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

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Authors:Mian, I.; Le Bas, M. J.
Author Affiliations:Primary:
The Univ., Dep. Geol., Leicester, United Kingdom
Volume Title:Mineralogical Magazine
Source:Mineralogical Magazine, 50 Part 2(356), p.187-197. Publisher: Mineralogical Society, London, United Kingdom. ISSN: 0026-461X
Publication Date:1986
Note:In English. 22 refs.; illus. incl. 3 tables
Summary: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. [R.A.H.]
Subjects:Amphibole group; Carbonatites; Chain silicates; Chemical composition; Clinoamphibole; Fenite; Fenitization; Igneous rocks; Magnesioriebeckite; Metamorphic rocks; Metasomatic rocks; Metasomatism; Mineral composition; Petrology; Processes; Silicates; Solid solution; Asia; Indian Peninsula; Pakistan; Clinoamphobole; Kabul River region; Khyber Pass; Loe Shilman; Magnesio-arfvedsonite; North West Frontier Province
Abstract Numbers:86M/3531
Record ID:1986086925
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland
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