Day #1 Deskcrop – Garnet-bearing Trondhjemite

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.

Garnet-bearing Trondhjemite Hand Sample

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.

garnet trondhjemite thin section - PPLgarnet trondhjemite thin section - XPL

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.

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