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Tonalites, trondhjemites, and diorites of the Elder Creek Ophiolite, California; low pressure slab melting and reaction with the mantle wedge

John W. Shervais
Tonalites, trondhjemites, and diorites of the Elder Creek Ophiolite, California; low pressure slab melting and reaction with the mantle wedge (in Ophiolites, arcs, and batholiths; a tribute to Cliff Hopson, James E. Wright (editor) and John W. Shervais (editor))
Special Paper - Geological Society of America (2008) 438: 113-132


The Elder Creek ophiolite, which crops out along the South, Middle, and North Forks of Elder Creek, is the largest exposure of mid-Jurassic Coast Range ophiolite in the northern Coast Ranges of California. The Elder Creek ophiolite contains almost all of the components of a classic ophiolite (mantle tectonites, cumulate ultramafics and gabbro, dike complex, volcanics), although most of the volcanic section has been removed by erosion and redeposited in the overlying Crowfoot Point breccia. It differs from classic ophiolite stratigraphy in that it has substantial volumes (25%-30% of the complex) of felsic plutonic rocks intimately associated with the other lithologies. The felsic lithologies include hornblende diorite, quartz-diorite, tonalite, and trondhjemite, which crop out in four distinct associations: (1) as rare, small pods within the sheeted dike complex, (2) as the felsic matrix of igneous breccias (agmatites), (3) 1-25-m-thick dikes that crosscut cumulate or isotropic gabbro, and (4) sill-like plutons up to 500 m thick and 3 km long that intrude the upper part of the plutonic section. Typical phase assemblages include quartz, plagioclase, hornblende, and pyroxene, in a hypidiomorphic texture. The Elder Creek tonalite-trondhjemite-diorite (TTD) suite spans a wide range in composition: 54%-75% SiO (sub 2) , 3.3%-14.3% FeO (super *) , and 2.7%-6.4% MgO; all are low in K (sub 2) O (<0.7%). The large sill-like plutons are generally higher in silica (average 69% SiO (sub 2) ) than the dikes, pods, and agmatite matrix (average 60% SiO (sub 2) ). Mg#'s range from 65 to 17, with Cr up to 227 ppm at 58% silica. High-Mg diorites with 4%-7% MgO at 53%-58% SiO (sub 2) are common in the dike suite, but relatively high MgO, Mg#, and Cr values are found in the large plutons as well. The major- and trace-element characteristics are consistent with partial melting of subducted, amphibolite-facies oceanic crust at relatively low pressures (5-10 kbar) outside the garnet stability field. Melting of subducted oceanic crust at these pressures can only occur during the collision and subduction of an active spreading center. Subsequent reaction of these melts with the overlying mantle wedge has increased their refractory element concentrations. The occurrence of zircons with inherited Pb isotope characteristics implies the involvement of subducted sediments containing an ancient zircon component Formation of the Elder Creek TTD suite was a transient event associated with ridge collision-subduction. This is consistent with previous models for the Coast Range ophiolite and other suprasubduction-zone ophiolites; it is not consistent with an ocean-ridge spreading-center origin.

ISSN: 0072-1077
EISSN: 2331-219X
Serial Title: Special Paper - Geological Society of America
Serial Volume: 438
Title: Tonalites, trondhjemites, and diorites of the Elder Creek Ophiolite, California; low pressure slab melting and reaction with the mantle wedge
Title: Ophiolites, arcs, and batholiths; a tribute to Cliff Hopson
Author(s): Shervais, John W.
Author(s): Wright, James E.editor
Author(s): Shervais, John W.editor
Affiliation: Utah State University, Department of Geology, Logan, UT, United States
Affiliation: University of Georgia, Department of Geology, Athens, GA, United States
Pages: 113-132
Published: 2008
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
ISBN: 978-0-8137-2438-6
Meeting name: Ophiolites, batholiths, and regional geology; a session in honor of Cliff Hopson; Geological Society of America 2005 Cordilleran Section meeting
Meeting location: San Jose, CA, USA, United States
Meeting date: 20050429April 29-May 1, 2005
References: 62
Accession Number: 2008-109990
Categories: Solid-earth geophysicsIgneous and metamorphic petrology
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, geol. sketch maps
N39°30'00" - N40°30'00", W122°30'00" - W121°45'00"
Secondary Affiliation: Utah State University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200840
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