There are two distinct kinds of rocks in this picture, the young faintly purple volcanic boulders are from the abundant Miocene ~ 15 million years basaltic lava beds upstream. An exception is the light-colored one in the upper right corner which is a felsic ash-flow tuff (~23-20 million). All the greenish hued rocks in the picture are indeed metamorphic and way older. They are mostly volcanic, the product of a volcanic arc (like Japan) that was added to the rim of the North America continent ~1700 million years ago. The chlorite content will vary from as low as 2-5% to 30-40% depending on what kind of volcanic rock was metamorphosed, and there are at least 12 different formations of green chloritic metamorphic volcanic and associated sedimentary rocks in that area. I can pick out at least four (Xcg: conglomerate – the lower left boulder, Xms: green argillite – upper middle, Xma: mafic lava – upper right, and Xfv: felsic lava – right center). The file I attach is one of the smaller of four pdfs you can download for free from our website (repository.azgs.az.gov) that cover the geology of the New River Mesa, AZGS OFR-98-12. This, the written report has detailed descriptions of many of the formations represented by the boulders in your picture; the others area a geologic map, a set of geologic cross-sections, and an explanation. There are several other maps in your area, including one for Cave Creek AZGS OFR-97-1, Humboldt Mountain AZGS OFR-98-11, and Bartlett Dam AZGS OFR-96-22.
Text by Charles Ferguson.
Photographer: Thomas McGuire