Garnetiferous Ultramafic Inclusions in Minette from the Navajo Volcanic Field

  1. F.R. Boyd and
  2. Henry O.A. Meyer
  1. Stephen N. Ehrenberg

Published Online: 19 MAR 2013

DOI: 10.1029/SP016p0330

The Mantle Sample: Inclusion in Kimberlites and Other Volcanics

The Mantle Sample: Inclusion in Kimberlites and Other Volcanics

How to Cite

Ehrenberg, S. N. (1979) Garnetiferous Ultramafic Inclusions in Minette from the Navajo Volcanic Field, in The Mantle Sample: Inclusion in Kimberlites and Other Volcanics (eds F.R. Boyd and H. O.A. Meyer), American Geophysical Union, Washington, D.C.. doi: 10.1029/SP016p0330

Author Information

  1. Department of Earth and Space Sciences, University of California, Los Angeles, California 90024

Publication History

  1. Published Online: 19 MAR 2013
  2. Published Print: 1 JAN 1979

ISBN Information

Print ISBN: 9780875902135

Online ISBN: 9781118664858

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Keywords:

  • Garnetiferous ultramafic inclusions;
  • Megacrystalline rocks;
  • Minette host rock;
  • Navajo volcanic field;
  • Peridotite xenoliths;
  • Xenolith equilibration

Summary

Ultramafic xenoliths at The Thumb, a small minette neck near Shiprock, New Mexico, are divisible into three overlapping textural/compositional groups: coarse garnet peridotites with minerals relatively low in Fe and Ti and high in Cr, sheared garnet peridotites with minerals relatively high in Fe, Ti, and Cr, and megacrystalline rocks (medium- to ultracoarse- grained intergrowths of clinopyroxene, orthopyroxene, olivine, garnet, phlogopite, and ilmenite) with minerals very rich in Fe and Ti and low in Cr. The megacrystalline rocks are compositionally similar to the “discrete nodule” suite of Lesotho kimberlites, but are fragments of polycrystalline aggregates rather than phenocryst-like single crystals. Pyroxene compositions indicate that the inclusions equilibrated at temperatures ranging from 930–1230°C within a narrow vertical interval at around 130 km depth.

The extreme textural and compositional heterogeneity of this restricted source terrane and the correlation between xenolith textures and mineral compositions are explained by the following model: The megacrystalline rocks are interpreted as precipitates from liquids introduced into an older, previously depleted terrane consisting of the coarse garnet peridotites. The Ti-Fe-rich sheared peridotites are thought to be the products of deformation and Ti-Fe-metasomatism of the coarse peridotite country rocks in tectonically disrupted aureoles surrounding the intrusive megacrystalline regions. The suggested metasomatizing medium is a residual liquid intermediate in composition between clinopyroxene and ilmenite.

The model for The Thumb's inclusion suite may also be extended to explain the texturally and compositionally analogous suite of xenoliths described by Boyd and Nixon (1974) from Lesotho kimberlites. The Lesotho sheared garnet peridotites are viewed as the products of extensive metasomatism of previously less enriched mantle in response to intrusion of magma from which the discrete nodule association crystallized.