Plutonic lunch: new understandings of Shap granite

Recently, we ate lunch at the Honest Lawyer pub outside Durham – an oxymoron for sure. Staring at me from the table was the belly of a possible volcano: a granite tabletop! It looked suspiciously like Shap granite from Cumbria, but unfortunately the quarry provenience was not known to the waitress.

As any volcanologist (but who else?) knows, granite is mainly formed in volcanic arcs where magma rises through the Earth's crust in 'diapirs';  most of these diapirs solidify as plutons 5 to 20km deep in the crust, but some make it to the surface where magma is extruded through volcanoes as lava or volcanic ash. The magma that doesn't make it out cools slowly in the chamber or pluton, allowing very large mineral crystals to grow. In this kind of pink granite, the large pink rectangular crystals are potassium (K-)feldspar, also known as orthoclase. Regular feldspar (plagioclase) forms the white crystals surrounding the large pink ones, while the grey bubble-like crystals are quartz, and the black grains may be biotite or amphibole.

Shap granite is featured as the Rock of the Month for December 2011 by the Open University. The Shap granite outcrop in the Lake District is intrusive into the Borrowdale Volcanic Group, which was once an island arc off the micro-continent of Avalonia in the Iapetus Ocean between 460 and 444 million years ago [1]. However, Shap dates later via processes quite different from the subduction zone granite emplacement described above. After Avalonia was accreted to Laurentia (North America), the area of the Iapetus suture in northern England (demarcated by the Solway tectonic line) was subject to magmatism on both north and south sides of the suture. It is thought that a period extensional tectonics, creating a pull-apart basin 21km wide, was able to generate higher temperatures that caused both mantle and crustal melts to rise into the upper crust [2]. These were emplaced in the subsequent period of Acadian Deformation between 400 and 390 million years ago. Shap granite, therefore, stands as the representative of a newly understood source of magma generation that is totally divorced from subduction tectonics.

Many buildings around England feature Shap granite floors or columns. So you can also walk on it  or bump into it as well as eat off it. Just keep those sharp eyes open for Shap and you will be rewarded by a trip to the center of the Earth (or at least 5 kilometers down where granite forms).

Oh yes, and the Honest Lawyer has great Eggs Benedict. Did I say lunch? Maybe it was brunch...

[1] Huff, WD; Bergström, & Kolata, DR (2010) "Ordovician explosive volcanism," pp. 13-28 in The Ordovician Earth System, ed. by FC Finney & WBM Berry. Geological Society of America Special Paper 466.

[2] Brown, PE; Ryan, PD; Soper, NJ & Woodcock, NH (2008) "The newer granite problem revisited: a trans-tensional origin for the early Devonian trans-suture suite". Geological Magazine 145.2:235-256.