G-94 Threlkeld microgranite
Purchase Code: QC-1
IAG members: £ 65
Non-Members: £ 100
This microgranite was collected from Threlkeld quarry, which is situated in an outcrop of felsic intrusive rock of Upper Ordovician/Lower Silurian age, dated at 468±6 Ma. Several outcrops of this rock, a quartz-feldspar phyric microgranite, apparently form part of a single, irregularly-shaped body intruded into the Ordovician Skiddaw Group, very close to the overlying contact with the Borrowdale Volcanic Group. Xenoliths of the Skiddaw Group sediments are common in the Threlkeld intrusion.
It is a strongly altered rock in which it is difficult to recognise primary minerals, especially the feldspars. Volume percentages of minerals were determined with the aid of a point counter but the severe alteration, particularly of feldspar minerals, hindered quantification; plagioclase and alkali feldspar were counted together. The results of this mineralogical assessment were: feldspar (66%); quartz (25%); alteration products, i.e. chlorite (9%), sericite, calcite; accessory minerals (titanite, epidote, zircon, rutile, ilmenite). The particle size distribution has been determined as 98.33% <63 μm, 1.64% between 63 and 125 μm, and 0.03% >125 μm.
The material is available in units of 35 g packaged in a polyethylene sachet.