Contributions to Mineralogy and Petrology

, Volume 95, Issue 1, pp 32–43 | Cite as

Monzo-anorthosite from the Tagueï ring-complex, Aïr, Niger: a hybrid rock with cumulus plagioclase and an infiltrated granitic intercumulus liquid?

  • Christian Moreau
  • William L. Brown
  • Jean -Paul Karche


An unusual hybrid rock composed of cumulus plagioclase with the interstices occupied by abundant quartzo-feldspathic material occurs in the Tagueï ring complex, Niger. The monzo-anorthosite consists of about 75% plagioclase in large tabular to elongate crystals with interstices occupied in two stages, firstly by clinopyroxene, titanomagnetite and apatite and then by an intergrowth of quartz and alkali feldspar associated with brown-green amphibole and zircon. Secondary green amphibole, chlorite, epidote and calcite may occur. Four stages in the crystallization history were identified:
  1. (1)

    Cumulus stage represented by the cores of the plagioclase laths (An56-An66 with reversed oscillatory zoning) and rare clinopyroxene (T ∼1,150° C),

  2. (2)

    Early intercumulus stage with a wide overgrowth zone on plagioclase (An62-An15 with oscillatory zoning and increase in Or), clinopyroxene, apatite, titanomagnetite (T ∼1,150-1,050° C),

  3. (3)

    Late intercumulus stage with alkali feldspar, quartz, opaque oxide, brown-green amphibole, apatite and zircon (T ∼750-700° C). Alkali feldspar gave cryptoperthite on further cooling.

  4. (4)

    Deuteric stage with development of turbidity in the alkali feldspar and plagioclase rims, and formation of patch perthite with microcline and secondary minerals (T ∼400° C).


The contrast in mineralogy, the large gap of ∼300° C between the early and late cumulus stages and the great abundance of quartz and alkali feldspar (∼20%) suggest that the late-stage liquid of granitic composition which filled the interstices was not a simple residual liquid which crystallized in situ. From chemistry (including REE) it is almost identical to the later radial granite dykes. From gravity measurements the intrusion has the form of a pipe with less dense rocks below the present exposure level. We propose that a pulse of granite magma rose within the pipe just before complete consolidation of the leucogabbro and replaced by infiltration the denser residual intercumulus liquid.


Apatite Oscillatory Zoning Granite Magma Alkali Feldspar Plagioclase Lath 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


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Copyright information

© Springer-Verlag 1987

Authors and Affiliations

  • Christian Moreau
    • 1
  • William L. Brown
    • 2
  • Jean -Paul Karche
    • 3
  1. 1.Département de GéologieFaculté des SciencesDakar-FannSénégal
  2. 2.CRPGVandoeuvre-lès-Nancy CédexFrance
  3. 3.Faculté des SciencesLaboratoire de PétrographieBesançon CédexFrance

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