The Mona Formation (formerly Mona Schist) is an extremely thick, metamorphosed, heterolithic unit of Neoarchean age in the Marquette-Ishpeming area of Michigan's UP.  The original rocks making up the Mona are inferred to be (& in many cases are provably) volcanic rocks.  The first sample shown below is a chlorite-sericite-quartz phyllite from the Lighthouse Point Member of the Mona Formation.  Its protolith was likely a volcanic tuff.


Mona Formation chlorite-sericite-quartz phyllite (field of view ~7.7 cm across) from Harvey Quarry, immediately south of Marquette South roadcut along Rt. 41, UP of Michigan, USA (see map).



The rock surface in the above picture is a foliation surface with superimposed crenulations that strike  from the upper right to the lower left.  The metamorphism and development of primary foliation occurred during the Kenoran Orogeny (~2.7 billion years).  The crenulations formed during the subsequent Penokean Orogeny (~1.84-1.88 billion years).



The Lower Member of the Mona Formation is famous for its well-preserved pillow basalt lavas, despite the rocks being subjected to multiple orogenies and metamorphic events in geologic history.  Pillow structures in basalt lavas are diagnostic of subaqueous/seafloor eruption (see modern example from Hawaii).


Mona Formation (lower Neoarchean) with gorgeous pillow basalt lavas, metamorphosed into greenstone (roadcut on northern side of Rt. 41/Rt. 28, just west of Rt. 502 intersection, W of Marquette & E of Negaunee, UP of Michigan, USA; see map).


Mona Formation pillow basalt lavas (metamorphosed).


Mona Formation pillow basalt lavas (metamorphosed)



Greenstone (metabasalt) (5.0 cm across) from pillow basalt roadcut shown above.  Published mineral analysis on Mona greenstone indicates that it is principally composed of plagioclase feldspar, chlorite, actinolite, epidote, and sericite.


Greenstone (metabasalt) (6.5 cm across) from pillow basalt roadcut shown above.



Some info. from:


Brown (1986) - Marquette district: Lower Proterozoic copper and iron.  in  Proterozoic sediment-hosted stratiform copper deposits of Upper Michigan and Belt Supergroup of Idaho and Montana.  Geological Association of Canada, Mineralogical Association of Canada, Canadian Geophysical Union Joint Annual Meeting, 1986, Ottawa, Ontario Field Trip Guidebook 1: 10-20.


Bornhorst & Johnson (1993).  Geology of volcanic rocks in the south half of the Ishpeming Greenstone Belt, Michigan.  USGS Bulletin 1904-P.  13 pp.



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