Petrogenesis of Pre-caldera mafic lavas, Jemez Mountains volcanic field

Abstract

The Miocene-Quaternary Jemez Mountains volcanic field (JMVF), the site of the Valles caldera, lies at the intersection of the Jemez lineament, a Proterozoic suture, and the Cenozoic Rio Grande rift. Parental magmas are of two types: K-depleted silica-undersaturated, derived from the partial melting of lithospheric mantle with residual amphibole, and tholeiitic, derived from either asthenospheric or lithospheric mantle. Variability in silica-undersaturated basalts reflects contributions of melts derived from lherzolitic and pyroxenitic mantle, representing heterogeneous lithosphere associated with the suture. The K depletion is inherited by fractionated, crustally contaminated derivatives (hawaiites and mugearites), leading to distinctive incompatible trace element signatures, with Th/(Nb,Ta) and La/(Nb,Ta) greater than, but K/(Nb,Ta) similar to, Bulk Silicate Earth. These compositions dominate the mafic and intermediate lavas, and the JMVF is therefore derived largely, and perhaps entirely, from melting of fertile continental Jemez lineament lithosphere during rift-related extension. Significant variations in Pb and Nd isotope ratios ( (super 206) Pb/ (super 204) Pb = 17.20-18.93; (super 143) Nd/ (super 144) Nd = 0.51244-0.51272) result from crustal contamination, whereas 87Sr/86Sr is low and relatively uniform (0.7040-0.7048). We compare the effects of contamination by low- (super 87) Sr/ (super 86) Sr crust with assimilation of high- (super 87) Sr/ (super 86) Sr granitoid by partial melting, with Sr retained in a feldspathic residue. Both models satisfactorily reproduce the isotopic features of the rocks, but the lack of a measurable Eu anomaly in most JMVF mafic lavas is difficult to reconcile with a major role for residual plagioclase during petrogenesis.

41 samples from this reference