PETROGENESIS OF THE MAFIC AND INTERMEDIATE MAGMAS AT THE MCDERMITT CALDERA CENTER, NORTHERN NEVADA AND SOUTHERN OREGON

We present new data and a first-order petrogenetic model for the mafic and intermediate magmas of the 16.7 to 16.3 Ma McDermitt caldera, a polygenetic eruptive center near the southern extent of the voluminous Columbia River Basalt Group (CRBG). Steens basalts, likely erupted from vents to the north, form the base of the mid-Miocene volcanic section at McDermitt. Local magmatism began no later than 16.69 ± 0.02 Ma and peaked with the 16.35 ± 0.03 Ma caldera-forming eruption of the ~ 400 km³ McDermitt tuff, with smaller eruptions resuming shortly thereafter. Mafic and intermediate lavas were erupted at McDermitt for the duration of magmatism and include basalts, icelandites, and mildly alkaline intermediate-composition lavas. Radiogenic isotopic compositions for the McDermitt magmas (⁸⁷Sr / ⁸⁶Sr_i = 0.704007 - 0.704530; ¹⁴³Nd / ¹⁴⁴Nd_i = 0.512925 - 0.512766) resemble the Imnaha (0.703782 - 0.70487; 0.512880 - 0.512741) more than the Steens magmas (0.703331 - 0.703842; 0.513167 - 0.512871). However, chemical and field evidence strongly suggest an evolutionary path for the McDermitt magmas with an origin at the Steens basalt source. AFC processes, using Cretaceous granitoids as assimilants, can accurately model the McDermitt magma evolution from Steens basalt to evolved intermediates (> 67 wt. % SiO₂). This relatively simple petrogenesis of the McDermitt magmas stands in contrast to the complex models for the nearby Santa Rosa volcanic field (Brueseke & Hart, 2009), which require multiple evolutionary mechanisms (e.g., AFC, partial melting of granitoids, magma mixing) to form the varied compositions of intermediate lavas found there. Our findings are consistent with a petrogenetic model involving Steens magmas underplating Cretaceous granitoids early in the CRBG main-stage magmatic episode and undergoing continuous AFC-dominated evolution to form the mafic and intermediate magmas at the McDermitt caldera center.

Brueseke & Hart, 2009, JVGR v. 188, p. 197 - 213.