By: magic-gigapans
License: Creative Commons Non Commercial ⧉

Uploaded: 5 May 2020
Last Updated: 13 May 2020

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There is one rock exposed along the Billy Goat Trail which is a bit more enigmatic. It is amphibolite, a metamorphic rock very rich in the mineral amphibole (a.k.a. hornblende), which shows up as black in this picture. The white portions of the rock are plagioclase feldspar. This is a very mafic metamorphic rock, with large crystals. Amphibolite occurs as large, folded tabular masses in the area of the Billy Goat Trail. An important question to ask here is "What was the protolith?" In other words, what were these amphibolites, before they were metamorphosed into amphibolites? Whatever it was had (1) a mafic composition and (2) large crystals. The answer which leaps to mind, I'm sure, is gabbro -- a coarse-grained mafic rock. Gabbro can occur as sills (large, tabular masses of mafic magma that squeeze between pre-existing sedimentary layers, in this case of metagraywacke), or it can occur as intrusive masses. If the latter, one place where we find a lot of gabbro is in the oceanic crust. If we interpret the amphibolite as being a metamorphosed slice of oceanic crust, then we are looking at the floor of the Iapetus Ocean here: the oceanic crust on top of which the graywacke was deposited!