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Journal of Petrology Volume 42 Number 11 Pages 2109-2143 2001
© Oxford University Press 2001
Early Proterozoic Calc-Alkaline and Middle Proterozoic Tholeiitic Dyke Swarms from Central–Eastern Argentina: Petrology, Geochemistry, Sr–Nd Isotopes and Tectonic Implications
1DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITY OF TRIESTE, VIA E. WEISS, 8, 34127 TRIESTE, ITALY
2DIPARTIMENTO DI INGEGNERIA CHIMICA, DELL’AMBIENTE E DELLE MATERIE PRIME, UNIVERSITY OF TRIESTE, PIAZZALE EUROPA 1, 34100 TRIESTE, ITALY
3INSTITUTO DE GEOCIENCIAS, UNIVERSITY OF SÃO PAULO (USP), PO BOX 11348, 01051 SÃO PAULO, SP, BRAZIL
4DIPARTIMENTO DI MINERALOGIA E PETROLOGIA, UNIVERSITY OF PADOVA, CORSO GARIBALDI, 37, 35137 PADOVA, ITALY
5INSTITUTO DE RECURSOS MINERALES (FCNyM-UNLP), CALLE 47 N° 522, 1900 LA PLATA, ARGENTINA
6DEPARTAMENTO DE GEOCIENCIAS, UNIVERSITY OF LONDRINA, PO BOX 6001, 86051-990 LONDRINA, PR, BRAZIL
7LABORATORIO DE ENTRENAMIENTO MULTIDISCIPLINARIO PARA LA INVESTIGACION TECNOLOGICA (LEMIT-CIC and FCNyM-UNLP), PO BOX 128, 1900 LA PLATA, ARGENTINA
Received January 2, 2001; Revised typescript accepted May 11, 2001
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL FRAMEWORK AND DYKE...CLASSIFICATION, PETROGRAPHY AND...GEOCHEMISTRYISOTOPE GEOCHEMISTRYPETROGENESISDISCUSSION AND CONCLUSIONSREFERENCES |
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The Rio de La Plata craton in Argentina (Azul and Tandil regions) is characterized by Early Proterozoic (2·0 Ga) calc-alkaline and Middle Proterozoic (1·6 Ga) tholeiitic dyke swarms intruding the crystalline basement involved in the Transamazonian Orogeny (2·2–1·9 Ga). The calc-alkaline dykes have andesitic and rhyolitic compositions and trend east–west, whereas the tholeiitic dykes mainly trend N30°W and are represented by basalts with low (0·9–1·7 wt %) and high TiO2 (up to 3·7 wt %). The calc-alkaline dykes have primitive mantle (PM)-normalized trace element patterns enriched in Rb, Ba, K, La, Ce and Nd, and significant negative Nb and Ti anomalies. These dykes are characterized by
t(Nd) values of -3 to -4, similar to those of the EMI mantle component. Low-TiO2 tholeiitic dykes have low incompatible-element (IE) contents and PM-IE patterns with slightly positive or negative Nb spikes. They have variable t(Nd) values (-0·5 to 12·1), which mainly reflect derivation from a depleted source mantle. High-TiO2 tholeiitic dykes have more enriched IE-PM patterns and are characterized by t(Nd) values (-1·4 to -7·5) typical of an enriched source mantle. Chemical and isotopic data and melting modelling indicate that both calc-alkaline and tholeiitic dykes originated by different melting degrees of a heterogeneous source mantle, the variable IE enrichment of which may have occurred in Late Archaean to Early Proterozoic times. The emplacement of the calc-alkaline dykes is associated with the transtensional stage of the Transamazonian Orogeny, whereas the tholeiitic dykes reflect extensional tectonics succeeding the Transamazonian event. The calc-alkaline and tholeiitic dykes are similar in emplacement age and characteristics to metamorphosed granites and volcanic rocks outcropping within the Namaqua fold belts of southwestern Africa (Richtersveld and Witberg–Aggeneys–Gamsberg provinces); this may indicate that the Rio de La Plata craton and southwestern Africa were contiguous in Early–Middle Proterozoic times. KEY WORDS: Argentina; geochemistry; petrology; Proterozoic dykes; Rio de La Plata craton
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL FRAMEWORK AND DYKE...CLASSIFICATION, PETROGRAPHY AND...GEOCHEMISTRYISOTOPE GEOCHEMISTRYPETROGENESISDISCUSSION AND CONCLUSIONSREFERENCES |
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Precambrian dyke swarms are common in cratonic blocks, providing useful correlations between different tectonic units and yielding information on related geodynamic processes (Halls, 1987
; Parker et al., 1990; Baer & Heimann, 1995).
The crystalline basement of the Azul and Tandil regions (Buenos Aires province, Argentina; Fig. 1) belongs to the Rio de La Plata craton and is intruded by unmetamorphosed Proterozoic mafic, intermediate and felsic dyke swarms of 2·0 and 1·6 Ga age, respectively (Teixeira et al., in preparation). Similarly, the portion of the Rio de La Plata craton in Uruguay is characterized by unmetamorphosed Proterozoic dyke swarms (Fig. 1) dated at 1·7 Ga (Florida region; Bossi et al., 1993; Teixeira et al., 1999) and 
0·7 Ga (Nico Perez and Treinta y Tres regions; Mazzucchelli et al., 1995; Rivalenti et al., 1995; Girardi et al., 1996).
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Study of the Argentinian dykes is important for an understanding of source mantle characteristics in Early–Middle Proterozoic times, as well as for better insight into the processes responsible for craton stabilization in the southernmost part of the South American platform.
In this paper we characterize the Proterozoic calc-alkaline and tholeiitic dyke swarms of central–eastern Argentina in terms of petrological and genetic implications, and relationships between tectonics and magmatism. We show that these dyke swarms originated from depleted- to incompatible element-enriched mantle sources, and that their emplacement occurred during the transtensional to post-collisional stages of the Transamazonian Orogeny (2·2–1·9 Ga).
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GEOLOGICAL FRAMEWORK AND DYKE OCCURRENCE |
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The southernmost Archaean–Proterozoic cratonic areas of South America (Fig. 1) are exposed in Uruguay and Argentina (Azul and Tandil regions) and belong to the Rio de La Plata craton, characterized by terranes involved in the tectonic–metamorphic and magmatic events of the Transamazonian Orogeny (2·2–1·9 Ga) and the Brasiliano cycle (0·8–0·5 Ga). The Rio de La Plata crystalline basement is mainly composed of gneisses, migmatite and granitoids. The Precambrian unmetamorphosed dyke swarms in Uruguay (Fig. 1; Bossi et al., 1993
; Mazzucchelli et al., 1995; Rivalenti et al., 1995; Girardi et al., 1996) intrude the crystalline terranes affected by both Transamazonian and Brasiliano events, and are dated as Early and Late Proterozoic, respectively: Florida 1·73 and 1·77 Ga (Teixeira et al., 1999) and Nico Perez and Treinta y Tres 0·7–0·6 Ga (Mazzucchelli et al., 1995; Rivalenti et al., 1995; Girardi et al., 1996). In contrast, the Precambrian unmetamorphosed dyke swarms in Argentina intrude exclusively crystalline terranes of the Rio de La Plata craton and are dated at 2·0 Ga and 1·6 Ga (biotite 40Ar/39Ar and baddeleyite U–Pb ages, respectively; Teixeira et al., in preparation). It is also noted that the Late Proterozoic sedimentary rocks of the Argentina ‘La Tinta Group’ (0·8–0·5 Ga, Fig. 1) in the southern Azul–Tandil regions are locally (Sierra de Los Barrientos) intruded by tholeiitic sills (0·5 Ga; Rapela et al., 1974).
The stratigraphy of the Azul and Tandil regions (Fig. 2) is not well constrained in terms of radiometric ages. The oldest rocks are gneisses, migmatites, micaschists and medium-grade amphibolites (Teruggi et al., 1974). They are intruded by granitoid bodies whose Rb/Sr ages cluster around 2·2–2·0 and 1·8–1·6 Ga (Varela et al., 1988; Dalla Salda et al., 1992). The ‘old granitoids’ (gabbroic to granitic rocks) outcrop in the northern part of the ‘Azul and Tandil Sierras‘; the ‘young granitoids’ (mainly monzogranites) are located in the southern part of these ‘Sierras’. The emplacement of both ‘old’ and ‘young’ granitoids has been related to the syn/late- to post-collisional stages, respectively, of the Transamazonian Orogeny (Varela et al., 1985, 1988; Dalla Salda et al., 1992). Mylonitic belts associated with the ‘old granitoids’ (Fig. 2) may reflect wrench or overthrusting tectonics (Dalla Salda, 1981; Ramos et al., 1990; Dalla Salda et al., 1992).
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The southern parts of the Azul and Tandil ‘Sierras’ are covered by Late Proterozoic sedimentary rocks of the ‘La Tinta Group’ (up to 400 m thick). These rocks correspond to those of the ‘Nama Group’ in South Africa (Fig. 1), suggesting a common sedimentary basin origin (Dalla Salda, 1982
; Kawashita et al., 1999). The unmetamorphosed dykes studied here intrude the crystalline basement terranes and the ‘old’ and ‘young granitoids’, and mainly strike N30°W in Azul, and N30°W and east–west in Tandil. The dykes (Fig. 2) are divided into two main groups according to field and chemical features (see below). The dykes of the first group (D1 and D2) mainly trend N30°W and have tholeiitic characteristics; those of the second group (D3 and D4) have an east–west strike and a calc-alkaline signature (see Fernandez & Echeveste, 1995).
Most of the tholeiitic dykes are low in TiO2 (<1·7 wt %, D1), although a few have TiO2 up to 3·7 wt % (D2). D1 dykes occur in both the Azul and Tandil ‘Sierras’, are subvertical, vary in thickness from 10 to 50 m, and extend for distances of up to 5 km. They are fine grained at the border and coarse grained in the centre. D2 dykes are exposed only in the Tandil ‘Sierras’, are subvertical, and trend east–west in the ‘Don Pedro’ quarry (Cerro Tandileufú) and in the southwestern part of the ‘Sierra del Tigre’, and NW–SE and NE–SW in the ‘Sierra Alta de Vela’. D2 dykes vary in thickness from 0·5 to 10 m and are medium to fine grained.
The calc-alkaline dykes have andesitic (D3) and rhyolitic (D4) composition. D3 dykes outcrop in the Tandil ‘Sierras’, mostly along the belt comprising Sierra del Tigre, Cerro Albion and Cerro Tandileufú. These dykes, 0·5–10 m thick, strike east–west, are vertical to moderately inclined (45°S), and have a medium–fine texture. D4 rhyolitic dykes are associated with, and sometimes intruded by, D3 dykes (Tandileufú quarry). D4 dykes strike east–west, are subvertical, and may reach 30 m in thickness (Brigitte quarry).
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CLASSIFICATION, PETROGRAPHY AND MINERAL CHEMISTRY |
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Total alkalis–SiO2 (TAS) and total alkalis–total Fe–MgO (AFM) diagrams (Fig. 3) reveal that dykes with SiO2 < 54 wt % (D1 and D2) are characterized by pronounced FeOtot enrichment and straddle the tholeiitic trend of Hawaii (Macdonald & Katsura, 1964
); dykes with SiO2 > 56 wt % (D3 and D4) plot in the calc-alkaline field of Irvine & Baragar (1971) (Fig. 3b). These data and the SiO2–(FeOtot/MgO) relationships (Fig. 3a) indicate that the Azul–Tandil dykes belong to different magmatic suites with tholeiitic and calc-alkaline characteristics, respectively.
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The dykes of the tholeiitic suite mainly correspond to basalts and basaltic andesites, the high-TiO2 (HTi) samples (D2) having lower SiO2 and higher incompatible element abundances (see below) than the low-TiO2 (LTi) analogues (D1). The dykes of the calc-alkaline suite plot in the fields of basaltic andesite and andesite (D3), and in those of dacite and rhyolite (D4).
LTi tholeiitic dykes (D1) have a subophitic texture dominated by plagioclase (Table 1) and pyroxenes (Table 2). Plagioclase is labradoritic (An71–52) and may be altered to clay minerals and sericite. Augite (Wo = 42–30), often associated with orthopyroxene (Wo = 4·5–4·8) or pigeonite (Wo = 8·7–11·3), has total compositional variations that mimic those of the Skaergaard intrusion (Fig. 4). The TiO2 content of augites is low and ranges from 0·25 to 0·70 wt %. Pyroxenes may be partly or completely replaced by hornblende, tremolite and chlorite. In general, olivine is scarce and restricted to low-Ca pyroxene-free samples. Olivine phenocryst compositions (Table 3) are either virtually constant (sample A34, Fo = 77·8–78·4%) or have a small range (sample A5) from 63·6 to 68·1%. The forsterite content of tholeiitic basalt A34 is in equilibrium (KdFe-Mg = 0·30; Takahashi & Kushiro, 1983) with the bulk-rock composition {100 x [at. Mg/(Mg + Fe2+)], mg-number = 51·3}, whereas this is not the case for the olivine of tholeiitic basalt A5 (mg-number = 60·2), which would require equilibrium olivine with Fo = 83%. Magnetite (ulvöspinel 26–63%) and ilmenite (R2O3 2·3–9·4%) are usually confined to the groundmass (Table 4). Quartz, apatite and epidote are common accessory phases. The crystallization temperatures (Kretz, 1982) of D1 pyroxenes range from 1130 to 1205°C, with Cpx–Opx equilibration at 1121°C (Iacumin, 1998). According to the Loucks (1996) thermometer, the olivine–augite equilibrium temperature for sample A34 is 1165°C. The homogeneous magnetite–ilmenite pairs yielded subsolidus equilibration temperatures ranging from 680 to 900°C (Table 4). The corresponding log(fO2) values range from -19·5 to -13·0 (Buddington & Lindsley, 1964), corresponding to conditions intermediate between fayalite–magnetite–quartz (FMQ) and wüstite–magnetite (WM) buffers (Fig. 5).
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, 
and •, D1, D2 (tholeiitic) and D3 (calc-alkaline) dykes, respectively. Fe* = Fe2+ + Mn + Fe3+.
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The scarce HTi tholeiitic dykes (D2) have ophitic textures. Labradoritic plagioclase is often altered to clay minerals and sericite and sometimes albitized (Ab up to 98%). Pyroxenes are augites (Wo 40–38) with high TiO2 content (1·2–1·4 wt %; Table 2) and pigeonite. Pyroxenes may be replaced by hornblende, actinolite–tremolite and chlorite. Ilmenite (Table 4), magnetite, rutile and pyrite (Echeveste & Fernandez, 1994), and apatite are abundant in the groundmass.
The basic–intermediate calc-alkaline dykes (D3) have porphyritic to intergranular textures. The porphyritic dykes have phenocrysts of plagioclase (An55–34, Table 1), often altered to sericite and clay minerals, and salitic–augitic pyroxenes (Table 2), almost completely replaced by amphiboles and opaque minerals. The holocrystalline matrix is composed of microphenocrysts of altered plagioclase, epidote, biotite–chlorite, opaques (ilmenite, Table 4; pyrrotite, Echeveste & Fernandez, 1994), altered alkali-feldspar and quartz. The dykes with intergranular texture have plagioclase and augite completely replaced by secondary minerals. Quartz–feldspar intergrowths are present in the groundmass.
The dacitic and rhyolitic dykes (D4) have a porphyritic texture, phenocrysts of altered plagioclase, alkali-feldspar and augite being replaced by amphibole–chlorite aggregates. The same minerals are found in the holo-microcrystalline matrix. Secondary zeolites or quartz and K-feldspars may fill vesicles or fractures.
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GEOCHEMISTRY |
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Major and trace elements were determined using a PW 1404 XRF spectrometer, following the procedures of Philips (1994)
for the correction of matrix effects. Results are accurate to within 2–3% for major elements and better than 7–10% for trace elements. Ferrous iron was determined by redox titration; loss on ignition (LOI) was determined at 1100°C (12 h) and corrected for Fe2+ oxidation. Rare earth elements (REE) were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES) at the Centre de Recherches Petrographiques et Géochimiques, CNRS, Vandoeuvre (France) (Govindaraju & Mevelle, 1987).
Low-TiO2 (LTi) tholeiites (Table 5) have SiO2 and MgO contents ranging from 48·7 to 53·7 wt % (mean 51·0 ± 1·2 wt %) and 8·8 to 3·8 wt %, respectively, and mg-number values from 61 to 36. As shown in the variation diagrams (Figs 6 and 7), with respect to MgO, the trend of SiO2 is rather flat, like that of P2O5 (0·12 ± 0·03 wt %), TiO2 (1·34 ± 0·13 wt %) and FeOtot (12·6 ± 1·0 wt %). The D1 dykes plotting in the basalt field have low contents of incompatible elements (IE), whereas those with basaltic andesite composition (D1a), for similar MgO, have higher IE (e.g. La, Ce, Nd, Zr; Fig. 7). D1a dykes also have La/Y (1·0) and Zr/Y (5·0) that are clearly higher (Fig. 8) than those of the D1 dykes (0·3 and 3–4, respectively), excluding the possibility that they derived from D1 basalt dykes through simple fractional crystallization. It should be noted that the D1 tholeiites have REE patterns (Fig. 9) similar to the E-MORB of Sun & McDonough (1989), with mean chondrite-normalized (La/Yb)CN of 1·55 ± 0·48, whereas the D1a-type tholeiite is more fractionated, i.e. (La/Yb)CN = 6·83. Considerable differences between D1 and D1a tholeiites are also apparent, excluding mobile elements such as Rb, Ba and K, in the multi-elemental diagram relative to the Primitive Mantle (PM) of Sun & McDonough (1989; Fig. 9), in which the (La/Nb)PM and (Zr/Ti)PM ratios of D1 dykes are lower than those of IE-enriched D1a dykes (1·06 ± 0·78 vs 2·92 ± 0·63, and 1·31 ± 0·36 vs 3·07 ± 0·04, respectively).
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High-TiO2 (HTi) tholeiites have MgO ranging from 6·9 to 3·9 wt % (mg-number 49–33) and relatively low SiO2 contents (48·8 ± 0·8 wt %). They are characterized by high concentrations of TiO2 (1·71–3·74, mean 2·50 ± 0·83 wt %), P2O5 (0·67–1·52 wt %), REE, Zr (254 ± 104 ppm) and Nb (25 ± 16 ppm). The REE pattern (A38) of a D2 dyke parallels that of the D1a basaltic andesite (LTi), and is similarly characterized by high (La/Yb)CN (6·72). In the multi-elemental diagram, the HTi dykes have enriched IE patterns with respect to LTi dykes, being characterized by positive P and negative Sr and Ti (Zr/TiPM = 1·93 ± 0·19) spikes. It should be noted that (La/Nb)PM ranges from 1·0 to 3·4 (mean = 1·92 ± 0·83).
The calc-alkaline basaltic andesites and andesites have MgO and SiO2 ranging from 7·0 to 3·7 wt % (mg-number 60–47) and 56·0 to 59·8 wt %, respectively. These dykes (D3), relative to the D1 tholeiites, have lower mean TiO2 (0·72 vs 1·20 wt %) and FeOtot (8·9 vs 11·9 wt %) and higher P2O5 (0·32 vs 0·14 wt %), Sr (552 vs 199 ppm), REE (e.g. La 26 vs 6 ppm; Nd 30 vs 11 ppm) and Zr (113 vs 86). The REE patterns of D3 dykes (Fig. 9) are different from those of tholeiitic D1 and D2 dykes, and are characterized by the highest (La/Yb)CN values, i.e. 10–12. In the multi-elemental diagram (Fig. 9), D3 dykes are distinct from the others by having pronounced negative spikes of Nb (La/NbPM = 3·39 ± 1·43) and Ti (Zr/TiPM = 3·18 ± 0·15).
The silicic dykes (SiO2 65–75 wt %) have the highest IE contents and are characterized by very high (La/Yb)CN (15–40) and a significant Eu negative anomaly (Eu/Eu* = 0·61–0·76).
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ISOTOPE GEOCHEMISTRY |
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Sr and Nd isotope analyses of selected samples were performed at the ‘Centro de Pesquisas Geocronológicas’ of São Paulo, following the procedure of Ludwig (1985)
. Rb, Sr, Sm and Nd concentrations were measured by isotopic dilution. The average 87Sr/86Sr for NBS-987 and 143Nd/144Nd for La Jolla standards were 0·710254(22) and 0·511857(46), respectively, with 2
standard deviations reported in parentheses. The initial 87Sr/86Sr (Sri) and 143Nd/144Nd (Ndi) were calculated to 1588 Ma (baddeleyite U–Pb age: Teixeira et al., in preparation) for D1 and D2 dykes, 2020 Ma (biotite 40Ar/39Ar age: Teixeira et al., in preparation) for D3 and D4 dykes, and 2150 Ma for the country rocks.
The studied dykes have Sri ratios (Table 6) higher than those of Bulk Earth (BE): tholeiitic dykes: 0·7030–0·7102 for D1 (Sr 5–107), 0·7030–0·7110 for D2 (Sr 6–118); calc-alkaline dykes: 0·7032–0·7050 for D3 (Sr 15–41) and 0·7029–0·7044 for D4 (Sr 12–32). The gabbroic to granitic country rocks have Sri ranging from 0·7025 to 0·7048 (Sr 8–30), as reported by Varela et al. (1988) for the ‘old granitoids’. The ‘young granitoids’ of the basement have Sri(1750 Ma) in the range 0·7181–0·7303 (Sr 221–395; Varela et al., 1985, 1988; Dalla Salda et al., 1992).
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The initial 143Nd/144Nd (Table 7) of the LTi tholeiitic dykes (D1) ranges from 0·51056 to 0·51121 and is slightly lower to higher relative to BE (Nd -0·4 to 12·2). In comparison, the HTi tholeiitic dykes (D2) are moderately (Ndi = 0·51052, Nd = -1·3) to much (Ndi = 0·51024, Nd = -6·71) lower than BE. D3 and D4 andesitic and rhyolitic dykes have similar Ndi (0·50980–0·50988) and are moderately lower than BE (Nd = -2·8 to -4·5).
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Sr–Nd relationships (Fig. 10) reveal that the HTi-D2 tholeiitic dyke A38 and the calc-alkaline andesitic D3 dykes plot in the enriched quadrant with respect to BE, suggesting an EMI-type isotopic signature, similar to the Florida (Uruguay) and Uauà (Brazil, São Francisco craton) Proterozoic tholeiitic dykes. Conversely, and particularly for sample A4, the LTi-D1 tholeiitic dykes plot in the enriched quadrant in terms of Sr, but in the depleted one in terms of Nd.
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PETROGENESIS |
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Crystal fractionation
A possible mechanism for interdyke differentiation is fractional crystallization. This process was tested by performing MELTS (Ghiorso & Sack, 1995
) and XLFRAC (Stormer & Nicholls, 1978) major element modelling. MELTS can model the crystal fractionation process of a magma on the basis of its major element composition, total pressure, fO2 and water content. A Rayleigh fractionation model was then tested to compare calculated vs observed trace element contents (Tables 9–12), using the solid–liquid partition coefficients listed in Table 8.
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MELTS results indicate that the mineral compositional variation of D1 LTi tholeiites is compatible with olivine solidus Ca-poor pyroxene + augite + plagioclase fractional crystallization at low pressure (0·1–0·4 GPa), low fO2 (FMQ – 1 log unit) and H2O < 0·3 wt % (Fig. 11). Similarly, XLFRAC results (Table 9) indicate that differentiation from dykes with 8 wt % MgO (A5 and A25) to those with 6–7 wt % MgO (M.E.D1) is compatible with 27–32% removal of plagioclase (9–12%), augite (15–16%) and olivine (2%, A5) or orthopyroxene (6%, A25). In general, calculated/observed trace element values are satisfactory, with the exception of Rb and Ba. It should be noted that, starting from parental magma A5 (Rb and Ba 7 and 31 ppm, respectively), it is impossible to use fractional crystallization to match the Rb and Ba concentrations of the mean composition of the moderately evolved D1 dykes (i.e. M.E.D1: Rb 106 ppm, Ba 166 ppm), probably as a result of alteration that did not affect elements less sensitive to alteration (e.g. Zr, Nb).
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D1a LTi tholeiitic dykes, characterized by high REE, Zr, Y and Nb contents, require parental magmas enriched in these elements. Among the D1 dykes with 8 wt % MgO, only sample A32 has the high content of incompatible elements. XLFRAC calculations (Table 10) indicate that this transition from A32 to the mean composition of D1a dykes is compatible with 47% fractionation of olivine (11%), clinopyroxene (8%), plagioclase (24%) and magnetite (4%). Calculated/observed trace element values are broadly consistent with such a differentiation process. The MELTS program (Fig. 11) indicates that this fractional crystallization may have occurred at low fO2 (FMQ – 1 log unit) and 0·1 GPa pressure, in near-anhydrous conditions (H2O 0·1 wt %).
MELTS modelling for the D2 HTi tholeiites (Fig. 11) indicates pigeonite as the dominant fractionating phase, at both ‘high’ (0·8 GPa) and low (0·1 GPa) pressures, for low H2O (0·1 wt %) and fO2 (FMQ – 1 log unit) conditions. XLFRAC mass balances, in conjunction with calculated/observed trace element values (Table 11), indicate that the transition from less evolved (MgO 7 wt %) to more evolved (MgO 4 wt %) tholeiites requires fractionation of pigeonite (19%) and plagioclase (8%).
The transition from the calc-alkaline D3 andesitic basalts (MgO 7 wt %) to andesites (MgO 4 wt %) requires removal of olivine or orthopyroxene. For the fractional crystallization of andesitic basalt T12, MELTS (Fig. 11) indicates that orthopyroxene is a liquidus phase even at 0·1 GPa [fO2 = FMQ + 2 log unit, H2O 1·0 wt %). Mass balance calculations and calculated/observed trace element abundances (Table 12) indicate that the transition from T12 (andesitic basalt) to A54 (andesite) is compatible with 26% removal of orthopyroxene (7%), clinopyroxene (5%), plagioclase (13%) and magnetite (1%).
XLFRAC results indicate that the silicic D4 dykes may derive from D3 andesites through 73% fractionation of clinopyroxene (36%), plagioclase (28%), magnetite (1%) and alkali-feldspar (8%), but there are great differences between calculated and observed trace element values (Table 12).
Variation diagrams (Figs 6 and 7) and Zr, La vs Ni relationships (Fig. 12) show that dykes D1, D1a, D2 and D3 are related to different parental magmas.
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Primary magma compositions for these dykes (P-D1, P-D1a, P-D2 and P-D3, respectively; Table 13) were calculated starting from samples A5 (mg-number 60), A32 (mg-number 58), A53 (mg-number 53) and T12 (mg-number 62), respectively, by adding olivine, clinopyroxene and orthopyroxene (in proportions 5:3:1, respectively) to achieve mg-number values of 72–73. Plagioclase was not included as fractionating phase, because the MELTS results (Fig. 11) show that plagioclase is not a liquidus phase in the early stages of crystal fractionation, even at low pressure. The relatively low Cr and Ni contents do not take account of sulfide (pyrite, pyrrotine, calc-pyrite, pentlandite) fractionation (Echeveste & Fernandez, 1994).
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Crustal contamination
In general, the Azul and Tandil dykes have Sri higher than that of BE. The calc-alkaline D3 and D4 dykes are characterized by IE and Sri values (0·7029–0·7050) lower than or similar to those of the ‘old granitoids’ (0·7034–0·7060; Varela et al., 1988) and therefore crustal contamination is not easy to detect. D1 and D2 tholeiitic dykes have Sri ratios ranging from 0·7031 to 0·7111. Figure 13 reveals that Sri increases in both D1 and D2 dykes are not correlated with SiO2, MgO, K2O, Rb, Ba, La, Zr and Nb. It should be noted that the country rocks have Sri ratios <0·7070 and cannot represent end-members in terms of simple mixing. Therefore, we modelled AFC contamination (DePaolo, 1981) for D1 and D2 dykes assuming the ‘old’ and ‘young granitoids’ of Varela et al. (1988) as contaminants and as starting compositions those of the D1 (A2, A5) and D2 (A53, A38) dykes with the lowest Sri values. Calculations were carried out with bulk D(Sr) partition coefficients of 0·5 and 0·9, considering that plagioclase was not a liquidus phase in the earliest stages of differentiation (Fig. 11). The results (Fig. 14) indicate that Sri variations in both LTi-D1 and HTi-D2 dykes are compatible with assimilation of small amounts (r = 0·2) of both ‘young’ and ‘old granitoids’ (Cavazzini, 1996). It should be noted, however, that D1 and D2 dykes with Sri values >0·707 do not conform to AFC mixing curves, even for residual liquid fractions as low as 0·2–0·3. This suggests that alteration may have contributed towards reaching the highest Sri values. In summary, major and trace elements, mass balance calculations and isotopic data all indicate that D1 and D2 tholeiitic dykes with Sri <0·7050 were not significantly affected by crustal contamination.
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Source mantle
The melting degree for the ‘primary’ D1 to D3 basalts was calculated (XLFRAC) using a peridotitic major element composition from Ringwood (1966; pyrolite), Chen (1971) and Ionov & Hofmann (1995). Calculations were carried out for dry and amphibole-bearing spinel- and garnet-peridotite mineral assemblages. The results (Table 14) indicate that the highest degrees of melting (15–9% and 14–6% for garnet and spinel peridotites) refer to the calc-alkaline D3 dykes, whereas the lowest ones (7–3% and 6–4%: garnet and spinel peridotites, respectively) apply to the high-TiO2 D2 tholeiitic dykes (Fig. 15).
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Incompatible trace elements (IE) in the peridotitic sources (Table 15) were computed (batch melting; Hanson, 1978) using the partition coefficients of Table 16 and the calculated degrees of melting and solid residua (Table 14). The multi-elemental diagrams of Fig. 16 reveal that the IE of D1 peridotitic sources are depleted with respect to PM of Sun & McDonough (1989), irrespective of whether such sources are enriched (i.e. pyrolite) or relatively depleted in terms of major elements. The relatively low IE contents of a major-element pyrolite-type mantle indicate that IE depletion of such peridotites reflects a low degree (e.g. <1%) of melting, which did not significantly change the original mineral assemblage or major element composition. D1a sources (Fig. 16b) have IE higher than D1 sources, are especially enriched in Rb, Ba and K, and display negative Nb and Sr spikes and positive P spikes. D2 source patterns (Fig. 16c) are similar to those of D1a and have distinct P and Ti positive spikes, as well as a more pronounced Sr negative anomaly. D3 source patterns (Fig. 16d) are globally enriched with respect to PM and are characterized by high Nb and less marked Ti negative anomalies. The high (LILE, LREE)/(HFSE, HREE) values of D3 calc-alkaline dykes with respect to D1 tholeiitic dykes do not necessarily imply different mantle peridotite residua, i.e. garnet- vs spinel-peridotites (see Fig. 15 for melting degrees). Such incompatible element ratio differences may reflect LILE–LREE enrichment in D3 mantle source as a result of ‘fluid/small-volume melts’ metasomatism that may have occurred in Archaean–Proterozoic times [TDM(Nd) 2·5 Ga].
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DISCUSSION AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL FRAMEWORK AND DYKE...CLASSIFICATION, PETROGRAPHY AND...GEOCHEMISTRYISOTOPE GEOCHEMISTRYPETROGENESISDISCUSSION AND CONCLUSIONSREFERENCES |
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Early Proterozoic dykes
The oldest Precambrian dyke magmatism of the Rio de La Plata craton outcrops in Argentina (Tandil region) and is dated at 2·0 Ga. The dykes crosscut the crystalline basement and are sub-coeval with the youngest ‘old granitoids’ that intruded the basement (2·2–2·0 Ga). This magmatism (andesites: D3; rhyolites: D4), intruded during the transtensional stages of the Transamazonian Orogeny, has calc-alkaline geochemical features including negative Nb–Ti spikes. The calculated D3 primary andesite magmas may be derived (major elements) from either anhydrous or amphibole-bearing peridotites by 5–15% batch melting, in the spinel and garnet facies. The high LREE/HREE ratios of the D3 dykes are compatible with garnet-peridotite residua as well as with spinel peridotites enriched in LREE by ‘metasomatic processes’ involving fluids and/or small-volume melts related to peridotites contaminated by eclogite-derived melts (see Rivalenti et al., 1998
). These ‘metasomatic processes’ imply interaction with subducted rutile-bearing crustal material (e.g. Ryerson & Watson, 1987; Brenan et al., 1994) and possibly ilmenite in the residua (Ayers et al., 1997; Ayers, 1998). The anhydrous primary mineral assemblages of the andesitic dykes indicate that melting occurred in near-anhydrous water-undersaturated conditions. The transition from andesites to rhyolites (major and trace elements) is only partly consistent with fractional crystallization.
The initial Sr isotopic ratios of the andesites and rhyolites are similar to those of the country rocks and the ‘old granitoids’, and are higher than that of BE; crustal contamination was not appreciable. In terms of Sr and Nd, these dykes, as well as those of Uauà (São Francisco craton, Brazil, 2·2–2·0 Ga), plot in the enriched quadrant and trend towards the EMI mantle component of Hart & Zindler (1989). Assuming that the Sm/Nd ratio was not significantly changed by melting, the TDM(Nd) of these Argentinian dykes indicate that they could be related to source mantle that underwent metasomatic processes responsible for their EMI Sr–Nd isotopic signature in Archaean–Proterozoic times (2·5 Ga).
Middle Proterozoic dykes
The youngest Precambrian dyke magmatism of the Rio de La Plata craton in Argentina (Azul and Tandil regions) dates at 1·6 Ga, an age similar to that (1·7–1·8 Ga; Bossi et al., 1993; Teixeira et al., 1999) of the oldest dykes from Uruguay (Florida region). The Argentina dyke swarms intrude both the ‘old’ and ‘young granitoids’ and have compositional characteristics typical of tholeiitic suites, with low and high TiO2 contents (LTi and HTi, respectively). Most LTi tholeiites (D1 dykes) have REE patterns similar to those of E-MORB, except for a few tholeiitic andesitic basalts (D1a dykes) characterized by higher LREE/HREE ratios (e.g. La/YbCN 7). D1 dykes have slightly positive or negative Nb spikes and HREE lower than those of E-MORB, whereas D1a dykes have a distinct negative Nb spike. The dykes with high TiO2 and incompatible elements (D2) are characterized by high LREE/HREE and slightly positive or negative Nb spikes. The Middle Proterozoic dykes cannot be related to the same parental magma, but require different primary melts originating from different mantle sources. The genesis of D1 tholeiites requires 5–13% melting of peridotites depleted in incompatible elements relative to PM; that of D1a (5–11% melting) and D2 (3–7% melting) dykes would be related to source mantle enriched in LILE and LREE with respect to PM. The tholeiitic dykes that can be considered unaffected by appreciable crustal contamination are characterized by initial 143Nd/144Nd ranging from slightly lower to higher than BE (D2 and D1 dykes, respectively).
In summary, petrological, geochemical and Sr–Nd isotope data support the hypothesis that the various Middle Proterozoic tholeiitic dykes require distinct mantle sources, ranging from IE-depleted (basalt melt extraction) to IE-enriched (‘metasomatic processes’) peridotites. It should be noted that, in a very restricted area, particularly in Tandil (Fig. 2), both calc-alkaline and tholeiitic dykes occur; this indicates important small-scale mantle heterogeneity.
Lastly, it is notable that the Middle Proterozoic tholeiitic dykes from Uruguay (Florida region, Rio de La Plata craton) have geochemical features (mainly trace elements) similar to their D1 Argentina analogues (Figs 6 and 7).
Tectonic implications
The Early Proterozoic calc-alkaline dykes of Tandil are sub-coeval (2·0 Ga) with the youngest intrusion of ‘old granitoids’ and represent transtensional stages of the Transamazonian Orogeny. On the other hand, the emplacement of Middle Proterozoic tholeiitic dykes in the Azul and Tandil regions relates to the development of the Rio de La Plata craton, where these dykes are sub-coeval (1·6 Ga) with the youngest intrusion of the ‘young granitoids’ and are associated with extensional tectonics during a post-collisional Transamazonian stage (Dalla Salda et al., 1988, 1992), 300–400 m.y. after the end of the Transamazonian Orogeny. Rogers (1996) proposed a time of 2·0 Ga for the existence of the ‘Atlantica’ continent, which includes the cratons of Amazonia (Guaporè and Guyana), São Francisco, Congo–Kasai, West Africa and, possibly, West Nile and Rio de La Plata. In our opinion, Rogers’ reconstruction should take into consideration the spatial position of the southwestern Africa region. Volcanic and granitic rocks from the ‘Richtersveld Subprovince’, within the Namaqua Fold Belts facing the Rio de La Plata craton in the Gondwana reconstruction (Fig. 1), have been related to the Eburnean Orogeny at 2·0 Ga (Hartnady et al., 1985; Reid, 1997) and are, therefore, coeval with the calc-alkaline rocks of Tandil and Azul. The African calc-alkaline rocks were subjected to metamorphic processes related to the Kibaran Orogeny (1·2–1·0 Ga; Hartnady et al., 1985; Reid, 1997; Colliston & Schoch, 1998), but not appreciable in the corresponding Argentinian rocks. Also, within the Namaqua fold belts, in the Witberg–Aggeneys–Gamsberg ore district, metamorphosed basic tholeiites similar to the Tandil and Florida dykes are present whose emplacement occurred at 1·6–1·7 Ga (Reid et al., 1987). The metamorphism of the African basalts (amphibolite facies) has been related to the Kibaran Orogeny (Reid, 1997). The similarities between the Proterozoic calc-alkaline and tholeitic rocks of Rio de La Plata and the southwestern Africa analogues suggest that these two regions were contiguous in Early Proterozoic as well as in Late Proterozoic times, consistent with the common sedimentary basin of La Tinta and Nama Groups (Dalla Salda, 1982).
In a global context, the generation of juvenile crust (‘granitoids’) within the Rio de La Plata craton occurred from 2·2 to 1·6 Ga. This time broadly corresponds to that attributed to the development of the Early Proterozoic Supercontinent proposed by Condie (1998) on the basis of the occurrence of greenstone belts. Yale & Carpenter (1998) partly confirmed this hypothesis, emphasizing the absence of giant dyke swarms in the periods that preceded and followed 2·4–2·2 and 1·6–1·4 Ga, respectively. These time gaps may well represent periods of continental dispersal after supercontinent break-up.
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ACKNOWLEDGEMENTS |
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Special thanks are due to A. Cundari and A. Marzoli for fruitful suggestions related to the manuscript, and to the constructive reviewer’s comments and criticism. L. Furlan (Trieste University) and A. Giaretta and A. Carampin (Padova University) are acknowledged for the valuable technical collaboration in this research. The authors acknowledge the financial support by FAPESP and CNPq (Brazilian Agencies) and CNR and MURST (Italian Agencies).
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FOOTNOTES |
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*Corresponding author. Present address: Dipartimento di Scienze della Terra, University of Trieste, Via E. Weiss, 8, 34127 Trieste, Italy. Fax: 0039-040-6762213. E-mail: jacumin{at}univ.trieste.it
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