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SUMMARY:REGIONAL METAMORPHISM IN THE BALLACHULISH AREA\, SW HIGHLANDS\, SC
 OTLAND: A NEW LOOK AT A FAMOUS OLD DEBATE - Dave Pattison - University of 
 Calgary
DTSTART:20231110T123000Z
DTEND:20231110T013000Z
UID:TALK207736@talks.cam.ac.uk
CONTACT:Lucas Measures
DESCRIPTION:The Ballachulish area of the southwest Highlands of Scotland w
 as the site of a famous debate in the 1920s between two of the giants of B
 ritish geology: Sir Edward Battersby Bailey\, a field geologist with of th
 e Geological Survey of Great Britain and University of Glasgow\, and Profe
 ssor Cecil Edgar Tilley\, a petrologist at the University of Cambridge. At
  issue were contrasting interpretations of the identity\, geometry and tim
 ing of regional metamorphic zones in the southwest Highlands\, and the imp
 lications these carried for the tectonic evolution of the region. The poin
 t of contention in the Ballachulish area was the cause of the abrupt disap
 pearance of garnet\, in the direction of increasing metamorphic grade\, ac
 ross a major fault\, the Ballachulish Slide. Bailey attributed the abrupt 
 disappearance of garnet to differing bulk composition of rocks of the same
  metamorphic grade on either side of the slide\, interpreting the slide to
  be pre-metamorphic\, whereas Tilley and later his Cambridge colleague Ger
 trude Elles and he attributed the abrupt disappearance of garnet to mechan
 ical juxtaposition of rocks of different metamorphic grade\, implying that
  the slide was post-metamorphic. New mineral assemblage data\, microstruct
 ural observations and rock and mineral compositional data are combined wit
 h phase equilibrium modeling to re-assess the regional metamorphism in Bal
 lachulish area. The newly revised chlorite\, biotite and garnet zones are 
 more similar to those of Bailey than Tilley. Compositional difference betw
 een lithologies accounts for the irregular distribution of mineral assembl
 ages in the region\, especially garnet. Isograd patterns and microstructur
 al observations indicate that metamorphism occurred in the latter stages o
 f the deformation history\, after development of the major slides (faults)
 \, in agreement with other studies. Pressure-temperature conditions of the
  garnet isograd are estimated to be ~7 kbar\, ~500 °C.
LOCATION:Tilley Lecture Theater\, Department of Earth Sciences
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