Evidence of a Late Jurassic Ridge Subduction Event: Geochemistry and Age of the Quartz Mountain Stock, Manastash Inlier, Central Cascades, Washington

James H. MacDonald Jr.1,* and Adam Schoonmaker2

1. Marine and Ecological Sciences, Florida Gulf Coast University, Fort Myers, Florida 33965, USA; 2. Geology Department, Utica College, Utica, New York 13502, USA

ABSTRACT Previous studies of rocks in California and Oregon suggest that a Late Jurassic ridge subduction event occurred. The geology of the Manastash inlier in Washington State supports this Late Jurassic ridge subduction. The biotite hornblende granodiorite and biotite hornblende tonalite of the Quartz Mountain stock and its cupolas are located in the Manastash inlier, central Cascades, Washington. This stock and its cupolas intrude the low-PT metamorphosed Look- out Mountain Formation turbidites—which are interpreted to have formed in a forearc depositional setting—and the suprasubduction zone Hereford Meadow amphibolite. A new single-crystal sensitive high-resolution ion microprobe– reverse geometry U-Pb age from stock zircons has an age of 157.4 5 1.2 Ma (2j). This new age corroborates the published age from the stock and is interpreted to be the crystallization age of the stock. New whole-rock major- and trace-element geochemistry suggests that the granitoids from this stock were generated by shallow mantle melting in a subduction zone setting. Geochemistry also suggests that the stock assimilated minor amounts of its wall rock and that the cupolas were generated from the same magmatic source as the main phase of the stock. The intrusion of the Quartz Mountain stock into the low-PT turbidites of the Lookout Mountain Formation and the suprasubduction zone Hereford Meadow amphibolite suggest that the magmas were generated by the subduction of a Late Jurassic spreading ridge. The geochemistry of the Quartz Mountain stock is similar to that of granitoids of the Taitao Peninsula, Chile, which were generated by the subduction of the Chile ridge during the late Neogene.

Online enhancements: supplemental tables.

Introduction Late Jurassic arc magmatism along the Cordillera ing the Late Jurassic. Late Jurassic arcs may have margin of North America was modest, compared to occurred in numerous tectonic settings, including other Cordilleran magmatic events. However, rem- a locus outboard of Late Jurassic basins (e.g., Yule nants of these Late Jurassic arcs are located through- et al. 2006; Dorsey and LaMaskin 2007), in retroarc out the Cordillera (e.g., Garcia 1982; Saleeby et al. settings (Coint et al. 2013), and in arc-arc collisional 1989; Miller et al. 1993; Allen and Barnes 2006; settings during the waning stages of a foundering Barth et al. 2008; Schwartz et al. 2011). Saleeby and slab (Schwartz et al. 2011). Therefore, better under- Busby-Spera (1992) suggest that Late Jurassic arcs standing of Late Jurassic arc rocks is critical to un- occurred during a period of distinctive change in derstanding the tectonic development of the Cor- plate motion. This idea is supported by Engebret- dillera during this time period. son et al. (1985), who suggested that the relative Late Jurassic arc rocks occur in Washington State motion between oceanic plates outboard of the North (fig. 1). These include the ca. 150–153 Ma Hicks American margin was changing dramatically dur- Butte Complex, Hicks Butte inlier (Miller et al. 1993; MacDonald and Pecha 2011; fig. 1); the ca. 154 Ma Manuscript received August 2, 2016; accepted March 9, Indian Creek Complex, Rimrock Lake inlier (Miller 2017; electronically published May 15, 2017. 1989; Miller et al. 1993; fig. 1); and the ca. 157 Ma * Author for correspondence; e-mail: [email protected]. Quartz Mountain stock, Manastash inlier (Miller

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This content downloaded from 069.088.190.011 on February 27, 2018 10:38:26 AM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). 424 J . H . M ACDONALD AND A. SCHOONMAKER et al. 1993; this study; fig. 1). Other arc rocks occur Manastash anticline is interpreted to be an active in Washington, but their Late Jurassic ages are poorly compressional growth fold (Ladinsky 2012). The north- constrained. These include 160–170 Ma arc rocks ern bounding fault, the Taneum Lake fault zone, is part in the Decatur terrane, San Juan Islands (Brown et al. of the Olympic-Wallowa Lineament (Raisz 1945; Ta- 1979; Brandon et al. 1988 and references therein; bor 1994; Pratt 2012; fig. 2). The anticlinal structure of fig. 1); minor ca. 163 Ma arc plutonic occurrences in this inlier exposed older units that are faulted against the Shuksan Greenschist, Easton Metamorphic Suite or unconformably overlain by mostly late to middle (Gallagher et al. 1988; Dragovich et al. 1998; fig. 1); Eocene sedimentary and volcanic rocks (fig. 2). The minor ca. 150–160 Ma arc rocks as faulted blocks in units in the Manastash inlier consist of the Helena- the western mélange belt (Tabor et al. 2002 and ref- Haystack mélange, the informally named Hereford erences therein; fig. 1); and minor, possibly ca. 160 Ma Meadow amphibolite, the Lookout Mountain Forma- arc rocks occurring at Point of Arches, Olympic tion, and the Quartz Mountain stock (Stout 1964; Peninsula (Snavely et al. 1993; Schasse 2003). Arc Goetsch 1978; Miller et al. 1993; Tabor 1994; Tabor rocks in Washington State are usually omitted et al. 2000; MacDonald 2006). The Helena-Haystack from regional Late Jurassic tectonic synthesis (e.g., mélange in the inlier has been offset by Eocene move- Saleeby and Busby-Spera 1992; Wyld et al. 2006). ment on the Straight Creek–Fraser River fault (fig. 1), This is most likely due to a paucity of data compared has poor age control—possibly Middle Jurassic to to other Late Jurassic arc rocks of the Cordillera Eocene—and may have been reactivated by move- and their involvement in Early Cretaceous transla- ment along the Taneum Lake fault zone (fig. 2; Ta- tion and Late Cretaceous thrust fault emplacement bor 1994; MacDonald 2006; MacDonald et al. 2008). (i.e., the Baja British Columbia [BC] hypothesis; The Darrington Phyllite occurrence in the Manas- Cowan et al. 1997). tash inlier has been interpreted to be a large block in This article presents new U-Pb single-crystal sen- the Helene-Haystack mélange (Goetsch 1978; Tabor sitive high-resolution ion microprobe–reverse geom- 1994; fig. 2). Outside of its fault contact, the Helena- etry (SHRIMP-RG) zircon dates with whole-rock Haystack mélange is not genetically related to the major- and trace-element geochemistry of the Quartz other units of the Manastash inlier. Although omit- Mountain stock to help fill in data gaps of Late Ju- ted from most terrane translation studies (e.g., Baja rassic arc rocks in Washington State. This will help BC), the units in the Manastash inlier have been constrain the diverse settings of Late Jurassic arc correlated with rocks farther south and have trans- rocks along the North American Cordilleran mar- lated, possibly a large distance, before their emplace- gin. New U-Pb zircon ages corroborate the published ment in Washington State (see Miller et al. 1993 or ca. 157 Ma age of this stock (Miller et al. 1993). Geo- MacDonald 2006). chemistry suggests that the stock has affinities that Lookout Mountain Formation. The Lookout Moun- are similar to modern island-arc settings. However, tain Formation consists of siltstone and shale, with the intrusive relationships into forearc ca. 160 Ma lesser sandstone, that were deformed into semi- sediments that underwent Buchan-type metamor- schist and biotite schist. This formation is intruded phism prohibit this setting. Instead, we suggest that by the Quartz Mountain stock, steeply faulted against the Quartz Mountain stock represents forearc mag- the Hereford Meadow amphibolite, and overlain by matism in an overriding plate that was produced by the late to middle Eocene Naches Formation (fig. 2). a downgoing oceanic ridge. The timing of this event Primary structures within sedimentary beds are gen- fits well with the Late Jurassic ridge subduction pro- erally apparent. They include composite lenticular, posed by Murchey and Blake (1993) and highlights wavy, and laminated bedding; graded bedding; and the complex and diverse tectonic settings that oc- cross laminae. Metamorphic minerals in this for- curred during the Late Jurassic Cordillera develop- mation include porphyroblasts and poikiloblasts of ment. staurolite, garnet, andalusite, and rare cordierite (Stout 1964; Goetsch 1978; MacDonald 2006). The metamorphic mineral assemblage of the Lookout Moun- Manastash Inlier Geology tain Formation suggests Buchan-type metamorphism. The pre-Cenozoic rocks of the Manastash inlier are Spotted, web, and decussate metamorphic textures exposed in the Manastash anticline. The Manastash of biotite commonly overprint older metamorphic anticline is part of the Yakima fold belt (Smith 1903; textures in the vicinity of Quartz Mountain stock in- Reidel 1984; Watters 1989; figs. 1, 2). The faults that trusions (MacDonald 2006). MacDonald (2006) re- bound this inlier are potentially active and may reported a youngest U-Pb detrital zircon age distribu- cord sinistral motion (Reidel et al. 1994; Blakely et al. tion of ca. 160 Ma from a moderately sorted, fine- 2011; Pratt 2012; fig. 2). Southeast of the inlier, the grained sandstone (location of the sample is in fig. 2).

Figure 1. Simplified geologic map displaying pre-Cenozoic tectonic elements of the central and northwest Cascades, modified from Miller et al. (1993). Jurassic arc rocks are displayed with the dark gray patterned fill. Ophiolitic and ultramafic rocks have a black fill.

Signiﬁcant middle to late Mesozoic, Devonian, and sandstones to have been volcanically derived. Mac- Precambrian detrital age distributions also occur Donald (2006) suggested that this formation was im- (MacDonald 2006). Goetsch (1978), Miller et al. (1993), mature and consisted of the middle part of a Bouma and MacDonald (2006) interpreted detritus from the sequence that was deposited in a forearc setting. For

Figure 2. Generalized geologic map of the Manastash inlier, modified from Miller et al. (1993) and MacDonald (2006). Age locations are from Miller et al. (1993), Tabor et al. (2000), MacDonald (2006), MacDonald and Harper (2010), and this study. Cz p Cenozoic; hbl p hornblende; Mz, D, & pC p Mesozoic, Devonian, and Precambrian. detailed structural information about the Lookout 2006) is a coarse- to fine-grained amphibolite, with Mountain Formation, see Stout (1964), Goetsch (1978), lesser felsic orthogneiss (fig. 2). This amphibolite is Miller et al. (1993), and MacDonald (2006). intruded by the Quartz Mountain stock and steeply Hereford Meadow Amphibolite. The informally faulted against the Lookout Mountain Formation, named Hereford Meadow amphibolite (MacDonald Helena Haystack mélange, and middle Eocene ba-

This content downloaded from 069.088.190.011 on February 27, 2018 10:38:26 AM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). Journal of Geology LATE JURASSIC RIDGE SUBDUCTION EVENT 427 salt of Frost Mountain (fig. 2). Tabor et al. (2000) and mon igneous texture, but ophitic, subophitic, and Tabor and Haugerud (2016) include this amphib- sieve textures also occur. The enclaves are com- olite unit as part of the Lookout Mountain Forma- monly ophitic to subophitic in texture. Plagioclase tion; however, Stout (1964), Miller et al. (1993), and is commonly twinned and displays compositional MacDonald (2006) utilize the different lithologies, zoning. Goetsch (1978) reported that the plagioclase higher metamorphic grade, and faulted contact (fig. 2) is oligoclase (An23–An28), and reverse-compositional to separate the amphibolite from the Lookout Moun- zoning is common. MacDonald and Schoonmaker tain Formation. Protolith ages from this unit are (2015) reported that the garnets have grossular-rich lacking, except a minimal age bracketing by the in- almandine compositions and interpreted them to trusion of the Quartz Mountain stock. MacDonald be igneous on the basis of their textures, lack of in- and Harper (2010) reported a 40Ar/39Ar total-gas age clusions, and geochemistry. Xenoliths of the Look- of 142.8 5 0.74 Ma (1j) obtained from hornblende of out Mountain Formation and amphibolite can be a medium-grained, well-foliated amphibolite. How- found in the main body of the stock and the large ever, this sample was most likely reset by Early Cre- northwestern cupola (fig. 3). The stock contains sec- taceous magmatic activity (MacDonald and Pecha ondary metamorphic minerals that overprint pri- 2011). MacDonald and Harper (2010) reported supra- mary igneous minerals. These include albite, actin- subduction zone geochemistry from the amphibo- olite, epidote, chlorite, and titanite (sphene). The lite and orthogneiss. On the basis of geochemistry metamorphic mineral assemblage of the stock is in- and field relationships, MacDonald and Harper (2010) dicative of greenschist-facies metamorphism. interpreted the Hereford Meadow amphibolite to have formed in a subducting-ridge tectonic environ- Methods ment. For detailed structural information about this amphibolite, see Stout (1964), Goetsch (1978), Miller Geochronology. Quartz Mountain stock zircons et al. (1993), and MacDonald (2006). were extracted from approximately 9 kg of a medium- Quartz Mountain Stock. The Quartz Mountain grained granodiorite from the main body of the stock primarily consists of medium-grained biotite stock (sample MAN-38; fig. 2; table S1; tables S1, hornblende granodiorite and biotite hornblende to- S2 available online). Standard density and magnetic nalite, with lesser quartz diorite, diorite, gabbro, and extraction techniques were conducted for this sam- rare trondhjemite. Aside from intruding the Look- ple at the Geochronology and Isotope Geochemis- out Mountain Formation and Hereford Meadow am- try Laboratory, Department of Geological Sciences, phibolite, the Quartz Mountain stock is unconform- University of North Carolina, Chapel Hill. At Stan- ably overlain by late to middle Eocene basalt and ford University, Palo Alto, California, approximately rhyolite of the Naches Formation and the Miocene 30 Quartz Mountain stock zircons were randomly Grande Ronde Basalt of the Columbia River Basalt selected, mounted in epoxy, polished to about half Group (fig. 2). The stock is weakly zoned, with mafic the mean grain thickness, and gold-coated. Second- samples more common along its margin. The main ary back-scattered electron and cathodoluminescence body of the stock is surrounded by numerous cu- images of the zircons were obtained with a JEOL JSM polas of various sizes, most of which are too small 5600 scanning electron microscope to identify in- to display on maps (fig. 2). The cupolas also intrude ternal structures. The zircons were uniform, and a the Lookout Mountain Formation and the Hereford few displayed minor compositional zoning. The zir- Meadow amphibolite (Goetsch 1978; MacDonald cons were broken during processing, so it was diffi- 2006). Fine-grained, oblong enclaves of gabbro and cult to determine whether rims or cores were being meladiorite 3–20 cm in length are common in the analyzed. The zircons did not appear to have inher- tonalite and granodiorite. A moderately defined mag- ited cores. matic foliation, which averages S11W, 547NW, can Uranium and thorium, in parts per million, and be seen throughout the main phase of the stock and uranium and lead isotopes from zircons were ana- the large northwestern cupola. Locally, the stock lyzed by SHRIMP-RG at the Stanford University– contains a metamorphic foliation, averaging N84W, US Geological Survey Micro Analysis Center (ta- 507NE, which is defined by a weak compositional ble S1). Count times of 12 min for each zircon were banding. used, and a total of 11 zircons were analyzed; how- Primary igneous minerals in the stock include ever, one analysis had a high 232Th/238U (1.28) and quartz, plagioclase, hornblende, biotite, alkali feld- was excluded from this data set (table S1). The pri- spar, and spinel, plus rare garnet and apatite. Horn- mary standard R33 (accepted age of 419 Ma) was blende and biotite crystallized before quartz and analyzed after every three unknown zircons from feldspar. Hypidiomorphic-granular is the most com- the Quartz Mountain stock sample (Charlier et al.

Figure 3. Annotated photograph of Lookout Mountain Formation sandstone xenolith in a hornblende tonalite of the Quartz Mountain stock. This sedimentary xenolith is from the main body of the Quartz Mountain stock.

2005). Zircon concentration standard CZ3 (U p 2j uncertainty (fig. 4A). Zircons from the stock have 550 ppm) was used as a calibration standard for the low common-lead values (table S1). Figure 4B dis- SHRIMP-RG (Williams 1998; Ireland and Williams plays a 206Pb/238U age of 157.4 5 1.2 Ma (2j; mean 2003). Data reduction was done with the SQUID 1.02 square weighted deviation [MSWD] p 2.5) for all and Isoplot 3.00 programs of Ludwig (2001, 2003). 10 zircons analyzed from the Quartz Mountain stock. Geochemistry. New whole-rock major- and trace- This age takes into account the 2j error of the stan- element analyses for 21 Quartz Mountain stock sam- dard R33. The MSWD greater than 1 suggests that ples were conducted with X-ray fluorescence (XRF) the error resides in the sample and is most likely and inductively coupled plasma–mass spectrometry not analytical. (ICP-MS) at the Peter Hooper GeoAnalytical Labo- Geochemistry. The stock and large northwest- ratory, Washington State University (WSU; table S2). ern cupola samples are grouped together with the Grinding of samples was done at WSU with a tungsten same symbol on geochemical diagrams because of carbide mill for XRF analyses and with iron equip- lithological similarities. The smaller cupolas—except ment for ICP-MS analyses (Johnson et al. 1999). Esti- sample MAN-17, because of its unique geochem- mates of accuracy and precision, as well as discussion istry, discussed below—have the same symbol on of analytical methods, for WSU XRF are given by diagrams. See the key in figure 5. The SiO2 values Johnson et al. (1999) and Kelly et al. (2016) and those of the Quartz Mountain stock samples are primar- for WSU ICP-MS by Knaack et al. (1994). All samples ily intermediate to felsic, with one mafic sample were crushed and hand-picked before grinding to en- (fig. 5A; table S2). The samples are primarily low-K sure that weathered material was not analyzed. Sam- and tholeiitic (fig. 5A). They plot in the calcic field ples were normalized to a loss-on-ignition-free basis on the modified alkali-lime index of Frost et al. before plotting on diagrams. (2001; fig. 5B). One cupola sample (MAN-62A), however, is medium-K and calc-alkaline (fig. 5A,5B). The majority of the samples have magnesian FeOT/ Results (FeOT 1 MgO) ratios (fig. 5C). All Quartz Mountain Geochronology. The Th/U ratios of the zircons are stock samples have molecular Al/(Na 1 K) ratios close to or less than 1 (table S1). The 206Pb/238U-207Pb/ that are greater than 1 (fig. 6). Thirteen Quartz Moun- 235U concordia diagram for zircons from the stock tain stock samples have aluminum saturation in- reveals that they are generally concordant within dex (ASI; molecular Al=(Ca 2 1:67P 1 Na 1 K); Frost

Figure 4. A, 206Pb/238U versus 207Pb/235U concordia diagram for 10 zircons from the Quartz Mountain stock. Er- ror ellipses are 2j. B, Average mean 207Pb/206Pb age of all 10 analyses from the Quartz Mountain stock. Error bars are 2j. 95% conf. p 95% conﬁdence.

et al. 2001) ratios that classify them as metaluminous (fig. 6). Eight samples have peraluminous ASI ratios (fig. 6). Two cupolas (MAN-9 and MAN-62A) have the highest ASI ratios (fig. 6). The four samples with the highest SiO2 (169 wt%) also have the highest FeOT/(FeOT 1 MgO) ratios, and are all peraluminous (figs. 5C, 6; table S2). The cupolas have higher A Cr, Ni, and V than most other Quartz Mountain Figure 5. ,SiO2 versus K2O weight percentage plot for Quartz Mountain stock and cupola samples. B,SiO stock samples (table S2). However, the two stock sam- 2 versus Na2O 1 K2O 2 CaO weight percentage plot for ples with the lowest SiO2 values have Cr, Ni, and Quartz Mountain stock and cupola samples. Magmatic V that are as high as those of the cupolas (table S2). series divisions are from Frost et al. (2001). C,SiO2 ver- The Quartz Mountain stock samples have pro- sus FeOT/(FeOT 1 MgO) weight percentage plot for nounced negative Ta, Nb, and Ti anomalies when Quartz Mountain stock and cupola samples. Ferroan and normalized to primitive mantle (fig. 7). Many of the magnesian divisions are from Frost et al. (2001). FeOT p samples also have a strong positive Pb anomalies, all iron expressed as Fe21.

Mountain stock—the Hereford Meadow amphibolite or Lookout Mountain Formation. Miller et al. (1993) reported a 206Pb/238U thermal ionization mass spectrometry age of 157 5 2 Ma from the large northwestern cupola of the stock. They interpreted this to be the crystallization age of the stock (ﬁg. 2). Our new 157.4 5 1.2 Ma age corroborates the Miller et al. (1993) U-Pb results (ﬁg. 4), and we also interpret ca. 157 Ma to be the crystallization age of the Quartz Mountain stock. This suggests that the crystallization age of the stock is Late Jurassic, according to the timescale of Gradstein et al. (2012). The 207Pb/206Pb age for the Miller et al. (1993) sample is 165 5 5 Ma, suggesting that their age is discordant and that the

Figure 6. Molar aluminum saturation index (Al/(Ca 2 1.67P 1 Na 1 K)) versus molar Al/(Na 1 K) plot for Quartz Mountain stock and cupola samples. Metalumi- nous and peraluminous divisions are from Frost et al. (2001). Symbols are the same as in figure 5. compared to primitive-mantle values (fig. 7). Sam- ple MAN-17 has a positive Sr anomaly when normalized to primitive mantle (fig. 7). All other elements are enriched compared to primitive mantle, with enrichment increasing with incompatibility (fig. 7). These samples have enriched light rare earth elements compared to chondrite values (fig. 7). They do not display significant Eu anomalies on the chondrite- normalized diagrams (fig. 7). Eu=EuÃ values average 0.996 (50.113 [1j]; table S2) from the stock and cupolas. The normalized patterns do not converge at the heavy rare earth elements (HREEs) on the primitive mantle– and chondrite-normalized diagrams (fig. 7). The majority of the Quartz Mountain stock samples have low Sr/Y ratios and moderate to high Y values (fig. 8). Cupola MAN-17 plots in the adakite field in figure 8. MAN-17 has SiO2,Al2O3,Cr,Ni,andYb values that are also consistent with those of adakites (table S2; Defant and Drummond 1990; Castillo 2012). The stock samples have low Rb/Sr and Rb/Ba ratios (fig. 9). The majority of the samples plot near the source of mafic protoliths for granitoid magmas and M-type granites in figure 9.

Discussion Figure 7. Primitive mantle–normalized (top) and chondrite- normalized (bottom) diagrams for Quartz Mountain stock The Th/U ratios of the analyzed zircons are close to and cupola samples. Primitive mantle–normalized val- or less than 1 (table S1). This supports the morphol- ues are from Sun and McDonough (1989), and chondrite- ogy interpretation that the zircons are igneous and normalized values are from McDonough and Sun (1995). not entrained zircons from the wall rock of the Quartz Symbolsarethesameasinﬁgure 5.

table S2). This could be the result of crystal fractionation driving the compositions to higher Fe and Al contents. Commonly, the geochemistry of island arcs becomes transitional between tholeiitic and calc-

alkaline at higher SiO2 values (e.g., Plank and Lang- muir 1988; Smith et al. 2003). Tatsumi and Kogiso (2003) suggested that island arcs that have predominantly low-K tholeiitic composition, with lesser transitional medium-K calc-alkali affinities, are in closer proximity to a trench. The Quartz Mountain stock samples generally have low large-ion lithophile elements (LILEs) and high-field-strength elements (HFSEs; figs. 5A,8,9; table S2). Pearce et al. (1984) and Pearce (1996) indicated that low Rb, Nb, Ta, Y, and Yb in granitoids resulted from partial melting of a mantle source in a volcanic arc setting. Brown et al. (1984) suggested Figure 8. Y, in parts per million, versus Sr/Y ratios for Quartz Mountain stock and cupola samples. Adakite that low Rb, Nb, and Y were distinctive of primitive field from Defant and Drummond (1990), Martin et al. arc settings. However, the Quartz Mountain stock (2005), and Castillo (2012). Taitao granitoids field (n p samples do not have the low La/Yb and Th/Yb ra- 80) from Guivel et al. (1999), Anma et al. (2009), and Kon tios found in primitive arc settings (table S2; Condie et al. (2013). Symbols are the same as in figure 5. 1989). The low K2O, Cr, Ni, Rb, and other LILEs and HFSEs are suggestive of granitoids derived from mantle sources (M-type; Whalen et al. 1987; Saito et al. sample might have experienced lead loss. Two of our 2004). analyses have high U, which may also suggest lead The enrichment of LILEs over HFSEs has been at- loss (table S1). Our MSWD of 2.5 suggests that the tributed to fluid mobility from dehydration of sub- age error is a result of our sample and not analytical. ducting plates in arc settings (Tatsumi et al. 1986; This high MSWD is in part the result of incorpo- Sun and McDonough 1989; Pearce and Peate 1995; rating the 2j error of the standard R33 into the Pearce 1996, 2014; Elliott et al. 1997; Tatsumi and Quartz Mountain stock zircons; however, it could Kogiso 2003). The enrichment of the LILEs Th and be the result of lead loss. The stock experienced Pb with respect to the HFSEs Nb, Ta, and Ce in greenschist-facies metamorphism. This moderate- temperature hydrothermal event could have resulted in lead leaching from metamict zircons (e.g., Mezger and Krogstad 1997). Regardless, the age agreement from two independent U-Pb studies, which utilized different analytical approaches, suggests that ca. 157 Ma is the crystallization age of the Quartz Mountain stock. The predominantly tonalite and granodiorite lithologies from the Quartz Mountain stock are consistent with the intermediate to felsic SiO2 values (fig. 5). These lithologies overlap the granitoid types that Maniar and Piccoli (1989), Barbarin (1999), and Frost et al. (2001) reported from modern island arc settings. The predominantly intermediate to felsic, low-K tholeiitic, calcic, magnesian, and metaluminous geochemistry of the stock and its cupolas are typical of island arc granitoids (figs. 5, 6; Barbarin 1999; Frost et al. 2001). Samples are slightly transitional to medium-K calc-alkaline, ferroan, and per- Figure 9. Rb/Sr versus Rb/Ba ratios for Quartz Mountain aluminous (figs. 5, 6). Typically, the ferroan and per- stock and cupola samples. Modified from Sylvester (1998). fi aluminous samples have higher SiO2 values (fig. 5C; Symbols are the same as in gure 5.

Quartz Mountain stock samples, when normalized to compositions, as well as low Rb/Ba and Rb/Sr ratios primitive-mantle compositions, is noticeable (fig. 7). (figs. 6, 9), disallow a sedimentary protolith for these This Th and Pb enrichment over HFSEs most likely igneous rocks. Finally, xenoliths of both the Lookout resulted from dehydration of a subducting plate as Mountain Formation and amphibolite can be found a mechanism for generating the magma of the stock. in the main body of the stock (fig. 3). Therefore, it is Magmas generated by partial melting in subduction suggested that the Quartz Mountain stock assimi- zone settings are typically rich in water and other lated moderate amounts of the Lookout Mountain volatiles. The crystallization of amphibole and bio- Formation and Hereford Meadow amphibolite dur- tite before quartz and feldspar occurs in granitic mag- ing emplacement. Conversely, contribution of sub- mas with H2O ≥ 4 wt% (Nédélec and Bouches 2015). ducted sediment to arc magmas may also enrich Pb Sun and McDonough (1989) noted that subduction with respect to HFSEs. Subducted sediment addi- enrichment will cause depletion of the refractory LILE tion can also enrich Sr and rare earth elements with Ti. The steep negative Ti depletion on the primitive respect to HSFEs (Plank and Langmuir 1998). The mantle–normalized diagrams for Quartz Mountain Quartz Mountain stock samples do not display stock samples (fig. 7) is the result of mineral fraction- strong enrichment of Sr and rare earth elements ation, most likely of oxides, as a result of a volatile- with respect to HSFEs (figs. 7, 8); therefore, the enrich magma generated in an arc setting. richment of Pb, as well as other geochemical af- The LILE primitive mantle– and chondrite- finities discussed above, is attributed to assimila- normalized patterns of the Quartz Mountain stock tion of the stock’s wall rock and not subducted samples display enrichment with increasing incom- sediment during generation of the magma. patibility (fig. 7). Pearce (1983) and Sun and Mc- Goetsch (1978), Tabor et al. (2000), and Mac- Donough (1989) suggested that an elevation of highly Donald (2006) all noted that the cupolas display a incompatible LILEs could have resulted from par- more penetrative metamorphic foliation than the tial melting of an enriched mantle source—similar large northwestern cupola and the main body of the to enriched mid-ocean ridge basalt. The normalized stock. Goetsch (1978), on the basis of textures, sug- patterns also do not converge at the HREEs (fig. 7). gested that the cupolas were mafic to intermediate. Pearce (1983) suggested that shallow mantle melt- Tabor et al. (2000) referred to the cupolas as “gab- ing will result in normalized patterns that do not bro,” suggesting that they are mafic in composi- converge at the HREEs, while melting of a deeper tion. Geochemistry indicates that the cupolas sur- mantle source will cause normalized patterns to con- rounding the stock are predominantly intermediate verge at the HREEs. This is due to the absence or in composition, while one is felsic (fig. 5A). Gen- presence of garnet in the mantle residue during par- erally, they are more intermediate than the sam- tial melting (Pearce 1983). An enriched mantle source ples from the stock (fig. 5); however, they plot along will have higher Rb/Sr ratios than a depleted mantle fractionation trends defined by samples from the source (Sun and McDonough 1989). The primitive stock (fig. 5). The cupolas have primitive mantle– mantle– and chondrite-normalized patterns, as well and chondrite-normalized patterns that are less en- as Rb/Sr ratios, for the Quartz Mountain stock sug- riched than but nearly identical to those of the gest that its magma was generated by partial melt- stock (fig. 7). These normalized patterns of the cu- ing of a shallow, possibly enriched, mantle source in polas and the stock do not cross each other (fig. 7). a subduction zone setting (figs. 7, 9). This suggests that the cupolas are derived from the Modest assimilation of the Lookout Mountain For- same magmatic source that produced the Quartz mation and Hereford Meadow amphibolite (fig. 3) by Mountain stock. Age data of the cupolas are lacking the stock is suggested by the transitional metaluminous- to test whether they were an early phase of the peraluminous affinities, enrichment of Pb, and mi- stock—which is suggested by their lower silica nor igneous garnet (figs. 6, 7). Assimilation is further values (fig. 5A). If the cupolas are an earlier mag- supported by high Cr, Ni, and V of the cupolas and matic phase of the stock, this could account for the stock samples MAN-28 and MAN-32 (table S2). The difference between their fabric and that of the rest garnet’s grossular almandine composition could be of the stock. the result of magmatic Fe enrichment from assimi- The geochemistry of the Quartz Mountain stock lation of the amphibolite and the biotite-rich Look- suggests that the granitoid magmas were generated out Mountain Formation (fig. 3). Further, igneous from shallow partial melting of a possibly enriched garnet occurs in both metaluminous and peralu- mantle source in an island arc setting (figs. 5–8). minous samples and is more abundant near the mar- However, the intrusion of the Quartz Mountain gin of the stock. The predominantly metaluminous stock into the Lookout Mountain Formation and

Hereford Meadow amphibolite does not allow for Anma et al. (2009) indicated that a subducting spread- a simple island arc setting (fig. 2). The Lookout ing ridge can generate “granitic” magma that in- Mountain Formation is a Late Jurassic, ca. 160 Ma, trudes into the forearc. The intrusion of the Quartz volcanically derived turbidite that is interpreted Mountain stock granitoids into the Buchan-facies to have formed in a forearc trench before under- forearc deposits of the Lookout Mountain Forma- going Buchan-type metamorphism (Goetsch 1978; tion—and the suprasubduction zone–affinity Her- Miller et al. 1993; MacDonald 2006). The Hereford eford Meadow amphibolite (fig. 2)—suggests that Meadow amphibolite is deformed pre–Late Juras- the stock’s magma intruded into a forearc setting as sic oceanic crust that has suprasubduction zone a result of a subducted spreading ridge (fig. 10). The geochemical affinities (MacDonald and Harper 2010). time frame between the deposition and lithification Granitoid magmas with arc affinities intruding de- of the Lookout Mountain Formation and the in- formed forearc sediments and oceanic crust occur trusion of the Quartz Mountain stock is similar to where spreading ridges form triple junctions with the time frame between granitoids formed by ridge convergent plate boundary trenches (e.g., Taitao Pen- subduction in the Taitao Peninsula, Chile, and their insula, Chile). wall rocks (Anma and Orihashi 2013). Bradley et al. DeLong et al. (1979), Brown (1998), and Sisson et al. (2003) and Cawood et al. (2009) indicate that ridge- (2003) note that low-PT, Buchan-type metamorphism trench interactions can result in forearc deforma- of forearc deposits occurs as a result of ridge sub- tion. If the cupolas represent a more intermediate duction. Forsythe et al. (1986), Guivel et al. (1999), and earlier magmatism of the Quartz Mountain and Sisson et al. (2003) note that accretion of fore- stock, then their more penetrative deformation could arc oceanic crust with suprasubduction zone geo- be the result of forearc deformation from the subducted chemistry can also occur during the subduction of a spreading ridge. spreading ridge. Further, DeLong et al. (1979) and The granitoid magmas are generated by shallow mantle melting resulting from spreading-ridge subduction and/or shallow melting of oceanic crust under garnet-free amphibolite conditions (DeLong et al. 1979; Guivel et al. 1996; Sisson et al. 2003; Anma et al. 2009; Kon et al. 2013). Martin et al. (2005) suggested that adakites should result from ridge subduction. There is only one adakitic sample from the Quartz Mountain stock (MAN-17; fig. 8; table S2); however, the lack of adakites from the Quartz Mountain stock does not preclude a ridge subduction setting for these magmas. Adakites do not occur in large volume in ridge-trench granitoids (e.g., Taitao granitoids; fig. 8; Bourgois et al. 1996; Guivel et al. 1996; Anma et al. 2009; Kon et al. 2013). Bourgois et al. (1996) suggest that the lack of numerous adakites from ridge-trench settings is the result of mineral fractionation decreasing the Sr content of the magmas. Anma et al. (2009) and Kon et al. (2013), alternatively, suggest that the lack of numerous adakites in granitoid magmas generated by ridge subduction is the result of shallow melting depths. The Quartz Mountain stock samples do not display pronounced Eu anomalies on the chondrite- normalized diagram and have Eu=EuÃ values that are essentially 1 (fig. 7; table S2). This suggests that shallow mantle melting, not plagioclase fractionation, generated the low Sr/Y ratios for the Quartz Mountain stock magmas. Figure 10. Tectonic cartoon displaying the genesis of the Shallow melting of a possibly enriched source is, Quartz Mountain stock as a result of a Late Jurassic ridge again, suggested by the dispersed HREE normalized subduction. patterns (fig. 7). The Rb/Ba and Rb/Sr ratios from

This content downloaded from 069.088.190.011 on February 27, 2018 10:38:26 AM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). 434 J . H . M ACDONALD AND A. SCHOONMAKER the Quartz Mountain stock are similar to those de- in California, was the result of a Late Jurassic ridge rived from melting of a mafic protolith (i.e., oceanic subduction. The effect of ridge subduction may be crust); however, they are also similar to those of recorded throughout the North American Cordillera. mantle-derived granites (M-type granites; fig. 9). Samples lack the high Cr, Ni, and Sr/Y ratios ex- Conclusions pected from the melting of subducted oceanic crust (fig. 8; table S2). Therefore, it is suggested that the The crystallization age of the Quartz Mountain Quartz Mountain stock magmas were derived from stock is ca. 157 Ma (fig. 4; table S1). This ca. 157 Ma shallow melting of a possibly enriched mantle source. age corroborates the Miller et al. (1993) age from the Radiogenic-isotope geochemistry would provide more large northwestern cupola (fig. 2). Although they are insight into the petrogenesis of the melting source more penetratively deformed, geochemistry suggests for the stock’smagmas—especially if the shallow that the numerous cupolas are petrogenetically re- mantle source that generated these magmas was lated to the main body of the Quartz Mountain stock truly enriched. (figs. 5–9). This is supported by the cupolas follow- Miller et al. (1993) correlated the Quartz Moun- ing fractionation trends similar to those of the stock tain stock with the Rogue-Chetco arc complex, Klam- samples and having identical yet lower primitive ath Mountains, Oregon-California, primarily on the mantle– and chondrite-normalized patterns that do basis of age. Wyld et al. (2006) suggested that Jurassic not cross those of the samples from the stock (figs. 5, rocks north of the Manastash inlier were in prox- 7). The magma that generated the Quartz Moun- imity to the Klamath Mountains after palinspastic tain stock and its cupolas originated from shallow restoration for Cretaceous strike-slip faulting. Al- melting of a mantle source above a subduction zone though the rocks of the Manastash inlier were (figs. 5–9). The contact relationships between the omitted from the Wyld et al. (2006) study, it is in- Quartz Mountain stock and its wall rock, the Look- ferred that they were in a similar position before out Mountain Formation and Hereford Meadow am- Cretaceous translation. However, the Rogue-Chetco phibolite (fig. 2), suggest that the stock was emplaced arc complex is suggested to be laterally continu- in the forearc as a result of the subduction of a Late ous with the Late Jurassic Galice Formation, while Jurassic spreading ridge (fig. 10). The geochemistry the Quartz Mountain stock intrudes the Late Ju- of the Quartz Mountain stock is similar to that of rassic sediments of the Lookout Mountain Forma- Neogene-age ridge subduction granitoid magmas in tion (fig. 2; Yule et al. 2006 and references within). the Taitao Peninsula, Chile (fig. 8; Bourgois et al. 1996; In the Big Craggies of southwest Oregon, 154 Ma Guivel et al. 1996; Anma et al. 2009; Kon et al. 2013). Rogue-Chetco-related amphibolites are intruded by The low-PT metamorphism and forearc depositional hornblende gabbros that are slightly metamorphosed, setting of the Late Jurassic Lookout Mountain For- suggesting that they were likely intruded shortly mation turbidites (Goetsch 1978; Miller et al. 1993) after amphibolite emplacement. These intrusions and the suprasubduction zone geochemistry of the have chemistries similar to those of the Quartz Moun- deformed ophiolitic Hereford Meadow amphibolite tain Stock (DePasquale et al. 2017). Further, Big (MacDonald and Harper 2010) support the ridge- Craggies gabbros are in close spatial position with trench setting for the Quartz Mountain stock (fig. 10). unmetamorphosed muscovite tonalites that have The Quartz Mountain stock and other rocks of the been dated at 149 Ma, indicating that the gabbros Manastash inlier (fig. 2) were laterally translated from were intruded before the tonalites. A temporal re- correlative rocks in California and Oregon before lationship similar to the Quartz Mountain stock their emplacement in Washington State (Miller et al. intrusion into the Lookout Mountain Formation 1993; MacDonald 2006). Therefore, the Quartz Moun- exists in the Sierran Foothills, California, and the tain stock and its wall rock may be one record of the Blue Mountains, Oregon. The ca. 153 Ma Guadalupe northern California Late Jurassic ridge subduction pluton, in the Sierran Foothills, intrudes the Late inferred by Murchey and Blake (1993). Jurassic volcaniclastic Mariposa Formation, and Late Jurassic metagranitoids of the Mountain Home Metamorphic Complex intrude the undated schists ACKNOWLEDGMENTS of Yellow Jacket Road in the Blue Mountains (Ernst et al. 2009; Anderson 2013). However, the tectonic The thoughtful reviews by J. Schwartz and D. Row- interpretations for the formation of these rocks do ley greatly improved the quality of this article. We not include ridge subduction (Ernst et al. 2009; thank them for their efforts. Some of the data for this Anderson 2013). Snow and Shervais (2015) suggested project were collected during J. H. MacDonald’sdis- thattheterminationoftheCuestaRidgeophiolite, sertation work. Funding for this dissertation work

This content downloaded from 069.088.190.011 on February 27, 2018 10:38:26 AM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). Journal of Geology LATE JURASSIC RIDGE SUBDUCTION EVENT 435 was supported by a USGS EdMap grant and the Uni- thank B. Kidd, B. Miller, and G. Harper for discus- versity at Albany Faculty Research Award Program sions that helped our interpretations in this article. grant to G. Harper, MacDonald’s PhD advisor. Addi- B. Miller was also kind enough to read an early draft tional funding for this study was supported by Flor- of this article. We thank Y. Kon for providing us with ida Gulf Coast University to J. H. MacDonald. We published data tables on the Taitao granitoids.

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