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The age and origin of a thick mafic–ultramafic keel from beneath the Sierra Nevada batholith

Ducea, Mihai N. and Saleeby, Jason B. (1998) The age and origin of a thick mafic–ultramafic keel from beneath the Sierra Nevada batholith. Contributions to Mineralogy and Petrology, 133 (1-2). pp. 169-185. ISSN 0010-7999. http://resolver.caltech.edu/CaltechAUTHORS:20140826-144105135

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Abstract

We present evidence for a thick (∼100 km) sequence of cogenetic rocks which make up the root of the Sierra Nevada batholith of California. The Sierran magmatism produced tonalitic and granodioritic magmas which reside in the Sierra Nevada upper- to mid-crust, as well as deep eclogite facies crust/upper mantle mafic–ultramafic cumulates. Samples of the mafic–ultramafic sequence are preserved as xenoliths in Miocene volcanic rocks which erupted through the central part of the batholith. We have performed Rb-Sr and Sm-Nd mineral geochronologic analyses on seven fresh, cumulate textured, olivine-free mafic–ultramafic xenoliths with large grainsize, one garnet peridotite, and one high pressure metasedimentary rock. The garnet peridotite, which equilibrated at ∼130 km beneath the batholith, yields a Miocene (10 Ma) Nd age, indicating that in this sample, the Nd isotopes were maintained in equilibrium up to the time of entrainment. All other samples equilibrated between ∼35 and 100 km beneath the batholith and yield Sm-Nd mineral ages between 80 and 120 Ma, broadly coincident with the previously established period of most voluminous batholithic magmatism in the Sierra Nevada. The Rb-Sr ages are generally consistent with the Sm-Nd ages, but are more scattered. The ^(87)Sr/^(86)Sr and ^(143)Nd/^(144)Nd intercepts of the igneous-textured xenoliths are similar to the ratios published for rocks outcroping in the central Sierra Nevada. We interpret the mafic/ultramafic xenoliths to be magmatically related to the upper- and mid-crustal granitoids as cumulates and/or restites. This more complete view of the vertical dimension in a batholith indicates that there is a large mass of mafic–ultramafic rocks at depth which complement the granitic batholiths, as predicted by mass balance calculations and experimental studies. The Sierran magmatism was a large scale process responsible for segregating a column of ∼30 km thick granitoids from at least ∼70 km of mainly olivine free mafic–ultramafic residues/cumulates. These rocks have resided under the batholith as granulite and eclogite facies rocks for at least 70 million years. The presence of this thick mafic–ultramafic keel also calls into question the existence of a “flat” (i.e., shallowly subducted) slab at Central California latitudes during Late Cretaceous–Early Cenozoic, in contrast to the southernmost Sierra Nevada and Mojave regions.


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http://dx.doi.org/10.1007/s004100050445 DOIArticle
http://link.springer.com/article/10.1007%2Fs004100050445PublisherArticle
Additional Information:© 1998 Springer-Verlag. Received: 27 December 1997; Accepted: 11 June 1998. This research was funded through the NSF grant EAR-9526859 (to J. Saleeby). M. Ducea acknowledges grant #5810-96 from the Geological Society of America. Earlier versions of this manuscript were reviewed by Roberta Rudnick, Lang Farmer, John Eiler, James Chen, and Julia Goreva. Journal reviews by Jay Ague and Drew Coleman have significantly improved the quality of the manuscript. This is California Institute of Technology Division of Geological and Planetary Sciences contribution # 8483.
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NSFEAR-9526859
Geological Society of America5810-96
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Caltech Division of Geological and Planetary Sciences8483
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ID Code:48942
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Deposited By: Tony Diaz
Deposited On:26 Aug 2014 21:54
Last Modified:26 Aug 2014 21:54

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