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GEOREF RECORD

Structure, Cr-chemistry, and age of the Border Ranges ultramafic-mafic complex; a suprasubduction zone ophiolite complex

Timothy M. Kusky, Adam Glass and Robert D. Tucker
Structure, Cr-chemistry, and age of the Border Ranges ultramafic-mafic complex; a suprasubduction zone ophiolite complex (in Tectonic growth of a collisional continental margin; crustal evolution of Southern Alaska, Kenneth D. Ridgway (editor), Jeffrey M. Trop (editor), Jonathan M. G. Glen (editor) and J. Michael O'Neill (editor))
Special Paper - Geological Society of America (2007) 431: 207-225

Abstract

A distinctive yet enigmatic suite of fault-bounded ultramafic massifs occurs within accretionary complex melange of the McHugh Complex on the Kenai Peninsula of southern Alaska. The largest and most significant of these include Red Mountain and the Halibut Cove Complex, consisting of dunite and pyroxenite with chromite seams and lesser quantities of garnet pyroxenite and gabbro. Several different hypotheses have been advanced to explain their origin. Burns (1985) correlated these fault-bounded ultramafic massifs with others known as the Border Ranges Ultramafic-Mafic Complex. Other parts of the Border Ranges Ultramafic-Mafic Complex are located several hundred kilometers away along the Border Ranges fault, marking the boundary between the Chugach terrane and the Wrangellian composite terrane in the northern and eastern Chugach Mountains. Burns (1985) suggested that this entire group of ultramafic bodies represents the deep roots of the Talkeetna arc developed on the southern margin of Wrangellia during Early Jurassic-Cretaceous subduction. In this model, bodies such as Red Mountain represent klippen thrust hundreds of kilometers southward over the McHugh Complex and now preserved as erosional remnants. Bradley and Kusky (1992) suggested alternatively that the Kenai ultramafic massifs may represent segments of a thick oceanic plate offscraped during subduction, and therefore might represent ophiolitic, oceanic plateau, or immature island arc crust as opposed to the roots of the mature Talkeetna arc. In this scenario, the Kenai ultramafic massifs would be correlative with the McHugh Complex, not the Talkeetna arc. A third hypothesis is that the Border Ranges Ultramafic-Mafic Complex may represent forearc or suprasubduction zone ophiolites formed seaward of the Talkeetna arc during early stages of its evolution and incorporated into the accretionary wedge during subsequent accretion tectonics. The implications of which of these models is correct are large because the Talkeetna arc section is the world's premiere example of a complete exposed arc sequence, including the volcanic carapace through deep crustal levels. Many models for the composition and evolution of the crust rely on the interpretation that this is a coherent and cogenetic section of arc crust. We report six new U/Pb zircon ages that show that at least some of the deep ultramafic and mafic complexes of the Border Ranges Ultramafic-Mafic Complex are Triassic (227.7+ or -0.6 Ma; Norian) and significantly older than structurally overlying Jurassic rocks of the Talkeetna arc (201-181 Ma, continuing plutonism until 163 Ma) but the same age as the surrounding Triassic-Jurassic-Cretaceous McHugh Complex. New geochemical data that show that rocks of the Border Ranges Ultramafic-Mafic Complex have ophiolitic affinities, with Cr-chemistry further indicating that the complex's rocks formed in a suprasubduction zone ophiolite. Regional and detailed and field observations show that rocks of the complex are similar to and can be structurally restored with other fault-bounded units in the McHugh Complex melange, and that a crude ophiolitic stratigraphy can be reconstructed through the Border Ranges Ultramafic-Mafic Complex and McHugh Complex. We suggest that the Border Ranges Ultramafic-Mafic Complex represents the forearc oceanic basement upon which the Talkeetna arc was subsequently built. The conclusion that the Border Ranges Ultramafic-Mafic Complex does not represent the base of the Talkeetna arc but instead contains remnants of a dismembered ophiolitic complex raises questions about the validity of mass balance calculations and bulk crustal compositions, as well as models of arc development used to understand the growth of continental crust.


ISSN: 0072-1077
EISSN: 2331-219X
Coden: GSAPAZ
Serial Title: Special Paper - Geological Society of America
Serial Volume: 431
Title: Structure, Cr-chemistry, and age of the Border Ranges ultramafic-mafic complex; a suprasubduction zone ophiolite complex
Title: Tectonic growth of a collisional continental margin; crustal evolution of Southern Alaska
Author(s): Kusky, Timothy M.Glass, AdamTucker, Robert D.
Author(s): Ridgway, Kenneth D.editor
Author(s): Trop, Jeffrey M.editor
Author(s): Glen, Jonathan M. G.editor
Author(s): O'Neill, J. Michaeleditor
Affiliation: Saint Louis University, Department of Earth and Atmospheric Sciences, St. Louis, MO, United States
Affiliation: Purdue University, Department of Earth and Atmospheric Sciences, West Lafayette, IN, United States
Pages: 207-225
Published: 2007
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
ISBN: 978-0-8137-2431-7
References: 89
Accession Number: 2008-063999
Categories: Solid-earth geophysicsGeochronology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., 2 tables, geol. sketch maps
N59°00'00" - N60°00'00", W152°00'00" - W150°00'00"
N59°19'30" - N59°23'60", W151°34'30" - W151°26'60"
Secondary Affiliation: Bucknell University, USA, United StatesU. S. Geological Survey, USA, United StatesDepartment of Geolgoical Sciences, GBR, United KingdomWashington University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200815
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