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 hetero­geneity of the phonolitic occurrence can be explained by in situ fraction­ation 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 paleo­zanen Alters altere Rotsedimente. GemiiB ihren Haupt- und Spurenele­mentgehalten (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 geo­chemisch primitive Abk6mmlinge eines granatführenden Oberen Erdmantel­Protolithes 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

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960 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, felde­spatoide, y apatita en cámaras magmáticas someras es un proceso adecuado para explicar la variación composicional de las fonolitas.

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 varia­tion withín single occurrences is very small and often within analytical error, special emphasis was laid on samplíng of very fresh and represent­atíve 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 sedi­ments (Fíg. 1). This graben-like zone is borde red to the NE by old crystalline basement and unconformably overlying Paleozoic c1astic sedi­ments, 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 (OE­GRAFF 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".

The Asuncion Alkaline 1

"

* 10 km

.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, 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. Oetaile 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

J. B. Presser

superior que contenían granate, y La cristalizaciém fraccionada de

ilinos, feldespato alcalino, felde­náticas someras es un proceso )osicional de las fonolitas.

~tion

e 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 varia­small and often within analytical pling of very fresh and represent­It locations and not to report on occurrences.

;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 sedi­clastic sediments (HARRINGTON marked by normal and antithetic 00 and 1100 m respectively (DE­extension 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".

The Asuncion Alkaline Province (Eastern Paraguay) 961

"

* 10km

.·.·:0 > ... ·:·:·:· :·:·:·:·:·:· ... ·

.SPDh9.renzo>:· ~t <

Quotern.

I Poleo ­Asunción

Phonolitem [1] Alkoli Bosolt gene

Asunción Tertiary-Sond-,Silt-,D Claystone Guozú Trio ssic

CaacupéClastic Sedim. Sil: Ordov.~ Eo-.[am br.CoopucúAcid Ma gma!.[2]

Metasediment. Zoto Ruqua Precombr. - -_ Lineament, Fault --- GeologicOl Boundory o Town -'- - Politicol Boundary ~Ri'/er

Fig. 1. Regional distribution and geologic setting of AAP volcanic rocks 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 described 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 alkaline 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).

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962 P. R. 8itschene and J. 8. Presser

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 occur­rences 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 accom­panied 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 volcani­genic 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 charac­teristics, this chapter reports separately on the alkali basalts and the phonolites.

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 assimila­tion. 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 (?) pheno­crysts. 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 hydroxi­des, 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

The Asunción Alkaline

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 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 cpx

Nemby C2b 15.3 51.~

Nemby C2a 12.4 56.í Pte. Remanso Al 9.5 46. ~

Tacumbú Tac 1 12.9 SU Cerro Lambaré L1 15.7 52.S Cerro Patiño Pla 12.6 52.S Cerro Patiño Plb 13.4 SU

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.____________~____________

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,rvations

alkaline rocks emerge as conical ro Nemby, Cerro Confuso, Cerro ndicators for not yet found occur­ndstones 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 accom­ever, 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

n ~

The Asunción Alkaline Province (Eastern Paraguay)

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 RN

Nemby C2b 15.3 51.3 27.3 6.1 0.1 73 100.1 ol-neph. Nemby C2a 12.4 56.7 25.5 4.7 0.7 76 100.0 ol-neph. Pte. Remanso Al 9.5 46.4 34.7 8.2 1.2 65 100.0 tephrite Tacumbú Tac 1 12.9 52.6 25.9 8.3 0.4 74 100.1 ol-neph. Cerro Lambaré L1 15.7 52.9 24.0 7.0 0.5 76 100.1 ol-neph. Cerro Patiño P1a 12.6 52.9 21.2 8.9 4.4 79 100.0 ne-bas. Cerro Patiño P1b 13.4 51.7 23.6 8.3 3.0 76 100.0 ne-bas.

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 (?) pheno­d 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 hydroxi­e latter paragenesis can also be to 30 cm thick.

nt mafic mineral with 46 to 53 and the opaques (4 to 9 %). The

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. Micropheno­crysts 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, some­times 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

963

964 P. R. Bitschene and 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.

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, xeno­lithic 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 XRF­analyses 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 Si02 ,

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

The Asuncion Alkaline

Tab. 2. Concentration of major el rocks (Sample num bers according from MILCH (1905; Mi 1) and STC (after HUGHES & HUSSEY 1976).

Al C1 L1

Si02 42.69 42.72 41.13 41 Ti02 2.24 2.19 2.60 Al, O) 14.05 12.22 11.93 13 Fe2 O) 4.36 3.89 4.55 ~

FeO 6.28 7.19 6.80 6 MnO 0.18 0.20 0.19 C MgO 9.39 11.37 12.72 1C CaO 11.60 11.36 11.12 11 Na,O 4.11 4.05 3.63 ~

K2 0 1.46 1.95 1.40 1 P 2 0 S 0.80 1.03 0.97 e H2 0 1.97 1.01 2.11 CO, 0.20 0.07 0.11

Sum 99.33 99.25 99.26 98

M-value 62 65 67 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 increa 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.

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.

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 %), Fe20 3 (3.9 to 4.6) dieating equilibrium of crystallized the residual melt. The Na20/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 Si02 , ,ions are plotted against MgO as d that true differentiation trends are different in age, location and diagrams are good indicators for

The Asuncion Alkaline Province (Eastern Paraguay) 965

Tab. 2. Concentration of major elements, CO, and H, O in AAP volcanic rocks (Sample numbers 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 C1 L1 P1 Tac 1 Sto Mi 1 B1 B2b

SiO, 42.69 42.72 41.13 41.83 42.03 41.52 40.95 53.25 51.75 TiO, 2.24 2.19 2.60 2.54 2.56 1.95 0.25 0.28 0.57 Al, 0 3

14.05 12.22 11.93 13.41 13.58 18.48 15.37 20.62 20.21 Fe, 0 3 4.36 3.89 4.55 4.19 4.48 6.03 6.36 2.80 2.94 FeO 6.28 7.19 6.80 6.18 6.28 6.62 4.38 0.89 1.43 MnO 0.18 0.20 0.19 0.17 0.18 0.36 traces 0.19 0.19 MgO 9.39 11.37 12.72 10.99 11.25 11. 52 10.46 0.10 0.37 CaO 11.60 11.36 11.12 11.80 11.56 5.30 11.67 0.91 2.12 Na,O 4.11 4.05 3.63 4.05 4.19 4.63 3.97 10.01 9. 79 K,O 1.46 1.95 1.40 1.17 1.86 2.70 1.26 5.62 5.14 P 2 0 S 0.80 1.03 0.97 0.72 0.76 0.09 0.09 0.07 0.15 H,O 1.97 1.01 2.11 1.58 1.23 0.40 3.93 1.66 2.84 CO, 0.20 0.07 0.11 0.09 0.08 n.d. n.d. 0.31 1.20

Sum 99.33 99.25 99.26 98.72 100.04 99.60 98.98 96.71 98.70

M-value 62 65 67 66 64 62 65 5 13

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 incom­patible elements are matched by highest concentrations in compatible elements, different degrees in partial melting or regional/local inhomo­genei ties of an infer red pe ridoti tic source rock are favoured as pet ro­genetic model rather than simple fractional crystallization of a common 64 Zb!. Geo!. Paliio nt. 1989. l.

966 P. R. Bitschene and J. B. Presser

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 amphi­bole 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 hetero­geneous suggesting garnet as a residual phase, where the HREE are stored (HASKIN 1984, FREY 1984), and imply regional, even local differ­ences 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.

Tab. 3. Trace element concentrations (ppm) of sorne AAP volcanic rocks (Sample numbers according to Fig. 1 and Tab. 2).

Al C1 L1 P1 Tac1 B1 B2b

Cr 317 413 505 387 376 14 7 Co 48 49 58 46 53 2 3 Ni 134 214 293 173 203 8 8 Cu 50 47 43 58 49 5 7 Zn 91 98 104 78 80 152 131 Ca 18 17 17 16 15 31 26 Rb 65 50 62 36 62 224 166 Sr 1197 1180 994 1080 1088 445 916 Yb 1.9 2.2 1.9 1.5 1.6 2.2 2.8 Y 26 30 28 24 24 27 31

The Asunción Alkaline

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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 incom­patibility 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) concentra­tions 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 phono­litic 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 el

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favour derivation of the alkali ­neous mantle source. Yb (1. 5 to .rions are low and rather hetero­lal phase, where the HREE are imply regional, even local differ­

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ppm) of sorne AAP volcanic rocks d Tab. 2).

Tac 1 B1 B2b

7 376 14 7 6 53 2 3 3 203 8 8 8 49 5 7 8 80 152 131 6 15 31 26 6 62 224 166 O 1088 445 916 5 1.6 2.2 2.8 4 24 27 31

the phonolites (3.3 for the more ss evolved sample B2b), however, :J1

lt of Zr due to its higher incom­agmatic 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 phono­on coefficients for Sr, but alkali ­:e Kd's for Sr (WOERNER et al.

and foids (and apatite ?) can and CaO are significantly higher

d

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The Asunción Alkaline Province (Eastern Paraguay) 967

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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 fractiona­tion of Ti, Ca, P, and Sr (V and REE ?) bearing phases.

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 alkali­basalts 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 incom­patible 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, com­positionally 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 charac­teristics, 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. VELAZ­QUEZ, Dr. F. WIENS and Dipl.-Geol. H. LECHNER-WIENS, and Lic. geol. M. YEGROS MARC. We are especially grateful to M. GRüN­HAUSER, Dr. j. ARETZ and ProL R. EMMERMANN, Inst. L Geowissen­schaften 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).

The Asuncion Alkaline 1

The latter is thanked for providing ing this work. Dr. H.. FERRIZ ar elaboration of the Spamsh abstraet

Ref

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 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

I J. B. Presser

phase, govern the differentiation ,e magma chamber. Compositional is therefore related to fractiona­

,) bearing phases.

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 alkali­Dw-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 incom­le source, in this case a garnet t e/amphibole. However, to model ~he AAP rocks additional REE and

:ierived from the alkalibasal ts by shallow magma chambers. The

d as an in situ crystallized, com­magma 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 charac­iUptive 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. GRÜN­EMMERMANN, Inst. f. Geowissen­versity of GieBen, for very helpful

geochemical work. Thanks are Igsgemeinschaft", who encouraged Dr. H.-J. LIPPOLT (Li 181/31-1).

The Asuncion Alkaline Province (Eastern Paraguay)

The latter is thanked for providing laboratory space and facilities supply­ing this work. Dr. H. FERRIZ and M. Se. J. DEHN are thanked for elaboration of the Spanish abstract and review of the English text.

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Addresses of the authors:

Dr. P. R. BITSCHENE, Institut UniversitatsstraEe 150, 0-4630 BOl

Lic.geo!. J. BAEZ PRESSER, Instit Paraguay.

1J. B. Presser

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The Asuncion Alkaline Province (Eastern Paraguay) 971

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Addresses of the authors:

Dr. P. R. BITSCHENE, Institut für Mineralogie der Ruhr-Universitat, UniversitatsstraBe 150, D-4630 Bochum, FRG.

Lic.geol. J. BAEZ PRESSER, Instituto de Ciencias Básicas, UNA, Asunción, Paraguay.