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The Round Mountain serpentinite melange, northern Coast Ranges of California; an association of backarc and arc-related tectonic units

F. Huot and R. C. Maury
The Round Mountain serpentinite melange, northern Coast Ranges of California; an association of backarc and arc-related tectonic units
Geological Society of America Bulletin (January 2002) 114 (1): 109-123

Abstract

The Round Mountain serpentinite melange, tectonically juxtaposed to the Franciscan Complex and to the Coast Range ophiolite, records structural and petrologic evidence for the tectonic processes that once affected late Mesozoic western North America. The melange mainly includes blocks of ultramafic and mafic compositions in a sheared ultramafic matrix. This matrix is thought to be essentially derived from shearing of highly serpentinized dunites, and more rarely harzburgites with or without clinopyroxene. Primary modal mineralogy of the ultramafic components, together with their spinel compositions and major and trace element chemistries, are consistent with both (1) backarc and (2) forearc and arc origins. Blocks of diabase, basaltic lavas, and rare gabbros display chemical characteristics similar to backarc magmas with specific Zr/Y ratios. The medium-(Zr/Y) group may be generated by approximately 23% of incremental melting from a fertile-mantle source. Rocks related to alkalic magmatism plot in the high-(Zr/Y) group, whereas those influenced by arc processes correspond to the low-(Zr/Y) group. Our data suggest that before the onset of the east-dipping Franciscan subduction, a Middle Jurassic backarc basin slightly affected by alkalic and arc-related magmatic processes (recorded by the first stage of the Coast Range ophiolite) lay west of the ensimatic Smartville island arc that was later involved in the Sierra Nevada orogeny. Subsequent collapse of the backarc basin initiated Franciscan subduction. The Late Jurassic arc and forearc domains (recorded by the second stage of the Coast Range ophiolite) were built on a part of this trapped backarc lithosphere. The Round Mountain serpentinite melange was formed by underthrusting of the upper part of the backarc crust followed by its tectonic accretion to the remaining backarc and the forearc- and arc-related mantle sections of the overriding lithospheric wedge.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 114
Serial Issue: 1
Title: The Round Mountain serpentinite melange, northern Coast Ranges of California; an association of backarc and arc-related tectonic units
Affiliation: Universite de Bretagne Occidentale, Institut Universitaire Europeen de la Mer (IUEM), Plouzane, France
Pages: 109-123
Published: 200201
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 82
Accession Number: 2002-011330
Categories: Structural geologyIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2002016
Illustration Description: illus. incl. sects., 2 tables, geol. sketch map
N39°52'30" - N39°55'30", W122°42'00" - W122°37'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 200205
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