Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: STAR5 on 14/06/2011 15:32:42
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These samples were both found in Southern Hew Hampshire. Pretty sure one is a tuff & the other breccia (maybe)
Both are heavier than normal, Metal detector = Ferrous. Can anyone help.... yes I am a newbie. Thanks
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Did you collect these near Ascutney Mountain?
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They were both collected in Candia, NH
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Sorry this took a while- I've been in the field with no computer connections.
Matrix supported breccia, with angular to subrounded clasts. At least 4 different clast lithologies. Matrix is uniform fine-grained material with small fragments. I didn't note any pumice shards or collapsed pumice in the photos. There are no alteration haloes around the clasts.
My best guess is some sort of mass movement (landslide, volcaniclastic flow, mudflow, etc). Any idea what the matrix is made of? How about the clasts?
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The first two photos, why do you call them tuffs?
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First, i appreciate the input & time, The breccia's matrix looks like very fine granite (from the granite state), feldspar visible in matrix. It has a weak magnetic attraction & sounds like glass/ceramic when tapped. It is very hard & heavy, as mentioned. So far all samples found in same general area, they are difficult to differentiate from surrounding rock as all are weathered & are partially lichen or moss covered. See matrix closeup picture.
My first impression on the Tuff was obtained by reviewing MANY websites, trying to learn what they were (newbie). These two rocks re-sparked an interest I had years ago. A wikipedia page was the closest match at the time i could find ( http://en.wikipedia.org/wiki/File:Ettringer_Tuff.jpg ). The clast were different minerals, shapes, & colors, some were weathered out of the matrix. Now. the outside maybe just weathering changing the appearance, probably one of the igneous rocks I have not found a picture of yet. One guess would be basanite? I'm still looking. [ Invalid Attachment ]
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The tuff appears to be a fine-grained igneous rock (possibly subvolcanic), probably a latite. I can see pheoncrysts of white blocky feldspar (possibly albite) and biotite in a fine grained matrix. Doesn't look like basanite, which would be a very odd rock to find. I didn't notice any quartz- but there are some odd shaped gray crystals that may be quartz. If it is quartz, this is probably a quartz latite or rhyolite-rhyodacite. (plutonic equivilents are monzonite or qtz monzonite).
Breccia clasts are from distinctly different parent rocks- which means that the breccia formed in a place where there were several different rock types exposed. My best guess is still some sort of lanslide or volcanic mudslide. I'm impressed with your photos- the matrix through the 30x is quite good. Looks like quartz plus chlorite and rock fragments- but as you can imagine, it's tough to identify rocks only through photographs. Hope this helps.
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Again, thanks for your help.