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Journal of Petrology | Volume 40 | Number 4 | Pages 653-678 | 1999
© Oxford University Press 1999

The Chemical and Isotopic Differentiation of an Epizonal Magma Body: Organ Needle Pluton, New Mexico

P. L. Verplanck1,*, G. L. Farmer1,{dagger}, M. McCurry2 and S. A. Mertzman3

1 Cooperative Institute for Research in Environmental Sciences (Cires) and Department of Geological Sciences Campus Box 216, University of Colorado, Boulder, CO 80309, USA
2 Department of Geology, Idaho State University Pocatello, Id 83209, USA
3 Department of Geosciences, Franklin and Marshall College Lancaster, PA 17604, USA

Received March 21, 1997; Revised typescript accepted September 30, 1998


  Abstract

Major and trace element, and Nd and Sr isotopic compositions of whole rocks and mineral separates from the Oligocene, alkaline Organ Needle pluton (ONP), southern New Mexico, constrain models for the differentiation of the magma body parental to this compositionally zoned and layered epizonal intrusive body. The data reveal that the pluton is rimmed by lower {varepsilon}Nd (~ –5) and higher 87Sr/86Sr (~0.7085) syenitic rocks than those in its interior ({varepsilon}Nd ~ –2, 87Sr/86Sr ~0.7060) and that the bulk compositions of the marginal rocks become more felsic with decreasing structural depth. At the deepest exposed levels of the pluton, the {varepsilon}Nd ~ –5 lithology is a compositionally heterogeneous inequigranular syenite. Modal, compositional and isotopic data from separates of rare earth element (REE)-bearing major and accessory mineral phases (hornblende, titanite, apatite, zircon) demonstrate that this decoupling of trace and major elements in the inequigranular syenite results from accumulation of light REE (LREE)-bearing minerals that were evidently separated from silicic magmas as the latter rose along the sides of the magma chamber. Chemical and isotopic data for microgranular mafic enclaves, as well as for restite xenoliths of Precambrian granite wall rock, indicate that the isotopic distinction between the marginal and interior facies of the ONP probably reflects assimilation of the wall rock by {varepsilon}Nd ~ –2 mafic magmas near the base of the magma system. Fractional crystallization and crystal–liquid separation of the crustally contaminated magma at the base and along the margins of the chamber generated the highly silicic magmas that ultimately pooled at the chamber top.

KEY WORDS: assimilation; enclave; epizonal pluton; isotope geochemistry; trace element geochemistry

{dagger} Corresponding author. Telephone: 303-492-6534. Fax: 303-492-1149. e-mail: farmer{at}terra.colorado.edu

* Present address: US Geological Survey, 3215 Marine Street, Boulder, CO 80303, USA.


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