(Moine) Calc-Silicate Rocks Represent Metamorphosed Tuffs? a Geochemical Re-Appraisal Richard A
Do Neoproterozoic (Moine) calc-silicate rocks represent metamorphosed tuffs? A geochemical re-appraisal Richard A. Batchelor School of Earth & Environmental Sciences, University of St. Andrews, St. Andrews, Fife, Scotland KY16 9AL Running heads: LH: RICHARD A. BATCHELOR RH: DO NEOPROTEROZOIC (MOINE) CALC-SILICATE ROCKS REPRESENT METAMORPHOSED TUFFS? Suggest: CALC-SILICATE ROCKS 1 ABSTRACT: Following the identification of grey quartz–albite–chlorite–calcite–muscovite rocks in Meso- to Neo-proterozoic sequences in Scotland as metamorphosed tuffs of intermediate composition, it has been shown that this lithology will generate calc-silicate rocks at higher metamorphic grades. Both rock types occur as thin beds with sharp contacts with their host, occur as multiple beds in isolated suites, and share chemical compositions suggestive of volcanic sources with tholeiitic andesite affinities. The failure to recognise calc- silicate rocks as tuffs might explain the apparent scarcity of volcanogenic material through c. 220 million years of early Earth history in Scotland. KEYWORDS: Calc-silicates, tuffs, intermediate magmatism, Moine 2 1 Calc-silicate rocks, which are metamorphic rocks composed mostly of plagioclase feldspar, amphibole, 2 garnet, quartz and occasionally epidote, are sporadically distributed throughout the Neoproterozoic 3 Moine Supergroup of Scotland. The Moine Supergroup comprises a thick sequence of sedimentary 4 rocks, deposited in fluviatile (Bonsor & Prave 2008) and shallow marine environments between c. 950 5 Ma and 870 Ma (Mendum et al. 2009), intruded by some minor igneous intrusions, all of which were 6 subjected to regional metamorphism and deformation events (Strachan et al. 2010). The sediments 7 were derived from the denudation of the c.1000 Ma Grenville Orogen and deposited off the eastern 8 margin of Laurentia into a foreland basin.
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