| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Journal of Petrology | Volume 44 | Number 6 | Pages 967-994 | 2003
© Oxford University Press 2003
The Effect of Reaction Overstep on Garnet Microtextures in Metapelitic Rocks of the Ilesha Schist Belt, SW Nigeria
1 MINERALOGISCHES INSTITUT DER UNIVERSITÄT WÜRZBURG, AM HUBLAND, D-97074 WÜRZBURG, GERMANY
2 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF CAMBRIDGE, DOWNING STREET, CAMBRIDGE CB2 3EQ, UK
Telephone: +49-931-888-5414. E-mail: armin.zeh{at}mail.uni-wuerzburg.de
Garnet-bearing assemblages of K-rich and K-poor metapelites from the Ilesha Schist belt, SW Nigeria, are investigated. K-rich samples contain the assemblages (A) garnet–staurolite–muscovite–chlorite–magnetite, (B) andalusite–garnet–staurolite–muscovite–chlorite–magnetite and (C) sillimanite–andalusite–garnet–muscovite–chlorite–magnetite. K-poor samples contain the assemblages (D) garnet–staurolite–cordierite–chlorite and (E) garnet–cordierite–chlorite ± staurolite. All assemblages contain quartz, plagioclase, biotite and ilmenite. P–T pseudosections calculated in the system CaO–Na2O–K2O–TiO2–MnO–FeO–MgO–Al2O3 –SiO2 –H2O ± O2 suggest peak metamorphism at 590 ± 20°C at 5 ± 0·5 kbar, followed by retrogression to 550°C at 3·0 kbar, in agreement with field evidence, domain assemblages, mineral compositions, modes and geothermobarometry. The absence of compositional zonation shows that garnet in all investigated rocks nucleated and grew at constant P–T–X in equilibrium with associated minerals on the thin-section scale. However, the garnet-in reaction did not begin until the establishment of a significant temperature overstep of 
80°C, with consequent rapid dehydration of chlorite- and muscovite-bearing assemblages resulting in interface-controlled garnet growth under locally hydrostatic pressure conditions. Garnet in rock types A and B formed with a characteristic pattern of biotite and quartz inclusions—a ‘house of cards’ texture. Additionally, clusters and aggregates of unzoned garnet grains formed in rock types A and D. Garnet resorption in rock types A–C resulted from prograde staurolite- and retrograde andalusite-forming reactions.
KEY WORDS: garnet microtexture; metapelite; THERMOCALC; pseudosection; reaction overstep; Nigeria
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  S. Chakraborty, S. Dasgupta, and S. Neogi Nucleation kinetics controlled by chemical overstepping and its tectonic implications: an example from the Sikkim Himalaya European Journal of Mineralogy, December 1, 2007; 19(6): 791 - 803. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. ZEH Calculation of Garnet Fractionation in Metamorphic Rocks, with Application to a Flat-Top, Y-rich Garnet Population from the Ruhla Crystalline Complex, Central Germany J. Petrology, December 1, 2006; 47(12): 2335 - 2356. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. A. Zuluaga, H. H. Stowell, and D. K. Tinkham The effect of zoned garnet on metapelite pseudosection topology and calculated metamorphic P-T paths American Mineralogist, October 1, 2005; 90(10): 1619 - 1628. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. ZEH, R. KLEMD, and J. M. BARTON JR Petrological evolution in the roof of the high-grade metamorphic Central Zone of the Limpopo Belt, South Africa Geological Magazine, May 1, 2005; 142(3): 229 - 240. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. ZEH Crystal Size Distribution (CSD) and Textural Evolution of Accessory Apatite, Titanite and Allanite during Four Stages of Metamorphism: an Example from the Moine Supergroup, Scotland J. Petrology, October 1, 2004; 45(10): 2101 - 2132. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
T. JOHNSON and M. BROWN Quantitative Constraints on Metamorphism in the Variscides of Southern Brittany--a Complementary Pseudosection Approach J. Petrology, June 1, 2004; 45(6): 1237 - 1259. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. ZEH, I. L. MILLAR, and M. S. A. HORSTWOOD Polymetamorphism in the NE Shackleton Range, Antarctica: Constraints from Petrology and U-Pb, Sm-Nd, Rb-Sr TIMS and in situ U-Pb LA-PIMMS Dating J. Petrology, May 1, 2004; 45(5): 949 - 973. [Abstract] [Full Text] [PDF]
|
|