OF THE ULTRAMAFIC COMPLEX AT DUKE ISLAND, T. N.

Transcription

1 MINERALOGICAL SOCIETY OF AMERICA, SPECIAL PAPER 1, 1963 INTERNATIONALMINERALOGICALASSOCIATION, PAPERS, THIRD GENERAL MEETING ORIGIN OF THE ULTRAMAFIC COMPLEX AT DUKE ISLAND, SOUTHEASTERN ALASKA T. N. IRVINEl M cm aster University, Hamiltn, Ontari ABSTRACT The Duke Island ultramafic cmplex crps ut in tw areas that ttal 9 square miles. Rcks within the cmplex are cmprised f livine, clinpyrxene, hrnblende and ferriferus xides and are classified as dunite, peridtite, livine pyrxenite and hrnblende pyrxenite; all are devid f plagiclase and rthpyrxene. A pegmatite f hrnblende and anrthite is clsely assciated and prbably cmagmatic, but abundant gabbr surrunding the cmplex has been altered near its margin and is apparently lder. Remarkable gravitatinal layering, shwing grain-size grading, ccurs intermittently in the livine-bearing ultramafic rcks thrugh a pssible thickness f 2 miles. Althugh the Fe/Mg ratis f livine and clinpyrxene increase slightly in the ultramafic units in the rder listed abve, cryptic layering is slight r absent. Cunterparts in the layering f crss-bedding, slide cnglmerates and slump structures suggest accumulatin under the influence f magmatic currents, and a marked structural break, lcally analgus t an angular uncnfrmity with basal cnglmerate, marks the cntact f livine pyrxenite and the dunite-pcridtite zne and indicates at least tw majr intrusins f magma int the cmplex. A variety f structural, mineralgic and petrlgic evidence suggests that the ultramafic rcks crystallized frm a magma f ultrabasic cmpsitin. Hrnblende pyrxenite generally frms a brder zne arund the livine-bearing rcks, and tgether with the alteratin aurele, seems t represent peripheral reactins that tended t prduce a successin f znes in lcal equilibrium alng thermal and chemical gradients between the ultrabasic magma and its cuntry rck. The pssible cmpsitin and temperature f the ultrabasic magma are discussed. INTRODUCTION mrphsed vlcanic and clastic sedimentary frmatins. These generally fringe the igneus cre f the island, which is cmprised f gabbric, ultramafic and granitic plutns that apparently were emplaced in that rder. The metamrphic rcks are nt in cntact with the ultramafic cmplex, but where they are clser than i-mile t the main gabbr mass, they have mineral assemblages indicative f the hrnblende hrnfels facies and, lcally, the pyrxene hrnfels facies (Fyfe et al., 1958). Elsewhere, they belng t the albite-epidte hrnfels r green schist facies. AND ACKNOWLEDGMENTS Duke Island is at the suthern end f sutheastern Alaska (55 55'N, 131 2'W) and has an area f 59 square miles. The ultramafic cmplex expsed n the island is a member f a belt f 35 r mre ultramafic bdies that ccur alng the 35-mile length f the Alaskan panhandle. A brief descriptin f the belt is available in Taylr and Nble (196). The present paper is a summary f the petrlgy f the Duke Island cmplex; a mre cmprehensive reprt is in preparatin. The infrmatin presented here was largely cllected while the authr was attending the Califrnia Institute f Technlgy, and the cntributins f many peple there by way f discussin, assistance, and criticism are gratefully acknwledged. Particular thanks g t Dr. James A. Nble fr spnsring the field wrk and supervising the study. During the academic year , supprt was prvided by the Fster Hewitt Fellwship in Field Gelgy. Grants frm the Gelgical Survey f Canada have cvered reprductin f sme f the diagrams. G. V. Middletn and D. M. Shaw have made several helpful suggestins with regard t expsitin. GEOLOGICAL DESCRIPTION OF DUKE Primary (pyrxene) gabbr. The primary gabbr is cmprised f -5 per cent plagiclase and 5-55 per cent pyrxene and hrnblende cmbined. Olivine and bitite are present lcally; ilmenmagnetite and apatite are cmmn accessries. Plagiclase ranges frm An t An95, An5_75predminating (Fig. 2). Orthpyrxene and clinpyrxene are generally bth present, but rcks having nly ne r the ther d exist. By ptical prperties (Figs. 3, ), rthpyrxene ranges frm En7 t En76, and clinpyrxene frm abut Ca37Mg53Fel t Ca3SMg?9Fe33. As shwn in Fig. 3, the Fe/Mg rati f each pyrxene increases as plagiclase becmes mre sdic. Olivine changes in the same way, its range f varia tin being F27 t FOn The pyrxene gabbr underlies mst f the eastern half f Duke Island and thus, is abundant. Unfrtunately, little is knwn abut its structure: the rck is largely massive; tpgraphic relief is lw; ISLAND Metamrphic rcks. The gelgy f part f Duke Island is shwn in Fig. 1. The ldest rcks are meta1 Present address: Gelgical Survey f Canada, Ottawa, Canada. 36

2 DUKE ISLAND ULTRAMAFIC COMPLEX 37 AN INTERPRETATION OF THE STRUCTURE OF THE DUKE ISLAND ULTRAMAFIC COMPLEX ~~/- cntact fault ~ -layering :';::::::---.<apprximate limit f _/ mafic pegmatite and - hrnblende gabbr FIG. 1 cntacts are prly expsed; and the pattern f mineral variatin is nt bviusly systematic. Rhythmic layering des, hwever, ccur at tw lcalities, and ne, dented "A" in Fig. 1, is ntable. The layering there is cntinuus and regular fr several tens f feet, and individual layers have thicknesses up t ne ft. Each layer has its base clearly defined by a marked cncentratin f pyrxene: prceeding upward, the prprtin f mafic minerals decreases in a sharp gradatin, and the rck becmes nrmal r slightly feldspathic and shws marked igneus laminatin. The layering is apprximately parallel t the nearby cntact f the gabbr against the ultramafic cmplex, and the directin f tps f layers, as indicated by grading, suggests that the gabbr underlies the ultramafic rcks. The same relatin is indicated fr this lcality and thers by the graded rhythmic layering that ccurs in the ultramafic cmplex as described belw. Descriptin f the ultramafic cmplex (a) Petrgraphy and mineralgy Mst f the ultramafic rcks crp ut in tw areas tha t t tal 9 square miles, but the areas are believed t be the expsure f ne large bdy at depth (Fig. 1). This in terpreta tin is based partly n an aermagnetic map and partly n gelgical inference, sme f which is mentined belw. If it is crrect, then it is imprecise t refer t each expsure as a "bdy." The wrd "utcrp" is therefre used in its brader sense, and the expsures are called the Hall Cve and Judd Harbr ultramafic utcrps.

3 38 T. N. IRVINE 6 C/)... c( 2 ~ ~ 8 ::IE Q: ~ 6 IL Q: 2 FIG. 2. Histgrams f the cmpsitin f plagiclase in the different types f mafic rcks ccurring at Duke Island, Alaska. In the hrnblende gabbr histgram, the dark part represents plagiclase frm hrnblende gabbr near the granitic intrusins; the rest represents plagiclase in hrnblende gabbr near the ultramafic rcks. ~ ::> z n z ORTHOPYROXENE VS PLAGIOCLASE MOLE PER CENT ANORTHITE _J APPROXIMATE MOLE RATIO Fe X M9+ Fe ny Q 25 APPROXIMATE 2 MOLE RATIO I Fe X 1 e 1-15 C+ M9 + Fe CLINOPYROXENE VS PLAGIOCLASE ~~ 1.675:t-r-~:--r-:::c---y--' 'r-""T' ,.-~_J MOLE 7 PER CENT 6 5 ANORTHITE FIG. 3. Relatin f the chemical variatins in the pyrxenes and plagiclase f the primary gabbr at Duke Islands as determined by ptical prperties. The plts f cexisting pyrxenes are jined by "tie lines." In the clinpyrxene, the prprtin f Ca is rughly cnstant near per cent.

4 DUKE ISLAND ULTRAMAFIC The ultramafic rcks are essentially cmprised f three minerals: livine; clinpyrxene (dipsidic augite); and hrnblende. The results f mdal analyses are given in Figs. SA and B, and the rck classificatin scheme is apparent in Fig. SA. The principal units are dunite, peridtite, livine pyrxenite, and hrnblende pyrxenite. Much f the hrnblende pyrxenite cntains per cent magnetite, 2-3 per cent ilmenite, and minr hercynitic spinel. Chrmite r chrmiferus magnetite is a sparse accessry in dunite and peridtite. Olivine is generally partly altered t serpentine and secndary magnetite, but therwise, the rcks are fresh. Orthpyrxene and plagiclase are ntable by their absence. Figure 5C shws the apprximate areal abundances f the different rcks. The histgrams were prepared by weighting each mdal analysis by the map area the specimen is believed t represent. Althugh perhaps nt statistically elegant, this prcedure des give prper weighting t specimens that cme frm small ccurrences f a particular rck type, and the result graphically prtrays distributin features that are very bvius in the field. Olivine pyrxenite cntaining 15-3 per cent livine (plus serpentine alteratin) is an extremely abundant rck type and has a histgram mde distinct frm that f livine-free hrnblende pyrxenite. Peridtite is rare by cmparisn, but a slight mde is present fr dunitic rcks in the Judd Harbr utcrp. Several f the ther ultramafic bdies in sutheastern Alaska have cmparable distributins in that their principal mdes crrespnd t dunite, livine pyrxenite, and livine-free pyrxenite. The refractive indices f clinpyrxene and livine increase slightly thrugh the rck sequence frm dunite t hrnblende pyrxenite (Fig. 6). Higher index is due primarily t a larger Fe/Mg rati, and the indicated cmpsitinal range fr livine is F78 t F85. Optics and chemical analyses shw clinpyrxene t range frm abut Ca5Mg8Fe7 t Ca5Mg3sFel2 (Fig. 7). The larger 2V f clinpyrxene in the ultramafic rcks as cmpared t that in gabbr (Fig. ) indicates a larger Ca/Mg+ Fe rati r, in ther wrds, less Ca-pr pyrxene in slid slutin. This suggests that Ca-pr pyrxene did nt reach its saturatin limit in the ultramafic rcks during their crystallizatin and, thus, is cmpatible with the absence f rthpyrxene. (b) Layering The utstanding feature f the Duke Island ultramafic cmplex is an abundance f stratificatin that COMPLEX 39 2VII~-'-'--'-~'--'-'~-'-' B tr 55 ~ dunile and peridtite livine pyrxenite --' hrnblende pyrxenite pyrxene gabbr FIG.. Plt f 2V against ny fr clinpyrxene frm Duke Island rcks. This is rughly equivalent t a plt f Ca cntent against Fe/ mg rati. has many f the features f the rhythmic layering in the Skaergaard, Stillwater, and ther intrusins, but which is distinct in that it is graded by grain size rather than mineral density (Fig. 8). The distributin and attitude f the layering is shwn in Fig. 1 by trend lines. It is almst exclusive t livine-bearing rcks and invlves nly tw primary-precipitate minerals, clinpyrxene and livine. (Hrnblende pyrxenite expsed n Kelp Island just suth f Duke Island shws magnetite layers, a few f which are graded.) Mst layers are 2-1 inches thick; the greatest thickness bserved fr a cntinuusly graded layer is 25 feet; the cmmn maximum thickness is 2- feet. Many layers can be traced la terally fr 3 feet, and pssibly, sme extend 9 feet. Thicker layers are generally carser, but in an average example, grain size is -1 mm at the base, decreasing t.2-2 mm at the tp. The bttm f each graded layer is sharp; the tp may be sharp but cmmnly is transitinal int finely laminated rck. Bth pyrxene and livine shw grain size srting, and livine tends t cncentrate near the tps f layers because it is generally finer (the tw minerals have abut the same density.) Layers lacking distinct grading cmmnly alternate with graded nes, but directin f grading is rarely reversed and, thus, is a reliable indicatr f tps where structures are cmpjica ted. Figure 9 shws a variant f graded layer that is cmparable t a cnglmera te bed. It is cmprised f fragments f livine pyrxenite in a matrix f peridtite. This type f layer ccurs nly in the peridtite zne f the Hall Cve utcrp. Assci-

5 T. N. IRVINE HALL COVE OUTCROP JUDD HARBOR OUTCROP HORNBLENDE HORNBLENOITE \ HORNBLENDE PYROXENITE HORNBLENDE OLIVINE PYROXENITE OLIVINE PYROXENITE PERIDOTITE DUNITE HORNBLENDE A 2.~ OliVINE + SERPENTINE CLINOPYROXENE OLIVINE + SERPENTINE OPAQUE OXIDES + HERCYNITIC SPINEL OPAQUE OXIDES + HERCYNITIC SPINEL B OLIVINE + SERPENTINE OOO<]C::t~O CLINOPYROXENE + HORNBLENDE OLIVINE + SERPENTINE CLINOPYROXENE + HORNBLENDE APPROXIMATE AREAL ABUNDANCE IN SQUARE MILES APPROXIMATE AREAL ABUNDANCE IN SQUARE MILES C OLIVINE + SERPENTINE CLINOPYROXENE + HORNBLENDE 2 OLIVINE + SERPENTINE ' CLINOPYROXENE -t HORNBLENDE ~ OLIVINE - BEARING ROCKS _ ROCKS FREE OF OLIVINE FIG. S. Mdal cmpsitin f the Duke Island ultramafic rcks in their tw main areas f utcrp.

6 DUKE ISLAND ULTRAMAFIC ated with it are ther structures characteristic f clastic sediments, including slump structures and crss-layering. It seems clear that the graded layers have frmed by gravity accumulatin f crystals under the influence f currents. A mechanism f irregular magma tic cnvectin cmparable t thse prpsed by Wager and Deer (1939) and Hess (196) is prbable. Hwever, at Duke Island, mechanical disruptin in the magma chamber and slumping have apparently played an imprtant rle in generating and perpetuating currents. Anther type f layering ccurring in livine pyrxenite at Duke Island is marked by a prly defined alternatin f pyrxene and livine and by thin, discntinuus bands f dunite. In appearance, it is nt unlike the "inch-scale layering" f the Stillwater Cmplex (Hess, 196, p. 133). The pyrxene crystals cmmnly shw preferred rientatin nrmal t the laminatin, pssibly because f grwth in situ r recrystallizatin, r bth. Mst f the layering in the Judd Harbr livine pyrxenite zne is this type. Cryptic layering is weakly develped in the Hall Cve peridtite zne but has nt been detected in the ther parts f the cmplex by ptical methds. (c) Structure f the cmplex The distributin f rck types in several f the ultramafic bdies in sutheastern Alaska is rughly cncentric and, in idealized develpment, features a dunite cre surrunded by successive rings f peridtite, livine pyrxenite, pyrxenite, and hrnblendite (Taylr and Nble, 196). The Unin Bay cmplex has the mst extensive sequence f znes, and the Blashke Islands cmplex, the mst symmetric zning. At Duke Island, the rck-type distributin in the Hall Cve ultramafic utcrp is vaguely cncentric in apprximately the abve sequence but primarily because f a remarkably cntinuus brder zne f hrnblende pyrxenite. In the Judd Harbr utcrp, rck units are arranged side by side, althugh here t, the main bdies f hrnblende pyrxenite are peripherally lcated. In the Hall Cve utcrp, abundant evidence shws that at least tw intrusins are invlved in the frmatin f the livine-bearing rcks. The peridtite zne is part f the yunger intrusin. It cuts sharply acrss livine pyrxenite layering, and itself has layering that dips abut 2 degrees flatter, giving a relatin nt unlike an angular uncnfrmity (Fig. 1, lcality B). Fragments and blcks f livine pyrxenite, sme f them layered, are included in the peridtite and are cncentrated in the lwer half f the expsed sectin like a basal cnglmerate. Many COMPLEX 1 HORNBLENDE OLIVINE PYROXENITE 2 ~ dl:l i= I ~ ~ I I ~ L 5) G, 1 <I ~,I, I 1 OLIVINE PYROXENITE ::;; :: 1 w 1 f- W " B LL 6 6 :: w ~ 2 ~DI,~,,I, ~~~ I DUNITE AND,1 PERIDOTITE 6 ~ en en ~ ~ g _ ill ~ ~~ s en ~ en 5 ~ ~ n, INDEX OF REFRACTION FIG. 6. Histgrams f the refractive indices f clinpyrxene and livine in the majr ultramafic rck units. f the smaller fragments ccur in fragmental layers (Fig. 9). Larger blcks, cmmnly having dimensins f tens t hundreds f feet, ccur individually r in jumbled clusters. Generally their weight and impact has caused intense defrmatin f peridtite layers n which they have fallen, and crss-layering is assciated and apparently was prduced by irregular currents accmpanying influx f the blcks. It is clear that bth intrusins were largely liquid, althugh the yunger may have cntained an appreciable prprtin f livine crystals. Emplacement f the yunger intrusin seems t have caused flding and faulting in earlier livine pyrxenite and, thus, was prbably frcible. In the Judd Harbr utcrp, layered structures in the dunite-peridtite zne are discrdant with thse in livine pyrxenite, and here t, relatins suggest that the frmer was frcibly emplaced int the latter. Thickness estimates fr the layered rcks are tenuus because f the cmplicated structure. Intermittently layered livine pyrxenite ttals abut 1,5 feet in the Hall Cve utcrp and 53 feet in the Judd Harbr utcrp. Abut 15 feet f cntinuusly layered peridtite are expsed in the Hall Cve utcrp. The interpretatin given in Fig. 1 shws dunite and peridtite as part f a yunger intrusin and

7 2 T.». IRVI1VE assumes that the cmplex is ne large bdy with a highly cntinuus peripheral zne f hrnblende pyrxenite. The vertical dimensin shwn fr the bdy is the minimum required t accmmdate the expsed layered rck. Mafic pegmatite. A remarkable pegmatite f hrnblende and anrthite (Fig. 2) ccurs in abundance in the suthwest part f Duke Island. Mst f it frms veins and dikes in gabbr, but sme ccurs in a swarm f dikes trending N. 5 W. thrugh the livine pyrxenite in the Judd Harbr ultramafic utcrp. The dikes range in width frm half an inch t 2 feet, and a few extend 12 feet alng strike. Grain size is cmmnly 1-6 inches and lcally reaches feet. A mre detailed descriptin and pictures f the rck have been published by Kschmann (1935). The very calcic plagiclase in the pegmatite precludes derivatin f the rck frm nrmal gabbric magma. The ultramafic cmplex is the prbable surce because it is rich in calcium and cntains hrnblende that is ptically and chemically similar t the pegmatite hrnblende. The pegmatite is mre cmmn near the better develped parts f the hrnblende pyrxenite zne, and cmparable pegmatite is assciated with several f the ther ultramafic bdies in Alaska. The abundance f the pegmatite at Duke Island, especially between the tw areas f ultramafic utcrp, suggests the presence f mre ultramafic rck at depth. Altered (hrnblende) gabbr. An appreciable part f the primary gabbr has been altered t hrnblende and plagiclase. Sme f the alteratin fringes Oi He y~ -,-, I Mg ATOM PER CENT Fe Clinpyrxene: ttl Ca:Mg:(Fe"+Fe'''+Mn). 11 Clinpyrxene: as abve, less Ca as hypthetical m Orthpyrxene. B' Pyrxene vein in peridtite. C Olivine pyrxenite. C' Olivin e pyrxenite. D Hrnblende pyrxenite. D' Hrnblende pyrxenite. x Trend f Ca- rich pyrxene Skaergaard intrusin. After y Trend f Ca-pr pyrxene Skaergaard intrusin. After Z Trend f clinpyrxenes in cmmn mafic magmas. After Hess,191. in the Brwn, in the Brwn, H Nri te FIG. 7. Plt f Duke Island pyrxenes n a Di-He-Mg-Fe diagram.

8 DUKE FIG. 8. Tw generatins f graded enite zne f the Hall Cve ultramafic n the left has been brken frm and, then, cvered by the layers the entire structure has been tilted inches. ISLAND ULTRAMAFIC layering in the livine pyrxutcrp. The angular blck a well-slidified accumulate n the right. Subsequently, t the right. The scale is six granite plutns and prbably has been caused by these yunger intrusins; its plagicalse ranges frm An2 t An57. Mst f the altered r hrnblende gabbr ccurs arund the ultramafic rcks, particularly between their tw main areas f utcrp. It has uneven prphyrblastic t pegmatitic texture and is intimately intermixed with and veined r intruded by mafic pegmatite. Its plagiclase ranges frm An t An98 and, as shwn in Fig. 2, is t a cnsiderable extent intermediate between the feldspars f pyrxene gabbr and mafic pegmatite. Evidently, this hrnblende gabbr is an alteratin f pyrxene gabbr caused by the highly calcic water-rich pegmatite frm the ultramafic cmplex. GENESIS OF THE ULTRAMAFIC COMPLEX Evidence f an ultrabasic magma. In the structural interpretatin presented in Fig. 1, it is implicit that the ultramafic rcks represent at least tw intrusins f magma f ultra.basic cmpsitin.' Such an interpretatin is, f curse, subject t serius questin, particularly in view f the abundance f gabbr expsed n the island. Hwever, attempts t interpret the ultramafic and gabbric rcks as 1 The adjective ultramafic shuld prbably nly be used in reference t mafic-rich rcks that are essentially free f feldspar. The magma's cmpsitin is described as ultrabasic, implying lw silica cntent, because the feldspar-bearing mafic pegmatite is prbably ne f its differentiates, and because chemical anajyses shw the Duke Island ultramafic rcks t cntain appreciable nrmative anrthite. COMPLEX 3 FIG. 9. Fragmental layers in the peridtite zne f the Hall Cve ultramafic utcrp. Small fragments f livine pyrxenite have been srted by size. They are surrunded by a peridtitic matrix. The scale is six inches. parts f a stratifrm bdy like the Stillwater cmplex have nt given reasnable results, and certain features are in direct cnflict (ntably, the relatins at lcality A). The idea f tw ultrabasic intrusins prvides a relatively cnsistent picture, and a variety f chemical and mineralgical evidence summarized belw seems in agreement. The ultramafic and gabbric rcks cntrast markedly in their mineralgy and chemistry. The primary gabbr generally cntains rthpyrxene and lcally is nritic. Ca-pr pyrxene des nt ccur in the ultramafic rcks and is absent frm their nrms (except in rck appreciably xidized during serpentinizatin f livine). Ultramafic clinpyrxene is undersaturated in Ca-pr pyrxene and, if anything, becmes mre s in the lwer-temperature differentiates (Fig. 7). It verlaps gabbr clinpyrxene in refractive index but has a differen t trend f ptical and, prbably, chemical variatin (Fig. ). Overall, the ultramafic rcks are extremely undersaturated in silica. Their nrms cntain nt nly livine, but nepheline, leucite and larnite. The chemical trend in ging frm dunite t mafic pegmatite is nt ne f appreciable silica enrichment, and mineral analyses indicate extensive substitutin f AI fr Si in the tetrahedral sites f clinpyrxene and hrnblende frm the lwer temperature rcks, due prbably, t sme extent, t a deficiency f silica. Sme f the mineralgical features that cntribute t the cntrast in degree f silica saturatin shwn by ultramafic and gabbric rcks might be ratinalized with frmatin f the rcks frm ne magma if it is assumed that, after accumulatin f

9 T. N. IRVINE the livine-bearing ultramafic units, the main bdy f residual magma underwent a brief perid f extensive xidatin. The change f irn frm the ferrus t the ferric state culd have caused precipitatin f the abundant magnetite in the hrnblende pyrxenite zne and, in additin, wuld reduce the FeH /Mg rati f the residual magma and effectively increase its silica cntent. As a result, the Fe/Mg ratis f the mafic silicates crystallizing frm the magma wuld drp abruptly and then prgressively increase again with differentiatin after xidatin ceased, and rthpyrxene might take the place f livine in the crystallizatin sequence, appearing bth as a distinct phase and in increased amunts in slid slutin in clinpyrxene. This wuld accunt fr sme f the pyrxene variatins shwn in Figs. 3 and. The effective prductin f silica by xidatin culd have led t the precipitatin f gabbr plagiclase in that it wuld prduce albite in place f the nrmative nepheline cmmn t the ultramafic rcks. The principal feature that seems t cntradict the abve interpretatin, in additin t the structural relatins, is that clinpyrxene in the rck richest in magnetite, hrnblende pyrxenite, has a higher Fe/Mg rati than that in the livine-bearing ultramafic units and, pssibly, less Ca-pr pyrxene in slid slutin (Figs. 6, 7). Furthermre, the envisaged perid f xidatin wuld prbably require a rather special event in the histry f the cmplex, which seems unlikely inasmuch as similar magnetiterich pyrxenites ccur in several ther ultramafic bdies in Alaska and British Clumbia (Taylr and Nble, 196). Clinpyrxene variatins in the ultramafic rcks at Unin Bay are cmparable t thse shwn in Figs. 6 and 7 (Ruckmick and Nble, 1959). If the ultramafic rcks were differentiated frm gabbric magma, they might be expected t cntain interstitial plagiclase as a prduct f interprecipitate magma. Als, ne might expect the ultramafic and gabbric rcks t have gradatinal cntacts. The cmplex is, hwever, devid f plagiclase, and its cntact with gabbr is everywhere sharp and apparently intrusive. Filter pressing might lcally remve pre magma but prbably culd nt cause a cmplete absence. It might be argued (in fact, in parts f the cmplex, it is prbable) that the pre magma was water rich and that the cmpnents f plagiclase crystallized in hrnblende; the ultramafic rcks have nrmative anrthite that, in sme, is due t Ab3 in hrnblende, and data btained by Yder and Tilley (1956) indicate that, in basic rcks at mderate t high water pressures, plagiclase is incmpatible with pyrxene and livine, their reactin prducing hrnblende. Nnetheless, unless tw distinct magmas are invlved, it seems frtuitus that interstitial plagiclase shuld be absent everywhere in the Duke Island ultramafic cmplex (and in the ther ultramafic bdies in S. E. Alaska; cf. Taylr and Nble, 196), especially when livine, pyrxene, and plagiclase cexist in the primary gabbr. On the ther hand, a high activity f water might well have been ne f the characteristics f the ultra.basic magma that distinguished it frm the gabbr magma. The mafic pegmatite is the nly feldspathic rck cnsidered t be directly related t the ultramafic cmplex. Althugh abundantly expsed it prbably is a prprtinately minr unit, and in terms f nrmative mineralgy, it t is very much undersaturated in silica. The mineralgic and textural relatins f hrnblende gabbr and mafic pegmatite are nt typical f cmagmatic rcks and wuld seem t preclude their frmatin by differentiatin f ne liquid. Mre likely, the hrnblende gabbr is the alteratin aurele f an ultrabasic magma emplaced in a cuntry rck f pyrxene gabbr. Cmpsitin and crystallizatin f the magma. Finegrained rcks that might represent chilled ul trabasic magma have nt been recgnized at Duke Island. Rugh estimates f the chemical cmpsitin f the average rck expsed in each f the ultramafic utcrps are given in Table 1. They were btained by weighting chemical and mdal analyses by the abundances shwn in Fig. 5 and are, therefre, clse t the cmpsitin f livine pyrxenite. The extent t which the estimates may represent the liquid part f the initial intrusins depends, f curse, n many factrs: e.g. the assumptins made in the cmputatins; the extent t which the expsed rcks represent the cmplex as a whle; the prprtin f crystals in the initial intrusins; the cmpsitin and amunt f mafic pegmatite and ther materials lst t the cuntry rck; and the extent t which the analyzed rcks represent cntaminatin by cuntry rck. Olivine pyrxenite is prbably a gravitatinally accumulated differentiate f the magma; it cnstitutes mst f the layered rck and apparently represents the prduct f ctectic precipitatin f livine and clinpyrxene inasmuch as the prprtins f these minerals are rughly thse f the eutectic prduct in the system CaMgSi26MgzSi (Bwen, 191). Hwever, the absence f cryptic layering may indicate that the rck is clse

10 DUKE ISLAND ULTRAMAFIC COMPLEX 5 in cmpsitin t its parent magma. Dunite and peridtite prbably frmed by fractinatin f an excess f livine ver the ctectic prprtin and/r by accumulatin f livine crystals suspended in the initial intrusins. The estimates in Table 1 include nrmative anrthite, hence the anrthite-bearing mafic pegmatite is a cnceivable late differentiate. The hrnblende pyrxenite zne, as expsed, is cnsiderably mre extensive in the Hall Cve ultramafic utcrp than in the Judd Harbr utcrp. Hwever, as shwn in Fig. SA, the livine-bearing rcks in the Judd Harbr utcrp cntain a larger prprtin f hrnblende, and this may indicate that hrnblende pyrxenite has frmed by sme prcess f differentiatin in situ that was mre effective in the nrthwest part f the cmplex. Hrnblende pyrxenite at the tp f the cmplex may be the result f gravity-cntrlled fractinal crystallizatin, livine having ceased t crystallize because f a peritectic reactin giving hrnblende r magnetite, r bth. The magnetite layering at Kelp Island culd be supprting field evidence, and perhaps ntable is that much f the hrnblende pyrxenite has abut the same prprtin f magnetite as the liquidus minimum f the jin CaMgSi 2 6 -Fe3 in air atmsphere (D. Presnall, persnal cmmunicatin, 1962). Hwever, the marginal (and basal?) brder zne requires a different explanatin. The cntinuity f the brder zne suggests that it is a reactin rim. In fact, the physical and mineralgic relatins f the hrnblende pyrxenite zne and the alteratin aurele f hrnblende gabbr culd be interpreted t represent an apprach t lcal equilibrium alng thermal and chemical gradients between a wa ter-rich ultrabasic magma and a pyrxene gabbr cuntry rck. The mechanics f the reactins are prblematical but are prbably related t the escape f mafic pegmatite frm the ultramafic cmplex. Ntable in this respect is that an appreciable amunt f hrnblende and magnetite ccurs as alteratin assciated TABLE 1. ESTIMATES OF THE COMPOSITIONOF THE AVERAGE ULTRAMAFICROCK EXPOSED IN EACH OF THE Tw MAIN AREAS OF ULTRAMAFICOUTCROP AT DUKE ISLAND. H 2 Is NOT INCLUDED I II e.i.p.w. Nrms Hall Judd Cve Harbr I II Si t } )3.9 Ab lfa.3 5. Fe [D' }.9} Clpx FeO lhe MgO Larnite eao An Na2.6. Ne K2.. Lc Ti2.8.5 Mt MnO.1.1 II Areas Represented; Hall Cve utcrp 5. sq. miles Judd Harbr u tcrp 3.6 sq. miles with the swarm f mafic pegmatite dikes in the Judd Harbr livine pyrxenite. Much f the perid f slidificatin f the ul trabasic magma must have been marked by ctectic crystallizatin f dipsidic pyrxene and frsteritic livine. A maximum temperature during this time wuld be that f the eu tectic in the system CaMgSiz6-Mg2Si (1387 C. at 1 atmsphere; Bwen, 191). Pssible excess livine in slutin during the frmatin f dunite culd require higher temperatures, but the ther cnstitutents evident in the ultramafic rcks (e.g. FeO, Fe23, AhOa and H 2 ) wuld lwer the liquidus. Thus the magma may initially have been at 125 C. r less and was prbably appreciably cler by the pegmatitic stage. BOWEN, N. L. (191) The system dipside-frsterite-silica. Am. Jur. Sci. 38, BROWN, G. M. (1957) Pyrxenes frm the early and middle stages f fractinatin f the Skaergaard intrusin, East Greenland. Mineral. Mag. 31, HESS, H. H. (191) Pyrxenes f cmmn mafic magmas. Am. Mineral. 26, , (196) Stillwater igneus cmplex: a quantitative mineralgical study: Gel. Sc. Am. Mem. 8,1-23. KOSCHMANN, A. H. (1935) Hrnblendite and related rcks f Annette and Duke Islands, sutheastern Alaska. Am. Gephys. Unin Trans., 16th Ann. Meet., REFERENCES RUCKMICK, J. C. ANDJ. A. NOBLE (1959) Origin f the ultramafic cmplex at Unin Bay, sutheastern Alaska. Gel. Sc. America Bull. 7, TAYLOR, H. P., JR. AND J. A. NOBLE, (196) Origin f the ultramafic cmplexes in sutheastern Alaska. 21st Int. Gel. Cng Rpt., Pt. XIII, WAGER, L. R. AND W. A. DEER (1939) The petrlgy f the Skaergaard intrusin, Kangerdlugssuag, East Greenland. Medd. Grentand, 15, YODER, H. S., JR. AND C. E. TILLEY (1956) Natural thleiite basalt-water system. Carnegie Inst. Wash. Yearbk. 55,