tectonics, yolo 12, no.2, pages 387-409, april ...pangea.stanford.edu/~mac/pdf/hacker+...
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TECTONICS, YOLo 12, NO.2, PAGES 387-409, APRIL 1993
GENESIS AND EVOLUTION OF A Klamath terranes [e.g., Ando et al., 1977; Burchfiel and Davis,PERMIAN-JURASSIC MAGMA TIC 1981; Wright, 1982; Miller and Saleeby, 1991], in some casesARC/ACCRETIONARY WEDGE, AND disproving the hypothesis ofallochthoneity and the need forREEV ALUA TION OF TERRANES IN THE large-scale displacement. This paper continues that trend byCENTRAL KLAMATH MOUNTAINS showing that a key group of Klamath 'terranes' are neither
bounded by faults with hundreds to thousands of kilometers ofBradley R. Hacker, W. G. Ernst, and Michael O. McWilliams displacement nor demonstrably allochthonous with res~t toDepartment of Geology, School of Earth Sciences, each other and thus are not terranes sensu stricto. This isStanford University, Stanford, California significant because the Klamath Mountains orogen, the
birthplace of the terrane concept, provides an archetype ofAbstract. Investigation of key areas of the central Klamath continental growth via long-term subduction of oceanic
Mountains demonstrates that genetic relationships connect lithosphere. A map of Klamath terranes is shown as Figure 1.several regionally significant lithologic units that were Understanding the processes of orogen construction from
, " previously considered to be separate tectonostratigraphic oceanic rocks is difficult because of the paucity of coherent-" terranes. The rocks embody an intraoceanic tholeiitic to stratigraphic sections. The so-called western Paleozoic and
alkaline magmatic arc and contiguous accretionary wedge Triassic belt has been the most difficult unit in the Klamathformed in Permian to Middle Jurassic time. The Eastern Mountains to understand, and yet its large outcrop area and
! Hayfork and St Claire Creek units comprise Permian-Jurassic central location-between terranes of the western and easternoceanic sedimentary rocks deposited nearby and coeval with Klamaths-make it a linchpin for the entire orogen. Thiseruption of the North Fork-Salmon River volcanoplutonic arc. importance of the western Paleozoic and Triassic belt impelledThe more inboard St Claire Creek unit consists of us to synthesize map data for the eastern portion of the beltstratigraphically coherent chert and minor argillite probably from its northernmost to southernmost extent (Figure 2). Thedeposited in a back-arc setting, whereas the more outboard map depicts one of the largest fossil intraoceanic magmatic arcEastern Hayfork sedimentary rocks were disrupted during and accretionary wedge pairs in the world. Along this belt,formation of an accretionary wedge. The volcanoplutonic younger faults expose different levels within the arc, affordingNorth Fork-Salmon River arc extends for 200 km in the an excellent opportunity to study at different structural levelscentral Klamath Mountains and was active episodically or the genesis, deformation, and metamorphism of immaturecontinuously from Permian through Middle Jurassic time. magmatic arcs, accretionary wedges, and ocean basins. ThisAlkalic members of the mafic igneous suite may have erupted paper integrates our new data with that of previous workers.following subduction of a spreading center. A varied heating We have attempted a detailed look at the birth, evolution, andhistory has produced an amalgam of submarine and regional assembly of a major magmatic arc and its associatedlow-pressure, moderate-temperature metamorphism, accretionary complex.overprinted locally by contact metamorphism. Geochronologyof weakly metamorphosed mafic dikes/sills (10 new REGIONAL GEOLOGY40 Ar/39 Ar analyses) suggests that significant heat was retainedregionally into Early Cretaceous time. The gross structure of the Klamath Mountains is a
structural stack of north-south trending units. Individual unitsINTRODUCTION have east dipping bedding and foliation and west vergent folds
and are generally separated by west-directed thrust faults.As an aid to understanding the geology of the Klamath Modeling of gravity [Mortimer, 1985], magnetic [Blakely et
Mountains, Irwin [1972, p. C103] introduced the term 'terrane' al., 1985], and seismic refraction data [Zucca et al., 1986; Fuisto refer to "an association of geologic features, such as et al., 1987] suggests that the thrust sheets dip moderately tostratigraphic formations, intrusive rocks, mineral deposits, and depths of -2-3 km and then dip gently (-3°-9°) eastward for
~ tectonic history, some or all of which lend a distinguishing more than 50 km.I' characteristic to a particular tract of rocks and which differ from Irwin [1966] divided the southern Klamath Mountains into
those of an adjacent terrane". Irwin [1972, p. CI09] also noted four major belts, the Eastern Klamath, central metamorphic,, that "The apparent juxtaposition of the terranes is most readily western Paleozoic and Triassic, and western Jurassic. Later,
explained by considering them to have been widely separated Irwin [1972] subdivided the largest of these, the westernwhen formed and subsequently to have been telescoped together Paleozoic and Triassic belt, into the Rattlesnake Creek,along"..."thrust faults." Since then, terrane nomenclature has Hayfork, and North Fork terranes (Figure 3). Hotz [1977]been applied worldwide. In the extreme, it has been deemed correlated blueschists in the northern Klamaths with the Stuart"more reasonable to assume allochthoneity among terranes" Fork Formation mapped by Davis and Lipman [1962] in the[Howell and Jones, 1984, p. 6] and that "minimum nominal western Paleozoic and Triassic belt farther to the south.displacements [between terranes] of a few hundred kilometers Mapping by Ando et al. [1977] led them to divide the Northappear to be required, but in fact, most displacements probably Fork into two terranes: (1) a chiefly tholeiitic igneouswere much larger" [Jones, 1991, p. 23]. basement thrust beneath (2) interbedded sedimentary and alkalic
However, the growing base of petrologic, structural, volcanic rocks. Blake et al. [1982] renamed the tholeiiticgeochronologic, isotopic, and mapping data has prompted the basement the Salmon River terrane and reserved the namerecognition of magmatic and depositional ties between North Fork for the sedimentary and alkalic volcanic section.
Wright [1981, 1982] subdivided the Hayfork into the volcanicCopyright 1993 by the American Geophysical Union; ~estern Hayfork and sedimentary Eastern Hayfork terranes.
Smce then, effort has focused on extending these terranes intoPaper number 92TC022S0. the northern Klamath Mountains. Rawson and Petersen0278-7407/93fJ2TC-0225OS10.00 [1982] and Donato et aI. [1981] carried the Rattlesnake Creek
)
,
388 Hacker et at. Klamath Magmatic Arc/Accretionary Wedge
123° 15' 123° 00' 122° 45'42°00'+ ° ,
e2a
N~ "41° 30 I -
0 krn 20
41 ° 00
anerraneane
ane
} SawyersBar .terrane
terrane 1
40° 30'+ - 40° 30' a123° 30'
Fig. 1. (a) Terrane map of the central Klamath Mountains in California. The "western Paleozoic andTriassic belt" includes the Rattlesnake Creek, Western Hayfork, Sawyers Bar, and Stuart Fork terranes.Herein, the Yreka-CaI1ahan, Trinity, and Redding terranes are jointly referred to as the "eastern Klamaths."The St Claire Creek unit south of 410 is mapped in reconnaissance only and should be considered aprobable mixture of St Claire Creek and North Fork lithologies. Areas of Figure 2 are shown byrectangles. (b) Sources used to compile the map.
and Hayfork terranes into the northern California Klamaths. Western Hayfork, Eastern Hayfork, North Fork, and SalmonBlake et aI. [1982] showed the Salmon River terrane reaching River terranes.the Oregon border, but based on lithological and fossil Although Mortimer demonstrated that the western Paleozoicsimilarities with the southern Klamath terranes, Mortimer and Triassic belt terranes of the southern Klamaths crop out in[1984] subdivided Blake et al.'s Salmon River terrane into the the northern California Klamaths, the designation of terranes
I ~
Hacker et aI. Klamath Magmatic Arc/Accretionary Wedge 389
123° 15' 123° 00' 122° 45'
Klein[197S).~. °30'
. 41°
,.i(
)",~---,I
41°00'j/ -41°00'
~i
~ Irwin [1963]
,. '"~
\..~, ' \ -'(-- '""
" "' ,\ "\ :>" '"1 i / ( b'""' .irwin [1974] ,:, [ "'.-. i400 30' " -40° 30'
123° 30' 123° 15' 123° 00'
Fig.1 (continued)
in the central KlIIInaths has remained a subject of considerable wholly fault-bounded entities and are therefore not terranes indisagreement For instance, it has been suggested that some the conventional sense. Instead, the units are intimatelyterranes, such as the North Fork, may not be presem in the related to one another by depositional, intrusive, or other typescentral Klamaths [Irwin, 1989], This paper demonstrates that of primary contact and form an important volcanoplutonicthe western Paleozoic and Triassic belt terranes in the northern arc/accretionary complex described in detail below. The Northand southern California Klamaths are contiguous through the Fork terrane is also herein subdivided into two units: thecentral Klamaths, This central area reveals new data for sedimentary section is called the St Claire Creek unit, and therecons1ruction of Klamath paleogeography, For example, the alkalic volcanic sequence retains the name North Fork. TheEastern Hayfork, North Fork, and Salmon River units are not purpose of this subdivision is to underscore the differences
.
390 Hacker et at. Klamath Magmatic Arc/Accretionary Wedge
N Y reka Area
PR: h,b 155 Ma
~
LllI68: h 164:1:4 Ma
Yr45: H 138 Ma
Yr61: H 154 Ma
Yr52: H 141 Ma
Yr84: H 164
.Yr82: H 157 . . . . .. . . .. . . . . . . . . ,
. . . . . . . . .
. . . . . . . . .. . . . . . . . .. . . . . .: : : : . . a
Fig. 2. Map of the North Fork-Salmon River arc and associated tectonostratigraphic units. Thesouthernmost portion of the arc, where late faulting has strongly disrupted primary contacts, has beenexcluded. Locations of bulk-rock analyses are shown; solid symbols from Ernst [1987], Ernst et al.[1991], and this study; open symbols from Ando [1983], Donato [1985], and Hotz [1979]. Radiometricages are called out. Age prefixes are: K-Ar hornblende age, h; K-Ar biotite age, b; Ar/Ar hornblende age,H; Pb/U zircon age, Z; samples dated by Lanphere et al. [1968], L1H68; sample dated by Paul Renne[personal communication, 1991], PR; samples dated by Wright and Fahan [1988], W&F88; sample agereported by Irwin [1985],185; and sample dated by Ando et al. [1983], ADS83; samples with "Yr"prefIXes and "M" suffIXes dated as part of this study. Radiolarian ages are shown by letters after referencescited in text. Unpatterned areas include: Quaternary alluvium, Cretaceous overlapping rocks, and theRattlesnake Creek, Stuart Fork, and Central Metamorphic terranes. (a) Yreka area; (b) Sawyers Bar area;(c) South Fork Salmon River-North Fork Trinity River area.
-,,
Hacker et aI. Klamath Magmatic Arc/Accretionary Wedge 391
Sawyers Bar Area
4tO
N
r
W&F88: Z ~164
W&F88: Z 159 Ma
448M: II 156 Ma
220M:II175Ma
4tO - b0 kin 20,- -i
Rock Types Radiolarian Ages Bulk Rock AnalysesE:] sedimentary rock Mz: Me~oroic * WP A sediment~bedded chert J: JurasSIc . PM VAB CAB sed" tB:E CaIbonate rock T: Triassic . . or " lmenm tholeiitic volcanic rock pz: Paleozoic C. MORB volcamc_alkalic volcanic rock P: Pelnlian o. VAB volcanic~ late plutonic rock IP: Pennsylvanian 9 . om volcanic
I I ' I: late, L-*Jear y p utomc rock 2' middle
I::;:J ultramafic rock 3: earl ~ Orientation of gradedc=J Rattlesnake Creek terrane' y bedding or pillowed flowsII]] Stuart Fork and Central Metamorphic terranes
Fig.2 (continued}
between the two sedimentary units, the Eastern Hayfork and St amphibolite, blueschist, quartzofeldspathic schist,Claire Creek, that are interbedded with the North Fork/Salmon recrystallized chert, and crenulated quartz-mica schist [Cox,River. 1956; Cashman, 1979; Charlton, 1979; Fahan, 1982; Burton,
1982; Wright, 1982]. The compositions of most mafic blocksEastern Hayfork Unit are unknown except for one vesicular volcanic boudin that is
similar to the North Fork unit (sample Yrl06 in Table I).The Eastern Hayfork unit varies from a broken formation to Cashman [1979] and Wright [1982] suggested that the
melange interpreted to represent an accretionary wedge [Wright, metamorphic blocks were derived from the more easterly Stuart1981]. The nonexotic (plausibly interbedded) component of Fork and Central Metamorphic terranes, and this suggestionthe Eastern Hayfork includes rocks grading from radiolarian has been strengthened by the geochronologic work of Goodgechert to tuff, pillow lava, and quartzose turbidites [Cashman, and Renne [1991]. Much of the Eastern Hayfork contains few1979; Charlton, 1979; Fahan, 1982; Wright. 1982]. if any exotic blocks and is instead chiefly disrupted chert,Tuffaceous material and clastic volcanogenic rocks are argillite, and mafic volcanogenic rocks.subordinate to detritus derived from continental or plutonic Limestone blocks in the Eastern Hayfork containprovenances. Exotic blocks are pebble- to house-sized, and Pennsylvanian, Permian, Early Triassic, Middle Triassic, andinclude ultramafic rock, basalt, gabbro, limestone, Late Triassic fossils, including Late Permian Tethyan
. .,
392 Hacker et al. Klamath Magmatic Arc/Accretionary Wedge
Salmon and Trinity River Area41"1
Yr19: H 155 Ma
AUS83:Z265-310MaYr13: H 201 Ma
..
N
f41"00
W&F88: h 141 Ma
W&F88: h 134 Ma
YI2:H149Ma
CW&F88: h 136 Ma123" IS'
Fig.2 (continued)
fusulinids and coral [Irwin, 1972, 1974; Cox and Pratt, 1973; Quartzose blocks contain detrital zircons with Pb/Pb agesIrwin et al., 1977, 1983, 1985; Gray, 1986; Miller and of 2.0-2.1 Ga, indicating that some debris was ultimatelyWright, 1987; Stevens et al., 1987, 1991]. Sedimentary derived from a mature Precambrian continental source [Millerstructures and fauna in limestone blocks suggest that the and Saleeby, 1991]. Chemical compositions of Easternlimestone was deposi~ed in shallow water [Cox and Pratt, Hayfork sedimentary rocks suggest that material was also1973; Cashman, 1979; Fahan, 1982; Wright, 1982]. The derived in part from the adjacent North Fork-Salmon Riverdiscovery of the same Late Permian Tethyan coral in the arc. Floyd et al. [1991] proposed that trace elementEastern Klamath terrane [Stevens et al., 1987] may imply that compositions of fine-grained quartzofeldspathic sedimentarythe limestone blocks are olistoliths derived from eastern paleo- rocks in favorable instances can be used to infer the tectonicPacific seamounts [Miller and Wright, 1987] or the Eastern environment of the source of the detritus. Many factors suchKlamath. Eastern Hayfork chert contains Late Permian, as grain size, sorting, diagenesis, and metamorphism may alsoMiddle Triassic, Late Triassic, and Late Triassic to Early affect the composition of sediment, so this approach is open toJurassic radiolarians, and Middle. Triassic and Late Triassic criticism. Using normalization factors of upper continentalconodonts [Irwin et 'al., 1982, 1983, 1985; Ando et al., 1983]. crust from Taylor and McLennan [1985] and average values for
I
Hacker et at. Klamath Magmatic Arc/Accretionary Wedge 393
*Davis and Lipman [X62] N western Paleozoic and Triassic belt
Irwin [1972] S Rattlesnake North Pork *StuartI Creek Pork
Hotz [1?17] N undivided western Paleozoic and Triassic belt Stuart
Ando et aI. [1977] S Rattlesnake North Pork PorkI Creek volcanics and sediments
N undivided western Paleozoic and Triassic belt
Wright [1981, 1982]R 1 ak NorthP k SptuartkI attesn e or or
. S Creek volcanics and sediments
- Donato et aI. [1981] N Salmon RiverBlak aI [1982] Rattlesnake Western Stuart
Rawson and P~:so~ [1982] S Creek Hayfork Eastern I Sa~on I ,North Por~ PorkI N Hayfork RIver vOlcanIcs and sedlffients
Mortimer [1984] Rattlesnake Western Eastern Salmon ~orth Pork, StuartS Creek Hayfork Hayfork River volcanICS and sediments Pork
Nth' Rattlesnake Western Eastern Salmon North StClaire Stuart18 T S Creek Hayfork Hayfork River Pork Creek Pork
V"Sawyers Bar terrane
Fig. 3. A history (top to bottom) of the naming and subdivision of terranes in the western Paleozoic andTriassic belt of the Klamath Mountains. The upper half of each box labeled "N" refers to the northernCalifornia Klamaths, and the lower half "s" refers to the southern Klamath Mountains.
sedimentary rocks derived from oceanic island arcs (V AB), Salmon River Unitcontinental arcs or active margins (CAB), oceanic within-platesoUrces (OIB), and passive margins (PM) compiled by Floyd et The Salmon River unit is composed of ultramafic rock,al. [1991], about half our analyzed fine-grained gabbro, diabase, and volcanic rock, which led Irwin [1972] toquartzofeldspathic sedimentary samples from the Sawyers Bar interpret it as oceanic crust. Others [Blake et aI., 1982; Andoarea (locations shown in Figure 2) have trace element et aI., 1983; Mortimer, 1984; Saleeby, 1990], have sinceabundances that can be matched to a particular tectonic setting concurred with this, but Ernst et al. [1991] proposed instead(Figure 4). Eight of our samples appear to have been derived that it is an immature intraoceanic arc.from a within-plate source, and 15 have element abundances Ultramafic rock in the Salmon River unit is serpentinizedappropriate for either V AB, CAB, or PM sources. Although, dunite and harzburgite tectonite [Ando, 1979]. The gabbro isin principle, it is possible to differentiate among rocks derived chiefly massive with local cumulus layering and containsfrom eroding oceanic island arcs, continental arcs, and passive andesine-bytownite and augite overgrown by igneousmargins, the somewhat disturbed abundance patterns precludes pargasite. It grades into massive diabase with similardoing so for most of our samples. The highly variable K, Rb, mineralogy. Overlying massive and locally pillowed volcanicand Sr abundances (Figure 4) are probably the result of similar rocks with thin volcaniclastic layers contain clinopyroxene,m~tasomatic addition or leaching [Hacker et al., 1992], and we hornblende, and plagioclase, minor spinel, apatite, andassume that these elements are no longer present in their possibly anorthoclase; carbonate and chert are present betweenoriginal concentrations. pillows [Cox, 1956; Ando, 1979; Mortimer, 1984; Ernst et
Thus, the Eastern Hayfork comprises broken formation and aI., 1991].melange formed prior to Jurassic time. Detritus was derived Salmon River volcanic rocks are similar to immature arcfrom a mixture of sources including continental rocks with lavas, including having high-field-strength elements depletedPrecambrian zircons, the eastern Klamaths, alkalic North below mid-ocean ridge basalts (MORB), and Th levels inFork-like rocks, and a Salmon River-like arc source. excess of MORB (Figure 5; Table 1; Ernst et al. [1991]).Deposition occurred at least from Permian to Late Triassic Their primitive nature is indicated by Cr and Ni contents astime (age range of radiolarians). The Eastern Hayfork is high as 1100 and 500 ppm, respectively. Flow rocks andinferred to have formed in an accretionary wedge [Wright, dikes show decreasing Cr and Ni with decreasing MgO,1982], and the fossils in limestone blocks suggest that oceanic suggesting fractionation of olivine, hornblende, orlithosphere of Devonian to Late Triassic age may have been clinopyroxene. Element-ratio diagrams rule out thesubducted beneath it Alternatively, the Eastern Hayfork may participation of olivine and plagioclase but support thebe the result of closure of a fairly small basin that never contention that clinopyroxene and/or hornblende wereinvolved significant subduction, in which case the limestones fractionating or accumulating phases [Ernst et ai., 1991].might be tectonic slivers or olistoliths. Decreasing Ti with decreasing Zr suggests fractionation of a
"
394 H
acker et al. Klam
ath Magm
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Hacker et al. Klamath Magmatic Arc/Accretionary Wedge 397
I North Fork volcanics"Passive margin" sedimentary rocb 100
~ -"""'V.2Ib, V"', V,34, V,IOIe .-'-. """,vu
1.a 108 PM ~u 0
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0.1K TiZrLaCeNd
100I ,V,"2
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-<:;l 0.1~ Sr K Rb Ba Th [Ta]Nb Ce P Zr [Hf][Sm]Ti Y Vb
41M, SSM, 87M, 93M, 134M, 159M, 166M, 355M Sahnon River flows and volcaniclastics0.1 100
K Rb Sr P Cr Ni Ti Zr La Ce Nd --- y'*,.;,",",V,3
- _V.2.. V..S, Vf12
10 ~ [1914]
"Arc" sedimmta ry rocks10
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1 ~~ ~ I »...""-';";.:::7:-:z-.:::,-, \ ,
~ I AB Maria VAB \, ,';3- [Wo<Ml et aL, 1980]
~ 0.1! VAB Sr K Rb Ba Th [Ta] Nb Ce P Zr [Hf)[Sm] Ti Y Vb
J Fig, 5. MORB-nonnalized abundimces of elements in the120M,I21M, 157M, 178M,I92M,I97M, North Fork unit, Salmon River hypabyssal rocks, and volcanic263M,294M,367M,370M,385M,503M ks bb .
k I ded. S Ern al [1991] d0.1 roc ; ga rolC roc S are exc u ee st et. anK Rb r P Cr Ni Ti Zr La Ce Nd Ernst [in press] for further analyses. Elements shown in
Fig. 4. Abundimces of elements in Eastern Hayfork parenthesc:s were ~ot analyzed. S~, K, Rb, and Ba abundimcessedimentary rocks nonnalized to the composition of upper were.m~~ dUrIng ~etamorphlsm [Hacker ~t al., 1992] andcontinental crust from Taylor and McLennan [1985]. Average provide lIttle Infonnauon about magma chemIstry.
values for sedimentary rocks derived from oceanic island arcs M ..(V AB) continental arcs or active margins (CAB) oceanic ost Salmon River cltnopyroxenes are strongly zonedwithin-~late sources (Om) and passive margins (PM) augite and plot within the intraoceanic arc field of Nisbet andcompiled by Floyd et al, [1991] are shown by heavy shaded ~earce:s [1977] discrirni~t diagram ~igure 6a). Cores arelines. Analyzed samples are listed by number in lower right nc~er In Cr2.o3 than the n~s: sugg~ung that the cores ~wcomer and shown by thin solid lines. Elevated K and Rb and while chroml~ was crystallIZing (FIgure .6b and 6c). Thisdepleted Sr concentrations probably reflect postdepositional trend a?pe~s In pyroxene from gabbro, diab~se, flow rocks,ddiu. d I h. U. I volcanIclastIC rocks, and tuffs. Igneous plagioclase crystals are
a onan eac Ing,respec vey. ca1. A . I . k dAD .as CIC as n60 In vo canlc roc s an 78 In gabbro.Plagiogranite within diabase along the Salmon River
yielded a strongly discordant Pb/U zircon age interpreted byTi-bearing phase as well. High Mg numbers in these locally Ando et al. [1983] to indicate crystallization in the range 265-sparsely phyric lavas argue for rapid ascent from the mantle, 310 Ma (pennsylvanian to Pennian). A gabbro body thatalthough the most magnesian (up to 18 wt% MgO) members intrudes the diabase gave a 40 Ar/39 Ar isochron age ofof the suite contain cumulus hornblende:t clinopyroxene 200,4:t1.4 Ma on igneous hornblende (fable 2). As discussed[Hacker et al., 1992]. below, hypabyssal rocks intruding the Salmon River unit
,
398 Hacker et at. Klamath Magmatic Arc/Accretionary Wedge
-1.2 F 1 .0.7 The volcanic rocks are massive breccias with angular toA -2.3 subangular clasts, tuffaceous cherts, pillowed flows, pillow
breccias, and hyaloclastites. Tuffs are crystal- to lithic-richA A . with angular to subangular clasts of basaltic rock, plagioclase,
A and clinopyroxene. The flows are typically aphyric to locallyWPA subtrachytic with forsterite + clinopyroxene + plagioclase,
A A A F plagioclase :t clinopyroxene, and plagioclase + hornblende +
2 clinopyroxene. Preservation of delicate quench textures
indicates that deformation was not penetrative and. F ... metamorphism was not pervasive. North Fork volcanic rocks
Salmon RIver North o.~ . ..8 Emstet aI. [1991] . thlslbJdy are ennched m light rare-earth elements, P, Hf, Zr, Ta, Nb, Fe,:~ort~[I98s] and Ti relative to mid-ocean ridge basalts, and resemble alkalic
-2.7 ocean-island basalts (Om) and alkalic arc rocks (Figure 5).Their primitive nature is indicated by Cr and Ni contents as ..
6 high as 600 and 400 ppm, respectively. Element-ratioB Salmon River North Folk diagrams suggest that clinopyroxene or hornblende were
5 ~=,~aL[I99I] Athls8\Udy /'~ fractionating or accumulating phases [Ernst et al., 1991]. The.M~ [198S] /':,1' lack of an Eu anomaly, high Sr contents, and positive
0' 4 /'::~A correlations between Sr, Zr and P, argue against significant~ ~:/~~ ' feldspar removal.~ 3 ,I ~~~: Pyroxenes from alkalic rocks tend to have high AI203,
2 ..,' \'~/ .,.,a i.,!.IJ~f"I TiO2, and.Na20 co~tents, and the North Fork pyroxenes are8 ~ ,:~";.:::~:.~ §.~\'!.f!!'.!1... no exception, reachIng >8 wt% Al203, 5 wt% Ti02, and 0.8
I l~. . D .~~~~'.-'-'.::'--,&" wt% Na20 (Table 3; Figure 6b and 6c). North Fork.,.- .~- pyroxenes are augites and salites higher in CaO and lower in
8 10 SiO2 content than Salmon River pyroxenes. As evident in0 2 4wt% A~O6 Figure 6a, they straddle the intraoceanic arc and within-plate1.2 3 magma fields in the discrimination diagram of Nisbet and
C Pearce [1977]. Igneous amphiboles in the North Fork are1.0 ~ : pargasites and contain more Na20 (>3.5 wt%) and less Ti02f '1' i i (SO.3:ovt%) (Table 3) than Salmon River hornblendes which
0' 0.8 ~ D\': contam less than 3 wt% Na20 and up to 3 wt% Ti02 [Ernst et~N I " , al., 1991].~ 0.6 . t~. rJ. D In the.Salmon River .~ea, a single mass of limestone up to
~ 0 4 ,~~, 600 m thIck rests deposItIOnally on North Fork volcanic rocks. D '~;:;""- and is overlain by St Claire Creek chen. This thick limestone
0.2 ~~ ,,',-, has a basal conglomerate of basalt and red chen cobbles and
D contains ooids and sparse volcanic clasts. Elsewhere,0.0 D 20 limestone beds interlayered with North Fork flows are usually0 5 WI% FeO a few meters thick and up to several hundred meters long
[Masson, 1949; Trexler, 1968; Hotz, 1977; Ando, 1979]. AtFig. 6. Salmon River and North Fork clinopyroxene some localities, oolitic limestone is intercalated withcompositions. (a) Discriminant diagram of Nisbet and Pearce volcaniclastic rocks and occurs as interpillow material[1977]. (b) and (c) Core-to-rim zonation shown by dashed [Masson, 1949; Ando, 1979; Wright, 1982]. Limestonearrows. Note subparallelism of Cr-Fe zoning in Cr-rich interbedded with volcanic breccia contains Late Permian algae,pyroxenes to compositional trend of comagmatic spinel and foraminifers, and bryozoans characteristic of low-energy,chromite shown by solid arrow in Figure 6c. shallow-water normal marine environments [Trexler, 1968;
Hotz, 1977; Ando, 1979; Mortimer, 1984]. Subroundedrange in age from 140 Ma to 173 Ma. These data could be ~olcanic bre.c':ias cemented with calcite su~gest subtropicalinterpreted to indicate the existence of a Pennsylvanian- IIt.to~ condI~ons, an~ le~ses of slumped IIme.ston~ bedsJurassic arc or the presence of a Jurassic arc built on older wIthin volcanIc rocks IndIcate local topographIC relief andbasement. Fossil ages of interbedded Eastern Hayfork unstable carbonate platforms. Limestone beds or exotic(?)sedimentary rocks suggest that construction of the Salmon blocks contain Late Pennsylvanian-Early Permian, and EarlyRiver suite began in Permian and extended at least through Permian fusulinids, and Early Carboniferous-Permian andLate Triassic time. Early Permian(?) foraminifers [Irwin, 1972, 1974; Irwin et al.,
1977].North Fork Unit Thus fat we have been unable to date the crystallization age
of the North Fork volcanics using the 40 Ar/39 Ar techniqueThe North Fork unit comprises up to 2 km of amygdaloidal because most rocks lack separable K-bearing phases.
volcanic rocks with rare limestone massifs [Ando, 1979; Sedimentary rocks of the Eastern Hayfork unit interbedded withWright, 1981; Mortimer, 1984]. It has been interpreted to the North Fork range in age from Late Permian to Laterepresent a seamount [Davis et al., 1978; Ando, 1979], Triassic. Late Permian fossils are present in limestonehowever, we present an alternative interpretation-that the intercalated with the North Fork volcanics, and the interbeddedNonh Fork is an alkalic pan of an immature arc. St Claire Creek unit contains Late Permian to Early or Middle
, '
Hacker et at. Klamath Magmatic Arc/Accretionary Wedge 399
TABLE 24. 40 Ar/39 Ar Data for Hornblende
T 139Ar 4°Ar. 37Ar/39Ar 4°Ar*/39ArK Age (Ma):t 1 0-
Sample Yr2900 0.174 0.155 2.0285 7.4270 142.1:t 111000 0.202 0.280 16.8423 8.0096 152.7:t 171060 0.358 0.698 9.8578 7.7114 147.3:t 3.81400 0.607 0.709 10.0174 8.0404 153.3:t 3.21120 1.000 0.850 9.8364 7.7374 147.8:t 2.2
Sample Yr13800 0.009 -0.000 14.5214 1.1564 22.5 :t 296900 0.017 0.120 29.7011 8.1060 151.8:t 591000 0.072 0.491 20.8547 10.2925 190.7 :t 8.11040 0.329 0.782 17.8869 11.0565 204.0 :t 2.81070 0.560 0.767 21.4978 10.7473 198.7:t 2.51100 0.639 0.765 19.1293 10.3611 191.9:t 5.21150 0.799 0.784 20.1233 10.8578 200.6:t2.91200 0.877 0.752 21.0896 10.7910 199.4 :t 4.91265 0.932 0.740 22.1728 10.9679 202.5 :t 5.71400 1.000 0.648 21.0959 10.5962 196.0:t 7.4
Sample Yr19800 0.015 0.004 2.6058 0.6920 13.5 :t 93900 0.024 0.132 4.1571 7.6318 143.3:t 46950 0.032 0.188 7.0100 8.3700 156.6:t 341000 0.042 0.231 5.0790 7.8663 147.5:t 301050 0.103 0.523 5.5263 8.1627 152.9 :t 6.21080 0.274 0.798 5.7391 8.2472 154.4:t 2.01100 0.599 0.900 5.8250 8.2249 154.0 :t 0.81120 0.756 0.914 5.8144 8.2411 154.3 :t 1.71150 0.800 0.913 5.9937 8.3934 157.0:t 5.81250 0.875 0.866 5.9674 8.0532 150.9 :t 2.71400 1.000 0.842 6.3047 8.1595 152.8:t 3.6
Sample Yr45800 0.060 -0.018 2.1636 -3.6753 -73.6:t 17900 0.107 0.050 1.3526 4.4953 86.5 :t 20950 0.134 0.062 6.5079 4.8767 93.6:t 311000 0.159 0.096 14.1794 5.1640 98.9:t 221030 0.209 0.473 13.3223 7.0614 133.9:t 121060 0.325 0.639 14.8268 7.1804 136.1 :t 4.21090 0.474 0.662 15.7687 7.5051 142.0:t 5.11150 0.684 0.693 16.1946 7.5168 142.2:t 3.01275 0.874 0.646 18.4827 7.4916 141.8:t 2.91400 1.000 0.599 11.7967 5.8904 112.4:t 5.4 \
Sample Yr52750 0.027 0.028 2.6867 6.2880 121.0:t 79900 0.052 0.219 3.3491 8.2158 156.5:t 37950 0.065 0.403 8.3738 7.4236 142.0:t 351000 0.086 0.439 9.4806 7.9788 152.2:t 241050 0.287 0.778 10.2988 7.3489 140.6:t 3.41100 0.668 0.806 11.0329 7.3294 140.3:t 2.01150 0.787 0.786 11.5269 7.3769 141.1:t 5.61225 0.824 0.689 13.1182 6.9650 133.6:t 151400 1.000 0.624 12.8426 7.3223 140.1 :t 4.6
Sample Yro1800 0.076 0.215 2.0189 9.1998 171.9:t 17900 0.106 0.661 1.1537 7.8060 146.9:t 2.51000 0.165 0.586 7.1922 7.4322 140.2:t 2.31100 0.594 0.851 7.2564 8.1446 153.0:t0.81200 0.896 0.866 7.0350 8.1227 152.6 :t 0.61400 1.000 0.776 8.0608 8.1830 153.7:t 1.7
I,
400 Hacker et aI. Klamath Magmatic Arc/Accretionary Wedge
TABLE 2a. (continued)--~-
T l39Ar 4°Ar* 37Ar/39Ar 4°Ar*/39ArK Age(Ma):t 1 CJ- . .
Sample Yr82900 0.066 -0.008 20.4592 -3.0563 -60.4 :t 521000 0.108 0.060 26.7716 7.3956 138.2:t 291030 0.153 0.267 13.8804 8.8452 164.1:t 141070 0.267 0.705 11.4153 8.4645 157.4:t6.21100 0.397 0.703 11.5029 8.2694 153.9:t3.61200 0.777 0.685 11.8487 8.3413 155.2:t2.21300 0.893 0.591 12.2392 8.2994 154.4 :t 4.71400 1.000 0.400 11.1983 7.9455 148.1:t 7.6 '
Sample Yr84800 0.039 0.034 2.8263 5.2934 100.2 :t 36900 0.082 0.527 2.0440 8.1870 152.6:t 11950 0.109 0.594 3.1071 8.1499 152.0:t 121000 0.135 0.563 4.8529 7.9139 147.8:t 111050 0.201 0.692 11.5377 8.3961 156.4:t 5.11100 0.796 0.755 14.4726 8.8526 164.5:t 1.71130 0.850 0.835 12.2150 9.0156 167.4 :t 6.71200 0.912 0.755 13.7384 8.6959 161.7:t 5.11300 0.944 0.674 12.7120 8.5099 158.4:t 101400 1.000 0.616 12.9926 8.7508 162.7 :t 8.2
Sample 220M900 0.134 0.019 4.6918 4.4466 84.7:t 631000 0.164 0.134 17.5776 6.5700 123.8:t 631050 0.344 0.649 12.5937 9.1124 169.6:t 8.61200 0.907 0.793 13.2877 9.3269 173.4:t 4.21400 1.000 0.422 .14.5766 9.2198 171.5:t 26
Sample 448M800 0.016 -0.036 8.3525 -25.2326 -583.8 :t 590900 0.028 0.151 11.5948 6.2603 119.7:t171000 0.079 0.256 11.5995 7.5786 143.9 :t 9.51050 0.281 0.763 6.7886 8.0825 153.1 :t 1.91100 0.516 0.825 6.5727 8.1497 154.3 :t 0.91200 0.886 0.816 6.6937 8.1721 154.7:t 1.0 ,1400 1.000 0.704 6.8305 8.2065 155.3 :t 2.1 '
Analytical details are provided by Hacker [1993].
TABLE 2b. 40 Ar/39 Ar Data for Hornblende
Sample J Weight, Grain Size, K20, Total Fusion Isochron MSWD Weighted Mean T steps 39 Ar,mg 11m wt% Age, Ma Age, Ma Plateau Age, Ma used, °C %-
yt'2 0.01103200 0.3 100 0.55 148.2:t2.4 149.3:t1.7 0.88 149.O:t1.6 all 100Yr13 0.01083200 5.4 200 0.30 197.6:t2.8 200.4:t1.4 0.83 199.8:t1.4 all 100Yr19 0.01083342 2.2 200 0.75 151.6:t1.6 154.2:t0.7 0.43 153.9:t0.6 all 100Yr45 0.01091362 3.2 80 0.30 120.O:t2.2 142.6:t1.7 0.55 142.O:tl.9 1090-1275 55Yr52 0.01103186 1.8 80 0.55 140.3:t2.9 140.5:t1.6 0.11 140.4:t1.5 all 100Yr61 0.01086971 2.0 200 0.80 153.5:t1.4 153.1:t1.5 7.1 152.8:t0.5 1100-1400 98Yr82 / 0.01076969 0.9 80 0.40 140.6:t3.7 155.9:t1.6 0.35 154.7:t1.6 1000-1400 93Yr84 0.01078490 2.4 200 0.45 159.9:t2.0 163.8:t1.5 0.71 164.2:t1.5 1100-1400 80220M 0.01081600 0.4 400 ... 159.4:t9.2 173.2:t3.7 0.17 172.5:t3.7 1000-1400 87448M 0.01095683 1.3 100 ... 144:t6.2 155.5:t0.7 2.1 154.4:t0.6 1000-1400 97
J, iITadiation parameter; MSWD, mean sum of weighted deviates of isochron; isochron and weighted mean plateau ages are based ontemperature steps and fraction of 39 Ar listed in the last two columns.
"
TABLE 3. North Fork Igneous Pyroxene and Hornblende Jurassic fossils. The oldest pluton intruding the North Fork isCompositions the English Peak pluton (;?:164 Ma Pb/U zircon age; Wright
S I /A I . and Fahan [1988]). Thus, eruption may have spanned Lateamp e na YSIS P . M.ddl J .. ermlan to I e uraSSIC time.
Yr51/62 Yr51/62 Yr51/65 Yr51/66 Yr51/67 Yr51/68 Although the North Fork rocks are alkalic ocean-islandrim core core core core core basalts, they need not represent seamounts. They may have
P asite erupted within the same immature arc as the Salmon River
SiO2 39.92 42.77 41.0:; 41.55 40.41 41.22 unitbecausethetwounitsi~terfmgerand~~rmissiblyAi203 14.95 12.90 13.40 13.51 16.18 14.24 coeval [Ernst, 1990]: Alkahne ma~~~ WIth hIgh-fieldTi02 0.30 0.22 0.17 0.25 0.18 0.23 stren.gth e1em~nt.enrlchment were mltially thoug~t to beFeO* 18.87 16.77 17.88 17.45 17.34 17 .63 re~tricted to wI~m-piate magmas gen.erated deep m.tile mantle.Cr203 0.07 0.04 0.05 0.04 0.09 0.00 It IS n~w re:ognlz~ ~at such v~lcanlc roc~s occur m a:arietyMnO 0.24 0.27 0.26 0.26 0.28 0.30 ?f settings, mcluding mtraoceanlc a~d continental margm arcs;MgO 8.04 9.75 9.46 9.47 8.28 8.66 msomearcstheyareeventhedomlnan~rocktype~e.~.,CaO 11.09" 11.44 11.39 11.68 10.88 11.34 Leemanetal., 1990]. Mo~ver,OIB-h~erockswlthmarcsNa20 4.03 3.70 3.82 3.85 3.88 3.83 may be related to subduction of young h~osphere. StoreyetK20 0.46 0.36 0.36 0.40 0.38 0.39 aI. [1989] p~po~ ~t when ~ no-~lab wmdow.appearedSum 97.95 98.23 98.73 98.46 97.91 97.84 beneath Baja California followmg ndge subduction, OIBs were
erupted because of the anomalously shallow heat Ormerod etSi 6.05 6.39 6.25 6.24 6.05 6.22 aI. [.1988] noted that o.m lavas were erupted in t~~ western
AllY 1.95 1.61 1.75 1.76 1.95 1.78 BasmandRan.ge~oVlnce2-3m.y.afterthetralh~gedgeofAlVI 072 067 061 063 091 076 su.bducted PacIfic h~osphere passed ben~th, leavmg ~ no-slab. . . . . .. wmdow. The Washmgton Cascades arc IS locally dommated
Tl 0.03 0.02 0.02 0.03 0.02 0.03 by OIBs, perhaps produced because the lithosphere beingCr2+ 0.01 0.01 0.01 0.01 0.01 0.00 subducted is young and hot and may devoiatilize beforeFe 2.36 2.09 2.20 2.18 2.14 2.22 reaching depths of magma generation, preventing bothMn 0.03 0.03 0.03 0.03 0.04 0.04 metasomatism of the mantle wedge and the development of aMg 1.82 2.17 2.10 2.12 1.85 1.95 normal arc signature [Leemanetal., 1990]. Thus,theNorthSum 4.97 4.99 4.97 4.99 4.97 4.99 Fork rocks, which appear to be an integral part of an immatureCa 1.80 1.83 1.82 1.88 1.75 1.83 arc may also have erupted following ridge s bd cti.Na 1.18 1.07 1.11 1.12 1.13 1.12' u u on.
K 0.09 0.07 0.07 0.08 0.07 0.08
TABLE 3. Sample/Analysis continued
Yr18/47 Yr18/48 Yr18/49 Yr18/50 Yr51/59 Yr51/60 Yr51/69 Yr51/70 Yr15/27 Yr15/29 Yr15/31 Yr15/32core rim core rim ~ rim core rim ~ rim ~ rim
PyroxeneSiO2 49.23 48.68 47.89 48.07 42.47 47.07 48.72 42.84 51.76 53.56 50.08 50.79AIZO3 5.45 5.32 6.85 5.57 8.38 5.32 4.51 8.66 3.03 1.62 4.01 3.58Ti02 1.80 2.06 2.35 3.19 5.11 2.80 2.06 5.05 0.58 0.30 1.00 0.90FeO* 13.64 11.26 8.68 10.16 9.91 9.53 8.68 10.93 6.66 6.08 8.44 8.78CrZO3 0.18 0.14 0.08 0.15 0.58 < < < 0.40 0.27 < <MnO 0.37 0.28 0.21 0.24 0.09 < 0.18 0.23 0.16 0.18 0.71 0.31MgO 11.41 13.93 14.26 13.10 10.82 12.29 14.04 10.36 17.76 18.29 15.08 16.31CaO 14.46 17.29 18.47 19.28 21.84 21.27 21.23 20.45 18.35 19.05 20.19 19.25NaZO 2.81 0.51 0.48 0.51 0.56 0.49 0.62 0.82 0.22 0.14 0.27 0.20KZO < < < < < < < < < < < <Sum 99.36 99.46 99.26 100.3 99.76 98.77 100.1 99.34 98.92 99.50 99.78 100.1
Si 1.87 1.83 1.79 1.80 1.63 1.80 1.82 1.65 1.91 1.96 1.87 1.88AllY 0.13 0.17 0.21 0.20 0.37 0.20 0.18 0.35 0.09 0.04 0.13 0.12AlVI 0.11 0.07 0.09 0.04 0.01 0.03 0.02 0.04 0.05 0.03 0.05 0.04Ti 0.05 0.06 0.07 0.09 0.15 0.08 0.06 0.15 0.02 0.01 0.03 0.02Cr 0.01 0.00 0.00 0.00 0.02 < < < 0.01 0.01 < <FeZ+ 0.43 0.35 0.27 0.32 0.32 0.30 0.27 0.35 0.21 0.19 0.26 0.27Mn 0.01 0.01 0.01 0.01 0.00 < 0.01 0.01 0.01 0.01 0.02 0.01Mg 0.65 0.78 0.79 0.73 0.62 0.70 0.78 0.59 0.98 1.00 0.84 0.90Ca 0.59 0.70 0.74 0.77 0.90 0.87 0.85 0.84 0.73 0.75 0.81 0.76Sum 1.85 1.97 1.97 1.96 2.02 1.99 1.98 1.98 2.01 2.00 2.01 2.00Na 0.21 0.04 0.03 0.04 0.04 0.04 0.05 0.06 0.02 0.01 0.02 0.01K < < < < < < < < < < < <
Less than signs denote compositions below detection. Sum refers to AIVI + Ti + Cr + Fe + Mn + Mg foramphibole and Al VI + Ti + Cr + Fe + Mn +Mg + Ca for pyroxene. Analytical details are provided by Hacker et
al. [1992].
,, '
402 Hacker et al. Klamath Magmatic Arc/Accretionary Wedge
St Claire Creek Unit recrystallization occurred at 500-900 MPa and 250°-400°CThis unit, infonnally named after exposures along the [Goodge, 1989a, 1989b; Hacker and Goodge, 1990].
South Fork of the Salmon River near St Claire Creek, TIES BETWEEN UNITScomprises chiefly coherent, bedded chert and minor argillite[Ando, 1979; Wright, 1981; Mortimer, 1984] unlike the Although Irwin [1972] initially postulated that Klamathdisrupted, argillite-dominated sequences of the Eastern Hayfork terranes might be unrelated far-traveled entities, more data ledunit. Previously it was included as part of the North Fork to the suggestion that the terranes were related to one anotherterrane, and postulated to represent open-ocean sediments [Davis et aI., 1978]. Demonstrated interrelationships among[Davis et aI., 1978; Ando, 1979; Wright, 1982], but herein we Klamath terranes have strengthened with time. An earlyseparate this sediment-dominated unit from th,e alkalic volcanic proposal that the St Claire Creek and Stuart Fork terranes arerocks of the North Fork. The new name emphasizes that the correlated [Davis and Lipman, 1962] has been discountedSalmon River and North Fork are interbedded with two [Wright, 1982] because, although older rocks in die St Clairedistinctly different types of sedimentary rock: disrupted chert- Creek could be equivalent to Stuart Fork protoliths, !
argillite of the Eastern Hayfork and coherent, well-bedded chert- radiolarians in the St Claire Creek postdate metamorphism ofargillite of the St Claire Creek unit. the Stuart Fork. Lithologic similarities shared by sedimentary
St Claire Creek chert contains radiolarians, sponge spicules, rocks of the Eastern Hayfork and St Claire Creek units ledfragments of plagioclase, quartz, clinopyroxene, brown Ando et al. [1977] to propose stratigraphic equivalence of these .
hornblende, chlorite, muscovite, and carbonaceous material two units.[Cox and Pratt, 1973; Wright, i981]. Radiolarians and sponge Many of the contacts between the Eastern Hayfork, Northspicules in chert have shape-preferred orientations suggesting Fork, Salmon River, St Claire Creek, and Stuart Fork unitsdeposition by currents [Mortimer, 1984]. Sandstone and are faults formed during Middle Jurassic deformation [Wrightargillite turbidites interlayered with the chert contain crystals and Fahan, 1988]. Accordingly, these units have been regardedof quartz, plagioclase, and rare potassium feldspar, and clasts of as terranes. However, not all contacts between the units areargillite, chert, recrystallized chert, mica schist, volcanic rock, tectonic, and the primary nature of some contacts is locallyand Permian McCloud limestone [Fahan, 1982; Wright, 1982; preserved. The two extensive volcanoplutonic units, theGray, 1986]. Chert contains Paleozoic, Late Permian, Middle Salmon River tholeiitic suite and North Fork alkalic suite,Triassic, Late Triassic to Early Jurassic, and Early to Middle interfinger and are permissibly coeval, indicating that they areJurassic radiolarians, and Middle Permian and Late Triassic part of the same magmatic arc [Ernst, 1990].conodonts [Irwin et aI., 1977, 1982; Blome and Irwin, 1983; The contact between the Eastern Hayfork and North Fork-Mortimer, 1984]. The composition of the detritus indicates Salmon River units is interpreted as the complex zone betweenthat the undisrupted chert-argillite of the St Claire Creek unit an accretionary complex (Eastern Hayfork) and an immaturewas derived from sources similar to those providing sediment magmatic arc (North Fork-Salmon River), as illustrated into the Eastern Hayfork unit [Wright, 1982]. The St Claire Figure 7. Parts of this zone show depositional contacts andCreek unit is also largely coeval with the Eastern Hayfork and parts are faults [Ernst, 1990]. Some of these faults may beNorth Fork-Salmon River units. syndepositional while others cut all observable bedding.Stuart Fork Terrane Gradational sequences of basal pillowed flows, overlying
volcaniclastic rocks, argillite and chert are characteristic ofThe Stuart Fork terrane, metamo~hosed in Late Triassic contacts where the North Fork-Salmon River volcanics overlie
time (two K/Ar ages and one 40 Ar/3 Ar age on phengite Eastern Hayfork sedimentary rocks. These sequences areaverage -219 Ma), is a blueschist-facies broken formation inverted where the Eastern Hayfork unit overlies the Northcomprising hemipelagic shale, chert, and mafic volcanic rocks Fork and Salmon River units. Crystal- to lithic-rich tuffs of "[Hotz 1977; Hotz et aI., 1977]. Detrital feldspars in the the North Fork-Salmon River suite are also interbedded withmetasedimentary rocks indicate proximity to a craton or radiolarian-bearing argillite and chert of the Eastern Hayforkmagmatic arc. The volcanic rocks are chiefly massive and unit. These depositional contacts are observed in the Sawyerslocally amygdaloidal pillowed flow rocks with interpillow Bar and Yreka areas. Furthermore, trace element abundancescarbonate. Trace element abundances suggest that the Stuart suggest that argillite of the Eastern Hayfork interbedded withFork volcanics are back-arc basin basalts [Goodge, 1990]. the North Fork-Salmon River volcanics received detritus from
Submarine low-pressure metamorphism of the Stuart Fork a magmatic arc-implying that the North Fork-Salmon Riverterrane was followed by blueschist and eclogite formation. arc was active during deposition of the accretionary wedge. AllEclogites suggest metamorphic conditions of 900-1100 MPa the "within-plate" Eastern Hayfork sedimentary rocks (Figureand 300°-400°C [Borns, 1980, 1984], and blueschist-facies 4) were collected near North Fork volcanic rocks (which have
E H yf k North Fork- .astern a or Salm Ri St Claire Creek Stuart Forkon ver
" "'~ ~ "" ,'.., '
accretionary magmatic back-arc ubdremnat,:,twedge arc basin s ZO~ Ion
Fig. 7. Inferred petrotectonic setting of the North Fork-Salmon River arc and associated sedimentarydeposits in Jurassic time.
,,
alkalic within-plate trace-element signatures), and all the "arc" warm at the time of hypabyssal intrusion because theor "passive margin" sedimentary rocks crop out near Salmon dikes/sills are spaced at least 100 m apart and would haveRiver volcanic rocks (which represent a tholeiitic immature cooled too rapidly for significant autometamorphism ifarc). This implies that interlayered volcanic and sedimentary intruded into cold wall rock. Metamorphism of the countryrocks in the Sawyers Bar area were derived from the same rock, commonly considered to be mid-Jurassic [Coleman et al.,source. Boudinaged hypabyssal North-Fork-affinity dikes 1988], thus probably lasted into Early Cretaceous time [cf.intrude the Eastern Hayfork accretionary complex. Harper and Wright, 1984], albeit at rather low temperatures.
The St Claire Creek and North Fork units are also Additional evidence for significant heat retention intointerbedded. Along the Salmon River, North Fork limestone Cretaceous time comes from the Condrey Mountain terraneis overlain by bedded red chert that passes upward into gray (Figure I), which yielded muscovite KlAr ages of 125-143 Machert of the St Claire Creek unit. Where the limestone is not and Rb-Sr isochron ages of 125-139 Ma [Helper et al., 1989].present, red chert lies directly on North Fork basalt and forms 2. Tholeiitic arc magmatism continued in the centrallenses between pillows [Wright, 1981; Ando et al., 1983]. California Klamaths from -200 Ma to -140 Ma. This activitySedimentological features mentioned earlier indicate that the may be a continuum but can be partitionedJor the sake ofundismpted chert-argillite St Claire Creek unit and the discussion into -200 Ma, -175 Ma, -164 Ma, -155 Ma, -149disrupted chert-argillite Eastern Hayfork unit were derived from Ma, and -140- Ma-old magmatic suites.. similar sources [Wright, 1981] and are coeval. The youngest ages come from dikes (samples Yr45 and
Previous maps have shown large exposures ofundismpted Yr52) intruding the Salmon River and North Fork units westalkalic volcanic rocks in the Eastern Hayfork [e.g., Mortimer, of Yreka (Figure 2). The age of 140.5:1:1.6 Ma likely reflects1985], and disrupted chert-argillite units in the North Fork and crystallization of dike Yr52, but the disturbed Ar isotopicSalmon River units [e.g., Ando et al., 1983; Mortimer, 1985]. systematics of dike Yr45 suggest that 142.6:1:0.7 Ma is aWe prefer to include all intact alkalic volcanic rocks in the (probably small) underestimate of the crystallization age.North Fork unit, and all disrupted chert-argillite rocks in the These diabases are probably the same suite of undeformed butEastern Hayfork. This subdivision, based on lithologies, weakly metamorphosed dikes and sills in the southern andrather than on groupings of unlike rock types is necessary central Klamaths that Fahan [1982] and Wright and Fahanbecause there is no basis for distinguishing dismpted chert- [1988] dated by KlAr method on hornblende as ranging fromargillite in one area (associated with alkalic North Fork rocks, 134 to 141 Ma. They are also the same age as numeroussay) from chert-argillite in another area (associated with plutons in the Klamaths (Coon Mountain, Jacksonville Goldtholeiitic Salmon River rocks, for example). Hill, Sugar Pine, White Rock, Grants Pass) [Lanphere ~t al.,
LATE JURASSIC-EARLY CRETACEOUS DIKES 1968; ~otz, 1971; Saleeby et al., 1982]. . .A slightly older (149.3:1:1.7 Ma) metadlabase dike (sample
In an attempt to measure emption ages of the North Fork Y~). was.collected from the Salm~n River uni~ near the .and Salmon River units, we determined 40 Ar/39 Ar ages of Tnmty River. Ig?eous rocks of ~1S age are widespread m ~ehornblendes from nine mafic hypabyssal dikes/sills and one KlamathMountam~. Late JuraSSIC (149.:1:6 Ma; !<IAr on SIXgabbro (Figure 8; Table 2). The samples were selected because bu~k~rock samples) Immature-arc volcanic rocks Intrude thethey contain relatively fresh hornblende and yet appear to have Tnmty terrane [Brouxel et al., 1989], and plutons with 149:1:2been metamorphosed to the same facies as the host rocks Ma Pb/U ages intrude the Galice Formation [Ammon Ridgebecause they contain metamorphic quartz, albite, epidote, and Glenn Creek plutons; Wright and Fahan [1988]) andcalcite, and trace actinolite. All analyzed dikes have tholeiitic Preston Peak ophiolite (Bear Mountain intrusive complex;i:fulk compositions similar to the Salmon River suite, except Saleeby et al. [1982]).for Yr84, which is alkalic. Except for the gabbro, all the dated Four mafic dikes yielded mid-150-Ma ages similar to therocks are rapidly cooled diabases that were never Ashland pluton [concordant 155 Ma KlAr ages on hornblendemetamorphosed above greenschist-facies conditions (S4000C; and biotite; P. R. Renne personal communication, 1991].Hacker et al. [1992]); thus each age spectrum should be Sample Yr19 from the North Fork unit gave an age ofdominated by a plateau that reflects the time elapsed since 154.2:1:0.7 Ma, sample 448M intruding the Salmon River
" igneous crystallization. In general, the Ar/Ar spectra obtained terrane yielded an age of 155.5:1:0.7 Ma, sample Yr61 cuttingdo have simple plateaus. There are no indications of excess Ar the Eastern Hayfork terrane has an age of 153.1:1: 1.5 Ma, andin the release spectra, and isochrons do not reveal trapped sample Yr82 from the Stuart Fork terrane gave an age of
. nonatmospheric Ar. The exception is sample Yr45, which has 155.9:1:1.6 Ma. These samples are -30 and...6() km south ofan age spectrum suggesting -20% argon loss and for which the Ashland pluton, however, indicating that the -155-Ma-oldisochron analysis yields a nonatmospheric 40 Ar/36 Ar ratio. magmatism was widespread.
Most of the intrusive samples proved to be surprisingly A 163.8:1:1.5 Ma age was obtained for an alkalic dikeyoung in spite of the fact that they have metamorphic (sample Yr84) in the North Fork unit. This rock is similar inparageneses similar to the enclosing country rocks. The age to the widespread Wooley Creek plutonic suite (159-164broadly 3QO-Ma zircon date from Salmon River plagiogranite Ma; Barnes et al. [1986], Wright and Fahan [1988]) which[Ando et al., 1983], the -159-164 Ma age of the cross-cutting intrudes the Rattlesnake Creek, Western Hayfork, EasternWooley Creek calc'-a1ka1ine plutonic suite (Russian Peak and Hayfork, Salmon River, Stuart Fork, and CentralEnglish Peak plutons; Wright and Fahan [1988]), and the Metamorphic units.Permian to Jurassic ages of interbedded chert, led us to expect The only rocks believed to represent part of the Salmonages in the range 160-300 Ma. Three of the samples yielded River suite are 220M, a metadiabase that yielded an age of 173ages that predate the -161-Ma-old Wooley Creek granitoids and Ma, and Yr13, a gabbro dated as 200 Ma.thus are plausible emption ages. Most of the dikes, however,are younger than the Wooley Creek suite and range in age from DEFORMAnON156-140 Ma (Table 2). This has two significant implications.
1. The dikes share the same grade of metamorphism as Subhorizontal contraction of the Eastern Hayfork, Norththeir host rocks; hence the latter must have been relatively Fork, Salmon River, St Claire Creek, and Stuart Fork units
,.
404 Hacker et aI. Klamath Magmatic Arc/Accretionary Wedge
200 i" Yr52 Yr45
::: . :~ s
I:: ",,:",::::a~~~I~~5~!.6~(::: I5110; 0.5 5 (71%V9Ar)~ '; ;~95i S ~
so ;: 03
65;50:~ 0.1 0.2 .6 0.7 OB 0.9 IJJ . 0.2 03 OA 0.5 0.6 0.7 OB 0.9 IJJ
Cumulative fraction Ar released Cumulative fraction 39 Ar released
Yr2 Yr61 t !IS5 . age=149.3:tl.7Ma(I00%V9Ar) I - ,I I --isochron age =152.7 :t 0.9 Ma (83% V9Ar)
~I """", """"»»»»"".'"'~: .," "~ :1I 0.5 II 0.5
03S S65
3500JJ . 0.2 0.3 OA 0.5 0.6 0.7 OB 0.9 IJJ . 0.1 0.2 0.3 0.4 0.5
Cumulative fraction 39 Ar released Cumulative fraction Ar re ease
Yr19 448M1S
1 isochron age =154.2:!:O.7 Ma(I00'Yo V9Ar) . 555 07M (97'" '"39Ar-Isochronage=l . :t. a 7o,"~:.. "' """"" ~" "00 00'"I '"II OA I S
03
6503
500.0 0.1 0.2 03 OA 0.5 0.6 0.7 OB 0.9 IJJ ..1 0.2 03 OA 0.5 0.6 0.7 o.s 0.9 1.0
Cumulative fraction 39 Ar released Cumulative fraction 39 Ar released
Yr82 Yr84IS5
I
';' I ,., """""""""""""""""""""""""""""" ';'
6 6 isochron age =163.8:t1.5 Ma (8 ," " -~ on age =155.9:t 1.6 Ma(93 :i .3 i 0.5
g; g; ~< S < .'03
0.2
. 0.2 03 OA 0.5 0.6 0.7 OB 0.9 IJJ 500.0 0.1 0.2 03 OA 0.5 0.6 0.7 0.8 0.9 1:8.1Cumulative fraction 39 Ar released Cumulative fraction 39 Ar released
3
~~ 11i!!!!:=::=J' ~2S ~""";:':AMo""""M 03" "00 00'" I '"- '55 I 03 ~ 15
J: 11:.. S ~ I Os
65 iii~ti:~~~i~"
"""""500:~""~:i 0.2 03 OA 0.5 .6 0.7 0.8 0.9 li.1 50.0 0.1 0.2 03 0.4 0.5 0.6 0.7 OB 0.9 1:8.1
Cumulative fraction 39 Ar released Cumulative fraction 39 Ar released
Fig. 8. 40 Ar/39 Ar and K/Ca spectra of dated samples. One standard deviation uncertainties are shown.
,I
Hacker et aI. Klamath Magmatic Arc/Accretionary Wedge 405
occurred early in the time period 170-161 Ma (probably pre- alteration comes primarily from consideration of bulk-rockEnglish &ak pluton, Pb/U age of ~164 Ma; Barnes et al. compositions and oxygen isotopes. Substantial, relatively[1986], Wright and Fahan [1988], Harper et al. [1993]). low-temperawre (l00o-2OQOC) submarine fluid-rock interactionIsoclinal folds are evident in sedimentary rocks but poorly caused enrichment of ~180 to -11 %0 from probable basalticdeveloped in massive igneous rocks [Cashman, 1979; igneous values of -6%0. Bulk compositional shifts (e.g.,Charlton, 1979; Mortimer, 1985; Goodge, 1989a]. depletions in Al203, Na20, and K20 in mafic volcanic rocks,
Younger normal faults, including the Scott Valley, Browns and K20 + Rb addition and Sr depletion in sedimentary rocks)Meadow and Vesa Bluffs faults, expose different levels of the may have occurred during submarine metamorphism as wellNorth Fork-Salmon Riyer arc (Figure 1). Northeast of the [Hacker et al., 1992].Scott Valley fault, met'!ffiorphic grade is subgreenschist facies, Regional metamorphic assemblages in the North Fork-relict igneous texwres are well preserved, and "higher-level" Salmon River arc span upper greenschist to prehnite-actinoliterocks such as pillowed flows and volcaniclastic rocks and pumpellyite-actinolite facies. Estimates of physicalpredominate; south of the fault, metamorphic grade locally conditions during greenschist-facies metamorphism indicate
, reaches upper greenschist facies, relict igneous textures are moderate to low pressures (-300 MPa) at temperatures ofrare, and "medium-level" rocks including massive volcanic 200°-450°C [Hacker et al., 1992]. The Eastern Hayfork unitrocks and dikes/sills predominate. "Deep-level" rocks such as is inferred to have experienced the same metamorphic event asgabbro and ultramafic rocks are more abundant south of the the North Fork and Salmon River rocks. RegionalBrowns Meadow fault The Vega Bluffs fault exposes the metamorphic minerals in Eastern Hayfork volcanic rocks arestructurally deeper Rattlesnake Creek and Salmon River units dominantly albite + epidote + actinolite:!: pumpellyite,north of the fault, and structurally higher Eastern Hayfork and although prehnite and zeolite minerals occur locally (Table 4).North Fork units to the south. The northward decrease in Biotite is prevalent in sedimentary rocks in the northernstructural level north of Vega Bluffs fault suggests that another Klamaths but occurs only sporadically in the southerneast-west trending, south dipping normal fault may have been Klamaths (Table 4). Goodge and Renne [1991] reported apresent, and was intruded by the Ashland pluton. Except for Permian bulk-rock Ar/Ar age from the matrix of the Easternthe Vega Bluffs fault, which must predate the Vega Bluffs Hayfork which they inferred was a neocrystallization age butpluton (164:!:4 Ma KlAr hornblende; Lanphere et al. [1968]), cannot be representative of the unit as a whole, which containsyounger age constraints on high-angle normal faulting are not Late Triassic fossils.available. Later contact metamorphism at 200-290:1:50 MPa and at
Cox [1956], Ando [1979], and Ando et al. [1983] suggested peak temperatures approaching 600°C around mid-Jurassicthat the North Fork, Salmon River, and Eastern Hayfork units cross-cutting plutons produced 3-4-km wide aureoles typifieddescribe an antiform, based principally on the outcrop pattern by gradual, systematic increases in the pargasite content ofof lithologies in the Salmon River unit and the postulated amphiboles, muscovite content of K-white micas, andexistence of blueschists on both limbs of the antiform. anorthite content of plagioclases in the mafic volcanic rocks.However, both of these assumptions are questionable. The Granitoid intrusion also increased ~180 metabasalt values byoutcrop pattern of igneous rocks in the core of the alleged an additional 5%0 within 3-4 km of plutons, probably byantiform probably reflects the nonplanar nature of intrusive exchange with intercalated metasedimentary rocks [Hacker etcontacts and not folding. The blueschist unit on the west limb al., 1992].of the antiform within the Eastern Hayfork has been located in Barton and Hanson [1989] employed thermal models todifferent areas along the Salmon-Trinity divide in two maps demonstrate that regional low-pressure metamorphism only[Ando et al., 1977; Davis et al., 1980]. Both localities were occurs in areas with a large fraction of relatively evenlyinvestigated in reconnaissance by B. R. Hacker and J. W. distributed intrusions-otherwise sufficient heat is notGoodge in 1990; although blue slates crop out in both areas, available at shallow depths. It is possible that regional
" no sodic-amphibo1e-bearing rocks are present Moreover, metamorphism of the North Fork-Salmon River arc was
Cashman [1979] and Ando et al. [1977] stated that blueschist caused by heat advected to shallow crustal levels by plutonsfloat, only, has been recovered from Hotaling Creek draining such as those that caused the contact metamorphism. The. the Salmon-Trinity divide. Substantial outliers of the Stuart regional and contact metamorphism may thus be geneticallyFork terrane (up to > 10 km2) occur within the North Fork and linked and nearly coeval. Heat for both metamorphic eventsEastern Hayfork (this study; Figure 2), and blueschist blocks may have come from the volcanoplutonic edifice representedor olistoliths occur within the Eastern Hayfork near the Trinity by the North Fork-Salmon River mafic magmatic arc or fromRiver [Cox, 1956; Fahan, 1982], but we question the the Wooley Creek calc-alkaline plutonic suite. The wide rangeexistence of a mappable Eastern Hayfork blueschist unit west of dike crystallization ages from 175 to 141 Ma indicates thatof the North Fork-Salmon River arc. magmatism potentially capable of producing regional
metamorphism spanned Middle Jurassic to Early CretaceousMETAMORPHISM time.
Because of their proximity to heat sources and fluids, TECTONIC HISTORYvolcanoplutonic arc rocks may show metasomatic and texturaleffects of several thermal events following igneous Although Davis et al. [1978] cautioned against fittingcrystallization: submarine metamorphism, regional Klamath terranes into a coherent plate tectonic setting, shortlymetamorphism, and contact metamorphism. Documentation thereafter enough data had been collected to enable Burchfielof this recrystallization and attendant metasomatism from the and Davis [1981] to suggest that the Eastern Hayfork unitNorth Fork-Salmon River arc in the Sawyers Bar area was formed in an accretionary wedge outboard of the Jura-Triassicprovided by Hacker et al. [1992]. Evidence for submarine volcanoplutonic arc in the eastern Klamaths. Wright [1981]
,, I
406 Hacker et at. Klamath Magmatic Arc/Accretionary Wedge
TABLE 4. Regional Metamorphic Assemblages in Mafic Rocks- .~ ~
oct pig ab mu bi chi ep pmp prh zeo qtz sph caI Reference- - - . Eastern Hayfork Unit
.. ... . Burton [1982]... . Charlton [1979]. . . . . Fahan [1982]. .. Fahan [1982].. .. . Fahan [1982]. .. .. . . . Cashman [1979] and Wright [1982]
North Fork Unit. ... . Mortimer [1985]. . Mortimer [1985] 0; I
... . Mortimer [1985] c.. ... Ando [1979]. . Wright [1981] .Salmon River Unit.. .. Cox [1956] and Cox and Pratt [1973]. ... .. Fahan [1982]. ... Ando [1979].. .. Cox [1956] and Cox and Pratt [1973]. Fahan [1982]. . . . Ando [1979].. .. .. Mortimer [1985]. .. Hacker et aI. [1992].. .. Donato [1989] and Hacker et aI. [1992].. . .. Hacker et aI. [1992]
, ,
Mineral abbreviations, act; actinolite; pig, plagioclase; ab, albite; mu, muscovite; bi, biotite; chi, chlorite; ep,epidote; pmp, pumpellyite; prh, prehnite; zeo, zeolite; qtz, quartz; sph, sphene; cal, calcite.
added further the idea that the North Fork and St Claire Creek River unit and may be a pan of Western Hayfork or Northunits might represent fore-arc mafic igneous crust and Fork-Salmon River magmatism. During the interval 170-sediments between the arc and wedge. This tectonic setting 165 Ma all these units were telescoped together, coincidenthas been modified slightly to include the Stuart Fork terrane as with crystallization of 166-167 Ma old plutons and 166-170a somewhat older accretionary wedge [Goodge, 1990], but in Ma deformation in the eastern Klamaths [Renne and Scott.general, this picture has undergone little further development. 1988]. Intrusion of arc tholeiitic dikes/sills was widespread
Our interpretation, based on exposures in the central until at least 141 Ma.California Klamath Mountains is considerably different. Wefollow earlier views of the Eastern Hayfork unit as an CONCLUSIONS ~accretionary wedge, but suggest that the North Fork andSalmon River units together represent an immature magmatic We completed a map delineating the rock units in thearc. The Salmon River unit is considered to be a tholeiitic central Klamath Mountains of California. The map, which is f
volcanoplutonic complex, whereas the alkalic, mafic volcanic based on the work of many geologists, permitted (1) the 'rocks are interpreted as related arc extrusives of somewhat delineation of a north-south belt of rocks that crops out fromatypical chemistry. The St Claire Creek unit represents one end of the Klamaths to the other [Irwin, 1972; Mortimer,pelagic sediments, and intercalated alkalic rocks suggest it was 1984], and (2) the identification of key areas for petrological,deposited near an intraoceanic arc. structural, and geochronological investigation. Detailed
North Fork-Salmon River volcanism was active at the studies in several of these key areas has resulted in a modifiedsame time as the eastern Klamath arc, and an immediate geologic interpretation of the central Klamath Mountains.question is, What is the relationship between these two Jura- The Eastern Hayfork, North Fork, and Salmon River units,Triassic magmatic arcs? Prior to Triassic time, radiolarians in conjunction with the St Claire Creek unit, make up thewere deposited in the St Claire Creek chert and plagiogranite Sawyers Bar terrane [Ernst, 1990]. The Eastern Hayfork iscrystallized in the Salmon River unit (Figure 9). High- interpreted as an accretionary wedge developed beginningpressure metamorphism ended in now-exposed portions of the perhaps in Permian time, but nevertheless completed by theStuan Fork terrane at -219 Ma, and at the same time, Middle Jurassic [Wright, 1982]. The North Fork and Salmonradiolarians were deposited in the Eastern Hayfork and St River units represent an immature volcanoplutonic arc thatClaire Creek units. At 200 Ma, gabbro crystallized in the was active from Permian through at least Middle Jurassic time.Salmon River unit, coincident with radiolarian deposition in The St Claire Creek unit apparently was deposited in a settingthe Eastern Hayfork and St Claire Creek units. At 174-169 similar to the Eastern Hayfork but on the inboard (back-arc?)Ma, the Western Hayfork arc fonncd. plulDns of which inn-ude side of the Nonh Fork-Salmon River arc. Volcanic rocks ofthe Eastern Hayfork. A 175 Ma dike/sill intruded the Salmon the intraoceanic arc are interbedded with sedimentary rocks of
, "
Hacker et aI. Klamath Magmatic Arc/Accretionary Wedge 407
""-01 ofNorth fu.k- ""_01 of St
Sol...,. Rive. a ClockPRB-TlIASSIC ? IIIIIIIIII? IIIIIIIIII?
Eutom NorthPMk- Stasi..HayfMk Salmon River c...k Sluort rork eutcrn Klamaths
c. 219 MA '1III!l1Im~~~~-~-
Rulem North PMk- SI CI Hayfork So1nmn River c...k Sluort rork -lem KI ths
c. 200 MA ?tmrn ~~~~~~m~~ ;- ~~-~ -~"~ ' y ~. ""'"' c;>c;>
We'lern Eutom North PMk- St au..Hayfork HayfMk Salmon Rive. ClOCk Sluort ro.k
-174-169MA ? n:I:rftJ~~~~~1;~~~~~~~~!~~m, ,-~,yy ~","~~ ' y ' ""'"'
'"~'" «~ J'~~ "" ,
-17~16SMA ..~ !..~ ~ ' ~~ ~
~~r:::-I.+-" ~~~~~-s-~~},"I~-()~~~~~Fig. 9. Inferred diagrammatic tectonic history of the North Fork-Salmon River arc.
the outboard accretionary wedge, and hypabyssal rocks intrude by primary depositional and magmatic ties. These three units,the sedimentary as well as volcanic rocks. The western edge of along with the newly designated St Claire Creek unit,the terrane is defined by a Middle Jurassic fault that places the constitute the Sawyers Bar terrane, which is bounded by faults.Middle Jurassic Western Hayfork calc-alkaline arc beneath the Many unanswered questions remain, but there is at least oneSawyers Bar terrane [Wright, 1982]. The eastern edge of the especially important unresolved aspect of each unit TheSawyers Bar terrane is a disrupted subduction wne thrust Eastern Hayfork is homogeneous on a large scale, yetjuxtaposing it against the Late Triassic Stuart Fork subduction remarkably varied at smaller scales. The delineation ofcomplex [Hotz, 1977]. mappable subdivisions within the Eastern Hayfork would be a
Late normal faulting and differential tilting have exposed notable improvement and might help resolve whetherdifferent levels of the volcanoplutonic complex from formation of the accretionary wedge was a lengthy or short-ultramafic tectonite at the base, up through plutonic and lived event Crystallization ages of the North Fork andhypabyssal rocks, and culminating in interbedded volcanic and Salmon River units remain poorly constrained. Whether thesedimentary rocks. Deeper levels of the arc dominated by Salmon River unit is a Jurassic arc built upon older oceanictholeiitic plutonic and hypabyssal rocks of the Salmon River basement or an arc that was active from Permian to Jurassic
" unit are interlayered with and overlain by mafic, alkalic time may be resolved through more detailed geochronology.
volcanic and volcanogenic rocks of the North Fork unit. A Although the Eastern Hayfork sedimentary rocks appear to beprolonged but low-rank heating history left the arc with a interbedded with North Fork-Salmon River volcanic rocks on. mixture of submarine and regional metamorphic effects of the the basis of field mapping, radiolarian ages in the Easternlow-pressure, moderate-temperature variety, overprinted locally Hayfork predate most radiometric ages of the North Fork-by contact metamorphism. Salmon River immature arc. This is clearly an issue that
Tholeiitic plutons and dikes/sills that range in age from 201 requires resolution. Our interpretation of the North Fork unitto 141 Ma intrude the entire terrane, indicating that arc as part of an immature arc hangs on two factors: thevolcanism continued until the Cretaceous. Moreover, weak interfingering relationship with Salmon River rocks, themetamorphism of the hypabyssal rocks suggests that transitional V AB- WP A pyroxene chemistry, and whole-rocksignificant heat was retained regionally into Early Cretaceous chemistry. Refutation or confirmation of this and other ideastime. advanced in this paper awaits future study.
The terrane concept was born in the Klamath Mountainsand led to the naming of at least 12 terranes-all fault bounded Acknowledgments. Our synthesis relies heavily upon theby definition. The value of the concept lies in the recognition earlier work of other geologists. Thanks to Nick Mortimer forthat adjacent rock units need not have originated next to one permission to include unpublished trace-element data. Weanother, and this has prompted a reexamination of whether the have benefited from discussions with Cal Barnes, Bobcontacts between terranes are really faults. This study expands Coleman, Mary Donato, and from reviews by W. P. Irwin, M.the work of earlier geologists to show that three "terranes," the M. Miller, D. Miller, J. L. Mosenfelder and J. E. WrightEastern Hayfork. North Fork, and Salmon River, are not This work was supported by Department of Energy grants DE-separated from one another primarily by faults, but are linked FGO3-90ERI4154 and 8802-121.
. ,!
408 Hacker et al. Klamath Magmatic Arc/Accretionary Wedge
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