QUARTZ DIORITE AND DIORITE PLUTONS OF THE ELDER CREEK OPHIOLITE, CALIFORNIA: FORE-ARC MAGMATISM IN A SUPRA-SUBDUCTION ZONE OPHIOLITE

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 re-deposited in the overlying Crowfoot Point breccia. It differs from classic ophiolite stratigraphy in having substantial volumes (25-30% of the complex) of felsic plutonic rocks intimately associated with the other lithologies. The dominant felsic lithologies are hornblende quartz-diorite and diorite, which crop out in four distinct associations: (1) sill-like plutons up to 500 m thick and 3 km long that intrude the upper part of plutonic section; (2) dikes 10-15 m thick that cross-cut dunite, wehrlite, or gabbro, (3) as the felsic matrix of igneous breccias (agmatites), and (4) as rare, small pods within the sheeted dike complex. Typical phase assemblages include plagioclase, quartz, and hornblende, with less common pyroxene (both cpx and opx), in a hypidiomorphic texture.

The Elder Creek diorites span a wide range in composition, with 54-75% SiO2, 3.3-14.3% FeO*, and 2.7-6.4% MgO; all are low in K2O (<0.7%) and could be classified as plagiogranites. 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 1000 ppm and Ni up to 200 ppm. High-Mg diorites with 4-6.4% MgO at 53- 63% SiO2 are common in the dike suite, but relatively high mg#s, Cr, and Ni are found in the large plutons as well.

The high MgO, Cr, and Ni exhibited by many of these diorites suggests a relationship to the dunite-wehrlite-clinopyroxenite cumulate suite – possibly as the evolved liquid fraction complementary to the cumulates. Alternatively, the similarity to sanukitoids and other high-Mg diorites suggests a possible link to ridge collision and flux of the mantle wedge with slab melts.