The Round Mountain serpentinite mé lange, 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 mélange 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 ∼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 mélange 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.

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