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Shervais, J.W., 2008, Tonalites, trondhjemites, and diorites of the Elder Creek ophiolite, California: Low-pressure slab melting and reaction with the mantle wedge, in Wright, J.E., and Shervais, J.W., eds., Ophiolites, Arcs, and Batholiths: A Tribute to Cliff Hopson: Geological Society of America Special Paper 438, p. 113–132, doi: 10.1130/2008.2438(03). For permission to copy, contact editing@geosociety.org. ©2008 The Geological Society of America. All rights reserved.

The Geological Society of AmericaSpecial Paper 438

2008

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

John W. ShervaisDepartment of Geology, Utah State University, 4505 Old Main Hill, Logan, Utah 84322-4505, USA

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 dif-fers 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 trond-hjemite, 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 plu-tonic 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 fi eld. Melting of subducted oceanic crust at these pres-sures 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 tran-sient 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.

Keywords: ophiolite, suprasubduction zone, slab melting, granitic rocks, arc magmatism.

114 Shervais

INTRODUCTION

Ophiolites have long been recognized as remnants of oceanic lithosphere, but the site and circumstances of their origins have been the subject of much controversy. Early studies focused on ophiolites as an analogue for oceanic crust formed at mid-ocean-ridge spreading centers (e.g., Moores et al., 1968; Moores, 1969; Dewey and Bird, 1971; Gass et al., 1975; Moores, 1975; Moore and Liou, 1979; Smewing, 1981). Later work proposed that most ophiolites, including the classic occurrences in Cyprus and Oman, formed above subduction zones in an environment related to island-arc volcanism (e.g., Miyashiro, 1973; Pearce, 1975, 1980; Alabaster et al., 1982; Shervais, 1982; Shervais and Kimbrough, 1985). It is now recognized that many ophiolites exhibit complex, multistage histories that include both oceanic-ridge and suprasubduction phases, although suprasubduction-zone processes are dominant (e.g., Shervais, 2001; Dilek, 2003; Smith and Rassios, 2003; Shervais et al., 2004a).

The Coast Range ophiolite of California has played a impor-tant role in the evolution of geologic thought on ophiolite genesis, from mid-ocean-ridge volcanism to back-arc basin spreading, and from simple suprasubduction-zone processes to complex multistage evolution involving suprasubduction-zone volcanism and spreading-center collision (Evarts, 1977; Hopson and Frano, 1977; Hopson et al., 1981; Shervais and Kimbrough, 1985; Shervais , 1990; Dickinson et al., 1996; Godfrey and Klemperer, 1998; Ingersoll, 2000; Shervais et al., 2004a, 2005a, 2005b). It is one of the most extensive ophiolite terranes in North America; it has an outcrop belt over 700 km long in California and cor-relative age terranes in Baja California, Oregon, and Washington (Metzger et al., 2002; Kimbrough and Moore, 2003). Its tectonic affi nity has remained controversial, despite the weight of data that require a suprasubduction origin.

In this contribution, I document the extensive occurrence of silicic plutonic rocks in the Elder Creek ophiolite, one of the larger and more complete remnants of Coast Range ophiolite (Fig. 1). These rocks represent the third magmatic suite common in many ophiolites, as documented by Shervais (2001), and pres-ent prima facie evidence against formation at an oceanic spread-ing center. Rather, these silicic plutonic rocks, which include tonalites, trondhjemites, diorites, and quartz diorites, are consis-tent with formation of the ophiolite in a suprasubduction-zone setting, such as a primitive island arc or forearc.

METHODS

Eight weeks of fi eld mapping were carried out at a scale of 1:12,000 in parts of two 1:24,000-scale U.S. Geological Survey quadrangles (Riley Ridge, Raglin Ridge), thin slivers along the western margin of the Paskenta and Lowry quadrangles, and the NE corner of the Hall Ridge quadrangle. The mapping formed part of a M.Sc. thesis by Joe Beaman at the University of South Carolina (Beaman, 1991). Additional mapping was carried out by Clark Blake and Angela Jayco of the U.S. Geological Survey (USGS).

Mineral chemistry for major and minor elements was obtained with a Cameca SX50 four-wavelength spectrometer automated electron microprobe at the University of South Caro-lina. Analyses were made at 15 kV accelerating voltage, 25 nA probe current, and counting times of 20–100 s, using natural and synthetic mineral standards. Analyses were corrected for instrumental drift and dead time, and electron beam–matrix effects using the φ–ρ-Z correction procedure provided with the Cameca microprobe automation system (Pouchou and Pichoir, 1991). Relative accuracy of the analyses, based upon comparison between measured and published compositions of the standards, is ≈1%–2% for oxide concentrations greater than 1 wt% and ≈10% for oxide concentrations <1 wt%.

I present new whole-rock geochemistry for 30 samples of diorite, tonalite, and trondhjemite. Representative samples were

38° N

36° N

40° N

124° 122°

40° N

38° N

36° N

Point Sal

Llanada

Cuesta Ridge

Quinto Creek

Mount Diablo

Sierra Azul

Geyser Peak / Black Mountain

Stonyford

SAF

SNF

Modoc Plateau

Coast Range Ophiolite

Great Valley Sequence

Franciscan Complex

Sierra Nevada

Klamath terranes

Tertiary

Salinia

Stanley Mtn

Leona Rhyolite

Del Puerto

Harbin Springs

Mt. St. Helena

Healdsburg

Elder Creek

124° W 122° W 120° W

Figure 1. Location map showing main lithotectonic units of Califor-nia and locations of largest exposures of Coast Range ophiolite. Elder Creek is the northernmost exposure and one of the largest. SAF—San Andreas fault, SNF—Sur Nacimiento fault. This fi gure was modifi ed after Shervais et al. (2004a).

Tonalites, trondhjemites, and diorites of the Elder Creek ophiolite, California 115

powdered in a tungsten carbide shatterbox and split for prepara-tion of fused glass discs and pressed powder pellets. Whole-rock major-element chemistry data were determined by X-ray fl uores-cence (XRF) analysis using a Philips 2400 XRF spectrometer at the University of South Carolina, using glass discs prepared from 1 g of pre-ignited sample fused with 5 g of lithium metaborate fusion fl ux. Zr and Sr were determined on these glass disks, and Ni was measured on pressed powder pellets, using the same instru-ment. All other trace elements (Th, U, Pb, Nb, Hf, Ta, Y, Rb, Cr, Sc, V, Ba, and the rare earth elements) were determined at Cente-nary College using a Perkin Elmer Elan 6000 quadrapole induc-tively coupled plasma–mass spectrometer (ICP-MS; 26 samples). Samples were dissolved in Tefl on bombs in a microwave diges-tion oven. XRF data were calibrated with a series of USGS rock standards. Analytical uncertainties were typically 2%–4% relative for major elements and 2%–10% relative for the trace elements. Major- and trace-element data are presented in Table 1.

FIELD SETTING

The Elder Creek ophiolite is the northernmost exposure of the Coast Range ophiolite in California (Fig. 1). It preserves a nearly complete ophiolite sequence, with cumulate mafi c and ultramafi c rocks, noncumulate gabbro and diorite, dike complex, and volcanic rocks (Shervais and Beaman, 1991, 1994; Shervais, 2001; Shervais et al., 2004a). Field relations and geochemistry indicate that four magmatic suites are present: (1) layered mafi c and ultramafi c cumulates, with associated isotropic gabbro, dike complex, and massive volcanics; (2) clinopyroxenite-wehrlite intrusions with less common gabbro and gabbro pegmatoid; (3) felsic plutonic rocks; and (4) basaltic dikes with mid-ocean-ridge basalt (MORB)–like geochemistry that crosscut rocks of the older episodes (Shervais and Beaman, 1991, 1994; Shervais, 2001; Shervais et al., 2004a). The ophiolite generally dips steeply to the east and plunges gently to the south (Fig. 2). As a result, cumulate plutonic rocks are exposed in the western and northern portions of the ophiolite, whereas upper-crustal lithologies (dike complex, vol-canics) crop out to the east and south. Felsic plutonic rocks make up ~25%–30% of the mapped lithologies, which generally crop out stratigraphically above the cumulate plutonic sequence. Upper-crustal lithologies were largely removed by erosion on the seafl oor prior to deposition of the overlying Crowfoot Point breccia , but they are preserved at the south end of the ophiolite and in fault blocks (Fig. 2). The Crowfoot Point breccia overlies the ophiolite unconformably along an erosion surface that cuts down-section to the north; as a result, the breccia rests on sill complex at the south end of the massif (Toomes Camp Road and Digger Creek sec-tions), on cumulate plutonic rocks in the central part of the ophio-lite (South Fork of Elder Creek), and on diorite at the northern end of the massif (Middle Fork of Elder Creek; Fig. 2).

Felsic lithologies crop out in four distinct associations: (1) small, meter-scale pods or dikes within the mafi c dike complex; (2) as the matrix of igneous breccias (agmatites) in diorite-gabbro intrusive complexes, (3) 1–25-m-thick dikes that

crosscut the cumulate plutonic series (and in places, agmatite complexes), and (4) sill-like plutons up to 500 m thick and 3 km long that intrude the upper part of the plutonic section; some of the plutons are bordered along their eastern (upper) margins by a complex of feldspar-phyric dikes that are compositionally identical to the adjacent plutons.

The meter-scale pods in the mafi c dike complex represent the classic “plagiogranite” association (Fig. 3A). These pods are rare, in part because much of the mafi c dike complex has been removed by erosion. Agmatites are more common, and they contain blocks of mafi c dike complex, isotropic gabbro, and layered cumulate gabbro in a matrix of felsic diorite (Figs. 3B–3D). In some agma-tites, the clasts are relatively large and angular and have sharp con-tacts with the adjacent matrix (e.g., Fig. 3D); in others, the clasts have disaggregated in part to form microxenoliths in the matrix, making it diffi cult to separate a “xenolith-free” sample for analy-sis (Fig. 3B). Less commonly, the xenoliths have sharp but curva-ceous contacts that suggest that the wall rock was still hot when it was invaded by the felsic magma and brecciated (Fig. 3C).

These igneous breccias comprise much of what had been mapped previously as isotropic gabbro. The agmatites may include signifi cant isotropic gabbro and form diorite-gabbro intrusive complexes. The most extensive diorite intrusive complex in the Elder Creek ophiolite is the Riley Ridge diorite complex, which is exposed on the northern fl ank of Eagle Peak, the south end of Riley Ridge, and along adjacent stretches of the South Fork of Elder Creek (Fig. 2). The agmatites document magmatic activity that postdates formation of the “classic” ophiolite stratig-raphy (layered gabbros, isotropic gabbros, mafi c dike complex).

The decameter-scale dikes (1–25 m thick) that crosscut the cumulate plutonic series and diorite-isotropic gabbro agmatite complexes are oriented at a high-angle to cumulate layering and to the unconformable contact of the Crowfoot Point breccia on the ophiolite. The thicker dikes are texturally zoned, with coarse-grained hypidiomorphic cores and fi ne-grained chilled margins. These dikes are more resistant to erosion than the lay-ered gabbros they intrude, so they form prominent ridges that stand out in relief where exposed along the South Fork of Elder Creek (Figs. 4A and 4B). The approximate locations of several prominent dikes along the South Fork of Elder Creek are shown on an oblique air photograph in Figure 4B. These dikes are ori-ented at a high-angle to the sill-like plutons that lie up-section from them (described below); as a result, they may represent feeders to the overlying plutons.

The largest felsic bodies in the Elder Creek ophiolite are sheet-like plutons up to 500 m thick and 3 km long that intrude the upper part of the cumulate plutonic section (Fig. 4B). These plutons are oriented subparallel to cumulate layering in the underlying gabbros and to the unconformable contact of the Crowfoot Point breccia. We interpret these plutons to represent horizontal tabular plutons (sills) intruded along the interface between the underlying cumu-late plutonic section (formed in the ophiolite magma chamber) and the superjacent mafi c dike complex prior to disruption of the ophio lite on the seafl oor to form the Crowfoot Point breccia.

TABLE 1. WHOLE-ROCK MAJOR- AND TRACE-ELEMENT ANALYSES OF ELDER CREEK TONALITIES, TRONDHJEMITES, AND DIORITES, WITH CIPW NORMS

Sample no. EC137–1 EC142–3 EC49–1 EC52–1 EC61–1 EC62–1B EC63–1 EC99–1 EC108–1 EC133–1 Rock type diorite diorite diorite diorite diorite quartz diorite diorite diorite quartz diorite diorite Occurrence agmatite

matrix xeno in

agmatiteRiley Ridge

complex Riley Ridge

complex Riley Ridge complex -

dike

Riley Ridge complex -

dike

Riley Ridge complex

Riley Ridge complex -

dike

Elder Creek -thick dike

Elder Creek -thick dike

SiO2 53.30 53.60 55.70 54.50 59.34 58.71 56.27 54.30 58.75 54.60 TiO2 0.99 0.54 0.78 0.80 1.13 0.81 0.79 0.83 0.90 0.99 Al2O3 17.16 13.59 14.06 17.14 15.19 18.59 16.70 15.85 17.78 14.36 FeO* 6.22 8.13 11.17 8.00 7.42 6.01 7.69 8.11 8.20 12.92 MnO 0.12 0.17 0.19 0.17 0.10 0.17 0.14 0.13 0.13 0.19 MgO 5.90 10.77 4.05 6.13 3.94 1.53 5.74 6.37 3.77 4.64 CaO 11.01 8.75 7.32 7.29 5.20 6.04 8.02 8.67 6.68 5.38 Na2O 3.76 2.72 5.13 4.43 7.04 5.36 4.92 4.66 3.66 5.27 K2O 0.67 0.62 0.24 0.50 0.05 0.48 0.42 0.06 0.05 0.03 P2O5 0.13 0.04 0.08 0.12 0.12 0.22 0.11 0.08 0.14 0.08 Sum 99.26 98.93 98.72 99.08 99.53 97.92 100.80 99.06 100.06 98.46 Mg# 62.9 70.2 39.3 57.7 48.6 31.2 57.1 58.3 45.0 39.0

Hf 1.02 0.91 1.43 1.4 0.95 1.57 1.89 1.28 Ta 0.28 0.22 0.14 0.31 0.31 0.21 0.37 0.12 Nb 1.7 0.47 0.78 2.45 1.26 4 2.64 0.64 Zr 80 41 63 78 94 50 64 66 132 55 Y 15.61 11.14 17.44 22.66 15.24 24 23.04 18.65 Sr 364 185 196 243 41 312 205 73 251 45 Rb 3.86 3.17 2.39 0.28 2.24 4 0.26 0.05 Sc 34.2 38.9 36.3 26.3 30.9 32 18.2 36.1 V 202.0 196.7 345.0 279 268.8 14 225.7 218 154.3 622.1 Cr 173.6 521.3 8.4 2.2 109.8 142 20.7 8.9 Ni 40 144 7 31 6 1 50 60 11 10 Ba 77.84 55.7 90.66 36.3 72.01 13.75 2.79 La 4.47 1.55 4.48 4.12 3.81 3.38 7.48 2.31 Ce 11.01 4.52 10.12 11. 68 9.27 9.05 18.33 6.13 Pr 1.71 0.76 1.51 1.97 1.42 2.79 1.01 Nd 8.28 3.93 7.23 9.88 6.85 5.74 13.1 5.23 Sm 2.48 1.37 2.2 3.11 2.1 2.1 3.66 1.84 Eu 1.02 0.52 0.63 1.08 0.78 0.87 1.16 0.69 Gd 3.07 1.87 2.82 3.82 2.67 4.31 2.67 Tb 0.51 0.33 0.49 0.64 0.44 0.43 0.7 0.51 Dy 3.19 2.13 3.25 4.15 2.88 4.43 3.48 Ho 0.67 0.46 0.73 0.9 0.63 0.93 0.79 Er 1.87 1.33 2.2 2.59 1.82 2.63 2.35 Tm 0.27 0.2 0.33 0.39 0.27 0.38 0.35 Yb 1.66 1.29 2.15 2.39 1.73 1.81 2.54 2.32 Lu 0.25 0.19 0.34 0.37 0.27 0.29 0.35 0.37 Pb 0.53 0.61 0.82 0.62 0.99 1.06 0.7 Th 0.44 0.21 0.64 0.88 0.36 0.9 0.38 U 0.15 0.07 0.23 0.24 0.15 0.29 0.16 Sr/Y 23.32 16.6 11.24 1.81 13.45 10.89 2.41 Cr/Y 11.12 46.78 0.48 0.1 7.21 5.92 0.9 0.48 La/Yb 2.7 1.2 2.08 1.72 2.2 2.94 0.99 La/Yb-ch 1.94 0.86 1.49 1.23 1.58 2.11 0.71

CIPW norm Quartz 0.02 1.22 0.82 8.55 0.31 13.02 0.15 Anorthite 27.26 22.60 14.15 24.74 9.31 24.70 21.19 21.45 30.94 14.96 Albite 32.90 23.90 44.56 38.69 60.20 47.32 42.07 40.55 31.82 45.87 Orthoclase 3.96 3.72 1.42 2.95 0.3 2.9 2.48 0.35 0.3 0.18 Corundum Diopside 21.01 16.36 17.94 8.23 12.61 2.81 13.62 16.52 0.25 9.12 Hypersthene 8.66 30.29 16.3 21.11 12.53 10.13 16.46 15.09 19.65 24.47 Olivine 2.49 0.52 0.6 Ilmenite 1.9 1.04 1.5 1.54 2.15 1.58 1.48 1.6 1.71 1.9 Magnetite 1.51 1.99 2.73 1.94 1.8 1.48 1.84 3.94 1.97 3.16 Apatite 0.3 0.09 0.19 0.28 0.28 0.53 0.25 0.2 0.32 0.19

(continued.)

Sample no. EC139–3 EC145–1 EC145–3 EC278–1 EC317–1 EC176–1 PK10–1 EC218–2A EC220–1 EC223–2 Rock type quartz diorite trondhjemite tonalite tonalite diorite trondhjemite trondhjemite trondhjemite tonalite trondhjemiteOccurrence Elder Creek -

thick dike Elder Creek -

thick dike Elder Creek -

thick dike Elder Creek -

thick dike Elder Creek -

thick dike Eagle Peak

felsiteEagle Peak

plutonPellows pluton

Pellows pluton

Pellows pluton

SiO2 58.50 68.20 64.30 71.91 55.40 69.38 76.42 74.42 61.82 62.42 TiO2 0.86 0.72 1.05 0.42 1.09 0.59 0.34 0.37 0.78 0.94 Al2O3 14.85 14.74 15.72 12.33 15.80 15.18 11.61 13.25 15.74 16.47 FeO* 6.96 4.24 5.97 3.25 6.77 4.67 2.52 3.55 6.23 9.12 MnO 0.08 0.07 0.11 0.12 0.07 0.04 0.05 0.04 0.09 0.05 MgO 5.77 2.01 1.88 1.12 7.17 1.20 1.30 0.94 2.80 3.42 CaO 7.74 2.37 4.71 4.62 7.96 1.61 1.93 0.94 7.66 1.57 Na2O 3.63 6.67 4.82 4.26 5.01 7.17 4.37 6.17 3.40 5.53 K2O 0.63 0.29 0.45 0.03 0.32 0.28 1.46 0.22 0.19 0.35 P2O5 0.14 0.16 0.30 0.09 0.14 0.11 0.00 0.08 0.12 0.14 Sum 99.16 99.47 99.31 98.15 99.73 100.23 100.00 99.98 98.83 100.01 Mg# 59.7 45.8 36.0 38.2 65.4 31.4 47.9 32.2 44.5 40.1

Hf 1.63 0.6 1.17 3.25 1.16 1.02 2.17 0.8 1.11 0.93 Ta 0.29 0.61 0.55 0.92 0.18 0.72 0.36 0.52 0.47 0.44 Nb 2.15 5.88 5.96 4.72 1.86 5.32 3.03 2.15 2.02 2.9 Zr 94 194 153 177 71 234 127 122 91 117 Y 23.28 26.55 32.57 28.7 18.99 28.09 21.24 10.3 16.07 26.02 Sr 259 215 269 174 255 121 100 88 170 107 Rb 3.69 1.08 1.61 0.32 2.22 0.8 7.11 0.74 1.4 2.47 Sc 24.2 12.7 16.3 7.4 29.3 10.3 8.4 7.9 17.9 16.8 V 171.4 23.0 47.8 22.4 192.0 27.3 24.8 7.8 205.7 199.2 Cr 227.5 1.4 1.4 16.7 115.7 1.4 1.6 1.4 21.1 0.9 Ni 58 2 1 1 Ba 50.14 54.7 77.48 10.97 37.41 68.68 425.37 19.62 29.41 31.12 La 5.67 9.11 9.14 10.91 4.32 8.44 7.66 4.64 6.09 9.84 Ce 15.28 19.03 23.47 27.23 10.84 16.8 17.57 8.49 14.16 23.03 Pr 2.51 3.55 3.85 3.99 1.72 3.29 2.82 1.65 2.05 3.45 Nd 12.15 16.56 18.88 18 8.39 15.08 12.94 7.18 9.13 15.53 Sm 3.56 4.67 5.52 4.8 2.61 4.29 3.62 1.81 2.51 4.23 Eu 1.1 1.35 1.65 1.4 1 1.49 1.02 0.83 1.02 1.29 Gd 4.24 5.42 6.53 5.36 3.3 5.09 4 1.98 2.99 4.83 Tb 0.69 0.88 1.04 0.87 0.55 0.87 0.67 0.32 0.49 0.81 Dy 4.41 5.55 6.54 5.51 3.59 5.68 4.3 2.12 3.04 5.14 Ho 0.95 1.19 1.36 1.16 0.78 1.25 0.91 0.47 0.65 1.07 Er 2.73 3.43 3.75 3.32 2.27 3.6 2.63 1.42 1.87 3.05 Tm 0.41 0.5 0.52 0.5 0.33 0.54 0.37 0.22 0.27 0.44 Yb 2.58 3.14 3.19 3.3 2.13 3.5 2.44 1.46 1.73 2.72 Lu 0.39 0.46 0.44 0.47 0.31 0.52 0.34 0.22 0.25 0.39 Pb 0.41 0.47 0.39 2.7 0.28 0.57 1.22 0.89 0.66 0.43 Th 0.93 1.18 0.88 1.26 0.49 1.42 1.04 0.99 1.11 1.05 U 0.25 0.22 0.23 0.46 0.19 0.23 0.4 0.18 0.3 0.4 Sr/Y 11.13 8.1 8.26 6.06 13.43 4.31 4.71 8.55 10.58 4.11 Cr/Y 9.78 0.05 0.04 0.58 6.09 0.05 0.07 0.13 1.31 0.03 La/Yb 2.2 2.9 2.87 3.31 2.03 2.41 3.14 3.17 3.53 3.62 La/Yb-ch 1.58 2.08 2.06 2.37 1.46 1.73 2.25 2.27 2.53 2.60

CIPW norm Quartz 9.59 18.78 19.76 36.19 18.99 38.29 32.43 19.93 15.58 Anorthite 21.77 9.00 19.30 14.13 18.97 6.84 7.42 3.89 26.59 6.48 Albite 31.73 57.17 41.78 37.25 43.22 60.88 37.28 52.46 29.86 47.10 Orthoclase 3.72 1.71 2.66 0.18 1.89 1.65 8.63 1.3 1.18 2.07 Corundum 0.42 1.34 4.47 Diopside 12.41 1.06 1.27 6.69 15.26 1.53 8.46 Hypersthene 17.12 9.51 11.06 3.72 11.7 8.72 5.6 6.84 10.68 19.99 Olivine 4.92 Ilmenite 1.65 1.37 2.01 0.82 2.07 1.12 0.65 0.7 1.5 1.79 Magnetite 1.7 1.03 1.45 0.8 1.64 1.13 0.61 0.86 1.52 2.2 Apatite 0.32 0.37 0.7 0.21 0.32 0.25 0.19 0.28 0.32

(continued.)

TABLE 1. WHOLE-ROCK MAJOR- AND TRACE-ELEMENT ANALYSES OF ELDER CREEK TONALITIES, TRONDHJEMITES, AND DIORITES, WITH CIPW NORMS (continued.)

Sample no. EC-223–1 EC254–2 EC309–1 EC246–1 EC247–1 EC272–1 EC276–2 EC322–1 EC147–1 EC148–1 Rock type trondhjemite trondhjemite trondhjemite trondhjemite quartz diorite tonalite tonalite trondhjemite trondhjemite tonaliteOccurrence Pellows pluton Red Mtn felsic

dikes Red Mtn felsic

dikes Red Mtn pluton

Red Mtn pluton

Red Mtn pluton

Red Mtn pluton

Red Mtn pluton/ chilled margin

Brush Mtn pluton

Brush Mtn pluton

SiO2 73.83 73.61 69.67 66.14 62.91 68.80 68.08 72.22 69.49 74.41 TiO2 0.42 0.53 0.48 0.96 0.54 0.70 0.58 0.38 0.45 0.33 Al2O3 13.82 13.86 14.45 15.30 15.34 13.81 14.99 13.82 15.42 13.71 FeO* 3.03 3.35 4.45 5.46 6.43 5.37 4.51 3.55 4.00 3.24 MnO 0.02 0.05 0.06 0.09 0.10 0.09 0.08 0.07 0.02 0.03 MgO 0.98 0.79 0.94 2.37 4.81 2.05 0.99 0.55 0.83 0.38 CaO 1.78 1.76 2.35 3.37 4.45 5.90 5.34 2.48 2.13 3.46 Na2O 6.13 6.17 6.36 5.75 4.95 3.69 4.90 5.61 6.72 4.73 K2O 0.24 0.13 0.20 0.04 0.67 0.02 0.17 0.43 0.52 0.29 P2O5 0.08 0.11 0.11 0.18 0.08 0.13 0.13 0.07 0.16 0.05 Sum 100.34 100.36 99.07 99.66 100.28 100.56 99.77 99.18 99.74 100.63 Mg# 36.8 29.6 27.3 43.6 57.2 40.4 28.1 21.5 27.0 17.4

Hf 0.93 2.37 1.97 1.13 1.17 2.29 1.01 0.65 Ta 0.57 0.56 0.57 0.53 0.37 0.61 0.79 0.54 0.9 Nb 3.3 3.74 5.12 2.95 1.74 3.18 4.57 1.34 4.14 Zr 137 140 176 98 70 116 173 154 78 189 Y 19.12 22.21 26.9 20.15 17.44 34.55 25.37 8.02 27.84 Sr 221 97 193 190 193 395 637 224 179 211 Rb 1.57 0.57 0.47 0.17 5.2 0.28 1.25 1.42 1.06 Sc 9.2 13.3 9.7 16.2 26 15.8 10.8 5.2 8.5 V 36.5 15.4 7.6 103.3 164.5 109.6 62 1.5 8.4 0.7 Cr 1.9 1.9 0.9 1.7 37.3 14.3 2.8 1.5 3.5 Ni 2 1 2 34 7 2 Ba 21.19 19.87 21.88 9.1 63.09 12.83 135.58 34.42 72.09 La 6.19 7.44 8.31 6.37 5.19 9.63 8.18 3.7 9.16 Ce 12.15 16.25 17.86 15.02 13.33 22.65 16.97 7.96 21.17 Pr 2.32 2.9 3.44 2.48 2.1 3.32 3.2 1.37 3.57 Nd 10.74 13.58 16.33 11.42 9.9 15.16 14.83 6.52 16.55 Sm 3.08 3.89 4.68 3.3 2.74 4.08 4.28 1.76 4.65 Eu 0.87 1.12 1.35 1.11 0.81 1.27 1.39 1.12 1.34 Gd 3.65 4.5 5.46 3.92 3.02 4.83 4.91 1.96 5.32 Tb 0.62 0.74 0.9 0.64 0.51 0.77 0.82 0.29 0.87 Dy 3.94 4.82 5.8 4.05 3.23 4.88 5.29 1.84 5.67 Ho 0.85 1.02 1.23 0.86 0.7 1.05 1.13 0.38 1.23 Er 2.38 2.94 3.38 2.48 2.08 2.99 3.26 1.07 3.6 Tm 0.35 0.43 0.48 0.37 0.32 0.43 0.49 0.15 0.55 Yb 2.17 2.86 2.92 2.34 2.13 2.69 3.12 0.96 3.48 Lu 0.29 0.41 0.39 0.34 0.33 0.39 0.47 0.14 0.52 Pb 0.32 3.13 0.71 0.98 0.64 1.6 2.01 1 1.16 Th 1.17 1.27 1.22 1.1 0.89 1.13 1.25 0.21 1.41 U 0.27 0.5 0.37 0.24 0.21 0.43 0.22 0.05 0.27 Sr/Y 11.19 4.37 7.18 9.43 11.07 11.43 8.83 22.31 7.58 Cr/Y 0.1 0.09 0.03 0.09 2.14 0.41 0.11 0.19 0.13 La/Yb 2.85 2.6 2.84 2.72 2.44 3.58 2.62 3.85 2.63 La/Yb-ch 2.04 1.87 2.04 1.95 1.75 2.57 1.88 2.76 1.89

CIPW norm Quartz 30.45 30.62 23.56 19.47 12.02 30.28 25.43 30.28 20.87 36.12 Anorthite 7.83 7.56 9.82 14.86 16.84 20.21 17.63 10.84 9.04 14.62 Albite 52.10 52.44 54.78 49.47 42.46 31.75 42.33 48.39 57.43 40.35 Orthoclase 1.38 0.77 1.24 0.24 3.96 0.12 1 2.54 3.07 1.71 Corundum 0.44 0.62 0.11 0.33 Diopside 0.58 3.17 6.11 6.15 0.53 1.32 Hypersthene 6.08 5.94 7.76 12.27 18.78 8.61 4.96 5.69 7.05 4.36 Olivine Ilmenite 0.80 0.99 0.91 1.84 1.03 1.33 1.1 0.72 0.85 0.63 Magnetite 0.73 0.81 1.09 1.32 1.55 1.29 1.09 0.87 0.97 0.78 Apatite 0.19 0.25 0.25 0.42 0.19 0.3 0.3 0.16 0.37 0.12

TABLE 1. WHOLE-ROCK MAJOR- AND TRACE-ELEMENT ANALYSES OF ELDER CREEK TONALITIES, TRONDHJEMITES, AND DIORITES, WITH CIPW NORMS (continued.)

Wacke and siltstone

Crowfoot Point Breccia

Volcanic rocks

Elder Creek Ophiolite

Dike complex

Felsic dikes

Quartz diorite & diorite

Isotropic gabbro

Cumulate gabbro

Wehrlite-Pyroxenite

Dunite & dunite broken formation

Sheared serpentinite

Volcanic blocks in melange

Foliated metasediments(Galice?)

South Fork Mountain schist

Franciscan Assemblage

Serpentinite mélange

Great Valley Series

vc

North

0 1 2miles

wc

GVS

fmg

ig

du

du

fdcvc

vc

fdc

du

du

fdc

vc

gs

gs

fmg

fdc

vc

gs

gs

gs

cg

sfms

dc

qdi

sfmscg

ssp

cpb

ig

wc

GVS

qdi

qdi

qdi

qdi

cg

cg

cg

cg

cgwc

wc

wc

wc

wc

ig

ig

ig

GVS

cpb

cpb

cpb

ssp

GVS

ssp

ssp

EC-148

EC107

ig

qdi

cg

wc

du

cpb

GVS

ssp

sfms

dc

Brush Mtn Pluton

Pellows Place Pluton

Red Mtn Pluton

Riley Ridge complex

Eagle Peak Pluton

Figure 2. Geologic map of the Elder Creek ophiolite, showing location of major tonalite-trondhjemite-diorite plutons (labeled with arrows).

Figure 3. Outcrop photos of plagiogranite and agmatites, Elder Creek ophiolite: (A) Trondhjemite pod, ~1 m across, in mafi c dike complex, South Fork of Elder Creek, EC-107 (zircon date). (B) Agmatite with xenoliths of mafi c isotropic gabbro in diorite matrix, Eagle Peak (PK-8). (C) Agmatite with xenoliths of isotropic gabbro (right) and mafi c lava or dike complex (left) intruded by felsic diorite matrix, South Fork of Elder Creek, EC-142. (D) Agmatite with xenoliths of layered gabbro in diorite matrix, South Fork of Elder Creek, EC-145.

Figure 4. Outcrop and low-altitude, oblique air photos of Elder Creek ophiolite. (A) South Fork of Elder Creek below Red Mountain, showing decameter-scale dikes of quartz diorite and tonalite that form rock ribs with light-colored soil cover; location EC-145 at right side of photo. (B) Oblique air photo showing South Fork of Elder Creek (mid-photo) and Riley Ridge (below South Fork of Elder Creek); also shown are major tonalite-trondhjemite plutons (Brush Mountain, Pellows Place, Red Mountain) and the Riley Ridge diorite-agmatite complex.

Tonalites, trondhjemites, and diorites of the Elder Creek ophiolite, California 121

Several of these plutons are bordered by felsic dike rocks that form extensive, mappable complexes along the margins of the plutons. The felsites consist of small plagioclase phenocrysts in an aphanitic matrix. They are compositionally identical to their adjacent plutons, and rare chilled contacts within the felsites sug-gest that they represent a sill complex, which formed a carapace above the underlying plutons.

There are four relatively large, discrete plutons (from south to north): Eagle Peak, Red Mountain, Pellows, and Brush Mountain (Figs. 2 and 4B). These may represent four separate plutons, or they may have originally formed contiguous bodies that have since been separated by faulting. The plutons stand out on aerial photographs by their contrast with the adjacent units: the cumulate gabbros and ultramafi c rocks that underlie the plutons have scant vegetation and light-colored soil, while the Crowfoot Point breccia, which overlies the plutons, has scat-tered vegetation and slightly reddish soils. In contrast, the felsic plutons are characterized by a cover of dense chaparral through which soil cannot be seen (Fig. 4), sometimes even when you are standing on it (which explains why these extensive felsic suites were not discovered earlier).

Shervais and coworkers (Shervais et al., 2005a) reported U-Pb zircon ages for two samples from the Elder Creek ophio lite: a plagio granite lens that intrudes the sheeted dike complex along the South Fork of Elder Creek (EC107–3) and a hornblende quartz diorite from the Brush Mountain pluton (EC148–2). Both samples are slightly discordant, with 238U/206Pb ages ranging from 161 to 172 ± 2 Ma and 207Pb/206Pb ages ranging from 165 to 198 Ma. Con-cordia plots have upper intercepts that suggest crystallization ages of 169.7 ± 4.1 Ma for EC107–3 and 172.0 ± 4.0 Ma for EC148–2. These ages are signifi cantly older than previous hornblende K-Ar dates from gabbro (154 ± 5 to 163 ± 5 Ma) ( Lanphere, 1971; McDowell et al., 1984), which probably represent cooling or Ar-loss ages (Shervais et al., 2005a). The 207Pb/206Pb ages imply an inherited zircon component (Shervais et al., 2005a), which has

been documented by Hopson et al. (this volume) to represent a Paleoproterozoic-age crustal component. This ancient inherited zircon component makes an oceanic origin improbable.

RESULTS

Petrography and Mineral Chemistry

Typical phase assemblages in the felsic plutonic rocks include plagioclase, quartz, hornblende, and opaque oxides; pyroxenes (both clino- and orthopyroxene) are found in the diorites and quartz diorites. The most common texture is hypidio morphic granular, and euhedral to subhedral hornblende prisms and plagio clase laths form a framework that encloses anhedral quartz and oxides. Grain size is generally ≤2 mm. Typical modal pro-portions are ≈50% plagioclase, 30%–35% hornblende, 0%–10% pyroxene, and 2%–25% quartz (Fig. 5).

Plagioclase forms elongate laths, 1–2 mm in length, that are relatively euhedral. Plagioclase composition is in the range An

32–65,

but albite An0–5

is common (Fig. 6A). Relict plagioclase is com-monly altered in part to smectite, giving the laths a dusty brown appearance in thin section (Fig. 5). Hornblende (Mg# 78–23) forms euhedral to subhedral prisms up to ≈3 mm in size; smaller grains are interstitial to plagioclase, whereas larger grains may partially enclose plagioclase laths subophitically. Pyroxenes include both clinopyroxene (augite to ferroaugite, Wo

38–49En

55–80) and ortho-

pyroxene (hypersthene, Wo04

En70

) (Fig. 6B). Quartz occurs as small (<1 mm) interstitial grains that stand out from adjacent plagio-clase because they are unaltered and optically clear (Fig. 5). In some samples, quartz is intergrown graphically with opaque oxides (Fig. 5B). Minor-element concentrations are extremely low in the pyroxenes: TiO

2 and Na

2O are typically less than 0.4 wt%, Cr

2O

3 is

≤0.05 wt%, and Al2O

3 is ≈1–3 wt%. Hornblende is enriched in all

of these elements, with TiO2 ≈ 0.5–3 wt%, Na

2O ≈ 0.5–1.8 wt%,

and Al2O

3 is ≈4–12 wt%, and Cr

2O

3 is ≈0.02–0.17 wt%.

Figure 5. Photomicrographs of Elder Creek diorites. Field of view = 5.2 mm across, all UPL. (A) EC-247, Red Mountain quartz diorite. (B) EC-148, Brush Mountain Pluton.

122 Shervais

Secondary phases include chlorite, prehnite, epidote, and zoisite. Chlorite is associated with the mafi c phases, whereas prehnite is found in clusters and along microveins and assumes both the radial and platy habits. Zoisite is rare and occurs inter-stitially. Sphene occurs as small, high-relief, orange grains.

Major- and Trace-Element Chemistry

The Elder Creek diorites span a wide range in composi-tion: 54%–75% SiO

2, 3.3%–14.3% FeO*, and 2.7%–6.4%

MgO (Fig. 7). All are extremely low in K2O (<0.7%), similar to

oceanic plagiogranites (Coleman and Peterman, 1976; Coleman and Donato, 1979). The plutons are higher in silica (62%–77% SiO

2) than the dikes, pods, and agmatite breccias (53%–59%

SiO2 in most samples; a few samples range up to 72% SiO

2).

Na2O increases with increasing SiO

2, whereas the other major

elements either decrease or show no change (Fig. 7). FeO* decreases with increasing Si (Fig. 7), and the suite defi nes a calc-alkaline trend on an AFM (alkalis-Fe)*-MgO) diagram (Fig. 8A). Based on their normative quartz-plagioclase-orthoclase com-positions, and their molar Ca-Na-K ratios, these rocks include diorites (<5% normative quartz), quartz diorites (5%–20% normative quartz), tonalites, and trondhjemites (>20% norma-tive quartz; Figs. 8B and 8C) (Barker, 1979; Le Maitre, 2002). Agmatite suites (e.g., Riley Ridge diorite complex) are largely

diorite or quartz diorite , whereas the decameter-scale dikes in Elder Creek include diorite , quartz diorite, and tonalite. The small plagiogranite pods, and the larger plutons (along with their felsite dike margins), are dominantly tonalite and trondhjemite. Many of the more Si-rich rocks are slightly corundum norma-tive, consistent with fractional crystallization of hornblende and plagioclase (e.g., Cawthorn and Brown, 1976).

The calc-alkaline trend of this suite resembles adakites from Baja California (Rogers et al., 1985) and plutons from the Aleutian arc (Kelemen et al., 2004) (Fig. 8A). In contrast to these suites, however, the Elder Creek tonalites, trondhjemites, and diorites are extremely low in K

2O, and somewhat higher in

Na2O. Similar low-K trends are seen in primitive island arcs of

the western Pacifi c, e.g., Tonga-Kermadec, Izu-Bonin. In partic-ular, the Elder Creek TTD suite displays strong similarities to the low-K, high-Ca dacites from Tonga-Kermadec (Bryan, 1979). A coeval suite of tonalites, trondhjemites, and grano diorites in the Smartville ophiolite is also similar (Beard, 1998).

High-Mg diorites with 4%–7% MgO at 53%–63% SiO2

are common in the Riley Ridge diorite complex and the dike suite, but relatively high Mg#, Cr, and Ni values are found in the large plutons as well. Mg# (100 × Mg/[Mg + Fe] molar) values range from 65 to 30 in rocks with <65% SiO

2, and even high-Si

tonalites and trondhjemites with 65%–75% SiO2 commonly

have Mg# = 30–48. The high Mg# values are accompanied by

0

10

20

30

40

50

100

90 80 70 60 50 40 30 20 10Ca

FeMg

PyroxeneHornblende

0

10

100

90 80 70 60 50 40 30 20 10Albite

Feldspar

Anorthite

OrthoclaseA

B

Figure 6. Mineral compositions: (A) feldspar ternary (lower portion) diagram showing relict igneous plagioclase (An32-An65) and secondary albite; and (B) pyroxene quadrilateral showing augite-ferroaugite, hypersthene, and hornblende (Ca-Mg-Fe) compositions.

Tonalites, trondhjemites, and diorites of the Elder Creek ophiolite, California 123

high compatible-element concentrations as well, with Cr up to 227 ppm in high-Si quartz diorites (Table 1). The high concen-trations of MgO and FeO* (4–9 wt% in most samples, with two high-Fe diorites ≈12 wt% FeO*) result in high normative mafi c contents, which include normative olivine in the more mafi c samples (Table 1).

Chondrite-normalized rare earth element (REE) concen-trations are enriched in the light rare earth elements (LREE) relative to the heavy rare earth elements (HREE) in all but two of the samples (Fig. 9). This enrichment is relatively modest, and chondrite-normalized La/Lu ratios are 0.7–2.1× chondrite in the diorites and quartz diorites (Fig. 9A) and 1.75–2.75× chondrite in the tonalites and trondhjemites (Fig. 9B), with total La concentrations of ≈10× to ≈50× chondrite. Discernible Eu anomalies are typically small and negative, but a few samples display small positive anomalies; two samples (trondhjemites EC147–1 and EC218–2A) display large positive Eu anomalies and lower HREE concentrations, consistent with plagioclase accumulation (Fig. 9).

Multi-element diagrams normalized to average normal MORB (Sun and McDonough, 1989) are moderately depleted in the less incompatible elements (middle to heavy REE ≈ 0.5–1× normal [N] MORB) and moderately enriched in the more incompatible elements (Ba to Sr ≈ 1× to 10× N-MORB; Fig. 10). The high fi eld strength elements Nb, Ta, Hf, and Ti display distinct negative anomalies relative to adjacent low fi eld strength elements of similar incompatibility, whereas Pb displays a small positive anomaly—all characteristics of subduction-zone magmatism. K and Rb are less enriched than comparable arc suites and may be depleted relative to N-MORB in some samples (Fig. 10). The depletion of large ion lithophile elements such as K and Rb is a common feature of suprasubduction tonalite-trondhjemite suites (e.g., Beard, 1998) and will be discussed later.

DISCUSSION

Petrogenesis of the Elder Creek TTD Suite

Major-element characteristics of the Elder Creek TTD suite are consistent with partial melting of a mafi c protolith, such as subducting oceanic crust or the deep roots of an island arc (e.g., Kay, 1978; Beard and Lofgren, 1989, 1991; Drummond and Defant, 1990; Rapp et al., 1991; Rapp and Watson, 1995; Martin et al., 2005; Thorkelson and Breitsprecher, 2005). Dehy-dration melting experiments on natural and synthetic green-stone-amphibolite compositions at pressures above and below the garnet-in reaction document a range in melt compositions, from trondhjemite through diorite, with increasing temperature and melt fraction (Beard and Lofgren, 1991; Rapp et al., 1991; Rushmer, 1991; Rapp and Watson, 1995). These melts are rela-tively high in alumina (14%–18% Al

2O

3), sodium (Na

2O ≈ 4%),

and iron (FeO* ≈ 7% at 60% SiO2), and low in K

2O (<1.5%).

The experimental melts are generally lower in MgO and

higher in TiO2 than those observed in Archean TTG (tonalite-

trondhjemite -granodiorite) suites or the Elder Creek TTD suite (e.g., Rapp et al., 1991; Rapp and Watson, 1995). Melt fractions increase and silica decreases with higher temperatures. There is little distinction in the major-element data between melts formed at high pressure and those formed at lower pressures, but Na/K ratio varies with protolith composition (Beard and Lofgren, 1991; Rapp and Watson, 1995). Both the experimental dehydration melts and the observed TTD suites are lower in alumina and higher in iron than water-saturated melts derived from similar protoliths (Beard and Lofgren, 1991).

Compared to adakites, bajaites, Archean tonalite-trondhjemite -granodiorite (TTG) suites, and similar rocks thought to represent melts formed in these settings, the Elder Creek TTD suite is characterized by relatively fl at chondrite-normalized REE patterns (Fig. 9), higher Y, and lower Sr (Fig. 7), and lower Sr/Y and Cr/Y (Fig. 11). The steep LREE-enriched patterns of adakites and related rocks (and the con-comitant depletion in HREE) is thought to refl ect equilibration with a garnet-rich, plagioclase-free, eclogite melting residue, which also lowers Y (partitioned into garnet) and raises Sr (no plagioclase in residue). The resulting high Sr/Y ratios are characteristic of adakitic and related melts. Reaction of these melts with the overlying mantle wedge enriches the melts in refractory elements (Cr, Ni, Mg), raising the Cr/Y ratio and, in extreme cases, resulting in the formation of sanukitoids, which are Mg-rich granites found in some Archean terrains that resemble magnesian andesites of the Setouchi belt in SE Japan (Tatsumi and Ishizaka, 1982).

Adakites, bajaites, Archean TTGs, and sanukitoids all refl ect melting of mafi c protoliths at relatively high pressures (>10 kbar) to form an eclogitic residue. In contrast, dehydra-tion melting of mafi c protoliths at lower pressures (5–10 kbar) results in melts with similar major-element characteristics, but with relatively unfractionated chondrite-normalized REE pat-terns, higher Y (no garnet in residue), and lower Sr (refractory plagioclase). These characteristics rule out melting of deep island-arc roots, which by defi nition imply pressures greater than 10 kbar. It is also inconsistent with fi eld relations that document a relatively thin crustal section throughout the Coast Range ophiolite (Shervais et al., 2004a).

Tulloch and Kimbrough (2003) discussed the occurrence of high Sr/Y and low Sr/Y granitoid suites in Baja California (Mexico) and New Zealand. They showed that low Sr/Y suites typically occur closer to the trench, whereas the high Sr/Y suites lie closer to the continental margin. They inferred a relationship between the position of these suites and thrusting of older arc crust under the continental margin to form the source of the high-Sr/Y suite granitoids (Tulloch and Kimbrough, 2003). Alternatively, I propose that the contrast in the position of these two suites may refl ect the inclination of the subducting slab, where low-Sr/Y suites are formed by melting of the mafi c slab at shallower depths closer to the trench, and high-Sr/Y suites are formed by melting at deeper levels farther from the trench.

0.4

0.6

0.8

1.0

12

14

16

18

AL 2

O3

4

8

12

FeO

*

2468

10

MgO

2468

1012

CaO

34567

Na 2

O

55 60 65 70 75SiO2

12345

K2O

555

Agmatite xenolithAgmatite matrixRiley Ridge Diorite ComplexDiorite DikesRed Mtn PlutonPellows PlutonEagle Peak PlutonBrush Mtn PlutonAleutian plutonsBaja lavas

50 80

TiO

2

A

B

C

D

E

F

G

Figure 7 (on this and following page). Harker diagrams, showing changes in major-element concentrations as function of silica con-tent: TiO2, Al2O3, FeO*, MgO, CaO, Na2O, K2O, Rb, Sr, Y, Zr, Sc, and V. Data for Aleutian island-arc plutons (gray cross) and Baja high-Mg andesites (+) are shown for comparison. Aleutian arc and Baja high-Mg andesite data suites were compiled by Peter Kelemen, Gene Yogadinski, and David Scholl (Kelemen et al., 2004).

100

200

300

Zr

(ppm

)

20

40

60

Y (

ppm

)

400

800

1,200

Sr

(ppm

)

255075

100125

Rb

(ppm

)

5101520253035

Sc

(ppm

)

100

200

300

400

V (

ppm

)

55 60 65 70 75

SiO2

100

200

Cr

(ppm

)

Agmatite xenolithAgmatite matrixRiley Ridge Diorite ComplexDiorite DikesRed Mtn PlutonPellows PlutonEagle Peak PlutonBrush Mtn PlutonAleutian plutons

50 80

H

I

J

K

L

M

N

Figure 7 (continued).

Agmatite xenolithAgmatite matrixRiley Ridge Diorite ComplexDiorite Dikes

.1

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Eagle Peak PlutonPellows PlutonRed Mtn PlutonBrush Mtn Pluton

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu.1

1

10

100

1000

A B

Figure 9. Chondrite-normalized rare earth element (REE) concentrations (C1 chondrite of Sun and McDonough, 1989). (A) Diorites and quartz diorites have relatively fl at to gently sloping patterns that range from slightly light (L) REE-depleted to slightly LREE-enriched (La/YbCH = 0.7–2.1), whereas (B) tonalites and trondhjemites have gently sloping patterns that are all moderately LREE-enriched (La/YbCH = 1.75–2.76). Two trondhjemites have small positive Eu anomalies and low heavy (H) REE concentrations, indicating plagioclase accumulation. Gray fi eld—Aleutian plutons, from Kelemen et al. (2004).

0

10

20

30

40

50

60

70

80

90

100

100

90 80 70 60 50 40 30 20 10

100

90

80

70

60

50

40

30

20

10

Alkali

FeO*

MgO

Calc-alkaline

Tholeiitic

10

20

30

40

50

60

70

80

90

100

100

90 80 70 60 50 40 30 20 10

100

90

80

70

60

50

40

30

20

10

Plagioclase

111111

Kspar

Quartz

Tonalite &Trondhjemite

Quartz Diorite

Diorite

0

10

20

30

40

50

60

70

80

90

100

100

90 80 70 60 50 40 30 20 10

100

90

80

70

60

50

40

30

20

10

Albite

000

00

Anorthite

Kspar

Diorites &Tonalites

TrondhjemitesAgmatite xenolithAgmatite matrixRiley Ridge DioritesDiorite Dikes

Red Mtn PlutonPellows PlutonEagle Peak PlutonBrush Mtn Pluton

A

C

B

Figure 8. Granitoid classifi cation ternary diagrams: (A) alkali-FeO*-MgO (AFM) ternary diagram, with calc-alkaline/tholeiite boundary of Irvine and Barager (1970); data for Aleutian island-arc plutons (cross) and Baja high-Mg andesites (+) are shown for comparison. (B) Norma-tive quartz-plagioclase-orthoclase ternary diagram, showing divisions into diorite (<5% quartz), quartz diorite (5%–20% quartz), and tonalite-trondhjemite (>20% quartz); note that the plutons are dominantly tonalite-trondhjemite, whereas the dikes and agmatites are largely diorite or quartz diorite. (C) Tonalite-trondhjemite ternary, after Barker (1979), based on normative anorthite-albite-orthoclase. Plutons are dominantly trondhjemitic with some tonalite, whereas the dikes and agmatites are dominantly dioritic with minor tonalite. Aleutian arc and Baja high-Mg andesite data suites were compiled by Kelemen et al. (2004).

Tonalites, trondhjemites, and diorites of the Elder Creek ophiolite, California 127

Setting for Slab Melting

It has long been recognized that most subducting slabs are too cool to melt at any depth, and that even deep melting to form eclogite residue (>35 km) requires relatively young oceanic crust (younger than 30 Ma) that has not yet cooled completely (Pea-cock, 1991). Melting at shallower depths (5–10 kbar pressure, 16–34 km depth) requires unusually hot conditions in the subduct-ing slab that can only be obtained when an active oceanic spread-ing center collides with a subduction zone and ridge subduction occurs (e.g., Thorkelson and Breitsprecher, 2005). Shervais and co-workers (Shervais, 2001; Shervais et al., 2004a, 2005a, 2005b) have proposed that such a collision ended formation of the Coast Range ophiolite and led to the intrusion of MORB composition dikes at Elder Creek and other Coast Range ophiolite locales, and to the eruption of MORB composition basalts at other locales (e.g., Cuesta Ridge).

Effect of Mixing and Assimilation in Agmatites

Diorites and quartz diorites that make up the matrix in agma-tites have silica contents that are lower than the tabular plutons and larger dikes that feed them (Fig. 7). This matrix may have formed by fractional crystallization of magmas parental to the associated isotropic gabbros. Alternatively, the matrix may have formed by the disaggregation and assimilation of more mafi c wall rock into an intrusive magma that was originally more tonalitic or trond-hjemitic in composition. This process is most clearly documented in the outcrops on Eagle Peak (Fig. 3B), where clasts of mafi c gabbro as small as 1 or 2 mm across are observed. This process may also account for the small positive Eu anomalies seen in a few samples, if the contaminant was a cumulate gabbro.

Comparison of the Elder Creek TTD Suite with Sanutikoids, Setouchiites, and Other High-Mg Diorites

The Elder Creek tonalite-trondhjemite-diorite suite exhibits relatively high concentrations of the refractory elements MgO, Cr, and Ni, and high Mg#, compared to normal calc-alkaline

.1

1

10

100

1000

Rb BaTh U NbTa K LaCePbPr Sr P NdSmZr Hf Eu Ti GdTbDy Y HoErTmYb Lu

Agmatite xenolithAgmatite matrixRiley Ridge Diorite complexDiorite Dikes

.1

1

10

100

1000

Rb BaTh U NbTa K LaCePbPr Sr P NdSmZr Hf Eu Ti GdTbDy Y HoErTmYb Lu

Eagle Peak PlutonPellows PlutonRed Mtn PlutonBrush Mtn Pluton

A

B

Figure 10. Mid-ocean-ridge basalt (MORB)–normalized multi-element plot. Concentrations are normalized to the normal (N) MORB average composition of Sun and McDonough (1989). Note the depletions in the high fi eld strength elements Nb and Ti, the enrichment in Pb, and the depletions in K and Rb in some samples. (A) Diorites and quartz diorites, and (B) tonalites and trondhjemites. Gray fi eld—Aleutian plutons, from Kelemen et al. (2004).

1

10

100

Sr/

Y

10 20 30 40 50 60 70 80 90Y (ppm)

.01

.1

1

10

Cr/

Y

Agmatite xenolithAgmatite matrixRiley Ridge Diorite complexDiorite DikesEagle Peak PlutonPellows PlutonRed Mtn PlutonBrush Mtn PlutonAleutian plutonsAleutian lavas

Figure 11. Sr/Y and Cr/Y versus Y for the Elder Creek tonalite-trondhjemite -diorite (TTD) suite, compared with Aleutian plutons and lavas, and high-Mg andesites from Baja California. Data suites were compiled by Peter Kelemen, Gene Yogadinski, and David Scholl (Kelemen et al., 2004). The Elder Creek TTD suite is characterized by lower Sr/Y and Cr/Y than comparable high-Mg plutonics and lavas. This refl ects melting at lower pressures, outside of the garnet stability fi eld.

128 Shervais

granitoids. The high concentrations of refractory elements sug-gest a relationship to the wehrlite-clinopyroxenite suite—possi-bly as the evolved liquid fraction complementary to the cumu-lates. They also suggest a petrogenetic relationship to sanukitoids and setouchiites. Sanukitoids are high-Mg granitoids found in Archean terranes that resemble high-Mg andesites found in Japan (Shirey and Hanson, 1984). Setouchiites are high-Mg andesites from the Setouchi Peninsula, Japan, that formed in response to subduction of an active spreading center (Tatsumi and Ishizaka, 1982). Like adakites, these high-Mg suites are characterized by low Y, high Sr, and high Sr/Y ratios that refl ect eclogite melting at pressures >10 kbar. They have even higher Cr/Y ratios than adakites, refl ecting extensive reaction of the primary slab melts with the overlying mantle wedge; some authors even suggest that sanukitoids and setouchiites represent partial melts of mantle wedge peridotites that have been metasomatized by slab-derived melts (Martin et al., 2005).

The high refractory element concentrations observed in the Elder Creek TTD suite imply a similar process, in which slab-derived partial melts formed at relatively low pressures (5–10 kbar) from amphibolite-facies metabasalts reacted with the overlying mantle wedge to become enriched in MgO, Cr, and Ni. Melts derived from amphibolite at these pressures will lack the depletion in HREE and Y observed in adakites, sanuki-toids, and setouchiites and will show a concomitant enrichment in LREE and Sr. In addition, melts formed at lower pressures will traverse thinner sections of the mantle slab, resulting in somewhat less refractory element enrichment. As a result, their Cr/Y ratios will be lower than corresponding ratios in sanuki-toids and setouchiites.

LILE Depletion in the TTD Suite

The Elder Creek tonalite-trondhjemite-diorite suite is char-acterized by exceptionally low concentrations of K and Rb—concentrations that are strongly depleted relative to other low fi eld strength elements (e.g., Fig. 7). This “overdepletion” of large ion lithophile elements is characteristic of oceanic plagio-granites (tonalities and trondhjemites). It is possible that this overdepletion in LILE results from secondary alteration. How-ever, there are two possible primary causes, both of which may be operative. First, dehydration melting experiments on natural metabasalt samples show that low-K

2O melts form in response

to melting of protoliths that are low in K2O to begin with, such

as oceanic greenstones (Beard and Lofgren, 1991; Rapp et al., 1991; Rushmer, 1991). These experiments show that protoliths with <0.3% K

2O are needed to form melts with the low-K

2O

concentrations observed here (<0.7%).Beard (1998) proposed that this overdepletion is caused by

the evolution of an alkali-rich vapor phase that is driven off dur-ing intrusion to permeate and alter the surrounding wall rock. The partitioning of alkalis into the vapor phase is controlled by the lack of early formed potassic mineral phases (due to low ini-tial alkali content), and the low partition coeffi cients for K and

Rb into the early crystallizing phases pyroxene, hornblende, and plagioclase (Barker and Arth, 1976). The data presented here support that interpretation: Ba does not correlate with Rb concentration, even though K, Rb, and Ba are all strongly depleted in the Elder Creek TTDs.

Tectonic Setting of the Elder Creek TTD Suite

The Elder Creek tonalite-trondhjemite-diorite suite resem-bles oceanic plagiogranite in many of its major- and trace-element characteristics, but other aspects of its chemical and petrologic composition require a suprasubduction-zone origin. These include the calc-alkaline fractionation trends, the frac-tionated REE and MORB-normalized multi-element charac-teristics, the nega tive high fi eld strength element anomalies, the extensive plutons of high-silica tonalite and trondhjemite, the common occur rence of modal hornblende and augite, and less common occurrence of modal hypersthene. This conclusion is supported by granite trace-element discrimination diagrams based on Y, Nb, and Rb (Pearce et al., 1984), which show that all of the rocks analyzed here have volcanic-arc granite system-atics (Fig. 12). It is also supported by tectonic discrimination diagrams for pyroxenes (Fig. 13; Leterrier et al., 1982). These diagrams show that the pyroxenes in these rocks are nonalka-line (Fig. 13A), orogenic (Fig. 13B), and largely calc-alkaline (Fig. 13C). The high Mg and Cr contents seen in many samples require that these melts reacted with a refractory mantle wedge during their ascent to the surface. Similar suites have been docu-mented in other suprasubduction ophiolites (e.g., Beard, 1998).

The suprasubduction-zone setting for slab melting and mantle-wedge reaction documented here is depicted in Figure 14. This model (which is scaled the same vertically and horizontally) assumes the subduction of relatively young (younger than 25 Ma) oceanic lithosphere, which is thermally buoyant. This results in relatively shallow subduction of altered oceanic crust beneath hot asthenosphere of the mantle wedge (Fig. 14). The infl ux of hot asthenosphere into the mantle wedge above the slab occurs during the earlier stages of subduction in response to sinking of the oceanic slab and formation of the main phases of ophiolite crust (e.g., Stern and Bloomer, 1992; Shervais, 2001). Melting of the subducting crust occurs at relatively shallow depths, cor-responding to pressures of 5–10 kbar, to form the TTD suite. The resulting melts rise through the mantle wedge, react to become more magnesian, and intrude ophiolite crust formed during the initial stages of subduction (Fig. 14). The occurrence of an ancient inherited zircon component seen in the Elder Creek TTD suite (Shervais et al., 2005a) requires the subduction of sediment bear-ing ancient zircon into the hot zone where melts are produced.

Formation of the TTD suite is a transient event that occurs only when relatively young, hot, buoyant lithosphere is sub-ducted—just prior to or after collision of the spreading ridge with the trench system. It represents stage three in the life cycle of suprasubduction-zone ophiolites (Shervais, 2001). This implies that TTD suite rocks must intrude older crustal elements of the

Tonalites, trondhjemites, and diorites of the Elder Creek ophiolite, California 129

ophiolite, and can only predate melts of oceanic basalt. It will also not be found in all ophiolite suites, only in those that exhibit a full “life cycle” (Shervais, 2001).

Support for the origin of the Elder Creek tonalite-trondhjemite-diorite suite by partial melting of subducted oceanic crust comes from amphibolite blocks in the underlying Francis-can complex. These blocks often contain small melt lenses of tonalitic or trondhjemitic composition that are mineralogically similar to the Elder Creek TTD suite (Fig. 15). We suggest that these represent the same process, but on a smaller scale.

.50 .60 .70 .80 .90 1.00Ca+Na (afu)

.00

.01

.02

.03

.04

.05

.06

.50 .60 .70 .80 .90 1.00Ca (afu)

.00

.01

.02

.03

.04

.00 .05 .10 .15 .20Al (afu)

.00

.01

.02

.03

Arc Tholeiite

Calc-alkaline arc

Orogenic(arc)

Nonorogenic(MORB)

Alkaline

NonalkalineTholeiitic & Calc-alkaline

Ti (

afu)

Ti+

Cr

(afu

)Ti

(af

u)

A

B

C

Figure 13. Tectonic discrimination diagrams for pyroxene (after Leterrier et al., 1982). (A) Low Ti = nonalkaline, tholeiitic, or calc-alkaline compositions, (B) low Ti + Cr at high Ca = orogenic (arc) geochemistry, (C) relatively high Ti relative to Al—calc-alkaline rather than arc tholeiite. MORB—mid-ocean-ridge basalt.

1 10 100 1000Y.1

1

10

100

1000

Nb

(ppm

)

Agmatite xenolithAgmatite matrixRiley Ridge DioritesDiorite DikesEagle Peak PlutonPellows PlutonRed Mtn PlutonBrush Mtn Pluton

VAG ORG

WPG

1 10 100 1000

Y+Nb (ppm)

.1

1

10

100

1000

Rb

(ppm

)

VAG

ORG

WPG

SynColG

A

B

Figure 12. Tectonic discrimination diagrams for granitoids, after Pearce (1984): (A) Y versus Nb, (B) Y + Nb versus Rb. Fields: VAG—volcanic-arc granite, ORG—ocean-ridge granite, WPG—within-plate granite, Syn-ColG—syncollisional granite. Elder Creek tonalite-trondhjemite-diorite (TTD) are classed as volcanic-arc granites on these plots.

130 Shervais

CONCLUSIONS

The Elder Creek tonalite-trondhjemite-diorite suite forms a signifi cant part of the Elder Creek ophiolite—up to 30% of the plutonic rock outcrop—that postdates formation of the ophiolite layered series and the sheeted dike complex. Field relationships and geochemical data show that it represents a suprasubduction magma suite, not oceanic plagiogranites formed at a spreading center. The major- and trace-element characteristics are consistent with partial melting of sub-ducted, amphibolite-facies oceanic crust at relatively low pres-sures (5–10 kbar) outside the garnet stability fi eld. Melting of subducted oceanic crust at these pressures can only occur dur-ing 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 involvement of subducted sediments is required by inher-ited Pb isotopes in zircon.

The Elder Creek tonalite-trondhjemite-diorite suite differs from adakites and Archean high-Mg granitoids (sanukitoids), which form by melting at higher pressures (>10 kbar) in the gar-net stability fi eld. Melting at higher pressures results in enriched LREE patterns, HREE and Y depletion, and Sr enrichment—none of which are observed in the Elder Creek TTD suite. However, the overall process of formation is similar: melting of young, hot subducting slab, followed by reaction of the primary tonalitic-

trondhjemitic melt with mantle peridotite during ascent. The pri-mary difference is that Elder Creek TTD suite melted at lower pressures in response to a ridge collision-subduction event.

The correlation of the Elder Creek tonalite-trondhjemite-diorite suite with a ridge collision-subduction event is consistent with previous models for the Coast Range ophiolite, which call

Trench Sediment with Ancient Zircons

Melt Zone5–10 kbar (16–34 km)

No Melt (Refractory: Low-T melt fraction extracted)

Spreading Center

Young, Hot Lithosphere

0

10

20

30

40

50

Calc-alkaline volcanics

Hot Asthenospherefrom

Slab Sinking

Melt Reaction with Wedge

Depth(km)

Thin Crust formed by Rapid Extension in Response to Slab Sinking

50 km 100 km0 km

Distance from Trench

Figure 14. Scale model depicting formation of Elder Creek tonalite-trondhjemite-diorite (TTD) suite. Subducting lithosphere is younger than 25 Ma and relatively hot at low pressures; this implies eventual collision of the spreading ridge with the trench. Main phase of ophiolite crust (≤6 km thick) formed earlier, during rapid extension of the forearc in response to sinking of the subducting slab. Subducting oceanic crust enters the melt zone at 5–10 kbar pressure or 16–34 km depth; shallow subduction also leads to a relatively wide zone of melting and intrusion. Ocean crust below 34 km is too refractory to continue melting. Note that formation of the TTD suite is a transient event that occurs only during subduction of young lithosphere, just prior to ridge collision. Vertical and horizontal scales are the same; nominal subduction angle is 20° (see Shervais, 2001).

Figure 15. Outcrop photo of Franciscan amphibolite block exposed in Elder Creek, showing melt lens of tonalitic composition. This melt lens is considered to be an analogue for the Elder Creek tonalite-trondhjemite -diorite (TTD) suite.

Tonalites, trondhjemites, and diorites of the Elder Creek ophiolite, California 131

on ridge collision to explain a late infl ux of MORB-like magmas that postdate other volcanic and plutonic activity (Shervais, 2001; Shervais et al., 2004b, 2005a). This model is also consistent with the relative ages of magmatic suites in many suprasubduction-zone ophiolites (Shervais, 2001).

ACKNOWLEDGMENTS

This paper would not have been possible without the pio-neering work and insights of Cliff Hopson, who introduced generations of geologists to the Coast Range ophiolite, and who has provided the inspiration for my continued work there. Clark Blake and Angela Jayko of the U.S. Geological Survey shared their mapping of the Elder Creek ophiolite and provided me with a detailed introduction to fi eld relationships among the ophio-lite, the Franciscan complex, and the Great Valley assemblage. This research was supported by National Science Foundation (NSF) grants EAR-8816398 and EAR-9018721 (Shervais) and EAR-9018275 (D.L. Kimbrough and B.B. Hanan). Geologic mapping of the Elder Creek ophiolite formed part of a Master’s thesis by Joe Beaman (Beaman, 1991) at the University of South Carolina. Eric Vinson assisted with the whole-rock analyses as part of an undergraduate research project, and Scott Vetter at Centenary College, Louisiana, provided the inductively coupled plasma–mass spectrometry (ICP-MS) analyses. James Beard and Alberto Patino-Douce are thanked for thorough reviews that substantially improved this manuscript.

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