Day #1 Deskcrop – Garnet-bearing Trondhjemite
posted in Geology, deskcrop |I’m starting off the year with one of my more uncommon deskcrops. This is a piece of garnet-bearing trondhjemite collected from the German Rancho Formation, near Stillwater Point State Park, in the Gualala block of coastal northern California. This rock was one of the most distinctive, and yet confounding from my Ph.D. thesis area. The view here is the cut face of a cobble-sized conglomerate clast that was deposited during the Eocene in a submarine fan channel deposit somewhere west of North America. The exact provenance of this particular type of clast could not be established because no exact matches are currently exposed in any of the likely source areas.
When I first encountered this type of clast in my first field season back in the summer of 1992, I immediately recognized little (1-3 mm) red soccer ball garnets and presumed the rock was some sort of metamorphic rock. Upon returning to Wisconsin and thin sectioning the rock the following fall I was surprised to discover that its mineralogy and petrography were distinctively igneous. Chemical analysis confirmed that this rock was a Fe, Mg, K-poor, Na-rich granitoid most resembling the plutonic rocks known as trondhjemites. As it turns out, the garnet was not Mn-rich as might be expected in such a leucocratic rock.
Such a distinctive rock seemed to be exactly the sort of clast one would hope to identify in a provenance study – one that might possibly be used to identify a unique source pluton and establish a sort of sedimentary “piercing point” to constrain early offset on the San Andreas and related faults. Only we never found the source pluton. Based on geochemical signature and U/Pb crystallization age of this clast and others like it (Early Cretaceous), and the distribution of geochemically similar rocks in the Cordilleran batholiths of western North America, and in consideration of other clasts from the basin, it seems most likely that the parent of this orphan is currently buried beneath sediments of the Great Valley Sequence or the Pacific Ocean along the western margin of the Sierran/Salinian batholithic belt, in a resting place currently known but to God.
I sit here with a garnet-bearing trondhjemite in my hand (makes typing difficult). Mine is about the same age that you mention, ~125 my but I hammered this sample out a nice exposure in the Chugach Mountains in Alaska. It was part of my MS research concerning a near trench pluton, the trondhjemite. I don’t remember the exact chemistry of the garnets but they visually resemble those in your picture. The data suggested some remelting of sediments during subduction of a ridge to account for the garnets and the chemistry of the trondhjemites. Beautiful rock and field area. I never make it back though due to the difficulty of access as we were helicopter supported.
I’ve read a bit about those garnet-bearing trondhjemites up in Alaska, particularly to get a handle on the tectonic affinities of such a rock. Given the Eocene depositional age of my clasts, though, I think a provenance link to the Alaskan terranes is impossible (even if one accepts the Baja BC hypothesis). Otherwise it’d make a hell of an interesting story!
I’d love to see one of those Alaskan garnet trondhjemites if you’re able to post an image of yours. Our rocks may not be twins, but they’re certainly members of the same extended family.
Have you got a reference for your publications on these rocks? It’s been a while since I worked on them, but I’d love to get caught up on the most recent work about them.
I’ll try to photograph the rock. In the meantime I tried some rapid research under my old advisor Terry Pavlis, currently at UTEP. The paper I was involved in is:
Pavlis, T.L., D.L. Monteverde, J.R. Bowman, J. Rubenstone, and M. Reason, Early Cretaceous near-trench plutonism in southern Alaska: A tonalite-trondhjemite intrusive complex injected during ductile-thrusting, Tectonics , 7, 1179-1199,1988.
Terry has some more recent work on Eocene examples I believe in:
V. B. Sisson, A. R. Poole, N.R. Harris, H. Cooper-Burner, T. L. Pavlis, P. Copeland, R. Donelick, and W. McClelland, Geochemical and Geochronological constraints for the genesis of a tonalite-trondhjemite suite and associated mafic intrusive rocks in the eastern Chugach Mountains, Alaska: A record of ridge-transform subduction, Geology of a transpressional orogen developed during ridge-trench interaction along the north Pacific margin, Geological Society of America Special Paper 371, edited by V.B. Sisson, S. Roeske, and T.L. Pavlis, p. 293-326.
Hope this helps…
Thanks for those refs, dmonte. I’ll take a look.
Oooh, I must look out for this. I don’t have a specimen of any tonalite, let alone a trondhjemite.
Are you looking for physical samples or just photos, Andrew? If the latter feel free to borrow liberally from my blog/Flickr stream. I can probably find and shoot a bunch more tonalites if you want them.
What else are you missing? I’ll see what I can do to help you fill in the gaps.
Ron, I was thinking of a real rock. But you’re right, I should survey Flickr for Creative Commons examples of this and other less-than-major rock types.
[...] addition, I’m going to follow Ron’s lead and have a weekly rock-on-my desk related post. However, I’m going to post photomicrographs instead of hand samples, just to [...]
Why would you think that the source is still out there? Couldn’t it just as easily have been entirely eroded away as have been buried by sediment?
That, of course, is entirely possible, as well.