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Bite my butt

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Frog-kind did not win this round.

 

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Bedrock of history

I am grateful for the opportunity to contribute to this blog. It is pretty exciting to actually get blogged, considering the state of the blogosphere these days. I appreciate this opportunity to share my research here, doubly so because my work relates to the geologic history of the area for which this blog is dedicated.

This research project was completed as part of my Master’s degree thesis work at Boston College during 2010-2012 under advisement of Dr. Christopher Hepburn and partially funded by a Geological Society of America Graduate Student Research Grant. It is titled “Implications of Silurian Granite Genesis to the Tectonic History of the Nashoba terrane, Eastern Massachusetts”. In this project, I studied two granites in eastern Massachusetts for the purposes of: determining the characteristics and crystallization ages of the ‘Sgr’ granites found in eastern Massachusetts, determining a new crystallization age for the foliated biotite granite phase of the Andover Granite, and synthesizing this new data with what we currently know of the Nashoba terrane and the surrounding region.

What is the Nashoba terrane?

The region I worked in is called the Nashoba terrane:

Map of Nashoba terrane

1: Hepburn and others (1995), 2: Loftenius (1998) and Wones and Goldsmith (1991), 3: Hill and others (1984), 4: Zartman and Naylor (1984), 5: Acaster and Bickford (1999), 6: Shride (1976).

A terrane is a collection of rocks that has its own tectonic history and are tectonically isolated from the rocks around it, commonly through faults. Nashoba terrane is composed of multiple granites, a couple of diorites, as well as a few gneisses and schists. The terrane is separated from the Merrimack terrane to the west and the Avalon terrane to the east by the Clinton-Newbury and Bloody Bluff faults, respectively. For a geographic perspective, the Nashoba terrane stretches from the town of Newburyport down through the town of Marlborough to the MA/CT border, forming a crescent-like shape roughly following I-95.

Granites Ho!

My two granites, the Sgr granites and the foliated biotite granite of the Andover Granite, are found along the eastern boundary of the Nashoba terrane. The Sgr granites are a group of three bodies of gray/pink to salmon colored granite found in the towns of Lincoln (Southern Zone), North Reading (Middle Zone), and Newburyport, MA (Northern Zone).

Hand samples of sgr

(Fun Fact: The Sudbury Granite, the southernmost body of the Sgr granites is the bedrock of Lincoln, MA.) The primary motivation to study the Sgr granites is that they are found adjacent to the Nashoba-Avalon terrane boundary. If these granites were found to crosscut the boundary, then their crystallization ages can be used to constrain the age of the boundary, thereby constraining the sequence of tectonic events here in eastern Massachusetts. (Fun Fact: I found that the Sudbury Granite forms the type locality of the famous Bloody Bluff in the Minute Man state park.)

The other granite I investigated was the foliated biotite phase of the Andover Granite:

Hand samples of foliated biotite AG

The Andover Granite is a large, and very complex, granite found in Andover, Massachusetts. The granite is complex because the it is actually composed of multiple smaller granites (phases), each slightly different from the others. I was particularly interested in the foliated biotite phase of the Andover Granite because of its previously accepted crystallization age. This age, ~450 million years old (Ma), was measured many decades ago using some of the early mass spectrometers available to geology. Today, we now know quite a lot about geochronology, so sometimes we revisit some of the older ages to see if the newer techniques change the age. Also, and more importantly, this 450 Ma age was problematic. In recent years a significant work has been done on understanding the tectonic history of southern New England, including eastern Massachusetts, (shameless plug for my adviser’s work). From this, a working geologic model arose telling the story of southern New England. The 450 Ma age does not fit with the latest iteration of this geologic model, so I wanted to revisit the age using modern geochronological techniques. If the 450 Ma age stands, then clearly our model would need to be updated.

Setting the stage.

The geologic history of New England is a fascinating story; however, my project only involves one chapter of it. New England, and much of the east coast of North America, formed between about 500 and 300 million years ago during the construction of the supercontinent Pangaea. Back then two continents existed, Laurentia (Proto-North America) and Gondwana (everything else). These two continents were separated by the Iapetus Ocean. (Fun fact: Iapetus was the father of Atlas, from where the Atlantic Ocean gets its name, hence the name choice). During this time, micro-continents and island chains (think Japan or the south Pacific Islands in terms of size and shape) collided with North America (then called Laurentia) in four separate mountain building events, or orogenies as we call them. Each orogeny built another section of the east coast extending out the eastern seaboard into the ocean. If you were to look at a geologic map the east coast you would see these stripes of tectonic realms that roughly trend North-South. Each stripe represents a different orogeny. If you take a look at the map you can kind of see what I mean. Each color relates to a specific realm of the Northern Appalachians. You can see how the realms roughly trend North-South. While the orogeny involved small island chains that were in the middle of the Iapetus Ocean, the latter three involved micro-continents that previously rifted away from Gondwana, the last orogeny involving Gondwana itself. (Fun Fact: Most of New England, and the east coast of North America, is originally from South America.)

The Nashoba terrane arrived on scene during what we call the Salinic orogeny. During this time, the passive margin of the micro-continent Ganderia crashed into Laurentia. It is called the passive margin because at this point Ganderia existed as two pieces, an active margin and a passive margin. The active margin is composed of a volcanic arc (think the Andes Mountains) that broke away from Ganderia during Cambrian time (~500 to 550 millions of years ago). The passive margin is the part of Ganderia that remained. The Salinic orogeny contributed a large amount of material to the construction of New England, building out CT from just east of the Hartford Basin to just west of I-395, MA from east of Springfield to around where I-495 is today, and then most of central NH and Maine.

Data and numbers

I used three techniques in my project: thin-section petrography, granite geochemistry, and Uranium-Lead radiometric age dating. (Fun Fact: Uranium-Lead dating is a lot like radiocarbon dating, but instead of counting the amount of carbon-14 you have relative to the amount of carbon-13, you are counting the amount of lead you have relative to amount of uranium. Unlike radiocarbon dating, which can only effectively go back 50 thousand or so years, Uranium-Lead dating can go back billions of years. Plus, there are two radiometric clocks inherent to the Uranium-Lead system: U-235 → Pb-207 and U-238 → Pb-206), allowing for a much higher degree of accuracy and precision to be attained since you are making two measurements simultaneously.) Petrography and geochemistry allow me to characterize the granites—which in turn allows me to determine how they formed—and the age dating allows me to place the granites inside our geologic model to see how the granites relate to the model, and the model to the granites.

Sgr Granites (including the Sudbury Granite)
• 420 million years old, give or take 0.5 million years
• Composed of gray/pink to salmon colored biotite granites
• Higher abundances of Fe, Mg, and K than typical granites
• Geochemistries common to rocks found in volcanic arc

Foliated Biotite phase of the Andover Granite
• 419 million years old, give or take 0.5 million years

What does this mean?

The significance of these ages is that these granites formed in the Nashoba terrane in the same time frame as what we call the Acadian orogeny which formed present day eastern Connecticut (past I-395), eastern Massachusetts (past I-95), Rhode Island on the east coast of what would become North America. Remember the Avalon terrane from earlier? It is part of Avalonia. (The name Avalonia gives it away doesn’t it?). The mineralogy and petrology of the Sgr granites and the Andover Granite (all the Andover Granites, not just the foliated biotite phase now) are consistent with rocks that formed inside a volcanic arc. Now this is not the volcanic arc that was on the active margin of Ganderia, but another arc that formed millions of years later (there were all sorts of volcanoes along the eastern seaboard of Laurentia during this time). What is great is that my data fits our model quite nicely. These new ages are similar to other crystallization ages that we have in the Nashoba terrane, and correlate well with the timing of events in the terrane. Also, this new data shows my granites can be correlated with other granites throughout New England (shaded yellow in this figure).

Regional Correlations

Yay—it works!

That’s pretty neat. What else is revealed?

What I find more interesting is how I think these granites likely formed. Granites are igneous rocks, meaning they formed from the crystallization of minerals inside a magma chamber. The most common method for a granite to form is through the metamorphism of continental rocks. In this case, the rocks are brought to high enough temperatures to trigger melting (this would be at ~650 ºC). The rock record of the Nashoba terrane shows that the highest temperatures the terrane reached were ~700 ºC.

The Andover Granite and Sgr granites are two different kinds of granites. The Andover Granite with its high abundance of muscovite and the presence of garnet indicates it was derived from the melting of continental rocks. Similar granites to the Andover Granite, or S-type granites, are commonly ‘minimum-melt’ granites. Meaning they formed at the lowest temperature possible for the region they are found in, so about 650 ºC. The Sgr granites, on the other hand, are I-type granites (based on their biotite content and geochemistry). I-type granites can form at much hotter temperatures (800 ºC). Well the peak metamorphic temperatures the Nashoba terrane achieved are sufficient to have the Andover Granite form, but what about the Sgr granites. How can you have 800 ºC granites forming in a terrane that did not get any hotter than ~700 ºC?

Well, we now know the granites formed inside a subduction zone 420 million years ago (more on this below). A rock is that is commonly formed in subduction zones is another igneous rock called a diorite. There just happens to be a very large diorite in the northern part of the Nashoba terrane, the Sharpners Pond Diorite. What makes diorites significant in this case is that they are quite hot when they are molten (~1000 ºC) so when they intrude into an area they heat up the surrounding rocks quite a bit. I believe this is how the Sudbury Granite formed. Both the granite and diorite are in close proximity to each other. I believe that when the Sharpners Pond Diorite intruded into the Nashoba terrane, it was the tipping point, so to speak. The intrusion of the diorite provided the extra heat required to trigger the melting that would go on to form the Sudbury Granite. Now that is remarkable!

So what’s the story?

Granite model

My project, as well as a significant amount of other geologic evidence collected and analyzed over the years, tells the story of the Acadian orogeny (420 to 400 million years ago). So the stage is set from earlier: We just had the Salinic orogeny with the passive margin of Ganderia crashing into Laurentia. While the Salinic orogeny was going on, Avalonia was still approaching Laurentia, (Avalonia was not going to just sit and wait for Ganderia to finish up,) To accommodate for the rapidly approaching Avalonia, the heavier oceanic crust that accompanied Avalonia on the same tectonic plate was pushed beneath Laurentia. This formed a subduction zone (A in the figure). As these rocks are pushed deeper into the Earth, they heated up and eventually melt. This melt rises through the Earth and erupts at the surface in a volcano and forms a volcanic belt (B in the figure). When this hot molten rock enters the upper crust it heats up the surrounding rock, triggering both the metamorphism of those rocks as well as melting and granite formation (C in the figure). At the end of the orogeny, Avalonia crashes into Laurentia causing widespread deformation all along the northern Appalachian Mountains (D in the figure). This was quickly follow by the Neo-Acadian orogeny, which when the micro-continent Meguma joined the party. Eventually (350 Ma to 250 Ma), Gondwana join in and Pangaea fully formed.

So… My granites, along with evidence from eastern New England and Canada, tell us the story of the Acadian orogeny.

Want to know more? Have other ideas? Don’t hesitate to contact me if you have any questions or comments. I always love to talk about rocks!

References

Acaster, M., and Bickford, M. E., 1993, U-Pb geochronology of metavolcanic and metavolcanogenic sedimentary rocks; Nashoba Block, eastern Massachusetts, Geological Society of America – Abstracts with Programs, vol. 25, p. A484.

Hepburn, J. C., Dunning, G. R., and Hon, R., 1995, Geochronology and regional tectonic implications of Silurian deformation in the Nashoba terrane, southeastern New England, U.S.A., in Hibbard, J. P., van Staal, C. R., and Cawood, P. A., eds., Current Perspectives in the Appalachian-Caledonian Orogen, Geological Society of Canada Special Paper 41, Geological Society of Canada, p. 349-366.

Hill, M. D., Hepburn, J. C., Collins, R. D., and Hon, R., 1984, Igneous rocks of the Nashoba Zone, eastern Massachusetts, in  Hanson, L., ed., Geology of the Coastal Lowlands: Boston, MA to Keenebunk, ME, New England Intercollegiate Geological Conference, 76th Annual Meeting, Salem State College, Salem, Massachusetts, p. 61-80.

Loftenius, C. J., 1988, The geochemistry and the petrogenesis of the Sharpners Pond

plutonic suite, NE Massachusetts, M.S. Thesis, Boston College, Chestnut Hill, MA, 284 p.

Shride, A. F., 1976a, Preliminary map of bedrock geology in the Newburyport East and West quadrangles, Massachusetts-New Hampshire, USGS Open File Report #76-488, U. S. Geological Survey, 1 p.

Wones, D. R., and Goldsmith, R., 1991, Intrusive rocks of eastern Massachusetts, in Hatch, N. L., Jr., ed., The bedrock geology of Massachusetts, U. S. Geological Survey Professional Paper 1366-E-J, United States (USA), U. S. Geological Survey, Reston, VA, United States (USA), p. I1-I61.

Zartman, R. E., and Naylor, R. S., 1984, Structural implications of some radiometric ages of igneous rocks in southeastern New England, Geological Society of America Bulletin, vol. 95, no. 5, p. 522-539.

Text and images copyright © D. Dabrowski 2014.

What lies beneath

Farrar Pond lies in a part of Lincoln that—unlike most of New England—is nearly devoid of visible rocks. (Colonial-era irony: the Massachusetts farmer exhausts his life prying boulders out of his fields, hauling them aside, and grunting them up into neat stone walls. So when he dies, they plant him with a rock on his head.) Even where exposed ledge doesn’t mock attempts to garden or farm, stones—through a combination of ice-lifting and erosion—continually grow right out of the ground.

But not so much around here, because there aren’t a lot of rocks in the underfoot mix. As per the various surface-geology and soils maps shown or referenced on the Geography page and parodied in this post, the land around Farrar Pond comprises lake- or river-bottom (“glaciolacustrine” and “glaciofluvial”) deposits of material scraped and milled from regions further north, now graded into valuable bank gravel and sand, weathered above and bio-degraded these past ten millennia or so into excellent light loams. Great for septic systems—hence at risk from illegal mining-type exploitation—and easily amended with leaf debris or wood chips into superb garden soil. For anyone interested in characteristic local glacial landforms (esker, drumlin, kettle, kame, delta, etc.), the work of great naturalist (and former Lincoln science teacher) Neil Jorgensen is highly recommended.

And yet naturally arrived rocks do turn up here on occasion, usually small:pebbles to melon-sized, well rounded and smoothed by abrasion. Often, these are of a fine-grained pink granite, quite unlike larger glacial erratics or gray ledge exposed outside this high-perc zone. However did they get here?

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One fine day a couple of years ago, a pleasant young man was seen wandering the small side-roads by the pond, peering around intently but with no obvious target. He appeared not at all lost, inquisitive but without the usual nature-watcher’s binoculars (or the burglar’s studied insouciance). Asked if he wanted assistance, he explained that he was a geology graduate student doing field work involving pink granite cobbles, and were any such to be found hereabouts? There were, and he promptly identified them as to type, formation and site of origin. He also provided several helpful references (some cited above), and agreed to share the results of his study. Degree now in hand, Dan Dabrowski is kindly providing a (relatively) lay-accessible summary of his findings, offered in the post to follow (above, in a normally configured browser) for our enjoyment and edification. For more serious readers, he has donated a copy of his actual thesis to the Lincoln Public Library.