Updates on Ages of Maine USA Pegmatites

For folks interested in Maine pegmatites and related localities, there has been quite a lot of good radiometric dating done in the past 5-10 years that is scattered about in a number of professional papers. So I figured it would be useful to collate some of it for general consumption. Also, I was told yesterday that we finally have a good Pb/U zircon date for the Litchfield, Maine sodalite/cancrinite nepheline syenite pluton. It is 321 Ma, + or - 1-2 Ma. The research and paper is now in press. Here's a pluton date list for central and southern Maine (error bars and paper refs. not included):

Topsham pegmatites 274
Brunswick granite 278
Sebago 300
Litchfield syenite 321
Lyman 322
Biddeford 354
Waldoboro 367
Peg. Dike at W. Gardiner 367
Mt. Waldo 371
Three Mile Pond 378
Lucerne 380
Rome/Norridgewock 380
Webhannet 391
Hornbeam Hill gneiss 393
Peg. Dike at Waterville 399
Lincoln Sill 418


Source: NEIGC field guides from 1995-2010, except Litchfield data (in press).

This is some great work indeed, Doug. The numbers correspond quite well to the Acadian Orogen mountain building event that occurred during the Paleozoic, with some being emplaced even after most of the orogenic processes were complete. Curiously, do the ages of the bodies seem to get younger the further south in Maine you get?

Nice list. Don't have the article handy, but we had results indicating that many granite pegmatites were slightly younger than the nearby plutons. (Foord, E. E., L. W. Snee, J. N. Aleinikoff, and King, V, 1995, Thermal Histories of Granitic Pegmatites, Western Maine, USA [abstract], Abstracts with Programs Geological Society of America v. 27:468.) Don't forget that many of Maine's granite pegmatites are partial melts in the thermal aureole of plutons rather than ejected fluids from the pluton chamber itself. ((Simmons, W. B., E. E. Foord, A. U. Falster, and King, V., 1995, Evidence for an Anatectic Origin of Granitic Pegmatites, Western Maine, USA [abstract], Abstracts with Programs Geological Society of America v. 27:411.) The implication that was published by Robert Moench is that the plutons were emplaced in a hot terrain (second sillimanite grade in some places) and that Moench found that the terrain he studied (Oxford/Franklin Counties) cooled at a rate of one degree Celsius per millions years based on Ar closure temperatures of coeval mineral associations. Therefore, the partial melts had a great deal of time before final crystallization into granite pegmatites or other rocks.

Best Wishes, Van King

Thanks Van -- The papers you have referenced help answer why some of the really big silica-rich plutons in eastern Maine (Mt. Waldo, Lucerne, Deblois, Bottle Lake) do not express a mineralogically diverse pegmatitic phase on their margins as seen in Androscoggin and Oxford Counties. It's not just that the Mineral Gods hate Cherryfield!

What I find most intriguing from this data list is how anomalously young the Topsham/Bowdoin pegmatites are (268-275 Ma). They are 25 Ma younger than the Sebago intrusion and 100 Ma younger than the Waldoboro pluton, their nearest neighbor to the northeast.

If nothing else, this helps to explain why the Topham/Bowdoin pegmatites are so mineralogically different from O/A counties, esp. in REE minerals. And at the same time, there is no nearby large contemporaneous pluton to be a 'source' for them. If it's anatexis of the host rock, we need to identify a heating event unique to the Topsham area around 275 Ma that melted the host rock but did not emplace a large pluton. I'm working on it now.

Paul,

Yes, if you restrict your dates to the Acadian orogeny and later (ie. 400 Ma), you do see kind of a north to south cline in Maine. What you mostly see is a very sharp pulse of pluton ages clustered around 390-370 Ma in central and eastern Maine; and then a very quiescent period for 50 Ma. Then, around 300 Ma, the Sebago pluton comes in and 25 Ma later the Topsham/Bowdoin pegmatites come in. And then nothing happens again for 50 Ma until the Triassic, which is when Pangaea was truly starting to break up.

By my understanding of the state of the art, the Carboniferous Sebago intrusion (and the Litchfield syenite intrusion) are best described as early Alleghanian, and definitely not late Acadian. There's 50-70 Ma of 'nothing' in between the two orogenies. The Acadian ended by 360.

Van King is right about the western Maine pegmatites. Their paragenesis is horrendously complex, and dates of the imputed 'parent' igneous bodies are not particularly useful, since it is still unclear if many or some of mineralogically diverse pegmatites ever actually received much 'fluid' from their respective large igneous bodies or instead just melted and then recrystallized. The 'residual fluid' model holds that the stuff in the parent melt spreads and becomes progressively fractionated and intrudes the country rock. But a second mechanism is that if ambient temperature is hot enough the country rock itself melts and re-aggregates within the mineralogic limits of its composition. The latter model seems to be a much better explanation for the observed diversity of Maine pegmatite bodies.