Palaeogene komatiites from Gorgona Island, East Pacific

G eochem ical Journal, V ol. 15, pp. 1 41 to 1 61, 19 81
141
Palaeogene kom atiites from G orgona Island,
East Pacific- A prim ary m agr na for
ocean fl oor basalts?
V . J. DIETRICH1, A. G ANSSER 2
J. SO M M ERA UER1 and W . E. C A M ER ON 3
Institut fur Kristallographie undPetrographiel,ETH-Ztirich, and Geologisches Institut2,
ETH-Ztirich, CH-8092 Zurich,Switzerlan d, Departm ent of Geology3,
Australian National U niversity,Canberra, Australia 2600
(R eceived M arch 30, 1981: Accepted M ay 13, 1981)
G orgona island is the only place in the w orld w here young ultram afic (pyroxenitic) kom atiitic lava
lf ow s are know n to occur. The island, w hich appears to be a sm all southern rem nant of the Colom bian
C oastal C ordm era, is buil t up of serpentinised peridotites, gabbros, basalts and associated Palaeogene oceanic
sed im ents.
Spinifex texured rocks occur w ith in the basal tic and doleritic com plex, closely associated with pillow
basalts of tholeiitic com position. D uring rapid cooling the order of cry stallisation w as: olivine (F090-91 ),
high A1-calcic pyroxene an d Cr-spin el as the m ajor quenched phases; then olivine (F086_82)' Iow A l-cal cic
pyroxene, plagioclase (A n75_81) and Ti-m agnetite as quenched products in the interstitial groundm ass.
Bulk chem istry: M gO - 16w t.% (M g/M g+ Fe2+ = .74), Cr - 1250ppm , Ni - 700ppm , Sc
33ppm ,
B a - 5 ppm , Zr - 30 ppm , Y - 15ppm ; fl at R E E pattern (5 to 8 x chondrites) with slight depletion of
L R E E. The associated basalts and dolerites yield sim il ar pattern, although sligh tly enriched in L R E E.
M odel calculations incorporating m ajor and trace elem ents show that fractionation of 20- 25% olivine
+ 0.5% C r-spinel from the kom atiitic liquid could produce olivine tholeiites sim ilar to those of Palaeogene
N azca plate. T he unique structural setting of the G orgona rocks probably results from a rapid uplift ofa
central part of the N azca ridge includin g im m ature m agm a cham bers.
applies particularly for the G orgona kom atiites
in respect to their occurrence in a Phanerozoic,
O ne of us (A.G.) visited the Pacific w est non-island ar c ophiolite com plex and to their
coast of C olom bia and m apped the G orgona chem ical affinities to ocean fl oor tholeiites.
island during the years 1944 and 1 945 w hile on
O ne of the m ajor goals of the present study
i
s
the
identification of the G orgona kom atiite as
an exploration assignm ent with the Shell Oil Co.
T he M esozoic to Palaeogene unm etam or- a parental liquid com position that m ay yield
phosed ophiolite com plex and the occurrence fractionated ocean fl oor tholeiites. T his is based
of the olivine spinifex rocks have already been on the assum ption that these liquids are derived
described by G ANSSER (1950). T he discovery by part ial m elting from a lherzolitic m antle
of their kom atiitic natur e in 1976 w as due to source,Ieaving residual harzbur gite behind,as has
the com parison with the high m agnesia boninites been suggested by BoETrcHER
___,_ _.(
et al. (1 975), M YSEN
dredged from the M ariana trench during the and K USHIRO (1977), BENDER !t al. (1978),
1976 ophiolite cruise of D m itri M endeleev G REEN et al. (1978) and PRESNALL al. (1 979).
O nly a few occurr ences of prim itive basa lts
(O BERHAENSLI et al., 1 977; D IETRICH et al., 1978;
PEIVE, 1980). Both quenched rock types have have been considered to be parental to m idsim ilar olivine (Fo_.o) and Cr-spinel com posi- ocean tholeiites (e.g. F REY et al., 1974;
tions and thus are good candidates for a discus- O'NIONS and CLARKE, 1972; ELTHON , 1979).
sion on prim ary basaltic m agrn as (G ANSSER et al, IRV_IN 's (1 977) calculated prim itive liquid com 1979; CAMERON et al., 1979, 1980). T his position of abyssal tholeiites requires olivine
INTR OD UCTION
142
V. J. DIETRICH et al.
(F o_ 90.=) as the first fractionated m ineral. T his
hypothetical liquid com position is sim ilar to
prim itive com positions of glass inclusions in
olivine and spinel from M id-A tlantic ridge basalts
(D ONALDSON and B Row N, 1977; D UNGAN and
R HODES,1978).
A m ajor tool in the determ ination of the
nature of prim ary m agm as besides phase equilibria and experim ental studies in the system
CaO-M gO-A1203-Si02 is a study of the chem ical
coherence (using m ineral chem istry as w ell as
m ajor, trace and rare earth elem ents) betw een
natural prim itive basaltic volcanics and the vast
am ount of fractionated tholeiites erupted at
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m agnetic anomalies (numbersin m.y.B.P.)after LONSDALE and KLITGARD (1978).
Palaeogene kom atiites
m id-ocean ridges. In this perspective the occurrence of pyroxenitic kom atiites interlayered
with basalts and dolerites on G orgona island
yields a unique situation. In order to clarify the
petrogenetic relationship betw een kom atiites
and basalts, the full scale of m afic and ultram afic rocks, such as gabbros, cum ulates and
picrites w ill be discussed.
TECTONlC SETTlNG AND G EOLOGY OF
G ORGONA ISLAND
G orgona island is located on the continental
slope (Fig. l) approxim ately 80 km off the
Colom bian coast at 3'N Lat. and 78.2'W Long.
T ectonically the island appears to be a sm all
southern rem nant of the Coastal C ordillera
(a northern segm ent of the A ndes: A UBOlN ,
1973; BUTTERLlN , 1973; G ANSSER, 1950, 1973),
w hich disappears into the Pacific O cean at
Cabo Corrientes. G orgona m easures - 14km 2
and is accam panied by the m uch sm aller
G orgonilla island (about I/12 of G orgona), I km
to the south-w est.
T he Coastal Cordillera between the G ulf of
San M iguel (eastern Panam ) and the Chongon
Colonche C ordillera (w estern Ecuador) consists
m ainly of serpentinised peridotites, gabbros,
tholeiitic pillow basalts and fl ows interbedded
with deep water sedim ents such as U pper
Cretaceous radiolarian cherts and lim estones.
Locally m iddle Eocene reef lim estones and
Palaeocene to Eocene siliceous shales form inclusions in the volcanic sequences. T hese ophiolitetype rocks are overlain by a volcano-sedim entary
form ation up to I,OOO m thick (M aestrichtian to
Low er M iddle Eocene in age) w hich contains
basaltic to andesitic fl ows. In addition m inor
dioritic intrusions crosscut the ophiolitic rocks
as w ell as the volcano-sedim entary sequences
(BANDY, 1970; D uQUE, 1972; FAUCHER and
SAVOYAT, 1973; G ANSSER, 1950, 1973; G oosENS
et al., 1977). W e consider there to have been
at least three U pper Cretaceous to L ow er
E ocene island arc sequences within the C oastal
C ordillera: Chongon Colonche Cordillera, Cabo
M arco area (Serrania de Baudo) and in the G ulf
of San M iguel area. T hese sequences are m arked
143
with I.A. (island arc) in Fig. I.
The G orgona islands are built of approxim ately ninety percent of m afic and ultram afic
rock sequences (T able I), arranged in several
tilted and faulted blocks. Tertiary and Q uaternary sedim ents cover the rest of the islands (for
com parison see m aps in G ANSSER, 1950 and
ECHEVERRIA, 1 980).
The oldest sedim ents are U pper E ocene
foram iniferal, arenaceous lim estones and pyroclastic, siliceous shales rich in radiolaria. T hey
are know n only from the southern tip of
G orgonilla island w here they dip to the south
and continue southw estw ards into the single
cliff of -EI Viudo. T he contact with the
G orgonilla gabbros m ay have been originally
intrusive, but has subsequently been tectonized.
O n the w est coast of G orgona faulted siliceous
and silty shales occur, containing radiolaria
and poorly preserved foram inifera. They are
com parable with shales in the C oastal C ordillera
and Atrato-San Juan belt of Lower Oligocene
age (Fig. 1). O n G orgona the tuffaceous intercalations are m issing. L ow er M iocene rocks are
exposed along the dividing channel, La Tasca,
betw een G orgona and G orgonilla islands. T he
section begins with a coarse conglom erate
containing basic igneous and chalcedony pebbles
and grades into a thick sandstone and shale
sequence. The L ow er M iocene is transgressive in
the C oastal C ordillera, but here the contacts are
faulted. M arine relicts of M iddle to U pper
M iocene, consisting of foram iniferal arenaceous
lim estones with som e gabbro and diabase
pebbles outcrop on the G orgona east coast,
covered by som e raised beach terraces. This
M iocene section represents the transgressive
tail end of the thick Atrato-San Juan basin
sedim ents.
U nm etam orphosed ultram afic rocks (m ainly
w eakly layered dunites and w ehrlites) occur
within the gabbros: in the m iddle part of
G orgonilla island arid in the m iddle part of
the w est coast of G orgona island (G ANSSER,
1950). F urther investigations of ECHEVERRIA
and PARIS in 1978 have proved the existence
of other occurrences, particularly in the
144
V. J. DIETRICH et al.
interior of the island along a m ajor axial
fault (ECHEVERRIA, 1980), w here m apping is
greatly handicapped by m ost luxurious tropical
rain forests.
T he gabbros, including som e olivine gabbro
and troctolite with pronounced cum ulate
textures, constitute the central part of G orgona
island and the southern part of G orgonilla
island. T he contacts to the underlying ultram afics are clearly transitional except in places
w here faulting has occurred. O livine and spinel
are poikilitically enclosed by clinopyroxene.
Plagioclase fills the interstices. O phitic textures
dom inate in the upper levels of the gabbroic
units, w hich gradually passinto dolerites.
A lteration and secondary rodingitisation are
present in m any outcrops of the ultram afic and
gabbroic rocks. Olivine is often serpenti.nised,
the rocks are typically m esh-texured consisting
of chrysotile, Iizardite, m agnetite, chlorite
assem blages.
Plagioclase-rich parts in the
cum ulates have been transform ed to a prehnitehydrogrossular assem blage; clinopyroxene show s
actinolitic fringes. In m ost cases the dark green
clinopyroxene-gabbros display a coarse grained
ophitic texture, with m inor occurrences of
pegm atitic patches.
O f special interest is the Eocene lim estone-
Fig. 2. Spimfex-textured kom atiite fr om Gorgona island.
Sam ple (49) from the upper surface of the ultram afic
lava fl ow at Punta Trinidad. Weakly serpentinised
skeletal olivine plates (F086-85) causing a pronounced
octahedral cleavage at the rock surface. The quenched
groundm ass consists of clinopyroxene aggregates, glass
with plum ose extinction and partly quenched m agnesiochromite.
gabbro contact on G orgonilla island. The
nodular, honeycoloured chalcedony lenses and
bands, usually 5--20cm thick, are aligned along
the contact. These rocks are know n from several
ophiolite occurrences in the Coastal C ordillera.
D iabases and basalts are concentrated along
both sides of G orgona island, and in the northern
part of G orgonilla island. O n m ap scale the
diabases are rather uniform , in detail, however,
m any m inor differences can be recognized.
T he subdivision frorn the gabbros is in
m any places arbitrary. O ften it is difficult to
decide w hether a rock should be called a coarse
fresh diabase, a partly altered dolerite with
subophitic textures or a fine-grained gabbro. T o
date sheeted dikes have not been recognized
within the gabbroic com plex. Sills and flow s
seem to be the predom inant structural features.
Pillow lavas are w ell exposed in the northern
part at B oca el H orno.
H yaloclastites form the southern part of the
G orgona island w here they are in faulted contact
with diabases to the north and L ow er Miocene
sedim ents to the w est. They dip gently southw ards, sim ilar to the Eocene sedim ents of
G orgonilla. This 500m thick section is m ost
varied, and consists of w ell bedded hyaloclastites,
pillow lavas and diabase-breccias. U nder the
m icroscope, the finertuffs exhibit a very irregular
Fig. 3. Coarse spimfex-textured innerpart of thesam e
kom atiiticfl ow (sam ple 48).Large,zonedskeletalolivine
plates (F091_83) and smaller skeleta/ hl h-AI clinopyroxenes in fi ne g7lained arborescent groundm ass of
plagioclase,clinopyroxeneand titanom agnetite.
Palaeogene kom atiites
and patchy netw ork of fine augite needles in a
palagonite and chlori te groundm ass. T he age of
the hyaloclastites is not clear. It m ay be related
to the northern basaltic com plex capping
the pillow basalts and m ost likely form ing the
base of the G orgonilla Eocene sedim ents.
K om atiitic fl ows, exhibiting typical spinifex
textures, have been found by G ANSSER (1 950) in
tw o localities of G orgona Island: on the east
coast at Punta Trinidad and on the w est coast at
La M ancura. In addition ECHEVERRIA (1980)
reported five other occurr ences from both sides
of the island within the basalt and diabase
com plex.
O f exceptional interest are the fresh
kom atiitic fl ow s at Punta Trinidad. There, the
fl ow s seem to be interlayered betw een gabbros
and basalts.
The distinctive textural habits of these
ultram afic rocks are readily apparent upon
inspection. The dark green, m assive fl ow s of
up to one m eter thickness are characterised by
145
tw o principal textures: A "coarse arborescent
spinifex" texture (Fig. 3), caused by shiny 5 to
8cm long skeletal olivine blades; and an
"octahedral" texture (Fig. 2), w here the rock
surface exhibits a pronounced octahedral
cleavage. The latter, actual surface on the outcrops, is also caused by the spinifex arr angem ent
of fine, random ly oriented (up to 2cm long)
olivine blades in an aphanitic m atrix of partly
altered glass, w hile coarse spinifex textures
dom inate the interior parts of these outcrops.
The bottom part of the fl ows are m ade up by
olivine and pyroxene with cu m ulate textures. In
this respect the G orgona spinifex fl ow s closely
resem ble the pyroxenitic and peridotitic
kom atiite fl ows at M unro T ow nship (O ntario)
described in detail by PYKE et al. (1973) and
A RNDT et al. (1977).
A NALYT1:CAL TECHNIQUES
Mineral com positions w ere analyzed using an
A R L electron m icroprobe type SE M Q equipped
Table 1. Mineralogy of theselected maficand ultram afic rocks from the Gorgona islands
rocknam e
texture
locality
K om atiite fl ow
spinifex interior
portion
Punta Trinid ad
sam e flo w
spini fex, glassy
m ar gin
Pillow b asal t
500 m N of
Punta Brava
Picrite basal t
intergranular/subophitic La V entara
D olerite
sub ophitic
Cabo del H orno
Gab br o
coar seintergran ular
Punta M ancura
G abbro
coar se ophitic
SE coast G orgonilla
O livine gabbro
cum ulate
SE coast G orgonilla
W ehrlite
cu m ulate
S of Pu nta M ancura
Sam ple
N o. olivine
48
blades
F0 91 -83
pyroxene
plagioclase
skeletal high-A l
clin opyroxene
intergr anular
low -A I clinop yroxene
(Table 3, Fig. 5)
A n 81 - 75
(in rim s A n60)
spinel
second ar y m inerals
m agnesiochrom ite
chrysotile
ilm enite
partly altered glass
m agnesiochrom ite
chrysotile
partly altered glass
49
blades
F086-8s
51
ol-pseu dom or phs
skeletal augite
plag-pseudoE n41 5W os9 s Fsl3.5-15.5 m orphs
m agnesiochrom ite
chlorite,albite
69
F 091-90
augi te
E n44 9W 039 ;2 Fsll.5-14
A n eo -78
m agnesiochrom ite
chr ysotile, chlorite
47
E n47W 039Fsl4
A n60-30
ilm enite
chlorite,actinolite
45
E n31 W o20 Fs43
A n25-10
titan om agnetite
chlorite,prehnite,
pum pellyite
60
oiko crysts:
E n47 }8W 041 2 FsIo-12
pig: Ens3W 08F s39
oiko crysts:
E n47W 043 Fslo
A n65-60
titan om agnetite
intersertal sm ectite
A n88-86
m agn esiochro m ite
oiko crysts:
E n45W 046 Fs9
A n co -78
m agnesiochrom ite
chr ysotile/lizardite
preh nite, hydrogrossular,
actin olite
chrysotile/1izardite
hy drogrossular
actinolite
59
Fo 81-79
46
F0 8S-83
l46
V. J. DIETRICH et al.
R epresentative olivine analyses
Table 2.
Sam ple
N o.
Si0 2
Fe O *
MgO
MnO
Ca O
NiO
Cr2 0 3
K om atiites
48
Centre of
blade
40.6
l0.0
48.6
0.16
0.3 3
0.44
0.12
48
"lanterni'
olivine
40.2
1 3.1
45.9
0.20
0.34
0.3 3
0.10
Total
lO 0.2
Si
Fe
Mg
Mn
Ca
Ni
Cr
[Y ]vr
0_99 6
0.20 5
1.776
0.004
0.009
0.009
0.003
2.006
1.0 01
0.27 2
1.7 04
0.004
0.009
0.007
0.002
1.998
F0 89.7
F o86.2
48
m argin of
blade
39.5
14.6
44.4
0.1 8
0.40
0.22
0.15
100.2
99.5
49
blade
Picrite b asalt
69
phenocrysts
40.3
13.7
45.1
0.20
0.42
0.30
0.13
41.2
8.7
49.6
0.1 3
0.32
0.41
0.12
W ehrlite
46
cum ulate
40.3
15.5
44.3
0.24
0.30
0.19
0.02
10 0.l
1O 0.5
100.8
0.997
0.308
1.6 71
0.004
0.0 12
0.0 04
0.0 03
2.0 02
1.007
0.287
1.682
0.004
0.011
0.007
0.003
1.994
1.00 3
0.17 8
1.79 8
0.00 3
0.00 8
0.009
0.00 2
1.998
1.00 7
0.32 3
1.650
0.005
0.008
0.003
0.000
l.989
F 084.4
Fo 85.4
F0 91.o
F0 83.6
Catio ns to 4 oxy gens
*Totaliron expressed as FeO.
Blades F 091-F o88 in centres
F087_83 in m argins
Table 3. R epresentative pyroxene analyses
Sam ple
N o.
48
48
skeletal pyroxene
Si0 2
Ti0 2
A12 03
FeO *
MnO
M gO
CaO
Na2 O
Cr2 03
45.0
l.1
11.5
6.8
0.10
1 1.9
22.4
0.20
0.1 8
Total
99.2
En
Wo
Fs
47.6
0.7 3
7.7
12.O
0.29
lI.7
18.9
0.2 1
0.0 8
99.2
48
48
intergranular
(with plagioclase
and Ti-m agnetite)
49.4
50.4
0.9 8
0.79
3.8
2.1
14,7
18.2
0,31
0.41
12,8
11.4
18,9
18.1
0,1 6
1O1.O
37.3 (47.5) 36.5 (4 1.0) 36.9
50.7 (43.0) 4 2.5 (36.5) 39.2
12.0 ( 9.5) 2 1.0 (22.5) 2 3.8
* Total iron expressed as F eO.
Values for E n W OFS in brackets
49
49
skeletal pyroxenes
49
69
intergranular
49.4
1.0 2
5.3
8.2
0.19
13.9
21.2
0.17
0.27
69
m icrophenocryst
53.2
0.26
2.6
7.3
0.23
17.7
19.1
0.16
0.31
100.7
99.7
1O0.9
99.6
32.9 4 6.3 (5 4.8) 42.2 (50.9) 3 3.9 (42.0) 45.2
37.7 3 7.3 (29.6) 41.2 (31.6) 5 2.7 (44.4) 39.4
29.4 16.4 (15.6) 16.6 (17.6) 1 3.4 (13.6) 15.4
41.3
45.0
1 3.7
49.7
38.8
1 1.5
43.9
42.0
14.1
1O 1.4
47.4
0.88
8.9
9.5
0.19
15.2
17.O
0.20
0.27
99.5
46.7
0.85
11.3
9.5
0.26
13.5
18.3
0.16
0.21
10 0.8
45.3
1.O
13.1
7.5
0.14
10.6
23.0
0.16
0.19
1 01.0
51
51
skeletal pyrosenes
50.9
0.86
4.5
9.4
0.25
15.4
18.8
0.21
0.35
50.3
0.63
4.8
8.6
0.19
15.0
19.8
0.24
0.11
CaA l2Si0 6 (Tscherm aks-m olecule)subtracted
with four m otor driven X-ray spectrom eters and
tw o fixed X-ray m onochrom ators (set for Si
and M g, respectively). The following operating
conditions w ere applied: 1 5K V acceleration
voltage, 20 nA sam ple current m easured on brass
and a m inim um electron beam size of 0.2,lm .
F or each analysis the IO elern ents as listed in
Tables 2,3 and 4 w ere determ ined. Forreference
intensities, natural silicates and oxides w ere
applied.
R easonable counting tim es w ere used to
yield a standard deviation of the counting
Palaeogene komatiites
147
Table 4. Representative spinelanalyses
Sam ple N o.
Si0 2
Ti0 2
Al20 3
Cr20 3
Fe20 3+
FeO
MgO
MnO
NiO
T otal
M agnesiochrom ites
49
48
0.09
0.41
20.28
46.4 1
6.06
9.62
16.67
0.29
0.30
100.13
51
69
46
0.1 2
0.45
22.3 8
43.31
6.0 6
9.99
16.6 1
0.24
0.25
99.4 1
0.11
0.56
21.02
40.83
7.00
15.34
12.81
n.d.
0.15
97.82
0.16
0.17
20.1 3
47.03
3.9 8
13.57
14.09
n.d.
0.17
99.30
0.07
0.80
20.26
35.32
13.15
17.99
11.20
0.3 8
0.15
99.3 2
Si
Ti
Al
Cr
Fe
F e2+
Mg
Mn
Ni
0.022 0.075
5.800
8.901
1.106
1.952
6.027
0.060
0.059
0.029
0.082
6.386
8.288
1.I04
2.021
5.992
0.049
O.049
0.028
0.106
6.255
8.147
1.329
3.239
4.819
n.d.
0.030
0.040
0.03 2
5.889
9.266
0.74 3
2.816
5.2 11
n.d.
0.034
0.01 8
0.15 3
6.075
7.10 2
2.517
3.826
4.245
0.08 2
0.030
M g/(M g+Fe2+)
Cr/(Cr+ Al)
Fe3+/(Fe3*+cr+A l)
0.755
0.605
0.070
0.748
0.565
0.070
0.598
0.566
0.084
0.649
0.610
O.047
0_526
0.539
0.160
Titanom agnetite
48
18.49
2.9 8
27.8 3
46.3 8
1.65
97.8 3
4.233
1.069
6.343
0.122
lI.807
0.425
Ulv56 M gt44
Fe20 calculated assuming stoichiom etry
n.d.= not determined
statistics better than I % for m inor elem ents. U p
to I O analyses w ere averaged to determ ine the
m ineral com positions. The accum ulated counts
w ere corr ected on-line for background, drift and
dead-tim e with a PD P-1 1/05 com puter. C orrection procedures for X-ray absorption, X-ray
fl uorescence (by characteristic and continuum
excitation, respectively), and atom ic num ber
effects were based on a ZA F corr ection program
w ritten for the C D C 6500 com puter system at
the ET H Ztirich.
M ajor elem ent bulk chem ical X-ray fl uorescence analyses (T able 5) of 9 glass beads fused
from ignited pow ers plus Li2B407 (1/5 ratio in
gold-platinum pan at 11 50' C: DIETRICH et al.,
1976) w ere obtained on an autom ated Philips
sequential spectrom eter PW 1450 at the EM PA
D tibendorf, Switzerland. Com puter program s
w ere applied for correction of drift, background
and m atrix effects.
Tw elve U S-G eological
Surv ey reference rock sam ples were used for
calibration. FeO and H20 determ inations w ere
done by wet chem ical m ethods. N b, Zr, Y, Sr,
U, Rb, Th, Pb, G a, Zn, Cu, Ni, C o, Cr, V, Ce,
N d, Ba, La, Sc, and S trace-elem ent abundances
w ere also analysed by X-ray fl uorescence on I Og
pow er sam ples using the synthetic background
m ethod, if m ajor elem ent contents w ere know n.
A com puter program w as used to calculate background, interference, m ass absorption and
standard deviations (NISBET et al., 1979). T he
sam e U SG S reference sam ples as above were
used for calibration. T he accuracy is :! 2- 3 %
at - lOOOppm , i 5- 10 % at - 100ppm and
:!: l0- 20% at lOppm . Using a Cr-tube, the
detection lim its are around 3 ppm for m ost of
the trace elem ents.
R EE abundances (T able 6) of nine selected
sam ples have been analysed by radiochem ical
neutron activation analysis (R N A A ) at the
EIR - W tirenlingen, Switzerland (BAJO and
W YTTENBACH, 1980). BC R-1 prepared in the
sam e m anner as the unknow ns yielded La = 26.6
:!: 0.2, Ce = 54.0 i 0.9, N d = 29.4 :! 0.5, Sm 7.1
i 0.04, Eu = 1.91 i 0.014, G d = 6.86 :!: 0.17, Tb
= 1.01 :!: O.018, Tm = 0.621 :!: 0.013, Y b =
148
V. J. DIETRICH et al.
Table 5. Chemical composition ofrepresentative rock types from Gorgona Island
w t. %
Sam ple N o.
Si0 2
Ti0 2
A120 3
Fe20 3
FeO
MnO
MgO
CaO
N a20
K 20
P2 0 s
H20+
Total
Basal ts
K om atiites*
D olerite
G abbros
W ehrlite
46
48
49
51
69
44.2
0.6 6
12.0
3.2
8.2
0.1 8
15.9
10.1
l.1 3
0.02
0.06
4.0
99.7
44.0
0.7 2
12.6
4.0
7.6
0.1 8
14.2
11.5
1.05
0.07
0.06
3.7
99.6
47.8
0.89
14.6
1.8
8.1
0.17
8.9
1 1.2
2.9
0.0 8
0.0 8
3.3
99.8
44.2
0.35
10.0
3.0
7.3
0.17
22.7
8.5
0.78
0_O1
O.04
2.8
99.9
47
5 2.2
1.3
13.6
3.5
8.1
0.17
6.6
10_1
3.0
0.24
0.12
1.2
10 0.1
45
45.O
0.91
17.2
2.1
8.1
0.17
6.1
14.7
1.9
0.17
0.09
3.6
100.0
60
47.7
1.1
15.5
1.9
7.9
0.17
7.4
11.8
2.8
0.08
0.09
2.3
98.8
42.1
0.33
12.7
1.8
8.6
0.16
20.6
8.9
0.70
0.02
0.04
4.0
99.9
39.5
0.28
5.5
2.5
9.9
0.1 9
30.2
5.3
0.1 0
0.0 0
0.04
6.0
99.4
0.72
0.65
0.82
o.54
0.55
0.60
0.80
0.83
5 60
11 80
72
29 0
36
14
63
117
15
33
0.5
1
50
6
95
150
390
53
290
43
14
72
100
14
41
1
l
128
19
505
1040
2180
103
180
29
10
50
70
8
8
<1
<1
20
4
51
68
85
56
380
49
17
92
167
23
67
3
3
104
31
64 0
54
160
47
290
38
16
65
14 3
21
43
2
2
132
41
820
10 0
15 0
47
3 30
42
16
57
198
12
41
2
710
84 O
109
11S
14
11
53
34
4
7
1
<1
71
13
30 8
11 80
12 80
1 38
90
14
8
53
43
1
5
M g/(M g+F e2+)
0.74
Trace elem ents(ppm )
Ni
7 20
Cr
125 0
Co
94
V
26 0
Sc
31
Ga
15
Zn
57
Cu
150
Y
14
Zr
30
Nb
0.5
Rb
1
Sr
58
Ba
3
S
109
115
51
450
59
<1
30
3
210
*A nalyses ofN o.48and 49are m ean of4 determ inations. Y,Zr,N b,Ba,R b on these sam ples also by B.W.
CHAPPEL (XR F). M g-values calculated on the basis ofFe3+/(Fe2+ + Fe3+)= 0.1.
3.360 i 0.03, Lu = 0.54 :!: 0.005. In addition
R EE from tw o kom atiite sam ples N os. 48 and
49 w ere determ ined by spark source em ission
spectrography at A N U Canberr a using the technique of TAYLOR and G ORTON (1977).
PETR OG RAPH Y AN D PH ASE C HEMISTR Y
N ine characteristic rock types (T ab. 1) exhibiting the least al teration have been chosen
from the ori ginal collection (G ANSSER, 1950) to
show the m ineralogi cal and chem ical variety of
the ultram afic and m afic rocks from G orgona
and G orgonillaislands.
Olivine
Selected olivine analyses for kom atiites, a picri te basalt and a w ehrlite are listed in
Table 2. The large (up to 5cm long olivine
plates (Fig. 3) in the spinifex part of the kom atiitic fl ows are zoned, with F09li38 in the centres
and F 087i}3in the m argins. Ni decreases tow ards
the ri m s w hereas Cr and Ca slightly increase.
High m agnesian olivine (F 091-89) is frequent in
the picri te basalt, Iow values are abundant in the
cum ulate rocks (F084 in wehrlite and F081-79 in
olivine gabbro).
Cry stallisation tem peratures of 1400- 1500
'C for the kom atiitic fl ows w ere calculated using
olivine-spinel geotherm om eter (ENdl, 1 978 or
R OEDER et al., 1 979). T he low er tem peratures
of ca. 900'C for the picri te and of 850- 900'C
for the cum ulates m ay be interpreted as subsolidus reequilibration tem peratures.
Clinopyroxene
Clinopyroxene com positions
are show n in T able 3, and the cry stallisation
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Palaeogene kom atiites
150
V. J. DIETRICH et al.
paths in Fig. 6. Rather unusual com positions
are present in the large, up to 2 m m "harpoon"type skeletal augites (Fig. 4), (G ANSSER, 1950).
These first crystallising pyroxenes yield extrem ely high Al203 (up to 13 wt.%), high CaO
(up to 23 w t.% ) and Ti02 (,up to 1.1 wt.% ). Ca
occupies M (2) sites, Al can enter both the tetrahedral coordination (Ca-Tscherm aks m olecule)
and the octahedral M (1) site and Ti can be
expressed as CaTiA 120 6' R ecent experim ental
investigations indicate m inim um tem peratures
on the liquidus of about 1 230'C for high Al
Fig. 4. Close up of the arborescent, intergranular
m aterial of sam ple 48. Skeletalhigh-AIclinopyroxene
and partly euhedral m agnesiochromite in aggregates of
plagioclase (A n75_81)' Fe-rich clinopyroxene and irregularly distributed titanom agnetite.
and Ti-beari ng clinopyroxenes (A KASAKA and
O NUM A, 1980; H UEBNER, 1980). In contrast the
fine-grained, arborescent, intergranular clinopyroxenes (Fig. 4) tend to be enriched in silica,
iron and m anganese, w hereas C a, Al, and Cr
values are low er. Thus these phases (transition
to ferroaugites) indicate a m ore evolved stage
during the process of rapid cooling. Clinopyroxene from the nodular bottom parts ofthe
kom atiitic fl ow s are low er in Al20 3 (0.3 to 2.6
w t.%; ECHEVERRIA, 1 980) indicating slow cry stallisation. Pigeonite or orthopyroxene have not
been found in the kom atiites. Subophitic and
intergranular clinopyroxenes from the cum ulate
rocks, gabbros, dolerites and basalts depict the
norm al trend of differentiation (stippled field
in Fig. 6). Their A 120 3-variation lies betw een 2
and 4w t.%. The cpx com positions reach
En38W 042Fs20, w ith respect to the iron enrichm ent in olivine. T he internal part of a pillowlava consists of sm all groundm ass skeletal
clinopyroxene (Fig. 6) revealing A l20 3 values up
to 5.3wt.% and CaO values of 21.2wt.% . T hese
com positions overlap with those of the first
WQ
l¥
l ¥
fA ﾖ
skelBtol pyroxenes [*=CaAl2SiObsubtracted)
in4B& 49
V¥
10 :::::
20 _ 50
F5 En
Fs
Wo
/
48 *-1:
intergranular
46. / :::1::':,:49¥
¥ pyroxenesin48
ultramafics
: :
withplag+Timt
gabbr05 ･,f ¥';${Il:=: ｭﾖ/:I¥ :ﾖ
dolerites , /=.:.j.ir:j,:
51 !
.// 48
40
/ // skeletalpyroxenecomposiPions
49 49 inArcheankomatiite5
70
Fig. 5. Photomicrograph of pillow basalt (sam ple 51
from Punta Brava) partly feathery and skeletal clinopyroxene microlites with few m agnesiochromites(black).
Fine grained plagioclase laths transform ed to albite,
minor olivine m icrophenocry sts replaced by chlorite.
In addition chloritefilled vescicles.
06 - _ En
50
40
30
30
Fig. 6. J)/rox ene com positions from the G orgona rocks
within portion of the system M gSi03 - FeSi0 3 - CaSi0 3.
A ll F e is expressed as Fe2+. Close stippled field.' com positionalvariation within ultram afic cum ulates, gabbros,
dolerites and basalts (e.g. N o. 46, wehrlite). N os. 69 and
51 represent extrem e clinopyroxene com positions in
picrite basalt and pillow basalt, respectively. W ide
stippled field.' skeletal pyroxenes in the kom atiite fl ow
48 and 49 and path of iron enrichm ent of intergranular
pyroxene. Stars.' sam e pyroxenes, but Tscherm aksm olecule C Al2Si02 subtracted.
F or com partson
skeletal pyroxene com position (enclosed field) in
A rchean kom atiites (CAM ER ON et al., 19 79).
Palaeogene kom atiites
stage quenched clinopyroxenesin the kom atiites.
The picrite basalt contains the m ost M g-ri ch
clinopyroxene m icrophenocry sts (En49:7 and
10w Ti0 2 Of 0.26w t.% ). Thus these m inerals
along with olivine (F091)representnearliquidus
phases.
Spinels
Spinel analyses are listed in T able 4.
The m agnesiochrom ites of the kom atiitic fl ow
occur as tiny skeletal (pseudo-euhedral) grains
often assem bled in patches within or close to the
large quenched olivine plates. Although only
tw o m agnesiochrom ites with relatively high M gO
(m ax. 16.7 wt.% ) and Cr203 (m ax. 46.4 wt.% )
but low FeO values are given,their com positional
vari ations in the centre (N O 48) and m argin (N o
49) of the kom atiitic fl ow are show n in Fig. 7.
Statistically, the rim s of the m agnesiochrom ites
are higher in Cr. A11 G orgona spinels have low er
Cr and Fe2+ values than those in the A rchean
kom atiites, but they are very sim ilar to the
spinels from aphyric olivine tholeiites of the
M id-Atlantic ridge.
In contrast to the quenched m agnesiochrom ites titanom agnetite is dissem inated as tiny
euhedral grains in the arborescent groundm ass
of plagioclase and Fe-rich clinopyroxene (Fig. 4).
Table 3 also show s s lected spinel analysis of
w ehrlite (N O 46), pillow-basalt (N O 5 1) and
picrite basalt (N o 69).In coincidence with olivine
F091 the spinels in the picrite basalt have Cr20 3
values of up to 47 wt.% , w hereas iron-rich
m agnesiochrom ites (Fig. 7) are present in the
cum ulate rocks (e.g. olivine gabbro N o 59: Cr20 3
= 34 to 42 wt.% and FeO totat = 15 to 33 wt.% ).
In the kom atiites, plagioclase
Plagio clase
form s the interstitial arborescent m aterial
together with iron-rich clinopyroxene and
titanom agnetite. The com positional range is
from A n81-75 in the cores and decreasing to
A n60 in the rim s of few zoned laths.
High - calcic plagioclase has been found only
in the cum ulate rocks (N O 59: A n88), picrite
basalt (A n80-78), and in the w ehrlite (A n80-78)'
In the pyroxene gabbros and doleri.tes,plagioclase
proves to be m ore sodic (gabbro N o 60: A n65-60,
151
.8
A A AA
<>
<> <' <>
｢｢
.7
.6
A A A A Belingw e
*
U¥
U*
G orgona
'. 49
48^^^.:::A':::
*
396.==:=:::;
kom atiites
M unro
. .69
334
=:=:::;::;:: ::=:=:=:;:::::[l
:j:=;::::1::=:::::.1
5 :::=:**
._
¥ 46
59 -
e
.5
332
.4
.3
.8
.6
.3
'7Mg/Mg+Fe2+ '5
Fig. 7. Chrom e spinel com positions expressed in term s
of Cr/(Cr+Al) and M g/(M g+Fe2+) for the Gorgona rocks:
filled triangles 48 and 49 = kom atiites; filled circles =
cum ulates (N o. 46 wehrlite,N o. 59 olivine gabbro,'open
circles = basalts (N o. 69 picrite basalt, N o. 51 pillow
basalt). Fe2+ was calculated assum ing stoichiom etry .
Forcom parison spinelsfrom Archean kom atiites(M unro,
Canada and Belingwe, Rhodesia,' data from CAMERON
et al., 1979) and ocean fl oor basalts from M id-A tlantic
ridge (M AR DSDP sites 332, 334, 395, 396, 411, 413)
and Nazca plate (D SDP Leg 34, site 319). R eferences in
Fig. 10 and SIG UR DSSON and SCHILLIN G (1976),
CLARKE and L O UBA T (19 77), SIG URD SSON (19 77).
gabbro N o 45: A n25-10 and dolerite N o 47:
A n60 -30 )'
B ULK C HEMISTRY
M ajor elem ents (Tables 5 and 7)
The basaltic
rocks are hypersthene-norm ative and contain up
to 20% olivine in the norm ; and the picrite
basalt even extend to 40% norm ative olivine.
T hey overlap in com position with the basalts
analysed by ECHEVERRIA (1980). T he doleri tes
(e.g. N o 47), w hich together w ith the basaltic
rocks fill the field of m id-ocean ridge basalts
are quartz-norm ative (T HOMrs oN et al., 1972) in
the quartz-diopside-hypersthene-olivine-nepheline no rm ative diagram
The kom atiitic rocks plot outside this basal tic
field, yielding higher norm ative olivine values of
27% (Table 7). O utstanding chem ical features
of these rocks are high M gO (1 2 and 16.6 wt.%
resp.; atom ic M g/M g+Fe2': 0.72 and 0.74), Iow
Ti0 2 (0.75 and 0.69 w t.% ), CaO IAl203 ratios
152
V. J. DIETRICH et al.
Table 7. Chemistry ofthe Gorgona komatiites and of a fractionalcry stallisation m odel
K om atiite flo w (Pu nta Trinidad)
Sam ple N o.
48 (centre)
M afor oxides,
A v. of 4
C alcd.
w t. %
determ inatio ns anhy drous
Si0 2
TiO2
Al20 3
Fe2 03
FeO
MnO
M gO
CaO
Na20
K2 0
P20 5
Cr20 3
NiO
H2 0+
Total
44.2
0.66
12.0
3.2
8.2
0.18
15.9
10.1
1.13
0.02
0.06
0.19
0.09
4.0
100.0
M g/(Mg+Fe2+)
xMO1g
46.2
0.69
12.6
1.3
10.3
0.19
16.6
10.5
1.18
0.0 2
0.06
0.20
0.09
100.0
O.74
92.0
48,A verages of
interstitial
quenched glass
49.2
0.78
11.6
n.d.
10.22 *
0.22
9.8
16.7
1.22
<.O1
n.d.
0.1 1
n.d.
47.6
0.54
12.7
n.d.
10.60 *
O.30
7.8
16.4
1.10
0.01
n.d.
0.14
n.d.
99.85
97.19
* Total iron
asFeO
4 9 (upper surface)
B asalt 48
A v. of 4
C alod. Fractional crystallisadeterm inations anhy drous tion m odel-25 % O1
44.0
0.72
12.6
4.0
7.6
0.18
14.2
11.5
1.05
0.07
0.06
0.17
0.07
3.7
10 0.0
45.8
0.75
13.1
1.3
10.5
0.19
12.0
12.0
1_09
0.0 7
0.0 6
0.1 8
0.07
100.0
0.72
47.8
0.89
16.22
4.32
8.15
0.24
7.09
13.64
1.53
0.27
0.07 8
10 0.2
0.53
83.8
Normative Mineralogy (CIP W norm , wt.%)
Or
Ab
An
Di
Hy
O1
Mt
Cm
Il
Ap
0.1
10.0
29.0
18.3
11.2
27.7
1.9
0.3
1.3
0.1
0.06
1O.4
26.2
46.3
5.9
8.3
1.4
0.06
9.6
30.6
43.8
7.8
5.6
1.4
1.5
1.1
0.4
9.2
30.7
23.0
6.4
26.6
1.9
0.2
1.4
0.1
1.6
12.9
36.5
25.2
10.4
9.4
l.6
1.7
0.2
Norm sand M g/(Mg+Fe2*) were calculated assuming Fe3++Fe3*) = 0.1.
Basalt 48: calculated com position extracting 25 % olivine in 5steps of(X ;g 92, 91, 90, 88, 86.5)
ol
betw een 0.92 and 0.87 and relatively low
alkaline concentrations (total average N a20 +
K20 - I.2 wt.%). T hese values fulfil the chem ical
characteristics of A rchean kom atiitic basalts
defined duri ng the Penrose Conference on
kom atiites (A RNDT and BRooKS, 1 980). Excep tional are the M gO contents w hich are below the
10wer lim its of 1 8wt.% in peridotitic kom atiites,
but sim ilar to those of pyroxenitic com position
(A RNDT et al., 1977; A RTH et al., 1977) and
probably close to basaltic type kom atiites
(N ESBITT et al., 1979) giving CaO IAl20 3 ratios of
0.8- 1.0 and Al2031Ti0 2 ratios of - 18. Few
higher M gO values have been reported by
ECHEVERRIA (1 980) from one kom atiitic fl ow on
G orgona island. T he basaltic and doleritic rocks
have M gO betw een 8.9 and 6.6 wt.% (M g-N o:
0.65 to 0.54), Ti0 2 betw een 0.9 and 1.3 wt.%
and Na20 -3 wt.%.
T he plutonic rocks indicate a rather wide
com positional range, although, petrographically
they consist m ainly of olivine, clinopyroxene
and plagioclase. The wehrlite (N O 46) and the
olivine gabbro (N o 59) represent cum ulate rocks
(M gO : 30.2 and 20.6 wt.% ; Ti02: 0.28 and 0.33
wt.% ) w hereas the gabbros (N o 45 and 60) are
m ore evolved plutonics (M gO: 6.1 and 7.4 w t.% ;
Ti0 2: O.91 and 1.1 w t.% ). Alteration (serpentinisation of olivine) is show n by the high H 20
contents and slightly high Fe20 3 values.
T he bulk com position of the picrite basalt
(N o 69) is significantly low in Si02, Ti02 and
Palaeogene kom atiites
alkaline contents, but high in M gO (22.7wt.% ).
Trace elem ents (T able 5)
The analysed
kom atiitic rocks yield high contents of Ni (560
and 720ppm ) and Cr (1 180 and 1250ppm ).This
is four tim es higher than Ni and C r valuesin the
pillow basalts. R b, Sr, B a, and Sc abundances
are in the sam e ranges asin the A rchean pyroxenite kom atiites from M unro Tow nship (A RTH
et al., 1977). R elative to basaltic and doleritic
rocks Sc and V are slightly depleted. The sam e
is evident for the incom patible elem ents, such
as Y, Zr, Nb, R b and G a. Zn and C u have interm ediate concentrations relative to cum ulate
rocks and basalts.
R are earth elem ents (T able 6)
The chondritenorm alized R EE patterns of the different rock
types (Fig. 8) sh ow sirn ilar characteristics
to each other. All the rocks have low total
R EE contents and show a depletion of the light
rare earth elem ents (LR EE): picrite basalt with
(Ce/Sm )N ratio of 0.187, cum ulate rocks 0.270.32, the kom atiites 0.34, gabbros 0.40- 0.67,
basalts 0.76 and dolerites 0.8. The R EE abundances from other kom atiitic occurrences of
G orgona island (ECHEVERRIA, 1980) are very
sim ilar, though L R EE values ar e som ew hat
153
The R EE pattern of the Archean
higher.
kom atiites overlaps the field of those from
G orgona. In general L R E E contents of the
pyroxenitic kom atiites of M unro T ow nship
(A RTH et al., 1977) are higher, thus the slopes
are fl attened. R elative to the kom atiltes, LR E E
of the G orgona basalts and dolerites are slightly
enriched. In contrast a w eak negative slope of
the heavy rare earth elem ent (H R E E) curv es
is evident in m ost of the rokcs. A gain the
picrite basalt is abnorm al with strong L R E E
depletion and H R E E enrichm ent. W eak Euanom alies are present in the dolerite and basalt.
C OM PARISON B ETW EEN G OR GO N A K O M ATIITES
AN D O CEA N F LO OR B ASA LTS
The sequence of crystallisation in the
kom atiitic fl ow during rapid cooling can be
divided in tw o stages:
- first stage of quenching: olivine (F 091) m agnesiochrom ite + high-AI clinopyroxene;
- second stage of quenching: olivine (F083) Fe-rich clinopyroxene + plagioclase + titanom agnetite - glass
T his sequence is com patible with the characteristic fractional crystallisation sequence
observ ed in m any ocean fl oor tholeiites. The
rem aining interplate glass in the spinifex kom atiites with typical plum ose extinction has a nearly
basaltic com position (T able 7 and Fig. 9).
G orgona Island ,_==:-- _
20
i
:N
A
:
ZC
i
A
ba
,sa
l
.s
l
;
Sim ilar glass com positions have been reported
1 i i
l
:/
/
'
:
:
dol
r
i
te
:
'
from
the other G orgona kom atiites by
/ ?'¥ I ' :i-¥' :'; '::'i J ¥ 'L ''_
47 ;1 1'f) :be
10
E'o'.7
' lt IJ'LiTTﾖTTTlﾖ
ECHEVERRIA (1 980). The glass is enriched in
:
Al
t
:
i
r¥
51 LT_--/ '' ,tj}¥;":
;i A'"/r
/ :JL JL L ALJJJA-_
60
¥A A I TTT7A$ !S
Si02, Ti02 and CaO but depleted in M gO and
J:L*I I l l
uｪ)
:5
'
f
i
A
l20 s･ The latter contents result from the nonl
'
/
ol-gabbro
Ooe
54 '
LV
'
Z
/
t
03
equilibrium
crystallisation of olivine and the
//iA
S ep////:C : / / : wehrlite
:
:E
U
49 J'//:'
unusual high-AI clinopyroxenes (T able 3) during
u) 2
48 A /./ v '
Ue
the first stage of quenching at tem peratures
O
o(
betw een 1400 and 1 500'C. This explains the
46 // '
1
59 '
shift of the average glass com position tow ards
e9 /
the pyroxene corner in the PL-PX-O L norm ative
La Ce Pr N d S m Eu G d Tb Dy H o Er
Y b Lu
diagram (Fig. 9).
Fig. 8. Chondrite normalised (N AKAM URA, 1974)rare
In order to estim ate an equilibrium crystalearth patterns of Gorgona kom atiites, cum ulate rocks, lisation, w hich could produce a basaltic liquid
gabbros, dolerites and basalts. Hatched field.' range of
reported values for Nazca plate basalts (D SD P Leg 34, from the pyroxenitic kom atiitic com position at
greater depth in a m agm a cham ber, a fractional
sites319A, 320B,321, THOMPSON etal.,1976).
V. J. DIETRICH et al.
154
60
I Spinifexrock5 GORGONA
pX
Pl-TholeiitesGORGONA
NAZCA (32T).
O
Coastal CordiileTa
A
/
O
'
GORGONA
50
10 i ':,. Oi-Tholeiites
P1F
' NAZCA (319)
PL
OL
f(/
7,l,'
(;,: Ii
hI
fract crY5tolli5ation
o:F '; :;::.OLIVINE
model fro m
L
r
'
:
:
;
,
:
･
:
:
THOLE
l
I
T
E
O
q 40 '
20 rl!L
O interplate
cli,, ,,,:;:: *
1
4
gla55in "
､
:
(
!
"
t
el
V ',..
L..¥
/ ! L"'
T"1'""
1'
(
"(
l
r
g5:O
¥O¥¥! L"".,'.;;,･
¥
b/ ( ¥
O ; ".::':･:,
30 q,
:Y'r:o / ,1:"::
30
/
infe rred ' L MAR
cotectic
2/
Iaphyric
40
basat5
332 395 396
411 413
80
70
60
50
40
-- NO MAT VE PLAC OCLASE
Fig. 9. N orm ative olivine-plagioclase-pyroxene relationships betw een the G orgona spinifex rocks and
aphy ric M id-A tlantic R idge basalts (L eg 37, site 332,
B LA N CH AR D et al., 19 76,･ L egs 45 and 46, sites 395 and
396, D UN GAN et al., 19 78 and R HO DES et al., 19 78, Leg
49, sites 411 and 413, W O OD et al., 19 79) and Tertiary
basalts from the Pacific (N azca plate L eg 34, site 321,
M A ZZULL O and BEN CE, 1976 and R H ODES et al., 1976,
Coastal Cordillera, G O OSSEN S et al., 19 77). The em pirically determ ined olivine-plagioclase cotectic line is from
SHIDO et al. (1971). The slope of the olivine control
line has been calculated extracting I O% of forsterite
from the G orgona spinifex rocks.
crystallisation m odel has been calculated. A stepwise extraction of a total am ount of 25% olivine
(K D value of about 0.3) yields a m id-ocean ridge
(M O R B) olivine basalt (Table 7, Fig. 9). T he
latter com position is slightly different from the
G orgona basaltic rocks with higher values of
CaO (- 13.6 wt.%) and A1203 (16.2 w t.% ) but
10w er values of N a20 and K20. A n additional
fractionation of 0.5% m agnesiochro.m ite as indicated by the Cr- and N i-distribution betw een
the kom atiites and basalts (Fig. 11) w ould increase Si0 2 as w ell as decrease A 120 3, M gO and
iron by a rather sm all portion. In order to reduce
the CaO and A l20 3 contents, total am ounts of
ca. 5% of both clinopyroxene (- E n45 Fs9 W 046)
and anorthite m ight be incorporated in the fractional crystallisation m odel. The olivine fractionation is show n by the direction of the control line in the norm ative pyroxene-plagioclaseolivine diagram (Fig. 9). T he arrow points to
olivine tholeiites and subsequently to m ore
evolved plagioclase tholeiltes close to theinferr ed
cotectic (SHIDO et al., 1971). B asalts from
G orgona as w ell as frorn the C oastal Cordillera
and N azca plate (D SD P Leg 34, sites 319A,
320B and 321) plot in the sam e area. For
com parison aphyric olivine tholeiltes from the
M id-Atlantic ridge (M A R) are also show n. Typical olivine tholeiites can therefore be derived
from pyroxenitic kom atiitic liquids by fractionation of 20 to 25% olivine and ca. 0.5% m agnesiochrom ite; plagioclase tholeiites by additional
fractionation of olivine, clinopyroxene and
anorthite.
Figures 10 an 11 dem onstrate the relationship betw een the G orgona kom atiites and
aphyric ocean fl oor basalts based on H arkertype diagram : M g/M g+Fe2+ (with Fe203/FeO
=0.1) versus critical m ajor and trace elem ents.
In the Si0 2 and CaO plots the olivine control
lines do not point exactly to the norm al basaltic
fields forthe reason discussed above.
The low CaO and A120 3 values of the evolved
G orgona dolerites (N o 47) m ay be explained by
an earlier accum ulation of clinopyroxene and
plagioclase. The picrite basalt (N o 69)seem s to
represent an early cum ulate carry in g Fo-rich
olivine, m agnesiochrom ite and M g-rich clinopyroxene.
In the trace elem ent vari ation diagram s
Fig. Il), the incom patible elem ents Zr and Y
show linear correlations betw een the kom atiites
and basalts, indicating norm al differentiation
trends. W ith respect to prim itive glasses and
rocks from oceanic environm ents, Cr and Ni
contents of the G orgona kom atiites are higher.
T hus a fractionation of m ax. 0.5% m agnesiochrom ite from the kom atiitic liquid is necessary
to produce the average Cr contents of 200- 250
ppm in tholeiitic basalts. Norm al M O R B affinity (N-type after W OOD et al., 1979) of the
G orgona kom atiitesis also indicated bythe abundances (Table 6) and ratios of H f, Th and T a:
H f/Th ratio = 20,La/T a = 16.5 and Th/T a = I.14.
T he lr content of 3.3 ppb is surprisingly high
and in the range of ultram afic m antle tectonites
(JAGOUTZ et al., 1979 and P. H AMLYN, Lam ont
pers. com m.). O cean fl oor tholeiites norm ally
contain only up to 0.1 ppb lr (G OTTFRIED and
G REENLAND, 1972;H ERTOGEN etal., 1980). This
Palaeogene kom atiites
14
5･
O
u
12
N
- - - T=
- OBT8 1
T .6 --48x
Cc]rribean .7
3
1
9
1
46
1
5
0
F
F
R
M AR
'L H
332 I ::::r
395
olcontrolline
:: , 527'-? ,49
l It+51 525_i."'411_1
8
155
1
'
CR
IR
ELBB
69
l
1
411-1
OB
tREt GR
*
4B
BB
.8
o contro ine
ABB
'OR
+69
^DB
46
44
L
4
411.1
^lR .GR
*EL
olcontrolline ¥ ,(4* 4e ^BB 69
+
Gorgona
.5
.6
Mg/(Mg+Fe+2)
.7
.8
4
5
.6
Mg/(Mg'Fe+2)
.7
.8
Fig 10 and 11. Com positions of Gorgona kom atiites (filled stars), fractional crystallisation m odel (open stars)
an basalts (crosses) relative to jresh aphyric ocean fl oor from M id-A tlantic ridge (M A R) and Nazca plate, few
primitive basaltic glass and rock com positions and hypothetical prim ary magm a com positions. A bscissa, atomic
proportions assum ing Fe203/FeO = O.1.
R eferences.'
BB = Baffin Bay tholeiite (O IONS and CLARKE, 1972)
DB = A verage com p. of m elt inclusions in Cr-AI spinel, D SDP Leg 37, sites 332, 335 (D ONALDSON and BRowN,
.
1974)
EL = High magnesia dyke N T23, Tortuga ophiolite com plex, Chile (ELTHON, 1979)
FF = DSDP 3-18glass (FREY et al., 1974)
GR = DSDP 3-18 + 1 7% olivine (GREEN et al. 1978)
IR = Calculated abyssaltholeiite (IR VINE, 1977)
R H = Reconstru cted composition of meltinclusion JSC 45G.I in olivine, DSDP Leg 45 and 46,site 395A
(R HODES etal., 1978,D UN GAN and RHODES, 1978)
N azca plate basalts: DSDP Leg 34,sites 319-321.
(K EMPE, 1976,M AZZULLO and BEN CE, 1976,R HODES et al., 1976 and THOMPSON et al., 1976)
M id-A tlantic ri dge basalts.'
JSC91: ol-basalt,D SDP Leg 37,site 332B (BLANCHARD et al., 1978)
411-1.･ DSDP Leg 49,site 411 (W OOD etal., 1979)
413-1: DSDP Leg 49,site 413 (W OOD etal., 1979)
525-5-1,･ Basalt glass,Fam ous area (BR YAN and M OoRE, 1977)
Sites 332 (A +B),' D SD P Leg 37 (FLOWER etal., 1977a, b)
Sites 395 and 396: DSDP Leg 45 and 46 (D UN GAN et al., 1978,･RHODES etal., 1978)
Basaltsfrom the CentralCaribbean:
Sites 146 and 150: D SDP Leg 15 (BENCE etal., 1975)
156
V. J. DIETRIC H et al.
suggests that lr has rem ai ned in the residual
ultram afics after separation of the basaltic m elts.
A problem seem s to exist in fractionating olivine
tholeiites from kom atiitic prim itive m elts in
respect to their depleted L R EE abundances.
A sim ple fractionation m odel of olivine and
m agnesiochrom ite, as it has been discussed on
the basis of m ineral chem istry, m ajor and trace
elem ents w ould only account for a slight increase of the LR EE contents (R E E contents in
olivine taken from STOSCH and SECK, 1980) but
not for the derivation of the norm al L R EE
patterns in olivine tholeiites.
M agm a mixing in ch am bers beneath midocean ridges has been discussed on the basis of
different com position betw een entrapped m elt
inclusions in olivine and plagioclase and the bulk
rock com positions as w ell as by disequilibrium
textures in plagioclase phenocry sts from M A R
basalts (D UNGAN and R HODES, 1 978). A m ixing
process betw een a pre-existing, LR EE enri ched
m elt and the prim itive kom atiitic liquid could
easily increase the m obile light rare earth elem ent contents of the m elt. T hus first stage
fractionated olivine tholeiitic m elts w ith slight
depletion of L R E E are getting progressively
enriched in the light rare earth elem ent abundances. The dolerites w ould require additional
fractionation of clinopyroxene and plagioclase
in order to yield the higher degree of LR E E
enrichm ent and the w eak negative Eu-anom aly.
Sim ilarities betw een basaltic ro cks from
Gorgona, the Coastal Cordillera and N azca
plate
Clinopyroxene com position from
G orgona basalts resem ble those of N azca plate
basalts (BUNCH and LA B ORDE, 1976), but they
are significantly different from clinopyroxene
from island arc basalts and from tholeiitic
basalts in oceanic islands (N ISBET and PEARCE,
l977). Phenocrysts and greater m odel am ounts
of augite are typical for evolved Pacific basalts
w hereas M A R basalts are characterised by phenocrysts of plagioclase and olivine (C AM ERON et al.,
19 80).
The norm ative schem e (Fig. 9) as w ell as the
variation diagram s (Figs. IO and 11) reveal
general chem ical sim ilarities of the basaltic rocks
from the different tectonic environm ents. A
closer look show s that a m ore detailed com parison betw een the G orgona rocks and basalts from
the ophiolite suites in the Coastal C ordillera is
rather difficult because of lack of data. G oosENS
et al. (1977) have described low potassic ocean
fl oor tholeiites sim ilar to those from G orgona
and highly evolved tholeiites as w ell as basaltic
andesites from island arc environm ents.
Palaeogene basalts from N azca plate have
been drilled in D SD P Leg 34, sites 319A, 320B
and 321. W hile the Oligocene olivine basalts of
site 3 1 9A are rather prim itive (M g/M g+Fe2+.
0.63- 0.53; Ti0 2: 1.1- 2.l w t.% , Fig. 10) the
Eocene basalts from site 321 are m ore evolved
plagioclase-pyroxene tholeiites (M g/M g+Fe2+.
0.52- 0.43; Ti0 2: 2.3- 2.7 w t.% ). Petrogenetic
relationships betw een ocean fl oor basalts from
different tectonic environm ents can be deducted
by the ratios of Ce/Zr, N b/Zr and Ba/Zr(TARNEY
et al., 1980). In this respect the G orgona
kom atiites, basalts, dolerites and gabbros are
very sim ilar to the N azca tholeiites of Leg 34,
site 31 9 but different from the M id-A tlantic
ridge basalts. This applies also to the R E E
abundances (Fig. 8).
To explain the different types of basalts in
the N azca plate, particularly those of site 321
with high Ti0 2, high LIL elem ent and R E E
abundances, m ore com plex processes have been
discussed. A m ixing process of depleted asthenosphere with a juvenile m antle plum e accom panied by shallow-depth olivine gabbro fractionation has been proposed by SCHILLlNG et al.
(1 976).
K o m atiites as products of prim ary m elt
The
discussion of the m antle source origin of the
kom atiitic liquids can be based on large-ionlithophile elem ents such as K, R b, Cs, Sr, Ba
(W HITE and SCHILLlNG , 1 978), on R EE abundances and on Sr, N d, and Pb isotope data.
O ceanic m antle origin is indicated by low K , Rb,
Sr, Ba and total R EE contents (5- 8 tim es
chondrites) as well as by L R EE depletion show n
by the ratio (Sm /N d)N> = 1.7- 1.9. T he Y b
Pal aeogene kom atiites
content of the kom atiites (- 15 ppm ) in the sense
of an indication for equiblibrium partial m elting
of a source peridotite (O'Nro NS et al., 1978)
requires approxim ately 1 8% partial m elting of a
lherzolite to produce the kom atiitic liquid. This
is the sam e value necessary to deri ve prim itive
tholeiites from Baffin Bay (com position show n
as BB in Figs. 10 and I l). G REEN et al. (1978)
inferred from olivine - addition studies to a
prim itive M A R glass (D SD P3 -19, corn position
FF in Figs. 10 and 1 1) that a picritic m agm a
(G R) close to the kom atiitic com position has
olivine and orthopyroxene as liquidus phases at
- 20 kbar and 1430'C, thus seggregated from
residual harzburgite.
Further evidence for an oceanic m antle
origin for the kom atiites has been supported by
87Sr/86Sr (0.7028- 0.7035) and 143N d/144N d
(0.5129- 0.5133) ratios (ECHEVERRIA, 1980).
T he tw o ratios give a negative oceanic correlation trend (O'Nro Ns et al., 1977). T he variation
of the ratios has been interpreted as due to
heterogeneous source regions in the m antle.
So far the available m ineralogical and chem ical evidence points to an explanation for the
G orgona kom atiitic fl ow s as being quenched
products of a very prim itive or prim ary m elt.
In addition the kom atiites plot w ellbetw een the
distribution fields of aphyric ocean fl oor tholeiites, prim itive glasses, entrapped m elt inclusions
and calculated prim ary m agm as (Figs. I O and
11). The basalts, dolerites and gabbros as w ell
as the residual cum ulate rocks of G orgona island
can therefore be derived from the initial kom atiitic m elt by fractionation and m ixing processes.
A rgum ents against sim ple fractionation
processes, deri ving the A rchean tholeiitic basalts
from prim ary kom atiitic m elts have been brought
up on the basis of trace elem ent contents and of
the m ism atch ofthe LR E E pattern. A RTH et al.
(1977) and N ESBITT et al. (1979) propose m ultistage m elting m odels of peridotitic m antle at
different depth producing prim itive tholeiites
apart from peridotitic kom atiites, the latter fractionating to pyroxenitic and basaltic kom atiites.
Gorgona island, an uplifted block of im m ature
157
magm a cham bers
T he reason for the existence of the kom atiitic fl ow s on G orgona island
is difficult to explain. Sim ilar occurrences have
never been reported from any Phanerozoic
ophiolite terrain or ocean fl oor. A structural
and com positional setting corn parable to that of
G orgona island have not been found: Sm all
kom atiitic lava fl ow s interlayered within a
basaltic and doleritic com plex along the m argins
of the island. In addition picritic rocks exist
on G orgona island and the num erous ultram afic
and m afic cum ulates outcrop in the central parts.
Intercalated oceanic sedim ents are m issing except for U pper E ocene arenaceous lim estones
from the southern tip of G orgonilla island.
M ineralogi cally and chem ically all G orgona
plutonics and basaltic volcanics are sim ilar to
O ligocene basalts from N azca plate oceanic crust.
U nfortunately, the structure and age of the
Pacific oceanic crust off Colom bia is rather
poorly know n (see Fig. I). Bathym etric contours show a rather wide shelf and the shallow
Colom bia trench. T his indicates that the topographic relief has been reduced over a long tim e
span. Further w est betw een Coiba, M alpelo and
the m ore southern Carnegie ridge m agnetic
anom alies m ark a spreading history of 25
to 8 m illion years (LONSDALE and K LITGARD,
1978). The age ofthe N azca plate crust betw een
Y aguina graben and C olom bia trench therefore
could be Eocene or Oligocene. T hisis consistent
w ith the oldest sedim ents that outcrop on
G orgona island, today incorporated in the
C olom bian shelf. T he Colom bia trench at least
m arks the trace of an earlier destructive continental m argin, along w hich the N azca plate has
been subducted since Early Tertiary or even
U pper Cretaceous.
A n island arc environm ent for the G orgona
island could therefore be envisaged, how ever,
neither structural nor geochem ical evidence has
been found to substantiate this hypothesis.
T he entire islands are m ade up of oceanic
tholeiites. Boninites, island arc tholeiites or
relicts of calc-alkaline rocks such as andesites
and dacites, com m on in the Pacific island arcs,
have not been found.
V. J. DIETRICH et al.
158
6
km
2
2
GORGO NA mode:
I komatiiticliquid
¥
¥
km
2
highleYel chamber
l*te5t 9e fra'tio"atio"
? mi*i g
!V
'1--i
i pBlad9s-alt'tioGLr!b*o }
4 ; :;i:;1J_;i " '=:._:
* * +-*.. -, }H2(
6
;ji.･ ::_ji. .j･1
:"i:::
6
km
sec
pjllQｪ
Fo"s
D*1**it'*
f.. " "
3.765
5263
___'_
:;,: 'i::'i }: : ::i:.
.==;;:;:' =i::i
6472
:::::G*bb'os::::: 697.2
:::i:iP it*s=:i:;::::
8
10
-,--4--
rl -
.,.. . ' ',
hL{. _･
-=:H:.ｭ(':/if
l
i'fr'ｪ30t015km l
_.: RESIDUAL 8j
MANTLE
HarzbU'git*
Fig. 12. Diagram m atic m odel of a central part of midocean ridge crust in order to explain the form ation of
small kom atiitic lava fl ows on Gorgona island. Intense
vertical and horizontal m ovem ents m ay have occurred
duri ng Eocene tim e. Th is process could have released
enclosed m elt from middle and high stage m agm a
cham bers and generated rapid ascend.
A sim ple m odel (Fig. 12) is proposed in
order to explain the unusual stru ctural setting of
m afic and ultram afic rocks intercalated with
m inor kom atiitic flow s: G orgona island itself
m ay represent an uplifted block of im m ature
m agm a cham bers within the oceanic crust of the
N azca plate. The process of the initial uplift
probably during Eocene from at least 2 -4 km
depths to the ocean fl oor is not yet understood.
B ut it w ould account for a rapid ascent along
faults and cooling of undifferentiated kom atiitic
liquids at high tem peratures betw een 1300 and
1500'C, for the form ation of picrite basalts and
for the existence of cum ulate ultram afics in the
central part of the island, M elting experim ents
on the G orgona kom atiites under pressures of
4, 6 and 8 kbar indicate crystallisation tem peratures of 1300 to 1400'C (DIETRICH et al. in
prep.). Intensive block faulting, as indicated by
earlier surv eilance (G ANSSER , 1 950; ECHEVERRIA
and PARrs , 1978 in ECHEVERRIA, 1980) w ould
confirm this m odel. G orgona island is com patible in term s of width and length with the sm all
dim ensions of the central part of the East Pacific
Rise (H ERRON et al., 1980) as w ell as with the
"infinite leek" m odel of the M id-A tlantic Ridge
in the F A M O U S area (NISBET and FOWLER,
1978). The latter m odel w ould also explain
m ore easily the m ineralogical and chem ical
variations of the tholeiitic basalts and dolerites
found on G orgona island. They m ay be interpreted as results of different degrees of fractionation within sm all sized and periodically refilled
m agm a cham bers (O'HARA, 1977) at high levels.
A cknowledgem ents- The X R F analyses have been m ade
with the assistance of A . ESENW EIN from theEidgen6sische M aterialpruf ungsanstalt (E M PA), D ub endorf. B. W .
C H APPEL (A ustralian N ational U niversity, Canberra)
and H. PALM E (M ax-Plan ck-Institut, M ainz) provided
additional trace elem ent data for th e kom atiites.
N eutron activation anal yses of the rare earth elem ents
w ere carried out by S. BAJO an d A. W Y TTENBACH at
the Eidgen6ssisches Institut fur R eak torforschung (EIR ),
Wh renlingen. S. R. TA YLO R (A ustralian N ational U niversity, C anberra) kindly supplied sp ar k source m ass
spectrom etry an al yses for the k om atiites.
W e th an k E. G. N ISBET (Cam bridge) for helpful
criticism and B. G UNTER and P. K OO NS (Zurich) for a
thorough review w hich greatly im proved the m anuscript.
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Chem istry of leg 4 5 basalts. In: Init. R ep. D eep Sea
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R H ODES, J. M ., B LAN CHA R D, D. P., R O DG ERS, K. V.,
JA COBS, J. W . and BR AN NO N, J. C. (1976) Petrology and chem istry of basalts from the N azca plate:
Part 2 - m ajor and trace elem ent chem istry. In: Init.
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R OEDER, P. L., C AM PELL, I. H. an d JAM IESON, H. E.
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SCHILLlNG, J. -G., A N DERSON, R. N. an d V OG T, P.
(1976) R are earth, Fe and Ti variations along the
G al apagos spreading centre, and their relationship to
the G alapagos m antle plum e. N ature 261, 108- 113.
SHIDO, F. A., M IY ASHIR O, A. and EWlNG, M . (1971)
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77-97.
FIGU RE FOR REPLACEM EN T
For th e paper "Palaeogene kom atiltes from Gorgona
Island, East Pacific - A prim ary m agm a for ocean floor
basalts?" by V.J.Dietrich et al., pleasereplace Fig. 12 on
page 158 ofthis volum e with this figure.
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Fig. 12 Diagram matic m odel of a central part of midocean ridge cru st in order to explain the form ation of
sm all kom atiitic lava fl ows on Gorgona island. Intense
vertical and horizontal m ovem ents m ay have occurred
during Eocene tim e. Th is process could have released
enclosed m elt from middle and high stage m agm a
cham bers and ge/terated rapid ascend.
RESID U A L
M A N TLE
Harzburgite