Geology and petrology of Floreana Island, Galapagos Archipelago, Ecuador

Journal of Volcanology and Geothermal Research, 52 ( 1992 ) 83-105

83

Elsevier Science Publishers B.V., Amsterdam

Geology and petrology of Floreana Island, Galapagos Archipelago, Ecuador Craig S. Bow~ and Dennis J. Geistb a12640 W. CedarDrive, Lakewood, CO 80228, USA bDepartment of Geology, Universityof ldaho, Moscow, ID 83843, USA (Received August 2, 1991; revised version accepted January 12, 1992 )

ABSTRACT Bow, C.S. and Geist, D.J., 1992. Geology and petrology of Floreana Island, Galapagos Archipelago, Ecuador. In: D.J. Geist and C.M. White (Editors), Special Issue in Honour of Alexander R. McBirney. J. Volcanol. Geotherm. Res., 52: 83-105. Floreana is the sixth largest of the Galapagos Islands and lies at the extreme southern margin of the Galapagos Platform. In sharp contrast to the other major islands, Floreana lacks a well-developed volcanic center, but instead is dominated by pyroclastic vents and derivative ash and cinder deposits. The majority of exposed lavas are xenolith-bearing alkali-olivine basalts that constitute a coherent suite referred to as the Main Series. The Flank Series comprises a group of isolated vents on the southern flank of the island which erupted xenolith-poor basaltic lavas that are generally poorer in incompatible trace elements and richer in plagioclase. Paleomagnetic measurements, supported by K-Ar age determinations, record a relatively limited timespan for subaerial volcanism. Shield development reached maximum dimensions by 1.0 Ma, with the final phases of pyroclastic activity at an end by 0.3 Ma. Main Series rocks are phenocryst-poor and typically contain clusters ofxenocrysts in an extremely fine-grained, granular matrix. Petrogenetic modelling indicates they represent products of partial melting of a LREE-enriched spinel-lherzolite source, little modified by subsequent fractional crystallization. Porphyritic Flank Series lavas experienced rather different ascent histories and may have resided in shallow-level magma chambers prior to eruption. Isotopic, geochemical and petrographic data demonstrate that Flank Series lavas are not cogenetic in any simple way with Main Series flows. Floreana lavas contain a diverse suite of ultramafic xenoliths. Al-augite group megacrysts are likely to be cognate and comagmatic with host alkali-olivine basalts. Transition metal contents of megacrysts and host lavas, however, preclude clinopyroxene fractionation as a mechanism of magmatic differentiation. It is proposed that megacrysts were products of passive crystallization and were never effectively separated from host magmas. Cr-diopside group nodules may have crystallized at very high temperatures from advanced partial melts and are less likely to bear any direct genetic connection to host basalts.

Introduction A series of integrated, island-by-island studies within the Galapagos Archipelago was initiated by Alexander McBirney and Howel Williams in the mid-1960's (McBirney and Williams, 1969; Swanson et al., 1974; Baitis and Lindstrom, 1980; Geist et al., 1985, 1986; Correspondence to: C.S. Bow, 12640 W. Cedar Drive, Lakewood, CO 80228, USA.

Cullen and McBirney, 1986; Vicenzi et al., 1990). This long-term project has resulted in a comprehensive picture of the complex magmatic and structural evolution of the Galapagos Archipelago through detailed analysis of the chemistry and temporal and spatial distribution of volcanism on individual islands. This paper describes results of geological and geochemical studies of Floreana Island, building upon the earlier reconnaissance of Williams and McBirney (1969). Floreana is of

0377-0273/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

84

C.S. BOW AND D.J. GEIST

particular interest, owing to the alkalic character of its lavas, an abundance of ultramafic xenoliths, and the observation that it represents an "endmember'" in most regional petrologic and geochemical studies (McBirney and Williams, 1969; White and Hofmann, 1979; Geist et al., 1988; Geist, 1992).

Geology Floreana is roughly circular in shape, with dimensions of 12 k m N - S by 15 k m E-W. It is the sixth largest island in the Galapagos Archipelago and lies at the southern margin of the Galapagos Platform (Fig. 1 ). Constructional slopes are little m o d i f e d by erosion, despite a somewhat wetter climate than that of islands to the north and west. The thorn- and brushcovered lava fields rise gently to a verdant

highland ringed by an impressive array of youthful cinder cones. The summit peak, Cerro des Pajas, stands at 920 m, more than 300 m above the surrounding lava plain. The subdued silhouette of the Floreana shield emphasizes the great height and steep slopes of the highland and flank cinder cones; pyroclastic rocks dominate on Floreana as compared to the other islands of the archipelago. In many respects, the island as presently exposed is simply a collection of pyroclastic vents and associated satellite flows and lacks any clear evidence of an ancestral central volcano similar to the imposing shields of the western Galapagos Islands. This is an important point, for there is little reason to accept the Hawaiian model of volcanic evolution as directly applicable to any of the Galapagos volcanoes. 90 °

0c

Femandin",,~ ,~ ~ 'inzon , ~ O ruz Alcedo~;'" "..~ " ~ S:nta ~ S a n Cerr~~ ~ f Cristobal AZUl~ _ ~ S~rra Roreana

I

I

I

I

Fig. 1. Location map for Floreana, the Galapagos Islands, and the nearby Galapagos SpreadingCenter. Floreana lies at the southern margin of the GalapagosPlatform (contour line marking 2000 m water depth).

GEOLOGY AND PETROLOGY OF FLOREANA ISLAND, GALAPAGOS ARCHIPELAGO, ECUADOR

The majority of Floreana lavas are xenolithbearing alkali-olivine basalts; differentiated rocks of hawaiitic composition are relatively sparse. On the basis of petrographic and geochemical criteria, most of the lavas constitute a coherent suite collectively referred to as the Main Series. The Flank Series comprises isolated vents on the southern flank of the island which erupted xenolith-poor basaltic lavas with distinct chemical and petrographic traits. Main-Series volcanism The oldest rocks exposed on Floreana are thinly-bedded, subaerial basalt flows. Outcrops of these oldest rocks are limited to sea cliffs cut into composite tuff-cinder-scoria vents and related flow sequences along the northwest, northeast, and southeast coasts (Fig. 2). All tested lavas are reversely polarized, and K-Ar dates from Pta. Daylight (Cox and Dalrymple, 1966) and Pta. Ayora (Bow, 1979) indicate shield development was welladvanced by approximately 1.0 Ma. These early shield-building lavas are primitive alkaliolivine basalts with 2-10% normative Ne and M g O / ( F e O ° + MgO) = 0.70-0.75. Basal lavas of the southeastern sea cliffs are 1.1-0.9 Ma xenolith-free rocks, whereas the vents at Pta. Daylight (0.77 Ma) include both xenolith-free and xenolith-bearing flows. The reversed-polarity lavas and basaltic scoria cone of Isla Champion once were contiguous with flows on the main island, evidence that maximum shield dimensions were attained during an early phase of volcanic evolution. Subsequent marine erosion severed the land bridge, converting the peninsula into an island. Normally-polarized basalts and hawaiites conformably overlie reversely polarized lavas of the earlier volcanic period (Fig. 2). Small flows (less than 10 km 2) erupted from numerous vents in the highlands and on the flanks and extended shorelines to the west and north but failed to inundate the reversely-polarized rocks around the volcanic centers at Pta. Day-

85

light and Pta Ayora. Normal-polarity basalts include both xenolith-free and xenolith-bearing types, although the majority carry some mafic or ultramafic crystal debris. Pyroclastic activity associated with the late stage of Main Series volcanism produced thick accumulations of airfall debris and built the first generation of large, highland cinder cones. These vents have not been directly dated but appear to be of intermediate age. They are distinguished from younger cinder cones by extensive erosional modification of crater walls and primary constructional slopes. Late in the active life of the volcano, explosive eruptions of alkali-olivine basalt magma produced two of the most impressive cinder cones found anywhere in the Galapagos Archipelago: Cerro Salinas and Cerro des Pajas (Fig. 2). The cones produced clouds of lapilli, cinder, and glassy basaltic shards which covered much of the island, further burying highland lavas already obscured by earlier pyroclastic events. The thickness of pyroclastic cover is difficult to estimate owing to the absence of significant erosion, but the rarity of outcrop at higher elevations testifies to the presence of up to several meters of cinder-rich volcanic soil. Initial pyroclastic outbursts were quickly followed by eruption of alkali-olivine basalts from vents located near the base of each cone. At Cerro Salinas, the western wall was breached, and a large lava lake developed. Lava issuing from this reservoir flooded the base of the cone and reached the eastern coastline along a 2.6km-long front. At Cerro des Pajas, a 1.7-kmlong NNE-directed rift zone propagated from the breached southern wall. A linear array of collapse pits, agglutinate-scoria cones and spatter ramparts mark the trace of the rift which ultimately fed 20 km 2 of basalt to the west and southwest. Eruption of the Salinas and Pajas lavas are the most recent volcanic events recognized on Floreana and mark the end of the active, volcanic phase of island development. A K-Ar age determination on the Salinas flow yielded a date of 0.6_+0.3 Ma (Bow

86

c . s . BOW AND D.J. GEIST

Pta. Cuervo

.1

Pta.

Daylight

Bahia Post Office

Marino /

Cerro

_Salinas

C>

I. C h a m p i o n . . . . 0 I. I = n a e r o y

j Cerro des P

Lo,

\

",~C,.evos

% Ayora

Q

Rada B l a c k ~ Beach _

Pta. Saddle

I. Caldwell

B Flank Series B Normally-Polarized Main Series B Reversely-Polarized Main Series

Mt. O l y m p u s

Pta. Sur !

I

5 km Fig. 2. Schematic geologic map of Floreana Island; contour interval is 100 m. Flank Series shields are shown by crosshatched pattern; reverse polarity lavas are shaded. Circled K-Ar dates are from Cox and Dalrymple (1966) and Bow (1979).

and Dymond, 1978); this determination may not be accurate, however, because the flow is scantly vegetated and has a very youthful appearance. Flank-Series volcanism

Two small monogenetic shields that vented Flank Series lavas are juxtaposed against the abrupt southern termination of the highland plateau (the subdued topographic highs within the 300-m contours of Fig. 2 ). Relatively rapid eruption of porphyritic basaltic lavas unaccompanied by explosive volcanism probably accounts for the broad form and low profile of these vents. This style of volcanic activity is atypical of Floreana, as are the petrography, major- and trace-element compositions of associated Flank Series lavas.

Structural controls on volcanism

The circular form of the Floreana shield and uniform density of satellite vents contrast markedly with the shapes and vent distributions of the other major islands, which have prominent alignments (McBirney and Williams, 1969; Chadwick and Howard, 1991 ). Floreana differs in other structural aspects from Santa Cruz, Baltra, N. Seymour, Santiago, Santa Fe, and San Cristobal (Swanson et al., 1974; Bow, 1979; Geist et al., 1985, 1986), most conspicuously by the absence of linear segments of faulted coastline and a dominant fissure trend. The extensive sea cliffs of the southwestern coast are not fault scarps but are erosional features due to prolonged exposure to wave action from the prevailing southeasterly winds. Minor faulting is sug-

GEOLOGY AND PETROLOGY OF FLOREANA ISLAND, GALAPAGOS ARCHIPELAGO, ECUADOR

gested by the presence of linear stream drainages in the southern and southeastern highlands, but this faulting seems to lack any regional tectonic significance. Several large highland cinder cones are arrayed in a crudely circular manner, which has suggested to previous workers the presence of an ancient caldera, obliterated by subsequent volcanic activity (Chubb, 1933 ). Williams and McBirney (1969) found no evidence for circular highland structures but conceded that a caldera might have existed at some point in the evolution of the island. The absence of residual topographic depressions, pit craters, or of petrologic evidence of significant shallow level fractionation further discount Chubb's hypothesis.

Petrology The majority of Floreana lavas are alkali-olivine basalts, according to the classification scheme of Irvine and Baragar (1971). Floreana's lavas are the most alkaline in the Galapagos Archipelago; the average normative (molecular) Ne content of the 20 analyzed flows is 3.9. Two normally-polarized flows (Fl18, Fl-19) are sufficiently evolved to be hawaiites. Rare hypersthene-normative lavas contain the same mineral assemblages as alkali-olivine basalts. The differences in normative mineral assemblage are due to slightly different silica and alkali contents and not to oxidation of iron. Main Series

Main Series alkali-olivine basalts are phenocryst-poor, aphanitic rocks that typically contain clusters ofxenocrysts in an extremely finegrained, granular matrix. Xenocrysts of olivine, clinopyroxene, plagioclase, and aluminous spinel are common; orthopyroxene xenocrysts are sparse, no doubt due to the rarity of low-calcium pyroxene in nodules as well as to reaction of these xenocrysts with silica-under-

87

saturated magma. Many lines of evidence indicate that much of the coarsely-crystalline material within Main Series flows is derived from disaggregated xenoliths: ( 1 ) Magnesian olivine occurs as single crystals and in spinel-bearing dunite and wehrlite nodules. Single crystals exhibit a variety of textures indicative of solid-state deformation. Several specimens clearly display the textural progression from optically-continuous cores through a zone of high strain to a rim composed of a fine grained aggregate of recrystallized olivine granules (Fig. 3a). (2) Microprobe analyses of xenolith olivines range from F095 to F073 with a mean value of Fo85. Groundmass olivines likewise vary in composition but are on average richer in iron (Fo74). Microphenocrysts of skeletal olivine may represent equilibrium crystallization, but the wide range in Mg/Fe exhibited by finely disseminated groundmass olivine emphasizes the extensive physical degradation and subsequent dispersal of xenolithic debris. (3) Aluminous sub-calcic augite and chromian diopside are preserved as single crystals and in multiphase nodules with olivine, plagioclase, and spinel. Jackets of purplebrown augite occasionally rim pyroxene xenocrysts, but more often disequilibrium is demonstrated by resorption and a sponge-like alteration of the relict crystal (Fig. 3b). The rarity of augitic rims on xenocrysts and of augite as a phenocryst phase indicate magmas were far from clinopyroxene saturation on eruption; this is confirmed by one-atmosphere melting experiments (Bow, 1979). (4) Plagioclase occurs as corroded xenocrysts in nearly every flow examined. Individual crystals range in size from 5 cm to microphyric chips that are difficult to distinguish from groundmass feldspar. The zoned cores of many crystals pass abruptly outwards into a sieve of minute silicate inclusions, rod-like spinel grains and droplets of glass. In most feldspars a sheath of clear, normally-zoned plagioclase, more calcic than the core of the

88

C.S. BOW AND D.J. GEIST

GEOLOGY AND PETROLOGY OF FLOREANA ISLAND, GALAPAGOSARCHIPELAGO, ECUADOR

89

Fig. 3. Textures of Floreana lavas and xenoliths. (a) Large olivine xenocryst within a Main Series Flow. Note the textural progression from optically-continuous core, through a zone of strain and undulatory extinction, to a rim composed of fine-grained recrystallized olivine granules. Field of view = 3.3 cm. (b) Fine-grained Main Series flow including a large, partly-digested spinel clinopyroxenite nodule (left side of photo). Note the ghost of a second pyroxene megacryst along the right side of the photo which has been almost entirely resorbed. Field of view = 3.3 cm. (c) Plagioclase xenocryst within a Main Series flow. The crystal exhibits a well-developed sieve texture rimmed by a more calcic equilibrium plagioclase. Field of view= 1.5 cm. (d) A Flank Series flow containing phenocrysts of magnesian olivine (right of center) and plagioclase. Field of view = 3.3 cm.

xenocryst, mantles the reaction zone (Fig. 3c ). Collectively, these textures are taken as evidence for disequilibrium between host alkaliolivine basalt magma and plagioclase, which is consistent with the observation that plagioclase is tens of degrees below the liquidus of most of these lavas at 1 bar (Bow, 1979). (5) Disequilibrium between pleonaste and basaltic liquid is indicated by conversion of the translucent spinel to an opaque, iron-rich oxide. A spectrum of spinel solid solutions has been incorporated as xenocrysts in Floreana lavas. Translucent green pleonaste (with compositions near the magnesian end of the hercynite-spinel series) is most often associated with aluminous augite while brown, chromian picotite is found in multiphase xenoliths with olivine and chromian diopside. The occurrence of skeletal olivine and microphyric plagioclase in the groundmass of Main Series lavas suggests that these minerals are liquidus or near-liquidus phases at one at-

mosphere. As discussed above, clinopyroxene phenocrysts are rare in Floreana rocks, although they are conspicuous in one flow (F l14 ). Melting experiments on F1-14 indicate the crystallization sequence olivine -plagioclaseclinopyroxene (Bow, 1979). This discrepancy between phenocryst assemblage and low-pressure melting relationships suggests the possibility that olivine and clinopyroxene nucleated at elevated pressures where pyroxene saturation preceded plagioclase. Flank Series

Flank Series lavas are characterized by a coarsely-crystalline groundmass, the presence of olivine and plagioclase phenocrysts (Fig. 3d) and rarity of xenoliths. These lavas are petrographically and geochemically similar to the slightly alkaline rocks of Santiago, Santa Cruz, and San Cristobal (Baitis, 1976; Bow, 1979; Geist et al., 1986). Conspicuous, euhe-

90

C.S. BOW AND D.J. GEIST

TABLEI

Major and trace element analyses of Floreana lavas. Major element oxides are in weight%, normalized to 100% volatile-free. Totals are measured analytical totals. Trace elements in ppm. Analytical techniques are discussed in the text. Flank Series speciments are italicized; all others are Main Series. F1-3 SiO2 TiO2 A1203 Fe203 FeO MnO MgO CaO Na20 K20 P205 Total Cr Ni Rb Sr Ba Sc Zr Ta Th Hf La Ce Sm Eu Tb Yb Lu

46.25 1.19 14.27 2.10 7.08 0.18 12.62 10.54 2.57 1.09 0.21 99.75

Fl-13 45.78 1.66 14.44 3.36 6.69 0.18 12.96 10.75 2.82 0.87 0.26 99.67

267 372 21 514

226 377 21 391

61

84

Fl-14

Fl-15

Fl-16

Fl-18

47.27 1.30 15.70 3.78 5.93 0.18 8.58 12.17 2.79 0.94 0.23 100.86

48.00 1.39 16.19 4.07 6.92 0.21 7.81 9.13 3.42 1.25 0.35 100.96

47.74 1.38 15.18 2.43 7.52 0.20 10.21 10.27 3.17 0.92 0.27 100.44

46.22 1.44 16.37 1.62 9.54 0.22 8.39 9.85 2.04 0.86 0.26 99.97

47.01 1.31 15.62 4.50 6.36 0.33 7.77 9.23 4.58 1.31 0.42 99.67

46.06 1.69 14.51 4.79 5.57 0.19 11.27 10.60 3.12 0.97 0.28 98.96

557 164 21 393 328 31 70 0.9 1.2 1.6 13.1 20.8 2.7 1.0 0.50 1.9 0.3

569 118 34 523 514 25 140 3.4 5.8 3.2 47.5 69.4 4.6 1.4 0.60 2.3 0.5

297 282 24 451 0 29 110 2.4 3.7 2.5 33.4 61.5 3.7 1.2 0.50 2.1 0.4

674 236 18 487 657 40 109 3.1 7.4 2.3 54.4 56.0 5.3 2.7 0.80 3.2 0.86

126 37 496 924 32 142 4.4 8.0 4.0 60.0 88.9 5.5 1.7 0.70 3.0 0.6

276 21 501 273 30 89 1.6 2.0 1.8 21.6 33.5 3.2 1.1 0.50 2.0 0.4

dral olivine phenocrysts, together with a groundmass assemblage dominated by radiating or stellate aggregates of magnesian olivine and labradorite, suggest these phases are at or near the liquidus for most lavas of this group. This interpretation is in general accord with experimental data, although plagioclase is the sole liquidus phase of one flow (FI-27) over a considerable temperature interval (Bow, 1979). Chemical and isotopic differences are also apparent, and suggest Flank Series basalts have origins or petrogenetic histories that are fundamentally different from those of Main Series lavas.

Fl-19

F1-20

F1-23 46.50 1.76 15.47 4.90 5.91 0.20 9.50 10.71 3.37 0.75 0.34 99.53

F1-24 48.22 1.80 15.58 3.05 6.92 0.18 8.93 9.54 3.10 0.96 0.31 99.26

726 210 21 457

642 225 24 542

121

123

F1-29

F1-30

45.39 1.80 15.58 3.05 6.92 0.18 13.07 10.23 2.86 0.93 0.22 99.36

46.39 1.65 14.55 1.63 8.18 0.17 12.62 11.26 2.54 0.76 0.25 99.27

912 354 20 427 462 32 62 0.9 1.0 1.5 12.4 20.1 2.6 0.9 0.40 1.8 0.3

359 19 382 274 33 82 1.0 1.4 0.0 13.6 21.4 3.0 1.0 0.50 1.8 0.4

Petrochemistry

Analytical techniques Major-element abundances, with the exception of and Na20, were determined by X-ray florescence, following the technique of Norrish and Hutton (1969). Na20 was determined by neutron activation analysis (Gordon et al., 1968) and ferrous iron contents by titration. Precision is estimated at 1-2% for S i O 2 , 2-3% for A1203, F e 2 0 3 , and CaO, 2-5% for TiO2 and Na20, 3-5% for MgO, and 5% for 1(20 and P 2 0 5 . Analyses for Zr, Sr, Rb, and Ni

GEOLOGY AND PETROLOGYOF FLOREANAISLAND,GALAPAGOSARCHIPELAGO,ECUADOR

FI-33 47.42 1.17 14.89 1.55 7.73 0.17 12.72 10.49 2.65 0.65 0.22 99.02 341 16 353 626 38 73

F1-48 46.82 1.62 14.98 1.36 7.99 0.19 11.41 10.57 2.58 0.92 0.22 98.76

F1-53 46.61 1.21 14.82 1.42 8.35 0.20 11.80 10.59 2.76 0.90 0.19 98.66

F1-59 47.38 1.18 14.63 1.29 7.61 0.18 12.35 9.68 3.15 1.12 0.23 99.73

F1-63 47.60 1.21 15.06 1.29 7.59 0.18 11.46 9.82 3.36 1.22 0.25 99.87

F1-82

F1-85

47.57 1.08 14.98 1.36 8.03 0.19 11.30 10.45 3.21 0.97 0.19 99.82

46.46 1.56 14.84 1.41 8.32 0.19 12.68 10.43 2.75 0.82 0.19 98.42

CH-1 47.38 1.28 15.56 2.59 6.46 0.18 10.92 9.99 3.19 1.03 0.25 98.94

267 20 438

308 14 483

380 27 457

320 25 322

336 28 405

427 339 17 392

229 27 401

89

64

88

85

77

85

35 94

11.0 2.7 0.8 0.50 1.9 0.44

were performed on undiluted pressed powder pellets by the techniques outlined by Norrish and Chappel ( 1967 ). The remaining trace-element data were obtained by instrumental neutron activation analyses (Gordon et al., 1968 ). Relative errors are estimated to be: 1-2% for Sc, Co, Hf, La, Sm, and Eu; 3-5% for Cr, Ta, Th, Ce, Tb, Yb, and Lu; and 5-20% for Ba (Lindstrom, 1976). Mineral analyses were conducted at the University of Edinburgh, using a Cambridge Microscan 5 microprobe. Mineral separates were hand-picked from xenoliths and megacrysts after rough crushing under a tungsten-carbide mortar. Purity of individual mineral separates is estimated to be

F1-25 46.50 1.52 15.55 2.80 7.54 0.19 9.98 12.31 2.46 0.26 0.15 100.37 369 212 8 300 130 36 60 0.6 1.4 9.0 15.6 2.7

1.7 0.4

91

F1-26 46.48 1.46 15.73 2.38 8.00 0.18 10.16 12.25 2.30 0.18 0.13 99.51 188 5 246 111 57 0.6 0.6 1.3 7.8 14.5 2.6 1.0 0.50 2.1 0.3

F1-27 48.30 1.76 16.85 3.44 5.73 0.17 7.11 11.76 3.42 0.76 0.27 99.22 24 84 20 336 34 92

F1-28 47.16 1.82 16.44 1.89 8.29 0.17 7.88 11.93 3.04 0.58 0.25 99.01 1250 116 13 338 190 41 116 0.8 1.1 2.4 10.1 15.6 3.1 1.1 0.50 2.1 0.4

F1-78 46.20 1.52 14.20 1.41 8.30 0.19 13.77 10.65 2.25 0.53 0.18 98.38 428 10 322 34 83

10.4 3.2 1.0 1.6 0.48

greater than 95%. Mineral separates were analyzed as above.

Major elements. Floreana lavas are alkali-olivine basalts with normative nepheline and abundant olivine (Table 1 ). Major-element MgO variation diagrams are largely controlled by the accumulation and disaggregation of ultramafic debris in the Floreana magmas (Fig. 4 ). Although there is a large amount of scatter in the data, most of the major-element variation falls between control lines constructed by assuming addition of typical xenolith olivine and augite in varying

92

C.S. BOW AND D.J. GEIST

÷ Main Sedes • Flank Sedes

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Fig. 4. MgO variation diagrams for Floreana lavas. Main Series samples shown as crosses, Flank Series as solid circles. Solid and stippled vectors show the effects of addition of 10% typical xenolith olivine and augite.

proportions. Although the same trends could be produced by fractionation of olivine and clinopyroxene, petrographic evidence for xenolithic contamination is convincing. Some of the scatter in the oxide variations is probably a manifestation of different parental magmas undergoing contamination by similar xenolithic debris. Flank Series lavas are distin-

guished by lower alkali and higher A l 2 0 3 and CaO abundances. The alkalic affinity of Floreana lavas is evident on an alkali-silica diagram (Fig. 5 ); most of the analyzed rocks plot well above the line separating Hawaiian tholeiitic and alkaline lavas (MacDonald and Katsura, 1964). It is evident that the alkali-silica diagram also effec-

GEOLOGY AND PETROLOGY OF FLOREANA ISLAND, GALAPAGOS ARCHIPELAGO, ECUADOR

4" 1234

O +

%

5 4, 4,

4"

t~

z

4, 1255

4.

1302

,% 4o122~ 4"1234

1260

4, 4,.4,. 0;225

3

2 45 ' ' '46 ....

417 ....

4!8 ....

z~9 ....

50

SiO 2

Fig. 5. Alkali-silica diagram for Floreana rocks. Line separates Hawaiian alkaline lavas (above line) from tholeiires, after MacDonald and Katsura (1964). Main Series (crosses) and Flank Series (circles) plot within (or very close to) the field of alkali basalts. Experimentally-determined 1 bar liquidus temperatures for nine lavas are shown (data from Bow, 1979 ). Open circle indicates the sole composition with plagioclase as the liquidus phase.

tively separates Floreana lavas that contain olivine as liquidus phase from the single studied composition with liquidus plagioclase (experimental data from Bow, 1979). This supports the suggestion of Delong and Hoffman (1975) that the MacDonald-Katsura line partly represents the trace of an important boundary surface within the basalt tetrahedron.

Trace elements Of all the incompatible trace elements (ITEs), Th has the greatest variation in the Floreana suite and is believed to have been the most incompatible element during petrogenesis of these magmas (Ferrera and Treuil, 1974; Allrgre and Minster, 1978). Moreover, all trace-element variation diagrams constructed with Th on the abscissa have positive intercepts on the ordinate (Fig. 6), indicating that these other elements are less compatible than Th (e.g., Clague and Frey, 1982). Elements such as the rare-earths (REEs) and the high-field strength elements (Ta, Zr) are well-correlated with Th (Fig. 6). The alkalis and alkaline-earth elements have less coherent trends with an apparent break in slope at Th

93

values of 1-2 ppm. Sc is negatively correlated with Th, except for highly variable Sc contents at high values of Th. Hank Series lavas are distinct from Main Series flows by virtue of lower Ni, Cr, Co, light REE, and incompatible-element abundances at a given Th content. All Floreana lavas are LREE-enriched, with (La/Sm)N ranging from 3.3 to 10.9 (Fig. 7). The HREEs are likewise enriched relative to chondrites but are not significantly fractionated ( S m / Y b ) N = 1.5-1.9. The REE signature of Flank Series lavas is distinct from the Main Series by virtue of lower La/Sm ratios and LREE contents. Incompatible element plots (Fig. 8), normalized to normal mid-oceanic ridge basalt (Sun and McDonough, 1989), also reflect the enriched nature of the Floreana suite. These elements are enriched in the Floreana lavas in order of their predicted incompatibility in the mantle (Sun and McDonough, 1989), except for small discrepancies in Rb/Ba, K/La, and Hf/Ti.

Isotopic data S7Sr/86Sr in five Floreana lavas ranges from 0.70338 to 0.70366 and 143Nd/144Nd ranges from 0.51291 to 0.51298 (Geist et al., 1988; W.M. White, pers. commun., 1992). The Floreana samples are amongst the most enriched from the Galapagos Islands, and the similarity in isotopic ratios within the suite suggests a source that is relatively homogenous relative to the archipelago-wide variability. Nevertheless, there is some suggestion that the Flank Series comes from a source which is slightly lessenriched than that of the Main Series.

Partial melting models and the source of Floreana magmas

The abundance of ultramafic xenoliths, absence of phenocrysts, and the high Mg/Fe, Ni,

94

C.S. BOW AND D.J. GEIST

and Cr contents of Floreana lavas restrict the degree of crystal fractionation experienced by the magmas. High levels of the incompatible trace elements within these same lavas likewise limit the extent of partial melting. In the following discussion, geochemical data are discussed which point to variable but low-degree partial melting as the controlling mechanism in generating the limited compositional variations in lavas and associated pyroclastic deposits. Several lines of evidence are qualitatively

consistent with the partial melting hypothesis. First, the pattern of Floreana lavas on the incompatible trace-element diagram (Fig. 8), with enrichment approximately in order of incompatibility, is consistent with partial ,melting being the principal mechanism for the generation of Floreana lavas. Second, plots of Th against partially-excluded elements (e.g., Sm, Zr, Yb) yields trends for Floreana lavas that are distinct from those of the other islands of the central and southern Galapagos (Fig. 9), but the slopes are similar to the partial melting

8O 60J

4o~ ,

4

Ce 8 76 Th + 11 8 R^2 = 0.87 "

"I

....

I"

"I

'"

' I ....

I"

"I

"'

"I"

"I

....

La/Sm = 1.05*Th + 3.32 R^2 = 0.89

I" ,',

s " /

1.2

2

÷

+

-

/ ~-----------"-"~ ÷

0.3

÷

0.8. 0.2

J 0.4.

P205

~

....

0.1

, ....

, . . , .,. . .

....

,....,

"''l""l'"'l

40.

....

;'"1'"'1""1

....

I' "'

I''"1'"'1""

•

I' "',

+ Sc

3s.: 30.

+ ~,o~;~ S ..~ +

" ++

4.

' - 140

Zr

i~2o i loo 80

+

4,

1

25.

: 60

+

20. 0

1

2

3

4

5 Th

6

7

8

9

1

2

3

I

• Flank Series

I

÷ Main Series

4 5 Th

6

7

8

9

95

GEOLOGY AND PETROLOGY OF FLOREANA ISLAND, GALAPAGOS ARCHIPELAGO, ECUADOR

Yb

/.'/

Ta 5,

4. 3. 2. 1.

4 . ~ * T h + 0.44 RA2 = 0.94 ,,

,.|

....

i,,

"l

'"

'l'

"'

I'"

"l

"'

'l"

"l

'"

Yb = 0.18*Th + 1.58 RA2 = 0.89 '1 ....

•" " 1 " "

"1

....

I ....

I"

"1

,,,

4.+ ~ - , . ,

500.

+

4.

400.

'1,'

,'1

""

'1'

,,,

Cr

"~

"~°'~°/L,~"--.-.... -

~,~

300

4.

300.

200

4. •

200. 100.

4.

4.

100

Sr ''

"1

"'

'l"

"1

""

'1'

'"

I"

"1

""

'1"

"1

~ , , ~ R +b

'"

'1'

"'

"

,,I

"'

'1"

"1

'"

'l'

'"

I"

"1

"'

'1"

"1

'"

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i,,

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e++ +

30

+

+Ni

600

20.

400 4" 4.

10.

4.

/ ' "1

i • Flank Series - '

• 1

'''

'1"

2

"1

3

'"

'|'

4

'''

I °" "1

5 Th

6

"'

'1"

7

"1

8

'"

'| ....

9

....

I""1",,I,,

1 2

I + Main Series 3

,I,

,',

I,"

4 5 Th

,'1

6

,,,

,I,,

7

,,I

8

,,,

,I,,

9

Fig. 6. Trace-element compositions of Floreana lavas, plotted on Th variation diagrams. Main Series (crosses); Hank Series (circles). Linear regressions are shown for those variations with strong correlations. Regressions from the alkaline lavas of the Honolulu Volcanic Series (Clague and Frey, 1982) are shown for comparison.

control lines of Ferrera and Treuil (1974). Third, the breaks in slope on the plots of Th versus P205, Sr, Rb, and K 2 0 can best be explained within the context of a partial melting model, whereby a major decrease in bulk distribution coefficients occurs at the point of inflection (at lower Th concentrations). Initial, high distribution coefficients could have resulted from retention of small amounts of re-

sidual phlogopite through low degrees of melting. Eventual elimination of phlogopite occurred as melting progressed, resulting in a sharp decrease in the bulk distribution coefficients for alkalis and phosphorous (e.g., Clague and Frey, 1982). Alternatively, the lavas could have multiple sources, with the most Thenriched magmas having been derived from a source relatively enriched in Th/P205 and Th/

96

C.S. BOW AND D.J. GEIST 10

o ~o ~o

i1

~eD

Main • Flank • Santa Cruz

[3

.~. 100-

II i

Sm

• San Cristoba 20

....

r ....

2

10

I

La

I

Ce

I I

i

Sm Eu

Tb

Flank ~

I

I

Ta

I

I

KLa

6

10

Fig. 9. Inter-island variations in Th versus Sm for volcanoes from the southern and central Galapagos. Data for Floreana plot as an entirely distinct array which mimics the partial melting curves of Ferrera and Trueil (1974).

tatively consistent with lower and more variable extents of partial melting in the Floreana suite, and the absence of garnet in the Floreana source but its presence in the Honolulu source. Second, the Honolulu suite does not have the observed decreased slopes in P205 and the alkalis at high Th abundances, indicating either apatite and phlogopite were never exhausted during melting of the Honolulu source or were never present in the first place.

O 10

I

Pinzon i .... 8

|O -i ....

Yb t.u

,00

I

Santiago

I

Th

I

Fig. 7. Chondrite-normalized REE abundances in Floreana lavas. Cross-hatched area contains field for Hank Series rocks.

Rb BaTh

~ ,,,

4

I0

I

I

CeSr

I

I

I

P SmZr

I

I

1

Hf Ti Tb

I

Yb

Fig. 8. ITE plots for Floreana lavas, normalized to typical MORB (Sun and McDonough, 1989). Cross-hatched area contains field for Flank Series rocks.

alkalis via mantle metasomatism or assimilation of the incipient partial melts that occur in some of the xenoliths (Bow, 1979). Trace-element variations of the Floreana suite are broadly similar to those of the Honolulu Volcanic Series (Clague and Frey, 1982), although the Honolulu Series is distinctly more alkalic. The correlations between the LREEs and Th are very similar, suggesting similar initial concentrations and extents of partial melting. The two series differ in two important ways, however. First, the Floreana suite has a much greater range in L a / S m and Yb abundances than the Honolulu suite; this is quali-

Partial melting models Geochemical and isotopic data summarized above strongly suggest Floreana lavas are interrelated via different extents of partial melting of a mantle reservoir containing constant or near-constant abundances of incompatible trace elements. This model can be tested quantitatively by utilizing REE data of Floreana basalts and equations which describe trace-element behavior as a function of progressive melting of a variety of possible mantle assemblages (e.g., Gast, 1968; Shaw, 1970). Concentrations of the REEs during progressive partial melting were calculated for garnet lherzolite, spinel lherzolite, amphibole lherzolite, plagioclase lherzolite, and plagioclase granulite assemblages (Fig. 10). Initial models assume a chondritic mantle, which is probably

97

GEOLOGY AND PETROLOGY OF FLOREANA ISLAND, GALAPAGOS ARCHIPELAGO, ECUADOR

Floreana Basalts

~

Spinel Lherzolite

100.

Plagioclase Lherzolite

OI Opx Cpx Sp Mode 58 30 10 2 , oo

10.

~

OI Opx Cpx PI Mode 55 20 15 10 Melt 10 20 40 30

~

__ 10%

20%

10% I

I

I

I

i

i

OI Opx Cpx Gar Mode 29 12 34 25 Melt 5 5 45 45

100

i

I

,

~

I

I

I

I

I

Lherzolite

I

I

I

Oi @px Cpx PI Mode 5 5 45 45 Melt 10 10 40 40

Hornblende Lherzolite

Garnet

_ 4%

i

OI Opx Cpx Hb Mode 30 30 20 20 Melt 10 15 35 45

Plagioclase Granulite

20%

10

I

La

Ce

S m E u Tb

YbLa

Ce

I

I

I

I

S m E u Tb

I

YbLa

I

Ce

I

I

I

i

S m E u Tb

i

Yb

Fig. l 0. Calculated REE profiles, from the batch melting equation of Shaw (1970), for partial melts of different mantle assemblages compared to observed concentrations from Floreana lavas. "Mode" refers to the modal proportion in the source, and "Melt" refers to the proportion of each phase that contributes to the melt. Distribution coefficients are from Lindstrom (1976), and a chondritic source is assumed. Numbers on each pattern are the extent of melting.

not strictly valid, because the Nd isotopic ratios of Floreana lavas indicate that the source has had a time-averaged LREE depletion relative to chondrites (White and Hofmann, 1978). Comparison of the REE profiles for these theoretical models with those of Floreana lavas develop basic constraints on the composition of the source mantle. First, the LREE enrichment, range in Yb abundances, and flat, unfractionated HREE pattern of the Floreana lavas argue strongly against the presence of garnet in the residual mantle assemblage. All garnet-bearing models yield liquids that are HREE depleted until advanced stages of partial melting, by which time Sm/Yb ap-

proaches unity. Second, the overall REE profiles of spinel lherzolite partial melts most resemble the Floreana data. Discrepancies persist in La/Sm, however; even very low-degree partial melts fail to attain the elevated La/Sm observed in Floreana basalts. Clearly, lavas with La/Sm > 3.2 could not have been derived directly by melting of chondritic mantle material, but must represent partial melts of a LREE mantle source. Partial melting of LREE-enriched sources has been postulated to explain REE profiles of alkali-olivine basalts worldwide (e.g., Sun and Hanson, 1975; Minster and All~gre, 1979 ). Although consistent with geochemical data, such

98

a mechanism is somewhat problematic in the case of Floreana, because Nd isotope data indicate the source has been depleted in LREEs on a time-averaged basis (White et al., 1978). Collectively, these observations would require that the Floreana source has been enriched in light REEs very recently, before new radiogenic 143Nd could have accumulated in the source. One possibility is that the LREE enrichment occurred in the lithosphere below Floreana, which formed about 10 Ma (Hey et al., 1977). Another possibility is that the asthenosphere beneath Floreana was recently enriched by mantle metasomatism, a possibility strongly supported by regional considerations (Geist, 1992). If Floreana lavas are interrelated via partial melting of a LREE-enriched source, then it is possible to construct a mathematical model which reproduces the spectrum of REE abundances observed in the rocks (Table 2 ). Such a model must duplicate, for different degrees of melting, the slope ratios La/Sm and Sm/Yb for the range of lavas. ITE-enriched Main Series lava F1-19 is taken to represent low degrees of partial melting and a primitive, ITE-depleted flow (Fl-29) is assumed to approximate an advanced melt. Calculated ratios of the light REE slopes are: ( L a / S m ) l g / ( L a / S m ) 2 9 = 2.3 and ( S m / Y b ) 1 9 / ( S m / Y b ) 2 9 - ~ - 1.3 Comparison of observed and calculated REE ratios for a spectrum of mantle sources indicates that lherzolite ( + spinel) is the only rock type capable of yielding the full range of Main Series REE upon melting (Table 2). These models predict that F1-19 and F1-29 represent 2% and 25% melting. The model actually provides little constraint on the absolute amount of melting required, and 0.2% to 2.5% melting could explain the data equally well. Substitution of the estimates of the extent of melting (F = 2% and 25%) into the appropriate melting equation yields L a / S m = 3 . 6 and S m / Y b = 1.4 for the source. If Fl-19 and Fl-29 are directly related by partial melting, the

C.S. BOW AND D.J. GEIST

TABLE2 REE ratios for mantle melting models. Best fit solutions for Main Series model (*) and Main Series/Flank Series Model ( + ) are indicated. Mantle

F1

F2

La/Sm F1 La/Sm F2

Sm/Yb F1 Sm/Yb F2

Plagioclase peridotite

0.02 0.02 0.02

0.30 0.20 0.10

1.6 1.6 1.4

1.2 1.2 1.1

Spinel lherzolite

0.02 0.02 0.02

0.30 0.25 0.10

2.3* 2.3 1.9

1.3* 1.2 1.1

Cpx-rich lherzolite

0.02 0.02 0.02

0.45 0.25 0.10

3.3 + 2.5* 1.6

1.1 + 1.0

0.02 0.02 0.02

0.40 0.20 0.10

3.9 3.3 2.2

4.0 1.9 1.3

Garnet lherzolite

Measured ratios from Floreana lavas: Main Series Model: (La/Sml)9/(La/Sm)29=2.3, (Sm/ Ybl)9/(Sm/Yb)29

= 1.3

Main Series/Flank Series Model: (La/Sml)9/(La/ Sm)25=3.3, (Sm/Ybl)9/(Sm/Yb)25= 1.3 Initial concentrations taken to be 3.1 ppm (La), 0.9 ppm (Sm), and 0.6 ppm (Yb). Distribution coefficients are from Masuda and Kushiro (1970), Lindstrom ( 1976 ), and Shimizu and Akimoto ( 1971 ).

source mantle most likely is a LREE-enriched lherzolite with a high modal clinopyroxene content. The timing of mantle enrichment can be constrained using these values of the LREE enrichment and the Nd isotopic ratios of Floreana lavas (White and Hofmann, 1978). If a strongly-depleted MORB source (~Nd= + 10 ) is taken as the initial mantle isotopic ratio, a model age for the enrichment event is 270 Ma. In the event the original mantle material was not so strongly depleted, enrichment could have occurred more recently. Equivalent trace-element modelling has been used to test the possibility that Flank Series magmas (represented by FI-25 ) are related to Main Series via progressive partial melting. In this case, the appropriate ratios of the LREE slopes are:

GEOLOGY AND PETROLOGY OF FLOREANA ISLAND, GALAPAGOS ARCHIPELAGO, ECUADOR

(La/Sm)19/(La/Sm)25=3.3 Yb )19/ (Sm/Yb )25= 1.2

and

(Sm/

Once again, it is evident that partial fusion of the three-phase lherzolite is the only mechanism that can accommodate the large range in La/Sm found in the rocks. Calculated La/ Sm and Sm/Yb ratios for the mantle source again indicate initial LREE enrichment. The high degree of partial melting implied by these model calculations (45%) is difficult to accept, however, given our present understanding of magma extraction in mantle melting systems (e.g., McKenzie, 1984).

Partial melting models: conclusions Major- and trace-element variation diagrams exhibit considerable scatter as a consequence of random addition of xenolithic debris to various parental magmas. Incompatible trace-element data, together with modelling of REE variations among Floreana lavas, suggest that the host lavas represent products of partial melting of a LREE-enriched lherzolite mantle. Isotopic data indicate that the mantle has been enriched within the past 270 Ma. The apparent lack of garnet in the Floreana source indicates relatively shallow depths of magma extraction ( < 60 km). This observation may have regional importance, because it is in striking contrast to Santiago, Santa Cruz, and San Cristobal, whose sources most likely do contain garnet (Lindstrom, 1976; Bow, 1979; Geist et al., 1986) and are therefore likely to be deeper. The Floreana source may have retained accessory phlogopite and apatite at the lowest stages of melting. Age and stratigraphic relationships require that repeated melting and extraction events occurred throughout the history of the volcano, continually providing relatively small volumes of near-primary magmas. Subtle variations in bulk distribution coefficients and in source composition are likely to have occurred and may in part account for the discrepancies between predicted

99

and observed trace element abundances in Main Series flows. Main Series and Flank Series lavas are distinct suites and are not readily linked to each other by simple melting models. This is consistent with accumulating petrographic and geochemical evidence that these magma series have somewhat different sources and magmatic histories. Ultramafic inclusion suite

Floreana is the only known source of ultramarie xenoliths in the Galapagos Archipelago. Gabbroic to quartz syenitic and granitic inclusions are common in lavas of Santiago, Rabida (McBirney and Williams, 1969), and Alcedo (Geist and Howard, in prep. ), and rare xenoliths of gabbro, troctolite, and norite are found in lava flows and tufts from Santa Cruz (Bow, 1979) and San Cristobal (Geist et al., 1986). These are low-pressure, igneous-textured xenoliths which reflect crystal/liquid equilibria attained during crystallization in high-level magma chambers. Xenoliths and xenocrysts collected from Floreana lavas and pyroclastic vents are of two principal types: ( 1 ) Ultramafic xenoliths, including wehrlite ( + spinel + plagioclase), clinopyroxenite megacrysts ( _+ spinel), dunite ( + spinel), and rare orthopyroxenite. Based on experimental studies in analogous systems (Chapman, 1976), these are high-temperature (1300°C), high-pressure (5-20 kbar) inclusions. (2) Mafic xenoliths compositionally akin to types described previously from other islands. Many of the Floreana variants exhibit tectonite or granulite fabrics rather than igneous textures, however, indicating a prolonged residence within the lower oceanic crust or upper mantle. Ultramafic xenoliths display a wide variety of textures. Roughly half of the wehrlites have protogranular (unfoliated) fabrics, with large olivine grains enclosed by an aggregate of

100

C.S. BOW AND D.J. GEIST

--?

d d.,~ ~ c5 c5 d d d ~ ' o ~

O

e

~

O

~

O

0

0

,,~

dd~,.~d~dd'~

~oO e.q e~

0

0 0

c5

c5

e~ IP-.-

o

•

0

00

o0 co

0

00

d

0

d

0

c~

0

0

0

~

~

0

0 ~

0

~

e~

~

e.q ' ~ e~ e~ 0

o

~,.

0

~ d 4 ~ d d ~ d d d d

88 e~

.<

~

~

z

~

QO e~

0

d

c5

e~

.~-c~

°

d

d

GEOLOGY AND PETROLOGY OF FLOREANA ISLAND, GALAPAGOS ARCHIPELAGO, ECUADOR

smaller polygonal clinopyroxenes. Spinel occurs as wormy, interstitial blebs and stringers or as euhedral inclusions within silicates. Many of the spinel grains are surrounded by plagioclase coronas. A smaller number of xenoliths have textures reflecting extensive subsolidus recrystallization: a porphyroclastic texture, characterized by mosaics of polygonal crystals enclosing large relict grains of olivine, and a mosaic equigranular texture, where the entire rock has recrystallized to equant, interlocking grains. A few of the xenoliths are severely deformed and display tabular equigranular fabrics characterized by the development of compositional segregations. Analytical data document a large degree of overlap in mineral and whole rock composition within various subgroups of the xenolith

101

population (Table 3). Variations in modal proportions and phase compositions between nodule types are partly due to subsolidus reactions, such as the exsolution of spinel from Alrich augite. Textural evidence of reheating of olivine and spinel, together with composite veining textures and the prolonged subsolidus history recorded in many inclusions render it unlikely that they are truly cognate. Floreana's xenoliths are similar to other clinopyroxene-rich suites from throughout the world in having both Cr-diopside and Al-augite compositions (Wilshire and Shervais, 1975), commonly referred to as Type I and Type II xenoliths (Frey and Prinz, 1978). Compositions of coexisting clinopyroxenes and olivines in primary assemblages from Floreana nodules (Fig. 11 ) indicate that there is a

2 TiO 2

Di

Hd -.~ 1 @ o

$

I

I

FeO

0.4

FeO + M g O @

| 0 0

0.2

I

I

0.1

0.3

Cr203 Cr203

+

AI203

Fig. 11. (a) Coexisting olivines and clinopyroxenesfrom Al-augite group (circles) and Cr-diopside group (squares) nodules, projectedinto the pyroxenequadrilateral. (b) Compositionaldata for nodule clinopyroxenes,delineatingfields for Al-augitegroup and Cr-diopsidegroup samples. Tie lines connectprimary nodule pyroxenes (circles) with coexisting groundmass pyroxenes (triangles).

102

C.S. BOWANDD.J. GEIST

narrow range of compositions from individual xenoliths, and that Type II xenoliths have marginally lower MgO/MgO+FeO* than the Type I wehrlites (Fig. 11 ). Floreana Type II nodules tend to be enriched in basaltic components, such as FeO and TiO2, relative to the Type I group, similar to trends observed worldwide (e.g., McDonough and Frey, 1989). The lherzolites that are an important component of other Cr-diopside group xenoliths do not occur in the Floreana suite. Cr-diopside group pyroxenes are enriched in Cr, Ni, and Sr, and depleted in the REE, Zr, Th, Ta, Y, Rb, Sc, Ti, A1, and Na relative to pyroxenes of the Al-augite group. Clinopyroxenes from Al-augite wehrlites and megacrysts exhibit differing degrees of LREE depletion with little overall variation in total REE abundances (Fig. 12). In contrast, the REE profile for a Cr-diopside megacryst is slightly LREE enriched, with lower overall abundances. Although this megacryst may not be representative of the entire Floreana Cr-diopside suite, its LREE-enriched character suggests that it is of the Type IB clan (Frey and Prinz, 1978).

1 0 0 ~

I

AIAugite

1

La

Ce

Sm Eu

Tb

Yb Lu

Fig. 12. REE compositions for clinopyroxenes and a spinel from Floreana xenoliths. The open triangles define REE profiles for liquids in equilibrium with Al-augite megacrysts, assuming the crystal/matirx distribution coefficents of Sun and Hanson ( 1975 ).

Calculated mineral/matrix distribution coefficients for Al-augites and host rocks are similar to values obtained from megacryst/basalt pairs from Ross Island, Antarctica (Sun and Hanson, 1975), Oki-Dogo Island, Japan (Nagasawa, 1973 ), and a worldwide sampling (Irving and Frey, 1984). This observation, together with experimental and petrographic data (Chapman, 1976; Bow, 1979 ), suggests Al-augite group xenoliths may be cognate in origin. Other trace-element data restrict the degree of Al-augite fractionation permissible in generating the range of Floreana magma compositions, however, and present a problem with respect to apparent distribution coefficients for chromium. Specifically, Sc is strongly partitioned into clinopyroxene (D > 2 based on Floreana megacryst/whole-rock abundance ratios), but the Sc contents of analyzed Floreana lavas show little systematic variation. These Sc data do not permit significant cpx fractionation, unless accompanied by additional minerals that reduce the bulk distribution coefficient to near unity, a mechanism for which there is no evidence. In addition, some explanation must be found for the crystallization of Cr-poor (0-500 ppm) clinopyroxene from a Cr-rich (500-1200 ppm) melt. Values of the distribution coefficient for Cr in clinopyroxene which would be required for crystallization under these conditions are up to three orders of magnitude lower than predicted for pyroxene crystallizing from liquids as Cr-rich as Floreana magmas. Systematic variations in temperature or pressure do not appear to provide the answer, because the distribution coefficients for Sc and Cr should not be strongly affected (e.g., Nielsen, 1990). One possible explanation is that available octrahedral sites in clinopyroxene are preferentially filled by other cations (i.e. A13+, Ti 4+, Fe 3+ ). The inverse correlation between Cr and other highly charged cations in Floreana clinopyroxene megacrysts (Fig. 13) and the relatively sharp A1203 break between high-Cr and low-Cr pyroxenes suggest a modification to the crystal

GEOLOGY AND PETROLOGY OF FLOREANA ISLAND, GALAPAGOS ARCHIPELAGO, ECUADOR

15

AI 2 0 3 -4TiO2 + 10 Fe203

@@ AI Augite

I &

AA Cr Oiopside I 5

I 10

• I 15

Cr z O a

Fig. 13. Variations in the contents of chromium and other, highly charged cations within clinopyroxenes from Floreana xenoliths and megacrysts. The double-ruled line separates Al-augite (solid circles) and Cr-diopside (solid triangles) groups.

lattice may occur under elevated pressures, perhaps as a consequence of accommodating very high levels of A1203. This could cause a distortion of the crystal structure, resulting in the unusual, glassy texture of Al-augite megacrysts and simultaneous exclusion of Cr. These apparently conflicting data can be reconciled via the model of "passive crystallization" (McBirney and Williams, 1969, p. 186 ) in which Al-augite group megacrysts are considered to be cognate, but non-fractionating components to the system. In other words, they may have crystallized from their host lavas but never effectively separated from them. Such a process can be viewed as approximating equilibrium crystallization, in that the bulk compositions of parental magmas remain essentially constant. The origins of Cr-diopside group xenoliths and their relationships to host lavas and to A1augite group inclusions are more obscure. The enigma of these rocks, like that of all Type IB xenoliths, is that they have very depleted major- and incompatible trace-element abundances (high MgO, Cr; low A1, Ti, REEs) but are somewhat enriched in LREEs. One possibility is that they crystallized from very primitive magmas that resulted from large extents

103

of partial melting, which would explain their relatively high Cr/(Cr+A1) and Mg/ (Mg+Fe) (Mysen and Kushiro, 1976). The proposition that Cr-diopsides crystallize from hotter, more advanced partial melts finds additional support in their ITE-depleted character, although such melts (and clinopyroxenes crystallized from them) should also have a LREE-depleted signature. Alternatively, these crystals could be part of a refractory residuum variable depleted by partial melting (e.g., Maaloe and Aoki, 1975 ) although, once again, their LREE enrichment is a problem. "Cryptic" mantle metasomatism has been called upon to resolve conflicting evidence provided by LREE-enriched Cr-diopsides (e.g., Menzies, 1983; Kempton, 1987). The salient feature of this model is that the xenoliths have experienced a complex history involving initial depletion during partial melting, followed by enrichment in incompatible trace elements by reaction with volatile-rich fluids or melts. Such a mantle metasomatic event is an attractive explanation for the depleted character of both Cr-diopside xenoliths and the strong, recent, LREE enrichment of host lavas. Overall, the Cr-diopside xenoliths are less likely to bear any direct genetic connection to host basaltic magmas.

Conclusions Geologic studies of Floreana Island have been integrated with geochemical and petrological observations and modelling to develop a general model for the origins of Floreana magmas and associated xenoliths. Main Series lavas contain much disaggregated xenolithic material and may be primary or near-primary magmas, little modified by crystal fractionation during ascent. Porphyritic Flank Series lavas experienced a rather different ascent history, as they resided in shallow-level magma chambers and crystallized olivine and plagioclase. Isotopic, geochemical and petrographic data demonstrate that Flank Series lavas are

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not cogenetic in any simple way with spatiallyassociated Main Series flows. REE models suggest Floreana lavas are the products of partial melting of a LREE,enriched, garnet-free lherzolite. Age and stratigraphic relationships require that repeated melting and extraction events occurred throughout the history of the volcano, which gave rise to relatively small volumes of derivative lavas. Floreana is unique amongst the islands of the Galapagos by virtue of having yielded a diverse suite of ultramafic xenoliths. Al-augite group megacrysts are likely to be cognate and an integral part of the magmatic system which ultimately generated olivine-normative, undersaturated lavas of the Main Series. Transition metal contents of megacrysts and host lavas, however, preclude clinopyroxene fractionation as being a significant mechanism of magmatic differentiation. It is proposed that these megacrysts were products of passive crystallization in a system that maintained constant bulk composition. Cr-diopside group nodules may have crystallized at very high temperatures from advanced partial melts, and are less likely to bear any direct genetic connection to host basaltic magmas.

Acknowledgements Thanks to Mac for originally suggesting this project, supporting it, and his patience in seeing it through. This work could not have been accomplished without the logistic support of the Charles Darwin Research Station and the permission of the Galapagos National Park (Ecuador). Stalin Benites was an invaluable field assistant. Tom Simkin and John Sinton provided helpful reviews. The work of DJG was supported by NSF grant EAR8804903.

References Allegre, C.J. and Minster, J.B., 1978. Quantitative models of trace element behavior in magmatic processes. Earth Planet. Sci. Lett., 38: 1-25.

C.S. BOW AND D.J. GEIST

Baitis, H.W., 1976. Geology, petrography, and petrology of Pinzon and Santiago Islands: Galapagos Archipelago. PhD thesis, University of Oregon, 256 pp. Baitis, H.W. and Lindstrom, M.M., 1980. Geology, petrography, and petrology of Pinzon Island, Galapagos Archipelago. Contrib. Mineral. Petrol., 72: 367-386. Bow, C.S., 1979. The Geology and petrogeneses of the lavas of Floreana and Santa Cruz Islands: Galapagos Archipelago. PhD thesis, University of Oregon, 308 pp. Chadwick, W.W., Jr. and Howard, K.A., 1991. The pattern of circumferential and radial eruptive fissures on the volcanoes of Fernandina and Isabela Islands. Bull. Volcanol., 53: 259-275. Chapman, N.A., 1976. Inclusions and megacrysts from undersaturated tufts and basanites, East Fife, Scotland. J. Petrol., 17: 472-498. Chubb, L.J., 1933. Geology of the Galapagos, Cocos and Easter Islands. BP Bishop Museum, Bull. 180, 67 pp. Clague, D.A. and Frey, F.A., 1982. Petrology and trace element geochemistry of the Honolulu Volcanics, Oahu: implications for the oceanic mantle below Hawaii. J. Petrol., 23" 447-504. Cox, A. and Dalrymple, G.B., 1966. Paleomagnetism and potassium-argon ages of some volcanic rocks of the Galapagos Islands. Nature, 209: 776-777. Cullen, A.B. and McBirney, A.R., 1986. Volcanic geology and petrology of Isla Pinta, Galapagos Archipelago. Geol. Soc. Am. Bull., 98: 294-301. Delong, S. and Hoffman, M., 1975. Alkali/silica distinction between Hawaiian tholeiites and alkali basalts. Geol. Soc. Am. Bull., 86:1101-1108. Ferrera, G. and Trueil, M., 1974. Petrological implications of trace element and Sr isotope distributions in basalt-pantellerite series. Bull. Volcanol., 16: 454-487. Frey, F.A. and Prinz, M., 1978. Ultramafic inclusions from San Carlos, Arizona: petrologic and geochemical data beating on their petrogenesis. Earth Planet. Sci. Lett., 38: 129-176. Gast, P.W., 1968. Trace element fractionation and the origin of tholeiitic and alkaline magma types. Geochim. Cosmochim. Acta, 32: 1057-1086. Geist, D.J., McBirney, A.R. and Duncan, R.A., 1985. Geology of Santa Fe Island: the oldest Galapagos volcano, J. Volcanol. Geotherm. Res., 26: 203-212. Geist, D.J., McBirney, A.R. and Duncan, R.A., 1986. Geology and petrogenesis of lavas from San Cristobal Island, Galapagos Archipelago. Geol. Soc. Am. Bull., 97: 555-566. Geist, D.J., White, W.M. and McBirney, A.R., 1988. Plume-asthenosphere mixing beneath the Galapagos Archipelago. Nature, 333: 657-660. Geist, D.J., 1992. An appraisal of melting processes and the Galapagos hotspot" major and trace element evidence. In: D.J. Geist and C.M. White (Editors), Special Issue in Honour of Alexander R. McBirney. J. Volcanol. Geotherm. Res., 52: 65-82.

GEOLOGY AND PETROLOGY OF FLOREANA ISLAND, GALAPAGOS ARCHIPELAGO, ECUADOR

Gordon, G.E., Randle, K.R., Goles, G.G., Corliss, J.B., Beeson, M.H. and Oxbey, S.S., 1968. Instrumental activation analysis of standard rocks with high-resolution x-ray detectors. Geochim. Cosmochim. Acta, 33: 1389-1414. Hey, R., Johnson, G.L. and Lowrie, A., 1977. Recent plate motions in the Galapagos area. Geol. Soc. Am. Bull., 88: 1385-1403. Irving, A.J. and Frey, F.A., 1984. Trace element abundances in megacrysts and their host basalts: constraints on partition coefficients and megacryst genesis. Geochim. Cosmochim. Acta, 48: 1201-1221. Irvine, T.N. and Baragar, W.R.A., 1971. A guide to the chemical classification of the common volcanic rocks. Can. J. Earth Sci., 8: 523-548. Kempton, P.D., 1987. Mineralogic and geochemical evidence for differing styles of metasomatism in spinel lherzolite xenoliths: enriched mantle source regions of basalts? In: M.A. Menzies and C.J. Hawkesworth (Editors), Mantle Metasomatism. Academic Press, London, pp. 45-90. Lindstrom, M., 1976. Geochemical studies of volcanic rocks from Pinzon and Santiago Islands, Galapagos Archipelago. PhD Thesis, University of Oregon, 174 pp. Maaloe, S. and Aoki, 1975. The major element composition of the upper mantle estimated from the composition of lherzolites. Contrib. Mineral. Petrol., 63: 161173. Masuda, A. and Kushiro, I., 1970. Experimental determination of partition coefficients of ten rare earth elements and barium between clinopyroxene and liquid in the synthetic silicate system at 20 Kb pressure. Contrib. Mineral. Petrol., 26: 42-49. McBirney, A.R. and Williams, H., 1969. Geology and petrology of the Galapagos Islands. Geol. Soc. Am., M em. 118, 197 pp. McDonald, G.A. and Katsura, T., 1964. Chemical composition of Hawaiian lavas. J. Petrol., 5:82-133. McDonough, W.F. and Frey, F.A., 1989. Rare earth elements in upper mantle rocks. In: B.R. Lipin and G.A. McKay (Editors), Geochemistry and Mineralogy of Rare Earth Elements. Rev. Mineral., 21: 99-146. McKenzie, D.P., 1984. The generation and compaction of partially molten rock. J. Petrol., 25:713-765. Menzies, M., 1983. Mantle ultramafic xenoliths in alkaline magmas: evidence for mantle heterogeneity modified by magmatic activity. In: C.J. Hawkesworth and M.J. Norry (Editors), Continental Basalts and Mantle Xenoliths. Shiva, Nantwich, pp. 92-I 10. Minster, J.F. and Allegre, C.J., 1979. Systematic use of trace elements in igneous processes, Part III: Inverse

105

problem of batch melting in volcanic suites. Contrib. Mineral. Petrol., 68: 37-52. Mysen, B. and Kushiro, I., 1976. Compositional variation of coexisting phases with degree of melting of peridorite under upper mantle conditions. Carnegie Inst. Washington, Yearb., 75: 546-554. Nagasawa, H., 1973. Rare earth distribution in alkalic rocks from Oki-Dogo Island, Japan. Contrib. Mineral. Petrol. 39: 301-308. Nielsen, R.L., 1990. Simulation of igneous differentiation processes. Rev. Mineral., 24: 65-106. Norrish, K. and Chappel, B.W., 1967. X-ray fluorescence spectrography. In: J. Zussman (Editor), Physical Methods in Determinative Mineralogy. Academic Press, London, pp. 161-215. Norrish, K. and Hutton, J.T., 1969. An accurate X-ray spectrographic method for the analyses of a wide range of geologic samples. Geochim. Cosmochim. Acta, 33: 431-453. Shaw, D.M., 1970. Trace element fractionation during anatexis. Geochim. Cosmochim. Acta, 34: 237-243. Shimizu, N. and Akimoto, S., 1971. Partitioning of strontium between clinopyroxene and liquid at high pressures: preliminary experiments. Earth. Planet. Sci. Left., 13: 134-138. Sun, S.S. and Hanson, G., 1975. Origin of Ross Island basinitoids and limitations upon the heterogeneity of mantle sources for alkalic basalts and nephelinites. Contrib. Mineral. Petrol., 52: 76-106. Sun, S.S. and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: A.D. Saunders and M.J. Norry (Editors), Magmatism in the Ocean Basins. Geol. Soc. Spec. Publ., 42:313-345. Swanson, EJ., Baitis, H.W., Lexa, J. and Dymond, J., 1974. Geology of Santiago, Rabida, and Pinzon Islands, Galapagos. Geol. Soc. Am. Bull., 85: 1803-1810. White, W.N. and Hofmann, A.W., 1978. Geochemistry of the Galapagos Islands: implications for mantle dynamics and evolution. Carnegie Inst. Washington Yearb., 77: 596-606. Wilshire, H.G. and Shervais, J.W., 1975. Al-augite and Crodiopside ultramafic xenoliths in basaltic rocks from the western United States. Phys. Chem. Earth, 9: 257272. Vicenzi, E.P., McBirney, A.R., White, W.M. and Hamilton, M., 1990. The geology and geochemistry of Isla Marchena, Galapagos Archipelago: an ocean island adjacent to a mid-ocean ridge. J. Volcanol. Geotherm. Res., 40: 291-315.