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

E-mail

 

Larry Heaman

Department of Earth and Atmospheric Sciences

University of Alberta, 1-26 Earth Sciences Building

Edmonton, Alberta T6G 2E3, Canada

 

Anthony N. LeCheminant*

Geological Survey of Canada, 601 Booth Street

Ottawa, Ontario K1A 0E8, Canada

*Present address: 5592 Van Vliet Road, Manotick, Ontario K4M 1J4, Canada

 

Wayne R. Premo

U.S. Geological Survey, P.O. Box 25046, MS 963

Federal Center, Denver, Colorado 80225, USA

 

Harlan, S.S., Heaman, L.W., LeCheminant, A.N., and Premo, W.R., 2003, The Gunbarrel mafic magmatic event: A key 780 Ma time marker for Rodinia plate reconstructions: Geology, v. 13, p. 1053-1056. PDF

 

Abstract

 

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 proto–Pacific 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.

 

Keywords: Neoprotoerozoic; Rodinia; mafic magmatism; geochronology; dike swarms; rifting

 

The Gunbarrel mafic magmatic event was featured as the February 2004 Large Igneous Province of the month by the Large Igneous Provinces Commission.

Location map showing distribution of Neoproterozoic dikes and sills with ages of 800–763 Ma in the northern Cordillera. Dark shaded areas are locations of Neoproterozoic sedimentary and volcanic strata. Northwest Territories: CC—Concajou 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.

 

 

U-Pb concordia plot for baddeleyite analyses from mafic sills and dikes of the Gunbarrel magmatic event.

 

U-Pb Analytical Data

Photograph of Mafic Dike at Mount Moran, Teton Range

Mafic Dikes of the Beartooth Mountains