February 2004 LIP of the Month

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(corresponds to "Gunbarrel" event, #63 in LIP record database)

The Gunbarrel mafic magmatic event: A key 780 Ma time marker for Rodinia plate reconstructions.

Stephen S. Harlan
Department of Environmental Science and Policy
George Mason University,
Fairfax, Virginia 22030, USA

Larry Heaman
Department of Earth and Atmospheric Sciences
University of Alberta, 1-26
Earth Sciences Building
Edmonton, Alberta T6G 2E3

Anthony N. LeCheminant
Geological Survey of Canada
601 Booth Street, Ottawa
Ontario K1A 0E8,

Wayne R. Premo
U.S. Geological Survey
P.O. Box 25046, MS 963 Federal Center
Denver, Colorado 80225, USA

Published in December 2003 issue of Geology

Download full article in PDF format.

Contributed by S. Harlan sharlan@gmu.edu

Location map showing distribution of Neoproterozoic dikes and sills with ages of 800-763 Ma in the northern Cordillera. Pink areas are locations of Neoproterozoic sedimentary and volcanic strata. Northwest Territories

CC--Carcajou Canyon sill; GB--Gunbarrel gabbro; CG--Calder gabbro; FL--Faber Lake gabbro. British Columbia: ML--Muncho Lake diabase dike. Alberta: SS--sill intruding Siyeh Formation in Purcell Supergroup. Washington: WS--volcanic rocks at base of Windermere Supergroup. Montana: BS--Belt Supergroup sill; WC--gabbro sill in Belt Supergroup strata near Wolf Creek; BT--Christmas Lake dike, Beartooth Mountains; TR--Tobacco Root Mountains. Wyoming: MM--mafic dike at Mount Moran, Teton Mountains.

U-Pb, K-Ar, Ar, and Sm-Nd represent apparent ages generated by uranium-lead, potassium-argon, 40Ar/39Ar, and samarium-neodymium methods, respectively.

Figure and caption corrected from published version.

Precise U-Pb baddeleyite dating of mafic igneous rocks provides evidence for a widespread and synchronous magmatic event that extended for >2400 km along the western margin of the Neoproterozoic Laurentian craton. U-Pb baddeleyite analyses for eight intrusions from seven localities ranging from the northern Canadian Shield to northwestern Wyoming-southwestern Montana are statistically indistinguishable and yield a composite U-Pb concordia age for this event of 780.3 ± 1.4 Ma (95% confidence level). This 780 Ma event is herein termed the Gunbarrel magmatic event. The mafic magmatism of the Gunbarrel event represents the largest mafic dike swarm yet identified along the Neoprotoerozoic margin of Laurentia. The origin of the mafic magmatism is not clear, but may be related to mantle-plume activity or upwelling asthenosphere leading to crustal extension accompanying initial breakup of the supercontinent Rodinia and development of the protoPacific Ocean. The mafic magmatism of the Gunbarrel magmatic event at 780 Ma predates the voluminous magmatism of the 723 Ma Franklin igneous event of the northwestern Canadian Shield by ~60 m.y. The precise dating of the extensive Neoproterozoic Gunbarrel and Franklin magmatic events provides unique time-markers that can ultimately be used for robust testing of Neoproterozoic continental reconstructions.