Tuesday, June 11, 2013

A Piece of Bradbury: Rebel Pegmatite

Some things in this world are predictable.  There is a very clear relationship between how big the crystals in an igneous rock are, and how long it took to form.  But some rocks rebel.

The rule of igneous rocks is simple. If crystals are small - as in a piece of basalt or rhyolite - the rock froze fairly quickly, having risen to the Earth's surface before dropping in temperature rapidly.  If the crystals are large, as is the case of this piece of granite I found at Bradbury Mountain, it means the rock formed deep underground, where the heat of the Earth allowed each crystal to gestate to full visible size before turning fully solid.
The rock that makes up Bradbury Mountain formed as Africa shoved its way underneath the North American plate - squeezing magma into Maine's underbelly.  Of course at that point the rock was covered with a thick blanket of rock thrown up by earlier parts of the collision.  The magma cooled slowly, and the crystals grew to the size you now see, following the rule to a tee.

Turn the rock around however, and you find these rebels, younger crystals, forming faster and larger than their granite brothers and sisters.  These crystals are pegmatite, effectively a large crystalled rock with the same make up as the granite on the reverse.  Pegmatite drew people to the site in the 1920s because the large crystals of the mineral feldspar were easily gathered for making ceramics.  But how did it get there?

As it solidified, the mass of magma that had squeezed under Maine, shriveled like a raisin.  The contraction created a gap with the surrounding rock, as well as spiderweb of cracks near the edge of the hardening granite.  These became a network of passageways through which left over magma could flow.  The remaining magma had all of the components of the original, but with a higher concentration of materials that couldn't form minerals, among others water.  This extra stuff became a super serum for crystal growth, filling in the empty space with megacrystals, a hundred times the size of their brethren.

Landes, Kenneth K. "The Paragenesis of the Granite Pegmatites of Central Maine."American Mineralogist 10 (1925): 355-411.

London, David. "Granitic Pegmatites: An Assessment of Current Concepts and Directions for the Future." Lithos 80 (2005): 281-303.

Simmons, Skip. "Pegmatite Genesis: Recent Advances and Areasfor Future Research." Proc. of Granitic Pegmatites: The State of the Art – International Symposium, Porto, Portugal. Web. 11 June 2013. <http://www.fc.up.pt/peg2007/files/simmons.pdf>.

Zieg, M. J., and B. D. Marsh. "Crystal Size Distributions and Scaling Laws in the Quantification of Igneous Textures." Journal of Petrology 43.1 (2002): 85-101.

"Bradbury Mountain State Park." Maine Bureau of Parks & Lands. 2009. Web. 10 June 2013. <http://www.maine.gov/cgi-bin/online/doc/parksearch/search_name.pl?state_park=12>.

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