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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% SiO2, 3.3%–14.3% FeO*, and 2.7%–6.4% MgO; all are low in K2O (<0.7%). The large sill-like plutons are generally higher in silica (average 69% SiO2) than the dikes, pods, and agmatite matrix (average 60% SiO2). 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% SiO2 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.

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