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U-Pb SHRIMP geochronology and trace-element geochemistry of coesite-bearing zircons, North-East Greenland Caledonides

By
William C. McClelland
William C. McClelland
Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844, USA
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Siobhán E. Power
Siobhán E. Power
Department of Geoscience, University of Iowa, Iowa City, Iowa 52242, USA
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Jane A. Gilotti
Jane A. Gilotti
Department of Geoscience, University of Iowa, Iowa City, Iowa 52242, USA
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Frank K. Mazdab
Frank K. Mazdab
U.S Geological Survey–Stanford Ion Probe Laboratory, 367 Panama Mall, Stanford, California 94305, USA
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Brigitte Wopenka
Brigitte Wopenka
Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130, USA
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Published:
January 01, 2006

Obtaining reliable estimates for the timing of eclogite-facies metamorphism is critical to establishing models for the formation and exhumation of high-pressure and ultrahigh-pressure (UHP) metamorphic terranes in collisional orogens. The presence of pressure-dependent phases, such as coesite, included in metamorphic zircon is generally regarded as evidence that zircon growth occurred at UHP conditions and, if dated, should provide the necessary timing information. We report U-Pb sensitive high-resolution ion microprobe (SHRIMP) ages and trace-element SHRIMP data from coesite-bearing zircon suites formed during UHP metamorphism in the NorthEast Greenland Caledonides. Kyanite eclogite and quartzofeldspathic host gneiss samples from an island in Jøkelbugt (78°00′N, 18°04′W) contained subspherical zircons with well-defined domains in cathodoluminescence (CL) images. The presence of coesite is confirmed by Raman spectroscopy in six zircons from four samples. Additional components of the eclogite-facies inclusion suite include kyanite, omphacite, garnet, and rutile. The trace-element signatures in core domains reflect modification of igneous protolith zircon. Rim signatures show flat heavy rare earth element (HREE) patterns that are characteristic of eclogite-facies zircon. The kyanite eclogites generally lack a Eu anomaly, whereas a negative Eu anomaly persists in all domains of the host gneiss. The 207Pb-corrected 206Pb/238U ages range from 330 to 390 Ma for the host gneiss and 330–370 Ma for the kyanite eclogite. Weighted mean 206Pb/238U ages for coesite-bearing domains vary from 364 ± 8 Ma for the host gneiss to 350 ± 4 Ma for kyanite eclogite. The combined U-Pb and REE data interpreted in conjunction with observed CL domains and inclusion suites suggest that (1) Caledonian metamorphic zircon formed by both new zircon growth and recrystallization, (2) UHP metamorphism occurred near the end of the Caledonian collision, and (3) the 30–50 m.y. span of ages records long residence times at eclogite-facies conditions for the UHP rocks of North-East Greenland. This spread in observed ages is interpreted to be characteristic of metamorphic rocks that have experienced relatively long (longer than 10 m.y.) residence times at UHP conditions.

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GSA Special Papers

Ultrahigh-pressure metamorphism: Deep continental subduction

Bradley R. Hacker
Bradley R. Hacker
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William C. McClelland
William C. McClelland
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J.G. Liou
J.G. Liou
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Geological Society of America
Volume
403
ISBN print:
9780813724034
Publication date:
January 01, 2006

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