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Westerly Granite / Narragansett Pluton

Last Updated: 4th Jan 2010

By Fred A. Schuster

I remember my last few trips to Westerly in the last two years. I think 2008 and 2007. Gosh, how time flies when your 57. I just went down for the 2009 Christmas holidays to Westerly to see my two daughters and grandson. Unfortunately in was colder than usual and looking outside was out of the question.
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Redstone Quarry Today
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Redstone Quarry Today


I was able to take some shots of the Cherenzia Quarry which is the old Redstone Quarry. I have seen some nice Carlsbad twinned tan orange microcline crystals when they were blasting route 78 through the hills upon which the old Redstone quarry sits. They are the Narragansett Pier Granite or Westerly granite intrusions. Being harder rock the intrusions often formed hills.
The State of Rhode Island is all on Avalonian Terrane, part of another continent, a lost continent if you will, that was thrust against a Proto North America named the proto continent Laurentia. There was an ocean named the Japetus Ocean that disappeared and would be analogous to the Atlantic. The Japetus Ocean closed as the continents Lauentia fused together and the small continent Avalon and Baltica, the precursor of Europe. So Avalon was fused, squeezed between the closing continents as the Japetus Ocean closed. Later a new rift formed in the super continent Pangea and the Atlantic Ocean separated Avalon in parts. Some of the continent Avalon is now the North East of the USA extending into Nova Scotia. The other part of Avalon is now in Europe and Africa, on the other side of the Atlantic Ocean. So here we have the state Rhode Island made of 100% Avalon.
The Narragansett Pier granite pluton was seething under the Avalon continent in the Permian Period. It was just after the continents collided to form Pangea and the last ocean closed. This collision about 350 to 300 million years ago caused the Allegheny Mountain building event, the Alleghenian Orogeny. The magma that cooled to become Westerly granite intruded the Avalonian terrane during the Alleghenian orogeny to form a batholiths of 5km x 40km under the terrane. There has been inclusions, xenoliths of the Pennsylvanian period sedimentary rock which you find to the east, exposed in Bristol Rhode and the north east of Narraganset Bay. These pieces had Pennsylvanian fossils dating the host rock to Pennsylvanian Period.
One thing that makes this Narraganset Pier pluton interesting is best described in the quotation from N.A., 1994, Geologic Map of Rhode Island, file name = bedrock_shp.zip (870kb) obtained from Rhode Island Geographic Information System, http://www.edc.uri.edu/rigis-spf/Statewide/state.html#geology
“Narragansett Pier Plutonic Suite - granite - Dark-pink to pale-gray, medium-grained equigranular granite, with lesser granodiorite and quartz monzonite. Composed of microcline, oligoclase, quartz, and accessory biotite, magnetite, ilmenite, apatite, sphene, zircon, monazite, apatite, and allanite; muscovite and garnet locally present; secondary chlorite and calcite. Mainly massive, but locally exhibits faint flow foliation. Cut locally by abundant pegmatite, aplite, and composite aplite-pegmatite of mineralogy similar to the host granite.”
There is an occurrence of cerium and other rare earths in these intrusions.
The granite occurs in fine grained granite to very coarse pegmatite veins sometimes forming microcline in sharp crystals as well as smokey quartz.
The granite was quarried in Westerly, Branford, westward into Connecticut, on Mason’s Island, Niantic and other places. It was used in many monuments such as, the Virginia Monument in Gettysburg, Pennsylvania and the Dover Patrol Monument in Hamilton Park, Brooklyn, NY.
The town of Westerly has many fine examples of the fine grained pink and gray varieties such as the Columbus statue next to the Westerly Library and local cemetery monuments.
During my visits It was difficult to find occasions to collect when I was in Westerly, except for rare occasions there was a little construction going on or a road cut.
A few years ago I was able to get permission to enter a construction site on Granite street while there was a strip of stores being built. The quarry behind was full of water but there was abundant pieces of the Westerly gray granite. This quarry was called the Smith Granite quarry. Today it is behind the Granite Hills Shopping Center and full of water.
I was trying to get permission to enter the Redstone Quarry through contacting the owners but I hadn’t enough time, so I took a photo from the public road. However, behind a friends property was an old slope made of dumped rock from this quarry. I was able get some samples from there. I plan to try to contact the owner to see if I could make a visit.

One piece was from a diabase like dike in the quarry which contains veinlets of satin spar calcite. Looking closely I was finding less than 1mm blue spots which I wish I could identify. The other interesting find was a coarser granite or pegmatite with nests of allanite crystals.
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Allanite


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Piece of gray fine grained Westerly Granite from the Smith Quarry

This type of diabase dike was also found in the Smith Granite Quarry which is now filled with water and is behind a strip mall on Granite Street Westerly. I was only able to find a piece of fine grained gray granite there, during the end of some construction.
Another dike of the Westerly granite was extensively worked in Bradford, about 4 miles east. The Sullivan and Crumb Granite Quarries have been working that dike for years and has produced some interesting specimens, especially well formed microcline Carlsbad twins. I hope to be able to visit this area in the future and possibly get permission to look around.
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Small Pit in Ashaway


There also is a number of small pits in Ashaway South Hopkinton on the side of Tomaquag Road (410 24’ 56.4” N, 710 45’ 14” W). I stopped there and took some photos. The granite was a reddish Westerly Granite with pegmatic veins.




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