JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. B9, PAGES 14,551-14,568, AUGUST 10, 1991

The GravinaSequence' Remnants of a Mid-MesozoicOceanic Arc in Southern Southeast Alaska

CHARLESM. RUBIN1 AND JASONB. SALEEBY

Divisionof Geologicaland PlanetarySciences, California Institute of Technology,Pasadena

Fragmentsof UpperJurassic to Lower Cretaceousvolcanic and basinal strata constitute the Gravinabelt in southeastAlaska. In the Ketchikanarea the Gravina belt is made up of two lithotectonicunits. The lower unit consistsof coarsemarine pyroclasticand volcaniclasticstrata, mafic flows, breccia,and fine- grainedtuff which are locally intrudedby hypabyssalbodies of diorite and quartz diorite. The volcanic rocksare characterizedby tholeiitic arc basalts,lack felsic volcanicstrata, and overlie Upper Triassicand older strataof the Alexanderterrane. Augire and/orhomblende-bearing porphydtic rocks are commonand locally intrude the Alexander terrane basement,where they are thought to representthe intrusive equivalentsof lavas within the section. Age constraintsfor the volcanicunit, basedon structuraland stratigraphicrelations with adjacentunits, are late Middle to Late Jurassic.The Gravinabelt upperunit consistsof fine- to coarse-grainedturbidRes and relatedconglomeratic channel-fill deposits. The basinal rocks unconformablyoverlie Permianand Triassic rocks of the Taku terrane and remnantsof the lower volcanicpart of the Gravinasequence which overlie the Alexanderterrane. The conglomerateunits contain mostlyvolcanic and plutoniclithic clasts,some of whichyield Pb-U zirconages of 154-158Ma. The predominanceof pyroclasticdeposits interbedded with massiveflows, tuff, breccia,and argillaceous turbidires,and the lithologic and chemical compositionof the volcanic rocks indicate a submarine volcanicarc settingfor the Gravinasequence. The basinalpyroclastic rocks are inferredto have beenshed from submarinestratovolcanos during the Late Jurassic.Epiclastic rocks were depositedas submarinefans, derivedin partfrom erosionof a magmaticarc. The presenceof fine-grainedtuffaceous turbidires implies ongoing,but distant,volcanism. The pyroclasticand volcaniclasticrocks represent remnants of a Late Jurassicoceanic arc constructedon a compositebasement consisting of the Alexanderand Taku terranes. The strataaccumulated in an intra-arcbasin on the eastemedge of the Alexanderterrane. The volcanic and basinal rocks were deformedduring a major mid-Cretaceousintra-arc contractionalevent, in conjunction with the emplacementof a distinctlyyounger, arc-related plutonic suite.

INTRODUCTION adjacent to North America [see summary by Rubin et al., 1990a]. This compositeterrane was accretedto North America The paleotectonic setting and stratigraphic affinity of in Early to Middle Jurassictime [Monger et al., 1982] and thus variably deformedand metamorphosedUpper Jurassicto Lower formed the western margin of the North American continent Cretaceousmetavolcanic and metasedimentarystrata exposed during late Mesozoic time. for nearly 750 km along the easternedge of the Alexanderand Recent tectonic syntheses of Mesozoic paleogeography Wrangellia terranes(Figure 1) [Berg et al., 1972)] have been have been limited by uncertaintiesregarding the stratigraphic the subjectof much debate. Theserocks are called the Gravina- and structuralevolution of the Gravina belt [e.g., Monger et Nutzotin belt [Berg et al., 1972] and consist of interbedded al., 1982; Pavlis, 1982; Plafker et al., 1989a]. The initial marine basaltic breccia, flows, tuff, and turbidires. The stratigraphic and tectonic relations between the Insular volcanic belt unconformablyoverlies both the Alexander and composite terrane and North America are obscured by mid- Wrangellia terranes [Berg et al., 1972; Monger and Berg, Cretaceouspolyphase deformation and metamorphism;thus 1987], which collectively form the Insular compositeterrane the initial tectonicboundary is highly modified. For example, [Wheeler and McFeely, 1987; terrane II of Monger et al., the timing and style of accretion of the Insular composite 1982]. The Insular compositeterrane, consistingof juvenile, terrane to the western North American continental margin mantle-derived volcanic arc and rift assemblages[Samson et remains enigmatic. Gravina belt rocks play a critical role in al., 1989], had no palcogeographicrelation to North America addressing these tectonic questions, due to their unique until Mesozoic time [Monger et al., 1982; Saleeby, 1983; stratigraphicand paleogeographicposition along the eastern Gehrels and Saleeby, 1987a]. Rocks that lie to the east of the margin of the Insular compositeterrane. If Gravina belt strata Insular composite terrane belong to the Intermontane overlap both the Insular and Intermontanecomposite terranes, compositeterrane [Wheeler and McFeely, 1987; terrane I of then a pre-LateJurassic tie betweenthe Insularsuperterrane and Monger et al. 1982], consistingof lower Paleozoic continent- the western margin of North America is implied. One derived slope-and-rise deposits, upper Paleozoic to lower objective of this study of the Gravina belt is to provide a Mesozoic ensimatic arc assemblagesthat probably formed critical tie betweenthe Insular superterraneand westernNorth America. 1Alsoat Departmentof Geology,Stanford University, Stanford, This paper presents new stratigraphic, structural, California. geochronologic and geochemicaldata on the Gravina belt in southern southeast Alaska. Although rocks of the Gravina Copyright1991 by the AmericanGeophysical Union. sequencehave been metamorphosedto greenschistand lower Papernumber 91JB00591. amphibolite facies, locally original bedding features are 0148-0227/91/91JB-00591 $05.00 preserved.This study is based on detailed geologic mapping

14,551 14,552 RUBINAND $.AI•Y: GRAVINASEQUENCE

Area of Figure I

tI i

i

Insular Superterrane

• accretionaryCretaceousoceanic complexes

•] arcUpper & riftPz assemblages- lowerMz primitive

• islandLower arcPz, assemblageslower Mz primitive i• CoastPlutonic Complex Intermontane Superterrane

,[• arcUpper assemblagesPz- lower Mz

• accretionaryUpperPz- lower complexesPz oceanic

ß ß ]• basinalUpperPz assemblages- lowerMz marginal ß e •..:•Lower Pzbasinal assemblages ß

ß

ß

i

Pacific

Ocean ß

ß

ß

ß

ß

ß

I 150km I

t 150ktm•

Fig. 1. Location map of the Gravina sequencein the northwesternCordillera, showingregions and features referred to in text. (Adapted from Beikman [1980] and Monger and Berg [1987]). Abbreviations are BI, Banks Island; CP, Chilkat Peninsula; EL, Etolin Island; J, Juneau; K, Ketchikan; P, Petersburg;and QI, Queen Charloue Islands. Regional extent of the Gravina-Nutzotin belt shown in the upper right inset. CV is Chitna Valley. Location of major lithotectonicelements in the northernCordillera shownin lower right inset. QC-F is the QueenCharloue-Fairweather Fault System; TT-NRMT is the Tintina -Northern Rocky Mountain Trench System;Ytt is the Yukon-Tanana terrane; At is the Alexanderterrane; and Wr is Wrangellia. RUBIN AND S^I •J•.•Y: (]RAVINA SEQUENCE 14,553

.r-•lYounger Intrusive Rocks • UDDerPalaezoic &Lower Mesozoic A!ava Seouence MiddleCretaceous Intrusive Rocks • •. argillite,Metamorphosedmarble, &maficquartzite pillow flows, tuff &breccia, -• Tonalite,granodiorite, diorite,&gabbro I• Lower Palaezoic KahShakes Seauence "•ii• Devonianorthogneiss, lowerPaleozoic quartz-bearing ?• Zonedultramafic complexes / Gabbro I• metapelite,psammitic rocks,quartzitesilicic & marble metavolcanic rocks,amphibolite, UDDer Jurassic & Lower Cretaceous Gravina Seauence . _ ,• Metamorphosedgreywacke,argillite, Palaezoic&Lower Mesozoic Alexander Terrane conglomerate,& pillow flows, basalt-andesite& hypabyssal intrusivetuff, breccia rocks r• Triassic conglomerate, limestone, basalt, & rhyolite

ßß 132ø • breccia,Ordovician-Silurian pillowed flows,basaltic & hypabyssalandesite tuff, rocks ß •Ordovician-Siluriantonalite,diorite, &gabbro U B ß Siluriantrondhjemite & local diorite 52 45 Strike& dip of foliation

Geologic contact (dashed where inferred 45 ' & dottedwhere covered)

• Thrust(dashedFault where inferred •1•& dottedwhere covered) ß ß 57 High(dashedAngle whereFault inferred • & dotted where covered)

34

Union Bay ultramaficcomplex

Gc = Gnat Cove

ß • • • • Bl=BackIsland

' • • • • • AI = AnnetteIsland

GI = Gravina Island

GI = George Inlet

Cl = Carol Inlet

10 k m ß TA = ThorneArm 22

Fig. 2. Geologicmap of ClevelandPeninsula, Revillagigedo Islands and adjacentislands. Adapted from Berg [1972, 1973] (partsof Annetteand GravinaIslands], Gehrels and Saleeby[1987b] (partsof Annette, Duke, and Gravina islands], C. M. Rubin (unpublishedmapping, 1985, 1986, 1987; ClevelandPeninsula and adjacentislands); C. M. Rubin and J.B. Saleeby(unpublished mapping, 1986, 1987, 1988; Revillagigedo and adjacentIslands). along the shorelinesof Annette, Gravina, and Revillagigedo comparisonto age-correlativevolcanic sequenceselsewhere in islands, adjacent smaller islands, and Cleveland Peninsula the Cordillera, and providesnew constraintson mid-Mesozoic (Figure 2). This paper characterizesthe alepositionalsetting of paleogeographyand paleotectonicsetting of the northwestern the Gravina sequence, provides a reference section for Cordillera. 14,554 RUBIN AND SALEEBY:GRAVINA SEQUENCE

PREVIOUS WORK slightly deformed and are not highly metamorphosed,except near the easternedge of the terranewhere they are overprinted Rocks of the Gravina belt in southeast Alaska were by late Mesozoic deformation [Rubin andSaleeby, 1987, originally describedby Buddington and Chapin [1929]. Saleeby, 1987]. East of and apparently depositionally Studiesby Gabrielse and Wheeler [1961] and Brew et al., overlying the Alexander terranelies the mid-Upper Jurassicto [1966] have characterizedthe regional geologic relations and Lower Cretaceous Gravina belt. providedsome of the first tectonicsyntheses of this region. It was not until the early 1970s that workers recognized the Ta• Terrane continuation of these Upper Jurassic to Lower Cretaceous volcanicand sedimentaryrocks across the ChathamStrait fault Structurally overlying the Alexander terrane and Gravina belt is the Taku terrane. In the Ketchikan area the Taku terrane into the Nutzotin belt of Yukon Territory and the eastern Alaska Range. In their pioneeringstudy, Berg et al. [1972] has two major lithologic components:(1) the upper Paleozoic named and defined the Gravina-Nutzotin belt, described the and middle MesozoicAlava sequenceand (2) the mid-Paleozoic sedimentaryand volcanic rocks along strike, providedfossil and older Kah Shakessequence. The Alava sequencecontains data of the age of the sequence,and provided a coherent Pennsylvanian,Lower Permian, and middle Triassic shallow geologicframework to interpret the belt using plate tectonic water limestone interbedded with and overlain by mafic models. In southern southeastern Alaska, near the Ketchikan aphyric flows, pillow breccia,pyroclastic strata, and quartzite area, sedimentaryand volcanicrocks assignedto the Gravina- [Silberling et al., 1981; Rubin and Saleeby, 1991]. Based on Nutzotin belt were describedby Brooks, [1902], Wright and the similarities in lithologies and fossil ages, the Alava Wright [1908], Smith [1915], Chapin [1918], Buddingtonand sequence may represent a highly metamorphosed and Chapin [1929], Berg [1973], andBerg et al. [1988]. structurallyfragmented part of the Yukon-Tananaand Stikine terranes [Rubin and Saleeby, 1991]. Locally, channel-fill GEOLOGIC SEITING depositsof the Gravina sequenceunconformably overlie the Alava sequence and thus form an overlap between the The Gravina-Nutzotin belt consists of a distinctive Alexander terrane and the Alava sequence. The Kah Shakes lithotectonic assemblageextending from the eastern Alaska sequencelocally occupieshigher structurallevels and consists Range to southernmostAlaska (Figure 1). It forms a narrow of Dev:Jnianorthogneiss, silicic metavolcanicrocks, quartz- belt which, in general, separates the Alexander and Taku ric• metasediments, metabasalt, marble, calc-silicate, and terranes[Berg et al., 1972;Monger and Berg, 1987]. The Taku quartzite. The quartz-rich metasedimentaryrocks may be terrane consistsof polydeformed and metamorphosedmid to correlative with the lower Paleozoic portion of the Yukon- upper Paleozoic and Triassic strata that have recently been Tanana terrane [Gehrels et al., 1990; Gehrels et al., 1991; correlatedwith parts of the Intermontanesuperterrane [Rubin Rubin and Saleeby, 1991; McClelland et al., submitted and Saleeby, 1991]. Gravina strata appear to depositionally manuscript, 1991a] and record deposition on a slope and overlie the Alexander terrane on Gravina Island [Berg, 1973; continental margin setting. Primary relations between the C.M. Rubin, unpublished mapping, 1985, 1986], Annette Kah Shakesand Alava sequencesare uncertain;however, the Island [CM. Rubin, unpublishedmapping, 1985, 1986], and presenceof Alava crinoidal marble interlayeredwith quartzite on Kupreanoff Island [McClelland and Gehrels, 1990]; that is lithologically similar to quartzite in the Kah Shakes however, the contact is nowhere exposed at any of these sequencessuggest an early depositionaltie. Thus the Gravina localities. To the east, Gravina strata unconformablyoverlie sequencemay overlapboth the Alexanderand Taku terranes. metamorphosedand deformedrocks of the Permianand Triassic parts of the Taku terrane [Rubin and Saleeby, 1991; W.C. GRAVINA SEQUENCE McClelland et al., Structural and Geochronologicalrelations along the western margin of the Coast Mountains batholith: Definition and Distribution Stikine River to Cape Fanshaw, central southeasternAlaska, A revised stratigraphy is presented here for the submittedto Journal of Structural Geology, 1991, hereinafter southernmostpart of the Gravina-Nutzotin belt, where it is referred as McClelland et al., submittedmanuscript, 1991a]]. exposedon Annette, Gravina, and Revillagigedo islands, and To the north on the Chilkat Peninsula, Gravina strata ClevelandPeninsula (Figure 2). Our data suggestthat the ages unconformably overlie Upper Triassic strata of the northern and lithologies of the assemblageexposed in the Ketchikan Taku terrane of Monger and Berg [1987], interpreted as a area are atypical of the Gravina-Nutzotinbelt exposedto the displaced fragment of Wrangellia by Plafker et al. [1989b]. Gravina belt strata have not been identified within or east of north in the Juneau area and in the eastern Alaska Range. Because of these differences, we informally use the term the extensivecrystalline rocks that underlie much of the Coast Gravina sequenceto include all the Upper Jurassicto Lower Mountains (Coast Plutonic Complex of Douglas et al. [1970]. Cretaceousrocks in the Ketchikan-PrinceRupert region. The Alexander Terrane revised stratigraphy includes the least deformed and metamorphosedrocks of the Gravina Formationand rocks that In southern southeast Alaska the Alexander terrane forms were previously thought to constitutepart the Taku terrane by structural basement for most of the rocks that lie west of the Berg et al. [1988]. Coast Plutonic Complex, including rocks of the Gravina The Gravina sequenceconsists of marine pyroclastic and sequence (Figure 2). The Alexander terrane consists of a volcaniclastic strata, argillite, greywacke, and conglomerate structurally intact lower Paleozoic ensimatic arc sequence [Berg et al., 1988] and is intruded by plutons that range in overlain by middle Paleozoicclastic and carbonatestrata, and a composition from diorite to granodiorite. Locally, Alaska- Upper Triassic rift-related assemblage[Gehrels and Saleeby, type zoned ultramafic bodies intrude epiclastic rocks of the 1987a]. In most areas,rocks of the Alexanderterrane are only Gravina sequence.In the Ketchikanarea the Gravina sequence RUBIN AND SALm•.nY:GRAVINA SEQUENCE 14,555

Gravina Sequence INFERRED PROTOLITH •V---V---V •V--I -*-* -*- I Interlayeredargillaceous & tuffaceousturbidites

Siliciclastic turbidites

Argillaceousturbidites with cobble to pebble argillitechannel-fill deposits

ß• 154 - 158 Ma granitoidcobbles (U-Pb zircon)

Early Cretaceoushiatus ?

Aphyricmetabasalt flows, breccia, & tuff

I•,--.Y- v v v.•_•.W_..yv 4

• r•Q ...... porphyriticPillowflowsdikes &breccia, &sills: tuffintruded breccia, bytuff,massive cpx-hb • hypabyssaldiorite

Interlayeredargillaceous & tuffaceousturbidites & siliclasticturbidites intruded by variedtexture

I v v v-v I _ ••.=--___ BuchiaRugosa (Kimmeridgian: 153-156 Ma) Argillaceousturbidites with channel-fill deposits.

• ß .•[;-:,•--:•I•71ClastsUpperderived Triassicfrom & olderthe underlying strata of theAlexander Alexander rraneterrane:intruded by cpx-hbporphyritic sills & dikesof the GravinaSequence

Hypabyssaldorite • Aphyricbreccia, metabasalt& tuff flows,• Tuffbreccia [• Pillowedflows & breccia • Tuffaceousturbidites •--•channel-fillArgillaceous deposits turbidires & L• Pyroxene-hornblende porphyrydikes&sills .F• Siliciclasticturbidites !• Alexanderbasement terrane

Fig. 3. Generalizedstratigraphic section of the Gravinasequence on GravinaIsland (Figure 2). The baseof the sectionis in fault contactwith the underlyingAlexander terrane, and the top of the section is not seen in this area. On northwestern Annette and northern Gravina islands, similar strata unconformablyoverlie the Alexanderterrane.

consistsof two distinctiveunits: a lower dominantlyvolcanic sequence remains uncertain due to deformation and thrust unit and an upperdominantly epiclastic unit (Figure 3). The faulting. lower unit consistsof coarsemarine pyroclasticstrata, mafic Lower Unit flows, breccia,and fine-grainedtuff which are locally intruded by hypabyssalbodies of diorite and quartz diorite. Fine- to Stratigraphy. The lower unit of the Gravina sequenceis coarse-grainedturbidires and related channel-fill deposits characterized by massive, cliff-forming basaltic breccia, composethe upper unit of the Gravina sequence. This unit flows, and tuff on Cleveland Peninsula and Gravina Island. appearsto lie unconformablyupon the Alava sequencein some This unit extends from the south on Annette Island to Gravina places, in other areas it lies on the lower volcanic unit of the Island and ClevelandPeninsula in the north (Figure 2). The Gravina sequence.The stratigraphicthickness of the Gravina lower contactof this unit is not exposed,and at most localities 14,556 RUBIN AND SALEEBY:GRAVINA SEQUENCE the base of the Gravina sequenceis in fault contactwith the Pyroclasticdeposits and lava flows dominatethe upper part Alexanderterrane [Brew and Karl, 1987]. On northwesternof the lower unit (Figure 3). Pyroclastic deposits overlie Annette Island and northern Gravina Island, clasts that are argillite and occur in units generally less than 20 m in lithologicallyand petrographically identical to the underlying structuralthickness, but thicknessesvaries up to 500 m. The TriassicHyd Groupof the Alexanderterrane are incorporatedin coarse-grained pyroclastic flow breccias contain clasts as Gravinaargillite (Figure 4). The contactbetween the Gravina coarseas 40 cm that are angularto subangularin shape. These sequenceand the Alexander terrane on these islands is depositsare generally monolithologic,with large hornblende interpretedas a faultedunconformity, based on the presenceof and augire phenocrystspresent in both the matrix and clasts; thesecoarse detritus in the Gravinasequence. The top of the phenocrystsare up to 3 cm in diameter. Lithic fragments lowerunit is designatedat the baseof the overlyingmffaceous within the pyroclasticdeposits consist of homblende-augite and argillaceous turbidites and conglomerate. This phyric angular to subangularclasts in a phyric or aphyric stratigraphiccontact is exposedon Gravinaand Revillagigedo tuffaceous matrix. Subordinatepyroclastic deposits with islands and on Cleveland Peninsula. aphyric matrix and clasts are also present. The proportion of The lower unit has a structural thickness of-1300 m and is matrix is generally50%, and clast-supportedfabrics are rarely easilydistinguished from similarlithologies in the Alexander observed. Bedding is characteristically massive; however, terraneby the predominanceof relatively fresh hornblende tectonic deformation probably obscures original bedding and augitephenocrysts and the lack of near-pervasiveFe- features. A second category of volcanic deposits include carbonate alteration in the flows, breccia, and ruffs. The lower crystal-richtuff and mffaceousargillite. These rocks consist unit consistsof four lithologicsubdivisions: (1) mudstone, of euhedral amphibole and augite, and rare feldspar calcareousargillite, conglomerate,and carbonate;(2) water- phenocrystsin a pale green tuffaceousmatrix containing laid Coarsepyroclastic deposits and raft; (3) lava flows;and (4) augire, hornblende, epidote-clinozoisite,albite, quartz, and intrusiverocks (Figure 3). white mica. Typically the phenocrystsmake up 10-15% of the A distinctive silicic- and carbonate-clastconglomerate is rock. Locally, crystalsoccur in discretelayers or horizons.In presentat the base of the lower unit. The conglomerate places, dikes of hornblende-augiteporphyry crosscut the consistsof matrix- and locally grain-supportedpebble- to interlayered argillite. cobble-sized,angular to subangularclasts of meta-rhyolite, Pillowed and massivemafic to silicic flows make up the metabasalt,limestone and dolomite that were derived from the remaining part of the lower unit and are best exposedon underlying Alexander terrane. Conglomerate beds are southernCleveland Peninsula. Typically, the mafic phyric lenticularin shapeand are interbeddedwith planar laminated flows are massive and are locally interlayered with black argilliteand siltstone. The conglomerateis overlainby thinly argillite. Hornblendeand/or augire-bearingporphyries intrude laminated, black to grey argillite, siltstone, and shale. parts of the lower unit argillite and mffaceousargillite. The Siliciclastic rocks display centimeter-sizedfining-upward phenocrystsand matrix of these dikes are compositionally sequencesof siltstone-mudstonecouplets. similar to the phyric flows and mff which are common within

Fig. 4. Deformedbasal conglomerateof the lower memberof the Gmvina sequenceon northwestem AnnetteIsland (Figure 2), with clastsfrom the underlyingTriassic Hyd Groupof the Alexanderterrane. RUBIN AND SALEEBY:GRAVINA SEQUENCE 14,557 the lower unit. On the northwest shore of Annette Island, accordingto the conventionssuggested by Thompson [1982], massive augite porphyry intrudes banded carbonaceous and Pearce [1982]. limestone, laminated felsic tuff, and limestone of the These discrimination diagrams suggestthat most of the Alexander terrane. Gravina sequencelavas and dikes closely resemblemodem Plutonic rocks. Structurally concordanttabular bodies of island arc tholeiites(IAT) (Figures5a and 5b). Ti/Zr ratios of hypabyssal diorite intrude lower unit Gravina rocks. The the Gravina volcanics fall within the arc tholeiite field; the diorite is texturally diverse and displays relict porphyritic, only exception is sample 85CR206, which falls in the ophitic,and granulartextures. The contactbetween the diorite continentalarc basaltfield. On a Cr/Y plot (Figure5b) the data and enclosing volcanic rocks is compositionally and plot either in the low Y part of the arc tholeiite field or in the texturallygradational and lacks a well-definedcontact aureole. high Y part of the MORB field. Ti/Zr ratiosare low and plot Attempts to extract zircon for U-Pb isotopic analyses from within the IAT and continental arc basalt field or arc tholeiite- varied textured diorite were unsuccessful. MORB overlapfields; however, these low TiO2 andZr values Age. The age of the lower unit is poorly constrainedand is are not typically seen in MORB [Pearce and Cann, 1973]. basedupon fossil agesreported by Chapin [1918], Berg et al. When Nb/Y is plotted againstZr/TiO 2, all of the valuesfall [1972], and Berg [1973]. Poorly preservedBuchia rugosa is within the andesite/basaltand overlap with the subalkaline present in argillite on southernGravina Island, indicating a basalt fields. Typically, these sampleshave low Nb/Y and Late Jurassic age (Kimmeridgian) (Figure 3). Abundant Zrfrio 2 ratios.REE and incompatiblelxace element data show belemnites(Cylindroteuthis)'which range in age from Middle typical patterns for basalts erupted in a oceanic island arc to Late Jurassic[Berg, 1973] occuron Blank Inlet on southern setting (Figure 6). These analysesindicated that they are Gravina Island. Large (up to 20 cm in diameter)pelecypods generally similar to the average island arc tholeiite basalt (Entolium), which range in age from Middle Triassic to Early [Pearce, 1982, 1983; Thompson, 1982] and are productsof Cretaceous,are alsopresent from Blank Inlet and Blank Island, subduction-relatedmagmatism. The Gravina volcanic rocks [Berg, 1973]. The lower age limit is post-UpperTriassic, the have low REE abundancesand minor LREE (light rare earth age of the youngest fossils in the underlying Alexander elements) enrichment (Figure 6c), which is characteristicof terrane. In summary,available fossil age data indicate a Late modem arc tholeiite suites [Gill, 1981]. Low TiO2 and Cr Jurassicage for at least part of the lower unit of the Gravina contentsare consistentwith this interpretation. sequence[Berg, 1973;Berg and Cruz, 1982]. Trace elementsalso provide information on both the source Geochemistry. A critical questionconcerning the Gravina and subsequentdifferentiation history of magmas. There is a sequencevolcanics is the identificationof their paleotectonic characteristic trace element signature for volcanic rocks environment. For mafic volcanic rocks which have undergone erupted at convergent plate margins; in particular, source low grade metamorphism,two useful methods classify and regionsare typically enrichedrelative to REE in both K group discriminatemagmas erupted in different tectonicsettings: (1) and Th group trace elementsand depletedin Ti group trace concentrations of trace elements such as Ti, Zr, Y, Cr, and Nb elements [Gill, 1981]. Ratios of highly incompatibletrace and (2) concentrationsof rare earth elements (REE) in the elements reflect source composition because they do not lavas anddikes [Pearceand Cann, 1973; Winchesterand Floyd, changeduring melting or fractionation. The Gravina sequence 1973; Garcia, 1978; Gill, 1981; Pearce, 1982, 1983; Pearce et mafic lavas are enriched relative to MORB in Ba, St, K, Rb, al., 1984; Thompson, 1982]. The criteria used to distinguish and Th (Figure 6); these patterns may be attributed to a tectonic affinities of magmas are derived from empirically subductedsedimentary component [e.g., Kay, 1980]; however, determined values from a variety of modern plate tectonic this enrichment can also record local variations of arc settings[Pearce, 1982, 1983; Pearce and Norry, 1979]. Trace basement [Moorbath and Hildreth, 1988]. Alternately, elements that are relatively immobile during seafloor seafloor and subsequentregional metamorphism may have alteration and greenschist facies metamorphism are used affected these values. A subducted-slab source for this [Cann,1970; Humphris and Thompson,1978]. Rareearth, enrichmentis unlikely, becauseof the absenceof negative Eu major,and traceelement analyses of lavasand dikes from the and Ce anomalies. The lack of Eu and Ce anomaliesis thought Gravinasequence are givenin Tables1 and2, and the dataare to reflect the relative absenceof a sedimentarycontribution to plottedon both traceelement discrimination diagrams (Figure the magma source [McLennan and Taylor, 1981; White and 5) and on a rare elementdiscrimination diagrams (Figure 6), Duprd, 1986]. The mafic rocks are enrichedin LIL (large ion

TABLE 1. Majorand Minor Element Analyses of GravinaSequence Volcanics

Sample

84JRa 84JRb 85CR104a 85CR206 85CR220a 85CR221 85CR227 87CR31 87CR38 Si02 49.40 52.10 47.90 46.90 48.70 48.44 49.40 47.90 47.60 A1203 16.90 17.70 16.50 18.20 10.60 18.50 18.50 12.50 13.20 Fe203 8.85 8.59 11.10 11.80 9.73 10.60 9.41 14.10 11.20 MõO 6.39 5.88 6.55 5.92 12.10 4.85 5.08 8.91 10.30 CaO 9.49 9.03 12.90 3.86 11.40 10.50 12.00 11.00 9.81 K20 0.66 0.91 0.83 0.51 0.61 0.86 1.20 0.24 0.15 Na20 3.19 3.40 1.73 4.80 2.91 2.91 2.00 1.29 3.12 TiO2 0.53 0.50 0.69 1.15 0.65 0.56 0.60 1.00 0.15 P205 0.09 0.09 0.11 0.49 0.21 0.12 0.12 0.28 0.83 MnO 0.19 0.19 0.21 0.19 0.15 0.19 0.15 0.19 0.28 All analysesdetermined by EnergyDispersive-X-Ray Diffraction; analysts: J. Taggan,A. Bartel,and D. Siems. 14,558 RUBIN AND S•Y: GRAVINASEQUENCE

Aw low-K islandarc tholeiitemagmas [Gill, 1981]. High Ba/La and Ba/Ta ratios, moderate La/Ta ratios, and moderate LIL elementscontents (relative to high field strengthand LRE elements)are alsocharacteristic of the Gravinasequence marie Ti (ppm) ß rocks(Table 2). Thesevalues suggest that the Gravinabasaltic 1000 Arc thole magmaswere controlledby the underlyingmantle wedge and 10-- locally influencedby its ensimaticarc basement. Based upon trace element abundancesfor Cr, Zr, Ti, multi elementand REE patterns,and combinedwith the presenceof 5 Cakc-alkaline•• significant amounts of pyroelastic volcanic rocks and basalt• interbedded turbidites, we interpret the lower unit Gravina sequencevolcanics as products of a oceanic volcanic arc I I I I complex. This interpretationis consistentwith the original 50 100 150 200 suggestionof Berg et al. [1972] that the Gravina-Nutzotinbelt Zr (ppm) formed in a magmaticarc setting. These compositionsand geochemical variations are similar to immature island arcs of a. the western Pacific [Gill, 1981]. Depositional setting. The base of the lower unit of the 1000 - Gravina sequencecontains pebble to cobble conglomerates thoughtto havebeen derivedfrom theunderlying Triassic Hyd Group. Based upon the presenceof laterally continuous, planar-laminated,normally graded sequencesof argillite, Or calcareousargillite, and siltstone, and the lenticular beds of (ppm) • MORB the predominantly matrix-supported conglomerate, the 100 - lowermost part of the Gravina sequenceis interpreted as _ __ Volcanic arc submarine turbiditc deposits. Lenses of coarse-grained basalt conglomerate represent associated channel-fill deposits. .... W/thin-plate Proximityto the Alexanderterrane is suggestedby two linesof I .I, basalt evidence:(1) the coarse-graineddetritus in the turbiditeswas % '1' derived from the underlyingAlexander terrane and (2) lower

10 ! unit hornblende-augiteporphyry intrudes Upper Triassicrocks I I ! of the Alexanderterrane on AnnetteIsland. Hornblende-augite 5 10 50 100 Y porphyry dikes represent the subvolcanic feeder to the (ppm) overlying volcanic rocks. Large volumes of marie to intermediate volcanic debris C. overlie the argillaceous turbidites. These volcanic rocks contain mostly coarse-grainedpyroelastic deposits, phyric tuff, and massiveflows. Monolithologicblocks in tuff breccia

0.10 - and blocky flows indicatelittle or no mixing of volcanogenie Rhyodacite debris. The compositionallyuniform crystal-rich mafic tuff matrix suggests that the deposits are a direct result of Zr (ppm) explosive eruptions. Locally, pillowed and massive flows TiO2 Andesite overlie and are intercalated with argillaceous turbidites.

0.01- Andesite / Deeperstructural levels are representedby the texturallyvaried basalt diorite. The diorite is compositionallyand texturallysimilar to the volcanic rocks, displays the same structural and Sub-alkaline metamorphicfeatures, and lacks a well-defined contactaureole, basalt substantiatingthe suggestionof Berg [1972] that the diorite and metavolcanic rocks are co genetic. The intrusion of I I 0.01 0.10 1'.0 100 hypabyssal diorite into the pyroelastic aprons probably Nb/Y caused sediment failure that resulted in volcanic slides, similar (ppm) to geologic relations observed in Jurassic volcaniclastic sedimentaryrocks on CedrosIsland [Busby-Spera,1988]. Figure5. Trace-elementdiscrimination diagrams showing data from Sedimentary and igneous structures of the pyroelastic Gravinasequence metabasalts. A) Ti/Zr discriminationdiagram of deposits and mafic lava flows indicate that both distal and Pearceand Cann [1973]; B) Cr/Y discriminationdiagram of Pearce [1982]; Zr/TiO2 vs Nb/Y discriminationdiagram of Winchesterand proximal volcanicfacies are present. Proximal lava flows are Floyd [1977]. pillowed and unbroken,whereas distal positionsare dominated by pillow breccia. The proximal pyroelasticdeposits contain large, juvenile monolithologic blocks; accidental fragments lithophile)elements relative to MORB and have high Ba/Ta are uncommon. Fine-grained crystal-rich tuff, lapilli-sized ratios (>24, except 87CR206) which indicate excess LIL to lithie tuff, and tuffaceous turbidites are more common in distal light rareearth (LRE) elementsrelative to MORB. High Ba/Ta areas. Northern and western source areas for the volcaniclastic ratios are commonin volcanicarcs and are representativeof apronsare supportedby stratigraphicand sedimentologicdata. RUBINAND SALEEBY:GRAVINA SEQUENCE 14,559

TABLE 2. TraceElement Analyses of GravinaSequence Volcarries

S•nple

84JRa 84 JRb 85CR 104a 85CR206 85CR220a 85CR221 85CR227

Ba 214.0 255.0 314.0 316.0 201.0 208.0 234.0 Rb 10.1 16.5 13.2 16.3 7.62 15.9 11.6 'Ih 0.7 85 0.771 0.57 3.71 0.96 0.97 0.68 K 547 8.0 7554.0 6891.0 4234.0 5064.0 7143.0 9962.0 Nb <10.0 <10.0 <10.0 <10.0 <10.0 <10.0 <10.0 Ta 0.201 0.203 0.0859 0.494 0.163 0.14 0.126 La 4.94 4.88 4.62 20.90 8.50 6.16 5.08 Lu 0.271 0.269 0.222 0.317 0.187 0.258 0.225 Ce 10.4 10.63 10.1 46.63 19.9 12.5 9.86 Fm 0.652 0.707 0.751 1.77 0.970 0.704 0.625 Sr 500.0 480.0 370.0 620.0 300.0 490.0 350.0 Nd 6.5 7.18 6.84 26.7 12.69 7.94 7.52 Sn 1.93 2.00 2.18 6.52 3.17 2.14 1.94 Ni 50.7 47.5 28.1 31.5 116 26 20.0 Cr 209 124 50.8 30.1 622 14.7 14.3 P 393.0 393.0 480.0 2138.0 916.0 524.0 524.0 Hf 1.09 1.08 0.805 2.54 1.14 1.16 0.961 Ti 3177.0 2998.0 4136.6.0 6894.0 3897.0 3357.0 3597.0 U 0.302 0.385 0.271 1.49 0.427 0.557 0.343 Y 12.0 12.0 10.0 20.0 12.0 12.0 10.0 Yb 1.77 1.81 1.62 2.14 1.14 1.69 1.46 Zr 55.0 55.0 40.0 110.0 55.0 60.0 45.0

Ba/La 43 52 68 15 24 34 46 BafFa 1065 4256 3655 640 1233 1486 1857 Ce/Yb 5.9 5.9 6.2 21.8 17.4 7.3 6.7 Hf/Lu 4.0 4.0 3.6 8.0 6.0 4.5 4.3 Hf/Ta 5.4 4.5 9.3 5.1 7.0 8.3 7.6 IMSm 2.5 2.4 2.1 3.2 2.7 2.9 2.6 La/Ta 24 24 54 43 52 44 40

Minor and rareearth elements abundances are given in partsper million.Rb, Nb, Sr, Y, andZr weredetermined by EnergyDispersive, X-Ray Fluorescence;other minor elements by NeutronActivation. Analysts:D. Fey,J. Budahn,T. Frost(trace elements) and S. Mac Pherson(wet chemistry).

The thicknessand overall grain size of mafic pyroclastic area, this unit extends from the south on Annette Island to depositsdecrease to the south and east. There are no known Cleveland Peninsula in the north (Figure 2). The best coarse-grainedpyroclastic rocks on the eastern and southern exposures are on southern Cleveland Peninsula, southwestern portions of the Gravina basin in the Ketchikan area. Vent RevillagigedoIsland and adjacentsmaller islands. The lower plugs, representedby hypabyssaldiorite of various textures, contact of this unit is not exposed; however, locally it is are present on the northern and western margin, which inferred to unconformably overlie both the lower volcanic supports the idea of a northern and western source for the unit of the Gravina sequenceand the Permian and Triassic volcaniclasticsedimentary rocks. The widespreadoccurrence Alava sequence (Figure 4). Cobble conglomerate locally of mafic flows indicatesthat fissuresmust have been present unconformablyoverlies Alava strata;no evidencefor faulting throughoutthe basin. is present. The top of the section is not exposed and the A simple progradational depositional sequence emerges highest observed levels are in fault contact with adjacent from facies relations within the lower unit. The basal unit was getfanes. deposited as turbiditic flows with channel-fill sequences The upper unit has an approximatestructural thicknessof containing clasts derived from the underlying Alexander 900 m. The unit is readily distinguishedfrom similar rock terrane. The turbidites are overlain by thick, submarine types in both the lower unit of the Gravina sequenceand the pyroclastic debris and lava flows. The predominance of Alava sequenceby the predominanceof argillaceousturbidites, pyroclastic depositsinterbedded with massive flows and tuff the presence of interbedded pebble to cobble conglomerate and argillaceous turbidites, together with the presence of interstratifiedwith the turbidite deposits,and the lack of mafic cogeneticintrusives and trace elementgeochemistry, indicates metavolcanic rocks. Sedimentary structures are generally a basinalvolcanic arc settingfor the lower part of the Gravina better preservedhere than in the lower unit. sequence. These basinal pyroclasticdeposits were shed from Lithologic units in the upper unit can be divided into two the flanks of submarinestratovolcanos during Late Jurassic groupsor facies: (1) argillaceousand tuffaceousturbidites and time. Hypabyssal intrusive rocks represent the intrusive (2) pebble to cobbleconglomerate. The argillaceousstrata are equivalentsof the lavas recognizedwithin the section. characterizedby very fine to fine-grained clastic beds which contain mostly dark grey, argillaceous siltstone, mudstone, and minor feldspathicsandstone and silty limestone. Sparse Upper Unit granulesand pebblesare presentwithin the argillaceouslayers Stratigraphy. The upper unit consistsof poorly exposed and are composedof very -fine grained felsic igneous clasts. siltstone,argillite, tuff, slate, and conglomerate. In the study Bedding is generally medium to thick (20-80 cm); however, 14,560 RUBIN AND SALEEBY:GRAVINA SEQUENCE

A recognized. The tuffaceousstrata consistof very fine to fine- grained, light gray to tan beds with very small crystals of feldspar and mafic minerals in a white mica-epidote-feldspar matrix. Pebble and cobble conglomerate is interbedded with the argillaceousturbidites (Figure 8). Conglomeratebeds occur in a distinctive mappable horizon up to 2 m thick exposed between southeastern Revillagigedo Island and southern •10 Cleveland Peninsulaand consistingof beds 0.5 to 2 m thick. Beds are commonly lenticular on an outcrop scale. The • 5 conglomerateoverlies finely laminated argillite and siltstone and grades stratigraphically upwards into grey argillite that contains fewer coarse clasts (Figure 7). The conglomerate- bearing horizon also containsthin layers of grey phyllite and siltstone with sparse granules of leucocratic fragments and 1 I I I i i i i i i i i i i i i i i i I argillite rip-up clasts. The conglomerateis matrix-supported, BaRbTh K NbTaLaCeSrNd PSmZrHf TiTb YTmYb and clastsare usually less than 18 cm in diameter,averaging 3- 8 cm. The matrix is argillaceous. On Revillagigedo and adjacentislands, boulders up to 40 cm in diameteroccur. The B clastsare mostly flattened as the result of youngerdeformation 100 -• _

-- (Figure 8) and consistof fine- to coarse-grainedgranodiorite,

_

-- quartz diorite and leucocratic diorite, volcanic porphyry,

-- argillite, and minor marble and vein quartz material. The

10-• matrix is fine-grained argillite containing quartz, albite, _ _ biotite, epidote,calcite, and white mica. _ _ Provename and U-Pb isotopicdata. The rock typesof clasts n' - o in the pebble to cobble conglomerateswere counted in the field (Table 3) and have been plotted according to major 8 - n' - constituentson ternary diagrams(Figure 9). The conglomerate units contain mostly volcanic and plutonic lithic clasts, with relatively few sedimentarylithic clasts. Mixing of these two rock types within the conglomeratesuggests that they were derived from a composite volcanic-plutonic source. The presence of plutonic and lesser volcanic clasts suggests 0.01 I I I I I I I I I I I I I I I I I I deposition near an uplifted region comprised of both its $r K RbBaTh TaNbCe P Zr HfSmTi Y Yb$cCr Ni volcanic cover and plutonic substrate. Ages of granodiorite and quartz diorite clasts within the c o 84Jra pebble to cobbleconglomerate have been determinedusing U- ß 84Jrb 100 -- Pb zircon methods(Tables 4 and 5). Zircon separateswere n 85CR104a preparedby standardmineral separationtechniques. Isotopic • []85CR206 data were determinedon a 35 cm VG 90ø extendedgeometry -• ,, 85CR220a sector multicollector and on a 30.48 cm 60 ø Lunatic IV mass •"'•-.-...•. '""1•_ "85CR2217spectrometer at Caltech. Lead was loaded with H3PO4 acid- silica gel [Cameronet al., 1969], and uraniumwas loadedwith H3PO 4 acid and graphite. Uranium and lead concentrations .8o were determined on solution aliquots from each sample by isotopedilution using a mixed205pb-230Th-235U spike. o 5 o - Seven fractions from four granodiorite and quartz diorite clastsfrom Gnat Cove and Back Island were analyzed(Tables 4 and 5). The zircon isotopic systematicsof the Gnat Cove samples(84JR28 C-1 and C-3; Table 5) are relatively simple and are interpreted as crystallization ages. These samples I I I I ! I I I I I I I I I I yield concordantages of 154 Ma and 158 Ma (Figure10). The La Ce Pr Nd PmSm Eu Gd Tb Dy Ho Er Tm Yb Lu zircon isotopicdata for clast C-3 are slightly discordant.The Fig. 6. Normalized incompatibleelement patternsfor the Gravina igneousage interpretationfor this sampleis highly dependent sequencemetabasalts. a) Chondrite-normalizedplot after Thompson on the discordancemechanism chosen to explain the data. The [1982], b) MORB-normalizedplot after Pearce [1983], c) Chondrite- normalizedREE plot. All data normalizedafter Nakamura [1974]. limited zircon yield from the clast prohibit an in-depth Multi elementplots generated by a computerprogram after Wheatley analysisof this question. Broad age boundsare assignedas and Rock [1988]. 172 to 154 Ma (Figure 10), based on possible minor disturbanceat a level which would permit the >62gm fraction massivebeds are up to 2 m thick are presentlocally. Argillite to retain its internal concordanceor, alternatively, minor rip-up clasts are common within the sandy layers. Graded inheritancewhich would dispersethe >62 gm fraction off of bedding,plane-laminated sandy layers, and amalgamatedbeds concordia from the internally concordantfraction. Thus we are presentlocally (Figure 7). Other bedformstructures are not interpret the age as 154 Ma. The igneous clast from Back RUBINAND SAL•EBY: GRAVINA SBQUENCE 14,$61

Inferred Protolith

Interlayered sandstone & argillite

Massive amalgamated sandstone & cobble to pebble sandstone with argillite rip-up clasts

Pebble to granule argillite & felsic tuff

Interlayered sandstone & argillite

Pb-U zircon ages of 154- 158 Ma for granitoid cobbles

Cobble to pebble sandstone & argillite with tuffaceous sandstone

Interlayeredsandstone & argillite

BACK ISLAND SECTION

Fig. 7. Measuredstructural-stratigraphic section of part of the uppermember of the Gravinasequence on Back Island.

Island (Figure2) yields an internallyconcordant age of 157 Ma 1982];however, intrusive bodies of similarage have yet to be (Figure 10). The closeagreement between the U-Pb agesof all found in the lower unit or its substrate.Despite the lack of four samples indicates that they represent Late Jurassic dated intrusive rocks in the lower unit, the lower unit does crystallizationages for the clastsand suggestsa homogeneous contain rock types compositionallysimilar to some of the or localized source terrane. Clasts from localities that occur plutonicclasts that have been interpreted as cogeneticwith the over 35 km along strike have common lithologic and U-Pb volcanic section, as discussed above. Furthermore, since zircon systematics. Gravina overlap strata and its basementwere structurally Several possible origins for the coarse plutonic detritus imbricatedduring mid-Cretaceous time, much of the arc is now exist. The clastswere previouslythought to have beenderived concealed beneath thrust sheets. from early Paleozoic plutons that are part of the Alexander Another possibility is that the granitic debris may have terrane[Berg et al., 1988], but this possibilityis ruled out by beenderived from a displacedarc terranethat hasbeen removed JurassicU-Pb age data. A likely possibilityis that the coarse- by strike-slipfaulting. It haslong been recognized that major graineddetritus was derivedfrom an upliftedand dissectedpart strike-slip faulting has affected the northwesternCordillera of the Gravina arc and its underlyingbasement complex. The [Gabrielse, 1985; Oldow et al., 1989]. Both southern and ages of the clasts are similar to those of Late Jurassicfossils northern source terranes exist which may have provided foundin the lower unit of the Gravinasequence [Harland et al., detritus for the upper unit conglomerates.To the south, in 14,562 RUBIN AND SALEEBY:(]RAVlNA SEQUENCE

Fig. 8. Highly deformedchannel-fill conglomerate of the uppermember of the Gravina sequenceon Betton Island. Plutonit clastsin the conglomerateyield U-Pb zircon ages of 154 - 158 Ma. The largestclast in the photographis approximately45 cm in diameter;clasts at Gnat Cove and Back Island are locally over 60 cm in diameter. northwesternBritish Columbia, Late Jurassicigneous activity and are part of the Tonsina-Chichagofbelt of Hudson [1983] has been recognized recently within the Coast Plutonic and the Chilkat-Chichagof belt of Brew and Morrell [1983]. Complex [Armstrong, 1988; Van der Heyden, 1989a]. The belt consistsof foliated hornblendequartz diorite, tonalite Although many of these plutons display internal discordance, and granodiorite. Age relationsfor much of the belt are poorly zircon from quartz diorite and granodiorite plutons yield known; however, hornblende K/Ar cooling ages for these crystallizationages from 154 Ma to 158 Ma [Van der Heyden, plutons range between 143 Ma and 170 Ma, which suggestsa 1989b]. For this Late Jurassicplutonic suite to have provided regionally extensive Late Jurassic magmatic belt in the clastic detritus to the upper unit of the Gravina sequence, southwestern Yukon and southeastern Alaska [Dodds and 150 km of post-Late Jurassicsinistral strike-slip displacement Campbell, 1988; Grantz et al., 1966; Loney et al., 1973; Karl is requiredalong inferred faults within the westernboundary of et al., 1987]. U-Pb zircon ages from tonalite in the Chitina the Gravina sequence.Strike-slip faults and associatedfabrics Valley region, are 153 + 4 Ma [Plafker et al., 1989a]. Plafker of this age in southeastern Alaska and western British et al. [1989a] cite evidence for offset of the early Mesozoic Columbia are lacking, and this hypothesisis difficult to test Talkeetna arc, truncation of the Chitina Valley plutons, and since critical contacts might lie underwater beneath Dixon large-scale sub horizontal ductile shear along the truncated Entranceor within the youngerCoast Plutonic Complex on the margin of Wrangellia as compatiblewith Late Jurassicto Early mainland. Cretaceous sinistral strike-slip motion of 600-1000 km in Alternatively, Late Jurassic metaplutonic rocks in the southeastern Alaska. A change in relative plate motions eastern Chugach and St. Elias mountains, exposed to the between the North American and Farallon plates [May and north, may represent the source terrane for the upper unit Butler, 1986] coincides with the initiation of sinistral strike- igneous clasts. These metaplutonic rocks extend slip faulting. The inferred fault contact in southeastAlaska southeastwardtoward ChichagofIsland in southeasternAlaska lies between Wrangellia and the Alexander terrane (Figure 1).

TABLE 3. Pebbleand cobble counts, upper member of the Gravinasequence, southern southeastern Alaska

Occurrence(Percent of clastsrandomly encountered)

AreaSampled N Li Lv Lvi Ls Lq Lsu

Cove 102 98 - 98 2 - 2 South Hume Island 102 96 - 96 2 2 2 Betton Island 269 63 27 90 7.3 2 7.3 Back Island 102 47 34 81 16 3 16 Gin Point 105 87 - 87 11 2 11 Guckers Cabin 109 60 - 60 34 6 34

N is numberof clastscounted, Li is lithic plutonicl;Lv is lithic volcanic;Lvi is total of Li andLv; Ls is argilliteand marble;Lq is quartziteand quartzvein; Lsu is undffferentiatedsedimentary rocks. RUBIN AND SALEEBY:GRAVINA SEQUENCL 14,:563

Li

Liv

+1 -H +1 +J. -H +1 +1

Lv Lsu Lq Fig.9. Compositionof pebble and cobble populationsfrom conglomeratewithin the uppermember of the Gravinasequence, Lsu, lithic sedimentaryundifferentiated; Liv, lithic volcanicand plutonic; Li, lithic plutonic;Lv, lithic volcanic,Lq, lithic vein quartz and quartzite.

If these plutons are the source for the upper unit clasts, approximately450 km of sinistralstrike-slip offset is required along inferredfaults locatedwithin or west of the Gravina arc strata(Figure 1). No suchfaults or associatedfabrics in post- Jurassicstrata, however, have been recognizedin the region. Therefore the three possiblesource terranes for the plutonic cobblesdetritus include: (1) uplifted and dissectedparts of the Gravina arc; (2) Late Jurassicplutonic rocks exposedto the southin northwesternBritish Columbia; and (3) LateJurassic plutonicrocks exposed to thenorth in theeastern Chugach and St. Elias mountains. The secondpossibility is unlikely, as no offsetsexist across hypothesized faults beneath Clarence Strait.Late Jurassic toEarly Cretaceous sinistral strike-slip removalof the deeperportions of the arc and in situ structural burialof theplutonic source terrane are both plausible scenarios. Age.No fossils have been found within the upper unit of the Gravina sequencein southernsoutheastern Alaska; thus, its depositionalage is poorly constrained. The Late Jurassic provenanceages for graniticclasts provide a maximumage for deposition. Locally, the upper unit depositionallyoverlies the UpperJurassic and Lower Cretaceousarc strataof the lower unit, which suggestsa post-EarlyCretaceous age; however,no primaryfossil ageshave been obtainedfrom this part of the Gravina sequence. North of the study area (Figure 1), on Etolin Island, ammonites(Arcthoplites belli andgrantziceras sp.) of early Albian age are present in interbeddedargillite, tuff, and volcanic breccia [Berg et al., 1972]. Basedon similaritiesin lithology and stratigraphicposition, the lower Albian strata on Etolin Island probablycorrelate with the upperunit of the

Gravina sequence. Similar ammonitesalso occur in lower v v A Albian strataof the WrangellMountains to the north [Imlay, 1960; Jones, 1967]. Based on geologic relations and sparse fossil data, an early Albian age is inferredfor the upperunit clastic and tuffaceous strata. Depositional setting. The upper unit of the Gravina sequencecontains epiclastic and tuffaceousturbidires which weredeposited in a submarinefan setting.The turbidireswere depositedon a compositebasement consisting of the Alava sequence,the Alexanderterrane, and the lowervolcanic unit of the Gravinasequence. Large volumes of epiclasticdebris were shed off the flanks of dissected volcanic centers in Early 14,564 RUBINAND S^I.g.•.BY:GRAVINA SEQUENCE

TABLE 5. U-Pb GeochronologicSample Locations From the Ketchikan Area and Descriptions of Clasts fromthe EpiclaticMember of theGravina Sequence

Sample Latitude Longitude Lithology ZirconProperties

88JR28 C-1 N55 ø 22'50" 131 ø 19' 24" Slightly altered,nonfoliated 2:1 to 3:1; Sub=An>Eu;irregular shapes; granodiorite colorless,grey-tint; inclusions common C-2 N55 ø 22'50" 131 ø 19' 24" Slightlyaltered, nonfoliated 2:1 to 3:1; Sub>Eu>An;irregular shapes; medium-grainedbiotite colorless,grey-tint; inclusions common in granodiorite somegrains C-3 N55 ø 22'50" 131 ø 19' 24" Slightlyaltered, nonfoliated fine- 3:1; Sub=Eu>An;irregular shapes; colorless, grainedleucograno-diorite grey-tint;inclusions common in all grains 88JR12 C-3 N55 ø 32'9" 131 ø 45'15" Slightlyaltered, nonfoliated fine- 2:1, Sub>Eu>An;colorless, grey-tint ndusions grained granodiorite commonin all grains

Abbreviationsare Eu, euhedral;Sub, subhedral; An, anhedral.2:1, length:widthratios of zircongrains. Colordetermined under reflected light. C- 1, clastnumber.

Cretaceoustime. Distal turbiditic strata locally interfingered Gravinaarc may have beenthe sourceterrane for the epiclastic with tuffaceous deposits suggesting ongoing volcanism. debris and this interpretationis consistentwith the presence Although volumetrically much less abundant, conglomerates of Upper Jurassicvolcanic arc stratain the lower unit. are present. Clasts in the conglomerateare commonly well rounded,indicating that detritusmust have been reworked in a DISCUSSION fluvial or beach environment prior to deposition by turbidity The Gravina sequencerepresents remnants of an oceanic flows. Well-rounded cobblessuggest that parts of the arc were island arc and its basinal sedimentarycover. The remnants subaeriallyexposed and subject to erosion. The sourceof the include marine pyroclastic and volcaniclastic strata and dominantly granitoid Upper Jurassic clasts is not known. basinal turbidites. The arc was constructedon a composite Epiclasticdebris was derived from the intermediateto shallow basement which consists of two elements, the Alexander levels of a dissectedmagmatic arc complex. The Late Jurassic terrane and the Taku terrane. The ensimatic nature of the arc basementis demonstratedby distinctive lithic components I I I I I [Gehrels and Saleeby, 1987a, b], juvenile, mantle-derived 165 160 155 150 145 Ma magmas [Samson et al., 1989], a relatively small crustal volume of volcanic strata, the dominance of tholeiitic arc basalts,and paucity of felsic volcanic strata. In southeastAlaska the oceanic island arc system was Gnat Cove I localized along the easternmargin of the Alexander terrane (Figure 1), where Upper Jurassicto Lower Cretaceousvolcanic and basinal depositsaccumulated on Upper Triassic and older strata of the Alexander terrane. Structures in the arc basement suggestthat strike-slip [McClelland and Gehrels, 1990] and Gnat Cove II possible convergentdisplacements occurred during the early and middle Mesozoic. Rocks of the eastern part of the Alexander terranewere deformedand disruptedprior to the depositionof the Gravina sequencealong the Duncan Canal Shear Zone in the Petersburgarea [McClelland and Gehrels, Gnat Cove I!! 1990]. Similar s.tratigraphicrelations may also be presenton Chilkat Peninsulawhere Upper Triassicbasalt, interpreted as a fragment of Wrangellia, underlies basinal turbidites that are tentatively correlatedto the Gravina sequence[Plaj7cer et al., 1989b]. Lower Mesozoic rocks of Wrangellia are thoughtto g Back Island 1 have formed in a rifted arc environment [Barker et al., 1989], analogous to the early Mesozoic history of the Alexander 165 160 155 150 145 Ma terrane. I I I I I The second basement component in the Ketchikan area Fig. 10. Concordiaplotted separately for eachsample, with scalefor consistsof the Taku terrane(Alava andKah Shakessequences). 207pb/206pbratio shown. Each segmentcontains data points and Upper Paleozoicbioclastic limestone, massive to pillowed error barsfor indicatedsamples of igneousclasts from Gnat Cove and metabasalticflows, and argillite of the Alava sequenceare Back Island. Bars at ends of concordiasegments show uncertaintyin unconformablyoverlain by upper unit basinal turbiditesand 207/206pbvalues of concordiabased on uncertaintiesin 238U and 235Udecay constants from Mattinson's [1987] treatment of Jaffeyet conglomerateof the Gravinasequence. The tectonicaffinity of al. [1971] data. Concordiadiagram after Tera and Wasserburg[1972]. the Alava sequenceis unclear; however, the primitive arc Linear regressionand errorsin lower and upperintercepts are adapted complexunderlain by continent-derivedclastic depositsmay from York [1969]. representdisrupted fragments of the late Paleozoicportion of RUBINAND SALEEBY:GRAVINA SEQUENCE 14,565 the Yukon-Tanana and early Mesozoic part of the Stikine changein the evolution of the Gravina basin. The turbidites terranes[Rubin et al., 1990a; Rubin and Saleeby, 1991]. Thus were depositedon a varied substrate,including the Taku terrane the Gravina basement is composite, consisting of an (Alava sequence)and the lower volcanic unit of the Gravina ensimatic rifted arc (Alexander terrane), a primitive arc sequence, indicating that the Alexander terrane and Taku complex(Stikine terrane),and continent-derivedslope and rise terrane were amalgamatedprior to the depositionof the upper deposits(Yukon-Tanana terrane). The originaldimensions of unit. The presence of Late Jurassicgranitoid cobbles in the arc system are obscuredby late Mesozoic deformation channel-filldeposits implies erosionand uplift of the older arc [Crawford et al., 1987; Rubin et al., 1990b; G.E. Gehrels et edifice, perhapscoeval with a changeof relative plate motions al., Geologicand tectonicrelations along the westernflank of during the latestJurassic to earliestCretaceous [Engebretsen et the Coast Mountains batholith between Cape Fanshaw and al., 1985]. Fine-grainedepiclastic debris was also shed from Taku Inlet, southeasternAlaska, submitted to Tectonics, 1991, the dissected arc. Ifiterstratified tuffaceous turbidites record hereinafter referred to as Gehrels et al., submitted manuscript, continuedvolcanism. Lack of quartz-richterrigenous deposits 1991; W.C. McClelland et al., The Gravina belt and Yukon- indicates bathymetric or paleogeographicisolation from a Tanana terrane on central southeastern Alaska: Protolith continental source area. relationsof metamorphicrocks along the westernflank of the Uplift and erosion of the older arc edifice and resulting Coast Mountains batholith, submitted to Journal of Geology, influx of epiclastic debris probably occurred in the Early 1991b] and fragmentarypreservation. Cretaceous.The Gravina sequenceis as young as Albian [Berg The localizationof oceanicisland arcs alongolder rifted and et al., 1972], and may be as young as Cenomanian(D. Brew, structurallydisrupted ensimatic basement fragments is well personal communication,1987). Uplift and erosion of the documentedin modemisland arc systemsof the westernPacific basin may have been related to tectonic instability along (e.g., the PhilippineArchipelago [Hawkins et al., 1985 and westernmargin of the Alexander terraneduring AptJan time. referencestherein]) and in ancientarc complexesof the North Mid-Cretaceous contractional deformation of the Gravina American Cordillera [Saleeby, 1983]. In the Philippine sequencehas beendescribed in southeastAlaska [Rubinet al., Islands the modem Mindanao arc nucleated above structurally 1990b; Gehrels et al., submitted manuscript, 1991; disrupted ensimatic basement fragments that marked the McClelland et al., submittedmanuscript, 1991a]. The uplift collision between the central and eastern Mindanao belts and erosion of adjacent parts of the Gravina belt may have [Hawkins et al., 1985; Karig, 1983]. The Mindanao marked the onset of mid-Cretaceouscompression and collapse composite arc basement includes crustal fragments that of the basinal sequence. Thick volumes of epiclastic debris originated at the Pacific-Eurasianplate boundary [Karig, recorda fundamentalchange in the basin evolutionand signals 1983], previouslyassembled by strike-slipand thrust faults. the onset of mid-Cretaceousorogenic activity. The similarity between the modern Mindanao arc with its On a regional scale, the timing of volcanismand epiclastic compositebasement and the Gravinasequence is striking. sedimentation in the Gravina sequence varies along strike. The inception of arc volcanismrecorded in the Gravina Farther to the north (Figure 1), in the Juneauregion, epiclastic sequenceprobably began in the middleLate Jurassic,based on strata at the base of the Gravina sequence are overlain by the oldest fossil ages on Gravina Island and to the north on metabasalticlavas and pyroclastic rocks [Brew and Ford, Cape Fanshaw [Gehrels and McClellan& 1988]. Initially, 1985]. Based on regional stratigraphic relations the mafic basalt and basaltic andesiteand pyroclasticflows of the lower volcanic strata may be as young as Early Cretaceousin age. unit accumulated on basinal sediments that unconformably Preliminary major element geochemicalanalyses suggest that overlie the Alexander terrane. The oldest basinal sedimentary the metabasalt formed in a rift environment [Ford and Brew, rocks consistof rhythmicallybanded fine-grained turbidites 1987]; however, a volcanic arc settingis also comparablewith interlayered with lenses of coarse detritus, derived from available data. Similar geologic relations are present in the Triassicparts of the Alexanderterrane. A LateTriassic through eastern Alaska Range where Upper Jurassic to Lower Middle Jurassichiatus in deposition and the presenceof Cretaceous volcanic and basinal strata form a thick and basement-deriveddebris in Upper Jurassicstrata suggestthat continuous section [Berg et al., 1972; Lowe et al., 1982; the basementwas locally subaeriallyexposed. Basementthen Richter, 1976]. The stratigraphically lowest rocks, subsided and accumulated fine-grained basinal sediments, consistingof Upper Jurassicshallow water marine sedimentary massivemafic lavas, and pyroclasticdebris of the lower unit. rocks overlain by turbidires and conglomerate,unconformably Progradationof pyroclastic aprons resulted in an upward- overlie Paleozoic and Mesozoic rocks of Wrangellia and coarsening sequence of volcaniclastic sedimentary rocks contain coarsedetritus derived from the underlyingbasement (Figure 3). Similar marine progradational sequences in [Berg et al., 1972]. Neocomian arc-related volcanic strata of volcaniclasticsediments have been recognizedin the modem the overlying Chisana Formation consist of submarine and Mariana Island arc [Karig, 1971], in marginal basins of the nonmarine mafic and intermediate flows, volcaniclastic rocks, southwesternPacific Ocean [Klein, 1985], and as an important and shallow marine argillite [Barker, 1987; Richter and Jones, component in marginal basins [Karig and Moore, 1975]. 1973]. Thus the inceptionof volcanic activity in the eastern Rapid buildup of pyroclasticaprons implies subsidenceof the Alaska Range and in the Juneauarea is perhapsyounger than basin, which allows for the transportof coarsevolcaniclastic volcanic activity recordedin the Ketchikanarea. debris into the basin. Alternatively, the progradational The deeper levels of the Upper Jurassicmagmatic arc are sequencemay have resulted from migration of volcanism exposedonly locally, in the St. Elias Mountains of the Yukon towards the basin. In either case, no simple pattern of and British Columbia, and possibly in Coast Range of volcanogenicsedimentation emerges from the available data. northwestern British Columbia. Recent studies conducted in A thick successionof epiclastic and tuffaceous turbidites the Coast Rangesof westernBritish Columbiahave revealeda blanketsthe lower volcanic sequence. The shift from volcanic deeperlevel of exposureof the Upper Jurassicmagmatic arc to predominantlyepiclastic depositionmarks a fundamental than in southern southeasternAlaska [Crawford et al., 1987; 14,566 RUBINAND SALEEBY: GRAVINA SEQUENCE

Hollister et al., 1987; Van der Heyden, 1989a; Woodsworthet thoroughand constructive review of the manuscript.Special thanks to al., 1983]. Here, Upper Jurassicplutons intrude the Alexander Fred Barker (U.S. GeologicalSurvey) for providingthe geochemical analysesfor the Gravinasequence volcanic rocks. terraneand representthe rootsof a magmaticarc [Armstrong, 1988; Van der Heyden, 1989b]. Only a thin metamorphic selvageof the Upper Jurassicvolcanic cover is preservedin REFERENCES northwestern British Columbia, whereas to the north in Armstrong,R. L. , Mesozoic and early Cenozoic magmaticevolution southeast Alaska, a relatively thick, volcanic section is of the CanadianCordillera, Spec. Pap. Geol. Soc. Am., 218, 55-91, preservedand is partly affectedby low-grademetamorphism. 1988. Barker, F., Cretaceous Chisana island arc of Wrangellia, eastern Deeperlevels of the arc also are preservedto the northin the Alaska,Geol. Soc.Am. Abstr. Programs,•9(7), 580, 1987. St. Elias Range and the southern Yukon Territory. Late Barker, F., A. Sutherland-Brown, J. R., Budahn, G., Plafker, Back-arc Jurassicto earliestCretaceous calcalkaline plutonism occurred with frontal-arc componentorigin of Triassic Karmutsenbasalt, throughout the Alexander terrane and locally within British Columbia, Canada, Chem. Geol. 75, 81-102, 1989. Wrangellia[Dodds and Campbell,1988]. Coevalcalcalkaline Beikman,H. M., Geologicmap of Alaska,Spec. Map, 1:5,000,000, U.S. Geol. Survey, RestonVa., 1980. plutonismalso extendedinto southernAlaska [MacKevett, Berg, H. C., Geologicmap of AnnetteIsland, Alaska, U.S. Geol. Surv. 1978; Hudson, 1983] and northernsoutheastern Alaska [Brew Misc. Invest. Map 1-684, 1972. and Morrell, 1983]. Volcanic strataof this age have not been Berg, H. C., Geology of Gravina Island, Alaska, U.S. Geol. Surv. recognizedat thoselatitudes; however, the UpperJurassic and Bull., 1373, 41 pp., 1973. Berg, H.C., and E. L. Cruz, Map showing locations of fossil Lower Cretaceousarc-related strata of the Gravina sequencein collectionsand relatedsamples in the Ketchikanand PrinceRupert southeastAlaska may representthe uppercrustal levels of the quadrangles,southeastern Alaska, U.S. Geol. Surv. Open-File Late Jurassicplutonic belt. The differencesin level of Report 82-1088, 1982. exposureof Early Cretaceousbetween southeast Alaska and Berg, H. C., D. L. Jones,and D. H. Richter, Gravina-Nutzotinbelt: Tectonic significance of an upper Mesozoic sedimentaryand westernBritish Columbiaare probablydue to Late Jurassicto volcanicsequence in southernand southeasternAlaska, U.S. Geol. Early Cretaceousarc-parallel strike-slip faulting, along strike Surv. Prof. Pap., 800-D, D1- D24, 1972. variations of mid-Cretaceousthrust faulting, and differential Berg, H. C., R. L. Elliott, and R. D. Koch, Geologic map of the post mid-Cretaceousuplift, extensionand erosion. This Ketchikanand PrinceRupert quadrangles,Alaska, U.S. Geol. Surv. interpretationmay explain the paucity of Upper Jurassic Misc. Invest. Map, 1-1807, 1988. Berg, H. C., D. L. Jones,and D. H. Richter, Gravina-Nutzotinbelt: volcanic rocks in western British Columbia, Yukon, and Tectonic significance of an upper Mesozoic sedimentaryand southern Alaska. volcanicsequence in southernand southeasternAlaska, U.S. Geol. Surv. Prof. Pap., 800-D, D1- D24., 1972. CONCLUSIONS Brew, D. A., and A. B. Ford, Preliminarygeologic map of the Juneau, Taku River, Aftin, and partsof the Skagwayquadrangles, southeast Basaltic to andesitic lavas and pyroclastic strata of the Alaska, U.S. Geol. Surv. Open File Rep., 85-395, 1985. Gravina-Nutzotin belt in southeastern and southern Alaska Brew, D. A., and S. M. Karl, Reexamination of the contacts and other features of the Gravina belt, southeastemAlaska, U.S. Geol. Surv. delineatea Late Jurassicto Early Cretaceousarc system. Arc Circ.,1016, 143-146, 1987. volcanismwas probably intermittent from middle Late Jurassic Brew, D. A., and R. P. Morrell, Intrusive rocks and plutonicbelts in to Late Jurassicthrough Early Cretaceous. The arc was southeastern Alaska, U.S.A., Circum-Pacific Plutonic terranes, constructedacross a compositebasement consisting of the editedby J.A. Roddick,Mem. Geol. Soc.Am. Mem., 159 171- 194, 1983. Alexanderand Wrangelliaterranes, and likely correlativesof Brew, D. A., R. A. Loney, and L. J.P. Muffler, Tectonic history of the Stikine and Yukon-Tanana terranes,requiring a pre-Late southeasternAlaska, in A symposiumon the tectonichistory and Jurassictie betweenInsular compositeterrane and the western mineral depositsof the westernCordillera in British Columbiaand margin of North America. The Jurassicvolcanic strata are neighboringpart of the United States,Spec. Vol. Can. Inst. Mining and Metall., 8, 149-170, 1966. dominatedby tholeiitic arc basaltsthat were shed from the Brooks, A. H., Preliminaryreport on the Ketchikanmining district, flanks of submarine volcanos. Fine- to coarse-grained Alaska, with a reconnaissance of Chickaman River, U.S. Geol. epiclasticturbidites were depositedin a submarinefan setting Surv.Prof. Pap., 1, 120 pp., 1902. adjacentto dissectedvolcanic centers. AbundantLate Jurassic Buddington,A. F., and T. Chapin,Geology and mineral depositsof graniticcobbles in the clasticbasinal deposits and the absence southeasternAlaska, U.S. Geol. Surv.Bull., 800, 398 pp., 1929. Busby-Spera,C. J., Evolutionof a Middle Jurassicback-arc basin, of AptJanstrata locally recorduplift and erosionof the arc. Cedros Island, Baja California, Evidence from a marine Volcanic activity was diachronousalong the island arc and volcaniclasticapron, Geol. Soc. Am. Bull., 100, 218-233, 1988. may reflect segmentation of the arc or incomplete Cameron,A. E., D. H. Smith, and D. H. Walker, Mass spectrometryof preservation.The volcanicand basinalrocks of the Gravina nanogram-sizesamples of lead,Anal. Chem.,41,525-526, 1969. Cann, J. R., Rb, Sr, Y, Zr, and Nb in some ocean-floor basaltic rocks, sequencewere deformedduring a major intra-arccontractional Earth Planet. Sci. Lett., 19, 7-11, 1970 event during mid-Cretaceous time, accompanied by Chapin, T., The structure and stratigraphy of Gravina and emplacementof a distinctlyyounger (90-100 Ma) arc-related RevillagigedoIslands, Alaska, U.S. Geol.Surv. Prof. Pap., 120-D, plutonic suite. 83-100, 1918. Chen, J. H., and G. J. Wasserburg,Isotopic determination of uranium Acknowledgments. Pans of this researchwere supportedby in picomoleand subpicomolequantifies, Anal. Chem.,53, 2060- National Science Foundationgrants EAR 86-05386 and EAR 88- 2067, 1981. 034834 (to Saleeby). Additionalsupport (to Rubin) was providedby a Crawford, M. L., L. S. Hollister, and G. J. Woodsworth, Crustal GeologicalSociety of AmericaPenrose Grant, a Sigma-Xigrant-in-aid, deformationand regionalmetamorphism across a terraneboundary, the U.S. GeologicalSurvey, Alaska Branch,and by the U.S. Forest CoastPlutonic Complex, British Columbia,Tectonics, 6, 343-361, Service,Ketchikan Ranger District. Jeff Marshallprovided excellent 1987. field assistanceduring the summerof 1987. We thank Fred Barker, Dodds,C. J., and R. B. Campbell,Potassium-argon ages of mainly Danel Cowan, Weecha Crawford, John Gatvet, GeorgeGehrels, Linc intrusive rocks in the Saint Elias Mountains, Yukon and British Holllister Bill McClelland, Megran Miller, Jim Monger, and George Columbia,Pap. Geol. Surv.Can., 87-16, 43 pp., 1988. Plafker for helpful discussions. George Plafker provided a very Douglas,R. J., H. Gabrielse,J. O. Wheeler,.D.F., Scott, and H. R. RUBIN AND SALEEBY:GRAVINA SEQUENCE 14,567

Belyea, Geology of western Canada, Rep. 1, Can. Geol. Surv., model for elementrecycling in the crust-uppermantle system,J. Ottawa, pp. 365-448, 1970. Geol., 88, 497-522, 1980. Engebretsen,D.C., A. Cox, and R. G. Gordon, Relative motions Klein, G. de V., The controlof depositionaldepth, tectonic uplift, betweenoceanic and continentalplates in the Pacific basin, Spec. and volcanismon sedimentationprocesses in backarcbasins of Pap. Geol. Soc. Am., 206, 59 pp., 1985. the western Pacific, J. Geol., 93, 1-25, 1985. Ford, A. B., and Brew, D. A., Major-element geochemistryof the Krogh, T. E., A low-contamination method for hydrothermal metabasaltsof the Juneau-Hainesregion, southeastAlaska, U.S. decompositionof zircon and extractionof U and Pb for isotopic Geol. Surv. Circ., 1016, 150-155, 1987 age determinations, Geochim. Cosomochim. Acta, 37, 485-494, Gabrielse, H., Major dextral transcurrentdisplacements along the 1973. Northern Rocky Mountain trench and related lineamentsin north- Loney, R. A., D. A. Brew, L. J.P. Muffler, and J. S. Pomeroy, central British Columbia, Geol. Soc. Am. Bull., 96, 1-14, 1985. Reconnaissancegeology of Chichagof, Baranof, and Kruzof Gabrielse, H. and J. O. Wheeler, Tectonic framework of southern Islands, southeasternAlaska, U.S. Geol. Surv. Prof. Pap., 792, Yukon and northwestemBritish Columbia, Pap. Geol. Surv. Can., 105 pp., 1973. 60-24, 37, 1961. Lowe, P.C., D. H. Richter, R. L. Smith, and H. R. Schmoll, Geologic Garcia, M. O., Criteria for the identification of ancient volcanic arcs, map of the Nabesha B-5 quadrangle,Alaska, U.S. Geol. Surv. Earth Sci. Rev., 14, 147-165, 1978. QuadrangleMap• GQ-1566, 1982. Gehrels,G. E., and W. C. McClelland, Outline of the Taku terrane and MacKevett, E. M., Jr., Geologicmap of the McCarthy quadrangle, Gravinabelt in the Cape Fanshaw-WindhamBay regionof central Alaska, U.S. Geol. Surv. Misc. Invest.Map, 1-1031, 1978. southeasternAlaska, Geol. Soc. Am. Abstr. Programs, 20, 163, Mattinson, J. M., U-Pb ages of zircon: a basic examinationof error 1988. propagation,Chem. Geol., 66, 151-162, 1987. Gehrels, G. E., and J. B. Saleeby, Geologic framework, tectonic May, S. R., andR. F. Butler,Noah AmericanJurassic apparent polar evolution and displacementhistory of the Alexander terrane, wander: Implications for plate motion, paleogeographyand Tectonics, 6, 151-173, 1987a. Cordilleran tectonics, J. Geophys. Res., 91, 11,519-11,544, Gehrels, G. E., and J. B. Saleeby, Geology of Annette, Gravina, and 1986. Duke islands, southeasternAlaska, Can. J. Earth Sci.,24, 866-881, McClelland, S. M., and S. R. Taylor, Role of subductedsediments in 1987b. island-arc magmatism:Constraints from REE patterns, Earth Gehrels, G. E., W. C. McClelland, S. D. Samson, P. J. Patcheu, and J. Planet. Sci. Lett., 54, 423-430, 1981. L. Jackson,Ancient continentalmargin assemblagein the northern McClelland, W. C., and G .W. Gehrels,Geology of the DuncanCanal Coast Mountains, southeast Alaska and northwest Canada, shearzone: Evidencefor Early to Middle Jurassicdeformation of Geology, 18, 208-211, 1990. the Alexander terrane, Geol. Soc. Am. Bull., 102, 1378-1392, Gehrels, G. E., W. C. McClelland, S. D. Samson, and P. J. Patchett, U- 1990. Pb geochronologyof detrital zircons from the Yukon crystalline terrane in the northern Coast Mountains batholith, southeastern Monger, J. W. H., and H. C. Berg, Lithotectonicterrane map of western Canada and southeastern Alaska: U.S. Geol. Surv. Misc. Alaska,Can. Jour. Earth Sci., in press,1991. Gill, J., OrogenicAndesites and Plate Tectonics,385 pp, Springer- Map MF-1874B, 1987. Verlag, New York, 1981. Monger,J. W. H., R. A. Price,and J. D. Tempelman-Kluit,Tectonic Grantz, A., D. L. Jones, and M. A. Lanphere, Stratigraphy, accretion and the origin of the two major metamorphicand paleontology,and isotopic ages of upper Mesozoic rocks in the plutonic welts in the Canadian Cordillera, Geology, 10, 70-75, 1982. southwesternWrangell Mountains, Alaska, U.S. Geol. Surv. Prof. Moorbath, W., and S. Hildreth, Crustal contamination to arc Pap.,550-C, C39-C47, 1966. Harland, W. B., A. V. Cox, P. G. Lewellyn, C. A. G. Pickton, A. G. magmatismin the Andes, Contr. Min. Petrol., 98,455-489, 1988. Smith,and R. Walters,A GeologicTime Scale, 131 pp., Cambridge Nakumara, N., Determinationof REE, Ba, Fe, Mg, Na, and K in UniversityPress, New York, 1982. carbonaceousand ordinary chondrites, Geochim. Cosmochim. Hawkins, J. W., G. F. Moore, R. Villamot, C. Evans, and E. Wright, Acta, 38, 757-775, 1974. Geology of the compositeterrane of east and central Mindanao, Oldow, J. S., A. W. Bally, H G. Av6 Lallemant, W. P Leemah, Earth Sci. Set., vol. 1, pp. 437-463, Circum-Pacific Council for Phanerozoic evolution of the Noah American Cordillera: United Energyand Mineral Resources,Houston, Texas, 1985. Statesand Canada,The Geologyof Noah America,in The Geology Hollister, L. S., G. C. Grissom, E. K. Peters, H. H. Stowell, and V. B. of North America - An Overview•vol. A, editedby A.W. Bally and Sisson,Confirmation of empiricalcorrelation of A1 in hornblende A.R. Palmer, pp. 139-232, Geological Society of America, with pressureof solidificationof calc-alkalineplutons, Am. Min., Boulder, Colo., 1989. 72, 231-239, 1987. Pavlis, T. J., Origin and age of the Border RangesFault of southern Hudson,T., Calc-alkalineplutonism along the Pacific rim of Southern Alaska and its bearing on the Late Mesozoic tectonic evolution of Alaska,in editedby J.A. Roddick,Circum-Pacific Plutonic terranes, Alaska, Tectonics, 1,343-368, 1982. Mem Geol. Soc. Am., 159, 159-1170, 1983. Pearce, J. A., Trace element characteristicsof lavas from destructive Humphris,S. E., and G. Thompson,Trace-element mobility during boundaries,in Andesites,Orogenic Andesitesand Related Rocks, hydrothermalalteration of oceanicbasalts, Geochim. Cosomochim. edited by R.S. Thorpe, pp. 525-547, John Wiley, New York, Acta, 42, 127-136, 1978. 1982. Iralay, R. W., Early Cretaceous(Albian) ammonitesfrom the Chitna Pearce, J. A., Role of the sub-continentallithosphere in magma Valley and Talkeetna Mountains,Alaska, U.S. Geol. Sum. Prof. genesisat active continentalmargins, in Continental Basalts and Paper 354-D p. 87-114, 1960. Mantle Xenoliths, edited by C.J. Hawkesworthand M.J. Norry, Jaffey, A. H., K. F. Flynn, L. E. Glendenin,W. C. Bentley, and A.M. pp. 230-259, BirkhauserBoston, Secaucus, N.J., 1983. Essling,Precision measurement of half-lives and specificactivities Pearce, J. A., and J. R. Cann, Tectonic setting of basic volcanic of 235Uand 238U, Phys. Rev. C, 4, 1889-1906,1971. rocks determinedusing trace element analyses,Earth Planet. Sci. Jones, D. L.,Cretaceous ammonitesfrom the lower part of the Lett., 19, 290-300, 1973. Matanuska Formation, southern Alaska, U.S. Geol. Surv. Prof. Pearce,J. A., and M. J. Norry, Petrogeneticimplications of Ti, Zr, Y, Paper547, 49 pp., 1967. and Nb variations in volcanic rocks, Contrib. Mineral. Petrol., Karig, D. E., Origin and developmentof marginal basinsin the 69, 33-47, 1979. westernPacific, J. Geophys.Res., 76, 2541-2561, 1971. Pearce, J. A., Lippard, S. J., and Roberts, S., Characteristicsand Karig, D. E., Accretedterranes in the northernpart of the Philippine tectonic significance of supra-subductionzone ophiolites, in Archipelago,Tectonics, 2, 211-236, 1983. Marginal basin geology: Volcanic and associatedsedimentary and Karig, D. E., and G. F. Moore, Tectonicallycontrolled sedimentation tectonicprocesses in modern and ancient marginal basins, edited in marginalbasins, Earth Planet. Sci. Lett., 26. 233-238, 1975. by B.P. Kokelaar and M.F. Howells, pp. 77-94, Blackwell Karl, S. M., B. R. Johnson,and M. A. Lanphere,New K-Ar agesfor Scientific Publications,Oxford, England, 1984. plutonson westernChichagof Island on on Yakobi Island, U.S. Plafker, G., W. J. Nockleberg, and J. S. Lull, Bedrock geology and Geol. Surv. Circ., 1016, 164-168, 1987. tectonic evolution of the Wrangellia, Peninsular, and Chugach Kay, R. W., Volcanicarc magmas:Implications of a melting-mixing terranesalong the trans-Alaskancrustal transectin the Chugach 14,568 RUBINAND SALEEBY: GRAVINA SEQUENCE

Mountainsand southernCopper River Basin, Alaska,J. Geophys. Thorpe,R. S., Andesites:Orogenic Andesites and Related Rocks,724 Res., 94, 4255-4295, 1989a. pp., JohnWiley, New York, 1982. Plafker, G., C. D. Blome, and N.J. Silbeding,Reinterpretation of Van Der Heyden, P., Jurassic magmatism and deformation: lower Mesozoic rocks on the Chilkat Peninsula, Alaska, as a Implicationsfor the evolution of the Coast Plutonic Complex, displacedfragment of Wrangellia,Geology, 17, 3-6, 1989b. Geol. Soc.Am. Abstr. Programs,21, 153, 1989a. Richter, D. H., Geologicmap of the Nabesnaquadrangle, Alaska, Van Der Heyden, P., U-Pb and K-At geochronometryof the Coast U.S. Geol. Surv. Misc., Invest. Map, 1-932, 1976. Plutonic Complex, 53øN-54øN, and implicationsfor the Insular- Richter,D. H , andD. L. Jones,Structure and stratigraphy of eastem Intermontane superterraneboundary, British Columbia, Ph.D. Alaska Range, Mem. Am. Assoc.Pet. Geol., 19, 408-420, 1973. thesis, Univ. of British Columbia, Vancouver, 1989b. Rubin, C. M., and J. B. Saleeby, The inner boundaryzone of the Wheatley,M., and N.M. S. Rock, SPIDER: A Macintoshprogram to Alexanderterrane in southernSE Alaska, A newly discoveredthrust generatenormalized multi-element "spidergrams,"Am. Min., 73, belt, Geol. Soc. Am. Abstr. Programs,19,455, 1987. 919-921, 1988. Rubin, C. M., and J. B. Saleeby, Tectonic framework of upper Wheeler, J. O., and P. McFeely, Tectonic assemblagemap of the Paleozoicand lower Mesozoic Alava sequence:A revisedview of the Canadian Cordillera. Open-File Rep.,1565, Can. Geol. Surv., polygeneticTaku terranein southernsoutheast Alaska, Can. J. Ouawa, 1987. Earth Sci., in press,1991. White, W. M., and B. Duprt, Sedimentsubduction and magmagenesis Rubin,C. M., M. M. Miller, andG. E. Smith,Tectonic development of in the Lesser Antilles: Isotopic and trace element constraints,J. Cordilleranmid-Paleozoic volcanoplutonic complexes: Evidence Geophys.Res., 91, 5927-5941, 1986. for convergentmargin tectonism,Spec. Pap. Geol. Soc. Am., 225, Winchester,J. A., and P. A. Floyd, Geochemicaldiscrimination of 1-16, 1990a. different magma series and their differentiation productsusing Rubin, C. M., J. B. Saleeby, D. S. Cowan, M. T. Brandon, and M. F. immobile elements, Chem. Geol., 20, 325-343, 1973. McGroder, Regionally extensive mid-Cretaceous west-vergent Winchester,J. A., and P. A. Floyd, Geochemicaldiscrimination of thrust system in the northwesternCordillera: Implications for differentmagma series and their differentiationproducts using continent-margintectonism, Geology, 18 (3), 276-280, 1990b. immobile elements, Chem., Geol., 325-343, 1977. Saleeby, J. B., Accretionary tectonics of the North American Woodsworth, G. J , M. L. Crawford, and L. S. Hollister, Cordillera, Annu. Rev. Earth Planet. Sci., 15, 45-73, 1983. Metamorphismand structureof the CoastPlutonic Complex and Saleeby, J. B., The inner boundary zone of the Alexander terrane in adjacentbelts, Prince Rupert and Terraceareas, British Columbia, southernSE Alaska: Part II southernRevillagigedo Island (RI) to Field Trip Guidebook14, 62 pp., Geological Associationof Cape Fox (CF), Geol. Soc.Am. Abstr.Programs, 19, 828, 1987. Canada, Waterloo, Ont., 1983. Samson, S. D., W. C. McClelland, P. J. Patchett,and G. E. Gehrels, Wright,F. E., andC. W. Wright,The Ketchikanand Wrangell mining Nd isotopes and Phanerozoiccrustal genesis in the Canadian districts,Alaska, U.S. Geol.Surv. Bull.• 347, 210 pp., 1908. Cordillera, Nature, 337, 705-709, 1989. York, D., Least-squaresfitting of a straightline with correlatederrors, Silbefiing,N.J., B. R. Wardlaw,and H. C. Berg,New paleontologic Earth Planet. Sci. Left., 5,320-324, 1969. age determinations from the Taku terrane, Ketchikan area, southeasternAlaska, U.S. Geol. Surv. Circ. 844, 117-119, 1981. C. M. Rubinand J. B. Saleeby,Division of Geologicand Planetary Smith, P.S., Notes on the geology of Gravina Island, Alaska, U.S. Sciences,Califomia Institute of Technology,Pasadena, CA 91125. Geol. Surv. Prof. Pap., 95, 97-105, 1915. Terra,F., andG .S. Wasserburg,U-Th-Pb systematics in threeApollo 14 Basaltsand the problemsof the initial Pb in lunar rocks,Earth Planet. Sci. Left., 14, 281-304, 1972. (ReceivedAugust 2, 1990; Thompson,R. N., Magmatismof the BritishTertiary province, Scott. revisedFebruary 19, 1991; J. Geol., 18, 49-107, 1982. acceptedFebruary 23, 1991.)