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Timing of anatexis in the eastern Adirondack Highlands; implications for tectonic evolution during ca. 1050 Ma Ottawan Orogenesis

M. E. Bickford, J. M. McLelland, B. W. Selleck, Barbara M. Hill and M. J. Heumann
Timing of anatexis in the eastern Adirondack Highlands; implications for tectonic evolution during ca. 1050 Ma Ottawan Orogenesis
Geological Society of America Bulletin (August 2008) 120 (7-8): 950-961

Abstract

U-Pb sensitive high-resolution ion micro-probe (SHRIMP) analyses of zircons from migmatitic metapelites in the eastern Adirondack Highlands demonstrate that substantial anatexis took place ca. 1050 Ma during the late, but still high-grade, ca. 1090-1050 Ma Ottawan orogeny. These results contrast with data from metapelites of the southwestern Adirondack Highlands and Adirondack Lowlands, which indicate that anatexis occurred ca. 1200-1160 Ma, during the Shawinigan orogeny and subsequent (ca. 1155 Ma) anorthosite-mangerite-charnockite-granite (AMCG) magmatism. Ca. 1180-1150 Ma zircons from this western regime do not contain ca. 1050 Ma (Ottawan) metamorphic overgrowths. The absence of ca. 1050 Ma Ottawan anatexis and overgrowths in the Adirondack Lowlands is explained by the observation that, consistent with titanite cooling ages, post-1150 Ma temperatures did not exceed approximately 700 degrees C. In the southwestern Adirondack Highlands, the absence of ca. 1050 Ma anatexis can be accounted for by earlier dehydration of metapelites during ca. 1180-1150 Ma Shawinigan-AMCG metamorphism. The occurrence of ca. 1050 Ma anatexis and formation of metamorphic zircons in the eastern Adirondacks cannot be explained by higher temperatures, because geothermometry indicates that all of the Adirondack Highlands reached a nearly uniform 750-800 degrees C during this period. Accordingly, the occurrence of ca. 1050 Ma Ottawan anatexis in the eastern regime is interpreted to be the result of: (1) influxes of hydrous fluids at ca. 1050 Ma, or (2) decompression melting during late extensional exhumation. A recently recognized low-angle late Ottawan (ca. 1050 Ma) fault system may have provided both fluid conduits and footwall decompression. The sense of displacement along the shear zone has not yet been unequivocally determined, but preliminary investigations of kinematic indicators demonstrate normal displacement. Thus, this structure may be an analogue of the ca. 1050 Ma northwest-dipping Carthage-Colton zone normal fault system that juxtaposed the Adirondack Lowlands against the Adirondack Highlands. In this context, these fault zones form a quasi-symmetrical core complex or gneiss dome, within which the high-grade core of the Adirondack Highlands was tectonically exhumed. A similar east-dipping, along-strike normal fault in Quebec (Tawachiche shear zone) exhumed the eastern Morin and Lac Taureau terranes at ca. 1050 Ma and may merge with the eastern Adirondack shear zone described here.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 120
Serial Issue: 7-8
Title: Timing of anatexis in the eastern Adirondack Highlands; implications for tectonic evolution during ca. 1050 Ma Ottawan Orogenesis
Affiliation: Syracuse University, Department of Earth Sciences, Syracuse, NY, United States
Pages: 950-961
Published: 200808
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 45
Accession Number: 2008-104171
Categories: Structural geologyGeochronology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2008086
Illustration Description: illus. incl. 2 tables, geol. sketch maps
N43°27'00" - N43°27'00", W74°15'10" - W74°15'10"
N43°44'03" - N43°44'03", W73°36'19" - W73°36'19"
Secondary Affiliation: Colgate University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 200836
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