"Rock of the Month # 29, posted for November 2003" --- A striking porphyritic granite from Shap Fell, GCW Old Series sample 111.25. Digital image, 07 April 2004. Scale is a 10-pence coin, diameter 24 mm.
This splendid granite is one of the best-known in England. A beautiful stone, long an attraction for visiting geologists (e.g., Hutchinson, 1926), figuring also in Harker's seminal review of contact metamorphism, which describes host rocks such as andesite and amygdaloidal lavas (Harker, 1932). The Shap intrusion, roughly 8 km2 in surface exposure but probably larger at depth, is a product of Caledonian magmatism, of lower Devonian date (393 Ma, Piper et al., 1978; Cumberland Geological Society, 1982, pp.15-20). The body lies in the Cross Fell inlier, also noted for a range of minor intrusions, such as lamprophyres (kersantites and minettes) thought contemporaneous with the Carrock Fell diabase dyke swarm.
This particular sample displays the most singular hand-specimen signature of this particular granite, namely the tabular, simple-twinned orthoclase alkali feldspar phenocrysts. Generally 15 to 30 mm in length, these pink microperthitic feldspars have been studied in considerable detail, as have the micas and other features of the Shap mass. The matrix is broadly equigranular (grain size 1-3 mm) and massive, composed largely of smoky quartz, white plagioclase feldspar and black biotite mica. The Shap rock has been referred to as an adamellite, but in modern petrographic terms such an orthoclase-rich lithology is better termed a monzogranite. Sulphides such as pyrite are often concentrated along joint planes.
CUMBERLAND GEOLOGICAL SOCIETY (1982) The Lake District. Cumberland Geological Society / Unwin Paperbacks, 136pp.
HARKER,A (1932) Metamorphism, a Study of the Transformations of Rock-Masses. Methuen & Co. Ltd., London, 1st edition, 360pp.
HUTCHINSON,A (1926) Northern Excursion (Min.Soc. Jubilee Celebrations Fieldtrip, 24-29 September 1926). Mineral.Mag. 21, 128-132.PIPER,JDA, McCOOK,AS, WATKINS,KP, BROWN,GC and MORRIS,WA (1978) Palaeomagnetism and chronology of Caledonian igneous episodes in the Cross Fell inlier and northern Lake District. Geol.J. 13, 73-92.
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