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Tuesday, 30 September 2014

Flat Holm Erratic Hunt (3): the pink granite erratic



More on the famous pink erratic boulder on the limestone rocks of Flat Holm's West Beach.  In 1936 a sample was collected from this boulder -- E17444 -- and a thin section was made.  It was examined by Dr AJ McGregor and identified as a pink leucocratic soda-rich granite; but he was unable to offer any possible provenance.

Another analysis (of the same thin section?) by Dr Stuart Baskerville of the University of Glamorgan was communicated to Chris Lee in 2011.  The petrology was described as "igneous quartz, with some slightly sutured boundaries; microperthite (quite fresh); minor hornblende; possible biotite."

Dr Baskerville said the sample was possibly from an alkali granite, granidiorite or adamellite, but he was concerned about a lack of significant amounts of mica.  All of the feldspar was thought to be microperthite -- and this might indicate equal amounts of plagioclase and potash feldspar, pointing towards adamellite (?).  The texture was described as holocrystalline, phanerocrystalline.

As far as a possible origin was concerned, Dr Baskerville suggested that the specific granite which best fitted the bill was the (Late Pre-Cambrian?) Coedana Granite of Anglesey.  But he emphasised that this identification was highly speculative.

All other suggestions gratefully received........

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