Adirondack Anorthosite

“The best geologist is the one who has seen the most rocks.”  – My professors and whoever invented the saying!

I would like to reiterate that I am still a student of geology and my “specialty” is by no means petrology, however I’d like to show some photographs from a recent escapade through the Adirondacks to look at some Anorthosites. This summer, I accompanied one of my professors on a few trips to scout out some rocks for future trips. For me, it was a chance to see some new rocks and learn a few things along the way.

For anyone who has a copy or would like to find one, my professor and I followed the trip guide in the “Field Trip Guide for the 67th Annual Meeting of the New York State Geological Association“, trip A2.

Unlike our previous expedition to Warrensburg, NY where we checked out some exceptional marble outcrops, this tripped lacked the visual wow-factor that the marble possessed (though the Adirondacks provide beautiful landscapes!). The anorthosites provide a different sort of intrigue. They are relatively uncommon (unless you want to travel to the moon), and they are poorly understood from what I have been able to gather from my studies.

Anorthosites are characterized as being predominately plagioclase feldspar, about 90% or more. For a better understanding of these rocks I’d refer you Ashwal 1993 where he describes the forms in which anorthosites are found (Archean plutons, Proterozoic massif plutons, lunar, etc) and possible explanation for the formation. I won’t bother trying to overextend my understanding of the topic. Time for photographs!

Stop 1 – Metanorthosite of the Westport Dome

Image of the Anorthosite at Stop 1.

Closer view of the Anorthosite at Stop 1. Note the tiny garnet in the center. The garnets were rare at this stop but more common at other stops.

Close up view of one of the large plagioclase crystals at Stop 1. Note the diagnostic striations! Parts of this crystal appear iridescent making me believe it is Labradorite.

Stop 2 – Anorthositic Gneiss

Anorthositic Gneiss at Stop 2. Similar in many respects to Stop 1 but here we see hints of foliation and much more garnet.

Closer view of the foliation at Stop 2. Note there is a higher concentration of Garnets than what we saw at the first stop.

This rock was located at another portion of Stop 2. Referred to in the guide book at a Jotunite.  It is described as being possible residual liquid from the Anorthosites.

Stop 5 – Gabbroic and Anorthositic Gneisses

Rock at Stop 5. For people like me not overly familiar with Anorthositic rocks, they began to all look alike at this point. The changes I did typically see was in garnet frequency and size. In this case, larger garnets and fewer of them.

Just a close up view of the garnets at Stop 5.

Alright so by now you’ve seen a number of photographs of Anorthosites (and garnets!) but you’re probably wondering what happened to Stops 3 and 4. Stops 3 and 6 were both Wollastonite mines requiring prior permission (which we did not have) to enter and look around. Stop 4 was supposed to be a Marble Xenolith in a Jotunite rock. This stop required scrambling up the outcrop off the side of US Route 9 and climbing over a fence to look at an outcrop  overlooking the Northway. We looked briefly but after not finding anything that looked like the marble xenolith we decided just to continue on.

I hope these photographs were of interest! I know I certainly enjoyed seeing anorthosites for myself. Cheers!