Greenstone (metabasalt) from the Precambrian of Michigan, USA. (5.0 cm across at its widest)
The rock shown above is greenstone from the Mona Formation (a.k.a. Mona Schist), an extremely thick, metamorphosed, heterolithic unit of Neoarchean age in the Marquette-Ishpeming area of Michigan's Upper Peninsula. The original rocks making up the Mona are inferred to be, and in many cases are demonstrably, volcanic rocks.
Greenstones occur in the Lower Member of the Mona Formation - these rocks were originally pillow basalts. Despite being multiply metamorphosed and subjected to several orogenies in geologic history, the geometry and morphology of the pillows are well preserved (see outcrop photos elsewhere in this photo album). Pillow structures in basalt lavas are diagnostic of subaqueous/seafloor eruptions.
Published mineral analysis on Mona greenstone indicates that it is principally composed of plagioclase feldspar, chlorite, actinolite, epidote, and sericite.
Stratigraphy: Lower Member, Mona Formation, Neoarchean, ~2.7 Ga (= metamorphic age)
Locality: roadcut on the northern side of Rt. 41/Rt. 28, just west of the Rt. 502 intersection, west of the town of Marquette & east of the town of Negaunee, Upper Peninsula of Michigan, USA
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