Geological and mineralogical investigations of the lithologies and their weathering products in a study area south-west of the field station "La Gamba", Golfo Dulce, Costa Rica

2008·16.3 MB·English

by  Scheucher, Lorenz E A

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Geologicalandmineralogicalinvestigationsofthelithologies
and their weathering products in a study area south-west
of the field station “La Gamba“, Golfo Dulce, Costa Rica
Investigación geológica y mineralógica de las rocas y sus
productos de meteorización, en un área al suroeste de la
estación “La Gamba”, Golfo Dulce, Costa Rica
Lorenz E. A. SCHEUCHER, Walter VORTISCH & Jorge LAGUNA-MORALES
Abstract: The study area belongs to the Golfito terrain. By geological mapping and mineralogical analyses five sedimentary
lithologiescanbedistinguished:fine-grainedgreenishtuffiticsiliciclastics(1),semi-consolidatedsands(2),blue-greymicritic
limestone(3),redmudstones(4)andgreyishclays(5).Asignificantvolcanoclasticinputforallsiliciclasticsedimentsisindicat-
edbythepresenceofsmectite.Additionally,basalt,basaltictuffsandbasalticvolcanicbrecciascanbeobserved.Mostofthestudy
areaconsistsoffine-grainedgreenishtuffiticsiliciclastics(lithology1),generallycomposedofsmectiteandquartzasthemain
constituents,withsmallproportionsofillite.Insomesamplesvaryingamountsofzeolite(probablyofheulandite-clinptilolitese-
ries),plagioclaseandclinopyroxeneweredetectedbyX-raydiffractometry.Lithologies2to5areintercalatedinonesectionof
about10m.Themircofossilcontentofthelimestone(3)suggestsaCretaceousage.Albitisationofplagioclasecrystalstogether
withtheoccasionaloccurrenceofcalcitecementindicatesmarineinfluenceonthebasalticlavas.Hydrothermalactivityissug-
gestedbyaniron-richtalcmineralobservedinsamplesofbasaltandbasaltictuff.Theweatheringproducts(partlylaterites)de-
velopedfromtheselithologiescanbedistinguishedbytheirquartzcontent.Soilsoverlyingthewidespreadgreenishtuffiticsilici-
clastics(lithology1)arequartz-bearing,whereasthoseoverlyingbasalticrocksaregenerallyquartz-free.Inmostsoilsamples,the
dominantmineralphaseisrepresentedbyinterstratifiedkaolinite-smectite.Insomesamples,disorderedkaoliniteispredominant.
Themaincrystallineironmineralsaregoethiteandhematite.Amemberofthemagnetite/maghemitegroupispresentinsome
samples.Alesserdegreeofweatheringisindicatedinsomesamplesbythepresenceofsmectiteandfeldspar.Inafewsamples,
veryintensiveweatheringisindicatedbysmallamountsofgibbsite.Recentvolcanicinfluenceisprovenbytheoccurrenceofam-
phiboleintheuppermostpartsofsomeweatheringprofiles.
Keywords:GolfitoTerrain,volcanicrocks,volcanoclastics,sedimentaryrocks,weathering,claymineralogy.
Resumen:EláreaestudiadaselocalizaenlazonadeGolfito.Conbaseenmapasgeológicosyanálisismineralógicos,sehanpodi-
dodistinguircincolitologíassedimentarias,asaber:(1)tufitassiliciclásticasverdosasdegranofino;(2)arenassemiconsolidadas;
(3)calizamicríticagrisazulada;(4)lodolitasrojas;(5)arcillasgrisáceas.Lapresenciadeesmectitaentodoslossedimentossilici-
clásticosindicalaexistenciadeunaimportanteperovariablecantidaddematerialvolcaniclástico.Además,basaltos,tufasybre-
chasbasálticas,puedenserobservados.Lamayorpartedeláreaestáconformadapormaterialessiliciclásticostufíticosverdososde
granofino(litología1),generalmentecompuestosdeesmectitaycuarzocomocomponentesprincipales,conunapequeñaporción
deilita.Análisispordifractometríaderayos-Xdealgunasmuestrasindicaroncantidadesvariablesdezeolitas(probablementedela
serieheulandita-clinoptilolita),plagioclasasyclinopiroxeno.Laslitologías2a5seencontraronintercaladasenunaseccióndeun-
os10m.Lamicrofaunaenlacalizamicrítica(litologiá3)indicaunedadCretácico.Laalbitizacióndealgunasplagioclasasdemues-
tralainfluenciadeaguamarinaenlasrocasbasálticas,locualcorrespondealaocurrenciaocasionaldecementodecalcitaenes-
tasrocas.Lapresenciademineralesdetalcoenriquecidoenhierroobservadosenmuestrasdebasaltosytufasbasálticas,indican
unaciertaactividadhidrotermal.Losproductosdemeteorización(parcialmentelateritas)desarrolladosdeestaslitologíassepueden
distinguirporsucontenidodecuarzo.Lossuelosquesobreyacenlaampliamentedistribuidatufitasiliciclásticaverdosa(litología1)
contienencuarzo;encambio,aquellosquesobreyacenalasrocasbasálticassongeneralmentelibresdecuarzo.Enmuchasmuestras
desueloslafasemineraldominantelarepresentaelinterestratificadocaolinita-esmectita.Enotrasmuestrasunacaolinitadesorde-
nadaocurrecomolafasedominante.Losprincipalesmineralesdehierrosongoethitayhematita.Enalgunasmuestrasesposible
observarmineralesdelgrupomagnetita/maghemita.Unbajogradodemeteorizaciónseindicaenalgunasmuestrasporlapresencia Stapfia88,
deesmectitayfeldespato.Unaspocasmuestrasrevelanunmayorgradodemeteorización,indicadoporpequeñascantidadesdegibb- zugleichKatalogeder
oberösterreichischen
sita.Unainfluenciavolcánicarecientesecompruebaporlaocurrenciadeanfíbolesenlaspartessuperioresdealgunosperfiles. Landesmuseen
NeueSerie80(2008):
Palabrasclave:TerrenodeGolfito,rocasvolcánicas,volcanoclásticas,rocassedimentarias,meteorización,mineralogíadearcillas. 31-45
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Fig.1:Pre-Neogene
tectonicunitsofthe
GolfoDulceregion
(fromDIMARCOetal.
1995).
Introduction Tothenorth-eastoftheRincónBlockarefluvatile,
lacustrineandmarinesiliciclasticsandcarbonatesofthe
The investigated area lies south-west of the La
FilaCosteñaforearcsediments(YUAN&LOWE1987).
Gamba field station and is bordered by the “Fila path”
(5kmlongpathstartingandendingnearthefieldsta- TheLaGambafieldstationandthusthestudyarea
tion).Withinthearea,alloutcroppingrocksandweath- arelocatedwithintheGolfitoTerrain,whichflanksthe
ering products were geologically mapped and samples RincónBlocktotheeastandtheFiláCosteñaforearc
werecollectedforlaboratoryanalyses,particularlyX-ray sedimentstothesouth-west(Fig.1).Withinthisterrain
diffractometry (XRD). Additionally, selected samples three geologic units can clearly be distinguished (DI-
were analysed by X-ray fluorescence. Thin sections of MARCO1994,Fig.2).Theigneousbasementisformed
somesampleswerestudiedbypolarisingmicroscopyand bymassiveoceanicbasaltsanddolerites,sometimesoc-
cathodoluminescence microscopy. An overview of the curringintheformofpillows.Thisbasementisoverlain
applied analytical techniques is given in the chapter
bysedimentsandvolcanicrocksoftheGolfitoForma-
“Laboratoryprocedures”.Theresultspresentedhereare
tion.Pelagiccarbonateswithminoroccurrencesofvol-
basedonthediplomathesisofSCHEUCHER(2006). canoclastic sediments are intercalated with doleritic
and basaltic flows and occasionally basaltic breccias.
Regionalgeologicalsetting
TheabsenceoflavaflowsmarksthebaseoftheQuebra-
AccordingtoDIMARCOetal.(1995)fourpre-Neo- daAchioteFormation,theuppermostunitoftheGolfi-
gene tectonic units are present in the Golfo Dulce re-
toTerrain.Largequantitiesofvolcanoclasticsediments
gion(Fig.1).
with few intercalations of pelagic limestones and
The southern fringe of the Osa Peninsula is repre- basaltic breccias characterise this formation. The Fila
sented by strongly deformed, late Cretaceous to Mi- Gamba Member, consisting of tuffites and tuffs, is the
coene sediments of the Osa-Caño Accretionary Com- uppermost unit of the Quebrada Achiote Formation.
plex, consisting of turbidites and other pelagic and ThestratigraphyoftheGolfitoTerrain,asstatedbyDI-
hemipelagic sediments, carrying boulders of oceanic MARCO(1994),isshowninFigure2.
basaltsandvarioussedimentaryrocks(DIMARCO1994,
DIMARCOetal.1995). Fieldsurvey(sampling)
TheRincónBlockformsmostofthelandareabor- Detailed geological mapping was conducted during
deringtheGolfoDulceandcomprisesbasalticbasement AprilandMay2004.Allpathssurroundingthefieldsta-
with overlying Cretaceous to Eocene sediments (DI- tion were mapped and transferred onto a topographic
MARCO1994). map. Primary lithologies were examined from natural
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and man-made exposures (the latter formed when the
pathswerecutintotherainforest).Evenwithintensive
tropicalweathering,someoftheoutcropsprovidedim-
portantinformationonsuchgeologicalfeaturesasstrat-
ificationandjointing.Representativesamplesfromvar-
iouslithologieswerecollected.Fortheinterpretationof
theweatheringproducts,informationonthesubsurface
stratigraphywasobtainedviacoresfromanaugerdrill.
Additionallyseveraltrenchesweredugforfurthersam-
pling.
Laboratoryprocedures
AllsampleswereanalysedbymeansofXRD,using
aPhilipsXpertsystem.Preparationofthesamplesasori-
entedmountsforXRDanalysesincludedair-drying,dis-
aggregation to ca. 5 μm and making a paste (0.085 g
powdermixedwith0.045mldistilledwater).Thepaste
wassmearedintoagroove(depth100μm,widthcorre-
spondingtothatoftheprimarybeam)ofaglasssample
holder(“smear-on-glass”,formoredetailsseeVORTISCH
1982). Because of the reaction during drying, some
smectite-rich samples had to be prepared without
adding water. The oriented mounts were measured se-
quentially in an untreated condition, after treatment
with ethylene glycol (EG) (according to BRUNTON
1955) and after thermal treatment (2 hours) at 350°C
and550°C.Thesetreatmentsarestandardproceduresin
claymineralogyandnecessarytodistinguishclayminer-
als with sufficient certainty (e.g. VORTISCH 1982). For
Fig.2:StratigraphyoftheGolfitoTerrane(fromDIMARCO1994).
reasons of clarity, all four measurements of one sample
were combined in one figure (from bottom to top: un-
Results
treated/EG-saturated/350°Ctreated/550°Ctreated,
seeFig.4).Determinationofinterstratifiedclayminer-
Primarylithologies(parentrocks)
alsfollowedMOORE&REYNOLDS(1997).
OnthebasisoffieldsurveyandX-raydiffractometry
Inordertodeterminechangesinthechemicalcom-
fivedistinctlithologieswererecognised:
position during weathering, selected samples were
analysedbymeansofX-rayfluorescence(XRF).Sample
Tuffite
preparation for XRF analysis includes grinding to
<100μm,dryingat105°,andheatingto1000°C.One Thisrocktypeisthemostprevalentintheinvesti-
gram of heated powder is mixed with six grams of gated area. In outcrops, the tuffites show variations in
lithiumtetraborate(fluxagent)andmelted.Themeltis colour, bedding and grain size. The colour ranges from
poured into a platinum mould and the resulting glass light green to light blue-green. Regarding grain size,
tabletisusedforanalysis. thesetuffitesappearasunconsolidatedtosemi-consoli-
datedclayeytosiltysediments.Grainsizeaswellasbed-
Thinsectionsofselectedsampleswereexaminedby
polarising microscopy and cathodoluminescene mi- ding vary over short distances. A relationship between
croscopy (CL; equipment: CITL Mk 3). Polished thin grain size and bedding is evident: the finer grained
sectionswereappliedforCL-microscopy. tuffites show mm-lamination whereas the coarser ones
are thickly bedded to massive. In thin sections of the
coarser grained tuffites (Fig. 3), grains of quartz (up to
100 μm) and glauconite (a green Fe/Mg-rich 2:1 clay
mineral),andradiolarianscanbeidentified.Theoccur-
rence of glauconite and radiolarians proves the marine
depositionalenvironmentoftheserocks.
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Basictuff
Inthestudiedoutcropsthesetuffsappearasbrown-
coloured, unconsolidated sands. They form homoge-
neous sequences with thicknesses of several metres
withoutanysedimentarystructures.Figure6showsthe
appearance of the basic tuffs in the field. An erosional
unconformitybetweenthegreentuffites(lithology1.1)
andtheoverlyingbrowntuffscouldbeobservedinone
section(Fig.7).
The basic nature of these tuffs is shown by their
mineralogical composition. The predominant mineral
phases are smectite and plagioclase. Microprobe analy-
sesoftheplagioclasesshowaNa:Caratioofabout1:1,
signifyingthattheircompositionislocatedattheande-
sine/labradorite boundary. Clinopyroxene, orthopyrox-
ene,andolivinewerealsoobserved.Thepresenceofin-
terstratified kaolinite-smectite indicates the beginning
Fig.3:Thinsectionofacoarse-grainedgreentuffite(quartz:yellowarrows, of weathering. Due to decreasing weathering intensity
glauconite:greenarrows,radiolarians:whitearrows;lengthofimage:2.67 with depth, the amount of this clay mineral decreases
.
mm,plane-polarisedlight)
downwards in weathering profiles. Samples taken from
deeper parts of some sections, additionally contain an
In addition to the macroscopic characteristics, dif-
iron-rich talc mineral, an indicator of possible hy-
ferencesinthemineralogicalcompositionofthetuffites
drothermalinfluence(seealsobelow,Figs.21and22).
also exist. All analysed samples contain varying
Figure 8 represents the mineralogical composition of
amountsofsmectite(withsomesamplesshowingpartial
onesampleofthebasictuffs.XRFanalysesprovideSiO
chloritisation of smectite), quartz and illite (Fig. 4). 2
(about 50% w/w) and MgO (6% w/w) data which are
Chloritewasprobablyformedbychloritisationofsmec-
typicalformaficrocks.
tite (loss of expansibility) through uptake of Al3+- or
Fe3+-hydroxide in the interlayer space (soil chlorite in Themineralogicalcompositionofthetuffsdirectly
overlyingthegreentuffitesdiffersfromthecomposition
thesenseofSCHWERTMANN&STANJEK2002).Itshows
described above with respect to the presence of quartz
low crystallinity and low thermal stability (amorphous
andtheabsenceoforthopyroxeneandolivine.
after heat treatment at 550°C/2 h). Besides the occur-
rence of illite as a clastic component, diagenetically
Sedimentarysequenceatlocationno.5
formed illite (1M-illite) has been found in some sam-
Onlyatonelocation(no.5)withintheinvestigat-
ples.
ed area, a complex sequence of sedimentary rocks and
Additionally, some of the samples contain anatase, unconsolidatedsedimentswasdetected.Thebaseofthis
plagioclase and zeolite (presumably of the heulandite- sequence is formed by a one metre-thick layer of light
clinoptilolite series). In one sample, small amounts of grey-coloured silty material, overlain by brown sand of
clinopyroxenewerefoundwhereasanothersamplecon- severalmetresthickness(Fig.9).Thecontactbetween
tainsnoticeableamountsofchlorite.Themineralogical these two layers is irregular and seems to be disturbed
compositionofazeolite-bearingtuffiteisshowninFig- (tectonicprocessesorsliding?).Thesandshowsinterca-
ure5.Thediscernibilitybetweenchloriteandsmectite lationsofgreysiltandisoverlainbyaonemetrethick
afterEG-andthermaltreatmentisclearlynoticeable. layer of a brown, massive, silty to clayey, consolidated
sediment(mudstone)withred-colouredcoatingsonfis-
Thechemicalanalysisofasamplecontainingsmec-
sures. Between this mudstone and the overlying grey
tite, illite, quartz and anatase shows an Fe-content
clay, a 10 cm thick intercalation of a bluish-grey
(Fe O ) of approx. 8.5% w/w. Because smectite is the
2 3 coloured limestone occurs (Fig. 10; XRD: essentially
only iron bearing mineral in this sample it must be of calcitic, minor amounts of quartz). This limestone is
nontroniticnature.Dioctahedralsmectite(nontronite) overlain by a grey-coloured clay of about 20 cm thick-
and zeolite (especially of the heulandite-clinoptilolite ness. Above this clay follows a 30 cm thick layer of
series)arecommonmineralphasesformedbydiagenet- brown sand. The spatial grain size distribution of this
ictransformationofglass-richvolcanicashesinmarine sandindicatesgradedbedding:normalgrading(i.e.up-
environments, as observed, for instance, in Costa Rica ward-decreasinggrainsize)inthelowerpartandinverse
byVORTISCH(1990). gradingintheupperone.
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Fig.4:Mineralogical
compositionofa
laminated,lightgreen
tuffite(sampleno.1in
Fig.25).
Fig.5:Mineralogical
compositionofa
coarse-grained,bedded
tuffite(sampleno.2in
Fig.25).
On the basis of microscopical investigation, the cur. Some samples additionally contain zeolite and
limestonewasclassifiedaswackestonetopackstone(ac- clinopyroxene. Clay mineralogy appears to be quite
cording to the nomenclature of DUNHAM 1962). The complexinsomesamplesofthesedimentarysequence,
microfaunasuggestsaCretaceousage(AxelvonHille- asshownforthegrey-colouredclayinFigure11.
brandt, pers. comm.). It is represented by radiolarians
(calcitised)andplanctonicforaminifera. Massivebasaltandbasalticvolcanicbreccia
Thevolcanoclasticinfluenceonthesiliciclasticsed- Besidesthebrownbasictuffmentionedabove,there
imentsofthissequencecanclearlybeseeninthemin- aretwoothertypesofbasicvolcanicrocksintheinves-
eralogical composition, as all analysed samples show tigatedarea:dark-coloured,massivevolcanitesandvol-
highamountsofsmectite,resp.smectite-richinterstrat- canic breccias. Due to their resistance to weathering,
ifiedclaymineralsasillite-smectiteandkaolinite-smec- themassivevolcanitesformadistinctivemorphological
tite. Moreover, considerable amounts of quartz and ridge(locationno.9).Macroscopically,theserocksare
varyingproportionsoffeldspar(mainlyplagioclase)oc- very fine-grained without visible phenocrysts. Under
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Fig.6:Outcropofbasictuff(location Fig.7:Erosionalcontactbetweengreentuffiteandbasictuff(location
no.3inFig.25). no.4inFig.25).
Fig.8:Mineralogical
compositionofa
sampleofbasictuff
(locationno.3in
Fig.25,sampling
depth:2.3m).
Fig.9:Brownsandoverlyinggrey-colouredsilt Fig.10:Carbonatelayerwithinthesedimentary
(locationno.5inFig.25). sequence(locationno.5inFig.25).
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Fig.11:Mineralogical
compositionofagrey
claywithinthe
sedimentarysequence
(samplefromlocation
no.5inFig.25).
Fig.12:Mineralogical
compositionofa
volcanicbreccia
(samplefromlocation
no.6inFig.25).
the microscope, they show a hypocrystalline fabric. parabletothoseofthemassivebasalts.Bothrocktypes
Lath-shaped plagioclases are embedded in a very fine- contain noticeable amounts of smectite (occasionally
grained matrix. Isolated grains of olivine and clinopy- chlorite-smectite).Microprobeanalysesofsomeplagio-
roxenecanalsobeobserved.Additionally,orthopyrox- clases show that they have undergone albitisation, a
enewasdetectedbyXRD.Sometimestridymiteandze- common process in marine environments. Traces of
oliteoccurasfillingsinjointsandcracks.Onthebasis magnetite/maghemitearealwayspresent.Asarepresen-
ofNa O,K O,andSiO content,thesevolcanitescan tativeexampleofthesebasicvolcanicrocks,theX-ray
2 2 2
beclassifiedasbasalts. diffraction patterns of an amphibole-bearing volcanic
brecciaareshowninFigure12.
The volcanic breccias consist of a variety of differ-
ent-colouredclasts,allofwhichareofvolcanicorigin. Someclastscontainsignificantamountsofvolcanic
The mineralogical composition of the breccias is com- glass.Theglassgrainsshowsignsoftransformationinto
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Fig.13:Thinsectionofvolcanicbreccia,showing Fig.14:CLimagefromthesameareaasFig.13,
calcitecementation(lengthofimage:6.85mm,plane- showingtypicalcalciteCLcolour(lengthofimage:
polarisedlight). 6.85mm).
Fig.15:Lateriticweathering Fig.16:Freshamphiboleinsampleofquartz-bearinglateritic
profile(quartz-bearing)inapit weatheringproduct(samplingca.0.3mbelowsurface,lengthofimage:
(locationno.7). 0.65mm,plane-polarisedlight,locationno.7)
agreen-colouredmineralphase(possiblynontroniteor intensively red-coloured weathering products. The ap-
chlorite). Nontronite, an iron-rich dioctahedral smec- pearance of these weathering products in the field is
tite,oftenfoundinalteredbasalticrocks,isindicatedby shown in Figure 15. Some of them can be classified as
XRD, as all analysed samples contain smectite. More- laterites in the sense of SCHELLMANN (1986). On the
over, radially crystallised zeolite occurs. XRD analyses basisofXRDanalysestwodifferenttypesofweathering
indicates the occurrence of analcime (a Na-zeolite). productscanbedistinguished.
The radial fabric is typical for rapid cooling. Structure
andchemistryofthiszeolitesuggestdepositioninama- Quartz-bearingweatheringproducts/laterites
rineenvironment.Anotherindicationformarineinflu- Nearlyallofthequartz-bearingweatheringprofiles
enceonthevolcanicbrecciaistheoccurrenceofcalcite contain small, light green rock pieces in the deeper
cement(althoughcalcite-freesamplesexistaswell),be- parts.Thesepiecesareconsideredtoberemnantsofthe
causeformationofcalciteislessprobableundertropical underlyinggreentuffites.Therefore,thetuffitescanbe
weatheringconditions(Figs.13and14).TheCLphoto seen as parent rocks for these weathering products.
(Fig.14)showsthetypicalorangeCLcolourofcalcite. Samplestakenfromthemostweatheredpartscanbere-
ferred to as laterites due to their mineral composition.
Weatheringproducts(laterites)
Generally,thedominantmineralphaseisinterstratified
CorrespondingtothelatitudeoftheGolfoDulcere- kaolinite-smectite,predominantlyconsistingofkaolin-
gion,mostoftheinvestigatedareaiscoveredbythick, itelayers(rangingfromca.77%indeepersectionstoca.
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Fig.17:Mineralogical
compositionofquartz-
bearinglaterite,sample
depth:0.3m(location
no.7inFig.25).
Fig.18:Mineralogical
compositionofquartz-
bearingweathering
product,sampledepth:
2.3m(locationno.8in
Fig.25).
94%nearthesurface).Besideskaolinite-richinterstrat- (smectite,illiteandfeldspar).Inonesection,smectiteis
ified kaolinite-smectite, disordered kaolinite (possibly replaced by non-expanding vermiculite in the deeper
metahalloysite)occurintheuppermostpartsoflaterit- parts(Fig.18).Tracesofmagnetite/maghemitearepres-
icweatheringprofiles.Asmentionedabove,inallsec- ent in some profiles. As already described by PAMPERL
tionsconsiderableamountsofquartzcouldbedetected. (2001a) noticeable amounts of an amphibole mineral
Thepresenceofsmallamountsofgibbsiteintheupper
arepresentnearthesurfaceinsomesections.Theam-
parts of some sections indicates very intensive tropical
phibolegrainscanbeupto300μminsize,asshownin
weathering.Theoccurrenceofhematiteandgoethiteis
Figure16.Theserelativelyfreshamphibolegrainsindi-
typicalfortropicalsoils.Inoneprofilenearthefieldsta-
caterecenttosubrecentvolcanoclasticinfluence.
tion traces of anatase were found. The less weathered
parts also contain residues of the primary, resp. diage- Themineralogicalcompositionoftwoselectedsam-
neticallyorhydrothermallyformedmineralassemblage plesofquartz-bearingweatheringproductsareshownin
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Fig.19:Mineralogical
compositionof
sample1,quartz-free
laterite(locationno.9
inFig.25,0.2m
belowsurface).
Fig.20:Mineralogical
compositionof
sample2(location
no.9inFig.25,0.6m
belowsurface).
Figures17and18(Figure17representingthemorein- Quartz-freeweatheringproducts/laterites
tensivelyweatheredmaterial,occurrenceofgibbsite). Macroscopically,theselateritescanhardlybedistin-
XRF analyses of one profile of the quartz-bearing guished from the quartz-bearing laterites. In the field
weatheringproducts(locationno.7)showedthatsam- theyappearmoreauburnthanredandmoreclayeythan
ples taken near the surface (more intensively weath- thequartz-bearinglaterites.Nofragmentsoftheparent
ered) are depleted in alkaline and earth-alkaline ele- rock have been observed in any section. So, these
ments (Na O, K O, CaO, and MgO) and enriched in weathering products could have been developed from
2 2
immobile elements like Al O and Fe O . However, thebasictuffs,basaltsorvolcanicbreccias.Somediffer-
2 3 2 3
the presence of an amphibole mineral near the surface encesregardingmineralogicalcompositioncanbeseen.
(Figs. 16 and 17) falsifies the results of XRF analyses Most noticeable is the absence of quartz. Mag-
withrespecttoalkaliandalkalineearthelements. netite/maghemite occurs with higher proportions than
40