesa samples are characterized by low compatible and incompatible trace elements (Nb, Rb, Ba, Zr, Ni, Cr) against La/Lu, similar to chemical trends for 7 to 4 Ma mafic dikes exposed in the western San Juan Mountains. The chondrite-normalized rare earth element patterns of the flow rocks show enrichment in LREE with relatively flat HREE patterns, while MORB-normalized multi-element plots show enrichment in incompatible LILE with subtle Zr-Hf anomalies. ⁸⁷Sr/⁸⁶Sr_(i) values are 0.70481 to 0.70483 and e_Nd(t) range from 0.5 to -0.2, indicate of an SCLM source with no crustal contamination.

The Specie Mesa basalt erupted along a series of northwest-trending faults that are related to incipient rifting in the western San Juan Mountains. On the basis of chemical and isotopic data we argue that the flow was formed by partial melting of a “depleted” lithospheric mantle that had undergone extensive melting from 28 to 26 Ma. This “depleted” SCLM was the dominant source of mantle magmas from 7 to 1 Ma in the western San Juan Mountains. Eruption of the Specie Mesa basalt preceded extensive erosion and landscape dissection of a westward flowing river system in the Holocene (gravel deposits in Disappointment Valley), as indicated by the presence of basalt clasts in the gravels.