A sulphur isotope study of iron sulphides in the late Precambrian Dalradian Easdale slate formation, Argyll, Scotland

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Authors:Hall, A. J.; Boyce, A. J.; Fallick, A. E.
Author Affiliations:Primary:
Univ. Strathclyde, Dep. Appl. Geol., Glasgow, United Kingdom
Scot. Univ. Res. and React. Cent., United Kingdom
Volume Title:Mineralogical Magazine
Source:Mineralogical Magazine, 52 Part 4(367), p.483-490. Publisher: Mineralogical Society, London, United Kingdom. ISSN: 0026-461X
Publication Date:1988
Note:In English. 16 refs.; illus. incl. 2 tables, sketch map
Summary:Pyritic slates from the late Precambrian, Middle Dalradian Argyll Group Easdale Slate Formation, contain mainly quartz, muscovite and chlorite with variable amounts of dolomite, albite and paragonite. Slates from Easdale Island and Cuan Ferry contain pyrite porphyroblasts with δ34S = + 12 to + 16 ppm. The pyrite grew during a post-tectonic retrogressive event at the expense ofpyrrhotine which formed during the main regional metamorphism of the Grampian orogeny by reduction of diagenetic pyrite. Slate from Oban contains abundant diagenetic framboidal pyrite and small syn-tectonic pyrite porphyroblasts with δ34S = +22 ppm. This pyrite was not all reduced to pyrrhotine on metamorphism so there was little retrogressive growth of pyrite. Metamorphism appears to have homogenized local (cm scale at least) isotopic inhomogeneities and preserved an average seawater-sulphate-sulphide isotopic fractionation value. Middle Dalradian seawater-sulphate had a δ34S value of about + 35 ppm, so the small fractionations are appropriate for bacteriogenic reduction in bituminous sediments, the heavier sulphide in the case of the Oban slate indicating more rapid reduction of sulphate. Lower Dalradian Appin Group, Ballachulish slate contains pyrite with δ34S = +15±2 ppm and is best interpreted as forming in the same manner as the Easdale slates of Easdale Island and Cuan Ferry; the sharp increase in late Precambrian ocean-sulphate sulphur isotope signature from +15 to > +30 ppm therefore occurred by Lower Dalradian Appin Group times. [R.A.H.]
Subjects:Dalradian; Isotopes; Metamorphic rocks; Metamorphism; Mineral assemblages; Petrology; Pyrite; Regional metamorphism; Retrograde metamorphism; S-34/S-32; Slates; Stable isotopes; Sulfides; Sulfur; Argyllshire Scotland; Europe; Great Britain; Scotland; United Kingdom; Western Europe; Easdale Slate; Evolution
Abstract Numbers:89M/0530
Record ID:1989002746
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland
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