The Hardwood Gneiss: Evidence for high P-T Archean metamorphism in the southern province of the Lake Superior region

Journal Article · Thu Mar 01 04:00:00 EST 1990 · Journal of Geology; (USA)

DOI:https://doi.org/10.1086/629398· OSTI ID:6250413

Peterson, J W ^[1]; Geiger, C A ^[2]

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1. Univ. of Chicago, IL (USA)
2. Universitaet Bayreuth (West Germany)

The Hardwood Gneiss is an areally small unit of Precambrian granulite-grade rocks exposed in the Archean gneiss terrane of the southern Lake Superior region. The rocks are located in the southwestern portion of the Upper Peninsula of Michigan and consist of a structurally conformable package of quartzitic, metapelitic, amphibolitic, and metabasic units. Three texturally distinct garnet types are present in the metabasites and are interpreted to represent two metamorphic events. Geothermobarometry indicates conditions of {approximately}8.2-11.6 kbar and {approximately}770C for M1, and conditions of {approximately}6.0-10.1 kbar and {approximately}610-740C for M2. It is proposed that M1 was Archean and contemporaneous with a high-grade metamorphic event recorded in the Minnesota River Valley. The M2 event was probably Early Proterozoic and pre-Penokean, with metamorphic conditions more intense than those generally ascribed to the Penokean Orogeny in Michigan, but similar to the conditions reported for the Kapuskasing zone of Ontario. The high paleopressures and temperatures of the M1 event make the Hardwood Gneiss distinct from any rocks previously described in the southern Lake Superior region, and suggest intense tectonic activity during the Archean.

• OSTI ID: 6250413
• Journal Information: Journal of Geology; (USA), Journal Name: Journal of Geology; (USA) Vol. 98:2; ISSN JGEOA; ISSN 0022-1376
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
FEDERAL REGION V
GARNETS
GEOBAROMETRY
GEOLOGIC AGES
GEOLOGIC FORMATIONS
GEOLOGIC HISTORY
GNEISSES
GREAT LAKES
HIGH TEMPERATURE
LAKE SUPERIOR
LAKES
METAMORPHIC ROCKS
METAMORPHISM
MICHIGAN
MINERALS
NORTH AMERICA
OXYGEN COMPOUNDS
PRECAMBRIAN ERA
RESERVOIR PRESSURE
RESERVOIR TEMPERATURE
ROCKS
SILICATES
SILICON COMPOUNDS
SURFACE WATERS
TECTONICS
TEXTURE
USA
VERY HIGH PRESSURE