Journal of Petrology Advance Access originally published online on December 10, 2004
Journal of Petrology 2005 46(2):407-439; doi:10.1093/petrology/egh082
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Journal of Petrology vol. 46 issue 2 © Oxford University Press 2004; all rights reserved
Petrogenesis of Pre-caldera Mafic Lavas, Jemez Mountains Volcanic Field (New Mexico, USA)
1 DEPARTMENT OF GEOLOGY, WASHINGTON STATE UNIVERSITY, PULLMAN, WA 99164, USA
2 DEPARTMENT OF GEOSCIENCES, OREGON STATE UNIVERSITY, 104 WILKINSON HALL, CORVALLIS, OR 97331, USA
3 DEPARTMENT OF GEOLOGICAL AND ENVIRONMENTAL SCIENCES, CALIFORNIA STATE UNIVERSITY, CHICO, CA 95929, USA
4 EARTH AND ENVIRONMENTAL SCIENCES, LOS ALAMOS NATIONAL LABORATORY, LOS ALAMOS, NM 87545, USA
5 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF CALIFORNIA, SANTA CRUZ, CA, 95064, USA
6 DEPARTMENT OF GEOLOGY, McMASTER UNIVERSITY, 1280 MAIN STREET WEST, HAMILTON, ONTARIO L8S 4M1, CANADA
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 (206Pb/204Pb = 17·20–18·93; 143Nd/144Nd = 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-87Sr/86Sr crust with assimilation of high-87Sr/86Sr 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.
KEY WORDS: Jemez Mountains volcanic field; Rio Grande rift; lithospheric mantle; crustal contamination; trace elements; radiogenic isotopes
* Corresponding author. Telephone: +1 509 335 2825. Fax: +1 509 335 7816. Email: jawolff{at}mail.wsu.edu
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