Calc-alkaline and tholeiitic dyke swarms of Tandilia, Rio de la Plata craton, Argentina: U–Pb, Sm–Nd, and Rb–Sr 40Ar/ 39Ar data provide new clues for intraplate rifting shortly

Calc-alkaline and tholeiitic dyke swarms of Tandilia, Rio de laPlata craton, Argentina: U�/Pb, Sm�/Nd, and Rb�/Sr 40Ar/39Ardata provide new clues for intraplate rifting shortly after the

Trans-Amazonian orogeny

W. Teixeira a,�, J.P.P. Pinese b, M. Iacumin c, V.A.V. Girardi a, E.M. Piccirillo c,H. Echeveste d, A. Ribot d, R. Fernandez d, P.R. Renne e, L.M. Heaman f

a Centro de Pesquisas Geochronological, Institute of Geosciences, University of Sao Paulo, Rua do Lago 562, 05422-970 Sao Paulo, Brazilb Department of Geoscience, University of Londrina, P.O. Box 6001, 86051-990 Londrina, Brazil

c Dipartimento di Scienze della Terra, University of Trieste, Via Weiss 8, 34127 Trieste, Italyd University of La Plata, INREMI and LEMIT, Calle 52, P.O. Box 128, 1900 La Plata, Argentina

e Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USAf Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alta., Canada T6G 2E3

Received 1 April 2001; received in revised form 28 November 2001; accepted 19 March 2002

Abstract

The Tandilia system, Argentina, southernmost part of Rio de la Plata craton (RLPC), is intruded by two

Paleoproterozoic unmetamorphosed dyke swarms which are: (i) intermediate (I) and acid (A); and (ii) basic (B1 and B2)

in composition. The latter dykes have tholeiitic characteristics, while for both I and A dykes major, minor, and trace

elements, including REE (e.g. the higher values of SiO2, K2O and Ba compared to tholeiitic dykes) are characteristic of

calc-alkaline suites. The calc-alkaline dykes (I and A) yielded 40Ar/39Ar step-heating plateau ages of emplacement of

20209/24 and 20079/24 Ma. These ages are within error in agreement with a Rb�/Sr errorchron of 19569/110 Ma (1s)

[initial 87Sr/86Sr�/0.70389/0.0025 (MSWD�/19)]. An upper intercept U�/Pb age on two baddeleyites from a tholeiitic

(B1) dyke places the intrusion at 15889/11 Ma. The tholeiitic dykes (B1 and B2) have K�/Ar whole-rock ages from 8039/

14 to 11939/18 Ma and a 40Ar�/39Ar plateau age of 8119/36 Ma (2s) on plagioclase. These discordant apparent ages

suggest variable Ar loss of the mineral systems. Calc-alkaline dykes mainly trend E�/W, and were emplaced during the

transtensional stage of the Trans-Amazonian orogeny during which the plutonic rocks of the Tandilia system were

formed. Such a scenario has similarities with the Eburnean evolution of the Richtersveld plutonic arc complex of the

southern African subcontinent (Namaqualand) that faces the RLPC in the West Gondwana reconstruction. The

significantly younger tholeiitic dykes of Tandilia (1.59 Ga) trend mainly NW-SE. Their presence constrains the time of

crustal extension at the Paleo-Mesoproterozoic boundary during which basin-formation tectonics and anorogenic

magmatism took place worldwide within a stabilized Paleoproterozoic lithosphere. Such an intraplate regime for the

emplacement the youngest dykes of Tandilia is consistent with transcontinental scale, diachronous extensional episodes

� Corresponding author. Tel.: �/55-11-3091-4274; fax: �/55-11-3091-4295

E-mail address: [email protected] (W. Teixeira).

Precambrian Research 119 (2002) 329�/353

www.elsevier.com/locate/precamres

0301-9268/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.

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within the South American continent which initiated shortly after the Trans-Amazonian orogeny, as illustrated by the

1.73 Ga Florida tholeiitic dyke swarm and coeval, anorogenic granitoids scattered across the RLPC (Uruguay and

Tandilia). Paleoproterozoic geologic features of the RLPC*/namely, the occurrence of plutonic arc rocks (2.14�/2.07

Ga) succeeded by emplacement of anorogenic granitoids and mafic dykes (1.73�/1.59 Ga)*/allow direct correlation

with the postulated Gondwana counterpart, mirrored by broadly contemporary plutonic rocks of the Richtersveld and

Bushmanland subprovinces of Namaqualand. The broad picture reinforces the idea that the Trans-Amazonian/

Eburnean orogenies played an important role for juvenile crustal accretion within the southern South America and its

southern Africa counterpart, which was followed by a tendency to dispersion of the stabilized continental fragments

during the Mesoproterozoic, preceding the assembly of the Rodinia supercontinent.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Geochronology; Paleoproterozoic dykes; Tandilia; Rio de la Plata craton; Africa

1. Introduction

Precambrian mafic dyke swarms record initial

stages of intraplate rifting or at least manifesta-

tions of crustal extension, which may be associated

with the orogenic collapse of mobile belt evolu-

tion. However, in most cases, mafic dykes cannot

be dated reliably by Rb�/Sr and K�/Ar methods

when minor post-crystallization alteration is pre-

sent. Fortunately, accurate dating of dyke empla-

cement can be obtained by the U�/Pb method

applied to baddeleyite and zircon. In general,

baddeleyite is better because, unlike zircon, it

tends to be concordant (i.e. 98�/100% concordant),

and since baddeleyite is rare in crustal rocks,

inheritance is not common in mafic rocks (Le-

Cheminant and Heaman, 1989; Heaman, 1991). In

addition, the 40Ar�/39Ar technique has been useful

for dating either primary biotite minerals from

mafic dykes or micas from contact-baked meta-

morphic country rocks in sharp contact with them

(Renne et al., 1990; Teixeira et al., 1999).

This paper presents results of isotopic dating

(40Ar�/39Ar mineral step heating, U�/Pb badde-

leyite, and Rb�/Sr and Sm�/Nd whole-rock ana-

lyses) on two distinct Proterozoic dyke swarms

that occur in the Tandilia system, Rio de la Plata

craton (RLPC)*/an igneous�/metamorphic com-

plex (Cingolani and Dalla Salda, 2000) formed

during the Trans-Amazonian orogeny (2.2�/2.0

Ga). Geochronologic data along with geochemical

results, constrain the origin of these dykes, as well

as their relationship with Paleoproterozoic crustal

evolution. Comparison of these rocks with coeval

geologic units within the southern African con-

tinent is presented, providing a better understand-

ing of the nature of the tectonism at the Paleo-

Mesoproterozoic boundary, as well as new insights

on the reconstruction of supercontinents during

the Proterozoic.

2. Geologic and geochronologic overview

The RLPC is one of the cratons in the contin-

uous network of mobile belts developed during the

Brasiliano/Pan-African orogenies of Western

Gondwanaland (Brito Neves and Cordani, 1991).Basement of the RLPC is known as the Piedra

Alta terrane in the western part of the Uruguayan

Shield (Fig. 1). This terrane includes three distinct

volcano-sedimentary sequences with low-grade

metamorphism and plutonic suites of TTG affinity

(Cingolani et al., 2001) all involved in the Trans-

Amazonian orogeny. The Florida dyke swarm

(Bossi et al., 1993; Mazzucchelli et al., 1995) wasemplaced into the Piedra Alta terrane at 1730 Ma

and was not later deformed, thereby constraining

the minimum age of tectonic stabilization of the

RLPC (Teixeira et al., 1999). The Piedra Alta

terrane is separated from the Nico Perez terrane

(Fig. 1) by an expressive subvertical shear zone

(Bossi et al., 1993). The latter terrane is allochto-

nous and has a different geologic history from itsneighbours*/the western Paleoproterozoic Piedra

Alta and the eastern Neoproterozoic terranes.

Piedra Alta granitoids display Rb�/Sr isochron

whole-rock ages between 2.4 and 2.0 Ga with

relatively low initial 87Sr/86Sr ratios (Preciozzi and

Bourne, 1992; Cingolani et al., 2001). SHRIMP

W. Teixeira et al. / Precambrian Research 119 (2002) 329�/353330

U�/Pb zircon analyses on volcanic rock from a

supracrustal belt yielded an age of 21409/10 Ma

while intrusive tonalite and granodiorite yielded

20809/15 Ma (Bossi et al., 2001). These plutons

are contemporary with granitic rocks of the Isla

Mala suite, as indicated by SHRIMP U�/Pb zircon

ages between 20659/9 and 20749/6 Ma (Hartmann

et al., 2000) and a conventional U�/Pb zircon age

of 20889/12 on granodiorite from this suite (Pre-

ciozzi et al., 1999). Migmatites, gneisses and

granitoids in the Piedra Alta terrane give Nd

model ages (TDM) between 2.45 and 2.06 Ga

Fig. 1. Distribution of main (Paleoproterozoic) cratons in the West Gondwana (a), showing the adjacent Meso- and Neoproterozoic

tectonic framework in the southern South America and southwest corner of Africa (b). (c) Geologic sketch map of the Rio de la Plata

craton (Piedra Alta (PA) terrane and the Tandilia system), showing the dyke occurrences and the adjoinging the Nico Perez (NP)

terrane and the Neoproterozoic Dom Feliciano belt (adapted from Bossi et al., 1993; Dalla Salda et al., 1988). See text for details.

W. Teixeira et al. / Precambrian Research 119 (2002) 329�/353 331

(Preciozzi et al., 1999). Therefore, the Trans-Amazonian orogeny was a period juvenile accre-

tion and regional metamorphism. Regional cool-

ing of the Piedra Alta terrane took place around

2.0�/1.9 Ga, based on K�/Ar and 40Ar�/39Ar ages

on the gneissic rocks (Teixeira et al., 1999).

The Nico Perez terrane has had a contrasting

crustal evolution compared with that of the Piedra

Alta terrane. Geologic mapping supported bySHRIMP U�/Pb zircon dating indicates the occur-

rence of an Archean nucleus of about 3100 Ma

(protoliths as old as 3410 Ma) which is surrounded

by a 2600 Ma granulite facies gneissic complex

(Bossi et al., 2001). This complex was further

subjected to reworking and crustal shortening at

2100 and 1200 Ma, respectively (Bossi et al., 2001;

Cingolani et al., 2002). Rapakivi granites (1.75 Ga)are also present in the Nico Perez terrane, as well

as felsic and mafic magmatism, dated between 0.70

and 0.56 Ga (Rivalenti et al., 1995; Girardi et al.,

1996; Preciozzi et al., 1999; Bossi et al., 2001). This

younger igneous activity is probably linked to the

Brasiliano orogeny (Dom Feliciano belt) that

affected the eastern region of the Uruguayan

Shield (Basei et al., 2000, for review).The RLPC continues to the south under the

Pampas (Chaco-Parana basin) sedimentary rocks

down to the Martin Garcia island and further to

the isolated ranges of Bayas, Azul, Balcarce, Alta

de Vela, Del Tigre and Tandil in Argentina (Figs. 1

and 2) where the basement rocks are collectively

known as the Buenos Aires complex (Marchese

and Di Paola, 1975; Cingolani and Dalla Salda,2000). This complex consists of gneisses, schists

and migmatites intruded by tonalitic�/granitic and

leuco-monzogranitic plutons, which make up a

WNW-ESE igneous-metamorphic belt*/the Tan-

dilia system (Dalla Salda et al., 1988, 1992).

The oldest tonalitic�/granitic plutonic suite of

the Tandilia system (based on Rb�/Sr isochron

ages between 2150 and 1970 Ma) occurs mainlyalong an east�/west shear belt in the northern part

of the complex, and is interpreted to reflect the

syn-collisional phase of the Trans-Amazonian

orogeny (Halpern and Linares, 1970; Ramos et

al., 1990; Dalla Salda et al., 1992; Dalla Salda,

1999; Cingolani et al., 2001). New SHRIMP U�/Pb

ages (2228�/2051 Ma) which are also comparable

with a Sm/Nd whole-rock isochron of 21409/80Ma on the same plutonic rocks, confirm that the

magmatic crystallization took place in the Paleo-

proterozoic. However, crustal-signature Nd model

ages are in the 2.7�/2.4 Ga range (Cingolani et al.,

2001; Pankhurst et al., 2001). Younger leucocratic

monzogranites (Rb�/Sr isochron age of 17709/88

Ma; Varela et al., 1988) occur in the Tigre and

Alta de Vela Ranges, together with acid metavol-canic rocks; their emplacement was associated

with wrench faults striking NNE-SSW (Dalla

Salda, 1981; Ramos et al., 1990). Transcurrent

tectonics have led to the appearance of several

wide W�/E shear zones in Tandilia (Fig. 2) that

produced cataclastic phenomena including mylo-

nitic fabrics in both the oldest plutonic suite and

monzogranites. The record of multiple deforma-tion events and intrusive magmatism in the

Tandilia system suggest that the Trans-Amazonian

orogeny involved continent�/continent collision

(tonalitic�/granitic plutons) as well as postcolli-

sional events including generation of leuco-mon-

zogranites and activity of strike-slip faults. The

geologic scenario is consistent with a plutonic arc

setting in late Paleoproterozoic times, which wasfollowed by crustal recycling within the southern-

most part of the RLPC (Dalla Salda, 1981; Dalla

Salda et al., 1992).

The tonalitic�/granitic suite yields initial87Sr/86Sr ratios between 0.702 and 0.706, suggest-

ing that Trans-Amazonian mantle-derived magma

was important in its genesis (Varela et al., 1988). In

contrast, the monzogranitic suite yields a highinitial 87Sr/86Sr value (0.7181), indicating it had an

anatectic origin (Varela et al., 1988; Dalla Salda et

al., 1988, 1992). The Montecristo leucogranite has

a contrasting origin, as supported by its TDM of

2.2 Ga (Cingolani et al., 2002). The above Sr and

Nd data suggest that, although the major period of

juvenile crust production was Paleoproterozoic,

late Archean crustal components have been im-portant during the history of the Tandilia rocks.

The Tandilia system contains two swarms of

unmetamorphosed Paleoproterozoic mafic dykes.

Most of the studied dykes occur in the vicinity of

Tandil (e.g. Del Tigre, Tandil, Alta da Vela ranges;

Fig. 2), but another swarm of dykes crops out in

the northwest part of study area, near Azul (Fig.


Fig. 2. Geologic sketch of the Tandilia system (after Dalla Salda et al., 1992). Keys: Tandilia dykes (1); Buenos Aires complex

(Paleoproterozoic): granitoids (2), migmatites, gneisses and schists (3), S-type leucomonzogranites (4); shear zones (5); faults (6). (a)

shows the main occurrences of dykes in Azul region.


2a). Previous K�/Ar ages for these dykes vary

from 1750 to 1070 Ma (Teruggi et al., 1974a,b;

Cortelezzi and Rabassa, 1976), suggesting that

variable Ar losses have affected the mineral

systems.

Field relations indicate that the relatively oldest

dykes exhibit dominantly E�/W trends, and are

related to the transational stage of the Trans-

Amazonian orogeny (Teruggi et al., 1973, 1974a).

The relatively youngest dykes have dominantly

NW trends and crosscut both the 1.77 Ga leuco-

monzogranites and the regional E�/W shear zones

(Dalla Salda, 1981; Teruggi et al., 1988; Dalla

Salda et al., 1988). Their emplacement is asso-

ciated with late extensional stress, following cra-

tonization of the Tandilia system (Fig. 2). Detailed

mapping of a thick N30W dyke in the Del Tigre

Range (Fernandez et al., 2001) reveals that,

locally, the late extensional stress may follow

pre-existing weakness planes of the E�/W shear

zone, so a few branches of coeval dykes trend

similarly E�/W (not shown in Fig. 2).

The Tandilia basement rocks have K�/Ar whole-

rock ages between 980 and 790 Ma (Linares,

1977), suggesting the influence of low-grade ther-

mal overprints related to the Neoproterozoic Dom

Feliciano belt that flanks the RLPC (see above).

The La Tinta Group*/a 400 m thick sedimentary

marine sequence*/partly overlies the Tandilia

system (Rapela et al., 1974; Dalla Salda and

Iniguez, 1979; Cingolani and Bonhomme, 1982).87Sr/86Sr measurements in La Tinta argillites show

that diagenesis took place between 7349/48 and

7259/36 Ma (Kawashita et al., 1999), in agreement

with a Rb�/Sr isochron age of 7239/19 Ma for

shales of the middle La Tinta Group (Bonhomme

and Cingolani, 1980; Dalla Salda et al., 1988).

However, a Rb�/Sr isochron age of 7699/23 Ma

was reported for pelites of the lower La Tinta

Group (Bonhomme and Cingolani, 1980). The

isotopic signatures and the stratigraphy of the La

Tinta Group support its correlation with the lower

Nama Group in South Africa, thereby suggesting

that a continuous sedimentary basin covered the

South America/Africa boundary during the Neo-

proterozoic (Dalla Salda et al., 1988; Kawashita et

al., 1999).

3. Petrography and geochemistry of the Tandiliadykes

In general, the Tandilia dykes are very fresh,

although minor alteration may be locally present.

Petrography and whole-rock geochemistry (Iacu-

min, 1998; Iacumin et al., 2001) allow distinction

of four types dykes in Tandilia, as summarized

below (Table 1): (1) basic dykes with subophiticand sometimes intergranular textures (designated

here as B1 type*/63% of the dykes); (2) basic

dykes with ophitic texture (B2*/12% of the dykes);

(3) intermediate dykes with porphyritic to inter-

granular texture (I*/10%) and (4) acid dykes with

porphyritic texture (A*/15%). The acid and inter-

mediate dykes may have a cataclastic fabric.

The (B1) dykes, which occur in the Azul andTandil ranges, are fine-grained at the borders and

coarse-grained in the center. They characteristi-

cally have labradoritic plagioclase, somewhat

altered to clay, and pyroxene (augite and minor

pigeonite) which may be locally replaced by

hornblende, tremolite, and chlorite. In general,

olivine is scarce and exclusive to low-Ca pyroxene-

free dykes. Accessory minerals include magnetite,ilmenite, quartz, apatite, and epidote. The scarce

(B2) dykes crop out in the Tandileiufu Hill and

Alta da Vela, Del Tigre and Tandil Ranges. They

are medium- to fine-grained, in contrast to the (B1)

dykes. Their mineralogy also differs from that of

the (B1) dykes, as they contain Ti-augite and have

no olivine. The labradoritic plagioclase in the (B2)

dykes is somewhat altered to clay and sericite, andis locally albitized. The clinopyroxene is locally

replaced by hornblende, actinolite�/tremolite and

chlorite. Accessory minerals are ilmenite, magne-

tite, and apatite.

The (I) dykes, which are exposed in the Tandi-

leiufu Hill and Del Tigre and Tandil ranges,

exhibit medium-fine grained texture. They have

phenocrysts of andesine-plagioclase and augite,both locally altered to clay minerals and amphi-

bole, respectively. The groundmass is made up of

plagioclase, epidote, biotite, opaques, alkali-feld-

spar, and quartz. Where the fabric is intergranular

the plagioclase and augite are partly altered to clay

and amphibole. Granophyric quartz�/feldspar in-

tergrowths are common. The (A) dykes are


Table 1

Main petrographic and geochemical characteristics of the Tandilia dykes

Tandilia dykes

Suite Tholeiitic Calc-alkaline

Type B1 B2 I A

Occurrence Azul and Tandil Tandil Tandil, Tigre, Albion and Tandileufu Tanges Tandil, Tigre, Albion and

Tandileufu ranges

Thickness (m) 10�/50 0.5�/10 0.5�/10 0.5�/30

Strike (domi-

nant trend)

NW-SE NW-SE E�/W E�/W

Petrography Subophitic and intergranular tex-

tures. Labradorite, augite or pigeo-

nite and olivine

Ophitic texture. Labradorite, au-

gite (high TiO2) and pigeonite

Porphyritic to intergranular textures. Labra-

dorite to andesine, salitic�/augitic pyroxenes

and Qz-Kf intergrowths

Porphyritic texture. Oligo-

clase, alkali-feldspar and

augite

Geochemistry Low TiO2 (0.87�/1.66), low contents

of IE. REE patterns similar to E-

MORB, with mean (La/Yb)N of

1.559/0.48

High TiO2 (1.71�/3.74 wt.%), high

contents of P2O5, Zr, Nb and other

IE. REE pattern: high (La/Yb)N of

6.72

Lower mean contents of TiO2 and higher P2O5,

Sr, REE and Zr, relative to B1. REE patterns:

higher (La/Yb)N mean values (10.939/0.64),

differing from the B1 and B2 types. Negative

spikes of Nb

SiO2�/65�/75 wt.%. High-

est contents of IE and very

high (La/Yb)N values (15�/

40)

Data from this paper and Iacumin et al. (2001). See text for details.

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parallel with and sometimes intruded by (I) dykes.

They contain phenocrysts of oligoclase�/plagio-

clase, alkali-feldspar, and augite, which also con-

stitute the groundmass. Secondary zeolites or

quartz and K-feldspars may fill vesicles or frac-

tures.

The (B1) dykes correspond to tholeiitic basalts

and subordinately to tholeiitic andesitic basalts

and latibasalts (Iacumin, 1998). The (B2) dykes

vary from tholeiitic andesitic basalts to latibasalts

and trachybasalts (Fig. 3). The (I) dykes plot in the

andesitic basalt and subordinately andesite fields,

while the (A) dykes fall in the rhyolite field. In an

AFM diagram (Fig. 4), (B1) and (B2) dykes plot in

the tholeiitic field whereas both (I) and (A) dykes

plot in the calc-alkaline field.

Averages of major, minor and trace elements (54

samples), including REE, reveal genetic differences

between the tholeiitic and calc-alkaline dykes (Fig.

5; Tables 2 and 3), as illustrated by the (I) dykes

that have higher SiO2, K2O, Na2O, Ba and Rb

than the tholeiites. The latter dykes have otherwise

higher mean contents of TiO2, FeOt, CaO and Ni

(Table 2). It is noteworthy that the tholeiitic dykes

with TiO2 higher than 1.7 wt.% (B2 type) are more

enriched, for a given MgO content, in P2O5, FeOt,

K2O, Ba, Sr, La, Ce, Nd, Zr, Y and Nb relative to

those with TiO2 B/1.7 wt.% (B1 type).

The (I, A) dykes are characterized by REE

patterns with (La/Lu)N much higher (I�/12�/10,

A�/14�/40) than those of the tholeiitic dykes

(B1�/1.3�/2.8, B2�/6.63; see average values in

Table 3). The genetic differences between the

calc-alkaline and tholeiitic dykes are also sup-

Fig. 3. R1 versus R2 diagram (de la Roche et al., 1980, modified

by Bellieni et al. (1981)) for the Tandilia dykes. B1 and B2,

basic, tholeiitic types. (I), intermediate dykes; (A), acid dykes (I

and A, calc-alkaline types. R1�/[4Si�/11(Na�/K)�/2(Fe�/Ti)]

and R2�/[6Ca�/2Mg�/Al].

Fig. 4. AFM diagram for the Tandilia dykes (A�/Na2O�/

K2O; F�/FeO; M�/MgO). Calc-alkaline-tholeiitic boundary

(solid curve) as proposed by Irvine and Baragar (1971);

tholeiitic field (dashed curve) after Macdonald and Katsura

(1964). Symbols and legend as in Fig. 3.

Fig. 5. Diagrams of REE abundance patterns normalized to

chondrite (after Boynton, 1984) for selected Tandilia dykes.

Symbols and legend as in Fig. 3.


ported by the significant negative Eu anomaly of

the (A) dykes, as well as the characteristic flat REE

pattern of the (B1) dykes (Fig. 5). The calc-alkaline

dykes are more enriched in LREE [(La/Sm)N

ranging from 3.1 to 6.3] relative to the (B1) types

(B1�/0.9�/1.9). The B2 (high-Ti) dykes also have

an enriched LREE pattern, but in conjunction

with the B1 (low-Ti) are generally more enriched in

HREE than the calc-alkaline dykes (Fig. 5; Table

3).

Calc-alkaline dykes have incompatible elements

(IE) contents and calculated 87Sr/86Sr initial values

(Sr0) typically lower than or similar to those of the

Tandilia tonalitic�/granitic suite (see previous sec-

tion and Table 6). Thus, crustal contamination is

not easy to detect. These dykes are characterized

by oNd values similar to those of an enriched

mantle component. Tholeiitic dykes with low-Ti

(B1 type) and high-Ti (B2) contents display sig-

nificant Sr0 variations up to 0.711 (Table 6), but

the Sr0 increases are not correlated with SiO2,

MgO, K2O Rb, Ba, La, Zr and Nd. This feature,

coupled with mass balance calculations, led Iacu-

min et al. (2001) to exclude appreciable crustal

contamination for (B1) and (B2) dykes having Sr0

values lower than 0.705. In addition, the Sr0 values

higher than 0.707 do not even conform to AFC

mixing contamination curves, thereby suggesting

that processes like low-grade alteration in some of

the studied dykes (see above) may have locally

contributed towards reaching these highest Sr0

values. Mass balance calculations, coupled with

chemical characteristics and differences in discri-

mination variation diagrams (Zr�/Ni and La�/

Table 2

Average major (wt.%) and trace (ppm) element contents for the Tandilia dykes

B1 B2 I A

N�/34 S.D. N�/7 S.D. N�/5 S.D. N�/8 S.D.

SiO2 50.66 0.05 49.06 0.85 57.51 1.82 73.19 3.16

TiO2 1.34 0.13 2.01 0.32 0.69 0.17 0.20 0.14

Al2O3 13.67 0.95 14.16 0.19 14.55 0.51 13.59 0.57

FeOt 12.57 0.95 14.80 0.43 8.79 0.50 2.54 1.34

MnO 0.22 0.01 0.22 0 0.17 0.01 0.06 0.03

MgO 7.53 0.09 6.36 0.56 5.80 1.62 0.54 1.09

CaO 11.22 0.49 9.01 0.22 7.51 1 1.62 0.94

Na2O 2.17 0.23 2.49 0.17 2.88 0.69 4.08 0.54

K2O 0.49 0.22 0.97 0.05 1.83 0.44 4.11 0.68

P2O5 0.12 0.03 0.85 0.15 0.28 0.11 0.05 0.06

LOI 2.92 0.83 2.71 0.62 2.62 0.72 1.62 1.24

Mg# 54.81 2.18 46.43 1.93 56.27 6.94 16.90 18.11

Cr 168 4. 105 24 103 67 12 24

Ni 100 0.7 87 12 17 4 3 4

Rb 22 11 46 4 88 48 149 31

Ba 78 69 482 56 529 166 1140 207

Sr 197 11 412 28 523 131 263 129

Nb 3 0.7 15 1.3 8 2.2 11 1.4

Zr 78 27 192 27 113 27 212 35

Y 18 5 24 0.50 21 6 26 6

La 4 0.7 30 1.6 23 8 54 7

Ce 10 4 67 9 53 14 100 12

Nd 8 2 40 4 27 9 45 6

See Iacumin et al. (2001) for details. Major elements recalculated to 100% on volatile-free basis. N , number of samples; S.D.,

standard deviations (1s). Symbols and legend as in Fig. 3. Major and trace elements were determined by X-ray fluorescence, at

University of Trieste (Italy), using a PW 1404 XRF spectrometer, following the procedures of Philips (1994) for the correction of

matrix effects. The accuracy of the results are within 2�/3% for major and minor elements and better than 7�/10% for trace elements.

Loss on ignition (LOI) was determined at 1100 8C (12 h) and corrected for FeO oxydation.


Ni) all support the idea that (B1) and (B2) dykesmust be related to distinct parental magmas. The

available oNd data (Table 7) reflect that (B1) dykes

may have been derived from a depleted source

mantle, whereas the (B2) dykes were derived from

an enriched mantle source (Iacumin et al., 2001,

for details).

In summary, chemical and isotopic data indicate

that calc-alkaline (I, A) dykes, low-Ti (B1) andhigh-Ti (B2) tholeiitic dykes of Tandilia derived

from heterogeneous source mantle. The variable

metasomatic enrichment may have occurred in

Late Archean to Early Paleoproterozoic times,

based on Sm�/Nd geochemistry (Iacumin et al.,

2001, for details).

4. Dating techniques

40Ar�/39Ar analyses (Table 4; see Iacumin, 1998)

were performed on biotite from two baked wall

rocks directly adjacent to the contact with (A) and

(I) calc-alkaline dykes, respectively. A plagioclase

from a tholeiitic (B1) dyke was also analysed. The

minerals were irradiated for 100 h in the Triga

reactor at Oregon State University, along with

Fish Canyon sanidine (FCs�/28.02 Ma; Renne et

al., 1998), a neutron fluence monitor. The fluence

parameter ‘J ’ was determined from individual

analysis of 15 sanidine grains in each of two

positions bracketing the unknown samples. The

separates were step-heated using furnace heating.

Ar isotopic compositions were measured at the

Berkeley Geochronology Center (USA) in static

mode by a MAP-215-50 spectrometer, using the

procedures described by Renne (1995).

Isotopic run data were corrected for mass

discrimination, radioactive decay and nucleogenic

interferences. Atmosphere-corrected data are

plotted as apparent-age spectra, which are accom-

panied by compositional Ca/K data derived from

corrected relative abundances of 37Ar and 39Ar for

Table 3

Average compositions of REE (ppm) and La/YbN, La/LuN, La/SmN, Gd/YbN and Eu/Eu� ratios normalized by chondrite (Boynton,

1984) for the Tandilia dykes

B1 B2 I A

N�/7 S.D. N�/1 N�/2 S.D. N�/4 S.D.

La 4.36 1.53 43.66 25.17 0.81 52.12 9.23

Ce 10.89 2.75 87.12 55.65 0.84 102.04 16.01

Pr 1.55 0.34 9.82 6.48 0.04 10.51 1.55

Nd 7.92 1.53 44.49 26.92 0.77 37.98 5.47

Sm 2.51 0.41 9.87 4.88 0.02 6.09 0.63

Eu 0.89 0.13 3.55 1.28 0.19 1.15 0.04

Gd 3.01 0.49 10.45 3.78 0.06 4.37 0.79

Tb 0.46 0.07 1.41 0.50 0.02 0.58 0.12

Dy 3.12 0.54 7.73 3.01 0.12 3.18 0.91

Ho 0.72 0.13 1.66 0.65 0.01 0.61 0.17

Er 1.86 0.34 3.95 1.56 0.04 1.53 0.37

Tm 0.27 0.05 0.61 0.23 0.01 0.23 0.06

Yb 1.90 0.30 4.38 1.55 0.04 1.60 0.47

Lu 0.28 0.04 0.68 0.24 0.01 0.25 0.08

(La/Yb)N 1.55 0.48 6.72 10.93 0.64 22.00 11.39

(La/Lu)N 1.61 0.54 6.63 10.60 0.76 21.78 11.65

(La/Sm)N 1.10 0.35 2.78 3.24 0.09 5.38 0.83

(Gd/Yb)N 1.28 0.06 1.93 1.97 0.03 2.21 0.38

Eu/Eu� 0.98 0.06 1.06 0.91 0.14 0.68 0.07

See Iacumin et al. (2001) for details. N , number of samples; S.D., standard deviations (1s). Symbols and legend as in Fig. 3. Rare

earth elements*/REE have been determined by the inductively coupled plasma atomic emission spectrometry (ICP-AES), at the

‘Centre de Recherches Petrographiques et Geochimiques’, CNRS, France Govindaraju and Mevelle (1987).


Table 440Ar�/

39Ar (selected steps) analytical data of tholeiitic dyke A4 (plagioclase) and baked country rocks (A54 biotite; A48 biotite) at the very contact with calc-alkaline dykes

(data from Iacumin, 1998)

T (8C) 40Ar (mol) 40/39 38/39 37/39 36/39 40�/39 %40 Age (Ma) 9/s (Ma)

Tholeiitic dyke A4 (plagioclase)

600 6.16E�/15 819.42 0.91785 5699.1 3.36600 �/83.6 34.2 1.00E�/20 4135.7

650 4.27E�/15 782.15 0.94617 3453.1 2.65834 �/166.48 34.9 1.00E�/20 4077.9

700 6.00E�/15 450.99 0.42421 855.50 1.12779 537.39 41.3 4882.2 1372.0

750 6.47E�/15 130.24 0.11633 187.91 0.34243 51.47 33.8 1532.4 37.5

800 6.27E�/15 43.00 4.60E�/02 90.02 0.10188 21.58 46.7 803.5 11.8

850 5.05E�/15 36.64 4.45E�/02 86.94 7.86E�/02 21.83 55.6 811.0 11.1

900 4.32E�/15 40.00 2.34E�/02 89.14 9.34E�/02 20.94 48.8 784.2 25.2

950 2.75E�/15 49.38 4.90E�/02 123.80 0.135672 21.19 38.9 791.9 103.8

1000 3.64E�/15 69.91 3.89E�/02 120.95 0.188898 26.18 34.0 936.8 63.9

1050 5.38E�/15 111.23 8.11E�/02 131.89 0.331039 26.64 21.5 949.6 55.7

1100 2.79E�/15 62.71 2.07E�/02 158.72 0.173709 27.39 38.4 970.2 45.3

1150 2.91E�/15 66.62 0.047534 135.19 0.132860 42.56 57.3 1344.3 22.3

1200 3.83E�/15 79.75 0.055627 139.15 0.187276 39.77 44.6 1280.9 40.1

1250 4.00E�/15 57.54 4.84E�/02 141.61 9.58E�/02 45.49 70.5 1408.2 27.7

1350 4.00E�/15 52.25 7.82E�/02 104.22 4.31E�/02 51.99 91.6 1542.9 14.1

1600 1.42E�/15 211.26 0.360343 93.52 0.466592 87.13 38.3 2134.9 31.7

A54 (biotite)*/baked country rock

600 4.74E�/14 682.76 0.427544 0 2.139767 50.46 7.4 1512.1 628.2

640 1.40E�/13 336.19 0.206281 0 1.032772 31.01 9.2 1066.7 131.1

680 2.38E�/13 72.22 4.16E�/02 4.59E�/02 0.144400 29.55 40.9 1028.6 10.8

710 6.07E�/13 72.54 2.03E�/02 9.96E�/02 3.68E�/02 61.68 85.0 1726.4 4.1

740 1.55E�/12 82.64 1.58E�/02 4.80E�/02 1.40E�/02 78.52 95.0 2006.9 6.8

770 2.74E�/12 79.47 1.38E�/02 6.70E�/02 3.86E�/03 78.34 98.6 2004.1 3.5

760 8.29E�/13 79.36 1.39E�/02 0.214999 3.26E�/03 78.43 98.8 2005.4 3.1

770 7.56E�/13 79.50 1.36E�/02 0.148473 2.62E�/03 78.74 99.0 2010.3 3.1

780 6.66E�/13 79.20 1.35E�/02 0.125110 1.81E�/03 78.68 99.3 2009.4 3.7

790 5.81E�/13 78.92 1.38E�/02 0.217852 1.67E�/03 78.45 99.4 2005.8 4.6

800 4.87E�/13 78.84 1.37E�/02 0 1.75E�/03 78.33 99.3 2003.8 5.2

820 4.63E�/13 79.21 1.36E�/02 0.203633 1.97E�/03 78.66 99.3 2008.9 4.6

840 3.65E�/13 79.10 1.39E�/02 0.150645 2.39E�/03 78.41 99.1 2005.2 5.2

860 3.38E�/13 78.79 1.39E�/02 0 2.61E�/03 78.02 99.0 1999.1 5.5

880 4.38E�/13 79.27 1.35E�/02 0.179101 2.54E�/03 78.55 99.1 2007.2 3.3

900 6.38E�/13 80.05 1.37E�/02 0.203168 2.40E�/03 79.37 99.1 2019.8 4.9

920 6.60E�/13 80.07 1.41E�/02 0.130750 2.02E�/03 79.50 99.3 2021.8 9.4

940 5.64E�/13 79.67 1.36E�/02 0.314538 2.37E�/03 79.02 99.2 2014.5 4.7

960 5.21E�/13 79.77 1.42E�/02 0.134934 2.32E�/03 79.10 99.2 2015.8 3.4

990 5.84E�/13 79.81 1.36E�/02 0.140124 2.67E�/03 79.04 99.0 2014.8 3.2

1020 3.95E�/13 80.09 1.40E�/02 0.261024 3.18E�/03 79.19 98.9 2017.15 4.5

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Table 4 (Continued )

T (8C) 40Ar (mol) 40/39 38/39 37/39 36/39 40�/39 %40 Age (Ma) 9/s (Ma)

1060 1.41E�/13 80.00 1.45E�/02 1.388759 4.95E�/03 78.73 98.3 2010.1 6.9

1100 6.36E�/14 81.54 1.75E�/02 6.938283 1.65E�/02 77.62 94.7 1992.9 11.7

1150 7.75E�/14 80.40 1.49E�/02 2.475953 5.32E�/02 79.18 98.3 2017.0 9.1

1270 3.25E�/14 87.45 1.80E�/02 2.361412 2.22E�/02 81.23 92.7 2048.1 20.0

1350 1.62E�/15 69.24 0.058738 124.1467 0.178192 29.29 38.3 1021.5 1356.4

1500 4.32E�/15 128.24 4.18E�/02 18.89543 0.408957 9.02 6.9 380.4 1119.1

A48 (biotite)*/baked country rock

600 9.04E�/14 352.89 0.184 3.9683 0.880437 93.32 26.4 2221.8 78.9

630 7.74E�/14 154.62 7.61E�/02 2.7505 0.319461 60.57 39.1 1706.3 33.7

660 5.89E�/14 74.69 3.03E�/02 1.3800 8.19E�/02 50.64 67.7 1515.8 15.5

690 2.57E�/13 65.73 0.018107 0.334615 2.53E�/02 58.29 88.7 1664.4 4.9

720 1.49E�/12 82.77 1.53E�/02 0.113612 1.31E�/02 78.91 95.3 2012.8 4.4

750 3.64E�/12 80.43 1.34E�/02 0.113765 4.20E�/03 79.21 98.5 2017.4 3.3

780 4.39E�/12 79.94 1.30E�/02 9.12E�/02 1.76E�/03 79.43 99.4 2020.8 3.4

810 3.14E�/12 79.96 1.30E�/02 0.158878 1.31E�/03 79.59 99.5 2023.3 3.0

850 1.64E�/12 80.05 1.27E�/02 0.184319 1.66E�/03 79.59 99.4 2023.2 6.2

890 2.53E�/12 79.86 0.012897 0.179104 1.65E�/03 79.39 99.4 2020.3 3.1

930 2.92E�/12 79.91 1.27E�/02 0.124480 1.42E�/03 79.50 99.5 2021.9 3.8

970 1.56E�/12 79.79 1.30E�/02 0.185273 1.67E�/03 79.33 99.4 2019.2 4.1

1010 1.62E�/12 79.76 1.31E�/02 0.231745 0.001688 79.30 99.4 2018.7 4.3

1050 1.99E�/12 81.79 1.31E�/02 0.382454 1.54E�/03 81.39 99.5 2050.6 3.0

1100 8.10E�/13 84.54 0.013439 0.952865 2.83E�/03 83.84 99.1 2087.0 3.9

1150 1.69E�/13 85.66 1.36E�/02 1.212999 9.22E�/03 83.11 96.9 2076.3 5.9

1220 1.06E�/13 86.32 1.59E�/02 1.037633 1.34E�/02 82.51 95.5 2067.4 8.5

1300 3.26E�/15 101.85 0.071091 54.7559 0.349681 3.02 2.8 136.7 889.9

1500 5.54E�/15 195.06 9.77E�/02 89.1162 0.648261 11.39 5.4 468.1 1340.6

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each step. Errors are reported at 2s level and donot include uncertainty in the age of FCs or 40K

decay constants (Min et al., 2000). The decay

constants recommended by Steiger and Jager

(1977) were used.

U�/Pb analyses for tholeiitic (B1) dyke A-17

were performed at the University of Alberta

Radiogenic Isotope Facility, Canada (Table 5). A

small number (B/50) of very small (B/30 mm in thelongest dimension) baddeleyite grains were recov-

ered from this sample. The two baddeleyite

analyses represent small multi-grain fractions (12

and 18 grains, respectively) of irregular tan to dark

brown fragments and parts of blades. The frac-

tions were initially dissolved with HF/HNO3 (10:1)

in teflon pressure vessels together with a mixed205Pb�/

235U tracer solution. This material was laterconverted to chloride form by evaporating to

dryness and re-dissolving in 3.1N HCl. U and Pb

were purified by standard anion exchange chro-

matography using micro-columns modified after

Krogh (1973) and closely following the procedure

outlined in Heaman and Machado (1992). The

total procedural blanks during the period of study

were less than 5 pg Pb and 0.5 pg U, respectively.The purified U and Pb were loaded together using

a silica gel-phosphoric acid mixture (Cameron et

al., 1969) onto zone-refined Re ribbon. All ana-

lyses were performed on a VG-354 mass spectro-

meter operated in single Faraday or Daly

(analogue) collector peak-hopping mode. All ana-

lyses were corrected for mass discrimination (Pb,

0.088% per a.m.u.; U, 0.155% per a.m.u.) based onreplicate measurements of the NBS-981 and U500

standards. In addition, all measurements obtained

with the Daly photomultiplier detector were ad-

justed for detector bias (Pb, 0.13% a.m.u.; U,

0.15% a.m.u.). The isotopic composition of com-

mon Pb in excess of analytical blank was calcu-

lated using the two-stage model of Stacey and

Kramers (1975). Discordia line calculation wasperformed using ISOPLOT (Ludwig, 1992) with

the 238U (1.55125�/10�10 a�1) and 235U

(9.8485�/10�10 a�1) decay constants recom-

mended by Jaffey et al. (1971). All errors reported

in Table 5 are quoted at 1s and were calculated by

numerical propagation of all known sources of

uncertainty. The error ellipses shown on the

concordia diagram and age uncertainties arereported at 2s.

Twenty-two Rb�/Sr whole-rock analyses (Table

6) were performed, using isotope dilution techni-

que, at the Geochronological Research Center

(CPGeo) of University of Sao Paulo, Brazil. The87Sr/86Sr ratios are listed with absolute errors (2s),

and have been corrected to the mean value of the

NBS-987 standard (0.7102549/0.000022 (2s)). Theoverall blank for the chemical procedure was 4 ng

for Sr. Isotope ratios were measured on VG-354

multicollector and single collector mass spectro-

meters, and the 87Sr/86Sr ratios were normalized to86Sr/88Sr�/0.1194. Isochron calculation followed

the procedure of Ludwig (1999), and the decay and

other constants used are from Steiger and Jager

(1977).Ten Sm�/Nd whole-rock analyses (Table 7) were

carried out at the CPGeo, using the two-column

technique, as described by Richard et al. (1976)

with modifications described by Sato et al. (1995).

The first ion-exchange resin was used for primary

separation of the REE, followed by the second

HDEHP-coated Teflon-powder column, for se-

paration of Sm and Nd elements. The laboratoryblanks for the chemical procedure during the

period of analyses yielded maximum values of

0.4 ng for Nd and 0.7 ng for Sm. The measured143Nd/144Nd ratio obtained for the La Jolla

standard was 0.5118579/0.000046 (2s).

5. Results and discussion

The emplacement age of the calc-alkaline dykes

was defined from the 40Ar�/39Ar analyses (Table 4)

of outgassed biotites from baked granitic rocks

sampled at a maximum distance of 10 cm from the

contact with the dykes. The biotite collected at the

contact with the A54 dyke (I type; Del Tigre

Range) was degassed, yielding a concordant spec-

trum with an integrated age of 19749/24 Ma and aplateau age of 20079/24 Ma (11 steps; Fig. 6a).

The biotite from the other granitic rock at sharp

contact with A48 dyke (A type; Tandileufu Hill)

yielded a comparable age spectrum with an

integrated age of 20199/24 Ma and a plateau age

of 20209/24 Ma (9 steps; Fig. 6b). The relatively


Table 5

U�/Pb baddeleyite results for A17 B1 tholeiitic dyke from Tandilia (RLPC)

Description� Weight

(mg)

U

(ppm)

Th

(ppm)

Pb

(ppm)

Th/

U

TCPb

(pg)

206Pb/204Pb 206Pb/238U 207Pb/235U 207Pb/206Pb 206Pb/238Pb 207Pb/235U 207Pb/206Pb %

Disc

Model ages (Ma)

A17 1 dark brown

108 Nm (12)

3 362.0 74.0 99.0 0.21 16 1122 0.2641�/5 3.525�/9 0.09681�/

¯

12

1510.8�/

¯2 1532.9�/

¯2 1563.5�/

¯2 3.8

2 tan 158 M (18) 4.0 91.4 7.5 25.8 0.08 9 613 0.2709�/

¯6 3.639�/

¯12 0.09741�/

¯

23

1545.8�/

¯2 1558.2�/

¯2 1575.1�/

¯4 2.1

Notes : 108 Nm refers to a non-magnetic fraction at a side tilt of 108 on a Frantz Isodynamic Separator. Number in parentheses refers to the total number of grains

analysed. Th concentration calculated from the amount of 208Pb and model 207Pb/206Pb age. All uncertainties reported at 1s.

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small and statistically insignificant difference (13

Ma) between the plateau ages of the biotites

suggests a moderate to rapid cooling after calc-

alkaline dyke emplacement (Baldwin et al., 1990),

or implies preferential Ar loss from the biotite

during a possible subsequent reheating. Following

the first interpretation the crystallization age is

taken as 20209/24 Ma.A plagioclase concentrate from tholeiitic dyke

A4 (B1 type; Azul Range) was also degassed,

yielding a plateau age of 8119/36 Ma (6 steps;

Fig. 6c; Table 4) which does not represent the

Table 6

Rb�/Sr (isotope dilution) analytical data and [87Sr/86Sr] initial ratios (Sr�/IR0) for the Tandilia dykes

Lab. no/sample Type Rb (ppm) Sr (ppm) 87Rb/86Sr 1s 87Sr/86Sr 2s oSr Sr�/IR0

13169/A-1 B1 40.89 166.93 0.7100 0.0099 0.72416 0.00010 �/75(1.6 Ga) 0.70787

13170/A-2 B1 16.50 149.62 0.3193 0.0044 0.71025 0.00008 �/4.2(1.6 Ga) 0.70292

13171/A-8 B1 16.70 157.18 0.3077 0.0043 0.71314 0.00012 �/49(1.6 Ga) 0.70608

13172/A-33 B1 29.79 195.91 0.4404 0.0061 0.71508 0.00009 �/33(1.6 Ga) 0.70497

13173/A-35 B1 42.09 171.38 0.7121 0.0107 0.72639 0.00010 �/106(1.6 Ga) 0.71006

13174/MT65 B1 29.71 192.44 0.4472 0.0063 0.71540 0.00007 �/36(1.6 Ga) 0.70513

13749/A-4 B1 16.85 166.20 0.2937 0.0027 0.71370 0.00007 �/62(1.6 Ga) 0.70696

13750/A-5 B1 5.74 134.16 0.1239 0.0011 0.70717 0.00009 �/24(1.6 Ga) 0.70433

13191/MT68 B2 38.68 370.56 0.3024 0.0042 0.71785 0.00008 �/118(1.6 Ga) 0.71091

13192/MT70 B2 33.21 393.17 0.2447 0.0041 0.71425 0.00017 �/86(1.6 Ga) 0.70863

13756/A-39 B2 37.53 453.13 0.2398 0.0026 0.71218 0.00009 �/58(1.6 Ga) 0.70667

13762/A53 B2 29.84 330.84 0.2611 0.0024 0.70895 0.00009 �/4.7(1.6 Ga) 0.70295

13751/MT67 B2 39.26 404.26 0.2813 0.0023 0.71705 0.00009 �/113(1.6 Ga) 0.71059

13752/MT69 B2 45.87 360.57 0.3684 0.0031 0.71228 0.00009 �/17(1.6 Ga) 0.70382

13175/A-40 I 42.00 577.84 0.2104 0.0035 0.70987 0.00007 �/24(2.0 Ga) 0.70381

13758/A-49 I 51.96 577.19 0.2606 0.0022 0.71125 0.00008 �/23(2.0 Ga) 0.70375

13761/A-52 I 43.76 507.00 0.2499 0.0021 0.71046 0.00009 �/16(2.0 Ga) 0.70327

13787/A-54 I 147.76 378.90 1.1319 0.0097 0.73667 0.00008 �/29(2.0 Ga) 0.70416

13759/A-45 A 129.55 186.57 2.0195 0.0167 0.75789 0.00009 �/30(2.0 Ga) 0.70001

13788/A-47 A 142.52 142.55 2.9161 0.0271 0.78779 0.00008 �/33(2.0 Ga) 0.70446

13789/BR-1 A 194.15 313.57 1.7998 0.0149 0.75290 0.00009 �/12(2.0 Ga) 0.70129

13786/BR-2 A 117.50 251.61 1.3572 0.0112 0.74389 0.00009 �/40(2.0 Ga) 0.70497

B1 and B2, Tholeiitic dykes; I and A, Calc-alkaline dykes. See text for explanation.

Table 7

Sm�/Nd whole rock analytical data of selected Tandilia dykes

Lab. no/sample Type Sm (ppm) Nd (ppm) 147Sm/144Nd 1s 143Nd/144Nd 2s TDM (Ga) fSm/Nd oNdt

1046/A-4 B1 1.90 7.64 0.1505 0.0005 0.512778 0.000039 �/ �/0.23 �/12.2 (1.6 Ga)

1047/A-5 B1 2.28 6.93 0.1991 0.0007 0.512893 0.000041 �/ �/0.01 �/4.5 (1.6 Ga)

1048/A-16 B1 2.57 9.63 0.1614 0.0005 0.512250 0.000020 �/ �/0.18 �/0.4 (1.6 Ga)

1049/A-38 B2 10.43 48.00 0.1314 0.0004 0.511895 0.000020 �/ �/0.33 �/1.2 (1.6 Ga)

1060/MT-70 B2 7.29 38.94 0.1131 0.0004 0.511394 0.000037 2.51 �/0.42 �/7.2 (1.6 Ga)

1050/A-41 I 4.93 26.68 0.1116 0.0004 0.511323 0.000040 2.59 �/0.43 �/3.9 (2.0 Ga)

1058/A-40 I 4.67 25.06 0.1126 0.0004 0.511375 0.000038 2.53 �/0.42 �/3.2 (2.0 Ga)

1061/A-54 I 5.18 26.70 0.1173 0.0004 0.511392 0.000010 2.63 �/0.40 �/4.0 (2.0 Ga)

1051/A-51 A 7.06 40.71 0.1049 0.0003 0.511190 0.000010 2.61 �/0.47 �/4.8 (2.0 Ga)

1059/A-48 A 4.26 22.53 0.1143 0.0004 0.511372 0.000040 2.58 �/0.42 �/3.7 (2.0 Ga)

Model TDM ages were calculated using DePaolo (1981) model parameters: a�/0.25; b�/�/3; c�/8.5 and 146Nd/144Nd�/0.7219 to

normalize the isotopic ratios [143Nd/144Nd�/Nd(Chur)0�/0.512638 and 147Sm/144Nd (Chur)0�/0.1967]. The oNd values were calculated

using the simplified equation oNd (T )�/oNd(0)�/QNdfSm/NdT , with the (Chur)0 values above and QNd�/25.09. See text for explanation.


crystallization age. The discordant age spectrum at

high temperature degassing (�/1150 8C) is accom-

panied by elevated Ca/K, which could reflect the

presence of cryptic secondary phases within this

concentrate. Additionally, the age spectrum shows

a ‘saddle-shaped’ pattern which is, in many cases,

evidence for samples containing excess 40Ar from

older, K-rich host rocks (Lanphere and Dalrymple,

1976). However, considering the typical low Ar

blocking temperature of plagioclase (�/176 8C;

Berger and York, 1981), the age spectrum may also

be related to episodic 40Ar loss at 810 Ma. In a

similar matter, two K�/Ar whole-rock apparent

ages of 8039/14 and 11939/18 Ma reported for (B2)

dykes (A38 and A39; Iacumin, 1998) reflect

variable Ar loss from the mineral systems.

Fig. 6. 40Ar�/39Ar spectra for the plagioclase of tholeiitic dyke and biotite from baked country rocks in sharp contact with calc-alkaline

dykes; Tandilia system: (a) and (b) baked country rocks; (c) tholeiitic dyke. Keys: biotite (bt), plagioclase (plg). See text for

explanation.


The tholeiitic (B1) dyke A17 intruded at 15889/

11 Ma (Fig. 7; Table 5), according to a discordia

upper intercept age (two baddeleyite analyses).

This is probably an estimate of crystallization age

for all the tholeiitic dykes of Tandilia, as they have

similar N30W trends oblique to the shear zones

(e.g. Del Tigre Range; see Fig. 2), and crosscutting

relations with the 1.77 Ga leuco-monzogranites

(see previous section).Rb�/Sr whole-rock determinations (Table 6)

were carried out on eight calc-alkaline (four

samples each of I and A types) and 14 tholeiitic

dykes (eight B1 and six B2 types) in an attempt to

estimate the age of crystallization of each swarm.

The calc-alkaline dykes yielded an errorchron of

19569/110 Ma (1s) and Sr0�/0.70389/0.0025

(Fig. 8). The MSWD value (19) reflects the data

scatter around the best line, as expected from

samples collected in a large area and by the

possibility that the intermediate and acid dykes

may have variable Sr0 ratios. Thus, the initial ratio

of the errorchron is a rough estimate of the average

Sr0 of the calc-alkaline magma. The individual

samples have Sr0 for t0�/2.0 Ga (40Ar/39Ar age)

ranging from 0.70129 to 0.70497 (Table 6). This

limited range suggests that Rb and Sr have not

been further reset at the whole-rock scale since the

dyke’s emplacement and is not compatible with the

possibility of appreciable crustal contamination, in

agreement with petrological and geochemical in-

ferences reported by Iacumin et al. (2001). The (I)

dykes have calculated Sr0 values between 0.70327

and 0.70416 (average 0.703759/0.00037), slightly

higher than the (A) dykes (0.70129�/0.70497), but

not significantly different within the errors (A

dykes have mean Sr0 values�/0.702689/0.00241).

In any case the Sr0 mean ratios are comparable

within the errors with those of the tonalitic�/

granitic plutons of Tandilia (with Sr0 as low as

0.7020), thereby reinforcing the idea that crustal

contamination, if present, was negligible for the

calc-alkaline dykes (see previous section).

(B1) and (B2) dykes did not yield an Rb�/Sr

isochron, as shown by the large data scatter in the

diagram (Fig. 9). This situation probably results

from low-grade hydrothermal fluids that may have

contributed towards increasing the Sr0 values

higher than 0.705 (Table 6, see previous section).

Fig. 7. U�/Pb concordia diagram for A-17 tholeiitic dyke.


It should be noted that not all of the studied dykes

contain secondary minerals (e.g. clay, sericite and

albite overprinting the plagioclase) although they

occur in a same area; this indicates that mobility of

Rb and Sr can be considered to be a local-scale

phenomenum, but linked with the late-magmatic

crystallization process of the tholeiitic dykes.

Ten Sm�/Nd whole-rock analyses were per-

formed on five calc-alkaline (I, A) and five

tholeiitic (B1 and B2) dykes. The oNd and oSr

parameters (samples A40 and A54), recalculated

for t0�/2.0 Ga (40Ar/39Ar age for the emplace-

ment; see above), vary from �/4.8 to �/3.2 and �/

24 to �/29, respectively (Table 7). These data plot

Fig. 8. Rb�/Sr whole-rock errorchron for the calc-alkaline dykes. Symbols as in Fig. 3.

Fig. 9. Rb�/Sr correlation diagram for the tholeiitic dykes. Symbols as in Fig. 3.


in the enriched quadrant with respect to bulkEarth (Iacumin et al., 2001), and suggest that the

calc-alkaline dykes may be derived from partial

melting of a mantle source with low Sm/Nd,

similar to those of an enriched source mantle

(Iacumin et al., 2001). The calc-alkaline dykes

display TDM from 2.53 to 2.63 Ga (Table 7),

indicating that such an enrichment of the source

mantle may have occurred in late Archean�/

Paleoproterozoic times.

Three tholeiitic (B1) dykes (A4; A5; A16) do not

allow calculation of single-stage Nd model ages, as

indicated by fSm/Nd values of �/0.23, �/0.18 and �/

0.01, respectively (Table 7). Sample A4 yields

anomalous high oNd(1.6 Ga) (�/12.2) and Sr0 values

(0.70696; Table 6), which probably reflect altera-

tion (see previous section). Samples A5 and A16have oNd(1.6 Ga) values of �/4.5 (Sr0�/0.70433) and

�/0.4, respectively. These results are interpreted to

reflect derivation from a depleted mantle source

which may have assimilated small amounts of

crustal components, as supported by the petrolo-

gical and geochemical characteristics of the low-Ti

tholeiites (see Iacumin et al., 2001, for details). In

contrast, (B2) dykes (A38 and MT70) have oNd(1.6

Ga) values of �/1.2 and �/7.2 (Sr0�/0.70863),

respectively Tables 6 and 7. The latter sample is

affected by late-magmatic alteration (see above),

whereas the Nd signature of A38 dyke can be

related to an enriched mantle source, in agreement

with the distinctive geochemistry of the high-Ti

tholeiites of Tandilia (Iacumin et al., 2001). In

addition, dyke MT70 yields TDM of 2.51 Ga that isotherwise comparable with TDM of the calc-alka-

line dykes (Table 7). Therefore, these two distinct

dyke suites may have originated by variable

enrichment of a late Archean�/early Paleoproter-

ozoic mantle source.

6. Summary and tectonic implications

Two distinct generations of Proterozoic dykes

crosscut the crystalline crust of the Tandilia

system. The 40Ar�/39Ar mineral plateau ages

indicate that the calc-alkaline dykes (I and A)

were emplaced between 2020 and 2007 Ma. The

tholeiitic (B1 and B2) dykes intruded at �/1590

Ma. Therefore, the previous published K�/Ar ageson the Tandilia dykes (1750�/1540 and 1070�/800

Ma) reflect variable Ar loss from the minerals of

these dykes.

The oldest calc-alkaline dykes have E�/W trends

and post-date Trans-Amazonian deformation and

metamorphism. These dykes were intruded essen-

tially subcoevally with the Tandilia tonalitic�/

granitic plutonism (Dalla Salda et al., 1988). Thisplutonic suite was formed during the Trans-

Amazonian orogeny, as supported by U�/Pb and

Rb�/Sr age determinations, and have major and

trace elements and Sr0 (0.7020�/0.7060) that re-

semble those of rocks generated in modern arc

environments (Dalla Salda et al., 1988). The calc-

alkaline dykes are therefore interpreted as trans-

tensional representatives of the Tandilia plutonicarc. This interpretation is also consistent with

geochemical characteristics of the calc-alkaline

dykes (I and A types), given the enriched mantle

source signature influenced by subduction-related

metasomatism (Iacumin et al., 2001). The geologic

scenario of Tandilia system fits with tectonic

interpretations of the Trans-Amazonian orogeny

in the Central Brazil Shield, in which this eventplayed an important role in mantle-differentiation

and crustal shortening processes, surrounding late

Archean domains (Cordani and Sato, 1999; Teix-

eira et al., 1999). In western Gondwanaland, these

late Paleoproterozoic stable remnants, including

the RLCP (Fig. 1) are consistent with the existence

of the Atlantica supercontinent at �/2.0 Ga

(Rogers, 1996).The youngest (B1) and (B2) tholeiitic dykes of

Tandilia have dominantly NW-SE trend and

crosscut both the E�/W mylonitic zones and the

1770 Ma leuco-monzogranites. This situation in-

dicates a significant change of the regional stress

field, post-dating the Trans-Amazonian orogeny.

The emplacement of these dykes at �/1.59 Ga

implies that extensional tectonics accompanied byanorogenic igneous activity predominated after

stabilization of the Trans-Amazonian orogeny,

within the RLPC. This assessment is also sup-

ported by the occurrence of the 1.73 Ga Florida

tholeiitic dyke swarm (western Uruguayan Shield)

and the coeval lllescas rapakivi granite (17489/4

Ma) in the Nico Perez terrane (Teixeira et al.,


1999). The crustally-derived leuco-monzogranites(17709/88 Ma) in Tandilia may also be related to a

continental rift scenario. It is worth noting that

such an intraplate setting characterizes the Stater-

ian period (1.8�/1.6 Ga) in South America, during

which diachronous mafic dykes, anorogenic

AMCG rapakivi suites (anorthosites, mangerites,

charnockites, acid volcanic rocks) and fault block-

basins developed across many Paleoproterozoiccrustal provinces (Brito Neves et al., 1995; Teixeira

et al., 1999).

The 1.59 Ga tholeiitic (B1 and B2) dykes of

Tandilia have geochemical and Nd signatures,

which are consistent with the presence of two

compositionally different magmas (Iacumin et al.,

2001). Also we speculate that such a situation may

be explained by rift dynamics during late Paleo-proterozoic times, since this regime could favour

interaction of the ascending asthenosphere with

variable amounts of the thinned subcontinental

lithosphere.

6.1. Correlations with the southern African

subcontinent

Geologic correlation between South Americaand southern Africa (Porada, 1979; Brito Neves

and Cordani, 1991), considering the Brasiliano/

Pan-African orogenies, allows prior tectonic link

with implications for the Paleoproterozoic evolu-

tion of the RLPC. In this regard, the geologic

framework of the Tandilia system shows many

similarities with the contemporary Eburnean crust

of the Richtersveld and Bushmanland subpro-vinces, within the 1.2�/1.0 Ga Namaqualand

(Hartnady et al., 1985; Colliston and Schoch,

1998), facing the RLPC in the West Gondwana

reconstruction (Figs. 1 and 10). In a global

context, the intercontinental scenario is consistent

with that of many late Mesoproterozoic orogenies

worldwide, representing the amalgamation of the

Rodinia supercontinent (Rogers, 1996), in re-sponse to diachronic rifting and dispersal of

descendants from ‘Atlantica’.

The Richtersveld subprovince, like the Tandilia

system, consists mainly of metamorphic equiva-

lents of igneous rocks (Barton and Burger, 1983).

A calc-alkaline volcanic pile (Orange River

Group) overlies the co-magmatic Vioolsdrif I-type suite that comprises separate intrusive phases

of gabbro, tonalite and granodiorite, dated at

19009/30 Ma (Reid, 1979, 1982). These granitoid

and volcanic rocks display TDM between 2.29 and

2.12 Ga (see Reid, 1997 for review). In addition,

late-tectonic granitoids (quartz monzonite, ada-

mellite, leucogranite) of the Vioolsdrif suite were

emplaced at 17309/20 Ma (Rb�/Sr isochron age;Reid, 1982) yielding Sr and Pb initial isotopic

signatures consistent with derivation from young,

unradiogenic crust similar in bulk composition to

the relatively older (Eburnean) tonalites and

granodiorites (Reid, 1997). It should be noted

that Orange River rhyodacites (see above) have

chondrite normalized REE abundance patterns

(ratios (La/Yb)N�/14.0; (La/Sm)N�/5.0; (Gd/Yb)N�/1.5; (Eu/Eu�)N�/0.6; Reid, 1997) that are

broadly comparable with those of the contempor-

ary calc-alkaline (A) dykes of Tandilia (ratios (La/

Yb)N�/22.0; (La/Sm)N�/5.4; (Gd/Yb)N�/2.2;

(Eu/Eu�)N�/0.7; Table 3).

The current preferred model for the Richters-

veld association involves the evolution of an

island-arc complex of Eburnean age and its latemobilization at depth along an active continental

margin (Hartnady et al., 1985). The available Nd,

Sr and Pb features of the rock suites are consistent

with the subduction process to produce metaso-

matism of a mantle wedge with an undifferentiated

bulk Earth signature, as well as with rapid

recycling of young crust to produce the granitic

components within the arc complex (Reid andBarton, 1983; Reid, 1979, 1997). Thus, the above

scenario resembles that envisaged for subcoeval

rocks of the Tandilia system.

Much of the Bushmanland subprovince (Fig. 10)

to the south of the Richtersveld Subprovince was

similarly formed and metamorphosed during the

late Paleoproterozoic, though severely obscured by

the Namaqua deformation and metamorphism at1.2�/1.0 Ga. In the Okiep terrane (Central Bush-

manland), the Koeris tholeiitic metabasalts (am-

phibolites) form a major volcanic horizon,

penecontemporaneous with the upper part of the

Bushmanland Group composed of quartzite, mar-

ble, schists, and gneiss with minor volcanic inter-

calations. These amphibolites are similar in age to


the Tandilia tholeiites, as supported by an Sm�/Nd

whole-rock isochron age of 16499/90 Ma (oNd(t)�/

�/0.48), interpreted as the time of eruption of the

basalt precursors (Reid et al., 1987). Additional

Rb�/Sr and U�/Th�/Pb work on the Koeris amphi-

bolites (Reid, 1997) has confirmed the important

role of the late Mesoproterozoic metamorphism

(Kibaran orogeny; Fig. 10).

Fig. 10. Crustal architecture of southern African subcontinent (a), showing the Paleoproterozoic Richtersveld and Bushmanland

subprovinces of Namaqualand (b) (adapted from Hartnady et al. (1985), Reid et al. (1987), Goodwin (1991)). Keys: intracratonic

sequences of Bushmanland (BS), Waterberg (WA) and Soutpansberg (SB). See text for details.


The Koeris amphibolites have trace element

geochemistry, including REE, consistent with the

existence of mantle heterogeneity, subduction

related metasomatism and crustal contamination

(Reid, 1997). The geochemical inference suggests a

back arc continental extension environment for

these mafic rocks, but it is worth noting that the

geochemical and isotopic signatures may also be

related to rift dynamics, as already proposed for

the origin of the Tandilia tholeiites. It should also

be noted that the Koeris amphibolites show near

flat REE patterns with low (La/Lu)N and high Ti/

Zr ratios (Reid et al., 1987), which compare closely

with those patterns shown by (B1) tholeiitic dykes

of Tandilia (Table 8). In a similar matter, the

Aggeneys/Witberg metabasalt types exhibit low Ti/

Zr ratios, more fractionated REE patterns with

LREE enrichment, negative Eu anomalies and

high (La/Lu)N ratio (Reid, 1997) which may be

compared with the characteristics of the (B2)

tholeiitic dykes, except for the Eu anomaly.

Salient geologic features of the Bushmanland

Subprovince include charnockitic granites, meta-

mafic rocks (gabbro, diorite, norite, anorthosite),

as well as characteristic metavolcanic-sedimentary

assemblages (e.g. Bushmanland Group) filling

intracratonic basins, probably accumulated be-

tween 1.7 and 1.6 Ga (Hartnady et al., 1985).

These intracratonic packages host several giant

stratiform CU�/Pb�/Zn�/Ag ore deposits (Reid et

al., 1987; Colliston and Schoch, 1998). Such a

tectonic framework again has similarities with

typical rock associations of the Statherian period

(Goodwin, 1991). In consequence, a major sig-

nificance for unrecognized intraplate 1.7�/1.6

events within Bushmanland subprovince can be

proposed, suggested also by occurrence of the

Koeris tholeiitic volcanism and the Vioolsdrif late-

tectonic granites (see above). Moreover, such a

scenario opens the possibility of a tectonic rela-

tionship with subcoeval leucogranitoid rocks of

the RLPC (see previous section). Also, we suspect

that a family of tectonically controlled �/1.75�/

1.70 Ga block-faulting basins (e.g. Waterberg�/

Soutpansberg groups) roughly distributed in a

NE-SW zone located within the Kaapvaal craton

(Fig. 10), as well as occurrences of poorly-studied

mafic dykes at the vicinities (Hartnady et al.,

1985), may similarly be relicts of Statherian

extensional tectonics in the southern African

subcontinent.

To conclude, correlation between the RLPC and

the southwest corner of Africa by comparing the

stratigraphy of pre-tectonic platform sedimentary

marine deposits of the La Tinta and the Nama

Group was established long ago (Porada, 1979;

Dalla Salda, 1982). From the geochronologic and

geochemical inferences now available for the

Tandilia system and the Richtersveld arc complex

there is also evidence for a Late Paleoproterozoic

(Trans-Amazonian/Eburnean) link, as already

proposed by Dalla Salda et al. (1988), and further

by some peculiar Paleo-Mesoproterozoic geologic

features of the Bushmanland subprovince.

Finally, the establishment of such an integrated

scenario may contribute to the debate on the

reconstruction of the Rodinia supercontinent and

its descendants (e.g. Atlantica supercontinent; see

Table 8

Average of La/Lu, La/Yb, La/Sm and Eu/Eu� ratios normalized by chondrite (Boynton, 1984) for amphibolites (Koeris tholeiitic

metabasalts) from the Bushmanland Subprovince, South Africa (Reid et al., 1987) and B1 and B2 tholeiitic dykes of the Tandilia

System (RLPC)

Ratios Central Bushmanland Subprovince (Southern Africa) Rio de la Plata Craton/Tandilia System (South America)

Gamsberg (N�/7) Aggeneys/Witberg (N�/4) B1 tholeiitic (N�/7) B2 tholeiitic (N�/1)

(La/Lu)N 1.649/0.27 4.899/0.15 1.619/0.54 6.83

(La/Yb)N 1.639/0.28 4.959/0.26 1.559/0.48 6.72

(La/Sm)N 1.279/0.19 2.539/0.05 1.109/0.35 2.78

(Eu/Eu�)N 0.999/0.06 0.769/0.04 0.989/0.06 1.06

Data from Reid et al. (1987), Iacumin et al. (2001). N , number of samples, standard deviations, 1s. Symbols and legend as in Fig. 3.

See text for explanation.


Rogers, 1996). Moreover, because the Bushman-land subprovince is unusually rich in base metal

sulphide deposits, metallogenetic intercontinental

correlations among comparable Paleoproterozoic

rock units are also relevant, as it opens new

possibilities of mineral exploration models and

potentials in the South American counterpart.

Acknowledgements

The authors thank the financial support of the

Brazilian Agencies FAPESP (grant no. 97/0640-5)

and CNPq (grant no. 522975/96-8), and CNR and

MURST (Italian Agencies). We also acknowledge

the technical staffs of the CPGeo (USP) and

Berkeley Geochronology Center (USA) for pro-vinding the Rb�/Sr and Sm�/Nd, and 40Ar�/

39Ar

dates. We are also greatfull to A. Marzoli, A. De

Min and L. Furlan (Dipartimento di Scienze della

Terra, Trieste, Italy) for their valuable collabora-

tion in this research. Special thanks are due to R.

Ernst (Geological Survey of Canada), C. Cingola-

ni (University of la Plata, Argentina) and U.

Cordani (USP, Brazil) for their fruitful suggestionsrelated to the manuscript, and to the constructive

reviewers’ comments and criticism.

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Documents

Calc-alkaline and tholeiitic dyke swarms of Tandilia, Rio de la Plata craton, Argentina: U–Pb, Sm–Nd, and Rb–Sr 40Ar/ 39Ar data provide new clues for intraplate rifting shortly