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Threlkeld Microgranite |
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Under
plane polarised light the clear patches show anhedral
quartz grains set in a groundmass of fine grained quartz and feldspar. Most
of the feldspar is sericitised, i.e. converted to
muscovite mica. |
Under
cross polarised light, euhedral plagioclase feldspar phenocrysts with varying
degrees of alteration are clearly visible, one towards the top left showing a
hint of twinning. The microgranite is reputed to contain garnets but none are
seen in this section, the dark patches being quartz grains in the extinction
position. |
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The following information is taken from Volume 17: Caledonian Igneous Rocks of Great Britain Chapter 4: Lake District and northern England Site:
BRAMCRAG QUARRY (GCR ID: 288) The age of the Threlkeld
Microgranite is around 451 Ma. Its relationship with the Ordovician country
rock and other Lake District intrusions has been the subject of debate for
many years. The relationships exposed at Bramcrag
Quarry are crucial in constraining the further interpretation of the isotopic
dates. The Bramcrag Quarry site is the only
locality that demonstrates that the Threlkeld
microgranite intrudes both the Skiddaw Group and the lowest part of the
Borrowdale Volcanic Group. The intrusion is considered, on petrographical and geochemical evidence, to be contemporaneous
with the thick ignimbrites of the upper part of the Borrowdale Volcanic Group.
The available Rb-Sr age of 438 ± 6 Ma post-dates
the cessation of volcanism and may represent either the age of emplacement of
the Threlkeld microgranite or a resetting event. Accurate
U-Pb dates on zircons may offer a resolution to
this problem. |
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