Geologic Setting and Petrogenetic Aspects

Zb!. Geo!. Palaont. Teil I
959-971
Stuttgart, November 1989
The Asunción Alkaline Province (Eastem Paraguay): Geologic Setting and Petrogenetic Aspects By PETER RENE BITSCHENE, Bochum (FRG), and JAIME BAEZ PRESSER, Asunción (Paraguay) With 2 figures and 3 tables in the text BITSCHENE, P. R. & BAEZ PRESSER, J. (1989): The Asunción Alkaline
Province (Eastern Paraguay): Geologic Setting and Petrogenetic Aspects.
- Zbl. Geol. Paliiont. Teil 1, 1989 (5/6): 959-971; Stuttgart.
Abstract: At the western border of the Paraná basin, in the region around
Asunción, Eastern Paraguay, alkalibasaltic and phonolitic
rocks of
Paleogene age pierce red bed sediments. According to their major and
trace element concentrations (e. g. Cr: 317-505 ppm; Nb: 96-117 ppm),
high M values (62-68) and low Zr/Nb ratios (1.8-2.2) the basaltic rocks
are considered to be geochemically primitive derivatives from low-grade
melting (3 to 6 %) of a garnet-bearing upper mantle protolith, which
mayas well have had phlogopite or amphibole. The compositional hetero­
geneity of the phonolitic occurrence can be explained by in situ fraction­
ation of sphene, alkalimafites, alkali-feldspar, foids, and apatite in a
shallow magma reservoir.
Zusammenfassung: Am Westrand des Paranábeckens, in der Umgebung von
Asunción, Ostparaguay, durchschlagen Alkalibasalte und Phonolithe paleo­
zanen Alters altere Rotsedimente. GemiiB ihren Haupt- und Spurenele­
mentgehalten (e. g. Cr: 317-505 ppm; Nb: 96-117 ppm), M-Werten (62-68)
und niedrigen Zr/Nb-Verhaltnissen (1.8-2.2) werden die Basalte als geo­
chemisch primitive Abk6mmlinge eines granatführenden Oberen Erdmantel­
Protolithes angesehen, welcher eventuell Phlogopit- oder Amphibol-führend
war. Ihre kompositionelle Heterogenitiit verdanken die Phonolithe der
In-situ-Fraktionierung von Titanit, Alkalimafiten, Alkalifeldspat, Foid und
etwas Apatit in einem oberfliichennahen Magmenreservoir.
Resumen: Basaltos alcalinos y fonolitas del Paleogeno atraviesan una
secuencia de capas rojas en el borde occidental de la cuenca del Paraná,
en las cercanías de Asunción/Paraguay Oriental. Las concentraciones en
elementos mayores y traza (e. g. Cr: 317-505 ppm; Nb: 96-117 ppm),
los contenidos altos de Mg en comparación con Fe (M#: 62-68), y los
valores del cociente Zr/Nb (1.8-2.2) sugieren que los basaltos alcalinos
son magmas relativamente primitivos, formados por una modesta fusión
0340-5109/89/1989-D959 $ 3.25
© 1989 E. Schweizerbart'sche Verlagsbuchhandlung, D-7000 Stuttgart 1
960
The Asuncion Alkaline 1
P. R. Bitschene and J. B. Presser
parcial (3 a 6 %) de rocas del manto superior que contenían granate, y
tal vez también f1ogopita o anfíbol. La cristalizaciém fraccionada de
esfena, minerales ferromagnesianos alcalinos, feldespato alcalino, felde­
spatoide, y apatita en cámaras magmáticas someras es un proceso
adecuado para explicar la variación composicional de las fonolitas.
"
*
10 km
1. Introduction
This pilot study contributes to the understanding of the geologic
setting and petrogenetic evolution of non-orogenic volcanic rocks of
Paleogene age in Eastern Paraguay. Results from geologic fieldwork,
pet rographic and geochemical investigations are presented. The scope of
the pet rologic part is to report on the compositional range of different
. small-volume, subvolcanic to volcanic, nephelinitic to basanitic rocks,
which are genetical!y related to each other. As the compositional varia­
tion withín single occurrences is very small and often within analytical
error, special emphasis was laid on samplíng of very fresh and represent­
atíve rock specímen from the dífferent locatíons and not to report on
the compositional homogeneity of síngle occurrences.
1. 1 Regional geology
The occurrences wíth Paleogene (60 to 38 Ma; BITSCHENE 1987),
alkalibasaltíc and phonolitic, volcanic rocks lie within an approximately
30 km wide, NW strikíng zone with Mesozoíc to Tertiary red bed sedi­
ments (Fíg. 1). This graben-like zone is borde red to the NE by old
crystalline basement and unconformably overlying Paleozoic c1astic sedi­
ments, and to the SW by Paleozoic c1astic sediments (HARRINGTON
1972). The NE and SW borders are marked by normal and antithetic
faults with vertical offset s of about 900 and 1100 m respectively (OE­
GRAFF et al. 1981), whereas the NW extension of this tectonic block is
obscured by young sediments of the Chaco basin, and the SE extension
is not yet c1ear due to swampy terrain and changes in tectonic and
magmatic style around the town of Paraguarí.
All the occur rences of the alkalíne volcanic rocks lie in cent ral
Eastern Paraguay, in the region around Asunción. They pierce red bed
sediments of Triassic to Early Tertiary age, they are al! Paleogene in
age, and geochemically and isotopically significantly different from the
Paraná basalts and their associated K-rich alkalíne rocks. Therefore the
alkalibasaltic and phonolitic occurrences can be grouped together and
form an independent geologic magmatic province, the volcanic "Asunción
Alkaline Province", hereinafter called AAP.
1. 2 Synopsis of previous work
Petrographic descriptions of "Ne basalts", "limburgites" and phonolites
from Eastern Paraguay were reported by PbHLMANN (1886), HIBSCH
(1891) and MILCH (1895). MILCH (1895, 1905) was the first to describe
petrography and geochemistry, and xenoliths from Tacumbú "limburgite".
.E:.: ............................
ASUNCIQN )
:::::: 0 :::::::::::::::::::::::::
Sqn Lo.rénzo
~d::
««;:;:: , ...
Fig. 1. Regional dist ribution and ge
and position of the AAP within f
letters indicate outcrops of volcani
Cerro Confuso, C = Cerro Nemby, 1
J = San Jorge, L = Cerro Lambaré, 1
V = Villa Hayes, X = Cerro Verde,
HARRINGTON (1950) listed several
Asunción, which were repeated b
mentioned basaltic sills from the )
the aboye mentioned rocks "Ne bas
rock and amygdale fillings. Oetailel
MIRAGLIA (1965), new occurrences
region are to be found in the II(
COMTE & HASUI (1971) and STO!<
K-A r measurements, microprobe dat
rat io f rom these rocks. Only a fe'
wrongly attributed to the Mesozoic
be found in the 1: 500.000 geologic
compiled by F. WIENS). DEGRAF
VELAZQUEZ (1982) provide descript
alkaline rocks from the AAP. COM !
OEMARCHI et al. (1989) studied t
compared them to Brazilian occum
mantle, where both Paraguayan and
heterogeneous on a regional scale. 1
al. (1985) point out the interrelatio
rifting and Tertiary alkaline magma'
geochemical and Sr isotope data, ar
were given by BITSCHENE et al. (19
The Asuncion Alkaline Province (Eastern Paraguay)
J. B. Presser
superior que contenían granate, y
La cristalizaciém fraccionada de
ilinos, feldespato alcalino, felde­
náticas someras es un proces o
)osicional de las fonolitas.
"
*
Quotern .
10km
m
D
e
;0 to 38 Ma; BITSCHENE 1987),
rocks lie within an approximately
esozoic to Tertiary red bed sedi­
~ is borde red to the NE by old
V overlying Paleozoic clastic sedi­
clastic sediments (HARRINGTON
marked by normal and antithetic
00 and 1100 m respectively (DE­
extension of this tectonic block is
haco basin, and the SE extension
ain and changes in tectonic and
¡guarí.
ne volcanic rocks lie in cent ral
d Asunción. They pierce red bed
y age, they are all paleogene in
y significantly different from the
rich alkaline rocks. Therefore the
es can be grouped together and
: province, the volcanic "Asunción
"'P.
;alts", "limburgites" and phonolites
by Pé'>HLMANN (1886), HIBSCH
S, 1905) was the first to describe
Jliths from Tacumbú "Iimburgite".
I Poleo ­
Phonolite
Asunción
[1] Alkoli Bosolt
~tion
understanding of the geologic
non-orogenic volcanic rocks of
Results from geologic fieldwork,
ions are presented. The scope of
compositional range of different
nepheliniti c to basanitic rocks,
Dther. As the compositional varia­
small and often within analytical
pling of very fresh and represent­
It locations and not to report on
oc currences.
961
.·.·:0 >... ·:·:·:· :·:·:·:·:·:·... ·
.SPDh9.renzo>:·
~t <
Sond-,Silt-,
Claystone
gene
Asunción
Guozú
TertiaryTrio ssic
Sil: Ordov.
~ Clastic Sedim . Caacupé
Eo-.[am br.
[2] Acid Ma gma!. Coopucú
Metasediment. Zoto Ruqua Precombr.
- -_ Lineament, Fault
o Town
- - - GeologicOl Boundory
~Ri'/er
- ' - - Politicol Boundary
Fig. 1. Regional distribution and geologic setting of AAP volcanic ro cks
and position of the AAP within Paraguay and South America. Capital
letters indicate outcrops of volcanic rocks; A = Puente Remanso, B =
Cerro Confuso, C = Cerro Nemby, O = Cerro Coi, E = Jardín Botánico,
J = San Jorge, L = Cerro Lambaré, P = Cerro Patiño, T = Cerro Tacumbú,
V = Villa Hayes, X = Cerro Verde, Y = Limpio.
HARRINGTON (1950) listed several occurrences of "olivine basalt" near
Asunción, which were repeated by ECKEL (1959), who furthermore
mentioned basaltic si lis from the Asunción area. PUTZER (1962) called
the aboye mentioned rocks "Ne basalts" and de scribed silicified country
rock and amygdale fillings. Detailed outcrop descriptions were given by
MIRAGLIA (1965), new occurrences oE "olivine basalts" from the Asunción
region are to be found in the "Cuadricula 40, ITA" (M.O.P.C. 1966).
COMTE & HASUI (1971) and STORMER et al. (1975) contributed first
K-Ar measurements, microprobe data on phenocrysts and one Sr isotope
ratio from these rocks. Only a few occurrenc es of alkali basalts, now
wrongly attributed to the Mesozoic Paraná basalts, around Asunción can
be found in the 1:500.000 geologic map provided by T. A. C. (1981,
compiled by F. WI ENS). DEGRAFF et al. (1981) and PALMIERI &
VELAZQUEZ (1982) provide descriptions of volcaniclastic sediments with
alka line rocks from the AAP. COMIN-CHIARAMONTI et al. (1986), and
DEMARCHI et al. (1989) studied the peridotitic xenoliths of the AAP,
compared them to Brazilian occurrences and concluded that the upper
mantle, where both Paraguayan and Brazilian occurrences come from, is
heterogeneous on a regional scale. DEGRAFF (1985) and BITSCHENE et
al. (1985) point out the interrelationship bet ween reactivated continental
rifting and Tertiary alkaline magmatism in Eastern Paraguay. K-Ar ages,
geochemical and Sr isotope data, and the introduction of the term AAP
were given by BITSCHENE et al. (1985) and BITSCHENE (1987).
962
P. R. 8itschene and J. 8. Presser
The Asunción Alkaline
2. Field observations
Some of the volcanic edifices with alkaline rocks emerge as conical
hills up to 60 m aboye terrain (Cerro Nemby, Cerro Confuso, Cerro
Lambaré, historically Tacumbú). Other indicators for not yet found occur­
rences of AAP rocks are columnar sandstones such as Cerro Coi near
Aregua (Fig. 1). Nemby and Tacumbú are volcanic necks. Near Puente
Remanso, a tephritic dike strikes NW and shows parallelepipedic jointing
and abundant zeolites within cm wide fissures. Sills are reported from
old drill holes within Asunción. Volcanism within the AAP was accom­
panied by major explosive phases. However, true volcaniclastic sediments
are mostly eroded and can only be observed near Lambaré, where the
stratigraphic sequence from bottom to top encompasses a basal volcani­
genic mass flow unit with alkalibasaltic blocks up to m' size. The upper
15 m of the Cerro Lambare stratigraphic sequence comprise several
layers of well stratified lapilli and ash layers with appreciable detrital
quartz-sand admixture.
3. Petrography
Since two different suites of volcanigenic rocks have been observed in
the field and each suite has its own macroscopic and microscopic charac­
teristics, this chapter reports separately on the alkali basalts and the
phonolites.
resulting color index is between 65
totals volumetrically between 21
(sheet silicates, amphibole, carbon
and 4,4 %. However, as the latter
it is mentioned only for compl l
Notably, the tephrite from Pte. f
content and the lowest color ind
Patiño is conspicuous because of il
light colored minerals. Tab. 1 sh
alkalibasalts from the AAP.
Tab. 1. Modal composltlOns of alka
c1inopyroxene, Ig = light colored
accessory minerals, CI = color inde:
RN = rack name (First capital le '
occurrences as marked in Fig. 1).
Occurrence
TS
01
Nemby
Nemby
Pte. Remanso
Tacumbú
Cerro Lambaré
Cerro Patiño
Cerro Patiño
C2b
C2a
Al
Tac 1
L1
Pla
Plb
15.3
12.4
9.5
12.9
15.7
12.6
13.4
cpx
51.~
56.í
46. ~
SU
52.S
52.S
SU
3. 1 Alkalibasalts
Macroscopically the basaltic rocks are black and aphanitic. Scoriaceous
and vesicular basalt is restricted to the volcanic blocks in the basal
volcaniclastic mass f10w near Lambaré and to Cerro Nemby lava sheets.
At Nemby, Lambaré and Tacumbú abundant green peridotitic xenoliths
of up to 35 cm diameter can be observed. Studies on these xenoliths are
reported in STORMER et al. (1975), COMIN CHIARAMONTI etal. (1986)
and DEMARCHI et al. (1989). Other xenoliths found comprise sandstones
and clayey siltstones . in all sizes and states of decomposition and assimila­
tion. Clearly, these rocks have been entrained from the sedimentary
count ry rock of Mesozoic or possibly Silurian age.
Microscopically the alkali basalts have hypocrystalline porphyritic to
seriate porphyritic textures. Fo-rich olivine, clinopyroxene with sometimes
green cores, and Cr spinel with corrosion features and overgrowth by
opaque , Cr-poor spinel are upper mantle xenocrysts and high-p (?) pheno­
crysts. Microphenocrysts and supposed true crystallization phases are
Fo-rich olivine, c1inopyroxenes such as Ti augite, diopsidic augite and
finally aegirin augite, opaque Fe-Ti ores, a few biotite f1itters, amphibole
(?), apatite, and a leucocratic matrix composed of nepheline, scarce
plagioclase, alkalifeldspar, glass, and zeolite (natrolite, analcite ?).
Secondary minerals comprise serpentine, smectite, Fe oxides and hydroxi­
des, iddingsite, carbonate, zeolite. The latter paragenesis can also be
found in amygdule fillings and veins up to 30 cm thick.
Clinopyroxene is the most abundant mafic mineral with 46 to 53
volume-%, next are olivine (9 to 16 %) and the opaques (4 to 9 %). The
~------------------------------
3. 2 Phonolite
The outcrop near the Confuso
distinct types of phonolitic rocks.
variety 81 has appreciable white
glass. Microscopically the phonolitE
display a marked f1uidal texture. So
of strongly resorbed former phlogo)
aegirine, biotite and opaque ore ne€
crysts are aegi rin augi te and pl
nepheline, accessory minerals are
pleochroic amphibole (arfedsonite ?
Fe hydroxides and abundant brown
ti mes zeolite develops towards vein
silicates, carbonate, and very few
are especially abundant in veins a
ri m med by brown glass are considel
found at Cerro Confuso is cernE
contains lapilli of alkalibasaltic e
green core pyroxenes with Ti-augiti,
______~--__________. .__~____----~Jl.____________~____________
~
a
l
n
J. B. Presser
,rvations
alkaline rocks emerge as conical
ro Nemby, Cerro Confuso, Cerro
ndicators for not yet found occur­
ndstones such as Cerro Coi near
are volcanic necks. Near Puente
and shows parallelepipedic jointing
fissures. Sills are reported f rom
1ism within the AAP was accom­
ever, t rue volcaniclastic sedi ments
lbserved near Lambaré, where the
top encompasses a basal volcani­
; blocks up to m' size. The upper
raphic sequence comprise several
;h layers wi th appreciable det ri tal
raphy
igenic rocks have been observed in
icroscopic and microscopic charac­
~Iy on the alkali basalts and the
re black and aphanitic. Scoriaceous
the volcanic blocks in the basal
and to Cerro Nemby lava sheets.
lundant green peridotitic xenoliths
ved. Studies on these xenoliths are
OMIN CHIARAMONTI etal. (1986)
(enoliths found comprise sandstones
ates of decomposition and assimila­
entrained from the sedimentary
lurian age.
lave hypocrystalline porphyritic to
vine, clinopyroxene with sometimes
osion features and overgrowth by
e xenocrysts and high-p (?) pheno­
d true crystallization phases are
15 Ti
augite, diopsidic augite and
'5, a few biotite flitters, amphibole
ix composed of nepheline, scarce
1 zeolite
(natrolite, analcite ?).
, smectite, Fe oxides and hydroxi­
e latter paragenesis can also be
to 30 cm thick.
nt mafic mineral with 46 to 53
and the opaques (4 to 9 %). The
~
The Asunción Alkaline Province (Eastern Paraguay)
963
resulting color index is between 65 and 79. The light colored groundmass
totals volumetrically between 21 and 35 % and the accessory minerals
(sheet silicates, amphibole, carbonate, apatite) amount to between 0,1
and 4,4 %. However, as the latter number comprises alteration minerals,
it is mentioned only for completion and has the lowest reliability.
Notably, the tephrite from Pte. Remanso has the highest light mineral
content and the lowest color index, whereas the basanite f rom Cerro
Patiño is conspicuous becausé of its high color index and low content in
light colored minerals. Tab. 1 shows the modal compositions of sorne
alkalibasalts from the AAP.
Tab. 1. Modal composltlOns of alkalibasalts in vol.-%. 01 = Olivine, cpx =
clinopyroxene, Ig = light colored groundmass, op = opaque ores, ac =
accessory minerals, CI = color index, S = sum, TS = thin section number,
RN = rock name (First capital letter of thin section number refers to
occurrences as marked in Fig. 1).
Occurrence
TS
01
cpx
Ig
op
ac
CI
S
Nemby
Nemby
Pte. Remanso
Tacumbú
Cerro Lambaré
Cerro Patiño
Cerro Patiño
C2b
C2a
Al
Tac 1
L1
P1a
P1b
15.3
12.4
9.5
12.9
15.7
12.6
13.4
51.3
56.7
46.4
52.6
52.9
52.9
51.7
27.3
25.5
34.7
25.9
24.0
21.2
23.6
6.1
4.7
8.2
8.3
7.0
8.9
8.3
0.1
0.7
1.2
0.4
0.5
4.4
3.0
73
76
65
74
76
79
76
100.1
100.0
100.0
100.1
100.1
100.0
100.0
RN
ol-neph.
ol-neph.
tephrite
ol-neph.
ol-neph.
ne-bas.
ne-bas.
3. 2 Phonolite
The outcrop near the Confuso river (Cerro Confuso) displays two
distinct types of phonolitic rocks. Sample B2b is dar k green, the lighter
variety B1 has appreciable white specks with zeolite, carbonate and
glass. Microscopically the phonolites are hypocrystalline porphyritic and
display a marked fluidal texture. Some mrn-wide, round aggregates consist
of strongly resorbed former phlogopite (?), now replaced by fine-grained
aegirine, biotite and opaque ore needles, and a glassy matrix. Micropheno­
crysts are aegirin augite and pure acmite, sanidine/anorthoclase and
nepheline, accessory minerals are significant sphene, and sorne bluish
pleochroic amphibole (arfedsonite ?). A few opaque ore f1itters, reddish
Fe hydroxides and abundant brownish glass comprise the matrix, some­
times zeolite develops towards veins and cavities. Zeolites, green sheet
silicates, carbonate, and very few f1uorite are secondary minerals and
are especially abundant in veins and bubbles. Zircon and quartz grains
rimmed by brown glass are considered to be xenocrysts. Tuffitic material
found at Cerro Confuso is cemented by carbonate and occasionally
contains lapilli of alkalibasaltic composition and crystal fragments of
green core pyroxenes with Ti-augitic rims, and brown amphiboles. Detrital
964
P. R. Bitschene and
The Asuncion Alkaline
J. B. Presser
quartz grains have a dirty rim and are homoaxially overgrown by clear,
authigenic quartz. The basaltic lapilli are important hints that at depth
alkalibasaltic rocks existed or still exist, and that there is a very c10se
relationship between primitive basalts and evolved phonolites.
Tab. 2. Concentration of major el
rocks (Sample num bers according
from MILCH (1905; Mi 1) and STC
(after HUGHES & HUSSEY 1976).
Al
4. Geochemistry
As this is a pilot study, only one representative sample was taken
f rom each of six occur rences. Sampling was done on f resh rocks, xeno­
lithic inclusions were carefully avoided on a macroscopical scale. Two
previous analyses, though not highly reliable, of AAP rocks by STORMER
et al. (1975) and MILCH (1905) have been included for completion of
the data. Major and trace elements have been analyzed on fused glass
disks (major elem ents) and powder pellets (t race elem ents) by XRF­
analyses against well known international standards. All standards were
reproduced within analytical error, which confirms the reliabil!ty of the
data presented. Yb was determined by ICP-AES methods. Analytical
procedures and operational set-up, detection limits and standard deviations
for all elements analyzed, and reproducibility and reliability of data of
the measured international standards are reported, for instance, in
BITSCHENE (1987). Analytical conditions followed strictly the procedures
in the geochemist ry depart ment at the "Institut für Geowissenschaften
und Li thosphiirenforschung, Uni versi tiit GieEen".
4. 1 Major elements
The SiO, concentrations of the alkalibasalts range from 41 to 43 %,
MgO from 9.4 to 12.7, Na,O from 3.6 to 4.2. K,O (1.17 to 1.95) and
Al, O) (11.9 to 14.1) concentrations have a wide range, which is matched
by P, Os (0.72 to 1.03 %) and TiO, (2.2 to 2.6 %). Rather constant
concentrations are observed for CaO (11.1 to 11.8 %), Fe, O) (3.9 to 4.6)
and FeO (6.2 to 7.2) respectively, indicating equilibrium of crystallized
Ti augite and perhaps magnetite with the residual melt. The Na, O/K, O
ratio is always > 1 (2.1 to 3.5) and the M values (after HUGHES &
HUSSEY 1976) lie between 62 and 68 (Tab. 2). Thus the alkalibasalts
are typically SiO, -undersaturated, Na-rich alkaline rocks of the Atlantic
family (after NIGGLI 1923) with a primitive geochemical character
typical of not significantly differentiated rocks from the upper mantle.
Two analyses from the phonolitic rocks have been made. Remarkable
are the high AI 2 O) (20.1 and 20.6 %) , Na, O (9.8 and 10.0 %), K2 O (5.1
and 5.6 %) , and low MgO (0.31 and 0.10 %) , CaO (2.12 and 0.91 %) and
P2 0 S (0.15 and 0.07 %) concentrations. This is due to the removal of
mafic mineral phases, apatite, and the crystallization of Na-rich minerals
(aegirine, nepheline, anorthoclase, zeolite) from the residual peralkaline
melt.
In Fig. 2 the major element concentrations are plotted against Si0 2 ,
whereas the trace element concentrations are plotted against MgO as
differentiation indexo It must be noted that true differentiation trends
cannot be deduced as the occurrences are different in age, location and
composition. Nevertheless the Harker diagrams are good indicators for
C1
L1
Si0 2
Ti0 2
Al, O)
Fe 2 O)
FeO
MnO
MgO
CaO
Na,O
K2 0
P 20 S
H2 0
CO,
42.69
2.24
14.05
4.36
6.28
0.18
9.39
11.60
4.11
1.46
0.80
1.97
0.20
42.72
2.19
12.22
3.89
7.19
0.20
11.37
11.36
4.05
1.95
1.03
1.01
0.07
41.13
2.60
11.93
4.55
6.80
0.19
12.72
11.12
3.63
1.40
0.97
2.11
0.11
41
Sum
99.33
99.25
99.26
98
62
65
67
M-value
13
~
6
C
1C
11
~
1
e
6
the compositional variety of the A
are furthermore very use fui for
According to their position in the
ne - 01 - q tet rahedron (YODEI
are alkalibasalts. Samples fitted
plot into the basani tic field 3b an<
4. 2 Trace elements
The trace element budget of
concentrations in both compatible
(293 to 134 ppm) and incompatib
(117 to 96 ppm) and Sr (1197 1
correlated with MgO (Fig. 2). O
and has high Kd' s for Ni (betwe
DOSTAL et al. 1983). Addition or
explains the observed trend and d
diopside and Cr spinel, which are
be the only element which inc rea
due to suppressed plagioclase cr
WEDEPOHL 1985) and thus enricl
small scale differentiation proc
elements show no obvious trend,
patible elements are matched b'
elements, different degrees in p,
geneities of an inferred peridotit
genetic model rather than simple
64
Zbl. Geol. Palaont. 1989. l.
The Asuncion Alkaline Province (Eastern Paraguay)
i J. B. Presser
, homoaxially overgrown by clear,
He important hints that at depth
;t, and that there is a very close
1d evolved phonolites.
Tab. 2. Concentration of major elements, CO, and H, O in AAP volcanic
rocks (Sample number s according to Fig. 1 and Tab. 1) including data
from MILCH (1905; Mi 1) and STORMER et al. (1975; Sto) and M values
(after HUGHES & HUSSEY 1976).
Al
representative sample was taken
g was done on fresh rocks, xeno-
I on a macroscopical scale. Two
!iable, of AAP rocks by STORMER
been included for completion of
lave been analyzed on fused glass
)ellets (t race elements) by XRF­
¡nal standards. All standards were
eh confirms the reliabillty of the
by ICP-AES methods. Analytical
2tion limits and standard deviations
leibility and reliability of data of
are reported, for instance , In
ns followed strictly the procedures
le "Institut für Geowissenschaften
}ieJ)en".
~libasal ts
range f rom 41 to 43 %,
¡ to 4.2. K, O (1.17 to 1.95) and
le a wide range, which is matched
(2.2 to 2.6 %) . Rather constant
l.1 t o 11.8 %), Fe 2 0 3 (3.9 to 4.6)
dieating equilibrium of crystallized
the residual melt. The Na 2 0/K2 O
the M values (after HUGHES &
l (Tab. 2). Thus the alkalibasalts
·ieh alkaline rocks of the Atlantic
pnmltlve geochemical character
d rocks from the upper mantle.
Jeks have been made. Remarkable
Na, O (9.8 and 10.0 %), K, O (5.1
10 %), CaO (2.12 and 0.91 %) and
;. This is due to the removal of
erystallization of Na-rich minerals
¡te) from the residual peralkaline
~ trations are plotted against Si0 2 ,
,ions are plotted against MgO as
d that true differentiation trends
are different in age, location and
diagrams are good indicators for
965
C1
L1
P1
Tac 1
Sto
Mi 1
B1
B2b
SiO,
TiO,
Al, 0 3
Fe, 0 3
FeO
MnO
MgO
CaO
Na,O
K,O
P20 S
H,O
CO,
42.69
2.24
14.05
4.36
6.28
0.18
9.39
11.60
4.11
1.46
0.80
1.97
0.20
42.72
2.19
12.22
3.89
7.19
0.20
11.37
11.36
4.05
1.95
1.03
1.01
0.07
41.13
2.60
11.93
4.55
6.80
0.19
12.72
11.12
3.63
1.40
0.97
2.11
0.11
41.83
2.54
13.41
4.19
6.18
0.17
10.99
11.80
4.05
1.17
0.72
1.58
0.09
42.03
2.56
13.58
4.48
6.28
0.18
11.25
11.56
4.19
1.86
0.76
1.23
0.08
41.52
1.95
18.48
6.03
6.62
0.36
11. 52
5.30
4.63
2.70
0.09
0.40
n.d.
40.95
0.25
15.37
6.36
4.38
traces
10.46
11.67
3.97
1.26
0.09
3.93
n.d.
53.25
0.28
20.62
2.80
0.89
0.19
0.10
0.91
10.01
5.62
0.07
1.66
0.31
51.75
0.57
20.21
2.94
1.43
0.19
0.37
2.12
9. 79
5.14
0.15
2.84
1.20
Sum
99.33
99.25
99.26
98.72 100.04
99.60
98.98
96.71
98.70
62
65
67
62
65
5
13
M-value
66
64
the compositional variety of the AAP rocks~ Major element concentrations
are furthermore very useful for the classification of aphanitic rocks.
According to their position in the ne-normative space within the cpx ­
ne - 01 - q tetrahedron (YODER & TILLEY 1962), the basaltic rocks
are alkalibasalts. Samples fitted into aTAS diagram (ZANETTIN 1984)
plot into the basanitic field 3b and into the phonolitic field 3e.
4. 2 Trace elements
The trace element budget of the alkalibasalts is marked by high
concentrations in both compatible elements Cr (505 to 317 ppm) and Ni
(293 to 134 ppm) and incompatible elements Zr (268 to 177 ppm), Nb
(117 to 96 ppm) and Sr (1197 to 994 ppm). Cr and Ni are- posi tively
correlated with MgO (Fig. 2). Olivine bears most of the Mg available
and has high Kd's for Ni (between 10 and 58 after IRVING 1978 and
DOSTAL et al. 1983). Addition or subtraction of a few percent of olivine
explains the observed trend and differences. The same holds true for Cr
diopside and Cr spinel, which are the Cr-bearing minerals. Sr seem s to
be the only element which increases with decreasing MgO and might be
due to suppressed plagioclase crystallization at depth (compare e. g.
WEDEPOHL 1985) and thus enrichment in Sr (and Al, 0 3 and SiO,), if
small scale differentiation processes are assumed. As incompatible
elements show no obvious trend, and highest concentrations in incom­
patible elements are matched by highest concentrations in compatible
elements, different degrees in partial melting or regional/local inhomo­
genei ties of an infer red pe ridoti tic source rock are favoured as pet ro­
genetic model rather than simple fractional crystallization of a common
64
Zb!. Geo!. Paliio nt. 1989. l.
P. R. Bitschene and J. B. Presser
966
The Asunción Alkaline
parent magma. Of particular interest are the low Zr/Nb ratios (2.2 to
1.8) which indicate a relative enrichment in Nb (FLOYO & WINCHESTER
1975). Relative high Nb concentrations could result from biotite or amphi­
bole or accessory mineral (garnet, rutile ?) partial melting which have
highest Kd's for Nb compared to other peridotitic source minerals. The
Zr/Nb ratio on the other hand is particularly insensitive to the degree
of partial melting, the differences, even small so, can therefore be
attributed to small-scale, local inhomogeneities of the source rock
(PEARCE & NORRY 1979). This would favour derivation of the alkali­
basaltic rocks from a locally heterogeneous mantle source. Yb (1.5 to
2.2 ppm) and Y (24-30 ppm) concentrations are low and rather hetero­
geneous suggesting garnet as a residual phase, where the HREE are
stored (HASKIN 1984, FREY 1984), and imply regional, even local differ­
ences in REE and thus incompatible element enrichment. Differentiation
of the alkalibasaltic rocks from an inferred common parent magma
source or significantly different degrees of partial melting of a uniform
protolith would require coupled incompatible element enrichment
compatible element depletion and no change in Zr/Nb ratios. As this is
not the case, similar low-grade partial melting of a heterogeneously
enriched mantle source is the most likely process to produce the observed
decoupled trace element concentrations and ratios.
wdd
C>
C>
m
N
00
~
N
~<=>
,
,
)(
'.J
)(
1
)(
0<
o
00
ON~O,.-O
,
)(
me
,
)(
:1
)(
X
X
•
x
X
X
X
Xx
'­
>«­
X
x)
X'
)
X
o
::l
LJ
Z
LJ
,
X
X
X
X
>­
U
X
X
X
X
Oc
00000000000 o
oooc>oc>or-L.f1(Y'\IJ)('r")("r"'I(
LI"'I...,:tmN(Y)N"­
~----wdd
Tab. 3. Trace element concentrations (ppm) of sorne AAP volcanic rocks
(Sample numbers according to Fig. 1 and Tab. 2).
Cr
Co
Ni
Cu
Zn
Ca
Rb
Sr
Yb
Y
Al
C1
L1
P1
Tac1
B1
B2b
317
48
134
50
91
18
65
1197
1.9
26
413
49
214
47
98
17
50
1180
2.2
30
505
58
293
43
104
17
62
994
1.9
28
387
46
173
58
78
16
36
1080
1.5
24
376
53
203
49
80
15
62
1088
1.6
24
14
2
8
5
152
31
224
445
2.2
27
7
3
8
7
131
26
166
916
2.8
31
The slightly higher Zr/Nb ratios of the phonolites (3.3 for the more
evolved sample B 1 and 2.9 for the less evolved sample B2b), however,
are attributed to the relative enrichment of Zr due to its higher incom­
patibility during advanced stages of magmatic differentiation. The more
mafic phonolitic sample B2b has lower Zr (748 vs. 1085 ppm), Nb (256
vs. 332 ppm), Rb (166 vs. 224 ppm) and Zn (131 vs. 152 ppm) concentra­
tions than the more evolved sample B 1. Sr, which in the case of the
alkali basalts behaved incompatibly, in the case of the phonolites seems
to behave compatibly. As clinopyroxenes, sphene and apatite from phono­
litic magmas commonly have low partition coeHicients for Sr, but alkali­
feldspars or even foides have appropriate Kd's for Sr (WOERNER et al.
1983), fractionation of alkali-feldspar, and foids (and apatite ?) can
explain the Sr decrease. Sr, Ti0 2 , P 2 0 S and CaO are significantly higher
in sam pie B 2b, thus sphene, alkali mafi tes, alkali -feldspar, foids, and
N
<=>
~
I
N
~
X
X
X
X
X
el
",
~
-«
Xx
X
X'
o
X
(Y"\
al"
U.
O
LJ~
~
o
X
el
¡.=
~
)(
X
O
O
X
~
X
N
Lrl
O
X
N
X
X
o..N
c>
o
.
X
o
X
+
N"-C>N,.-c>~~rn~
------%-~M
>
o
.
~
1. . 1
o
r
J
The Asunción Alkaline Province (Eastern Paraguay)
J. B. Presser
~----wdd
.re the low Zr/Nb ratios (2.2 to
t in Nb (FLOYO & WINCHESTER
ould result from biotite or amphi­
le ?) partial melting which have
peridotitic source minerals. The
icularly insensitive to the degree
,en small so, can therefore be
nogenel tles of the source rock
favour derivation of the alkali­
neous mantle source. Yb (1. 5 to
.rions are low and rather hetero­
lal phase, where the HREE are
imply regional, even local differ­
ement enrichment. Di fferentiation
inferred common parent magma
; of partial melting of a uniform
mpatible element enrichment
lange in Zr/Nb ratios. As this is
al melting of a heterogeneously
)' process to produce the observed
and ratios.
OOC"'"'l
M
N,
0("'""")
00_0
~~r] ~':I ~'~
N,
o
g~
o
22
I
oC>
00
S?ooo
g,.~~ [.~ ~~'~I ~'~I
N"\ ' "
J
967
o
C>
...j-
'"
coa:
.S e; w
~~ rn
x
x
x
x
x
x
x
x
x .-~
x
", 3:
i ~o
x
x
><x
x
.,;<
'­
x
x
o
::J
:z:
w
x
Xx
x
x
w
.,;<
x
x
x
000000000000
oc>c>OOOOt-UlrrlLI"'lfT"l
lI1-:f"mN('I")N
o
o
rn '"
"'..:: "
..... o ID
CÜ.-\G
ID
....
X
E c ro
ID'- E
Q)
..
'"
~oO'
-----wdd
..c:­ . . . ..... =+
'" ..c:
.S..o
~E::;
-ID
ID ID ....
ro
_00000000
LI1fT"1000ll1N
o -
<:> o
000
" .... o....
ro"'ro
..... ....
ro"
O"
cro
bJj
:¿: ID _ o
'"
I
~""'f-<
.2EU)
::::2 E
ro c -­.....
1­
x
x
x
c..c:O
..c
..o
:z:
'­
N
O:::
x
x
::L
Xx
x·
>s.; .
x
x
x
..o
'­
V)
en
x
xx
x
x
>­
w
X
1><
..... :¿:
~
..... IDa:
"
ü
~
.....
ro
....
_
0'-'
ppm) of sorne AAP volcanic rocks
d Tab. 2).
" CID~
o-,
ro .... ......­
ro
:r:
N
7
6
3
8
8
6
6
O
5
4
Tac 1
B1
B2b
376
53
203
49
80
15
62
1088
1.6
24
14
2
8
5
152
31
224
445
2.2
27
7
3
8
7
131
26
166
916
2.8
31
the phonolites (3.3 for the more
ss evolved sample B2b), however,
lt of Zr due to its higher incom­
agmatic differentiation. The more
Zr (748 vs. 1085 ppm), Nb (256 Zn (131 vs. 152 ppm) concentra­
1. Sr, which in the case of the
the case of the phonolites seems
;, sphene and apatite from phono­
on coefficients for Sr, but alkali­
:e Kd's for Sr (WOERNER et al.
and foids (and apatite ?) can
and CaO are significantly higher
afites, alkali-feldspar, foids, and
Ir)
..... 0..0
O"'U
--..::: -l
U) ro­
~----O/olM
-.
'"
-'"
o
o
-'"
LrI
x
x
...-OCJ'.....a
..
-.
rn
...j-
x
x
x
_.
...j-0",
x
-.
..... ~:¿:
"'..o
c._ E
.@ (;J o
bJj.-\G ....
ro-ro ....
Vl
ro
..... ID .....
C..c: ro
ID ..... "
E
ID C ­
-ID o-x
_
.... ro
o ..... ;>,
'c;¡'~ ..o
~
x
)(
rn
Ln
)(
)(
x
)(
E
.- ro ro
x.
Lñ~
:J1
...j- ....
...j-3:
o
In
o'"
¡=
LrI
o
~N
~
w~
x
iX
o
o
x
+
+
_.
d
8'
In
Xx
¡<
x
o
x
d
)!:
x
x
~
u..
o
~
o
~
«
x x
+
+
x
x
o
o
x
x
+
x
+
N
rnV)
::Lx
x
x
x
o
x
)If.
Xx
X
o
o
en
">t::.'"
x
x
o
x
+
...j­
x
N
...j­
::;~ E
Vl
-.
.-
-.
-.
-
-.
>
E ro
ID
ro _ ~
~'"@
ro
...,ro =o
'O ro ro
c: e " ..c:
.~
o E C e;
;::; ::l e;
ro-O~
-- oÜ • r--..
ro
'" o-,
>CID~
Ir)
¡....¡
::;
-
'o
.
"'-._ o..
ID
..... ......­
N
.-.-.-.-'-~..--.-<r-'-
~----O/o~M
:z:
o
ID "ID
..c:-.-\G
..... ..0 ....
'"
Ln
X
x
)(
x C (;J
olJ~ o
.- c..c: .....
968
P. R. Bitschene and J. B. Presser
apatite, though only a very subordinate phase, govern the differentiation
processes within the shallow phonolitic magma chamber. Compositional
heterogeneity of the Confuso phonolite is therefore related to fractiona­
tion of Ti, Ca, P, and Sr (V and REE ?) bearing phases.
The Asuncion Alkaline 1
The latter is thanked for providing
ing this work. Dr. H.. FERRIZ ar
elaboration of the Spamsh abstraet
Ref
5. Geologic and petrogenetic model
The outcrops studied reveal Paleogene alkalibasaltic and phonolitic,
volcanic activity at the western fringe of the Paraná basin. The volcanic
rocks pierce Triassic to Tertiary red bed sediments and form an own
volcanic province of intracontinental, Na-rich alkaline magmatism, the
"Asunción Alkaline Province, AAP" related to reactivated rifting processes
within the South American continent during Early Tertiary times.
The volcanic rocks are modally 01 nephelinites, Ne basanites, tephrites
and phonolites. Major and trace element data indicate that the alkali­
basalts are geochemically primitive, low-grade melting products from a
h~terogeneous mantle source. Trace element data indicate that the source
rock is garnet-bearing and might have had additional phlogopite/amphibole
phases. Compared with data from GAST (1968), GREEN (1970), FREV
et al. (1978), and SUN & HANSON (1975) the AAP alkalibasaltic rocks
can be generated by low-grade partial melting (3 to 6 %) of an incom­
patible element enriched upper mantle source, in this case a garnet
Iherzolite with ± additional phlogopite/amphibole. However, to model
quantitatively multistage evolution of the AAP rocks additional REE and
isotope studies are required.
The phonolitic rocks may have derived from the alkalibasalts by
fractional crystallization processes in shallow magma chambers. The
Cerro Confuso occurrence is interpreted as an in situ crystallized, com­
positionally heterogeneous, phonolitic magma chamber, where prior to
magma solidification, maybe during for mation of a zoned magma chamber,
sphene, alkalimafites, alkali-feldspar/foid and minor apatite fractionation
were responsible for the observed magma heterogeneity.
Future work should extent the knowledge on REE and isotopic charac­
teristics, on volcanology, especially eruptive processes, within the AAP,
on the ages and their geographic distribution, and on the relationship of
the AAP rocks to the older but nearby Paraná basalts and associated
K-rich alkaline rocks in Eastern Paraguay.
Acknowledgements: Field work was made possible by a full DAAD
scholarship to the first author and through logistic assistance in Paraguay
by the "Instituto de Ciencias Basicas, Universidad Nacional de Asunción"
and its crew, especially ProL Dr. N. GONZALEZ ROMERO , Dra. G.
LEON DE LOZANO, Lic. geol. D. ALVARENGA, Lic. geol. J.-e. VELAZ­
QUEZ, Dr. F. WIENS and Dipl.-Geol. H. LECHNER-WIENS, and Lic.
geol. M. YEGROS MARC. We are especially grateful to M. GRüN­
HAUSER, Dr. j. ARETZ and ProL R. EMMERMANN, Inst. L Geowissen­
schaften und Lithospharenforschung, University of GieBen, for very helpful
and professional assistance with the geochemical work. Thanks are
extended to the "Deutsche Forschungsgemeinschaft", who encouraged
this pilot study through funds to ProL Dr. H. -J. L1PPOL T (Li 181/31-1).
BITSCHENE, P. R. (1987): Meso
Magmatismus in Ostparagu~y:
zweier Alkaliprovinzen. - D1SS.
[ UnpubI.J
BITSCHENE, P. R., L1PPOLT, H. J
kanismus in Ostparaguay (AsUI
Westrand der südamerikanischel
26; Stuttgart.
COMIN-CHIARAMONTI, P., DEMAF
VALLE, F. & SINIGOI, D. (.19
and heterogeneity from pende
and Paraguay. - Earth Planet. S
COMTE, D. & HASUI, Y. (1971):
the Potassium - Argon Method
Janeiro.
DEGRAFF, J. M. (1985): Late ME
the western edge of the Paran
Abstr.: 1 p.; Washington.
DEGRAFF, J. M., FRANCO, R. &
fisica y geologica del valle de
Assoc. Geol. Argent., Rev., XX
DEMARCHI, G., COMIN-CHIARAW
CASTILLO, C. A. M. (1989): L
Paraguay: Petrological Constn
PICIRILLO, E. M. & ,MEL~I ,
Volcanism of the Parana Basln
Aspects; Sao P~ulo (Inst. Astro
DOSTAL, J., DUPUV, c., CARRO
& MAURY, R. C. ~1983): Pi
application to volcamc rocks I
Cosmochim. Acta, 47: 525-533:
ECKEL, E. B. (1959): Geology a
Reconnaissanee. - U.S. Geol. ~
FLOVD, P. A. & WIN.CHES-r:ER,.
setting discriminatlOn USlDg 1
Lett., 27: 211-218; Amsterdarr
FREY, F. A. (1984): Rare Eart
Rocks. - In: HENDERSON, P. ,
153-204; Amsterdam (EIsevier
FREY, F. A., GREEN, D. H. &
basalt pet rogenesis: A study.
from South Eastern AustralH
pet rological data. - J. Petrol.
GAST, P. W. (1968): .Trace e!
tholeii tic and alkahc magm
1057-1082; London.
I J. B. Presser
phase, govern the differentiation
chamber. Compositional
is therefore related to fractiona­
,) bearing phases.
,e magma
The Asuncion Alkaline Province (Eastern Paraguay)
969
The latter is thanked for providing laboratory space and facilities supply­
ing this work. Dr. H. FERRIZ and M. Se. J. DEHN are thanked for
elaboration of the Spanish abstract and review of the English text.
References
rogenetic model
~ene
alkalibasaltic and phonolitic,
of the Paraná basin. The volcanic
bed sediments and form an own
Na-rich alkaline magmatism, the
ted to reactivated rifting processes
lfing Early Tertiary ti mes.
lephelinites, Ne basanites, tephrites
ent data indicate that the alkali­
Dw-grade melting products from a
,ment data indicate that the source
had additional phlogopite/amphibole
lST (1968), GREEN (1970), FREY
1975) the AAP alkalibasaltic rocks
melting (3 to 6 %) of an incom­
le source, in this case a garnet
t e/amphibole. However, to model
~he AAP rocks additional REE and
:ierived from the alkalibasal ts by
I
shallow magma chambers. The
d as an in situ crystallized, com­
magma chamber, where prior to
¡nation of a zoned magma cham ber,
id and minor apati te fractionation
la heterogenei ty.
ledge on REE and isotopic charac­
iUptive processes, within the AAP,
ibution, and on the relationship of
rby Paraná basalts and associated
ay.
made possible by a full DAAD
lugh logistic assistance in Paraguay
Universidad Nacional de Asunción"
l. GONZALEZ ROMERO, Dra. G.
ARENGA, Lic. geo!. J.-C. VELAZ­
> H.
LECHNER-WlENS, and Lic.
>specially grateful to M. GRÜN­
EMMERMANN, Inst. f. Geowissen­
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