rocks from the Captains Bay pluton which is zoned from a narrow gabbroic rim to a core of quartz mon- zodiorite and granodiorite. The chemical variations.
Contributions to Mineralogy and Petrology
Contrib. Mineral. Petrol. 73, 69 87 (1980)
9 by Springer-Verlag 1980
Trace Element and Isotopic Variations in a Zoned Pluton and Associated Volcanic Rocks, Unalaska Island, Alaska: A Model for Fractionation in the Aleutian Calcalkaline Suite * Michael R. Perfit 1, I iannes Brueckner 2.3, James R. Lawrence 3, and Robert W. Kay 4 1 Research School of Earth Sciences, Australian National University, Canberra, ACT 2600, Australia 2 Department of Earth and Environmental Sciences, Queens College of the City University of New York, Flushing, NY 11367, USA 3 Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY 10964, USA Department of Geological Sciences, Cornell University, Ithaca, NY 14853, USA
Abstract. Trace elements, including rare earth ele-
ments (REE), exhibit systematic variations in plutonic rocks from the Captains Bay pluton which is zoned from a narrow gabbroic rim to a core of quartz monzodiorite and granodiorite. The chemical variations parallel those in the associated Aleutian calcalkaline volcanic suite. Concentrations of Rb, Y, Zr and Ba increase as Sr and Ti decrease with progressive differentiation. Intermediate plutonic rocks are slightly enriched in light REE ( L a / Y b = 3.45 9.22), and show increasing light REE fractionation and negative Eu anomalies (Eu/Eu*=l.03-0.584). Two border-zone gabbros have similar REE patterns but are relatively depleted in total REE and have positive Eu anomalies; indicative of their cumulate nature. Initial 87Sr/S6Sr ratios in 8 samples (0.70299 to 0.70377) are comparable to those of volcanic rocks throughout the arc and suggest a mantle source for the magmas. Oxygen isotopic ratios indicate that many of the intermediate plutonic rocks have undergone oxygen isotopic exchange with large volumes of meteoric water during the late stages of crystallization; however no trace element or Sr isotopic alteration is evident. Major and trace element variations are consistent with a model of inward fractional crystallization of a parental high-alumina basaltic magma at low pressures ( < 6 kb). Least-squares approximations and trace element fractionation calculations suggest that differentiation in the plutonic suite was initially controlled by the removal of calcic plagioclase, lesser pyroxene, olivine and F e - T i oxides but that with increasing differentiation and water fugacity the removal of sub-equal amounts of sodic plagioclase and hornblende with lesser F e - - T i oxides effectively drove residual liquids toward dacitic compositions. Major and trace element compositions of aplites which intrude the pluton are not adequately explained by frac*
LDGO Contribution no. 2964 Address offprint requests to: M.R. Perfit
tional crystallization. They may represent partial melts derived from the island arc crust. Similarities in Sr isotopcs, chemical compositions and differentiation trends between the plutonic series and some Aleutian volcanic suites indicates that shallow-level tractional crystallization is a viable mechanism for generating the Aleutian calcalkaline rock series.
Introduction
The combination of refined gcophysical, chemical and experimental data has resulted in a number of hypothcscs regarding the origin and chemical evolution of calcalkaline rock series along continental margins and island arcs. Much of the data has been obtained from volcanic rock suites where the exact physical and genetic rclationships among different rock types are often obscured or enigmatic. In an attempt to understand more completely the geochemical variations in volcanic rocks along the Aleutian Island arc, we have undertaken a program of detailed field and laboratory investigations of the plutonic bodies and individual volcanic centers (Perfit 1977, 1978, in preparation; S. Kay et al. 1977; Citron 1979 a). Plutonic bodies, often of batholithic size, have been considered to represent slowly cooled magma chambers that fed overlying, coeval volcanoes (Branch 1967; Larsen 1948; Bateman 1974; Schweickert 1976; Myers 1975; Cobbing and Pitcher 1972; Pitcher 1978 ; Snoke et al. in press). Similar elemental abundances and systematic elemental variations in contemporaneous volcanic and plutonic series have suggested analogous origins (Bateman et al. 1963; Hamilton 1969; Forbes et al. 1969; Gulson and Botinger 1972). During the past decade, a wealth of trace element and isotopic data has been obtained from volcanic rocks erupted at plate margins. Only
00t0-7999/80/0073/0069/$03.80
70
M.R. Perfit et al. : Trace Element Variations in a Zoned Pluton, Alaska 15OoW
iTfPW
,5~
Fig. 1. Simplified geologic map of the Captains
3~-2N 166~
166~
166~ I
5
I
Imi
I
I
I
0
I
5
I
I
I
km
I
I
0
recently, however, have the associated plutonic rocks been investigated in such detail (e.g., L o p e z - E s c o b a r 1974; Kesler et al. 1977; Perfit 1977; Silver and Early 1977; M a s o n and M c D o n a l d 1978; Price and Sinton 1978 ; B a t e m a n and Nokelberg 1978 ; Frey et al. 1978 ; Bateman and Chappell 1979; G r o m e t 1979; Snoke et al. in press). In this paper we present rare earth element, selected trace element, 8~Sr/S6Sr, ~sO/160 and m a j o r element data f r o m the zoned Captains Bay pluton on Unalaska Island (Captains pluton o f Drewes et al. 1961) in the Aleutian Islands (Fig. 1). M o r e than 100 samples f r o m the pluton and 45 associated volcanic rocks were analysed for Rb, Sr, Y, Zr and Ti a n d over 300 samples were petrographically examined. The data permit us to trace the chemical evolution o f the pluton and provide useful information regarding the generation o f the Aleutian calcalkaline series. A detailed discussion o f the phase chemistry and chemical c o m p a r i s o n with Aleutian volcanic rocks is presented elsewhere (Perfit 1977, in preparation). The Captains Bay pluton ( ~ 100 k m a) is crudely zoned f r o m a n a r r o w gabbroic rim ( < 100 m) to a heterogeneous central region o f quartz diorite, quartz monzodiorite and granodiorite (Fig. 1). N u m e r o u s aplite dikes and granitic apophyses intrude the central portion of the pluton and are a b u n d a n t along some
Bay pluton on Unalaska Island in the Aleutian Islands (see inset). Specific sample localities are indicated. Horizontally lined regions are primarily gabbroic; vertically lined regions are primarily felsic with abundant granophyre and aplite; the remaining shaded area consists of quartz dioritic to granodioritic intermediate plutonic rocks. The unshaded regions are part of the Unalaska formation or covered by Quaternary alluvium. Along contacts solid lines are gabbroic border zones and dotted lines are either hidden or inferred contacts. Internal contacts are typically gradational
intrusive contacts. The pluton intrudes slightly metam o r p h o s e d Tertiary volcanic and sedimentary rocks on Unalaska Island and is petrographically similar to the larger, Shaler pluton on the island (Drewes et al. 1961) (see Fig. 2). The larger plutons in the Aleutians are primarily quartz diorites to quartz monzodiorites and exhibit similar inward petrologic zonations. Small gabbroic intrusives c o m m o n l y border the m a j o r plutons and larger, older gabbroic intrusives crop out on a few islands (Fraser and Snyer 1959; Coats 1956; Byers 1959; Drewes et al. 1961 ; Gates et al. 1971).
Tectonic and Geologic Setting Unalaska Island is the third major island west of the Alaskan Peninsula and second largest in the Aleutian Island arc: The islands represent the subaerial expression of the Aleutian Ridge upon which an arcuate chain of Quaternary stratovolcanoes has been built. The Aleutian Trench lies 170 to 300 km to the south of the islands and deepens westward to about 7500 m around 178~ (Marlow et al. 1973; Jacob et al. 1977). The Pacific plate is moving in a northwesterly direction with respect to the North American plate at a rate of 6.4-7.4 cm/year (Minster et al. 1974). The direction of motion is nearly perpendicular to the northeasterly trend of the eastern and central Aleutians. Recent seismic models show that the 80 km thick Pacific plate underthrusts the arc at a shallow angle (~30 ~ until it reaches
M.R. Perfit et al. : Trace Element Variations in a Zoned Pluton, Alaska a depth of 40 km where the dip abruptly increases to about 70~ at 250 km at which point earthquake foci are no longer recorded (Jacob and Hamada 1972; Engdahl 1977). Along the length of the arc, at depths greater than 40 kin, the shape and location of the Benioff zone relative to the active volcanoes is nearly constant (Davies and House 1979). Seismic studies indicate that there is a gradual crust-mantle transition between 20 and 40 km beneath the arc and that low velocity, low Q asthenosphere exists below 70 km to a depth of 100-110 km where the high velocity, high Q subducted slab is encountered (Jacob and Hamada 1972; Grow 1973; Engdahl 1977). The Aleutian Basin which lies directly north of the Aleutian Ridge is believed to be normal oceanic crust a remnant of the non-consumed Kula plate (Cooper et al. 1976). Apparently, the Aleutian arc has been formed in a compietely oceanic environment commencing in the earliest Tertiary (Scholl et al. 1975). Generally the rocks that form the Aleutian Islands conform to three informal units (Marlow et al. 1973; Delong et al. 1978): (1) An 'early series' composed of marine clastics, volcaniclastics and volcanic flows (primarily submarine) that have been slightly deformed and metamorphosed which may be as old as Eocene; (2) A middle unit of plutonic rocks with radlometric ages primarily between 10 and 15 m.y.b.p.; (3)A 'late series' ( < 5 m . y . b . p . ) consisting of interbedded basaltic and andesitic volcanics and volcaniclastics that are unmetamorphosed and lie unconformably over the older units. Recent K - A r dates from the Hidden Bay pluton on Adak, indicate that there was another magmatic event around 33 m.y.b.p. (Citron et al. 1979). Amphibole separates from the Shaler pluton, just west of the Captains Bay pluton, yielded a K A r a g e of lt.l_+3.0m.y.b.p. (Marlow etal. 1973). Lankford and Hill (1979) quote an unpublished K - A r age of 13 m.y. (D.W. Scholl) for the Captains Bay pluton. The Captains Bay pluton intrudes the 'early series' Unalaska Formation which is composed of fine to coarse sedimentary and pyroclastic rocks intercalated with basaltic to dacitic flows and sills. Numerous basaltic and andesitic dikes, some genetically related to the pluton, intrude the older sequence, The Unalaska formation is at least as old as early Miocene based on Molusca fossils found in sedimentary rocks surrounding the pluton (Perfit 1977; Lankford and Hill 1979) and some questionable remains of desmostylid (sea cow relative) east of Unalaska Lake (Drewes et al. 196i). Field relationships suggest that the Captains Bay pluton was both forcefully and passively intruded into the Unalaska Formation at shallow levels. The surrounding country rocks have been domed, faulted and chaotically fractured in proximity of the pluton. Secondary mineralization, hydrothermaI alteration, explosive brecciation and stoping are evident along forceful contacts. Late-stage intrusion of aplites and pegmatites along brecciated contacts is also common. There is little physical (and chemical) evidence that indicate significant assimilation of country rock by the pluton. The presence of fine-grained gabbroic rocks, gabbros with igneous layering and cumulate textured rocks in sharp contact with hornfelsed country rock suggests that rapid cooling, crystal accumulation and marginal accretion of crystals occurred along passive contacts. Comb-layering in one gabbro indicates that supercooling and possible water-saturation may have also taken place during the initial stages of crystallization (Taubeneck 1967 ; Lofgren and Donaldson 1975; Walawender 1976). These processes apparently occurred before the main body of magma solidified because intermediate plutonic rocks intrude the gabbros and mafic borderzone but never vice-versa. Porphyritic, hypabyssal rocks on the edges and top of the pluton may represent coeval subvolcanic rocks that were partially engulfed by the piuton as it rose toward the surface, or crystal-rich liquids that were quenched as volatiles excaped along border faults and fractures (Drewes etal. 1961; Schweikert 1976; Bussell and McCourt 1977).
71
Petrography The gabbroic and dioritic rocks are typically rather fine-grained and have hypidiomorphic-granular to panidiomorphic-granular textures. Subophitic texture, igneous lamination or cumulate texture are obvious in some gabbros. They are primarily composed of 50% to 70% euhedral, slightly zoned plagioclase crystals (Anss_a6) and lesser amounts of granular augite, orthopyroxene (Enst-7o) and F e - T i oxides. Phlogopite, hornblende and less commonly orthoclase and quartz are present in minor to trace amounts. Apatite is a common accessory phase. Mafic constituents can make up as much as 50% of the border zone gabbroic rocks but the proportions of each mineral vary considerably. Overgrowths of hornblende on pyroxene, sodic plagioclase on catcic plagioclase and phlogopite on Fe - T i oxides are suggestive of intercumulus growth due to trapped intercumulus melt. Olivine is only present in a gabbroic plug from Needle Point on Amaknak Island 8 km north of the main pluton (Fig. 1). Drews and others (1961) describe olivine and green spinel in a border zone gabbro from the Shaler pluton. Aggregates (< 10 mm) of pyroxene and F e - T i oxides that have orthopyroxene-magnetite intergrowths in their cores are commonly observed in the Captains Bay pluton. Remarkably similar symplectites observed in the Waterange mafic intrusion (Australia) have been shown to replace olivine through a reaction of olivine with intercumulus liquid during the late stages of crystallization (Ambler and Ashley 1977). Quartz diorite and quartz monzodiorite are the dominant rock types found in the interior of the Captains Bay pluton, although locally, granodiorite is volumetrically more abundant (Fig. 2). The intermediate granitic rocks are hypidiomorphic-granular to porphyritic and generally coarser grained than the gabbroic rocks. Boundaries between rock types are nearly always gradational and mineralogical variations are subtle. Oscillatory zoned plagioclase crystals (An60 -Anzo) are the most abundant constituent (generally more than 50% of the mode) and rarely have rims of orthoclase of antiperthite. Augite and hypersthene coexist but hornblende and/or biotite are more abundant in more felsic rocks. Hornblende ubiquitously replaces hypersthene first and later augite. Textural relations suggest that biotite is the last mafic mineral to crystallize.
Quartz
50
20
5 7Gabbro 189H -~20je
Plogi0close
Monzodiorile
K-feldspar
Fig. 2. Modal compositions (quartz, k-feldspar, plagioclase) of plutonic rocks from the Captains Bay pluton (solid circles) and Shaler pluton (open squares from Drewes et al. 1961) on Unalaska Island
72
M.R. Perfit et al. : Trace Element Variations in a Zoned Pluton, Alaska
Quartz and orthoclase (Org0 91) are always interstitial and exhibit a variety of textures (Perfit and Lawrence 1979). They typically exist as micrographic or granophyric intergrowths and less commonly exhibit a granitic texture. Rarely, they form separate, anhedral grains. Low-Ti magnetite, lesser ilmenite and rare titanomagnetite comPrise less than 5% of the mode. Accessory minerals include: chlorite, sphene, epidote, apatite and rare traces of calcite, zircon, allanite and monazite. Granophyres (leucocratic granodiorite) occur as small, irregular masses and apophyses that grade imperceptibly into the intermediate plutonic rocks. They are predominantly composed of intergrown quartz and orthoclase with minor albite, mafic silicates and F e - T i oxides. Aplite dikes, that intrude all of the plutonic rock types and into the country rocks, have variable proportions of orthoclase and quartz with minor to trace a m o u n t s of albite (often altered to orthoclase) and mafic silicates.
Analytical Results Major element compositions of the samples selected for REE and isotopic analysis are presented in Table 1. A compilation of Rb, Sr, Ti, Zr and Y contents of the plutonic and volcanic rocks from Unalaska
that were not analyzed for major elements is available from the senior author. The analytical methods employed are discussed in Appendix 1. The variation of MgO, FeO (total) and K20 + Na20 in the plutonic suite are shown in comparison to volcanic rocks from Unalaska and other well known calcalkaline suites (Fig. 3). The differentiation trend parallels calcalkaline suites from other Aleutian Islands as well (Coats 1962, 1959; Forbes et al. 1969; Byers 1959). A few gabbros (e.g., 189H, 66B) fall off the main trend but plot near to some of the more mafic basalts from Makushin volcano on Unalaska Island (Perfit 1978). On an alkali-silica diagram, nearly all of the plutonics plot within the field of high-alumina basalts defined by Kuno (1966). The overall major element variations and field relationships strongly support the hypothesis of in situ differentiation by inward fractional crystallization from the walls and possibly the floor of the plutonic mass (Vance 1961 ; Ragland and Butler 1972; Perfit 1977). A detailed examination of trace element variations within the pluton can be of great use in
Table 1. Major and trace element abundances: Captains Bay pluton, Unalaska No.
1
2
3
4
5
6
Sample I.D.
189H
66B
169H
t60
32BGR 32
7
8
9
10
11
12
13
175
UN36
92A
211
175AP
M K 1 0 A 131
14
15
U5-133
UM4
SiO2 TiO2 AlzO3 FeO" MgO MnO CaO Na20 KzO PzO5
48.41 0.21 22.13 6.55 8.23 0.06 13.20 1.18 0.12 0.25
50.92 1.00 19.47 8.12 6.39 0.29 10.39 3.i3 0.22 0.32
51.07 1.02 18.66 9.38 6.10 0.16 10.18 3.14 0.55 0.21
51.45 1.09 17.82 9.18 5.92 0.14 10.28 3.00 0.54 0.11
56.78 0.92 16.68 7.01 4.65 0.11 7.21 3.79 1.44 0.16
59.30 0.79 17.02 6.01 4.50 0.06 7.02 3.48 1.50 0.27
60.29 0.95 16.86 6.04 3.46 0.06 6.44 3.68 1.92 0.28
62.48 0.89 16.46 5.00 3.00 0.15 5.36 3.65 2.30 0.16
63.76 0.73 16.32 4.80 2.64 0.20 5.20 3.70 2.57 0.12
70.61 0.50 15.40 1.46 1.94 0.00 2.03 4.14 3.75 0.12
74.23 0.17 13.34 1.09 0.18 0.04 1.09 3.13 5.54 0.08
53.75 0.97 19.77 7.80 3.32 0.12 8.72 4.12 1.47 -
54.45 0.78 18.16 7.13 5.32 0.15 9.74 3.10 0.89 0.13
57.48 0.92 18.05 7.33 4.66 0.14 8.01 3.38 0.60 0.28
51.70 1.12 17.70 7.80 5.45 0.15 10.11 3.12 0.97 0.20
Total
100.35
100.25
100.48
99.52
98.78
99.95
99.99
99.44
100.03
99.95
98.88
100.34
99.84
100.85
98.31
0.80 72.5 9.83
t.27 62.3 14.23
1.54 57.7 5.71
1.55 57.5 5.56
1.51 58.2 2.63
1.34 61.1 2.32
1.75 54.6 1.92
1.67 55.7 1.59
1.81 53.6 1.44
0.75 73.6 1.10
6.06 25.7 0.57
2.35 47.2 2.80
1.34 61.0 3.48
1.57 57.2 5.63
1.43 59.5 3.22
FeO/MgO Mgr Na20/KzO
Trace elements (ppm) 1.80 4.59 4.50 12.0 12.5 15.9 19.1 21.3 31.1 46.0 12.2 7.39 4.98 9.49 K ( x 1,000) 1.00 3.5 13 12 28 34 37 55 47 67 147 17 15 7 19 Rb 3 174 310 300 310 388 489 539 534 786 350 337 300 Ba 105 610 456 437 394 426 404 345 377 282 71 510 464 580 442 Sr 640 9 20 22 27 22 22 28 23 25 32 25 24 24 23 Y 5 9 68 80 126 125 140 211 220 215 125 90 95 174 140 Zr 22 6,000 6,120 6,540 5,220 4,740 5,700 5,340 4,380 3,000 1,020 5,800 4,680 5,520 6,720 Ti 1,260 0.005 0.006 0.03 0.03 0.07 0.08 0.09 0.16 0.12 0.24 2.07 0.03 0.03 0.01 0.04 Rb/Sr 333 514 353 375 429 368 430 347 453 464 313 717 493 711 505 K/Rb 5.55 6.24 6.92 3.45 5.84 6.08 6.08 7.63 8.34 9.22 14.2 4.83 7.73 4.17 La/Yb F e O " = T o t a l Fe as Fe 2+ ; Sample 12: Andesite from Pakushin cone Unalaska; 13: Andesite Unalaska formation; 14: Dike in Unalaska formation; 15: Basalt, U m n a k Is. (Kay 1977): short dash means not analyzed
M.R. Perfit et al. : Trace Element Variations in a Zoned Pluton, Alaska
73
FeO*
Japanese Calc-alkaline
BGR
Ix I ~ K I 5
AI
x~x
L.California Batholith
Fig. 3. AFM diagram showing chemical trends of plutonic and volcanic rocks from Unalaska Island compared to calcalkaline suites from Japan and the Lower California Batholith. Specific samples discussed in the test are shown as solid triangles. Other samples are from Drewes and others 1961, and Perfit 1977. FeO* is total Fe as FeO
9 /|
/ x
99
K20
P
cJ
u ton
MgO
+ Na20
[
1000
=
e
r
f I
[
I
J
t
I
I
I
I
=
I
i
|
o
|
E
Volcanic
500 9
|
9
|
9
9
9
L_
.
(/)
.
9
200
I00
Fig. 4. Log-log Rb-Sr diagram showing the trend of plutonic rocks from the Captains Bay pluton and the volcanic rocks (including flows, sills and dikes) from Unalaska Island (solid triangles). The dotted field is that of low-Ti, colorless ashes recovered in deep-sea cores around the Aleutian Islands (Chia 1973, Edsall 1976), The star represents a rhyolite from Umnak Island (Walker 1974). Note that some of the aplites plot below the limits of the diagram
|174174149 e 9 ",
|
,,
J
t
*
*I
Plutonic |
\
Volcanic 9
5(3
1000~C) occurred under relatively reducing conditions ( f O 2 = 1 0 - 9 t o 1 0 - 1 ~ During the late stages of solidification however, the appearance of sphene, hematite and rutile, together with f O 2 calculated from exolved F e - T i oxides in the intermediate rocks, suggest that crystallization occurred under more oxidizing conditions (Perfit 1977). These more oxidizing conditions, approaching the Mag-Hem buffer, are reflected in a drop in calculated opluag. To a first approximation, the D's used in the calculations, with the possible exception of the HREE, are appropriate for our models. If we assume the mineral assemblage in cumulate gabbros is representative of initial proportions of minerals fractionating from some parental magma, and we use the same distribution coefficients as above; a crude estimate of the trace element content of the parental magma can be calculated using Equations 5 and 12 of Paster et al. (1974). Petrographic evidence suggests that trapped intercumulus liquid comprises approximately 10% to 30% of the cumulate gabbros. Making a reasonable assumption that the most mafic gabbro (189H) represents the solid residue after 10 percent
20(
0 r 0
~0 -i
crystallization and that it trapped 20% of the parental magma, then the calculated initial concentrations in the parent liquid are: 12 ppm Rb, 310 ppm Ba, 580-400 ppm Sr L.USr(rlplag 1.8-3.0), 31 ppm Ce, 4.0 ppm Sin, 3.5 ppm Dy, and 2.1 ppm Yb. These values are in good agreement with the concentrations in the samples proposed to be parents in our models (160, 169H and UM4, see Figs. 12 and 13). =
Genesis of the Plutonic Series
Gabbroic Border Zone and Contamination Mineralogic, field and chemical data support the premise that many of the gabbroic rocks represent early crystal cumulates of near liquidus phases that were separated from a parental high-A1 basaltic magma. In particular, the low LIL and REE abundances, and positive Eu anomalies are unlike any volcanic rocks in the Aleutians and consistent with a cumulate origin. Drewes and others (1961) suggested that the mafic border zones surrounding the Shaler and Captains Bay plutons were formed by contamination of granodioritic magma with more mafic country rock. Even the most mafic rocks in the Unalaska Formation have LIL element concentrations higher than those in the border zone gabbros which means that a mixing hypotheses is untenable. The REE patterns of these border zone gabbros are comparable to those in pyroxene gabbros in the Peninsular Ranges that Gromet (1979) believes are genetically related to the voluminous tonalites.
0.5PT. At low pressures (
