Oxygen isotopic evidence for meteoric water interaction with the Captains Bay pluton, Aleutian Islands

Earth and Planetary Sctence Letters, 45 (1979) 16 22 © Elsevier Scientific Pubhshlng Company, Amsterdam - Printed In The Netherlands

16

[2]

OXYGEN ISOTOPIC EVIDENCE F O R METEORIC WATER INTERACTION WITH THE CAPTAINS BAY PLUTON, A L E U T I A N ISLANDS MICHAEL R P E R F I T

Research School of Earth Sctences, Austrahan Nattonal Umverstty, Canberra, A C T 2600 (Austrahaj and JAMES R LAWRENCE

Lamont-Doherty Geologtcal Observatory, Pahsades, NY 10964 (U S A )

Recewed March 12, 1979 Rev]sed version received June 26, 1979

Low 180/160 values have been found in rocks from the Captains Bay pluton and surrounding country rocks on Unalaska Island In the Aleutian Islands The pluton IS surrounded by a metamorphic aureole in which the country rocks have been domed, faulted and chaotically fractured Samples with lower 180/16 0 values (8180 = - 4 1 to +2 7°Ao,SMOW) commonly have lnterstmal mlcrographlc Intergrowths of quartz and turbid potassium feldspar These facts suggest extensive interaction of orculatmg meteoric waters with the pluton Unalaska Island must have been subaerially exposed at or shortly after the time of intrusion of the pluton The pervasive low-grade metamorphism of the "early series" volcanic rocks of all of the Aleutian Islands might be a result of the interaction of intrusive bodies with meteoric waters rather than the consequence of ridge subductlon NI4 ~/2*

1 Introduction

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The Captains Bay pluton is one of three wellexposed plutons that intrude shghtly metamorphosed, Tertiary volcanic and sedimentary rocks on Unalaska Island m the Aleutian Islands (Fig 1) The petrography, composition and field relations of the Captains Bay pluton ( " d 00 km 2) are typical of the larger granodlorlte plutons along the arc [ 1 - 6 ] Generally, the Aleutian island arc consists of (1) an "early series", as old as Eocene, composed o f marine elastics, volcanoclastlcS and volcanic flows (predominantly submarine) and associated plutons that have been slightly deformed and metamorphosed to greenschist-grade, (2) a middle unit of plutonxc rocks with radlometrxc ages primarily between 10 and 15 m y Lamont-Doherty Geolog]cal Observatory Contribution No 2862

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Fig 1 Location map of plutomc rocks on Unalaska Island in the Aleutian Islands Area mapped In Fig 2 is enclosed an the dotted lines

17 B P , and (3) a "late series" (<5 m y B P ) consisting of interbedded "andesltlC" volcanic rocks and volcanoclastlcs that are unmetamorphosed and lie unconformably over the older units [7,8] The formation of this sequence of units and the tectonic history of the Aleutian arc have been related to the progressive subductlon of the Kula plate, Kula Radge and Pacific plate into the Aleutian trench during Tertiary tune [9-13] A lack of good paleontological data, detailed field work and the tenuous correlation of sedimentary rock units has hampered the refinement of these tectonic models The problems of timing and cause of the low-grade thermal metamorphism along the arc also remains unresolved

[81 The oxygen isotope and petrographic data presented m this paper help to resolve some of these problems We document large-scale Interaction of meteoric water with the epizonal Captains Bay pluton and surrounding "early series" country rock S:milar interactions have been described in other volcanoplutonlc locahtles (e g [11-14])

2 General geology and petrography The Captains Bay pluton intrudes the Unalaska Formation which is composed of fine to coarse sedimentary and pyroclastlc rocks intercalated with basaltic to dacltIC flows and sills Numerous dikes (some genetically related to the pluton) intrude the sequence The Unalaska Formation is at least as old as early Miocene based on Mulusca fossils found in breccia and sandstone south of the pluton [2] Amphibole separates from the Shaler pluton, west of the Captains Bay pluton, yielded a K-At age of 11 1 -+ 3 0 m y BP [7] The Captains Bay pluton is crudely zoned from a narrow run of two-pyroxene gabbro and diorite to a heterogeneous central region of hornblende-biotite granodiorite that is intruded by apllte dikes (Fig 2) Two smaller, more mafic plutons crop out on the northeast coast of Amaknak Island north of the main pluton (Figs 1 and 2) The uppermost, porphyritic surface of the pluton is exposed beneath the Unalaska Formation on Pyramid Peak (~650 m) Here the country rocks have been vertically displaced by approximately 450 m

Surrounding the pluton, the country rocks have been domed, faulted and chaotically fractured The intrusive contacts are characterized by (1) inequlgranular to porphyritic plutonlc rocks that exhibit irregular and indistinct contacts with the wall rock, (2) locally abundant xenohths and schlleren, (3) extensive brecciatlon (exploslveg), (4) secondary mmerahzatlon including quartz, potassium feldspar, tourmaline, pyrite and calcopyrlte, (5) vein pegmatlte and mlarolitic cavity development, and (6) hydrothermal alteration of wall rock evidenced by the Irregular enrichment of volcanic rocks with potassium and/or silica Although xenohths may be abundant at these "forceful" contacts, there is no physical or chemical evidence to suggest assimilation Border zones composed of gabbro and diorite do not have these features and give the appearance of more "passive" intrusion Metamorphic grade diminishes away from the contacts in a fashion similar to that around the larger Shaler pluton [1] Rare hornblende-hornfels is in contact with, or included In, the Captains Bay pluton but rapidly grades into albite-epldote hornfels and then into low-grade greenschlsts, which lack epldote and Include zeolltes At distances beyond 5 km from contacts, alteration IS minimal The gabbrolc to dlorltlC rocks are typically equlgranular, have a hypldlomorphlc-granular texture and occasionally have cumulate textures or igneous lamination They are primarily composed of small ( 1 - 2 mm) euhedral plagloclase crystals and lesser, granular (<1 mm) auglte, hypersthene and Irontitanium oxides Biotite, hornblende and quartz are present in minor to trace amounts Modal abundances are presented In Table 1 Hornblende and biotite increase in abundance at the expense of pyroxene in more slhclc rocks Quartz diorite and quartz monzodlorite are the dominant rock types found In the interior of the Captains Bay pluton although locally, granodiorite is volumetrically significant These intermediate granit:c rocks are hypldlomorphlc-granular to porphyritic and coarser grained than the gabbros Boundaries between rock types are nearly always gradatlonal and mmeralogIcal variations are subtle Zoned plagloclase crystals are the most abundant constituent (generally comprising more than 50% of the mode) and rarely have runs of potassium feldspar or antlperthltlC rater-

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Fig 2 Geologacmap of the Captains Bay pluton with sample locahtles from this study 180/160 values for each sample are shown below the bold faced sample numbers Sample numbers are hsted m Table 2 growths Auglte and hypersthene coexist m the intermediate rocks, but hornblende or/and biotite are more abundant m more felslc rocks Uralmc hornblende is a ublqmtous replacement of pyroxene Quartz and orthoclase are always interstitial and exhibit a variety of textures (1) most commonly as mlcrographlc or granophyric lntergrowths, (2) xenomorphic granular or granitic texture, (3) least commonly as separate interstitial anhedral grams In some altered plutomc rocks chlorite, epldote, tourmaline and/or sulphldes are present Granophyres occur as irregular small masses and apophyses that grade unperceptlbly rata the surrounding plutonlc rocks They are composed of graphically lntergrown quartz and cloudy potassium

feldspar which surround altered phenocrysts of plagloclase and relaUvely minor ferromagneslan and oxide phases Aphtes, which typically intrude interme&ate plutonic rocks, appear to have been late-stage liquids They are xenomorphlc-granular and contain variable amounts of quartz and potassium feldspar with relicts of alblte and traces of mafic minerals

3 Analytical results Oxygen isotope analyses of eight rocks from the Captains Bay pluton and three country rocks from the Unalaska Formation are presented in Table 2

19 TABLE 1 Modal analyses of typical plutomc rocks from the Captains Bay pluton * Rock type

Sample No

Qtz

Plag

K-spar

Brat

Amph

Pyx

Op

Acc

Gabbro Gabbro Gabbro Norlte Quartz diorite Quartz monzodmnte Quartz monzodlorlte Quartz monzodlorlte Quartz monzodmnte Granodlonte Grano&onte Granophyre Aphte Aphte

189 H 66 C 201 169 H 32 175 181 245 U5-185 236 UN36 211 174AP 32B-AP

0 Tr 1 Tr 5(g) 7 12(g) 16(g) ll(g) 18(g) 21 26(g) 33 39

57 68 63 55 55 50 50 45 46 45 40 23 10 13

0 Tr 0 0 9 17 15 18 22 20 18 39 56 42

Tr 1 0 23 6 7 6 6 4 4 3 10(Chl) Tr 2

0 23 Tr 5 13 7 4 10 5 4 15 Tr 0 Tr

33 9 30 10 2 5 10 1 9 5 3 0 0 0

I0 3 6 6 8 4 3 4 3 4 1 Tr 1 2

Tr Tr(Chl, E) 1 1 l(Chl, E) 1 1 l(Chl, E) 1 1 1 2(E, T) Tr 2(E, T)

Note Tr = trace, Acc = accessory, Op = opaques, Chl = chlorite, E = epldote, T = tourmahne g = abundant graphic lntergrowths * Over 1000 points counted for each thin-section

(also, see Fig 2) A majority of the samples exhibit highly depleted ISo/160 values (relative to SMOW) compared with "normal" igneous rocks Isotoplcally "normal" granitic rocks have whole-rock 6 ~80 values in the range +6 to +10 with a tendency to increase from about +6 to +9 m the gabbro to granite fractlonatlon sequence [15] Numerous studies (e g [ 1 1 - 1 4 ] ) have shown that low 6180 values, such as those reported here, are produced by circulation of

TABLE 2 618 O values of rocks from Unalaska Island Sample No

Rock type

618OsMo w (*/~)

189H 66C 32 175 181 245 211 174AP U5-157 192

gabbro gabbro quartz diorite quartz monzodlonte quartz monzodlorlte granodlonte granophyre aphte dike metabasalt (amphlbohte) volcamc breccia (amphlbole-epldote-bmtlte hornfels) andeslte (greenschlst)

+5 7 +4 9 _+0 1 - 10 +6 2 +2 7 +_0 2 -0 8 -4 1 +7 0 - 3 5 _+0 1

131

-1 9 ± 0 4 -0 5

meteoric water m a convective cell around eplzonal mtrusive bodies as they cool Isotopic depletion has also been observed m country rocks several kilometers from magma chambers, particularly where the surrounding rocks are highly 1orated or fractured volcamcs (1 e [11,14]) On the basis of the 180/160 analyses, the plutonlc rocks can be divided into two categories (1) those with "normal" 6180 values, and (2) those with depleted values The two groups exhibit distract textural differences The most noticeable feature is the presence of interstitial mlcrographlc lntergrowths of quartz and turbid potassium feldspar which is ubiquitous m the 180-depleted mtermedlate rocks Less commonly, the I aO-depleted group will also show the effects of late-stage alteration such as chlorite replacement of mafic minerals, replacement of plagloclase by potassium feldspar, the formation of epldote, rutlle, hematite, sulphldes and/or tourmalme These characteristics are usually observed m samples from altered contact zones The presence or absence of mlcrographlc intergrowths appears to be directly related to the 6180 value of a rock For example, low-61~O quartz monzodlorlte (181) differs mlneraloglcally from "normal" quartz monzodlorlte (175) only by the abundant interstitial mlcrographxc mtergrowths it contams

20 Granophyre (211), that has the lowest 81 s o value ( - 4 1) is an extreme example of this In sharp contrast to the granophyre, a chemically similar apllte (174AP) has a rather normal 81 s o value (+7 0) and has few graphic lntergrowths One border zone gabbro (189H) has a normal 8180 value (+5 7), whereas a shghtly metamorphosed gabbro (66C) has a somewhat low value (+4 9) At least m this case the effects of metamorphism do not have a great effect on lowering the I So/160 ratio in the plutomc rocks It as also of interest to note that the chemical composition of individual minerals m "normal" and depleted rocks are essentially the same The irontitanium oxides, however, exhibit the effects of latestage re-eqmhbratlon under highly oxidizing conditions, probably associated with the influx of large volumes of water [2] Marked 180 depletions m the country rocks are related to their proximity to the pluton An amph:bohtlzed basalt sampled from the intrusive contact has the lowest value (and also the highest grade of metamorphism), whereas a slightly altered andeslte, approximately 3 km from the contact, is the least depleted (81 s O = - 0 5) A volcamc breccm w:th intermediate metamorphic grade, half a kdometer from the contact, has a 8180 value between the two metavolcanlc rocks

4 Discussion Plutomc rocks from the Captains Bay pluton that exhibit 180 depletions have textural, mineralogical and field related characteristics common to 180depleted plutonlc rocks from other regions [15] These features suggest that a great deal of volatile/ hydrothermal fired mteractmn occurred with the coohng plutomc magma and surrounding country rocks In particular, the formation of mlcrographlc textures in gramtIc rocks has been ascribed to the coexastence of an aqueous gas phase with the magma

[21 Using the model of Taylor [17] it can be shown that plutomc rocks and volcanic country rocks can become depleted by lnteractmn of a magma with a convectmg cell of meteoric water Fig 3 shows the 6180, 8rf, of a rock that has undergone oxygen lSO-

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Fag 3 Plot of 8 18 0 values of hydrothermally altered rocks vs calculated water/rock (W/R) ratxo for the open system (o) and closed system (c) models of Taylor [17] assuming an lnlUal 8 1 8 0 for the rock before alteration of 6~o The W/R values are based on the ratio of water oxygen to rock oxygen The upper curves (dashed) assume an mmal 818 O value of 0%o, that of seawater The lower curves (sohd) assume an anmal 8 1 8 0 value of -10%o, that of meteoric water in the Aleutian Islands The 818 O values of three of the samples studied are represented by dotted hnes

tope exchange wlth either seawater (8180 = 0%o) or meteoric water (8a80 = - 1 0 -+ 2%o, determined from 180/16 O and D/H analyses of precipitation from Adak and Bethel, Alaska by the International Atomic Energy Agency m Vienna, Austria) at the temperatures of 200,300 and 400°C as a function of the water/rock (W/R) ratio It can be seen that very low 8180 values, <3%o, can only result by interaction with meteoric water Interaction with seawater simply cannot produce 6 a80 values much lower than 3%o Because most of the rocks depleted in 180 from the Captains Bay pluton and surrounding country rocks have/5180 values lower than 3%o the clcrulatlng fluid must have been meteoric water Water wxth a 818 O like that present today (-10%0) as appropriate Exam1nation of Fig 3 shows that higher temperatures and hagher water/rock ratios yield lower 8180 values for the lSOtoplcally exchanged Igneous rocks As previously noted, the three country rocks analyzed have progressively lower 6 1 8 0 values as the pluton is approached The observed metamorphic assemblages indicate that temperatures could have

21 been above 400°C adjacent to the pluton and less than 300°C at a distance 3 km from the contact This means that water/rock ratios m the country rocks would have been between 1 2 and 2 0 during isotopic exchange The rate of convection, however, would have been higher closer to the phiton yielding higher water/rock ratios Varlablhty of ~ 18O values observed within the pluton may be explained by examining the history of cooling, crystallization and fracturing Border-zone gabbros which were formed by early fractmnal crystallization, accumulation of crystals and rapid coohng along the margins of the intrusion, could have been sealed from isotopic exchange with meteoric water Furthermore, we suggest that the i 80 depletions in the intermediate rocks are prtmarlly due to oxygen exchange of meteoric waters with late-stage residual magma which crystalhzed as mlcrographic ratergrowths or granophyres, which did not affect the previously sohdxfied gabbros The geometry of fluid circulation has been shown to be slgmficantly controlled by fracture systems [ 18] Thus, fracturing due to thermal contraction would have provided avenues for efflcent hydrothermal circulation and consequently maximized water/rock ratios (1 e , lower 6180) in more fractured and jointed regions Aphte dikes were intruded into the Captains Bay pluton after it sohdlfied The aphte investigated exhibits httle 1s O depletmn or granophyrlc ratergrowths At the tune when the aphtes were formed, either hydrothermal clrculahon was severely restricted or the pluton may have cooled to such an extent that a convectwe cell was no longer operatwe It ~s of interest to note that although water-rock interaction has occurred on a large scale, there has been little or no disturbance of major and trace element abundances or of Sr isotopic ratios m the plutonlc rocks Major and trace element contents vary predictably with dlfferentmtlon and show no evidence of extenswe mobility [2] Likewise, aTSr/S6Sr ratios, although variable, do not correlate with ~ 180 values 5 Tectonic imphcatlons We have shown that the Captains Bay pluton and surrounding "early series" country rocks have rater-

acted with a great volume of 180-depleted groundwater This necessitates that at least part of Unalaska Island be subaerlally exposed at or before the time of intrusion ( - 1 5 - 1 0 m y B P ) Uphft due to pluton intrusion along with volcanism may have occurred (<15 m y B P ) to produce the extensive exposure of eplzonal plutons and thousands of meters of Miocene sedimentary rocks on Unalaska Island We further suggest that the pervasive low-grade metamorphism observed on Unalaska Island (and possibly on other islands within the arc) is a consequence of hydrothermal circulation induced by plutonlc intrusion, not a result of ridge subductlon as DeLong et al [8] have suggested The effects of a younger thermal-isotopic overprinting cannot be overlooked Oxygen isotope studies of plutonlc and volcamc rocks from the central Aleutian Islands are in progress and should further define the thermal consequences of intrusion and the extent of subaerlal exposure along the Aleutian arc

6 Conclusions (l) Oxygen isotope data obtained from the Captains Bay pluton and surrounding country rocks clearly indicate that many of the rocks have been severely depleted in 180 relatwe to "normal" igneous rocks by ISOtOpic exchange with low -180 meteoric water (2) Circulation of a hydrothermal cell was enhanced by the abundant fractures and faults in the well-bedded volcanic terrain around the pluton (3) This meteoric-hydrothermal convection system produced extensive low-grade metamorphism at distances up to 5 km from the intrusive contacts (4) Water/rock ratios were estunated to have been between 1 2 and 2 0 within 3 km of the pluton but may have been much higher close to the mtruswe contacts (5) 180 depletions m the plutonic rocks correlate with the presence of late-stage mlcrographic ratergrowths, alteration, mineralization and highly oxidized iron-titanium oxades (6) Spatial 1518O variations within the pluton may be due to crystaUlzatlon history and proxtmity to fluid-transporting fractures (7) The commonly observed replacement of py-

22 r o x e n e s b y h o r n b l e n d e does n o t have a s u b s t a n t i a l effect o n t h e (51 s O values o f t h e p l u t o n l c rocks w h i c h suggests t h a t this is a m a g m a t l c r e a c t i o n u n r e l a t e d to late-stage a l t e r a t i o n (8) T h e A l e u t i a n arc was at least partially above sea level d u r i n g t h e p l u t o n l c event in M i o c e n e t i m e (9) T h e pervasive low-grade m e t a m o r p h i s m o f t h e " e a r l y series" m a y simply be a result o f i n t e r a c t i o n w i t h h e a t e d m e t e o r i c g r o u n d w a t e r s related to p l u t o m s m along t h e arc, r a t h e r t h a n t h e c o n s e q u e n c e o f ridge s u b d u c t l o n

Acknowledgements The a u t h o r s t h a n k R K a y , I Pddley, D E l t h o n a n d R K O ' N l o n s for h e l p f u l discussions The research was s u p p o r t e d b y N a t i o n a l Science F o u n d a t i o n grants O C E - 7 6 - 8 1 9 5 2 , O C E - 7 5 - 2 0 3 6 0 , E A R - 7 6 82456, and EAR-76-20945

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6 G P CItron, RW Kay, SM Kay a n d J k Sutter, Dlscovery of an early Ollgocene magmatlc event on Adak Island, central Aleutian Islands, Alaska, Trans Am Geophys Union 60 (1979) 413 (abstract) 7 M S Marlow, D S Scholl, [ C Bufflngton and T R Alpha, Tectonic history of the central Aleutian arc, Bull Geol Soc Am 84 (1973) pp 1555-1574 8 S E Delong, P J I ox and I W McDowell, Subductlon ot the Kula Ridge at the Aleutmn Trench, Bull Geol Soc Am 89 (1978) 8 3 - 9 5 9 D E Hayes and W C Pitman, IIt, Magnetic llneatlon in the North Pacific, in Geological Investigations of the North Pacific J D Hays, ed, Mem Geol Soc Am 126 (1970) 291-314 10 D S Scholl, E C Buffmgton and M S Marlow, Plate tectonics and the structural evolution of the Aleutian-Bering Sea region, In Contributions to the Geology of the Bering Sea Basin and Adjacent Regions, R B Forbes, ed, Spec Paper Geol Soc Am 151 ( 1 9 7 5 ) 1 - 3 1 11 I1 P Taylor and R W Forester, Low-180 igneous rocks from the Intrusive complexes of Skye, Mull and Ardnamurchan, western Scotland, J Petrol 12 (1971) 4 6 5 497 12 H P Taylor, Oxygen isotope evidence for large-scale interaction between meteoric ground waters and Tertiary granodlorite intrusions, Western Cascade Range, Oregon, ! Geophys Res 76 ( 1 9 7 1 ) 7 8 5 5 - 7 8 7 4 13 H P Taylor, Oxygen and hydrogen ISOtope evidence for large-scale circulation and Interaction between ground waters and igneous intrusions, with particular reference to the San Juan volcanic field, Colorado, m Geochemical Transport and Kinetics, A W Hofmann et al, ed, Carnegie Inst Wash Publ 634 ( 1 9 7 4 ) 2 9 9 - 3 2 4 14 R W Forester and tt P Taylor, 1 8 0 / I 6 0 , D/H, and laC/ 12 C studies of the Tertmry igneous complex of Skye, Scotland, Am J ScI 277 ( 1 9 7 7 ) 1 3 6 - 1 7 7 15 tf P Taylor, Oxygen and hydrogen isotope studies of plutomc granite rocks, earth Planet Scl Lett 38 (1978) 177-210 16 R tI Jahns and C W Burnham, Fxperlmental studies of pegmatlte genesis, 1 A model for the derivation and crystalhzatlon of gramte pegmatltes, Fcon Geol 64 (1969) 843-864 17 H P Taylor, Water/rock lnteractmns and the origin of H20 m granmc bathohths, J Geol Soc London 133 (1977) 509 558 18 D Norton and J Knight, Transport phenomena m hydrothermal systems cooling plutons, Am J Scl 277 (1977) 937-981