Metamorphic reactions in the biotite zone, eastern Scotland: high thermal gradients, metasomatism and cleavage formation |
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Authors: | T J Dempster A E Fallick C J Whittemore |
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Institution: | (1) Division of Earth Sciences, Gregory Building, University of Glasgow, Glasgow G12 8QQ, UK e-mail: T.Dempster@earthsci.gla.ac.uk; Fax: +44-141-3304817, GB;(2) Isotope Geosciences Unit, Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QF, UK, GB;(3) Division of Earth Sciences, Gregory Building, University of Glasgow, Glasgow G12 8QQ, UK, GB |
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Abstract: | Prograde metamorphic reactions involving the growth of phyllosilicates and accompanying cleavage development have been investigated
in Dalradian metasediments from the biotite zone of eastern Scotland. Crystallization of muscovite within the psammites of
the Southern Highland Group is linked to the replacement of plagioclase porphyroclasts. This reaction is triggered by significant
alkali metasomatism during active deformation and plays an important role in the formation of a prominent spaced cleavage
within the psammites. The Si content of most of these early-formed muscovites is partially buffered by the quartz content
of the rock, although close to the Highland Boundary Fault, evidence of greater influence from externally derived fluids on
muscovite compositions is preserved. Locally higher fluid fluxes adjacent to the fault are also indicated by a relatively
high δ
18O(SMOW) signature in the rocks. The biotite-producing reaction in these greenschist-facies rocks is linked to the later production
of a celadonite-poor muscovite which formed as overgrowths around pre-existing white micas. This reaction is sensitive to
the initial composition of muscovite and preferentially occurs in quartz-rich metasediments containing a celadonite-rich muscovite.
A systematic increase in the progress of the biotite-producing reaction northwards across the biotite zone confirms the presence
of high geothermal gradients along the southern margin of the Dalradian block, adjacent to the Highland Boundary Fault.
Received: 26 February 1999 / Accepted: 8 November 1999 |
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