Tonalites, trondhjemites, and diorites of the Elder Creek ophiolite, California: Low-pressure slab melting and reaction with the mantle wedge

Abstract

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 ultramafi cs 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 2 , 3.3%-14.3% FeO*, and 2.7%-6.4% MgO; all are low in K 2 O (<0.7%). The large sill-like plutons are generally higher in silica (average 69% SiO 2) than the dikes, pods, and agmatite matrix (average 60% SiO 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 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 fi eld. 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.

Inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone

“…1; Hopson et al, 1981Hopson et al, , 2008Shervais and Kimbrough, 1985). This ophiolite consists largely of crustal rocks, including both plutonic rocks (i.e., gabbros and diorites) and volcanic or hypabyssal rocks (i.e., boninites, basalts, basaltic andesites, and dacites) (Giaramita et al, 1998;Shervais et al, 2004Shervais et al, , 2005aHopson et al, 2008;Shervais, 2008). It has been dated to between 165 and 172 Ma using U-Pb zircon (Kimbrough in Shervais et al, 2005a;Hopson et al, 2008;Mattinson and Hopson, 2008) and 39 Ar-40 Ar on basaltic glass (Shervais et al, 2005a), and as middle to early late Jurassic (Bajocian through Kimmeridgian) using radiolarian assemblages (Murchey in Shervais et al, 2005a).…”

Subduction initiation along transform faults: The proto-Franciscan subduction zone

“…The most common suites are similar to those found in the Coast Range ophiolite: (1) a primary suite of cumulate dunite and layered gabbros composed of plagioclase and clinopyroxene, with less common olivine, orthopyroxene, or hornblende; (2) a later intrusive suite of wehrlites, pyroxenites, and primitive isotropic gabbros; and (3) a fi nal suite of calc-alkaline dikes and plutons composed of tonalitetrondhjemite-quartz diorite, which may grade into isotropic gabbro as modal quartz declines (Shervais, 2001(Shervais, , 2008Shervais et al, 2004).…”

“…Nonetheless, its origin is controversial and three primary hypotheses have been advanced: (1) formation at a mid-ocean-ridge spreading center at low paleolatitudes, and subsequent rapid drift northward to collide with North America (Hopson et al, 1981Pessagno et al, 2000); (2) formation as a backarc basin behind an east-facing volcanic arc that collided with North America during the Late Jurassic Nevadan orogeny (Godfrey and Klemperer, 1998;Ingersoll, 2000); and (3) formation by forearc or intra-arc rifting along the western margin of North America, in response to nascent or renewed subduction of oceanic plates beneath North America (Shervais and Kimbrough, 1985;Shervais, 1990Shervais, , 2001Stern and Bloomer, 1992). The consensus of most recent studies support the suprasubduction-zone model (Stern and Bloomer, 1992;Shervais et al, 2004Shervais et al, , 2005aShervais, 2008 Subduction initiation along transform faults: The proto-Franciscan subduction zone | RESEARCH Choi et al, 2008a;Jean et al, 2010), but models based on backarc basins and oceanic spreading centers still persist (e.g., Hopson et al, 2008).…”

“…Suite 2 wehrlites, clinopyroxenites, and isotropic gabbros are intrusive into the layered gabbros of suite 1, documented by massive wehrlitepyroxenite dikes that crosscut layering in the gabbro, and by xenoliths of layered gabbro in wehrlite and suite 2 isotropic gabbro (Hopson et al, 1981;Shervais, 2001;Shervais et al, 2004). The TTD suite (suite 3) forms small intrusive bodies, thick (10-15 m) dikes, and fl at-lying, kilometerscale plutons that intrude all of the older plutonic suites and parts of the overlying volcanic-hypabyssal complex (Shervais, 2001(Shervais, , 2008Shervais et al, 2004). Radiometric U-Pb zircon ages for the Coast Range ophiolite represent this fi nal intrusive suite; 238 U-206 Pb ages typically range from ca.…”

Geological, petrological, and geodynamical characteristics of the Karacaali Magmatic Complex (Kırıkkale) in the Central Anatolian Crystalline Complex, Turkey

“…Wenge et al (2005) experimentally showed that tonalitic and granodioritic melts can be formed by solid dehydration melting of amphibolite at 2.0 GPa and 850-1000 °C. On the other hand, tonalitic arc magmatism was reported from oceanic subduction zone settings (Giunta et al, 2006;Shervais, 2008). Yan et al (2010) also suggested 2 possible sources for the origin of tonalitic magma in the granitic rocks from the Nansha microblock (South China Sea): i) partial melting of Precambrian continental basement and Precambrian rocks, and ii) mantle-derived magma, which results from the interaction of released fluids from subducted slab and the overlying mantle wedge.…”