JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. B7, PAGES 15,507-15,520,JULY 10, 1997
Nicoya Peninsula, Costa Rica: A single suite of Caribbean
oceanic plateau magmas
Christopher
W. Sinton
1 andRobertA. Duncan
Collegeof OceanicandAtmospheric
Sciences,
OregonStateUniversity,Corvallis
Percy Denyer
EscuelaCentroamericana
de Geologia,Universidadde CostaRica, SanJose,CostaRica
Abstract. The pre-Tertiaryoceaniccrustexposedon the westcoastof CostaRica hasbeen
broadlyreferredto astheNicoyaComplex.Thisstudywasdesigned
to determine
theageof the
NicoyaComplex
in theNicoyaPeninsula,
PlayaJac6,andtheQuepos
Peninsula
using4øAr-39Ar
radiometric
datingandto assess
thepetrologic
relationships
betweenthedifferentlocalitiesusing
majorelement,traceelement,andSr,Nd, Pbisotopic
data.Radiometric
agesof basalts
and
diabases
fromtheNicoyaPeninsula
are88-90Ma (witha weightedmeanof 88.5Ma), andthoseof
twointrusiverocks(a gabbroandplagiogranite)
areboth83-84Ma. Thecombined
geochemical
dataindicatethatthesampled
NicoyaPeninsula
rocksbelongto a singlesuiterelatedby fractional
crystallization
of similarparental
magmas.Nd andPbisotopic
ratiosindicatea common
mantle
sourcedistinctfromthatof mid-oceanridgebasalts.Boththeageandcomposition
of theNicoya
rocksareconsistent
with theideathattheyarea partof theCaribbeanCretaceous
oceanicplateau
[Donnelly,1994]. TheJac6lavasaregeochemically
similarto theNicoyaPeninsula
suite,anda
singleageof 84 Ma is identical
to theageof theNicoyaPeninsula
intrusives.
Theoneanalyzed
Quepos
basalthasa radiometric
ageof ~64Ma, andit isenriched
in incompatible
elements
relative
to theNicoyarocks.Similarities
in Nd andPbisotopic
ratiosindicatethattheQuepos
and
Nicoya/Jac6
lavaswerederived
froma similarmantlesource
tothatwhichproduced
theNicoya
rocks,possiblythe Galapagos
plume.
Introduction
Obductedoceaniccrustexposedon the west coastof Costa
Rica is broadly referredto as the Nicoya Complex [Dengo,
1962] (Figure 1). Theserocksare composedof intermingled
basalts (massive and pillowed flows, dikes, and breccias),
gabbros and plagiograniteswith interspersedpieces of
radiolarianchert [e.g., Kuijpers, 1980;Alvarado et al., 1993;
Denyer and Arias , 1993]. Outcropsare generallylimited to
coastalpeninsulaswith the most extensiveexposureson the
NicoyaPeninsula.The full extentof the obductedcrustis not
preciselyknown,but it is possiblethat the Nicoya Complex
is part of a belt of accretedMesozoic oceaniccrust that
extendssoutheastward
throughPanamainto westernColombia
and Ecuador[e.g., Escalante, 1990].
On the basis of the fossil ages of the radiolarian chert
associatedwith the igneous rocks, most previous workers
have divided the oceanic igneous rocks of the Nicoya
Peninsulainto an upper and a lower unit [Schrnidt-Effing,
1979; Kuijpers, 1980; Bourgoiset al., 1984; Gurskyet al.,
1984; Wildberg, 1984; Frisch et al., 1992]. The lower unit
1Nowat Graduate
Schoolof Oceanography,
University
of Rhode
Island, Narragansett.
Copyright1997by theAmericanGeophysical
Union.
Papernumber97JB00681.
0148-0227/97/97 JB-00681 $09.00
was described as normal oceanic crust of Jurassic to Early
Cretaceousage, and the upper unit was subdivided, based on
composition, into a suite of Late Cretaceousoceanic rocks
associatedwith the Caribbeanoceanicplateau and a primitive
arc series. However, our re-examination of contacts between
chert and basalt demonstrated only intrusive (basalt into
sediment) rather than depositional (sediment on basalt)
relationships,so the sedimentage cannot be used to constrain
the basalt age. The division of the igneous rocks into two
seriesof different agesis thereforeopen to question.
The Caribbean oceanic plateau (also called the Caribbean
Cretaceous Basalt Province by Donnelly et al. [1990]) is
composedof the thick oceaniccrustthat makesup the core of
the Caribbeanplate and numerouspiecesof obductedoceanic
crust along the plate margin [Donnelly et al., 1990].
Debatable aspects of the Caribbean plateau include its
formation location (Pacific or in situ) and the plate tectonic
setting of origin (i.e., intraplate, ridge centered, back arc).
The preexistingoceanic lithospherewhich, basedon seismic
reflection profiles [Diebold et al., 1981] and plate
reconstructions[Duncan and Hargraves, 1984], purportedly
lies beneath the plateau has not yet been definitively
identified in any of the on-land exposuresnor has it been
sampledby deep sea drilling. The exposedoceaniccrust in
western Costa Rica may play a very important role in
understandingthe natureof the plateauif, indeed,somepart of
the Nicoya Complex representsthis preexistingoceaniccrust.
This possibility can be evaluatedby determiningthe age and
compositionof the lavas.
15,507
15,508
SINTONETAL.'THENICOYAPENINSULA
83.2 _+1.3
Peninsula
92.5 _+ 5.4
10030 '
.2
1el0• 90.3_+3.2
Nico'
Peninsula
PI. Jac6!
;00
PI. Herradura
89.7 + 1.7:
/
•-•
19
18
83.8_+1.1
88.0_+0.7
84.2 _+ 1.8
1-33
63.9 _+0.5
Sta. Cruz
22
Nicoya
21
89.9 _+2.8
10 ø
23
Carmona
85 ø 30'
Figure 1. Schematicmap of the Nicoya Peninsulawith inset showinglocationof Playa Jac6 and Quepos.Solid
circlesaresample
locations.
The4øAr-39Ar
plateau
agesof basalts
anddiabases
aregiven,withtheagesof the
intrusive rocks in italics. Thin lines on the Nicoya Peninsulaare major roads.
This projectwasdesignedto (1) determinethe radiometric similar mantle sourceand that all can be related by fractional
ageof rocksfrom the NicoyaComplex,(2) testthe ideaof an crystallizationof similar parentalmelts. Nd and Pb isotopic
upper and lower serieswithin the Nicoya Peninsulausing ratios from the Nicoya Peninsula,Playa Jac6, and Queposare
4øAr-39Ardatingandgeochemistry,
and(3) betterconstrain similar but are distinct from normal mid-ocean ridge basalts
the origin of the Nicoya rockswith an improvedgeochemical
data set. In addition, some samples were collected from
coastalexposuresof oceanicbasaltsat Playa Jac6 and the
QueposPeninsulafor comparison
to the Nicoya rocks. We
eruptedon the East Pacific Rise. Altogether,thesedata do not
support the division of the Nicoya Peninsula igneous rocks
into two or more suites; rather, the rocks appear to be
genetically related and are part of the Caribbean oceanic
report4øAr-39Ar
radiometric
agesaswell asmajorelement, plateau.
trace element, and Sr, Nd, and Pb isotopic data from these
three localities. Our data show that rocks from the Nicoya
Peninsulaerupted at 88-90 Ma, which is contemporaneous
with other on-land Caribbeanplateausitesin Haiti, Curaqao,
and western Colombia [Sinton, 1996]. The geochemicaldata,
in conjunctionwith previouslypublisheddata, show that all
of the rocks collectedfrom the Nicoya Peninsulacome from a
Previous
Work
Donnelly [1994] summarizedthe previousstudiesof the
Nicoya Complex and the reader is referred to his paper for a
more thorough description of these. The majority of the
exposuresof igneousoceanicrocks and oceanicsedimentsare
SINTONET AL.:THE NICOYAPENINSULA
on the Nicoya Peninsula, and smaller exposuresare found
along the coast to the north, at the Santa Elena Peninsula,and
to the south, at Playa Jac6/PlayaHerradura,and the Quepos
Peninsula(Figure 1). For clarity, the rockswill be referredto
by their locality, so the term Nicoya in this paperwill refer to
the Nicoya Peninsula(samplesdesignatedNC in Table 1).
In general, previous authorshave subdividedthe Nicoya
Peninsula igneous rocks into two general units, termed the
Matapalo and Esperanza units by Kuijpers [1980] and
Bourgoiset al. [1984]. This division was originally basedon
fossil age and field observations [Schrnidt-Effing, 1979;
Baurngartner et al., 1984], although no contact between the
two units was defined [Kuijpers, 1980]. Later studiesby
Wildberg [1984] and Gurskyet al. [1984] referredto thesetwo
units as the Lower and Upper Nicoya, with the units separated
by a discontinuouslayer of radiolarianchert. However, there
are no observeddepositionalcontactsbetweenthe radiolarian
chert and the igneous rocks. This observation as well as
considerable differences in the interpretation of the
stratigraphyand origin of the rocks led Donnelly [1994] to
suggestthat the Nicoya Peninsulaigneous rocks may be a
single unit.
Previous radiometric ages of the Nicoya Peninsula rocks
have not been useful in constrainingtheir magmatichistory.
Alvarado et al. [1993] compiledK-Ar agesthat rangefrom 30
[Gradstein
et al.,
15,509
1994])
Sabana Grande
sediments
conformably overlie the igneous rocks [Bourgois et al.,
1984], placinga minimumage on eruptionand crystallization
of the magmas.This formationis a deepwaterassemblage
of
a sedimentary
basalt/radiolarian
chertbrecciabaseoverlainby
thin siliceousmudstones
gradinginto limestone[Bourgoiset
al. , 1984].
There are no reportedradiometricor fossil ages for the
Playa Jac6/Herradurarocks. The basalts at Quepos are
interbeddedwith Paleoceneforaminiferaand are overlainby
early Eocenesediments[Schmidt-Effing,1979;Baumgartner
et al., 1984], somewhatolder than the K-Ar reportedage of
46.2 + 4.4 Ma [Alvaradoet al., 1992 and referencestherein].
Sample Descriptionsand Mineralogy
Rock sampleswere collectedfrom the Nicoya Peninsula,
PlayaJac6,and Queposin March 1993. Samplelocationsare
shownin Figure 1, andbrief descriptions
are givenin Table 1.
Samplingwas concentrated
in the northernNicoya Peninsula
because
the goalwasto identifyandcharacterize
the proposed
oldernormaloceaniccrustin thisarea. The bestexposures
are
along the coast and road cuts. However, even in large
exposures,beddingor paleohorizontal
directionis impossible
to73Ma,reflecting
theeffects
ofalteration
and40Arloss
and to determineowing to faulting,surficialweathering,and the
not the actual time span of volcanism. There have been oftenlimitedoutcroparea. Someoutcropsshowspheroidally
several reports of fossil ages from the radiolarian chert weatheredrocks in which well-preserved,roundedclasts of
assemblagesthat occur as inclusionswithin the igneousrocks diabaseare foundin a matrix of heavilyweatheredand friable
throughout the peninsula. These assemblageshave been material. Pillow structures are rarely observed in the
assignedages of Early Jurassic(Callovian) to Late Cretaceous northwesternpeninsula.
(early Santonian)[Galli-Olivier, 1979; Bourgoiset al., 1984;
Fractured,massivebasaltflows exposedat the northernand
Baurngartner et al., 1984]. However, becauseof the intrusive southernendsof Playa Jac6were alsosampledas well as at a
relationshipsof the igneousrocks into the radiolarian chert, road cut on the coastroadjust southof Playa Jac6 (Table 1).
the fossil ages provide only a maximumcrystallizationage of
All the basaltsexaminedalongthe coastat Queposare heavily
the igneous rocks. The late Campanian (-75-70 Ma,
altered and only three samples with the least apparent
Table 1. CostaRicaSampleDescriptions
Sample
NC93-2
LocationDescription
RockType
Condition
massivebasalt in Rio Brasil, Pilas
mediumgrain
slightalteration,moderatealteration
NC93-3
exposedhill nearroadside,2 km westof Sardinal
basalt
medium-fine
slightalteration,
plagioclase
0-10%
NC93-11
smallroadcutacrossfromentranceto HotelPlaya
Azucar,spheroidalblocks
roadcut 3 km northof Tempate
NC93-13
NC93-14
northof El Ed6n,follow QuebradaSaltoto hill whererock
exposed,greatvariationin grainsizeovershortdistance,
NC93-18
quan'y2 km westof Coyolito,overlainby radiolarite,contact
grainbasalt
mediumgrain
basalt
diabase
gabbro
with contact unknown
inaccessible
mediumgrain
basalt
NC93-20
NC93-34
roadcut at top of Vista al Mar
NC93-26
Coastalexposure,southof Pl.ayaMonte del Barco(northof
PlayaPanama)
JC93-1
roadcut 1.2 km southof lastentranceto PlayaJac6, massive, mediumgrain
basalt
fracturedbasalt,no sediments,
20 m high
northendof PlayaJac6 Basaltcropsouton cliffsandflat
mediumgrain
basalt
lying on beach,similarto S end
southendof beach1 at Quepos,basaltin cliff andlarge
fine grainbasalt
"islands"
onbeach,massivewithcolumnar
joints,some
JC93-3
QP93-1
fine grainbasalt
diabase
northsideof PuntaSabana,
flat-lyingandcliff-exposed
basalt/diabase
andradiolarite,
basaltis massive,with
igneous
andfaultcontactw/radiolarite,primarybeddingof
radiolarite(redandgray/white)preserved,
cutby dikes
alteration and veins
plagiogranite
near veins
replacedby clays
slightalteration,plagioclase10%
replacedby clays
slightalteration,
someplagioclase
cores
beginningto alterto clays
altered,plagioclase
partiallyreplacedby
clays,someprimarytwinningseen,
graphicquartzandplagioclase
lightalteration,somequartzveins
light to moderatealteration
lightalteration,
plagioclase
0-25%
replacedby clays,depending
on
proximity to veins
altered,plagioclase
partiallyreplacedby
albite+ clay,pyroxenewith fractured
texturealteringto secondary
oxides
variablyaltered
variablyaltered,heavyveinpenetration
light to moderatealteration,somefresh
butpartiallyresorbed
plagioclase
phenocrysts
15,510
SINTON
ETAL.:THENICOYAPENINSULA
alteration were taken from the beachjust north of the entrance
to Manuel Antonio National Park (Table 1).
Thin sectionswere made of all of the samplesin order to
determine their mineralogy and suitability for geochemical
and geochronologicalanalyses. The rocks were divided into
five categories based on petrographic observations:basalt,
contamination
(4øAr/36Ar
= 295.5).Fromtheheating
steps,
agespectrum
diagrams
(ageversus% 39Arreleased)
were
generated (Figure 2).
Plateau ages were calculated from
consecutive
stepsthat are concordant
within 1(• errorusing
the proceduredescribedby Dalrympleet al. [1988], in which
stepageswere weightedby the inverseof their variance(Table
basaltic breccia, diabase,gabbro, and plagiogranite.Basaltic
2). Isotope
correlation
diagrams
(36Ar/4øAr
versus
39Ar/4øAr)
samplesare fine grainedand generallyaphyricand consistof
were also made for each analysis,in which the slopeis
plagioclase, pyroxene, Fe-Ti oxides, and altered mesostasis. proportional
to the age (isochron)and the inverseof the y
Only one basaltic breccia was collected (NC93-25), and it is
interceptgivesthe initial 4øAr/36Arcomposition.The
composed of a heavily altered matrix surrounding basalt
isochron
agesarecalculated
usingthe sameheatingstepsasin
clasts. Diabasic samples have the same mineralogy as the
the plateau ages.
basalts except that they are holocrystalline, are coatset
We definea reliablecrystallization
agebasedon the criteria
grained, and have trace amounts of tabular apatite.
of Pringle [1993]asfollows:(1) a well-defined
agespectrum
Spheroidally weathered samples are usually diabasic in
plateauof at least three concordantconsecutive
stepsthat
represents
at leastof 50%of thetotal39Arreleased;
(2) an
texture.
Isotropic gabbroic samples have the same
isochronage concordant
with the plateauage andwith a value
mineralogyas the basaltsand diabasesbut have grain sizesup
for theZ2-statistic
SUMS/N-2[York,1969]belowthe95%
to 10 mm. The plagiogranitesall have coarsetexturesand are
level;and(3) a 4øAr/36Arinterceptthat is
composedof myrmeketic plagioclase and quartz with lesser confidence
statistically indistinguishablefrom the atmosphericvalue
amounts of green clinopyroxene (hedenbergite) and Fe-Ti
295.5.
oxides. A cumulategabbro (NC93-100) containslarge (-4 cm
The SUMS/N-2 cutoffvaluesare takenfromPringle [ 1993,
diameter) unzoned plagioclase (An85_86), clinopyroxene
(Ens1) [Sinton, 1996], and trace olivine (completelyreplaced Table 2]. For three-, four-, and five-point isochrons,the
cutoffvaluesare 3.84, 3.00, and 2.60, respectively.
by iddingsite) set in a diabasic matrix of plagioclase,
Many of the samples
from the NicoyaPeninsula
displayed
clinopyroxene, and Fe-Ti oxides.
erratic
step
ages,
from
which
no
age
plateaus
or
isochrons
The rocks show varying degrees of alteration with most
could be discerned. This can be attributedto a combinationof
samples having relatively pristine pyroxene and relatively
alteration
effects,37Arand39Arrecoil,andthelowK20
fresh to partially altered plagioclase (to colorlessclays). In
the basalts, the mesostasis has altered to chlorite and fine-
grained secondary oxides. Thin veins of prehnite and rare
calcite are found in most of the samples,and the density of
veins generally correlateswith the degree of alteration. The
plagiogranitesare all moderatelyto heavily altered.
contentsof the rocks. Someof the plagioclaseseparates
(NC93-36 and NC93-9) gave step agesthat increasedwith
Results
increasingtemperature,
up to 180 Ma. Evidentlythey
containedsomecomponent
of excessradiogenic4øArof
whichevensmallamounts
canproduce
erroneous
ages,given
the low K20 content(<0.10%)of theserocks. Plagioclase
separatesfrom all but one of the analyzedplagiogranite
samples
gavenonconcordant
stepagesthatwereusuallyless
Geochronology
than 80 Ma. In this section,we reportonly the resultsfrom
the samplesthat yieldedreliablecrystallization
ages(Table
2).
Age determinations for whole rock and plagioclase
separatesfrom the least altered sampleswere performed at
Of 13 analyzed
samples
fromtheNicoyaPeninsula,
7 gave
reliablecrystallization
ages(Figure2). Five (fourbasaltsand
OregonStateUniversity(OSU) usingstandard
4øAr-39Ar one diabase)gaveplateauagesrangingfrom 88.0 to 92.5 Ma
incrementalheatingtechniques[Duncanand Hargraves, 1990;
Duncan and Hogan , 1994]. Whole rock sampleswere either
crushed in bulk in a ceramic jaw crusher and sieved to a
uniform 0.5-1 mm grain size or cut into mini cores.
Plagioclase crystals were magnetically separated from
diabase, gabbro, and plagiogranite samples, checked for
purity under a binocular microscope,and washedfor 30 s in
5% hydrofluoricacid. All sampleswereultrasonicallycleaned
in distilled water. Sampleswere sealedin evacuatedquartz
glassvials and irradiatedfor 6 hoursat the OSU TRIGA nuclear
reactorfacility. Neutron flux during irradiationwas monitored
by FCT-3 biotite (27.7 Ma, [Hufforal and Hammerschmidt,
1985]). Ar isotopic compositionsof the crushedwhole rock
samplesand someof the plagioclaseseparates(-•500 mg) were
determined using an Associated Electrical Industries (AEI)
MS-10S mass spectrometer. The mini cores and the other
plagioclaseseparates(-•100 mg) were analyzedusing a Mass
AnalyzerProducts(MAP) massspectrometer,
equippedwith a
(a weightedmeanof the agesis 88.5 Ma anda nonweighted
meanis 90.1 Ma). Of particularinterestis the diabaseNC93-
34 (PuntaSabana),
whichwastakenneara largeinclusion
(50
m) of radiolarianchert. The plateauage of 88.0 + 0.7 Ma
indicatesthe age of intrusionand confirmsthat magmatism
occurredwell after the Jurassic-EarlyCretaceousradiolarian
chert. Plagioclase
separated
froma gabbro(NC93-14B)anda
plagiogranite
(NC93-26)gave statisticallyindistinguishable
plateau ages of 83.8 + 1.1 Ma and 83.2 + 1.3 Ma,
respectively. The significantlylower agesof the intrusive
rocksrelative to the analyzedbasaltsand diabaseindicatethat
somemagmaticactivityoccurredat least 5-7 m.y. after initial
basalt eruptionsat 88-90 Ma.
One of two analyzedbasaltsfromJac6(JC93-1)yieldeda
reliablecrystallization
agewith a three-step
plateauof 84.2 +
1.8 Ma, an isochron of 83.4 + 2.5 Ma, and an initial
4øAr/36Ar
ratioof 295.6+ 6.9(Figure
2). Thelowerinitial
step age indicatessomeradiogenic4øAr loss from less
low-blank, double-vacuum resistance furnace.
retentive (alteration) sites. The consistentplateau and
Individual
agesforeachof the4øAr-39Ar
temperature
steps isochron ages of this sample indicate a volcanic event
were calculated after corrections for background, mass youngerthan the eruptionof the Nicoya Peninsulabasaltsbut
fractionation, isotopic interferences,and atmosphericargon
perhapsrelated to the intrusions.
SINTON ET AL.: THE NICOYA PENINSULA
o
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•-•
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15,511
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SINTONETAL.:THENICOYAPENINSULA
Table2. The4øAr-39Ar
Plateau
andIsochron
Ages
forBasaltic
Rocks
FromtheNicoya
Complex,
Costa
Rica
Sample
Material
Plateau
Age 39Ar
Percent Isochron
Age,m.y.
by 1/02,m.y
ofTotal
N
4øAr/36Ar
Intercept
_+
10
SLIMS
N-2
Nicoya PeninsulaBasalt and Diabase
NC93-2
NC93-3
NC93-18
NC93-20
NC93-34
whole rock
whole rock
whole rock
whole rock
whole rock
90.3
92.5
89.7
89.9
88.0
+
+
+
+
+
3.2
5.4
1.7
2.8
0.7
100
100
100
100
97
NC93-14B
NC93-26
plagioclase
plagioclase
83.8 + 1.1
83.2 + 1.3
JC93-1'
wholerock
84.2+ 1.8
92
QP93-1
?
whole
rock
63.9+ 0.5
51
87.8 +
86.8 +
92.3 +
85.9 +
86.4 +
0.3
7.4
3.0
3.3
1.3
4
5
5
5
5
296.3
302.0
294.0
299.7
308.1
+
+
+
+
+
2.6
6.6
4.4
6.7
5.5
1.00
0.52
0.47
0.16
0.29
Nicoya PeninsulaIntrusives
100
80.9 + 1.1
100
83.2 + 2.0
7
7
301.5 + 1.1
295.2 + 2.3
2.00
2.20
83.4+ 2.5
3
295.6+ 6.9
0.08
60.1+ 0.7
3
376.7
+ 20.7
0.08
Ages
were
calculated
using
thefollowing
decay
constants:
•.e=0.581
x3910-l0
yr-1,
•.•=4.963
x 410-løyr
-1.Isotopic
interferences
forthe
36
37
-4
37
40
39
Oregon
State
University
TRIGAreactor
are( Ar/ Ar)ca= 2.64x 10 '' ( Ar/37Ar)ca= 6.73x 10-, and( Ar/ Ar)K=1.00x 10-3. Errors
for
bothstepageplateaus
andisochrons
arereported
to 2•. Agesarecorrected
for Ar decay,half-life= 35.1days.N referstothenumber
of steps
usedin theisochroncalcuation.SUMS/(N-2) is thegZ-statistic
from York[1969].
*Sample
from
Playa
Jac6.
?Sample
from
Quepos.
Only one Quepossample(QP93-1) was consideredsuitable powderedsamplesgave higherTa values. Accordingly,Ta
for analysis,althoughit is moderatelyalteredand very fine concentrations
for WC-powderedsamplesare not reported.Pb
grained. A three-step(of six) plateaugave an age of 63.9 +_ concentrationswere reliably measuredin only some of the
0.5 Ma, although the isochron age derived from the same samples. ICP-MS analysesof the basaltstandardW-1 agree
with acceptedvalues [Sinton, 1996].
steps is slightly younger (60.1 +_0.7 Ma) and the initial Ar
Someof the major and traceelementdata are listedin Table
isotopic composition is considerably higher than
atmospheric(376.7) which is an artifact of fitting only three 3, and the completedatasetis reportedby Sinton [ 1996]. The
points(Figure 2). The youngeragesof the highertemperature basalts,diabases,and gabbrosfrom the Nicoya Peninsulaand
stepscanbe attributed
to recoilof 39Atduringirradiation,
a Jac6 have tholeiitic compositions with MgO contents
problem often encounteredwith very fine grained samples.
We considerthe plateauage to be the bestestimateof the time
of crystallization of the Quepos basalts,and it is consistent
with the Paleocene age of sediments intercalated with the
basalts[Schrnidt-Effing,1979; Baurngartncret al., 1984].
ranging from 10.7 to 4.0%. The plagiogranites have
andesiticto dioritic compositions
(52 - 69% SiO2). The two
analyzedQueposrocks are transitionalto alkalic basalts.
Our major and traceelementdata and thosefrom previous
studies[Wildberg, 1984;Frischet al., 1992] from the Nicoya
Peninsulaare plottedon severalvariationdiagramsof MgO
Mineral
Chemistry
Analyses of pyroxene and plagioclase are reported by
Sinton [1996]. The trend of the pyroxenesis roughly similar
to that of the Skaergaardintrusion clinopyroxenes[e.g., Deer
et al. , 1992] and is consistent with the Nicoya Peninsula
magmas being derived from fractional crystallization of
tholeiitic parental melts.
Major
and Trace Element Geochemistry
All the sampleswere analyzed for major elementsand Ni,
Cr, Sc, V, Sr, and Y by X ray fluorescence (XRF) at
Washington State University using a Rigaku 3370
spectrometer[Hooper et al. , 1993]. A subsetof the samples
was analyzed for Rb, Y, Zr, Nb, Ba, Hf, Ta, Pb, Th, U, and the
rare earthelements(REE) usinga V.G. Plasmaquadinductively
coupled plasma mass spectrometer(ICP-MS) at Oregon State
University [Pyle et al. , 1995]. Sampleswere powderedfor the
XRF and some of the ICP-MS analyses using a tungsten
carbide (WC) mill. For some of the ICP-MS analyses,cleaned
rock chipswere powderedin a ceramicmortar. There were no
apparentdifferencesbetweenthe ICP-MS resultsfrom sample
duplicatesusing the two preparationmethods,exceptthe WC-
versus major elements and the relatively immobile trace
elements(Figure 3). Elementsthat are affectedby alteration
and are present in low abundances,such as K and Rb, show
considerablescatter and are not plotted. These plots, in
additionto illustratingthe wide rangein MgO contentfor the
Nicoya rocks, demonstratea single coherent trend for all
samples. A first-order interpretationof this trend is that the
rocks are derived from similar parent magmasby fractional
crystallization. The Jac6 rocks fall along the Nicoya trend
while the Queposbasaltsshow distinctly different trendsfor
some of the elements [Sinton, 1996].
The role of fractional crystallizationin the evolutionof the
Nicoya Peninsularocks can be assessedusing the variation
diagrams. The decreasingconcentrationsof certain elements
or element ratios with decreasingMgO content reflect the
crystallizationand removalof particularminerals. All of the
major elementvariationdiagrams,with the exceptionof those
for FeO and TiO2, show continuous,roughly linear trends.
FeO and TiO2 both increasewith decreasingMgO until -4%
MgO, at which point they decrease.This inflectionis likely
due to the appearanceof Fe-Ti oxides on the liquidus. At
greater MgO contents, there are no obvious inflections in the
trend and therefore the fractionating mineral assemblage
SINTON ET AL.: THE NICOYA PENINSULA
15,513
Table 3. Major andTraceElementAnalysesfor the CostaRica IgneousRocks
NC93-2
NC93-3
NC93-11
NC93-13
NC93-14B
NC93-18
NC93-20
NC93-26
SiO2
AI203
TiO2
50.18
13.58
1.68
50.50
14.44
1.04
50.05
14.27
1.00
50.47
14.26
1.26
51.43
13.38
1.87
50.83
13.99
1.03
50.6
14.34
0.98
FeO
13.31
10.55
10.38
10.77
13.46
10.25
9.65
MnO
0.23
0.21
0.18
0.19
0.21
0.19
0.19
CaO
10.57
12.66
12.11
12.35
9.11
11.66
NC93-34
63.62
11.18
1.02
JC93-1
51.19
14.72
1.01
51.38
14.63
1.13
13.51
9.44
0.16
0.20
12.54
4.06
12.01
JC93-3
QP93-1
49.57
14.43
1.23
49.44
13.29
2.94
9.86
11.56
11.77
0.18
0.22
0.19
12.19
10.89
7.68
MgO
6.59
8.82
8.93
8.26
5.51
8.82
8.73
0.39
8.55
8.79
8.68
7.33
K20
Na20
P205
0.11
2.73
0.13
0.07
1.59
0.07
0.07
1.86
0.07
0.10
2.33
0.09
0.14
3.11
0.14
0.10
2.13
0.07
0.06
1.82
0.07
0.14
5.74
0.29
0.38
2.54
0.07
0.05
2.08
0.08
0.13
2.77
0.09
0.87
5.38
0.27
L.O.I.
1.56
3.31
3.29
2.11
4.42
3.59
3.89
1.20
2.90
3.26
3.64
2.83
Total
100.67
103.26
102.20
102.19
102.79
102.66
102.87
101.31
103.01
103.62
103.21
102.00
Ni
48
91
84
87
18
93
106
0
100
88
97
120
Cr
70
288
200
242
8
284
209
I
296
311
202
269
$c
V
46
52
55
49
47
50
50
23
53
54
52
34
406
319
323
330
415
324
319
2
333
335
343
383
83
Sr
140
171
100
146
298
218
178
Rb
1.10
0.54
0.42
1.31
2.17
0.47
0.80
139
87
76
275
3.23
0.41
0.87
11.78
Y
34
22
15
21
34
16
15
17
18
21
36
Zr
107
58
31
43
69
49
44
39
50
55
194
19.0
Nb
5.8
3.4
3.0
4.3
7.2
3.3
3.0
3.5
4.5
3.4
Ba
20
10
11
66
41
31
7
66
12
22
159
La
3.99
2.31
1.98
3.20
5.16
2.16
2.15
2.43
2.73
2.73
13.54
Ce
11.43
6.89
5.40
8.50
13.73
5.87
5.59
6.44
7.46
7.26
34.14
Pr
1.93
1.13
0.91
1.43
2.24
0.96
0.91
1.07
1.21
1.20
4.92
Nd
9.99
5.83
4.52
7.13
11.42
4.95
4.76
5.30
5.95
6.24
22.58
6.00
Sm
3.39
1.92
1.55
2.33
3.68
1.69
1.60
1.77
1.89
2.00
Eu
1.20
0.72
0.59
0.87
1.34
0.65
0.61
0.68
0.71
0.75
1.99
Gd
4.19
2.65
2.16
3.19
4.59
2.26
2.15
2.30
2.56
2.64
6.77
Tb
0.89
0.51
0.41
0.60
0.95
0.46
0.43
0.46
0.47
0.52
1.21
Dy
5.35
3.27
2.65
3.71
5.64
2.79
2.62
2.82
3.11
3.34
6.53
Ho
1.22
0.75
0.59
0.82
1.19
0.62
0.56
0.62
0.68
0.76
1.28
Er
3.38
2.17
1.51
1.92
3.30
1.64
1.51
1.56
1.77
2.01
2.96
Tm
0.52
0.33
0.25
0.32
0.47
0.27
0.24
0.26
0.30
0.34
0.48
Yb
3.34
2.03
1.44
1.81
3.19
1.60
1.55
1.54
1.72
2.06
2.62
Lu
0.51
0.31
0.23
0.31
0.51
0.23
0.24
0.26
0.29
0.32
0.41
Hf
2.98
1.57
0.94
1.36
2.22
1.29
1.18
1.14
1.29
1.52
4.62
Ta
0.23
0.18
0.29
0.20
0.18
0.23
0.30
0.23
1.24
Pb
0.32
Th
0.38
0.27
0.11
0.18
0.31
0.14
0.16
0.20
0.28
0.26
1.38
U
0.11
0.04
0.03
0.04
0.09
0.05
0.06
0.05
0.07
0.07
0.34
Majorelements
andSc,Cr, Ni, V, andSrdetermined
by X rayfluorescence;
all othertraceelements
weredetermined
by
inductivelycoupledplasmamassspectrometry.
remained relatively constant.
The fractionation of
clinopyroxene is evident from the continuous decreasein
CaO, Sc, and Cr with decreasingMgO. The decreasein A1203
and Ni can be attributed to the removal of plagioclase and
olivine, respectively. Together, the variation diagrams are
consistent with the removal of clinopyroxene, plagioclase,
and olivine from --9% MgO to --4% MgO, at which point Fe-Ti
oxides join the fractionatingassemblage.
Petrographic observationsare generally consistentwith
this interpretation. As previouslystated,all samples(except
NC93-100) are aphyric, and petrographicobservationsare of
limited help in determining the fractionating mineral
assemblage. However, the cumulate gabbro NC93-100
containslarge crystalsof clinopyroxene(Ens1), plagioclase
(An85) [Sinton, 1996], and what appear to be olivine
pseudomorphs set in a basaltic matrix.
This cumulate
assemblagemay representthe early crystallizing phasesof
the Nicoya suite.
The major and trace elementdata indicatethat the Nicoya
Peninsulaplagiograniteswere derivedfrom the more primitive
15,514
SINTONET AL.: THE NICOYA PENINSULA
20
AI 2
(wt. %)
ß
'16
I•
•2/
•
•
•
10
••• I I•
I
TiO• (•t. %)
2
•
•
6
4
2
Cr (ppm)
Sc (ppm)
6O
[]
-.
500
.
400
50
40
_ 300
3O
.
200
20
100
10
o
i
i
i
i
i
120
•• Y(ppm)
400
[]
lOO
[]
[]
ß
300
Zr (ppm)
•:
•n
[]
[]
0
I
0
2
I
4
I
6
I
8
I
l0
12
I
I
2
4
200
[]
100
0
6
8
10
12
MgO (wt. %)
MgO (wt. %)
Figure3. Variationdiagrams
of somemajorandtraceelements
versus
MgO for theNicoyaPeninsula
rocks
(solidcircles)usingdatacompiled
fromthispaper,Sinton[1996],Wildberg[1984],andFrischet al. [1992]and
EastPacificRiseMORB (opensquares)
from13øSto 5øN[Langrnuir,
1988].
tholeiitic
basalts.
This is consistent with a model described
by Wildberg [1987], in which the plagiogranitesare derived
by a two-stage processof fractional crystallizationand later
filter pressing of a tholeiitic magma. The latter process
leaves a residual Fe-rich diabase/gabbro,such as samples
NC93-7
the Nicoya rocks, the MORB data show an inflection in CaO
andScbetween
6% and8% MgO,indicating
theappearance
of
clinopyroxeneon the liquidus at that time and thus low-
pressure fractionation.
As discussed previously,
clinopyroxene
in the Nicoyasuiteappears
to havebeenpart
and NC93-15.
of the fractionatingmineralassemblage
from at least -9%
MgO.
Because
the
primary
effect
of
high-pressure
fractional
Comparison to Mid-Ocean Ridge Basalts
crystallization is to increase the temperature at which
The Nicoya Peninsularocks are comparedto mid-ocean clinopyroxene
appearson the liquidus[e.g., Benderet al.,
ridge basalt (MORB) glassesfrom the East Pacific Rise from 1978; Grove et al., 1992;Langmuiret al., 1992], fractional
13øSto 5øN [Langrnuir, 1988] in Figure 3. The MORB data crystallization
of the Nicoyamagmasappearto haveoccurred
show relatively coherenttrends for all elements;these trends
are distinct from the Nicoya suite for most elements. Unlike
at pressuressignificantlygreaterthan that for MORB.
To examine whether the differencesbetween the MORB and
SINTON ET AL.: THE NICOYA PENINSULA
15,515
100
.........................
•...-z:•.'•................................................................
!aS•••
ra•n•
t•[
abases
1
I
I
I
I
I
I
I
I
I
I
I
I
I
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Figure 4. C1 chondrite-normalized
rareearthelementdiagrams.Basaltanddiabasedataarefrom thispaperand
Sinton [1996]. Plagiogranite
data are from Wildberg [1984]. Chondritevaluesare from McDonoughand Sun
[1995].
Nicoya magmasare a resultof differencesin the depthof
fractional crystallization(from parental melts of similar
majorelementcomposition)
or of differences
in parentalmelt
composition,the two data sets are comparedat 9% MgO.
Both data setshavecomparable
CaO and FeO, but A1203,
Summarizing the major and trace element results, the
Nicoya Peninsula rocks appear to be related by fractional
crystallizationfrom similar parental melts. The Jac6 basalts
are compositionallysimilar to the Nicoya rocks while the
Quepos basalts are distinct as well as significantly younger.
which behavesincompatiblyand is depletedfrom the mantle The Nicoya rocks are compositionallydistinct from MORB
during partial melting [e.g., Forsyth, 1992], is higher in the lavas in terms of some major and trace elements. These
MORB magmas. Other incompatibleelementsin the Nicoya differencesmay be partly attributedto a differencein the first
suite, such as TiO2, Zr, and Y, are also slightly to
appearanceof clinopyroxeneon the liquidus,possiblydue to a
differencein the pressureat which crystallizationoccurred,as
significantly lower than in the MORB suite at 9% MgO
well as differences in the degree of partial melting of the
(Figure 3). These differences cannot be attributed to
differences in the fractionating mineral assemblage. Rather, source mantle. The flat REE patterns of the Nicoya rocks
they must reflect a depletionof the Nicoya parentalmagmas indicate a sourcethat less previousmelting than the depleted
relativeto MORB parentalmagmas. Sucha depletioncouldbe upper mantle that is the sourcefor normal MORB, and this is
confirmed by the isotopic analyses.
causedby higherdegreesof partialmeltingof the mantle.
Rare
Earth
Elements
Figure 4 showsthe chondrite-normalized
rare earth element
analyses. All show relatively flat, parallel patterns,
consistent with the Nicoya Peninsula rocks belonging to a
single,cogeneticsuite. All of the Nicoya rocks(includingthe
matrix of the NC93-100 cumulategabbro) have slightly light
rare earth element(LREE) -depletedto slightly LREE-enriched
chondrite-normalized REE patterns (La/Yb n = 0.77-1.21,
Ce/Ybn = 0.85-1.23) with the absoluteabundances
of the REE
increasingwith decreasingMgO. The two analyzedJac6rocks
also have relatively fiat REE patterns(LaJYbn = 0.90-1.08)
which, along with the variation diagrams,suggeststhat they
and the Nicoya Peninsula rocks were derived from similar
parental magmas. The fiat patterns are different from the
LREE-depleted patterns displayed by most normal MORB.
The Quepos basalt has a LREE-enriched pattern (LaJYbn=
3.51). This observation and the differences in the variation
diagramsand the younger age demonstratesthat the Quepos
rocks were derived from different parental magmas than the
Nicoya and Jac6 rocks.
Sr-Nd-Pb
isotopic
Ratios
Six of the freshestNicoya Peninsulasamplesas well as two
Jac6 basaltsand one Queposbasaltwere analyzedfor Sr, Nd,
and Pb isotopes,and the data are presentedin Table 4. Both
measuredand age-corrected
initial ratiosare reported. Sr, Nd,
and Pb isotopic ratios were determinedby M. Gorring at the
Departmentof GeologicalSciences,Cornell University, using
a V.G. Sector 54 mass spectrometer[White and Patchett,
1984; White and Duprd, 1986]. Approximately 500 mg
aliquotsof rock were leachedfor 2 hoursin 6 N HC1 prior to
dissolution. A separatealiquot of NC93-20 was not leached
for comparison. The leachedand unleachedaliquotsof sample
NC93-20 did not give significantly different results(Table 4).
Replicateisotopicanalysesfor Sr and Pb ratios of someof the
sampleswere performedon separatesampledissolutionsand
show excellent reproducibility(Table 4).
The 87Sr/86Srratios showconsiderable
scatter,while the
143Nd/144Nd
ratiosarerelativelyconstant
for all of theCosta
Rica samples(Figure 5). It is possiblethat the original Sr
isotopic signature of the rocks has been skewed to more
15,516
SINTONET AL.:THE NICOYA PENINSULA
Table4. Sr,Nd, andPbIsotopic
Analyses
forBasaltic
RocksfromtheNicoyaComplex,CostaRica
87Sr
Sample 86Sr
m
87Sr
143Nd
143Nd
206pb
206pb
207pb
bm 204p
bi 204p
bm
86Sr
i 144Nd
m 144Nd
i end/ 204p
NC93-20u
0.70346
0.70345
............
NC93-20
0.70341
0.70340
0.512967
0.512843
6.26
207pb
208pb
208pb
204pb
i
204pb
rn
204
Pbi
19.23
19.16
15.56
15.56
38.87
38.81
19.28
19.21
15.59
15.58
38.97
38.90
NC93-18
0.70393
0.70392
0.512990
0.512863
6.65
19.28
19.02
15.58
15.57
38.93
38.71
NC93-34
0.70450
0.70442
0.512977
0.512853
6.45
19.62
19.12
15.63
15.61
39.39
38.75
NC93-13
0.70329
0.70326
0.512981
0.512860
6.59
19.34
19.15
15.63
15.62
39.14
38.84
NC93-11
0.70316
0.70314
0.513014
0.512886
7.10
19.68
19.22
15.60
15.58
39.47
38.89
NC93-3
0.70361
0.70360
0.512985
0.512863
6.65
19.33
19.20
15.60
15.60
39.08
38.81
NC93-3R
0.70346
0.70344
19.26
19.13
15.57
15.56
38.91
38.64
JC93-1
0.70347
0.70345
19.37
19.21
15.58
15.57
39.04
38.84
JC93-1R
0.70344
0.70342
19.34
19.19
15.56
15.55
38.98
38.78
JC93-3
0.70313
0.70310
19.38
19.06
15.60
15.58
39.06
38.69
19.40
19.09
15.60
15.59
39.12
38.75
19.43
19.21
15.61
15.60
39.06
38.77
19.40
19.18
15.58
15.57
38.94
38.66
JC93-3R
QP93-1
QP93-1R
............
0.512955
0.512848
6.36
............
0.512987
0.512878
6.94
....................
0.70460
0.70450
0.512916
0.512850
6.40
....................
All samples
wereacidleached
priortodissolution
except
NC93-20u.The"m"subscript
denotes
measured
values;
the"i"subscript
denotes
values
agecorrected
usingconcentrations
frominductively
coupled
plasma
mass
spectrometer
analyses
and90MafortheNicoyarocks,
84Ma
fortheJacorocks,
and64Ma fortheQuepos
sample.Replicates
aredenoted
with"R"andarecomplete
replicates,
thatis,theyareanalyses
of
separaterocksamplealiquots.
radiogenic values by Sr addition, consideringthat the most
altered of the analyzed samples (QP93-1) has the highest
Discussion
87Sr/86Sr.However,thisrelationship
is notclear,giventhat A Single Magmatic
sample NC93-34 from the Nicoya Peninsula is virtually
unalteredand its high 87Sr/86Srratio cannotbe readily
explained by seaflooralteration. This samplewas taken near
a large intrudedbody of radiolarianchert, so it is possiblethat
the Sr isotopic signature has been increased by the
assimilation of pelagic sediments. Considering these
possiblecontaminationeffects, Sr isotopescannot be used as
Suite
Ideally,the interpretation
of the petrogenetic
historyof a
suiteof igneousrocksshouldbe basedon the integrated
observations
from fieldwork,petrography,
geochronology,
and chemistry. In westernCosta Rica, individual flow units
are virtually impossibleto identify and stratigraphic
relationships between the different outcrops are
reliable indicators of mantle sources in this suite.
unconstrained,
so we mustrely moreon the geochemical
and
The contaminationfrom pelagic sedimentsthat is evident agedatain interpreting
themagmatic
historyof thesuite. We
from the Sr isotopesis not apparentin the Nd and Pb isotopes havemadea casethatthe rocksof theNicoyaPeninsula
are
(nor, for that matter, in the trace element data). This is not
unexpected for Nd, as Nd concentrationsin seawater and
pelagic sediments are very low. The range of initial
part of a singlesuiteof variablydifferentiatedrocksderived
from similarparentalmagmas.On the basisof the 4øAr-39Ar
studies,the magmascrystallizedat 88 to 90 Ma while some
143Nd/144Ndratios for the samplesis small (0.51285- intrusive
magmatic
activityoccurred
at 84 Ma. Theprevious
designations
of a lower unit of normalmid-ocean
ridge
0.51289). The Pb isotopic ratios also show a restrictedrange
and show no evidence of a mixing trend toward pelagic
sedimentPb isotopic compositions[Doe , 1970] (Figure 5).
On 2ø6pb/2ø4pb vs. 2ø7pb/2ø4pb and 2ø6pb/2ø4pb vs.
generatedcrustand an upperunit partiallymadeof oceanic
plateau lavas [Schmidt-Effing, 1979; Kuijpers, 1980;
Bourgoiset al., 1984;Gurskyet al., 1984;Wildberg,1984;
2ø8pb/2ø4pb
diagrams,
theage-corrected
CostaRicarocksplot Frisch et al., 1992] appearto be untenable.
in a group that is more radiogenic than present day East
The primitivearc seriesproposed
by Wildberg[1984]and
Pacific Rise basalts. Consideringthe analyticalerror, the Pb
and Nd isotopic ratios of all of the analyzed samplesare not
appreciably different, although NC93-11 and NC93-34 have
Frisch et al. [1992] was based on the observationthat some
rockshaverelativelylow concentrations
of certainhighfield
strength
elements
(HFSE)(e.g.,Zr andTi) andsomehavehigh
ratios,whichcanbe a characteristic
of arc lavas.
2ø6pb/2ø4pb
and2ø8pb/2ø4pb
ratios considerably
more 87Sr/86Sr
radiogenicthan the others. Together,the Nd and Pb isotopic
data show that the mantle sourcesfor the Nicoya Peninsula,
Jac6, and Quepos magmas were similar. These sourcesare
enriched relative to the depleted, upper mantle source for
normal MORB
from the East Pacific Rise.
However, these studies did not note that the low HFSE lavas
alsohaverelativelyhighMgO contents
andthe depletion
of
HFSE (as well as otherincompatible
elements)is an effectof
dilution (that is, less fractionatedsampleshave lowest
absoluteincompatibleelementconcentrations).As we have
SINTON
ETAL.:THENICOYAPENINSULA
0.5132
¸¸
o Nicoya
• ß
0.5131
15,517
0
0.5130
-{-Quepos
ß&
ßß•,ß
•
•
ß Galapagos
o E Pacific
MORB seawater
•teration
ß
•
or•lagic
sediment
-- .
•::::::•::•
•::•
::::::::::::::::::::::::::::::::::
:•::::::::::•::
:::::::::::::::::::::::::::::::::::
:::::::::::::::::::::::::::::::::::::
:::::::•:::
:::::::•::::
:::::::::::::::::::::::::
0.5129
0.5128
0.7020
0.7025
0.7030
0.7035
0.7•0
0.7•5
0.7050
87Sr/86Sr
0.5132
0.5131
0.5130
ß ß
0.5129
0.5128
17.8
I
I
I
18.3
18.8
19.3
19.8
206pb/204pb
15.70
15.65
-
15.60
-
15.55 - 15.50 -15.45 - -
I
15.40
17.8
'
18.3
I
I
18.8
19.3
ß
19.8
2O6pb/2O4pb
39.4
38.6
38.2
¸ o
37.8
37.4
17.8
I
I
I
18.3
18.8
19.3
19.8
206pb/204pb
Figure5. Sr,Nd, andPbisotopic
ratiocorrelation
plots.Typicalanalytical
uncertainties
areindicated
by the
sizeof thesymbols.
EastPacificMORBdataarefromWhiteet al. [1987]andPrinzhofer
et al. [1989]. Pelagic
sediment
valuesfromDoe [ 1970]. C is thecommonplumecomposition
reportedby Hananand Graham[1996].
NC93-34is an unaltered
diabase
thatmayhaveincorporated
radiogenic
Sr frompelagicsediment
contamination.
15,518
SINTONET AL.:THENICOYAPENINSULA
shown, 87Sr/86Srratios cannotbe reliably used as a
geochemicaldiscriminantwith the Nicoya lavas and therefore
conclusionsbasedon theseare suspect.
We cannot definitively state that all of the pre-Tertiary
rocks of the Nicoya Peninsula are from the same Late
Cretaceous
suite.
The radiolarian
chert must have been
depositedon oceaniccrustthat is at leastof Jurassicage (H.-J.
Gursky, personal communication, 1994). What we have
shown is that the rocks that have thus far been analyzedare
not
Jurassic
MORB.
We
consider
that
the
blocks
of
radiolarian chert were either entrained by massive submarine
Relationship
to the Caribbean
Oceanic Plateau
It has been previously suggestedthat a portion of the
Nicoya Complex is related to the thick CretaceousCaribbean
crust [e.g., Wildberg, 1984], which is consideredto be the
remnant of an oceanic plateau produced by initial, massive
volcanismover the Galapagosplume [Duncanand Hargraves,
1984]. This is supportedby the fact that the -90 Ma agesof
the Nicoya Peninsulabasaltsfall within the narrow range of
radiometric ages from basaltic rocks in Haiti, Cura•jao,
Colombia,and GorgonaIsland [Sinton, 1996]. The relatively
flat REE patternsand Nd and Pb isotopicratios of the Nicoya
lava flows or heavily penetratedby sills and dikes. Implicit
in this conclusionis that the Late Cretaceousmagmaserupted and Jac6 rocks are also similar to some of the Caribbean
in an intraplate setting through preexisting oceanic crust. plateau lavas [Sinton, 1996]. Thus we conclude that the
Nicoya Peninsula rocks are one of many pieces of the
Some of the older oceanic igneous crust over which the
Caribbeanplateauexposedby collisionaltectonicsaroundthe
radiolarian chert was deposited may be found to be
marginsof the Caribbeanplate.
cornmingled with the plateau lavas described here by
Sr, Nd, and Pb isotopicratiosare excellenttracersof mantle
continued sampling and analysis.
source
composition because they remain unchanged during
Eruption through preexistinglithosphereprovidesa ready
model to explain the greater depth of crystal fractionation, partial melting and fractional crystallization. Differences in
comparedto that at which normalMORB magmasare pooled. these ratios among magmascan be ascribedto the mixing of
Becausethe lithosphericthicknessesat mid-oceanridges are magmas or mantle sources. The Nd and Pb isotopic ratios
negligible, melts pool at shallow depths (1-2 km below indicate that the Nicoya rocks and probably the Jac6 and
seafloor). In contrast,the parentalmelts of the Nicoya suite Quepos basalts were derived from similar mantle sources
which are more enrichedthanthe depleteduppermantlefrom
may have pooled beneath the pre-existing, perhaps 20-50
whichnormalMORB are derived. The isotopicratiosplot
m.y. old oceanic lithosphere, resulting in fractional
within
the field of presentday Galapagoslavas [White et al.,
crystallizationat depthsexceeding20-30 km below seafloor.
1993], consistentwith derivationfrom the Galapagosmantle
However, we cannot discount the possibility that the
described
by isotopic
relatively early onset of clinopyroxene crystallization was plume. In termsof mantleend-members
due to a greater diopsidecomponentin the Nicoya parental composition[e.g., Zindler and Hart, 1986], the sourcefor the
Costa Rica rocks could be interpretedas a mixturebetween
magmasrelative to thoseof normal MORB.
depleteduppermantle(DMM) andan enriched(HIMU) source.
Relationship of Jac6 and Quepos Lavas
to the Nicoya
Peninsula
Suite
In the field, the Jac6 rocks are identical to the Nicoya
Peninsula suite; massive, faulted, and fractured basalt flows;
but the one radiometricage indicatesthat magmatismoccurred
at 84 Ma. It is not clear at this point whether volcanismin
this location was continuous from 88 to 84 Ma, but it is
possiblethat the Jac6 basaltand 84 Ma Nicoya intrusivesare
related to a common magmatic phase, based on their
similarity in age and composition. The compositional
similarity between all of the Nicoya and Jac6 rocks indicates
that there was no change in mantle source or conditions
between 90 and 84 Ma.
The Quepos basalts are considerably younger than the
Nicoya and Jac6 suite and have major and trace element
compositionsthat reflect smaller degreesand perhapsdeeper
partial melting. It is possiblethat the Queposlavasrepresent
an accretedseamount(M. Meschede,personalcommunication,
1994) that originatedon the subductingFarallon (now Cocos)
plate. As indicated by the isotopicratios, the Queposlavas
appearto have been derived from a mantle sourcesimilar to
that of the Nicoya and Jac6 rocks. If this is the case, then
there must have been a long-term,active thermal anomalyin
the area. An attractivecandidateis the Galapagoshotspot(see
following section). It is likely that the Queposlavas were
part of an older, now subductedextensionof the aseismic
CocosRidge, which was producedby the northeastmotion of
the Farallon plate over the Galapagoshotspot[Hey et al.,
1977].
Alternatively,the dataplotnearor withinthe generalrangeof
a common mantle source(FOZO, [Hart et al., 1992]; PHEM,
[Farleyet al., 1992];C, [Hananand Graham, 1996])(Figure
5). Accordingto Hanan and Graham [1996], this common
mantle sourcemay be located in the transitionzone between
theupperandlowermantleor at thecore-mantle
boundary.
Melting
Conditions
The mantlemeltingconditionsthat producedthe Nicoya
lavas can be constrainedusing the compositionaldata.
However,most mantlemeltingstudiesusinggeochemistry
have focusedon mid-oceanridge environments
[e.g., Klein
and Langmuir , 1987; Johnson et al. , 1990] and not on
oceanicplateaus. Partial melting of the mantle below mid-
oceanridgesappearsto be controlledby polybaricmeltingof
a columnof mantlewith poolingof discretemeltpacketsfrom
different depths[e.g., Klein and Langmuir, 1987;Johnsonet
al. , 1990; Grove et al. , 1992]. It is likely that a similar
process,but operatingon a largerscale,producedthe Nicoya
lavas and oceanicplateausin general. Oceanicplateaus,
which have characteristics
indicativeof rapidly constructed
submarineflood basaltprovinces,are probablyproducedby
relatively high degreesof partial melting [Storeyet al.,
1991; Mahoneyet al. , 1993], and so the generalmelting
regimewas significantlylargerand moreephemeralthanthat
of mid-ocean ridge melting. The depletion of some
incompatibleelementsin the Nicoya rocksrelativeto MORB
cannotbe attributedto a moredepletedmantlesource,given
thattheREE andisotopicdataindicatea relativelyundepleted
SINTONETAL.:THENICOYAPENINSULA
15,519
mantle source. Therefore the depletionsare best explained by
higher degreesof partial melting of the mantle.
Higher degrees of melting are also indicated by the highMgO lavas that are found in the SantaElena Peninsula(low-Ti
basalt of Wildberg
[1984]) and to the north of Playa
References
Jac6/Herradura [Alvarado et al., 1997]. An 4øAr-39Ar
Alvarado, G. E., P. Denyer, and C. W. Sinton, The 89 Ma Tortugal
komatiitic suite,CostaRica: Implicationsfor a commongeological
origin in the Caribbean and Eastern Pacific region from a mantle
plume, Geology,in press,1997.
Baumgartner,P. O., C. R. Mora, J. Butterlin, J. Sigal, G. Glagon, J.
Az6ma, and J. Bourgois,Sedimentaci6ny paleografiadel Cret•cico
y Cenozoico del litoral pacffico de Costa Rica, Rev. Geol. Am.
radiometricage of plagioclasefrom a lava in the latter of these
sections is 89.7 + 1.4 Ma [Alvarado et al., 1997], which
indicates anomalously high thermal conditions in the mantle
at this time.
Sinton [1996] used batch melting equationsto show that a
potential primary magma for the Nicoya basalt suite could be
producedby 30% melting of a mixed depleted and primitive
mantle. Although it is possibleor even likely that fractional
or near-fractional melting occurred during formation of the
primary magmas, if the fractional melts accumulated before
eruption, then the aggregate melt compositionswould have
been identical to batch melting [Shaw, 1970; Johnsonet al. ,
1990]. Such a high degree of melting was also reported by
Mahoney et al. [1993] for the Ontong Java oceanic plateau
lavas.
The pattern of QP93-1 indicates that either the mantle
Alvarado, G. E., S. Kussmaul,S. Chiesa, P.-Y. Gillot, H. Appel, G.
W6rner, and C. Rundle, Resumencronoestratigr•ficode las rocas
igneas de Costa Rica basadoen datacionesradiom6tricas,J. South
Am. Earth Sci., 6, 151-168, 1993.
Cent., 1, 57-136, 1984.
Bender, J. F., F. N. Hodges, and A. E. Bence, Petrogenesisof basalts
from the project FAMOUS area: Experimentalstudy from 0 to 15
kbars, Earth Planet. Sci. Lett., 41,277-302,
1978.
Bourgois,J., J. Azema, P.O. Baumgartner,J. Tournon,A. Desmet,and
J. Auboin, The geologic history of the Caribbean-Cocosplate
boundarywith special referenceto the Nicoya ophiolite complex
(Costa Rica) and DSDP results (Legs 67 and 84 off Guatemala): A
synthesis,Tectonophysics,108, 1-32, 1984.
Dalrymple,
G.B.,D.A. Clague,
T. L. Vallier,
andH.W.Menard,
4øAr39Ar
age,petrology,
andtectonic
significance
ofsome
seamounts
in
the Gulf of Alaska, in Seamounts,Islands, and Atolls, Geophys.
Monogr. Ser., vol. 43, edited by B. H. Keating, P. Fryer, R. Batiza
andG. W. Boehlert,pp. 297-315, AGU, Washington,D.C., 1988.
sourcewas enrichedin LREE or that garnet was residualin the
source mantle or both. Sinton [1996] showed that a calculated
Deer, W. A., R. A. Howie, and J. Zussman,An Introduction to the Rock
primary magmafor this samplecould have been generatedby
mixing a 5% melt of a garnet-bearing
mantlewith a 10% melt
of the spinel-bearingmantle.
Forming Minerals, 696 pp., Longman,White Plains,N.Y., 1992.
Dengo, G., Tectonic igneoussequencein Costa Rica, in Petrologic
Studies:A Volume to Honor A.F. Buddington,edited by A. E. J.
Engel, H. L. Jamesand B. F. Leonard,pp. 133-161,Geol. Soc. Am.,
Conclusions
Denyer, P., and O. Arias, Geologiadel nortede la Peninsulade Nicoya,
Boulder, Colo., 1962.
Our combined
data indicate that the mafic oceanic rocks of
the Nicoya Peninsulabelong to a single magmaticsuite, the
vast majority of which formed within a shortperiod at 88-90
Ma.
The
data are not consistent
with
the division
of the
Nicoya Peninsularocks into separatesuites. Two intrusive
rocks (a gabbroand a ptagiogranite)are slightly youngerat
84 Ma, suggestingintrusive activity occurredafter the initial
volcanic eruptions. We cannot discountthe possibilitythat
fragmentsof Jurassicto Early Cretaceousoceanic crust exist
in the Nicoya Peninsula and that these have not been
identified. The coherenttrends in the variation diagrams,the
similarity in the REE diagrams,and the similarity in Nd and
Pb isotopesprovide strongevidencethat the rocks collected
thus far from the Nicoya Peninsula are related to the same
event. This event is most likely the formation of the
Caribbean oceanic plateau, based on similarities in age and
compositionsto exposedsectionsof the plateau around the
Caribbean plate margins [Donnelly et al., 1990; Sinton,
1996]. The Playa Jac6/Herradurabasaltsare slightlyyounger
(84 Ma) than the Nicoya Peninsulabasaltsand diabasesbut are
compositionally identical.
The Quepos basalts are
significantlyyounger(64 Ma) and formed by smallerdegrees
of partial melting of a mantle sourcesimilar to the one from
which the Nicoya magmaswere derived. All of the obducted
mafic material in western Costa Rica rocks could be products
of the Galapagosmantle plume.
Acknowledgments. We would like to thankD. Christie,D. Graham,
and A. Grunderfor their helpful commentsand input and M. Gorring
for doing the isotopicanalyses. We are indebtedto K. Cameron,T.W.
Donnelly, and D. Elthon for helpful reviews. M. Meschedeand H.-J.
Gursky providedvaluableadvice during the planningstagesof this
project. This researchhas been supportedby the National Science
Foundation.
Costa Rica, Rev. Geol. Am. Cent., 16, 69-84, 1993.
Diebold, J. B., P. L. Stoffa, P. Buhl, and M. Truchan, Venezuela basin
crustalstructure,J. Geophys.Res.,86, 7901-7923, 1981.
Doe, B. R., Lead Isotopes,137 pp., Springer-Verlag,N.Y., 1970.
Donnelly, T. W., The CaribbeanCreataceousbasaltassociation:A vast
igneousprovincethat includesthe Nicoya Complexof CostaRica,
Profil, vol. 7, pp. 17-45, Univ. of Stuttgart,Stuttgart,Germany,1994.
Donnelly, T. W., et al., History and tectonic setting of Caribbean
magmatism,in The Caribbean Region, vol. H, edited by G. Dengo
and J. E. Case,pp. 339-374, Geo. Soc.Am., Boulder,Colo., 1990.
Duncan, R. A., and R. B. Hargraves, Plate tectonic evolution of the
Caribbeanregion in the mantlereferenceframe, in The CaribbeanSouthAmericanPlate Boundaryand RegionalTectonics,
editedby
W. E. Bonini, R. B. Hargravesand R. Shagam,pp. 81-93, Mem.
Geo. Soc. Am., 16, 81-93, Boulder, Colo., 1984.
Duncan,
R. A., andR. B. Hargraves,
4øAr/39Ar
geochronology
of
basementrocks from the MascarenePlateau, ChagosBank, and
Maldives Ridge, Proceedingsqf the Ocean Drilling Program,
Scientific Results,115, edited by R. A. Duncan, J. Backman and L.
C. Peterson,pp. 43-52, OceanDrill. Program,College Station,Tex.,
1990.
Duncan,R. A., and L. G. Hogan,Radiometricdatingof youngMORB
using
the4øAr-39Ar
incremental
heating
method,
Geophys.
Res.
Lett., 21, 1927-1930, 1994.
Escalante,G., The geology of southernCentral America and western
Colombia,in The CaribbeanRegion,vol. H, editedby G. Dengoand
J. E. Case,pp. 201-230, Geol. Soc. Am., Boulder,Colo., 1990.
Farley, K. A., J. H. Natland,and H. Craig, Binary mixing of enriched
and undegassed
(primitive?)mantlecomponents
(He, Sr, Nd, Pb) in
Samoanlavas, Earth Planet Sci. Lett., 111, 183-192, 1992.
Forsyth, D. W., Geophysical constraintson mantle flow and melt
generation beneath mid-ocean ridges, in Mantle Flow and Melt
Generationat Mid-Ocean Ridges,,Geophys.Monogr. Ser., vol. 71,
editedby J. PhippsMorgan,D. K. BlackmanandJ. M. Sinton,pp. 165, AGU, Washington,D.C., 1992.
Frisch,W., M. Meschede,and M. Sick, Origin of the CentralAmerican
ophiolites: Evidence from paleomagneticresults, Geol. Soc. Am.
Bull., 104, 1301-1314, 1992.
Galli-Olivier, C., Ophiolite and island arc volcanism in Costa Rica,
Geol. Soc. Am. Bull., 90, 444-452, 1979.
Gradstein,F. M., F. P. Agterberg,J. G. Ogg, J. Hardenbol,P. van Veer,
15,520
SINTON
ETAL.:THENICOYA
PENINSULA
J. Thierry,andZ. Huang,A Mesozoictimescale,
J. Geophys.
Res., Prinzhofer, A., E. Lewin, and C. J. Allegre, Stochasticmelting of the
99, 24,051-24,074, 1994.
Grove, T. L., R. J. Kinzler, and W. B. Bryan, Fractionationof midoceanridgebasalt(MORB), in MantleFlow andMelt Generationat
Mid-Ocean Ridges,,Geophys.Monogr. Ser., vol. 71, editedby J.
PhippsMorgan,D. K. Blackmanand J. M. Sinton,pp. 281-310,
AGU, Washington,D.C., 1992.
marble cake mantle: Evidence from local study of the East Pacific
Rise, Earth Planet Sci. Lett., 92, 189-206, 1989.
Pyle, D. G., D. M. Christie, J. J. Mahoney, and R. A. Duncan,
Geochemistryand geochronologyof ancientsoutheastIndian and
southwestPacific seafloor,J. Geophys.Res.,100, 22,261-22,282,
1995.
Gursky,H.-J., M. Gursky,R. Schmidt-Effing,
andH. Wildberg,Karten
zur Geologie von Nordwest Costa Rica (Mittelamerika) mit
Schmidt-Effing, R., Alter und genesedes Nicoya-Komplexeseiner
ozeanischen P/ileokruste (Oberjura bis Eoz/in) im siidlichen
Efi•iuten, Geol. Palaontol., 18, 173-182, 1984.
Hanan, B. B., and D. W. Graham, Lead and helium isotopicevidence
from oceanicbasaltsfor a commondeepsourceof mantleplumes,
Science,272, 991-995, 1996.
Hart, S. R., E. H. Hauri, L. A. Oschmann,andJ. A. Whitehead,Mantle
Zentralamerika, Geol. Rundsch., 68, 457-494, 1979.
Shaw, D. M., Trace element fractionation during anatexis, Geochim.
plumeentrainment:
Isotopic
evidence,
Science,
256,517-519,1992.
Hey, R., G. L. Johnsonand A. Lowrie,Recentplatemotionsin the
Galfipagos
area,Geol.Soc.Am.Bull., 88, 1385-1403,1977.
Hooper,P. R., D. M. Johnson,and R. M. Conrey,Major and trace
Cosmochim. Acta, 34, 237-243, 1970.
Sinton, C. W. 1996. A tale of two large igneous provinces:
Geochronologicaland geochemicalstudiesof the North Atlantic
Volcanic Province and the Caribbean Oceanic Plateau, Ph.D. thesis,
Oreg. StateUniv., Corvallis, 1996.
Storey, M., J. J. Mahoney, L. W. Kroenke, and A.D. Saunders,Are
oceanicplateaussite of komatiiteformation?,Geology,19, 376-379,
1991.
element analysisof rocks and mineralsby X-ray spectrometry,
OpenFile Report,Geol.Dept.,Wash.StateUniv., Pullman,Wash., White, W. M., and 13.Dupr6, Sedimentsubductionand magmagenesis
1993.
in the Lesser Antilles: Isotopic and trace element constraints,J.
Geophys.Res.,91, 5927-5941, 1986.
Hurford,
A. J.,andK. Hammerschmidt,
4øAr/39Ar
andK-Ardating
of
theBishopandFishCanyonTuffs:Calibration
agesfor fission-track White, W. M., and P. J. Patchett,Hf-Nd-Sr and incompatibleelement
abundances
in islandarcs:implicationsfor magmaoriginsandcrustdatingstandards,
Chem.Geol.,58, 23-32, 1985.
mantle evolution, Earth Planet Sci. Lett., 67, 167-185, 1984.
Johnson,K. T., H. J. B. Dick and N. Shimizu,Melting in the oceanic
uppermantle:An ion microprobestudyof diopsidesin abyssal White, W. M., A. W. Hofmann, and H. Puchelt,Isotopegeochemistry
of Pacific mid-oceanridge basalt,Earth Planet Sci. Lett., 92, 4,881peridotites,
J. Geophys.
Res.,95, 2661-2678,1990.
4,893, 1987.
Klein, E. M., and C. H. Langmuir,Globalcorrelations
of oceanbasalt
chemistrywith axial depthandcrustalthickness,
J. Geophys.
Res., White, W. M., A. R. Mc13irney, and R. A. Duncan, Petrology and
geochemistryof the GalfipagosIslands:Portrait of a pathological
92, 8089-8115, 1987.
mantleplume,J. Geophys.Res.,98, 19,533-19,563,1993.
Kuijpers,E. P., The geologichistoryof theNicoyaOphioliteComplex,
Wildberg, H.,Der Nicoya-Komplex, Costa Rica, Zentralamerika:
Costa Rica and its geotectonicsignificance,Tectonophysics,
68,
Magmatisme und Genese polygenetischenOphiolith-Komplexes,
233-255, 1980.
vol. 62, 123 pp., Miinsterisches
Forsch.Geol. undPal/iontol.,1984.
Langmuir,C., EastPacificRiseData Synthesis
(petrologydatabase),
York, D., Least-squares
fitting of a straightline with correlated
errors,
vols. I and2, JointOceanogr.Inst.,Washington,
D.C., 1988.
Langmuir,
C. H., E. M. Klein,andT. Plank,Petrological
systematics
of
mid-oceanridge basalts:constraintson melt generationbeneath
oceanridges,in Mantle Flow and Melt Generationat Mid-Ocean
Ridges,,Geophys.
Monogr.Ser.,vol.71, editedby J. PhippsMorgan,
D. K. Blackmanand J. M. Sinton,pp. 183-280,AGU, Washington,
D.C., 1992.
Earth Planet Sci. Lett., 5, 320-324, 1969.
Zindler, A., and S. R. Hart, Chemical geodynamics,Annu. Rev. Earth
Planet. Sci., 14, 493-571, 1986.
P. Denyer, Escuela Centroamericanade Geologia, Universidadde
Mahoney,J. J., M. Storey,R. A. Duncan,K. J. Spencer,
andM. Pringle, CostaRica,Apdo. 35-2060, SanJose,CostaRica.
Geochemistry
andageof the OntongJavaPlateau,in TheMesozoic
R.A. Duncan, College of Oceanic and Atmospheric Sciences,
PacificGeology,Tectonics,
and Volcanism,
Geophys.
Monogr.Ser., OregonStateUniversity,Corvallis,OR 97331
vol. 77, editedby M. Pringle,et al., pp. 233-261,AGU, Washington,
C.W. Sinton, Graduate School of Oceanography,University of
D.C., 1993.
McDonough,W. F., andS.-s.Sun,The composition
of theEarth,Chem.
Geol., 120, 223-253, 1995.
Pringle,M., Age progressive
volcanismin the MusiciansSeamounts:
A
testof thehotspothypothesis
for theLateCretaceous
Pacific,in The
MesozoicPaco'i'cGeology,Tectonics,and Volcanism,Geophys.
Monogr. Ser., vol. 77, editedby M. Pringle,et al., pp. 187-215,
AGU, Washington,D.C., 1993.
Rhode Island, South Ferry Road, Narragansett, RI 02882. (email:
csinton@gsosunl.gso.uri.edu)
(ReceivedSeptember3, 1996; revisedFebruary25, 1997;
acceptedFebruary 26, 1997.)