MONA FORMATION
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.