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Early to middle Miocene backarc magmas of the Neuquen Basin; geochemical consequences of slab shallowing and the westward drift of South America

Suzanne Mahlburg Kay and Peter Copeland
Early to middle Miocene backarc magmas of the Neuquen Basin; geochemical consequences of slab shallowing and the westward drift of South America (in Evolution of an Andean margin; a tectonic and magmatic view from the Andes to the Neuquen Basin (35 degrees -39 degrees S lat), Suzanne Mahlburg Kay (editor) and Victor A. Ramos (editor))
Special Paper - Geological Society of America (2006) 407: 185-213

Abstract

New (super 40) Ar/ (super 39) Ar, major and trace element, and isotopic data for ca. 24-15 Ma backarc volcanic rocks from the Sierra de Huantraico, Sierra Negra, and Sierra de Chachahuen-La Matancilla regions (36 degrees S-38 degrees S) in the Neuquen Basin shed light on the early Miocene evolution the the south-central Andes. A model calling for incipient shallowing of the subducting slab under the northern Neuquen Basin and an increase in the rate of westward motion of South America relative to the underlying mantle at ca. 20 Ma can explain many regional features. Early Miocene magmatism in the Neuquen Basin began with the eruption of ca. 24-20 Ma alkali olivine basalts from monogenetic and simple polygenetic centers located up to 500 km east of the trench. Their characteristics (Ta/Hf>0.45, epsilon (sub Nd) = +3.6 to +4.2; La/Ta<14; Ba/La<16; (super 87) Sr/ (super 86) Sr = 0.7037-0.7040) indicate a backarc mantle devoid of arc-like components. These basalts erupted at a time of extension all along the margin during a period of rapid, near-normal Nazca-South America plate convergence when spreading ridges between the Pacific, Nazca, and Antarctic plates were becoming more parallel to the Chile Trench. Ridge rotation along with slab roll-back in response to slow relative motion between South America and the underlying mantle can explain why isotopically enriched magmas erupted far to the east of the trench in a generally extensional regime. Subsequently, 19-15 Ma basaltic to trachyandesitic backarc lavas with weak arc-like La/Ta (15-26), Ba/La (15-32), and Ta/Hf (0.2-4.5) ratios and a more depleted isotopic signature (epsilon (sub Nd) = +3.9 to +4.7; (super 87) Sr/ (super 86) Sr = 0.7033-0.7037) erupted in a contractional regime. Their chemical features fit with incipient shallowing of the Nazca plate under the northern Neuquen Basin. A contractional regime that extended all along the margin can be explained by westward acceleration of South America over the underlying mantle as Nazca-South America plate convergence slowed.


ISSN: 0072-1077
EISSN: 2331-219X
Coden: GSAPAZ
Serial Title: Special Paper - Geological Society of America
Serial Volume: 407
Title: Early to middle Miocene backarc magmas of the Neuquen Basin; geochemical consequences of slab shallowing and the westward drift of South America
Title: Evolution of an Andean margin; a tectonic and magmatic view from the Andes to the Neuquen Basin (35 degrees -39 degrees S lat)
Author(s): Kay, Suzanne MahlburgCopeland, Peter
Author(s): Kay, Suzanne Mahlburgeditor
Author(s): Ramos, Victor A.editor
Affiliation: Cornell University, Department of Earth and Atmospheric Sciences and Institute for the Study of the Continents, Ithaca, NY, United States
Affiliation: Cornell University, Department of Earth and Atmospheric Sciences, Institute for Study of the Continents, Ithaca, NY, United States
Pages: 185-213
Published: 2006
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
ISBN: 978-0-8137-2407-2
ISBN: 0-8137-2407-4
References: 69
Accession Number: 2007-020827
Categories: Igneous and metamorphic petrologySolid-earth geophysics
Document Type: Serial
Bibliographic Level: Analytic
Annotation: Includes three appendices
Illustration Description: illus. incl. sects., 3 tables, sketch maps
S37°45'00" - S36°00'00", W70°15'00" - W68°30'00"
Secondary Affiliation: Universidad de Buenos Aires, ARG, ArgentinaUniversity of Houston, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200712
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