Polymetamorphism of the Precambrian Baltimore Gneiss in southeastern Pennsylvania

Wagner, Mary Emma; Crawford, María Luisa

doi:10.2475/ajs.275.6.653

Cited by 32 publications

(12 citation statements)

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“…1978-Tracy, 1978ITD Scharbert & Kurat, 1974in press) - Perkins & Chipera, 1985ITD Grew, 1981IBC Berg, 1977IBC Harley, 1985aIBC Ellis, 1980;IBC Sandiford, 1985a,6;ITD Harley, 1985ft IBC Waters, 1985IBC Janardhan, Newton & Hansen, 1982-Harley, unpubl. -Perkins, Essene & Marcotty, 1982IBC Sanders, Daly & Davies, 1987 Lai, Ackermand & Upadhyay, 1987 ITD 16-2 IBC IBC --ITD ITD ITD ITD ---ITD, IBC ITD, IBC ITD, IBC ITD ITD IBC -ITD ITD ITD ---IBC, ITD IBC --ITD ITD S. L. H A R L E Y Original reference Bohlen, 1987Arima & Barnett, 1984Wagner & Crawford, 1975Zhang & Cong, 1981(in Newton & Perkins, 1982) Black et at. 1987Ellis, 1983Harley, 1988Harley, 1988Johannson & Moller, 1986Karsef al.…”

Section: B Distributionmentioning

confidence: 97%

The origins of granulites: a metamorphic perspective

1989

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Although many recent reviews emphasize a uniformity in granulite pressure-temperature (P-T) conditions and paths, granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation. A thorough survey of over 90 granulite terranes or occurrences reveals that over 50% of them record P-Tconditions outside the 7.5+ 1 kbar and 8OO + 5O°C average granulite regime preferred by many authors. In particular, an increasing number of very high temperature (900-1000 °C) terranes are being recognized, both on the basis of distinctive mineral assemblages and geothermobarometry. Petrogenetic grid and geothermobarometric approaches to the determination and interpretation of P-T histories are both evaluated within the context of reaction textures to demonstrate that the large range in P-Tconditions is indeed real, and that both near-isothermal decompression (ITD) and near-isobaric cooling (IBC) P-T paths are important. Amphibolite-granulite transitions promoted by the passage of CO 2 -rich fluids, as observed in southern India and Sri Lanka, are exceptional and not representative of fluid-related processes in the majority of terranes. It is considered, on the contrary, that fluid-absent conditions are typical of most granulites at or near the time of their recorded thermal maxima.ITD granulites are interpreted to have formed in crust thickened by collision, with magmatic additions being an important extra heat source. Erosion alone is not, however, considered to be the dominant post-collisional thinning process. Instead, the ITD paths are generated during more rapid thinning (l-2mm/yr exposure) related to tectonic exhumation during moderate-rate or waning extension. IBC granulites may have formed in a variety of settings. Those which show anticlockwise P-T histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion, with or without additional crustal extension. IBC granulites at shallow levels ( < 5 kbar) may also be formed during extension of normal thickness crust, but deeper-level IBC requires more complex models. Many granulites exhibiting IBC at deep crustal levels may have formed in thickened crust which underwent very rapid (5 mm/yr) extensional thinning subsequent to collision. It is suggested that the preservation of IBC paths rather than ITD paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust, and that hybrid ITD to IBC paths should also be observed.Most IBC granulites, and probably many ITD granulites, have not been exposed at the Earth's surface as a result of the tectonic episodes which produced them, but have resided in the middle and lower crust for long periods of time (100-2000 Ma) following these events. The eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation.

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Section: B Distributionmentioning

confidence: 97%

The origins of granulites: a metamorphic perspective

1989

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“…The Baltimore gneiss farther north near Philadelphia retains some granulite facies mineral assemblages (Wagner and Crawford, 1975), but here in the Maryland Piedmont, no relict granulite facies assemblages have been found. This probably reflects a shallower depth of the Baltimore gneiss during the Precambrian episode in this area, not an influx of the hydrolyzing fluid: there is no textural or mineralogic indication of a large-scale fluid activity in these rocks.…”

Section: Summary and Discussionmentioning

confidence: 67%

Petrology of the Baltimore gneiss in the northeast Towson dome, Maryland Piedmont

Olsen¹

1999

The Mid-Atlantic Piedmont: Tectonic Missing Link of the Appalachians

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“…The basement and cover sequences both contain amphibolite-facies assemblages, but the coarsely segregated migmatitic aspect of the basement complex suggests that the Grenville metamorphism was more intense than that of the Paleozoic. Relict granulitefacies mineral assemblages are common in similar Baltimore Gneiss lithologies near Philadelphia (Wagner and Crawford, 1975), but no granulite-facies assemblages have been found in the Baltimore area. Consequently, we suspect that the gneisses at Baltimore were originally metamorphosed under amphibolite-facies conditions only slightly higher than the conditions of the Paleozoic orogeny.…”

Section: Summary Of Conclusionmentioning

confidence: 98%

The Baltimore Gneiss domes of the Maryland Piedmont

Fisher

Olsen²

2004

Gneiss Domes in Orogeny

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Gneiss domes in the Maryland Piedmont near Baltimore contain a wide variety of metamorphic rock types, all metamorphosed to amphibolite grade and collectively named Baltimore Gneiss. Most were derived from interlayered sedimentary and volcanic rocks into which granitic plutons were emplaced in both Precambrian and Paleozoic time. Migmatitic features are common in the Baltimore Gneiss. Most seem to refl ect either original sedimentary layering or original layering that has been enhanced by later metamorphic differentiation, but some involved partial melting, perhaps during the Precambrian metamorphism. We fi nd no evidence for widespread melting in rocks at the present level of exposure during the Paleozoic. Limited fl uid-driven K-metasomatism may well have occurred locally, but we fi nd no evidence for metasomatism on a regional scale. Pressure-temperature conditions for the Baltimore Gneiss during the Paleozoic metamorphism are estimated to be 7-9 kb and 625-675 °C.For half a century, the anticlinal structures exposing basement gneiss at Baltimore have been cited as classic examples of mantled gneiss domes, but more recent fi eld studies and geophysical investigations suggest that they are the result of interference between late upright folds and early nappe structures formed by a combination of isoclinal folding along subhorizontal axial surfaces and thrust faults rooted in a regional décollement.

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Polymetamorphism of the Precambrian Baltimore Gneiss in southeastern Pennsylvania

Cited by 32 publications

References 0 publications

The origins of granulites: a metamorphic perspective

The origins of granulites: a metamorphic perspective

Petrology of the Baltimore gneiss in the northeast Towson dome, Maryland Piedmont

The Baltimore Gneiss domes of the Maryland Piedmont

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