Sm-Nd and U-Pb Isotopic Evidence of Juvenile Crust in the Adirondack Lowlands and Implications for the Evolution of the Adirondack Mts

Sm-Nd investigations of Hyde School Gneiss yield $T_{DM}$ ages only slightly older than U-Pb zircon emplacement ages thus indicating substantial addition of juvenile, calcalkaline crust to the Adirondack lowlands at ca. 1300 Ma. Juvenile crust of similar age has been recognized within the Adirondack...

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Bibliographic Details
Published in:The Journal of geology 1993-01, Vol.101 (1), p.97-105
Main Authors: McLelland, James M., Daly, J. Stephen, Chiarenzelli, Jeffrey
Format: Article
Language:English
Subjects:
Geochronology
Geological time
Geology
Gneiss
Granite
Highlands
Lowlands
Magmatism
Monazites
Mountains
Orogeny
Rocks
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Summary:Sm-Nd investigations of Hyde School Gneiss yield $T_{DM}$ ages only slightly older than U-Pb zircon emplacement ages thus indicating substantial addition of juvenile, calcalkaline crust to the Adirondack lowlands at ca. 1300 Ma. Juvenile crust of similar age has been recognized within the Adirondack highlands, the Elzevir terrane further to the northwest in Canada, and in the Green Mts. of Vermont suggesting the presence of an extensive magmatic arc at that time. While the precise nature of the arc remains uncertain, an Indonesian analogue is favored. U-Pb zircon ages of ca. 1150-1160 Ma are reported for lowlands samples of syenitic and granitic rocks belonging to the anorthosite-mangerite-charnockite-granite (AMCG) suite that also occurs in the highlands, as well as in the Canadian sector of the Frontenac terrane. The regional occurrence of these anorogenic intrusions indicates that the Adirondack highlands and Frontenac terrane, including the Adirondack lowlands, constituted a single, contiguous entity by ca. 1160 Ma. Monazite ages from AMCG rocks and metamorphic zircons from Hyde School Gneiss indicate that the last high grade metamorphism (i.e., upper amphibolite facies) in the Adirondack lowlands occurred at ca. 1150 Ma. The presence of 1050 Ma granulite facies assemblages east of the Carthage-Colton mylonite zone (CCMZ) in the highlands indicates the presence of a discontinuity across the CCMZ, which may manifest ca. 1050 Ma downfaulting of the lowlands block along the northwest dipping CCMZ in response to verthickened crust during the Ottawan orogeny.
ISSN:0022-1376
1537-5269
DOI:10.1086/648198