Canadian Minerulogist
Vol. 17, pp. 871-887 (1979)
SPILITESAND OPHIOTITEMETAMORPHISM
SERPENTINITES,
JUDITH B. MOODY
Departtnent of Geology, University ol
North Carolina, Chapel Hill,
N.C. 27514, U.S,A.
Sotr.rvrenE
ABSTRACT
Ophiolite metamorphism occurs before, during
Le m6tamorphismed'un cortdgeophiolitique peut
pr6c6der, accompagnerou suivre l'obduction sur
or after obduction onto the continental margin.
Models of submarine hydrothermal alteration for
la plateforme continentale. Pour les milieux sousmod€rn ridge-crests have been developed from
marins, les modlles d'alt6ration hydrothermale aux
heat-flow measurements, seismic data and direct
cr6tes oc6aniquessont construits a partir des mesampling of oceanic crust and mantle from DSDP
sures du flux de chaleur, des donn6essismiqueset
drilled and dredged samples. The heat-flow data
d'un 6chantillonage direct de la cro0te et du
require convective heat-trander to explain the low.
manteau (carottes DSDP et 6chantillons dragrr€s).
measured values, tiu$ requiring the development
Le flux calorique observ6implique un transfert de
of a fracture system from thermal contraction of
chaleur par convection, requ6rant un systdme de
the cooling crust and the tensional environment.
fractures i la cr6te, par contrastion de la crotte
The hydrothermal activity will persist if the frac- ' au refroidissementet pa,rtensionstectoniques.L'acture system remains open to accommodate fluid
tivit6 hydrothermale poraisteratant que le systlme
access a$ the orust ages. We do not know the
de fractures reste ouvert aux fluides au cours du
distribution and depth of oceanic fracture systems
vieillissement de la cro0te. On ignore la distribua\pay from the ridge crest and transform faults.
tion des cassuresocdaniques et leur profondeur
Extent of metamorphism of the oceanic crust is
loin de la cr€te et des failles transformantes.L'6strongly dependent on: (1) age, (2) crustal tlicktenduo du m6tamorphismede la crotte oc6anique
ness, (3) permeability of sediments and crust, (4)
d6pend fortement des facteurs suivants: (1) 6ge
fluid-flow parameters, (5) conductive vr. convec- et (2) 6paisseurde la cro0te, (3) permfubilit6 de
la crotte et des s6diments,(4) paramOtresdu flux
tive heat-flow regime, (6) development of major
des fluides, (5) importance relative de la conducfault and fracture syst€ms, and (7) spreading rate.
tion et de la convection, (6) d6veloppementde
Metamorphism of the upper part of a continenta[y
systdmesimportants de failles et de fractures et
emplaced opbiolite could have occurred within the
(7) vitesse de s€paration des plaques. Les suites
oceanic environment. The difficulty with deterophiolitiques que I'on trouve sur les plaques con'
mining an oceanic provenance from the wholetinentales lrcuvent avoir 6t6 mdtamorphis6esl leur
rock basali gBochemistry is that (l) significant
partie sup6rieureen milieu oc6anique,mais il est
geochemical variations occur for oceanic basalts
difficile de le prouver I partir du g6ochimisme
global des roches, vu: (1) la grande variation des
from different tectonic environments, and (2)
6asaltes en fonction du milieu tectonique et (2)
major-element chemistry can be significantly afla
mobilit6 des 6l6mentsmajeurs pendant le m6tafected by metrmo4rhism. Trace-element data have
morphisme. A cet 6gard, les 6l6mentstraces sont
been more successful in demonstrating the oceanic
plus utiles pour 6tablir le caractOreoc6aniquedes
character of ophiolitic basalts. Seawater involvebasaltes ophiolitiques. k rdle de l'eau. de mer
ment in their metamorphism has been demonstrated
dans leur m6tamorlrhisme est d6montr6 par un
by Li- and Na-enrichment pattems in the metaeorichissementen Li et Na et par les isotopesstables
basalts and stable isotopic measurements on indes roches intrusives et extrusives. L'enrichistrusive and extrusive mafic rocks. The presence sementeq S dans les dykes et les coul6esmafiques
of sulfide mineralization in dykes and mafic volI Troodos montre que les systdmeshydrothermaux
oc6aniquespeuvent donner des gisements-de sul'
canic rocks at Troodos argues for the ability of
Ir m6tarnorphisme des niveaux ultramafifures.
produce
sulfide
system
to
hydrothermal
an oceanic
ques suit probablement fobduction; les moddles
deposits. Metamorphism of the ophiolitic ultramafie
de flux de ihaleur ne pr6voient pas de p6n6tration
layer probably occurs after obduction; heat-flow
de l'eau jusqu'au mant€au' Dans la plupart- des
models do not predict penetration of water to
cas, le m6tamorphisme disparait dans le niveau
oceanic mantle, and in most ophiolite sequences, gabbroique. Pour comprendre les 6v6nementstecmetamorphism d.tes out in the gdbbro layer. ThereioniqueJ de fhistoire d'une ophiolite, il faut con'
forp, unraveling the metamorphic history of an
naltre son 6volution m6tamorphique.
opbiolite is critical to understanding its tectonic
(Iraduit par la R6daction)
evolution.
871
THE
CANADIAN
IxtnonucrroN
Ophiolites are stratigraphically sonnected sequencesof igneous and sedimentaryrocks (Conference Participants 1972) that are usually considered to be part of the oceanic crust (Fig. 1).
These rocks are partly to completely metamorphosed (Table 1) and have been tectonically
emplaced onto continental crust.
The purpose of this paper is to examine the
.metamorphism of individual
ophiolite sequences to ascertain the relative timing of
metamorphism and to establish criteria for recognizing the metamorphic effects on the
primary igneous stratigraphy. In the process of
doing this, various models of the oceanic crust
will be examined to determine whether metamorphism of the oceanic crust could occur in
sl'/a prior to tectonic emplacement, or whether
metamorphism of the obducted oceanic slab
occurs during or af,ter tectonic transport. This
work will build on the discussion of ophiolite
metamorphism summarizedby Coleman (t977).
OcsANrc HvonorgeRMlr,
MooBr,s
The degree of metamorphism and alteration
of oceanic crust has been a topic of considerable recent interest (Francis 1976, Muehlenbachs & Clayton 1976, Stern et aI. 1976, Cann
1978, Cann & Moore 1978). Approaches have
included: (l) experimental work on basalt-seawater interaction (Hajash 1975, 1977, Mottl
1976, Bischoff & Soyfried 1978, Mottl & Holland 1978) and on peridotite-seawater interaction (Seyfried & Dibble 1978), (2) petrographic studies of oceanic metaigneous rocks
(Melson & van Andel 1966, Aumento & Loubat
1971, Aumento et al. 1971, Miyashiro et c/.
1971, Bonatti et al. t975, Bonatti & Honnorez
1,976,Prinz et al. 1976, Scott & Hajash 1976,
Humphris & Thompson 1978a, b), and (3)
models of circulbtion patterns of fluids within
the oceanic crust (Ribando et al. 1976, Rosendahl 1976, Wolery & Sleep L976, Lister 1977,
1978, Fehu 1978, Sleep & Rosendahl 1979).
Evidence for the presence of hydrothermal
circulation in the oceanic environment is derived
from: (l) the presence of metalliferous sediments at oceanis ridges (Bostrom et aI. 197I,
Dymond et al. 1913, Piper 1973, Francheteau
et al. L979) and in the Red.Sea brines (Bischoff
1969, Shanks & Bischoff 1977), (2) heat-flow
measruements at ridge crests and flanks (Williams e/ al. 1974, Wolery & Sleep 1976 Anderson & Langseth 1977, Herman et al, 1977,
Anderson et al. 1979), (3) decay in intensity
MINERALOGIST
of magnetic remanence away from a ridge axis
(Irving L97O), (4) microearthquake swarms at
mid-ocean ridges (Sykes 1971), (5) presence
of an active hydrothermal system such as at
the Galapagos rift (Corliss et aX. 1977, L978,
Lonsdale L977, von Herzen et aI. 1977) and
TAG hydrothermal field (Lowell & Rona 1976).
The sediment mounds associated with the
Galapagos hydrothermal $ystem are inferred
to have for.med from slow percolation of hydrothermal fluids circulating through the basaltic
crust and overlying sedimenb (Corliss ef al.
1978), though direct vertical connection between
tho mounds and the hydrothermal vents has
not been shown to occur (Natland et al.. t979).
Hydrothermally circulating seawater also has
been directly observed from the subaerial geothermal fields at Reykjanes, Iceland (Bjdrnsson
et al. t972, Mottl e/ al. 1975). Under the assumption that ophiolites are ancient oceanic
crust, arguments have'been put forth to use
the occurrence of massive sulfides within the
ophiolite (Par,mentier & Spooner 1978, Rona
1973) dnd whole-rock isotope geochemistry to
infer the presence of a former hydrothermal
circulation system (Spooner L978), ostensibly
while at or near an oceanic ridge.
Heat-flow mod.els
In his heat-flow model for oceanic ridges'
Sleep (1975) postulated that vertical heat-conduction from the ridge axis is a function of
spreading rate. Cooling due to conduction (heat
generated from magma accreting at ridge axis)
is more rapid near a iidge axis with slow
spreading-rates besause the heat is conducted
almesl vertically. Ilowever, heat-flow measurements indicate that conduction is not the predominant mode of heat transfer in young
oceanic crust.
Vertical heat-transfer at the ridge can be
acceleratedby hydrothermal circulation syslems,
as they add a connective term to the total
heat-dissipation. Williams et aI. (1974) and
Wolery & Sleep (1976) postulated that hydro'
thermal circulation is responsible for the
anomalously low heat-flow.observed at ridges'
Wolery & Sleep (t976) estimated the rate of
hydrothermal heat advection by computing the
difference between a theoretical heat-production
associated with sea-floor spreading and the observed heat-flow. This calculation leads to the
extremely high calculated flow-rate of seawater
through the oceanic hydrothermal system of
1.3 - 9.0 x 107 g/y. Wolery & Sleep (L976)
postulated that hydrothermal activity would
SBRPENTINITES,
SPILITES
AND OPHIOLITE
METAMORPHISM
873
BAY O F I S L A N D S
TROODOS
Tertiorysediments
LoyerI
Sediments
UpperPillowLovos
Loyer 2
PillowLovos
LowerPillowLovos
ffi
SheetedDikes
Sheetedlntrusive
Compler
l-oyer3
Gobbros
Pluionic
Compler
Gronophyres
ond Gobbroa
Ullromofic
cumuloles
Seismic
Moho
PehologicolMoho
Horzburglie
(depleted
montle)
ffi
Anorthosife, Troclolife
Dunite
Feldspothic
te Chromiie
Clinopyroxeni
Florzburgite
Mantle
ffi
FIG. l. Stratigraphy of two well-described ophiolite sequenc€scompared to oceanic layer l, 2,3 and 4
(mantle).
for Galapagos spreading-centre,10-15 m.y. for
the East-Pacific rise, 5G-70 m.y. on the Midlbt@rphlc
Equl valent
Uretamrphosed R@ks
Atlantio ridge). This transition is dominated
chert
mrlne sedirents
oceanlc layer'l
by the development of a certain sediment-thickspil'lte (keratophyres)*
basalt
oceanlc layer 2
gabbmr
mtagabbrc or amphlbolite
Neanlc la.!€f 3
nlss (2 300-,m) and a change in composition
serpentlnlte (rodlngltes)s
rcck
ultramfic
mntle
from carbonate to siliceous deposition, which
(harzburglt€'
dunlte)
'sheoted
effectively decreases the bulk permeability of
gabbrolc
present
bet{€n the lntruslve
dytes my or my not be
layer and the extrirslve basalt layer (Flg. l).
tho sediment cover. AIso, hydrothermal flow
*
comnly assoclatedmcks prcduceddurlng mtarcrphisn.
decreaseswith time owing to the clogging of
ceaseafter 15-20 m.y. for fast-spreadingridges fracture systems within the crust by alteration
and,2L-26 m.y. for slow-spreadingridges. These minerals (Sleep & Wolery 1978). Therefore,
time periods are derived from the convergence other physical variables, such as permeability
of theoretical and observed heat-flow curves; and flow parameters, are also strongly a function of age and spreading rate. By means of
Wolery & Sleep further postulated that after
that interval, heat flow should occur purely by very closely spacedheat-flow surveys,Ande-rson
conduction. flowever, Anderson & Langseth et al. (19i9) have been able to calculate both
(1977) have shown that the transition from
the conductive and convective components of
predominantly convective to conductive heat- heat flow through oceanic crust and sediments
flow in the oceanic crust is higttly dependent (Indian ocean), further substantiating the imon the age of the crust and the spreading rate portance of convective procesies in the heat(e.g., 40-160m.y. for Indian Ocean, 4-6 m.y. flow regime of young oceanic crust.
TABLE I.
ROCKSOF THE CLASSICOPHIOLITESEqUENCE
874
THE
CANADIAN
Depth ol fluid penetration
Heat-flow models (Lister L972, L974, L977,
Ribando et al. L976) have been postulated in
which the depth pf penetration of water is approximately 5 km, based on the wavelength
of the heat-flow variation observed at the
Galapagos spreading-centre (Williams el cl.
1974r. Permeability alone limits the access of
fluids to about tle upper 2OG-300 m of the
oceanic basement (Wolery & Sleep t976).
Therefore, the permeability of the oceanic crust
and sediments is not sufficient to explain the
heat-flow measruements;most heat-flow models
also call upon a fracture system, developed
througb cooling and static fatigue (Lister 1974),
to explain the anomalously low heat-flow values.
Widespread porous flow has been shown to be
an unlikely mechanism for fluid circulation
(Sleep & Wolery 1978).
DSDP drilling results in the Atlantic oseanic
crust show a general absence of hydrothermal
alteration regardless of crustal age, which led
Hall & Robinson (1979) to conclude that the
MINERALOGIST
hydrothermal sirculation system must occur
at depths greater than 600 m and that it is
probably irregularly distributed. However, in
the Pacific, large fault-sca4ls about 1 km apart
(Williams et al.1974, Rosendahl 1976) dominate
the topography and provide zones of high
permeability. Also, in DSDP leg 54, Hekinian
et al. (L977) reported intensely fractured and
brecciated oceanis crust on the southern flank
of the Galapagos spreading-centre.The development of a fracture syste,m in tfe oceanic
crust is related to the thickness of the basaltic
crust, its brittleness (a function of composition)
and the spacing of major fault patterns (Rosendahl, pers. comm. 1979).
Fluid penetration obviously determines the
depth to which metamorphism can occur. Heatflow models do not predict penetration to the
oceanic mantle. The seismic velocity data
(Rosendahl 1.976,Fig. 5) and oceanic crustal
models generated from those data (Clague &
Straley 1977) cannot distinguish betwben pristino igneous rock in layer 3 and the uppermost
mantle (Fig. 1) and metaigneorn rocks and
Thin sheet
dlte conplex
10
10
AXIS,KM
DISTANCE
FROMRIDGE
15
Fro. 2. Ocean-ridge model for a fast-spreading centro from Sleep & Rosendahl (t979). The magma
chamber (dots), which is probably filled with cumulate mush, and tle conduit of upwelling mantle
material (angles) are very wide.'The walls of the conduit may consist of cooler material at the
sides (half circles) or material segregated from tho partial melt. The inner intrusion-zone (dense
vertical lines) probably occurs over that part of the magma chamber where melt is uzually present.
Downwarp of the base of the magma chamber and the density difference between the cooled
gabbroic crust (slanted lines) and the magma chamber are possible anomalous masses. The gpotherms are calculated assuming conductive heat-transfer only.
SERPENTINTTES,
SPILITES
AND OPHIOLITE
875
METAMORPHISM
has been emphasized not only with respect to
heat flow, but also to chemical and stratigraphic differences in the ocpanic crust generated (Rosendahl 1976, Lister 1977, Sleep &
Rosendahl 1979). Fracture patterns are distinctly different for the two rates of spreading:
fast-spreading ridges are marked by axial-block
topography and slow+preading ridges by axial
valleys (Rosendahl 1976). The salient structurai difference between the two typ€s of
oceanic ridge is the presence or absence of
deeply penetrating faults; their presence seems
to characterize slow-spreading ridges. The dif'
ference in the fault patterns between the slowand fast-spreading ridges can also be inferred
from tle common occurrence of metaigneous
rocks dredged from slow-epreading ridges (Mid'
Atlantis ridge) and the absence of meta'morphic
rosks from fast-spreading ridges (Pacific oceanbasin) (Rosendahl 1976).
Sleep & Rosendahl (1979) presentedpossible
geothermal gradients away from the ridge axis
based on evidence that a fast-spreading ridge
has a relatively large, wide magma-chamber
(Fig. 2), and a slow-spreadingridge has a very
narrow magma-chamber (Fig. 3). The slope of
the isotherms is much less steep at the fast-
serpentinite. Epp & Suyenaga (1978) do not
agree with Sleep & Wolery (1973) that hydrothermal precipitation will close the fracture
system and stop circulation in the crust older
than the heat-flow transition zone (Williams et
al.1974). They argued that thermal contraction
continues as the crust continues to cool, which
allows the fracture system to remain open and
to accommodate the volume occupied by the
formation of hydrothermal minerals. Thus,
fractures would increase in depth with age and
therefore would increase the extent of metamorphism with age, perhaps allowing fluid penetration to the mantle. Serpentinites have been
dredged from Atlantic oceanic ridges and fracture-zones (Aumento et al. 1971, Bonatti &
Honnorez 1976\. Bonatti & Honnorez (1976),
Bonatti (1976), Clague & Straley (1977) and
Epp & Suyenaga (1978) proposed diapiric intrusion of serpentinite into fault zones parallel
to oceanic ridge-axes, which implicitly assumes
that fluid access to the lower crust and mantle
occurred through deep fractures produced by
tle tensional environment at the ridge.
Fast versus slow spreding-rates
The importance of fast versus slow spreading
0
it
*T, o,ri-T"'
I Thick
|
|
|
'WttVVrtlZ |
'/zzi/zi/m.
//Z//Z
ffiffi
xl'e:t;J:JJil}5
?'ji,
-JJ.i^1\
lili*;"':.*\
:'J_(
1..)9^."-r..i-r
-)
-J
iili:a.?""s
sili\ii,Yir:..Ii:lr,.i.rh
-{'r
L)
::l
.I^-"JL' :'..,5 -*rt-,i
1.1-.,?li;.\?ri
10
-'t
L\
v!'1.'^u
15
DISTANCEFROMRIDGEAXIS, KM
Fro. 3. Ocean-rirlge model for slow-spreadrng centre from Sleep & Rosendahl (1979). Symbols are same
as Figure 2. Contrast with fast-spreading centres indicates a very small magma chamber, a thick
sheeteddyke complex, a wide zone of cooler material segregated from melt and a narrower intrusive
zone. The geotherms are calculated assuming couductive heat-transfer only.
876
THE
CANADIAN
spreading ridge because of the large magmachamber and reduced vertical heat-loss by conduction. The Nisbet & Fowler (1978) model
for a slow+preading ridge suggeststhe absence
of a maema chamber and injection of magma
by crack propagation. I{owever, the postulated
geothermal gtadients for both types of spreading centre are high enough to drive a hydrothermal circulation system and to be the cause
of metamorphism of the oceanic crust. The
presence of a hydrothermal circulation system
at tle oceanic ridge would sipificantly alter
the geotherms (FigS. 2, 3) as calculated by
Sleep & Rosendahl (1979). Parmentier &
Spooner (1978) have attempted to model such
a convective system, and have demonstrated
that the magnitude of the geotherms can decreasesignificantly as a function of the boundary
conditions chosen.
The preceding discussion has shown that
oceanic hydrothermal systems exist and that
their activity is determined by such factors as:
(1) age of oceanic crust, (2) crustal tlickness,
(3) permeability of sedimenb and crust, (4)
fluitl-flow parameters, (5) conductive yersus
convgctive heat-florr regme, (6) development
of major fault- and fracture-systems,and (7)
spreading rate. Unfortunately, the extent of
oceanic metamorphism, exclusive of fault and
fracture zones that allow fluid access, is unknown because of the technical inability to
sample deep oceanic crust away from those
-of
zones. Therefore, an understanding
oceanic
hydrothermal systems is critical to an understanding of the extent of oceanic metamorphism
TABlt 2.
ELEI.ENTAL@ILITIES
ErperlEntalr
si%
n%
4t203.
F€0
F€203
nn0
1490
Ca0
la?o
K2o
Pz%
lzo
U'RT!|G LOX.GRADE
!T'AIiIORPTT${ OF EASALTS
0e0lc
lbtabasalts
ophl ol ltlc
lietabasalts3
tgl-
MINERALOGIST
because fluid access to rock determines the
extent of meta.morphism. Further rvork is
needed on the proces'sesof formation, spacing,
depth and frequency of occurrence of fracture
systems in the crust, since they are a critical
part of the hydrothermal system, both at and
away from the oceanic ridge.
OPHIoLITES
The stratigraphic sequence of rocks making
up the complete ophiolitic sequence is relatively well defined (Conference Participants
1972). The major problem in correlating
ophiolites with oceanic lithosphere is that the
oceanic provenance of tle ophiolite is uncertain. Various models have been proposed suggesting ophiolites can be correlated with mature
oceanic crust and mantle (Gass 1977), rtrilginal-basin type of crust and mantle, or immature oceanic crust, i.e., that from a ridge
crest (Rosendahl 1976).
Geochemistry ol ophiohtic rocks
Experiments with basalt and seawater (Table
2) shoJr major-element exchangebetween basalt
and fluid. Table 2 also illustrates element
changes in oceanic and ophiolitic basalts that
have undergone greenschist-facies metamorphism. A good correlation exists between the
direction and change of element mobilities be.
tween oceamc metabasalts and experimentally
derived analogues.The metabasaltsof ophiolite
suites show the greatest variation in the majorelement mobility when compared to the experimentally derived metabasalts and to the oceanic
metabasalts.
Experimental work on mineral stabilities and
IAII.E 3.
n.d.
COIIPARISO{OF ELB'IEI{TALiIOBTLIIIES DURINELO'I.ONADEMFTANORPIIISiiI
OF PERIDOTITE
tup€rl@ntalr
oeMtc
tleta-Parl dotl te."
ophlolttlc
ibt!-Pe rl dot l te.
sr%
+
+
n .! .
(-) los durlnq @t@mhls
(+) goln turlns @t@rihls
(-, no chege
n.d. not det€rlned
'Hojd:h (197s),
(1976), Hottl & Holland (1978)
itlttl
rotlos), c@arlson to un!ltered
{lq-rcck/soeter
bsalt
Ti 02
n.o.
41203
n.o.
Fe0
n .d .
FeZ03
n .o .
Fe0t
lin0
ugo
Ca0
Na20
startlng@tedrl
\0
lzf
+
zcot"t
1tszz1, [email protected]& Th@psd 0978a), c@partson to prtsttne basalts
ln s@ potrologlc Drmne@
to
"cot.h (lgzz), Grapes (1976), tbEtes et al. 09ll), c@arlsq
unalt€red ba$lts In Indlvldual opTioTtte seguen@'o u average.
for 0altered ophlolltlc brsalts
(+) gsln durlng @ta@rphl$,
(-) loss durlng rta@rphlsEr
(r) no
change, n.d. not detoElned. I S€yfrled t Dlbble (1978)i 2 A@nto t
(1966), ibores t Ylne
Loublt (lgn), Prlnz et al. (1976)i 3 ilnshlrc
(1971), cole@n t KBl6'(1971), ltntlgnt€t
al. (1973). coqar'lson of
Iosaes and galns rlti avengo por'ldotlte (t{@kolG 1954, ct.w 1967).
SPILITES
SERPBNTINITES,
TAELE4.
NaZ0
Kzo
Heo
OPHIOLITE
877
METAMORPHISM
AIID
I.IOBILITIESFORI4ETABASALTS
OF DIRECIIOIIOF IIIA,OR-ELBIENT
COT,IPARTSON
EI{VIROI{}'IEI{TS
FROIiIDIFFEREI{T
METAPERIDOTITES
ErPlgg'
si02
Ti 02
A1203
Fe0
Fe203
Nn0
!190
ca0
AND
d
o
s
nd
Exp/oph*
dd
ds
.@bLoc'
Elp/gc*
dsd
q
ndndd
no
nd
d
s
s
o
d
d
s
s
d
9phlq.g'
ndnds
d
s
Exp/0ph*
d
no
nd
s
no
o
q
s
d
(l
d
q
d
s
s
q
d
q
5
s' sas.'
oritrcmcks'
'ph' 'phr
rocks'
:'?;Ji:i"fi:"loT'ilt'e"S-';;3anrc
metamorphism of ultramafic rocks (Chernosky
lg7', Irioody L976a, b, Seyfried'& DibbG
t97B) documents some-major compositional
changes during serpentinization, involving seawateiunder gr-eens&ist-faciesconditions iTable
3). These ciunger chiefly involve an addition
oi magnesium ind watei and a loss in total
iron, S1Or, AlaOa and FeO.
Tables 2 and 3 show the direction of majorelement mobilities for metamorphic rocks' of
the oceanic crust and ophiolite suites. The experimental results preseited ur" for basalt and
ieridotite reacted ivith seawater under greenschist-faciesconditions. Comparisons of the direction of element mobility for metamorphic
rosks from different environments are presented
patterns of element mobilities
in Table 4. Similar -and
oceanic rocks could inbetween ophiolitic
dicate similar metamorphic conditions (e.g.,
in ophiolitic and oceanic metaperidotites). A
of the rock-seawatir experiments
"o-puri.oo
and the element-mobility patterns in metamorphic rocks might indicaie ieawater involvement
in metamorpliism (e.g., experimental and oceanic metabalsalts).Howevei, Table 4 indicates
the difficulty in trying to use the major-element
geochemistry to deteimine the time and place
if metamoiphism, as the correlation between
tho three diiferent groups, except for the two
examples given aboie, iJ inconclusive.
Thi degiee of element mobility during metamorphism- largely determines the relevince of
geochemical ituai". performed on ophiolitic
iocks. Whole-rock cheinistry of basaltsf,as been
used to demonstrate that ophiolites are related
to oceanic crust. For eiample, Miyashiro
(1973, 1975a,b) has used majoi-elemeni chemistry to suggest
-an that the Troodos ophiolite was
istanA-arcenvironment. However,
formed in
his claim has been refuted by Gass-et al.
Og75), Hyngq (1975), Moores (1975) and
Church & Coish (L976). Some of the disagree'
ment between Miyashiro (I975a, b) and the
other workers results from Miyashiro's (1975c)
contention that element mobility during metamorphrsm is minor. The difficult! in using
major-element data to support arguments of
tectonic environment is shown in Tables 2 and
3. These tables clearly illustrate significant
major-element changes during low-grade metamorptrism of both basalts and ultramafic rocks
in ophiolite sequences;thus, care must be exercised in an extrapolation from major-element
chemistry to the tectonic setting of the igneous
rocks. The weight of evidence for the Troodos
would favor an oceanic origin of the basalts'
bul not chiefly oq the basis of the whole-rock
chemistry. Therefore, Miyashiro's alSuments
based on major-element chemistry of the.basalt
are not very convincing evidence -of an islandarc origin for the Troodos complex. ,
Miyashiro (1975a) has also used whole-rock
chemistry to classify opliolites into -three different groups based on the inferred presence
of calc-alkaline or tholeiitic trends as shown
Howby the basalts of ophiolitic se_quence^s._
ever, Pearce & Gale (1977) and Sun-&-Nesbitt
(1978) suggested,as did Miyashiro (1975a, b'
variation exists
c), that nome real geochemical_
among oceanic basalts from the different tectonic environments within oceanic lithosphere
(e.g., oceanic ridge-crest versas island arc ver'
sus back-arc basin). Therefore, the petrological
origin of ophiolitic basalts is further compli'
catid not only bv metamorphism,-but. also by
real geochemical differenc€s reflecting tectonic environments'
The direction of element mobility for the
878
THE
CANADIAN
metamorpho$ed oceanic yeruru ophiolitic ultramafic rocks is the same except for SiOr, Al:Oa
and trGO (Table 4). However, the preliminary
results of peridotite-seawater experiments (Seyfried & Dibble 1978) conflict with tle geochemistry of metamorphic rocks, especially
with respect to CaO. Calciu.m is lost from
ultramafic rocks during metamorphism in both
oceanic and ophiolitic rocks, and the leached
calcium is deposited in the rocks associated
with serpentinites, e.9., rodingites (Barnes &
O'Neil 1969,Barneset al. L972, 1978, Coleman
1977, Pf.erfer 1977, Wenne.r 1979). Honnorez
& Kirst (1975) described rodingites dredged
from the Mid-Atlantic-ridge fracture zones;
these formed by the alteration of a noritic
gabbro. The element-mobility data support tle
formation of rodingites by fluids involved in
the metamorphism of ultramafic rocks. CaO
is also shown to be lost during metamorphism
of basalts.
Tables 2 and 4 indicate that Na-enriched
metabasalts (i.e., spilites) can form from the
interaction of seawaterwith basalt. Both oceanic
and ophiolitic metabasalts demonstrate an en-s
richment in sodium, which agrees with the experimental data on basalt-seawaterinteraction.
A comparison of the direction of mobility for
the other elementsin the experimentally derived
metabasalts with the direction of those in the
oc.eanicmetabasaltsdemonstrates very good to
excellent agreement. A comparison of oceanic
and ophiolitic metabasalts shows a much
greater variation in patterns of element mobility.
Further evidence for seawater involvement
in the formation of spilites could be inferred
from the data of Shaw et aI. (1977), Enriched
MINERALOGIST
in sodium and water, spilites are also enriched
six-fold in Li comparedto the unmetamorphosed
basalt. Seawater, brines and volcanic waters
are obvious choices for the Li- and Na-enrichment.in the spilites.
The data in Tables 2 and 4 indicate that
the metamorphism of basaltic rocks could occur
in the oceanic crust. Also, the wider variability
in the major-element-mobility data for ophiolitic
metabasalts may indicate the effects of more
than one metamorphic event, with involvement
of a fluid other than seawater. The data for
the ultramafic rocks (Tables 3, 4) are inconclusive.
Pearce& Cann (L971) used the Ti,Zr andY
contents of four different ophiolitic basalts to
demonstrate that the volcanic rocks have oceanfloor provenance.Smewing et aI. (1,975) argued
that the major- and trace-element contents of
Troodos metabasalts are significantly different
from those formed at present-day construetive
plate-margins,and proposed an origin at a slowspreading ridge within a small, marginal oceanbasin. Coish (1977) assertedthat metamorphism
had little effect on TiOz, PzOo, 7-r, Y, Cr or
Ni contents of metabasaltsfrom the Betts Cove
ophiolite, Newfoundland, and proposed an
ocean-floor origin for the basalts. Beccaluva
et al. (1977) determined 12 different traceelement contents for the rocks within the
ophiolitic sequence from eastern Corsica and
concluded that the ophiolitic metabasaltsshowed
an ocean-floor affinity.
Rare-earth-element(REE) data from ophiolitic rocks are sparse; rocks from the Pindos
suite, Greece (Montipy et al, 1973), the
Troodos complexo Cyprus (Kay & Senechal
TABLE5. GTOCHE|TCAL
CRITERIATJSED
BY BLAND(1978) IO DETER}IINE
NIE ORIGINAL
CHAMCTER
OF
APPALACHIAN
METABA5AI.IS
Chenical
DiscrJnlnator
12-20 vt.
cao a t190a
>20UCao+ lilgo'
alkalles
K20 & NaZ0/190ratlo
Fl-F2 dlscrlminate function
FZ-F3dlscrlmlnate functlon
Tl !gZr, ppm*
Zr vs. Tll100 vs. (3)Y, ppm
Caort g J: Y, ppn
r
*
Rock Types
otstinlliriinea
selects rocks of slmJlar
tectonJc affinities &
ellmlnates ultrmaflc cumlates
OFB& M frm
(LKt, CAB,st1o,0r)
IPB, SHo, oFB,
(LKT & CAB)
5H0, LKT, CAB,oFB
OFB,CAB, LKT
llPB, CAB,LKT, and
oFgtCAB+LKT
LKTE SHoplot to left
of SCAI; 0I, CoN,oFB
plot to r'lght of SCATT
CABplot within SCAT
coupledwith Tl02 content
coupled wlth Cao + Mgo content
Reference
Pearce& Cann(1973)
Pearce (1976)
Jakes & t{hite (1972)
RosersglLql. (1974)
Pearce(1976)
Pearc (1976)
Pearce& Cann(1973)
Pearce& Cann(1973)
L m b e r t & H o l l a n d( 1 9 7 4 )
SYMBOLS:
oFB= ocean-floor basalts
LKT. lm-potasslum tho'leiites
CAB. calc-alkaline basalts
SHo= shoshonltes
0l - oceanlc-island basalts
C0ll. continentalrifting basalts
RA= rear-arc basalts
UPB= within-plate basalts
SCAT= standard calcalkaline trend
SERPENTINITES,
SPILITES
AND OPHIOLITB
1976, Smewing & Potts 1976) and the Point
Sal ophiolite, California (Menzies et al. 1977)
have been examined. Unfortunately, total agreement on the effect of metamorphism on the
primary basalt-REE distributions is not evident.
Menzies et al, (t977) summarized evidence
that indicates light-REE mobility during zeolitefacies metamorphism of ophiolitic basalts. This
mobility results in an increase in the total con'
centratibn of" REE and a change in the profile
characteristics due to Ce or La mobility. At
the same time, greenschist-faciesmetamorphism
has not changed the REE patterns of the
primary basaltic lavas of ophiolite c,omplexes
(e.g., Point Sal ophiolite). Conversely,Montigny
et AL ('973), Kay & Senechal(1976) ' Smewing
& Potts (t976), Coish & Church (1978) present
evidence that REE abundances of ophiolitic
basalts were not modified by metamorphism for
the Pindos, Troodos and Betts Cove complexes'
Clearly, more REE data are needed for the
total ;tratigtaphy of an ophiolite sequence be-
METAMORPHISM
fore the question of. REE mobility during metamorphism can be resolved.
,A- combination of major- and trace-element
data is more convincing in demonstrating -an
ocean-floor origin for the ophiolitic metabasalts'
Bland (1978) has confirmed that the tectontc
origin of a metabasalt can be determined, but
thal more than one geochemical test must
be applied in order to see through the metaoverprint. Table 5 summarizes the
-otfh-i"
diffJrent criteiia used bv Bland (1978) to determine the tectonic setting of many different
ases of the metabasaltsin the Appalachian
BTue Ridge and Piedmont. A more definitive
statemeni about the origin of the ophiolite
basaltscould be made if a complete geochemical
study were done on these rocks, similar to
Bland's (1978), using a statistically significant
sample size. Pearce & Gate (1977) have used a
com-bination of whole-rock chernistry, traceelement and REE data of metabasalts to classify the tectonic envtonment (oceanic) asso-
oceonicserPenliniles
oceonicmetobosolls
oceonicgobbro
oceonicbosolt
ophiolitemelobosolls
F#
ophiolite serPentinites
0-{
t-l
ophioliteulfromoficrocksl
portiollY serPentinized
* vein serpentine
Troodos serPentinates
Troodos metobosolts
to SMOW)
DrsO %o(relotive
(1973' 1977a' b)'
data obtained from Spooner et al'
Frc. 4. Summary of whole-rock oxygen-isotope
(
1
e
7
4
)
.
layror
&
Wenner & Taylor (1973), Magaritz
880
TIIE
CANADIAN
ciated with the formation of massive-sulfide
deposits related to ophiolites.
Orygen-isotope data have been used to indicate the type of water involved in ,metamorphism as well as the temperature of metamorphism. Spooner et al. (L974, 1977a) argued
for seawater involvement in the metamorphism
ol eastern Liguria, Pindos and Troodos opnioiltic basalts. Ifowever, Figure 4 shows that the
ophiolitic metamorphic rocks are more enriched
in I'tO compared with the oceanic rocks. Some
olerlap is present in the dati, especially for
the partly serpentinized ultramafic rocks. However, these oxygen-isotope data alone do not
necessarilyprovide conclusive evidence for seawater involvement.
. Sheppard (1977) and Heaton & Sheppard
(1,977) presented both D18Oand gD isotopic
data for the metamorphosed Troodos pillowlavas, sheeted intrusive complex, trondhjemites
and ulper gabbro. The 8D values for Tioodos
rocks overlap with the few available measurements of oceanic metamorphic rocks. Mineral
data are combined with whole-rock values to
calculate models for 8D and DtsOfractionation
assuming equilibrium, with the isotopic differences reflecting different temperatures (i.e.,
different rnetamorphic grades. Heaton & Sheppard (1977) concluded that a large component
of seawaterwas involved in the metamorphism
of Troodos rocks.
Wenner & Taylor (1973) and Magaritz &
Taylor (1974) documented a marked difference
between oceanic and continental ophiolitic serpentinites on the basis of both oxygen- and
hydrogen-istotope compositions. They argued
against a seawater origin of the watei involved
in the metamorphism of the ophiolitic ultramafic rocks.
Recent measurementsby Sakai & Tsutsumi
(1978) of serpentine-water D/H fractionations
between 100-500oC at 2 kbar suggest that
deuterium exchange,in particular, may be controlled by nonequilibrium processes.'The experimentally derived D/H fractionation factors
indicate that a mixed fluid containing 50-75%
magmatic water and temperatures higher than
300oC were involved in the formation of
oceanic lizardite-chrysotile serpentinites; this
result is not in agreement with previous work
(Wenner & Taylor 1973), Sakai\ & Tsutsumi,s
(1978) work illustrates the difficulty in using
whole-rock isotopic anabses to determine the
nature of the fluid involved in metamorphism
or alteration if equilibrium isotopic fractionations are assumed.
The model of Spooner & Fyfe (1973) and
MINERALOGIST
Spooner et al, (1977a) for hydrothermal metamorphism of ophiolite metabasalts from eastern
Liguria, Italy, is based on oxygen-isotope data,
water contents and Fe3+/(Fe'++Fes+) ratios
of the rocks. The metamorphic mineral assemblages indicate that temperature increased
with depth in the basaltic lavas. The SrEOvalues
decreasewith depth but do not reach the 8r8O
composition of pri.mary basalt. The 8'0 profile
was assumedto be controlled by the temperature
gradient. Different oxygen-isotope values and
oxidation ratios were measured for cores versns
rims of metabasalt pillows, indicating that isotopic and chemical equilibrium was not achieved
during metamorphism. Spooner et aI. (1977a)
combined the oxygen-isotope data with the
oxidation ratio Fe,+r/(Fe2++Fes+) to calculate
the amount of water involved in the alteration
of the basalt (ld-l$:
1 water:rock ratio).
Their calculations assu.methat the fluid involved
in the alteration was sealrater.
Measurements of sTSrlsSr ratios in the
Troodos mineralized and metamorphoseddykes
and mafic volcanic rocks (Chapman & Spooner
1977, Spooner et aI. 1977b) yield values enriched in 87Srrelative to fresh oceanic basalts.
Theso authors suggested that seawater was the
ore-forming fluid for the sulfide ore-deposits
of the Troodos complex, a hypothesis substantiated by fluid-inclusion data from vein
quartz coexisting with the sulfides (Spooner
& Bray 1977), They gave the ocsurrence of
the deposits as further evidence of a major
oceanic hydrothermal-circulation system within
the basalts.
Spooner & Fyfe (1973), Andrews & Fyfe
(1976), Coleman (1977) and Rona (1978)
have documentedthe evidencefor hydrothermal
systems at spreading centres with regard to
seawater leaching of the basaltic oceanic crust
to derive the metals for the massive sulfides
of ophiolitic complexes. Spooner (1977) and
Parmentier & Spooner (1978) have developed
a model for Cyprus deposits, where a strong
case can be made for the location of the sulfide
deposits within the metabasalt layer as a result
of oceanic hydrothermal circulation. A total
geochemical study of ophiolite metabasalts associated with massive sulfides in Cyprus, Oman
and Betts Cove has led Pearce & Gale (19V7)
to postulate that the basalts formed at d spreading axis in an oceanic back-arc basin rather
than at a major oceanic ridge-crest. Unfortunately, the subsequent metamorphism and deformation of the ophiolitic metabasalts of the
Whalesbackcupriferorn iron deposit,NewfoundIand, do not permit the relationship between
SERPBNTINITES,
SPILITES
AND OPHIOLITB
881
METAMOSPITISM
TABLE 7. ]iETAIORPHICHISMRY OF OPHIOLIIESI
I{ETABASALTS*
OF OPHIOLITIC
TYPICATNINEMLOGY
Deqr€eof lrtamrohls|[
ND METAPERIDOTITES*
lbtanorphlsu ln oceanlccrust
chlorite,epidote,albite,actinolite
sDilites:
6rn to'gabbrc lalEr onlYi
c
a
l
c
i
t
e
! s p h e n e ,z e o l i t e s '
serDentlnlzationdurlng or
aftar tectonlc ea[placecnt
Serpentinites: lizardlte, chrysoti le' magnetite
I brucite, antigorite
TABLE6.
* Essential [Etamorphicnineralogy of oceanic retaigneous rocks and continental ophiolltic rocks
is san€.
sulfide mineralization and sub'sea-floor metamorphism to be determined (Bachinski 1977)'
Metamuphic
mineralogY
As shown previously (Table 1), metaigneous
lesks similnr to those in ophiolite suites have
been recovered in dredge samples from oceanic
ridges and frachrre zones. Table 6 summarizes
thJ essential mineralogy of serpentinites and
spilites (layers 2 and. 4 in the stratigraphy -of
tn€ ophioiite sequences). The metamorptric
mineriogy is not very helpful in contrasting
the oseailc var,trrscontinental ophiolitic environment - they are the same. The interesting
problem is that both the serpentinites and
spilites (Na-enriched metabasalts) can undergo
in the samerange-oftem-peraturemetamorphism
-sonditions
(Moody 1976a, -b, Mogdy
pressure
b Meyer 1978, Meyer 1978). If that seltinc
is defined in terms of zeolite- and greenschistfacies metamorphism, then the distinction be'
tween the timing of metamorphism and occurrence of serpentinites and spilites together in
the ophiolite sequence can be made.
Miny models of sub-sea-floor meta'morphhal
(Spooner & Fyfe 1973, Bonatti et'al. 1975,
Co'leman 1977, Elthon & Stern 1978, Humphris
& Thompson 1978a, b) indicate, increasing
grade with increasing depth'
metamorfhic
-metamorphism
is depicted in these
Oceanic
models as dying out in the gabbroic layer at
conditions oi upper-greenschist or amphibolite
metamorphic facies.
Ophiolitic rocks that show this increasing
grade of metamorphism down through the
6phiolite sequence,from sedimentsto extrusive
and then to intrusive mafic rocks (Fig. 1) are
found in (1) eastern Liguria, Italy (Spooner
et ol. 1974, L971a)' (2) the Sarmiento
complex, Chiie (Elthon & Stern 1978), (3)
the bhenaillet massif, France (Mevel er al'
1978), (4) Darvel Bay' Borneo (Hutchison
lg78)'. f"Ue Z summarizes the best described
of the known ophiolite sequenceswith respect
to metamorphism. The difficulty in preparing
the table was that information on the meta-
Examles
Vourlnos, Northern 6r€ece
lhcquarl€ lsland
Bay'of IslanG' NHfoundland
Blor-l'le-tbyn, t{dfoundl and
Betts CoYe, tleYfoundland
Tmodos, CyPrus
Saniento comPler' Chller
Polnt Sal. Callfornla'
S€nall' ornan
Pspua-l{€t{Guinea Chenaillet. Francej
E. Llgurla. ItalyqlbrYel Bay, Bomeo'
Dunn ittn., l,ler Zealahd6
PoIymtrrmrPhlsm befora
Southern quEb€-c'
and after eqlaceEnt'
Eastern Taimnu
lncludlng r€gional
Kanmy, Nolqys
uBtamtphlsn
*
nely published lnfor0Etlon mdifles 8nd adds to ttls-otlg-.
tglt"n
& salliburv(lsls;'
r"iii-iirniiiiiion of chrlstenson
o
;m"iisltti':ti;l;:
tt#i.'l;llii':s'tl#h*dlt3'
& Hotsrt(1e78)_.
,iiJigioi-riuiiint (t9rsa,'o).Laqrent
Eiilt
;t'cf.-(ii?al I al-io,;&-Ernsd(leTe)'esturts!.sL'
(r97e).
morphism of the total stratigraphic sequence is
eeneiallv unavailable. Those ophiolites that give
E"ia"o"" for a possible oceanic episode of
metamorphism reich upper-greenschist facies,
although-Mevel et aI. (1978) described a1Prybolite-facies metamorphism in an ophiolite
eabbroic laver. Amphibolites formed from metailorphism of the intrusive mafic layer have
been observed in the oceanic environment
(Table 1). These amphibolites are,not to be
confused with those produced at the base of
the ophiolite in a meiamorphic aureole that is
clearly related to its tectonic emplacement
(Dewey & Bird 1971, Jamieson L979, McCaig
& Kemp 1979).
Most-ophiolitic serpentinites are composed of
antigorite
than
rather
lizarditihrysotile
(Wenner &'Taylor 1973, Maga1ig & Tay^tor
i974, wi"k. & tttlhittater L977,Prichard 1979),
indicating a lower temperature of metamorphism
than the antigorite field of stability (N!,odv
'1976a,Evans 1977). However, Liou & Ernst
(l97gi reported antigorite in ultramafic rocks
of the East-Taiwan ophiolite, and other uppergreenschist-faiies minerals from the meta6asalts. Clearly, depth of penetration of circulating ocean waters is important, 1:. are
metam6rphic pressure-temperature conditions'
In the case of most ophiolitic ultramafic rocks,
amphibolite-facies metamorphism is too high
a grade of metamorphism to have taken place
in oceanic environments.
Amphibolite-grade oceanic metamorphism of
ultranrafic ro"k. is uncommon; Bonatti et al'
(1970) have described anthophyllite and cummingtonite in a dunite. In fact, prograde.metaof serpentinite with formation of
rnotfttit*
"meiamorphic" olivines has been postulated to
882
TrrE SANADTANMINERALOGIST
occur at the greenschist-amphibolite boundary
(Vance & Dungan 1977, Dungan 1977) with
the coexistence of antigorite and olivine.
Johannes (L975) & Evans et at. (1976) determined the equilibrium breakdown of antigorite
to forsterite * talc to occur at approximately
500"C at 2 kbat total pressure. Prograde metamorphism of a serpentinite to the amphibolite
facies would produce the mineral assemblage
forsterite l- talc t tremolite (Vance & Dungan
1977, Evans t977), not the common metamorphic assemblagefound in metamorphosed
ophiolitic ultramafic rocks (Table 6). Therefore, the presenceof an amphibolite-faciesmetamorphic assemblagein the ultramafic layer of
an ophiolite probably indicates a prograde regional metamorphism of the ophiolite after e.mplacement of the ophiolite on the continent.
The major consensus (Tables 7, 8) from the
studies of both oceanic and ophiolitic rocks is
that metamorphism of the ophiolitic sequence
can occur within layers L and 2 but ceases at
various different levels in layer 3 (gabbro,
Fig. 1). The sessation of metamorphism in
layer 3 is directly related to the limit of fluid
penetration into the oceanic crust, which is unknown except for oseanic ridge-crests and
transform faults. As further information about
fluid penetration away from crests and faults
is unlikely to emerge because of drilling limitations, quantitative modeling of the soolirrg, contracting crust will be required to shed light
9n the major oceanic-fracture systems away
from the ridge crests.
Evidence gathered to date strongly suggests
that serpentinization of the ultramafic layer
occurs after and uot before emplacement of
Mrteruonpruc Moosrs non OpnrolrrBs
the ophiolite. Certainly, the physical properties
The discussionpresented in the two previous of peridotite yerurs serpentinite (Moody 1976a)
sections indicates that ophiolitic basalts have are sufficiently different during deformation to
an oceanic character. Metamorphism of layer l,
suggest this (i.e., the tectonic emplacement of
layer 2 and part of layer 3 (Fig. 1) probably a serpentinite would take place with the prooccurs in the oceanic crust before tectonic duction of a m6lange and complete disruption
emplacementof the ophiolite, whereasmetamor- of tie serpentine layer). Mdlange production
phism of the lower part of hyer 3 and all of
on a large scale has been described for sublayet 4 occurs during or after emplacement duction-zone ophiolites (Coleman 1977) but
(Moody 1978). Detailed models of hvdro- is not a widespread feature
of other ophiolites.
thermal circulation syste,ms at mid-ocianic
Therefore, tectonic-emplasement models point
ridgeg are in the process of being evaluated by
to the ophiolite meta.morphism of the lower
geophys.icists,geologists and petrologists. More layers 3 and 4 occurring during
or after eminformation is needed before an internall.v con- placement, not before. The tectonic emplacesistent model for both fast- and slow-sprlading ment of some ophiolites (e.g., at Bay of Islands
ridges can be developed. The extent of meta- and Semail) is marked at their basesby metamorphism at the ridge crest, and as a function morphic aureoles of medium- to high-grade
of age of the oceanic crust; is still unknown. rocks including mylonites.
TAELE8.
ophiolte
Rocks
SCHEME
OFIiIETAI.IORPNISM
FOROPHIOLITESI
0ceanic
Metilorphism
l4arlneSedirent
Zeolite
Basalt
Zeol ite-greenschl st
( s p ll l t e s )
0bductlon
No retmorphlsn, or
mtamorphismin oceanic
envlronrent before tectonJc transport
Reglonal*
Zeollte
Zeolite-greenschlst
0labase
0reenschist
creenschlst
Gabbro
creenschist-amphibol lte
Greenschlst-mphlbot
lte
Perldotite
Unretmorphosed,except
l o c ai s e r p e n t i n i z d t i o n
along fracture zonest
rcdlngjtes assoclated
with serpent'inites
1 contact retamrphlc
aureole [6lange,
nryIonltes,serpentinlzatlon ! rodJngjtes
SerpentlnizatJon,
possible prograde
mtamrphlsn of
serpentlne
Condltionsof
l,letmorphlsm
No defomtlon
Defomation at base of
ophlolite slab w,ith
development
of follatlon there
Ibve loprent of
penetratlve
follatlon
*
*
modlfied frm Colemn (1977)
multlple stages of retmorphisn poss.lble,superpos.ltionof ocean.tcrlth continental
;;t;:|!i"sl#"[lli#;,]li],ot.n
srmpre
burlarretamrphtsm
wrthour
devetopmnt
SERPBNTINTTES,
SPILITES
AND OPHIOLITB
METAMORPIIISM
883
(1969): The relationship
I. & CfNpn, J.R.
Element mobilities during metamorphism are BARNEs, fluids in some fresh alpine-type. ultrabetween
in
re-exercised
be
must
sienificant. Care
possible modern serpentinization'
;;ft;;-and
cJnstructing parent igneous rocks from their
Unite<t States. Geol' Soc' Amer' BulI'
**t.to
supposed melamorphic progely. Several lines
80, 1947-t960.
of-eeochemical and petrographic evidence must -,
& Tnpscesss, J.J' (1978)-: Present
be lssessed before definite conclusions can be
dav serpentinization in New Caledonia, Oman
reached about the parent igneous rock and
u"i iuiottavia. Geochim. Cosmochirn' Acta 42'
14+145,
its origin.
Rep, J.B. & O'Nrn, J.R. (1972): -Meta'
Tho metamorphic history of an ophiolite. is -,
-otohi" assemblagesand the direction of -flow
an integral part if its geological history'.Critical
oi- m"ta-o"pttic fluids in four instances of serto an inderstanding of this metamorphism is
Petrology 35'
Contr. Mineral'
pu"tioi"rtioti.
events
geological
the ability to decipher the
263-276.
undergone by the ophiolite after emplacement
L., OnNrNsrnrtER, D. OHNENSTETTER'
--Vf.
oo ttti continent, id emplacement history and BEccALUvA,
G. (L977): The trace, elee
VnNiurnr-r-r,
tt
provenance'
then its possible oceanic
it :iI"
Corsican ophiolites' Contr'
of
geochemistry
rnent
to sav that the unique imPortance of ophiolites
Mineral. PetrologY 64' 1l-31'
in thi understandingof global tectonicProcesses'
-BIscHoFF,J.L. (1969): Red Sea geothermal-brine
couplea with the aift"utty in unraveling their
theii mineralogy, chemistry and- gen-i;
deposits:
histbries, will assure them a continuing central
Brines and- Recent Heavy Metal
Hot
;;.
position in plate-tectonic studies in the forein the Red Sea (E' T' Degens- &
ilposits
seeable future.
n.'.C" R.*' eds.), Springer-Verlag, New York'
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of seawater from 25" to 350'C'
chemisiry
AcrNowr-nPGBMENTS
Amer. J. Sci. 278, 838-860'
S., Anr'l6nssoN, S. &-To:vrAssoN' J'
This work was supported by National Science BJiirNssoN,
'Economic
author
The
evaluation of Reykjanes thergrant
-6.t""
EAR75-21846'
l9lZl:
Foundation
aiea, Iceland, Amer' Assoc' Petrol'
il
appreciates the opportunity provided by F' J'
Geol. Bull. 56' 2380-2391.
to investigate this interesting topic'
fii.ks
Element Geochemistry
Bruce Rosendahl provided stimulating discus- BLeNo, A.E. (1978) t Traceof Maryland, Virginia'
Sequences
of
Volcanic
manuscript
of.iL" unA help
-I at iarious stages
i'nd North Carolina and its Bearing on the,Tec'
am deeply- grateful to T'..R'
preparation.
of the Central Appalachians'
evomttn
iiit
provided
support,
who gave critical
W-tt"y
Ftt.o. tn"ti., Univ. Kentucky, Irxington' Ky'
discussion, and reviewed the manuscript'
in
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