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Sodic amphiboles in fenites from the Loe Shilman carbonatite complex, NW Pakistan

Published online by Cambridge University Press:  05 July 2018

I. Mian  and
M. J. Le Bas
I. Mian
Affiliation:
Department of Geology, The University, Leicester LE1 7RH, UK
M. J. Le Bas
Affiliation:
Department of Geology, The University, Leicester LE1 7RH, UK
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Abstract

The carbonatites at Loe Shilman, near Khyber in NW Pakistan, fenitize their country rocks to form a metasomatic zone c.100 m wide of alternate dark blue (mafic) and pale grey (felsic) banded fenites which grade into unfenitized bedded slates and phyllites. The Na-amphiboles in the banded fenites form a complete solid solution series between magnesio-arfvedsonite and magnesio-riebeckite which coexist with varying proportions of aegirine, albite, and K-feldspar, with or without phlogopite or biotite.

The amphiboles show a gradual decrease in Na2O, K2O, Mg ratio [100Mg/(Mg + FeT + Mn)] and iron oxidation ratio, and an increase in total iron away from the carbonatite contact. The pleochroism correlates with the chemistry and distance from the carbonatite contact.

The Mg ratio decreases from 74 to 35 away from the carbonatite contact. The iron oxidation ratio [100Fe3+/ (Fe3+ + Fe2+)] decreases in the magnesio-arfvedsonite for the first 30 metres from the carbonatite contact, and then increases in the magnesio-riebeckite from 40 to 60 metres from the carbonatite contact. K relative to Na decreases away from the contact in the amphibole, and the decrease in K causes an increase in vacancy in the A site. The main variation in the chemistry in this solid solution series is due to (K,Na)A+(Mg,Fe2+)c ⇌ □ + (Fe3+)c substitution.

Keywords

amphibolesfenitescarbonatitesLoe ShilmanPakistan
Type
Petrology
Information
Mineralogical Magazine , Volume 50 , Issue 356 , June 1986 , pp. 187 - 197
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1986

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