Use of trace element abundances in augite and hornblende to determine the size, connectivity, timing, and evolution of magma batches in a tilted batholith N. Coint, C.G. Barnes, A.S. Yoshinobu, M.A. Barnes, and S. Buck Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA ABSTRACT The occurrence of large volumes of volcanic record the evolution of the melt composition in rocks ejected during single eruptive events (e.g., magmatic systems and thus have great potential to The tilted Wooley Creek batholith (Klam- Ritchey, 1980; Bacon and Druitt, 1988; Lipman provide information that allows reconstruction of ath Mountains, California, USA) consists of et al., 1997; Bachmann et al., 2002, 2005; Chris- the history of intrusive and extrusive complexes. three main zones. Field and textural relation- tiansen, 2005; Hildreth, 2004; Christiansen and In the case of calc-alkaline magmas, augite and ships in the older lower zone suggest batch- McCurry, 2008) suggests that large volumes of hornblende are of particular interest because they wise emplacement. However, compositions of magma are stored in the middle to upper crust. can crystallize at or close to the liquidus, depend- augite from individual samples plot along indi- Recent high-precision U-Pb dating of zircon ing on the amount of water present in the system vidually distinct fractionation trends, confi rm- indicates that it is not uncommon for intrusive (Piwinskii, 1968; Eggler and Burnham, 1973), ing emplacement as magma batches that did systems to grow over several millions of years and because they occur throughout a wide range not interact extensively. The younger upper (e.g., Glazner et al., 2004; Matzel et al., 2006; of compositions, from basaltic andesite to rhyo- zone is upwardly zoned from tonalite to gran- Grunder et al., 2008; Memeti et al., 2010). Ther- lite. By studying the compositions of these min- ite. Major and trace element compositions mal modeling, however, indicates that even large erals, we can also track the chemical evolution of of hornblende show similar variations from batches of magma should not remain above their the melt, which is generally not accessible when sample to sample, indicating growth from a solidus for >1 m.y. (Glazner et al., 2004; Paterson working with bulk-rock data on intrusive rocks single magma batch that was homogenized et al., 2011). These modeling results have led to because many plutonic rocks are partial cumu- by convection and then evolved via upward emplacement models in which plutons form in lates (e.g., Deering and Bachmann, 2010). percolation of interstitial melt. Highly porphy- increments that do not interact extensively with In this study we utilize major and trace ele- ritic dacitic roof dikes, the hornblende com- one another, and have also been interpreted to ment abundances in augite and hornblende to positions of which match those of upper zone indicate that plutons are not necessarily related to reconstruct the assembly of a tilted calc-alka- rocks, demonstrate that the upper zone mush volcanic rocks (Coleman et al., 2004; Mills and line pluton, the Wooley Creek batholith (WCb; was eruptible. The central zone contains rocks Coleman, 2010). These emplacement models also Barnes et al., 1986b; Barnes, 1987). Tilting and of both lower and upper zone age, although imply that important processes in the chemical subsequent erosion have exposed ~9 km of in most samples hornblende compositions evolution of magmatic suites such as fractional structural relief through the intrusion, making it match those of the upper zone. The zone is crystallization, assimilation, and magma mixing a good candidate for study of the organization rich in synplutonic dikes and mafi c mag- (e.g., De Paolo, 1981; Bohrson and Spera, 2007; of intrusive bodies. Furthermore, the presence of matic enclaves. These features indicate that Ohba et al., 2007; Claiborne et al., 2010; McLeod roof dikes in the structurally highest part of the the central zone was a broad transition zone et al., 2010; du Bray et al., 2011) are restricted to system provides samples of magma that escaped between upper and lower parts of the batho- the lower part of the crust (Annen, 2009; Mills the system (Barnes et al., 1986a), enabling us to lith and preserves part of the feeder system to and Coleman, 2010; Tappa et al., 2011). Incre- address the problem of the connection between the upper zone. Homogenization of the upper mental emplacement of batholith-scale intrusions volcanic and plutonic rocks. zone was probably triggered by the arrival of has been demonstrated through high-precision mafi c magma in the central zone. Continued geochronology (Glazner et al., 2004; Matzel GEOLOGICAL SETTING emplacement of mafi c magmas may have pro- et al., 2006; Walker et al., 2007; Grunder et al., vided heat that permitted differentiation of the 2008; Memeti et al., 2010; Miller et al., 2011) in Klamath Mountains upper zone magma by upward melt percola- cases where emplacement spanned hundreds of tion. This study illustrates the potential for use thousands to millions of years. However, some The WCb is situated in northern California, of trace element compositions and variation in batholith-scale plutons may be emplaced in time USA, in the Klamath Mountains geologic prov- rock-forming minerals to identify individual scales of <1 m.y. (Miller et al., 2011; Coint et al., ince. The province consists of a series of north- magma batches, assess interactions between 2013). In such cases, alternative methods must be south–oriented accreted terranes bounded by them, and characterize magmatic processes. used to identify magma batches and assess their regional east-dipping thrust faults, resulting in size and evolution. preservation of a record of more than 400 m.y. INTRODUCTION High-temperature mafi c minerals crystallize of active subduction along the western North early in magmatic systems and are capable of American margin (Snoke and Barnes, 2006). The ways in which plutons are assembled incorporating trace elements in abundances large The WCb is one of a series of plutons (Wooley are currently the subject of vigorous debate. enough to be measured in situ. These minerals Creek suite) emplaced from Middle to Late Geosphere; December 2013; v. 9; no. 6; p. 1747–1765; doi:10.1130/GES00931.1; 9 fi gures; 5 supplemental fi les. Received 13 March 2013 ♦ Revision received 17 July 2013 ♦ Accepted 29 August 2013 ♦ Published online 23 October 2013 For permission to copy, contact [email protected] 1747 © 2013 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/6/1747/3346579/1747.pdf by guest on 25 September 2021 Coint et al. Jurassic time (167–156 Ma) into rocks of the western Paleozoic and Triassic belt, central Legend metamorphic belt, and eastern Klamath belt Late Jurassic 123°20’00”W Ji (Allen and Barnes, 2006). The Slinkard plu- plutons ton, which crops out northeast of the WCb, Western is related to WCb magmatism (Barnes et al., Klamath terrane 1986a). However, extensive subsolidus recrys- c. 152 Ma tallization prevented us from including the Orleans thrust Slinkard pluton in this study. At the time of the wHt Western/eastern plutonic event, the area was undergoing exten- eHt Hayfork terranes Lower zone sion associated with formation of the backarc Josephine ophiolite ca. 159–164 Ma (Harper 351 RCt Rattlesnake Creek et al., 1994; Hacker et al., 1995). The Wooley terrane Creek suite was emplaced east (modern coordi- Ukonom Lake nates) of the Josephine ophiolite. At the same Central zone time, subduction-related magmatism was active west of the Josephine ophiolite and is now rep- 6209 resented by the Chetco plutonic complex and 5209 100 Pigeon Rogue Formation. The Chetco-Rogue arc, the 6309 132 7109 Roost Josephine ophiolite, and its cover sequence, the 594 Galice Formation, form the western Jurassic 133 belt, which was thrust underneath the western 30 Paleozoic and Triassic belt along the Orleans Ten 5709b 6809 thrust (Harper et al., 1994) during the Neva- Bear Cuddihy dan orogeny (153–150 Ma; Allen and Barnes, 397 Mtn. Lakes basin 128a 777b 4809 208 2006). This thrusting truncated the base of at 4909 least some Wooley Creek suite plutons (Barnes, Deadman Z5 Lake 1982, 1983; Jachens et al., 1986). Exhumation 317 of high-pressure rocks of the Condrey Mountain Schist through a structural window tilted the 833 41°30’30” N WCb ~15°–30° toward the southwest (Barnes et 394 al., 1986b), and subsequent erosion has resulted in ~9 km of structural relief through the pluton. 687 Medicine Mtn. WCb Upper zone The WCb (Fig. 1) was emplaced between Gradational 159 and 155 Ma (Coint et al., 2013). The batho- compositional lith intrudes three host terranes of the western boundary Orleans thrust fault Paleozoic and Triassic belt; from structurally (teeth on HW) higher to lower, these are (1) the eastern Hay- Thrust fault fork terrane, a chert-argillite mélange, (2) the (teeth on HW) western Hayfork terrane, a volcaniclastic sand- 590 Field area stone and argillite unit, and (3) the Rattlesnake Klamath Creek terrane, a serpentine matrix to block-on- 471 N Mts. block ophiolitic mélange overlain by a coher- CA ent cover sequence (Wright and Wyld, 1994). 377b The WCb can be divided into three zones, all of which display gradational contacts (Fig. 1). The region of abundant lower zone is composed primarily of biotite ± 548 “Roof zone” dikes hornblende two-pyroxene diorite and/or gabbro 551 164b and biotite ± hornblende two-pyroxene tonalite. 236a Two U-Pb (zircon, chemical abrasion–ther- mal ionization mass spectrometry, CA-TIMS) 2408 ages from this unit are identical with analytical 257a 0 5 km uncertainty at 158.99 ± 0.17 Ma and 159.22 ± Ji 0.10 Ma (Coint et al., 2013).