Tonalites, Trondhjemites, and Diorites of the Elder Creek Ophiolite, California: Low-Pressure Slab Melting and Reaction with the Mantle Wedge

Home , Granitoid, Quartz diorite, Tonalite

The Geological Society of America Special 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. Shervais Department 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 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 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.

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

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 [email protected]. ©2008 The Geological Society of America. All rights reserved.

113 114 Shervais

INTRODUCTION 124° 122° 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- 40° N Elder Creek ocean-ridge spreading centers (e.g., Moores et al., 1968; Moores, 40° N 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 Stonyford and Oman, formed above subduction zones in an environment Harbin Springs related to island-arc volcanism (e.g., Miyashiro, 1973; Pearce, Geyser Peak / Black Mountain Mt. St. Helena 1975, 1980; Alabaster et al., 1982; Shervais, 1982; Shervais and Healdsburg Kimbrough, 1985). It is now recognized that many ophiolites Mount Diablo exhibit complex, multistage histories that include both oceanic- 38° N 38° N Leona Rhyolite ridge and suprasubduction phases, although suprasubduction- zone processes are dominant (e.g., Shervais, 2001; Dilek, 2003; Sierra Azul Del Puerto Smith and Rassios, 2003; Shervais et al., 2004a). The Coast Range ophiolite of California has played a impor- Tertiary Quinto Creek tant role in the evolution of geologic thought on ophiolite genesis, Modoc Plateau from mid-ocean-ridge volcanism to back-arc basin spreading, and from simple suprasubduction-zone processes to complex Salinia SNF Llanada multistage evolution involving suprasubduction-zone volcanism 36° N 36° N and spreading-center collision (Evarts, 1977; Hopson and Frano, Great Valley Sequence 1977; Hopson et al., 1981; Shervais and Kimbrough, 1985; Coast Range Ophiolite SAF Shervais , 1990; Dickinson et al., 1996; Godfrey and Klemperer, 1998; Ingersoll, 2000; Shervais et al., 2004a, 2005a, 2005b). It Franciscan Complex Stanley Mtn Cuesta Ridge is one of the most extensive ophiolite terranes in North America; Sierra Nevada it has an outcrop belt over 700 km long in California and cor- Klamath terranes Point Sal 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 124° W 122° W 120° W that require a suprasubduction origin. Figure 1. Location map showing main lithotectonic units of Califor- In this contribution, I document the extensive occurrence nia and locations of largest exposures of Coast Range ophiolite. Elder of silicic plutonic rocks in the Elder Creek ophiolite, one of the Creek is the northernmost exposure and one of the largest. SAF—San larger and more complete remnants of Coast Range ophiolite Andreas fault, SNF—Sur Nacimiento fault. This fi gure was modifi ed after Shervais et al. (2004a). (Fig. 1). These rocks represent the third magmatic suite common in many ophiolites, as documented by Shervais (2001), and present prima facie evidence against formation at an oceanic spreading center. Rather, these silicic plutonic rocks, which include Mineral chemistry for major and minor elements was tonalites, trondhjemites, diorites, and quartz diorites, are consis- obtained with a Cameca SX50 four-wavelength spectrometer tent with formation of the ophiolite in a suprasubduction-zone automated electron microprobe at the University of South Caro- setting, such as a primitive island arc or forearc. lina. Analyses were made at 15 kV accelerating voltage, 25 nA probe current, and counting times of 20–100 s, using natural METHODS and synthetic mineral standards. Analyses were corrected for instrumental drift and dead time, and electron beam–matrix Eight weeks of fi eld mapping were carried out at a scale of effects using the φ–ρ-Z correction procedure provided with the 1:12,000 in parts of two 1:24,000-scale U.S. Geological Survey Cameca microprobe automation system (Pouchou and Pichoir, quadrangles (Riley Ridge, Raglin Ridge), thin slivers along the 1991). Relative accuracy of the analyses, based upon comparison western margin of the Paskenta and Lowry quadrangles, and the NE between measured and published compositions of the standards, corner of the Hall Ridge quadrangle. The mapping formed part of is ≈1%–2% for oxide concentrations greater than 1 wt% and a M.Sc. thesis by Joe Beaman at the University of South Carolina ≈10% for oxide concentrations <1 wt%. (Beaman, 1991). Additional mapping was carried out by Clark I present new whole-rock geochemistry for 30 samples of Blake and Angela Jayco of the U.S. Geological Survey (USGS). diorite, tonalite, and trondhjemite. Representative samples were Tonalites, trondhjemites, and diorites of the Elder Creek ophiolite, California 115 powdered in a tungsten carbide shatterbox and split for prepara- crosscut the cumulate plutonic series (and in places, agmatite tion of fused glass discs and pressed powder pellets. Whole-rock complexes), and (4) sill-like plutons up to 500 m thick and 3 km major-element chemistry data were determined by X-ray fl uores- long that intrude the upper part of the plutonic section; some cence (XRF) analysis using a Philips 2400 XRF spectrometer at of the plutons are bordered along their eastern (upper) margins the University of South Carolina, using glass discs prepared from by a complex of feldspar-phyric dikes that are compositionally 1 g of pre-ignited sample fused with 5 g of lithium metaborate identical to the adjacent plutons. fusion fl ux. Zr and Sr were determined on these glass disks, and The meter-scale pods in the mafi c dike complex represent the Ni was measured on pressed powder pellets, using the same instru- classic “plagiogranite” association (Fig. 3A). These pods are rare, ment. All other trace elements (Th, U, Pb, Nb, Hf, Ta, Y, Rb, Cr, in part because much of the mafi c dike complex has been removed Sc, V, Ba, and the rare earth elements) were determined at Cente- by erosion. Agmatites are more common, and they contain blocks nary College using a Perkin Elmer Elan 6000 quadrapole induc- of mafi c dike complex, isotropic gabbro, and layered cumulate tively coupled plasma–mass spectrometer (ICP-MS; 26 samples). gabbro in a matrix of felsic diorite (Figs. 3B–3D). In some agma- Samples were dissolved in Tefl on bombs in a microwave diges- tites, the clasts are relatively large and angular and have sharp con- tion oven. XRF data were calibrated with a series of USGS rock tacts with the adjacent matrix (e.g., Fig. 3D); in others, the clasts standards. Analytical uncertainties were typically 2%–4% relative have disaggregated in part to form microxenoliths in the matrix, for major elements and 2%–10% relative for the trace elements. making it diffi cult to separate a “xenolith-free” sample for analy- Major- and trace-element data are presented in Table 1. sis (Fig. 3B). Less commonly, the xenoliths have sharp but curva- ceous contacts that suggest that the wall rock was still hot when it FIELD SETTING was invaded by the felsic magma and brecciated (Fig. 3C). These igneous breccias comprise much of what had been The Elder Creek ophiolite is the northernmost exposure mapped previously as isotropic gabbro. The agmatites may include of the Coast Range ophiolite in California (Fig. 1). It preserves signifi cant isotropic gabbro and form diorite-gabbro intrusive a nearly complete ophiolite sequence, with cumulate mafi c and complexes. The most extensive diorite intrusive complex in the ultramafi c rocks, noncumulate gabbro and diorite, dike complex, Elder Creek ophiolite is the Riley Ridge diorite complex, which and volcanic rocks (Shervais and Beaman, 1991, 1994; Shervais, is exposed on the northern fl ank of Eagle Peak, the south end of 2001; Shervais et al., 2004a). Field relations and geochemistry Riley Ridge, and along adjacent stretches of the South Fork indicate that four magmatic suites are present: (1) layered mafi c of Elder Creek (Fig. 2). The agmatites document magmatic and ultramafi c cumulates, with associated isotropic gabbro, dike activity that postdates formation of the “classic” ophiolite stratig- complex, and massive volcanics; (2) clinopyroxenite-wehrlite raphy (layered gabbros, isotropic gabbros, mafi c dike complex). intrusions with less common gabbro and gabbro pegmatoid; The decameter-scale dikes (1–25 m thick) that crosscut the (3) felsic plutonic rocks; and (4) basaltic dikes with mid-ocean- cumulate plutonic series and diorite-isotropic gabbro agmatite ridge basalt (MORB)–like geochemistry that crosscut rocks of the complexes are oriented at a high-angle to cumulate layering older episodes (Shervais and Beaman, 1991, 1994; Shervais, 2001; and to the unconformable contact of the Crowfoot Point breccia Shervais et al., 2004a). The ophiolite generally dips steeply to the on the ophiolite. The thicker dikes are texturally zoned, with east and plunges gently to the south (Fig. 2). As a result, cumulate coarse-grained hypidiomorphic cores and fi ne-grained chilled plutonic rocks are exposed in the western and northern portions of margins. These dikes are more resistant to erosion than the lay- the ophiolite, whereas upper-crustal lithologies (dike complex, vol- ered gabbros they intrude, so they form prominent ridges that canics) crop out to the east and south. Felsic plutonic rocks make stand out in relief where exposed along the South Fork of Elder up ~25%–30% of the mapped lithologies, which generally crop Creek (Figs. 4A and 4B). The approximate locations of several out stratigraphically above the cumulate plutonic sequence. Upper- prominent dikes along the South Fork of Elder Creek are shown crustal lithologies were largely removed by erosion on the seafl oor on an oblique air photograph in Figure 4B. These dikes are ori- prior to deposition of the overlying Crowfoot Point breccia, but ented at a high-angle to the sill-like plutons that lie up-section they are preserved at the south end of the ophiolite and in fault from them (described below); as a result, they may represent blocks (Fig. 2). The Crowfoot Point breccia overlies the ophiolite feeders to the overlying plutons. unconformably along an erosion surface that cuts down-section to The largest felsic bodies in the Elder Creek ophiolite are sheet- the north; as a result, the breccia rests on sill complex at the south like plutons up to 500 m thick and 3 km long that intrude the upper end of the massif (Toomes Camp Road and Digger Creek sec- part of the cumulate plutonic section (Fig. 4B). These plutons are tions), on cumulate plutonic rocks in the central part of the ophio- oriented subparallel to cumulate layering in the underlying gabbros lite (South Fork of Elder Creek), and on diorite at the northern end and to the unconformable contact of the Crowfoot Point breccia. of the massif (Middle Fork of Elder Creek; Fig. 2). We interpret these plutons to represent horizontal tabular plutons Felsic lithologies crop out in four distinct associations: (sills) intruded along the interface between the underlying cumu- (1) small, meter-scale pods or dikes within the mafi c dike late plutonic section (formed in the ophiolite magma chamber) complex; (2) as the matrix of igneous breccias (agmatites) in and the superjacent mafi c dike complex prior to disruption of the diorite-gabbro intrusive complexes, (3) 1–25-m-thick dikes that 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 xeno in Riley Ridge Riley Ridge Riley Ridge Riley Ridge Riley Ridge Riley Ridge Elder Creek - Elder Creek - matrix agmatite complex complex complex - complex - complex complex - thick dike thick dike dike dike 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.) TABLE 1. WHOLE-ROCK MAJOR- AND TRACE-ELEMENT ANALYSES OF ELDER CREEK TONALITIES, TRONDHJEMITES, AND DIORITES, WITH CIPW NORMS (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 trondhjemite Occurrence Elder Creek - Elder Creek - Elder Creek - Elder Creek - Elder Creek - Eagle Peak Eagle Peak Pellows Pellows Pellows thick dike thick dike thick dike thick dike thick dike felsite pluton pluton pluton 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 tonalite Occurrence Pellows pluton Red Mtn felsic Red Mtn felsic Red Mtn Red Mtn Red Mtn Red Mtn Red Mtn pluton/ Brush Mtn Brush Mtn dikes dikes pluton pluton pluton pluton chilled margin pluton 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 Great Valley Series

GVS GVS Wacke and siltstone du cpb Crowfoot Point Breccia wc

du cg Elder Creek Ophiolite wc Brush Mtn Pluton vc Volcanic rocks du Dike complex

ig fdc fmg qdi Pellows Place Pluton Felsic dikes EC-148 GVS qdi Quartz diorite & diorite ig du fdc fdc ig Isotropic gabbro cg vc cg Cumulate gabbro sfms vc wc cg Wehrlite-Pyroxenite wc Red Mtn Pluton cg du Dunite & dunite qdi fdc broken formation ig EC107 cpb dc qdi Serpentinite mélange wc cg ssp Sheared serpentinite cg vc

gs Volcanic blocks ig wc Riley Ridge complex in melange

qdi fmg Foliated metasediments sfms ssp wc cg (Galice?) gs Eagle Peak Pluton Franciscan Assemblage qdi South Fork sfms ssp Mountain schist gs cpb GVS

gs wc cpb ssp gs North

0 12 vc cpb miles

ssp

GVS

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 been documented by Hopson et al. (this volume) to represent a that form extensive, mappable complexes along the margins of Paleoproterozoic-age crustal component. This ancient inherited the plutons. The felsites consist of small plagioclase phenocrysts zircon component makes an oceanic origin improbable. in an aphanitic matrix. They are compositionally identical to their adjacent plutons, and rare chilled contacts within the felsites sug- RESULTS gest that they represent a sill complex, which formed a carapace above the underlying plutons. Petrography and Mineral Chemistry There are four relatively large, discrete plutons (from south to north): Eagle Peak, Red Mountain, Pellows, and Brush Typical phase assemblages in the felsic plutonic rocks include Mountain (Figs. 2 and 4B). These may represent four separate plagioclase, quartz, hornblende, and opaque oxides; pyroxenes plutons, or they may have originally formed contiguous bodies (both clino- and orthopyroxene) are found in the diorites and that have since been separated by faulting. The plutons stand quartz diorites. The most common texture is hypidiomorphic out on aerial photographs by their contrast with the adjacent granular, and euhedral to subhedral hornblende prisms and units: the cumulate gabbros and ultramaﬁ c rocks that underlie plagioclase laths form a framework that encloses anhedral quartz the plutons have scant vegetation and light-colored soil, while and oxides. Grain size is generally ≤2 mm. Typical modal pro- the Crowfoot Point breccia, which overlies the plutons, has scat- portions are ≈50% plagioclase, 30%–35% hornblende, 0%–10% tered vegetation and slightly reddish soils. In contrast, the felsic pyroxene, and 2%–25% quartz (Fig. 5). plutons are characterized by a cover of dense chaparral through Plagioclase forms elongate laths, 1–2 mm in length, that are

which soil cannot be seen (Fig. 4), sometimes even when you relatively euhedral. Plagioclase composition is in the range An32–65,

are standing on it (which explains why these extensive felsic but albite An0–5 is common (Fig. 6A). Relict plagioclase is com- suites were not discovered earlier). monly altered in part to smectite, giving the laths a dusty brown Shervais and coworkers (Shervais et al., 2005a) reported appearance in thin section (Fig. 5). Hornblende (Mg# 78–23) forms U-Pb zircon ages for two samples from the Elder Creek ophio lite: euhedral to subhedral prisms up to ≈3 mm in size; smaller grains a plagiogranite lens that intrudes the sheeted dike complex along are interstitial to plagioclase, whereas larger grains may partially the South Fork of Elder Creek (EC107–3) and a hornblende quartz enclose plagioclase laths subophitically. Pyroxenes include both

diorite from the Brush Mountain pluton (EC148–2). Both samples clinopyroxene (augite to ferroaugite, Wo38–49En55–80) and ortho- 238 206 are slightly discordant, with U/ Pb ages ranging from 161 to pyroxene (hypersthene, Wo04En70) (Fig. 6B). Quartz occurs as small 172 ± 2 Ma and 207Pb/206Pb ages ranging from 165 to 198 Ma. Con- (<1 mm) interstitial grains that stand out from adjacent plagio- cordia plots have upper intercepts that suggest crystallization ages clase because they are unaltered and optically clear (Fig. 5). In of 169.7 ± 4.1 Ma for EC107–3 and 172.0 ± 4.0 Ma for EC148–2. some samples, quartz is intergrown graphically with opaque oxides These ages are signiﬁ cantly older than previous hornblende K-Ar (Fig. 5B). Minor-element concentrations are extremely low in the

dates from gabbro (154 ± 5 to 163 ± 5 Ma) ( Lanphere, 1971; pyroxenes: TiO2 and Na2O are typically less than 0.4 wt%, Cr2O3 is ≤ ≈ McDowell et al., 1984), which probably represent cooling or 0.05 wt%, and Al2O3 is 1–3 wt%. Hornblende is enriched in all 207 206 ≈ ≈ Ar-loss ages (Shervais et al., 2005a). The Pb/ Pb ages imply of these elements, with TiO2 0.5–3 wt%, Na2O 0.5–1.8 wt%, ≈ ≈ an inherited zircon component (Shervais et al., 2005a), which has and Al2O3 is 4–12 wt%, and Cr2O3 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

A Orthoclase Feldspar

0 100 90 80 70 60 50 40 30 20 10 Albite Anorthite Ca 50 Pyroxene B Hornblende 40

0 100 90 80 70 60 50 40 30 20 10 Mg Fe

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.

Secondary phases include chlorite, prehnite, epidote, and diorite or quartz diorite, whereas the decameter-scale dikes in zoisite. Chlorite is associated with the maﬁ c phases, whereas Elder Creek include diorite , quartz diorite, and tonalite. The prehnite is found in clusters and along microveins and assumes small plagiogranite pods, and the larger plutons (along with their both the radial and platy habits. Zoisite is rare and occurs inter- felsite dike margins), are dominantly tonalite and trondhjemite. stitially. Sphene occurs as small, high-relief, orange grains. Many of the more Si-rich rocks are slightly corundum normative, consistent with fractional crystallization of hornblende and Major- and Trace-Element Chemistry plagioclase (e.g., Cawthorn and Brown, 1976). The calc-alkaline trend of this suite resembles adakites The Elder Creek diorites span a wide range in composi- from Baja California (Rogers et al., 1985) and plutons from

tion: 54%–75% SiO2, 3.3%–14.3% FeO*, and 2.7%–6.4% the Aleutian arc (Kelemen et al., 2004) (Fig. 8A). In contrast to

MgO (Fig. 7). All are extremely low in K2O (<0.7%), similar to these suites, however, the Elder Creek tonalites, trondhjemites,

oceanic plagiogranites (Coleman and Peterman, 1976; Coleman and diorites are extremely low in K2O, and somewhat higher in

and Donato, 1979). The plutons are higher in silica (62%–77% Na2O. Similar low-K trends are seen in primitive island arcs of

SiO2) than the dikes, pods, and agmatite breccias (53%–59% the western Paciﬁ c, e.g., Tonga-Kermadec, Izu-Bonin. In partic-

SiO2 in most samples; a few samples range up to 72% SiO2). ular, the Elder Creek TTD suite displays strong similarities to the

Na2O increases with increasing SiO2, whereas the other major low-K, high-Ca dacites from Tonga-Kermadec (Bryan, 1979). A elements either decrease or show no change (Fig. 7). FeO* coeval suite of tonalites, trondhjemites, and granodiorites in the decreases with increasing Si (Fig. 7), and the suite deﬁ nes a calc- Smartville ophiolite is also similar (Beard, 1998). alkaline trend on an AFM (alkalis-Fe)*-MgO) diagram (Fig. 8A). High-Mg diorites with 4%–7% MgO at 53%–63% SiO2 Based on their normative quartz-plagioclase-orthoclase com- are common in the Riley Ridge diorite complex and the dike positions, and their molar Ca-Na-K ratios, these rocks include suite, but relatively high Mg#, Cr, and Ni values are found in the diorites (<5% normative quartz), quartz diorites (5%–20% large plutons as well. Mg# (100 × Mg/[Mg + Fe] molar) values

normative quartz), tonalites, and trondhjemites (>20% norma- range from 65 to 30 in rocks with <65% SiO2, and even high-Si

tive quartz; Figs. 8B and 8C) (Barker, 1979; Le Maitre, 2002). tonalites and trondhjemites with 65%–75% SiO2 commonly Agmatite suites (e.g., Riley Ridge diorite complex) are largely have Mg# = 30–48. The high Mg# values are accompanied by Tonalites, trondhjemites, and diorites of the Elder Creek ophiolite, California 123

high compatible-element concentrations as well, with Cr up to higher in TiO2 than those observed in Archean TTG (tonalite- 227 ppm in high-Si quartz diorites (Table 1). The high concen- trondhjemite-granodiorite) suites or the Elder Creek TTD suite trations of MgO and FeO* (4–9 wt% in most samples, with two (e.g., Rapp et al., 1991; Rapp and Watson, 1995). Melt fractions high-Fe diorites ≈12 wt% FeO*) result in high normative mafi c increase and silica decreases with higher temperatures. There contents, which include normative olivine in the more mafi c is little distinction in the major-element data between melts samples (Table 1). formed at high pressure and those formed at lower pressures, Chondrite-normalized rare earth element (REE) concen- but Na/K ratio varies with protolith composition (Beard and trations are enriched in the light rare earth elements (LREE) Lofgren, 1991; Rapp and Watson, 1995). Both the experimental relative to the heavy rare earth elements (HREE) in all but two dehydration melts and the observed TTD suites are lower in of the samples (Fig. 9). This enrichment is relatively modest, alumina and higher in iron than water-saturated melts derived and chondrite-normalized La/Lu ratios are 0.7–2.1× chondrite from similar protoliths (Beard and Lofgren, 1991). in the diorites and quartz diorites (Fig. 9A) and 1.75–2.75× Compared to adakites, bajaites, Archean tonalite- chondrite in the tonalites and trondhjemites (Fig. 9B), with total trondhjemite -granodiorite (TTG) suites, and similar rocks La concentrations of ≈10× to ≈50× chondrite. Discernible Eu thought to represent melts formed in these settings, the Elder anomalies are typically small and negative, but a few samples Creek TTD suite is characterized by relatively fl at chondrite- display small positive anomalies; two samples (trondhjemites normalized REE patterns (Fig. 9), higher Y, and lower Sr EC147–1 and EC218–2A) display large positive Eu anomalies (Fig. 7), and lower Sr/Y and Cr/Y (Fig. 11). The steep LREE- and lower HREE concentrations, consistent with plagioclase enriched patterns of adakites and related rocks (and the con- accumulation (Fig. 9). comitant depletion in HREE) is thought to refl ect equilibration Multi-element diagrams normalized to average normal with a garnet-rich, plagioclase-free, eclogite melting residue, MORB (Sun and McDonough, 1989) are moderately depleted which also lowers Y (partitioned into garnet) and raises Sr in the less incompatible elements (middle to heavy REE ≈ (no plagioclase in residue). The resulting high Sr/Y ratios are 0.5–1× normal [N] MORB) and moderately enriched in the characteristic of adakitic and related melts. Reaction of these more incompatible elements (Ba to Sr ≈ 1× to 10× N-MORB; melts with the overlying mantle wedge enriches the melts in Fig. 10). The high fi eld strength elements Nb, Ta, Hf, and Ti refractory elements (Cr, Ni, Mg), raising the Cr/Y ratio and, display distinct negative anomalies relative to adjacent low in extreme cases, resulting in the formation of sanukitoids, fi eld strength elements of similar incompatibility, whereas which are Mg-rich granites found in some Archean terrains Pb displays a small positive anomaly—all characteristics that resemble magnesian andesites of the Setouchi belt in SE of subduction-zone magmatism. K and Rb are less enriched Japan (Tatsumi and Ishizaka, 1982). than comparable arc suites and may be depleted relative to Adakites, bajaites, Archean TTGs, and sanukitoids all N-MORB in some samples (Fig. 10). The depletion of large refl ect melting of mafi c protoliths at relatively high pressures ion lithophile elements such as K and Rb is a common feature (>10 kbar) to form an eclogitic residue. In contrast, dehydra- of suprasubduction tonalite-trondhjemite suites (e.g., Beard, tion melting of mafi c protoliths at lower pressures (5–10 kbar) 1998) and will be discussed later. results in melts with similar major-element characteristics, but with relatively unfractionated chondrite-normalized REE pat- DISCUSSION terns, higher Y (no garnet in residue), and lower Sr (refractory plagioclase). These characteristics rule out melting of deep Petrogenesis of the Elder Creek TTD Suite island-arc roots, which by defi nition imply pressures greater than 10 kbar. It is also inconsistent with fi eld relations that Major-element characteristics of the Elder Creek TTD suite document a relatively thin crustal section throughout the Coast are consistent with partial melting of a mafi c protolith, such Range ophiolite (Shervais et al., 2004a). as subducting oceanic crust or the deep roots of an island arc Tulloch and Kimbrough (2003) discussed the occurrence (e.g., Kay, 1978; Beard and Lofgren, 1989, 1991; Drummond of high Sr/Y and low Sr/Y granitoid suites in Baja California and Defant, 1990; Rapp et al., 1991; Rapp and Watson, 1995; (Mexico) and New Zealand. They showed that low Sr/Y suites Martin et al., 2005; Thorkelson and Breitsprecher, 2005). Dehy- typically occur closer to the trench, whereas the high Sr/Y suites lie dration melting experiments on natural and synthetic green- closer to the continental margin. They inferred a relationship stone-amphibolite compositions at pressures above and below between the position of these suites and thrusting of older arc the garnet-in reaction document a range in melt compositions, crust under the continental margin to form the source of the from trondhjemite through diorite, with increasing temperature high-Sr/Y suite granitoids (Tulloch and Kimbrough, 2003). and melt fraction (Beard and Lofgren, 1991; Rapp et al., 1991; Alternatively, I propose that the contrast in the position of these Rushmer, 1991; Rapp and Watson, 1995). These melts are rela- two suites may refl ect the inclination of the subducting slab, ≈ tively high in alumina (14%–18% Al2O3), sodium (Na2O 4%), where low-Sr/Y suites are formed by melting of the mafi c slab at ≈ and iron (FeO* 7% at 60% SiO2), and low in K2O (<1.5%). shallower depths closer to the trench, and high-Sr/Y suites are The experimental melts are generally lower in MgO and formed by melting at deeper levels farther from the trench. 1.0 A 2 0.8 0.6 TiO 0.4

18 3 B

O 16 2 14 AL 12

12 C

8 FeO* 4

10 8 D 6

MgO 4 2

12 10 E 8 6 CaO 4 2

7 F

O 6 2 5

Na 4 3

5 4 G O

2 3

K 2 1 50 555 60 65 70 75 80

SiO2 Agmatite xenolith Agmatite matrix

Figure 7 (on this and following page). Harker diagrams, showing Riley Ridge Diorite Complex changes in major-element concentrations as function of silica con- Diorite Dikes tent: TiO2, Al2O3, FeO*, MgO, CaO, Na2O, K2O, Rb, Sr, Y, Zr, Sc, Red Mtn Pluton and V. Data for Aleutian island-arc plutons (gray cross) and Baja high-Mg andesites (+) are shown for comparison. Aleutian arc and Pellows Pluton Baja high-Mg andesite data suites were compiled by Peter Kelemen, Eagle Peak Pluton Gene Yogadinski, and David Scholl (Kelemen et al., 2004). Brush Mtn Pluton Aleutian plutons Baja lavas 300 H 200

Zr (ppm) 100

60 I 40

Y (ppm) Y 20

1,200 J 800

Sr (ppm) 400

125 100 K 75 50 Rb (ppm) 25

35 30 L 25 20 15

Sc (ppm) 10 5

400 M 300 200 V (ppm) 100

200 N

100 Cr (ppm)

50 55 60 65 70 75 80

SiO2

Agmatite xenolith Agmatite matrix Riley Ridge Diorite Complex Figure 7 (continued). Diorite Dikes Red Mtn Pluton Pellows Pluton Eagle Peak Pluton Brush Mtn Pluton Aleutian plutons FeO* 100 Quartz 100 A 10 90 B 0 90 1 80 20 0 Anorthite 80 2 70 30 100

70 30 C 40 10 60 MgO 90 60 40 20 50 Tholeiitic 50 80

50 50 30 40 60 70

40 60 70 Kspar 40 30 60

0 30 Tonalite & 7 50 20 Calc-alkaline Trondhjemite80 500

20 80 60 10 90 400 Quartz Diorite 90 10 0 7 0 100 30 100 90 80 70 60 50 40 3 20 10 Diorites & Alkali Diorite0 Tonalites 100 1001 90 80 70 60 50 40 3 20 10 20 0 80 Plagioclase 90 10 Trondhjemites Agmatite xenolith Red Mtn Pluton 0 100 Agmatite matrix Pellows Pluton 1 90 80 70 60 50 40 30 20 10 00 Kspar Riley Ridge Diorites Eagle Peak Pluton Albite Diorite Dikes Brush Mtn Pluton 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).

1000 1000 A B

100 100

10 10

Agmatite xenolith Eagle Peak Pluton 1 Agmatite matrix 1 Pellows Pluton Riley Ridge Diorite Complex Red Mtn Pluton Diorite Dikes Brush Mtn Pluton

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

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

1000 A 100 Agmatite xenolith Agmatite xenolith Agmatite matrix Agmatite matrix 100 Riley Ridge Diorite complex Riley Ridge Diorite complex Diorite Dikes Diorite Dikes Eagle Peak Pluton Pellows Pluton 10 Sr/Y 10 Red Mtn Pluton Brush Mtn Pluton Aleutian plutons 1 Aleutian lavas

1 .1

Rb BaTh U NbTa K LaCePbPr Sr P NdSmZr Hf Eu Ti GdTbDy Y HoErTmYb Lu 10

1000 B 1 Cr/Y 100 Eagle Peak Pluton Pellows Pluton Red Mtn Pluton .1 Brush Mtn Pluton 10

.01 10 20 30 40 50 60 70 80 90 1 Y (ppm) Figure 11. Sr/Y and Cr/Y versus Y for the Elder Creek tonalite- .1 trondhjemite-diorite (TTD) suite, compared with Aleutian plutons and lavas, and high-Mg andesites from Baja California. Data suites Rb BaTh U NbTa K LaCePbPr Sr P NdSmZr Hf Eu Ti GdTbDy Y HoErTmYb Lu were compiled by Peter Kelemen, Gene Yogadinski, and David Scholl (Kelemen et al., 2004). The Elder Creek TTD suite is characterized by Figure 10. Mid-ocean-ridge basalt (MORB)–normalized multi-element lower Sr/Y and Cr/Y than comparable high-Mg plutonics and lavas. This plot. Concentrations are normalized to the normal (N) MORB average refl ects melting at lower pressures, outside of the garnet stability fi eld. 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 Effect of Mixing and Assimilation in Agmatites plutons, from Kelemen et al. (2004). Diorites and quartz diorites that make up the matrix in agmatites have silica contents that are lower than the tabular plutons and Setting for Slab Melting larger dikes that feed them (Fig. 7). This matrix may have formed by fractional crystallization of magmas parental to the associated It has long been recognized that most subducting slabs are isotropic gabbros. Alternatively, the matrix may have formed by too cool to melt at any depth, and that even deep melting to form the disaggregation and assimilation of more mafi c wall rock into eclogite residue (>35 km) requires relatively young oceanic crust an intrusive magma that was originally more tonalitic or trond- (younger than 30 Ma) that has not yet cooled completely (Pea- hjemitic in composition. This process is most clearly documented cock, 1991). Melting at shallower depths (5–10 kbar pressure, in the outcrops on Eagle Peak (Fig. 3B), where clasts of mafi c 16–34 km depth) requires unusually hot conditions in the subduct- gabbro as small as 1 or 2 mm across are observed. This process ing slab that can only be obtained when an active oceanic spread- may also account for the small positive Eu anomalies seen in a ing center collides with a subduction zone and ridge subduction few samples, if the contaminant was a cumulate gabbro. occurs (e.g., Thorkelson and Breitsprecher, 2005). Shervais and co-workers (Shervais, 2001; Shervais et al., 2004a, 2005a, 2005b) Comparison of the Elder Creek TTD Suite with have proposed that such a collision ended formation of the Coast Sanutikoids, Setouchiites, and Other High-Mg Diorites Range ophiolite and led to the intrusion of MORB composition dikes at Elder Creek and other Coast Range ophiolite locales, and The Elder Creek tonalite-trondhjemite-diorite suite exhibits to the eruption of MORB composition basalts at other locales relatively high concentrations of the refractory elements MgO, (e.g., Cuesta Ridge). Cr, and Ni, and high Mg#, compared to normal calc-alkaline 128 Shervais

granitoids. The high concentrations of refractory elements sug- Rb into the early crystallizing phases pyroxene, hornblende, gest a relationship to the wehrlite-clinopyroxenite suite—possi- and plagioclase (Barker and Arth, 1976). The data presented bly as the evolved liquid fraction complementary to the cumu- here support that interpretation: Ba does not correlate with lates. They also suggest a petrogenetic relationship to sanukitoids Rb concentration, even though K, Rb, and Ba are all strongly and setouchiites. Sanukitoids are high-Mg granitoids found in depleted in the Elder Creek TTDs. Archean terranes that resemble high-Mg andesites found in Japan (Shirey and Hanson, 1984). Setouchiites are high-Mg andesites Tectonic Setting of the Elder Creek TTD Suite from the Setouchi Peninsula, Japan, that formed in response to subduction of an active spreading center (Tatsumi and Ishizaka, The Elder Creek tonalite-trondhjemite-diorite suite resem- 1982). Like adakites, these high-Mg suites are characterized by bles oceanic plagiogranite in many of its major- and trace- low Y, high Sr, and high Sr/Y ratios that refl ect eclogite melting element characteristics, but other aspects of its chemical and at pressures >10 kbar. They have even higher Cr/Y ratios than petrologic composition require a suprasubduction-zone origin. adakites, refl ecting extensive reaction of the primary slab melts These include the calc-alkaline fractionation trends, the frac- with the overlying mantle wedge; some authors even suggest that tionated REE and MORB-normalized multi-element charac- sanukitoids and setouchiites represent partial melts of mantle teristics, the nega tive high fi eld strength element anomalies, wedge peridotites that have been metasomatized by slab-derived the extensive plutons of high-silica tonalite and trondhjemite, melts (Martin et al., 2005). the common occurrence of modal hornblende and augite, and The high refractory element concentrations observed in less common occurrence of modal hypersthene. This conclusion the Elder Creek TTD suite imply a similar process, in which is supported by granite trace-element discrimination diagrams slab-derived partial melts formed at relatively low pressures based on Y, Nb, and Rb (Pearce et al., 1984), which show that (5–10 kbar) from amphibolite-facies metabasalts reacted with all of the rocks analyzed here have volcanic-arc granite system- the overlying mantle wedge to become enriched in MgO, Cr, atics (Fig. 12). It is also supported by tectonic discrimination and Ni. Melts derived from amphibolite at these pressures will diagrams for pyroxenes (Fig. 13; Leterrier et al., 1982). These lack the depletion in HREE and Y observed in adakites, sanuki- diagrams show that the pyroxenes in these rocks are nonalka- toids, and setouchiites and will show a concomitant enrichment line (Fig. 13A), orogenic (Fig. 13B), and largely calc-alkaline in LREE and Sr. In addition, melts formed at lower pressures (Fig. 13C). The high Mg and Cr contents seen in many samples will traverse thinner sections of the mantle slab, resulting in require that these melts reacted with a refractory mantle wedge somewhat less refractory element enrichment. As a result, their during their ascent to the surface. Similar suites have been docu- Cr/Y ratios will be lower than corresponding ratios in sanuki- mented in other suprasubduction ophiolites (e.g., Beard, 1998). toids and setouchiites. The suprasubduction-zone setting for slab melting and mantle-wedge reaction documented here is depicted in Figure 14. LILE Depletion in the TTD Suite This model (which is scaled the same vertically and horizontally) assumes the subduction of relatively young (younger than 25 Ma) The Elder Creek tonalite-trondhjemite-diorite suite is char- oceanic lithosphere, which is thermally buoyant. This results in acterized by exceptionally low concentrations of K and Rb— relatively shallow subduction of altered oceanic crust beneath hot concentrations that are strongly depleted relative to other low asthenosphere of the mantle wedge (Fig. 14). The infl ux of fi eld strength elements (e.g., Fig. 7). This “overdepletion” of hot asthenosphere into the mantle wedge above the slab occurs large ion lithophile elements is characteristic of oceanic plagio- during the earlier stages of subduction in response to sinking of granites (tonalities and trondhjemites). It is possible that this the oceanic slab and formation of the main phases of ophiolite overdepletion in LILE results from secondary alteration. How- crust (e.g., Stern and Bloomer, 1992; Shervais, 2001). Melting ever, there are two possible primary causes, both of which may of the subducting crust occurs at relatively shallow depths, cor- be operative. First, dehydration melting experiments on natural responding to pressures of 5–10 kbar, to form the TTD suite. The

metabasalt samples show that low-K2O melts form in response resulting melts rise through the mantle wedge, react to become

to melting of protoliths that are low in K2O to begin with, such more magnesian, and intrude ophiolite crust formed during the as oceanic greenstones (Beard and Lofgren, 1991; Rapp et al., initial stages of subduction (Fig. 14). The occurrence of an ancient 1991; Rushmer, 1991). These experiments show that protoliths inherited zircon component seen in the Elder Creek TTD suite

with <0.3% K2O are needed to form melts with the low-K2O (Shervais et al., 2005a) requires the subduction of sediment bear- concentrations observed here (<0.7%). ing ancient zircon into the hot zone where melts are produced. Beard (1998) proposed that this overdepletion is caused by Formation of the TTD suite is a transient event that occurs the evolution of an alkali-rich vapor phase that is driven off dur- only when relatively young, hot, buoyant lithosphere is sub- ing intrusion to permeate and alter the surrounding wall rock. ducted—just prior to or after collision of the spreading ridge with The partitioning of alkalis into the vapor phase is controlled by the trench system. It represents stage three in the life cycle of the lack of early formed potassic mineral phases (due to low ini- suprasubduction-zone ophiolites (Shervais, 2001). This implies tial alkali content), and the low partition coefﬁ cients for K and that TTD suite rocks must intrude older crustal elements of the Tonalites, trondhjemites, and diorites of the Elder Creek ophiolite, California 129

1000 .06 A A .05 100 WPG .04 Alkaline

.03 10 VAG ORG Nonalkaline .02 Tholeiitic & Calc-alkaline Agmatite xenolith Nb (ppm) Agmatite matrix Riley Ridge Diorites 1 Diorite Dikes .01 Eagle Peak Pluton Pellows Pluton Red Mtn Pluton .00 Brush Mtn Pluton .50 .60 .70 .80 .90 1.00 .1 Ca+Na (afu) 1 10 Y 100 1000 1000 .04 B B SynColG .03 100 WPG Nonorogenic (MORB) .02 VAG Orogenic 10 (arc) Ti+Cr (afu)Ti+Cr (afu) Ti

Rb (ppm) .01

1 ORG .00 .50 .60 .70 .80 .90 1.00 Ca (afu) .1 1 10 100 1000 .03 Y+Nb (ppm) C Figure 12. Tectonic discrimination diagrams for granitoids, after Pearce (1984): (A) Y versus Nb, (B) Y + Nb versus Rb. Fields: Calc-alkaline arc VAG—volcanic-arc granite, ORG—ocean-ridge granite, WPG— .02 within-plate granite, Syn-ColG—syncollisional granite. Elder Creek tonalite-trondhjemite-diorite (TTD) are classed as volcanic-arc granites on these plots. Ti (afu) Ti .01 Arc Tholeiite 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). .00 Support for the origin of the Elder Creek tonalite- .00 .05 .10 .15 .20 trondhjemite-diorite suite by partial melting of subducted oceanic Al (afu) crust comes from amphibolite blocks in the underlying Francis- can complex. These blocks often contain small melt lenses of Figure 13. Tectonic discrimination diagrams for pyroxene (after Leterrier et al., 1982). (A) Low Ti = nonalkaline, tholeiitic, or calc- tonalitic or trondhjemitic composition that are mineralogically alkaline compositions, (B) low Ti + Cr at high Ca = orogenic (arc) similar to the Elder Creek TTD suite (Fig. 15). We suggest that geochemistry, (C) relatively high Ti relative to Al—calc-alkaline rather these represent the same process, but on a smaller scale. than arc tholeiite. MORB—mid-ocean-ridge basalt. 130 Shervais

Spreading Trench Center Sediment with Ancient Zircons Thin Crust formed by Rapid Extension in Response to Slab Sinking Depth Calc-alkaline volcanics (km)

10 Young, Hot Melt Reaction with Wedge Lithosphere Hot Asthenosphere Melt Zone from 20 5–10 kbar Slab Sinking (16–34 km) 30

40 No Melt (Refractory: Low-T melt fraction extracted) 50

0 km 50 km 100 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).

CONCLUSIONS trondhjemitic melt with mantle peridotite during ascent. The primary difference is that Elder Creek TTD suite melted at lower The Elder Creek tonalite-trondhjemite-diorite suite forms pressures in response to a ridge collision-subduction event. a signifi cant part of the Elder Creek ophiolite—up to 30% of The correlation of the Elder Creek tonalite-trondhjemite- the plutonic rock outcrop—that postdates formation of the diorite suite with a ridge collision-subduction event is consistent ophiolite layered series and the sheeted dike complex. Field with previous models for the Coast Range ophiolite, which call 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 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 involvement of subducted sediments is required by inherited 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 garnet stability fi eld. Melting at higher pressures results in enriched LREE patterns, HREE and Y depletion, and Sr enrichment—none Figure 15. Outcrop photo of Franciscan amphibolite block exposed of which are observed in the Elder Creek TTD suite. However, in Elder Creek, showing melt lens of tonalitic composition. This melt the overall process of formation is similar: melting of young, hot lens is considered to be an analogue for the Elder Creek tonalite- subducting slab, followed by reaction of the primary tonalitic- 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 Dewey, J.F., and Bird, J.M., 1971, Origin and emplacement of the ophiolite suite; Appalachian ophiolites in Newfoundland: Journal of Geophysical that postdate other volcanic and plutonic activity (Shervais, 2001; Research, v. 76, no. 14, p. 3179–3206. Shervais et al., 2004b, 2005a). This model is also consistent with Dickinson, W.R., Hopson, C.A., Saleeby, J.B., Schweickert, R.A., Ingersoll, the relative ages of magmatic suites in many suprasubduction- R.V., Pessagno, E.A., Jr., Mattinson, J.M., Luyendyk, B.P., Beebe, W., Hull, D.M., Munoz, I.M., and Blome, C.D., 1996, Alternate origins of the zone ophiolites (Shervais, 2001). Coast Range ophiolite (California); introduction and implications: GSA Today, v. 6, no. 2, p. 1–10. ACKNOWLEDGMENTS Dilek, Y., and Newcomb, S., eds., 2003, Ophiolite Concept and the Evolution of Geological Thought: Geological Society of America Special Paper 373, This paper would not have been possible without the pio- 504 p. neering work and insights of Cliff Hopson, who introduced Drummond, M.S., and Defant, M.J., 1990, A model for trondhjemite-tonalite- dacite genesis and crustal growth via slab melting: Archean to mod- generations of geologists to the Coast Range ophiolite, and who ern comparisons: Journal of Geophysical Research, v. 95, no. B13, has provided the inspiration for my continued work there. Clark p. 21,503–21,521. Blake and Angela Jayko of the U.S. Geological Survey shared Evarts, R.C., 1977, The geology and petrology of the Del Puerto ophiolite, Diablo Range, central California Coast Ranges, in Coleman, R.G., and their mapping of the Elder Creek ophiolite and provided me with Irwin, W.P., eds., North American Ophiolites: Oregon Department of a detailed introduction to fi eld relationships among the ophio- Geology and Mineral Industries Bulletin 95, p. 121–139. lite, the Franciscan complex, and the Great Valley assemblage. Gass, I.G., Neary, C.R., Plant, J., Robertson, A.H.F., Simonian, K.O., Smewing, J.D., Spooner, E.T.C., and Wilson, R.A.M., 1975, Comments This research was supported by National Science Foundation on “The Troodos ophiolitic complex was probably formed in an island (NSF) grants EAR-8816398 and EAR-9018721 (Shervais) and arc,” by A. Miyashiro and subsequent correspondence by A. Hynes EAR-9018275 (D.L. Kimbrough and B.B. Hanan). Geologic and A. 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