EARLY (MIOCENE) BASALTS OF THE JEMEZ VOLCANIC FIELD, NEW MEXICO (USA): A MODEL FOR CONTINENTAL MAGMATISM

Early (~14-7 Ma) rocks of the Jemez volcanic field (JVF) comprise a range of tholeiitic and alkalic primary compositions, with associated intermediate and silicic derivatives (Lobato Formation). Basaltic rocks are most voluminous in the northern volcanic field, where they include several broad, low-relief shield volcanoes not recognized in the original mapping. These shield edifices are now partially dissected by faulting and erosion, which in some cases expose the subvolcanic intrusions, and are obscured by heavy vegetation. The basalts are important in elucidating the various magmatic processes by which a long-líved continental volcanic field develops and evolves.

Nd and Sr isotopic data for basalts are compatible with an origin from compositionally heterogeneous, Proterozoic (1.8-1.6 Ga) lithospheric mantle (E_Nd ~0 to + 2.2). Most of the basaltic and related rocks contain evidence for lower crustal contamination, with E_Nd +1.1 to -5.5 and ⁸⁷Sr/⁸⁶Sr 0.70405-0.70455; at least some are contaminated with upper crust (⁸⁷Sr/⁸⁶Sr up to about 0.706). Small volumes of dacitic to rhyolitic rocks that accompany the basalts have Nd identical with the inferred mantle source region, indicating that they were derived by fractional crystallization from mantle-generated magmas and/or by melting of young lower crustal mafic rocks (with mantle signatures). High La/Yb (36) suggests that garnet may be present in the source, in which case the melt must be derived from the base of the crust (12-15 kb).

An origin for the evolved rocks dominantly by fractional crystallization (single-stage model) implies that significant deposits of ultramafic cumulates must be present in the lower crust (but magma mixing/mingling and crustal contamination also played important roles). The developing Española basin of the Rio Grande rift could easily accommodate significant volumes of cumulates in the extending crust. Alternatively, melting of lower crust (two-stage model) implies that a mafic protolith was already present, presumably from early magmatism associated with the JVF, which was melted by subsequent intrusions. These results demonstrate that considerable interaction with lower to upper crust occurred early in the development of the volcanic field.