Petrology of the Proterozoic mafic dyke swarms of Uruguay and constraints on their mantle source composition

Pretumbrinn Resenrth ELSEVIER

Precambrian Research 74 (1995) 177-194

Petrology of the Proterozoic mafic dyke swarms of Uruguay and constraints on their mantle source composition 1 Maurizio Mazzucchelli a, Giorgio Rivalenti a Enzo Michele Piccirillo Vicente Antonio Vitorio Girardi c, Lucia Civetta d, Riccardo Petrini b

b,

"Dipartimento di Scienze della Terra, Universit~ di Modena, P.le S. Eufemia, 19, 141100 Modena, Italy b lstituto di Mineralogia e Petrografia, Universitd di Trieste, P.le Europa, 1, 1 34127 Trieste, Italy lnstituto de Geoci~ncia, Universidade de Sdo Paulo, Rua do Lago, 562, 05506-900 Sdo Paulo, Brazil a Dipartimento di Geofisica e Vulcanologia, Universit& di Napoli, Largo S. Marcellino, 10, 180138 Naples, Italy

Received 15 August 1994; revised version accepted 8 March 1995

Abstract Three dyke swarms in Uruguay range in age from Palaeoproterozoic ( 1.86 Ga, Florida region) to Neoproterozoic ( ~ 0.7 Ga, Nico Perez and Treinta y Tres regions). The Florida and Nico Perez swarms are basalts, basaltic andesites and andesites with tholeiitic affinity, characterized by LILE and LREE enrichment with respect to HFSE, Nb depletion with respect to K and La, K / R b < 260, B a / R b < 19, Z r / N b > 13, ( L a / Y b ) n > 4.5. Both swarms have positive esr and negative eNa, but the Florida isotopic array is dominated by eNO variations (EM1 type) and that of Nico Perez by esr variation (EM2 type). The Treinta y Tres swarm consists of slightly ne-normative transitional or alkaline basalts with geochemical characteristics resembling those of OIB (e.g., Gough Island), but suggestive of a less enriched OIB source, and have slightly positive end and eSr.

Crustal contamination does not appear to be important in the petrogenesis of the swarms. It is proposed that the geochemical characteristics of the Florida swarm derive from melting of lithospheric mantle infiltrated by hydrous fluids released from the thermal breakdown of hydrous phases, either contained in underplated oceanic crust or formed by interaction of asthenospheric fluids with the lithospheric mantle. The EM 1-type isotopic features are considered as unrelated with possible fluid addition, but as a pre-existing feature. The geochemical and isotope characteristics of the Nico Perez swarm are attributed to time-integrated enrichment processes, which affected the Nico Perez lithospheric mantle during the Palaeoproterozoic Florida episode. The geochemistry of the Treinta y Tres swarm may be explained by partial melting of a lithospheric mantle which did not suffer Palaeoproterozoic LILE and LREE enrichment but which was isotopically reset in Palaeoproterozoic times. Alternatively, a residual mantle related to the Florida event is required. Mantle melting and dyke intrusion probably occurred in an ensialic environment. The geochemical and isotopic characteristics of the Uruguay dyke swarms are similar to those of the Mesozoic basalts related to the Gondwana break-up, supporting the possibility that the latter derived from a heterogeneous lithospheric mantle source which recorded Proterozoic enrichment processes. This paper is dedicated to the memory of Giovanna and Italo.

0301-9268/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10301-9268(95)00014-3

178

M. Mazzucchelli et al. / Precambrian Research 74 (1995) 177-194

1. Introduction Three Precambrian mafic dyke swarms ranging in age from Palaeo- to Neoproterozoic occur in Uruguay (Bossi et al., 1993; Rivalenti et al., 1994; Girardi et al., 1994). These dykes provide important information on the geochemical and isotopic composition of their mantle source(s) at distinct time intervals. Since these dykes occur within the old Gondwana supercontinent, they may have also monitored the effects of Proterozoic mantle processes on the basalt source(s) related to the Gondwana break-up during the Mesozoic. These dykes, located along the southern part of the Paranfi basin, may have derived from similar mantle material(s) to that which produced the Early Cretaceous Paranfi flood tholeiites ( 1 3 3 + 1 Ma; Renne et al., 1992). This is of special interest because the Paranfi tholeiites and those from Etendeka (Namibia) are closely related to the opening of the South Atlantic. However, it is still debated if these basalts originated from lithospheric or asthenospheric mantle sources (e.g., Petrini et al., 1987; Hawkesworth et al., 1988; Piccirillo et al., 1989; White and McKenzie, 1989).

2. Geological outline The Precambrian geology of Uruguay is very complex and it is not well constrained in terms of radiometric ages. The crystalline basement comprises terrains of Palaeo- to Neoproterozoic age (Fig. 1; Dalla Salda et al., 1988). High-grade granite-gneiss terrains of the Rio de La Plata Craton, metamorphosed during the Palaeoproterozoic (2.2 Ga; Dalla Salda et al., 1988; Cordani and Soliani, 1990), constitute the basement of the Florida region of western Uruguay. To the east this region is separated from terrains related to the "Brasiliano Cycle" (BC) (De Almeida et al., 1976, 1981 ), which largely corresponds to the Pan-African Cycle, by a major dextral transform fault. This cycle represents a period of continental collision which created the Gondwana continent (Ramos, 1988; Shukowsky et al., 1993). In the Nico Perez region (Taquarimb6 block) the Rio de La Plata craton was intruded by granites of the BC, whereas in its eastern part (Sierra da Encantada

block) it is strongly sheared and metamorphosed during the BC (Fragoso Cesar, 1987, 1993). The easternmost Uruguay (Treinta y Tres region) consists of 0.90.6 Ga gneisses and migmatites, calc-alkaline tonalites and granites (Paratin] block), intruded by 0.6-0.5 Ga sub-alkaline granites (Dom Feliciano granites; Dalla Salda et al., 1988). According to Fragoso Cesar et al. (1993) the granites of the Taquarimb6 block could be related to subduction linked to the closure of the Charrua ocean and having a trench zone located about 500 km northeast of the present region. The deformation of the Sierra da Encantada block and the intrusion of the Paratin] granites could represent obducted basement and Andeantype magmatic rocks, respectively, related to the closure of the Adamastor ocean at 0.8 Ga (Hartnady et al., 1985), which separated the Rio de La Plata and the Kalahari cratons. In the Florida region, the mafic dykes crosscut the Rio de La Plata terrains with an E - W trend, turning southeast close to the transform fault. In the Nico Perez region, the swarm trends northeast-southwest where cutting the BC granites of the Taquarimb6 block and east-west when traversing the sheared Rio de La Plata terrains of the Sierra da Encantada block. In the Treinta y Tres region the dykes intersect the granites and gneisses of the Paratin] block with a N-S trend. In all cases the mafic dykes are vertical and have widths of a few tens of metres.

3. Age of the dykes Analytical methods for Rb-Sr and Sm-Nd isotopes are reported in Bossi et al. (1993) and Rivalenti et al. (1994). Absolute concentrations of Rb and Sr were determined by atomic absorption spectroscopy. Sm and Nd were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and, in a few samples (Rivalenti et al., 1994) by isotopic dilution mass-spectrometry, which is within a 5% error relative to ICP-AES results. According to Bossi et al. (1993) the Florida dykes have a Rb-Sr age of 1.86 + 0.12 Ga. A slightly younger 4°Ar/39Ar age of 1.7 Ga was obtained for the same dyke swarm by Teixeira et al. (1994).

179

M. Mazzucchelli et al. / Precambrian Research 74 (1995) 177-194

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M. Mazzucchelli et al. / Precambrian Research 74 (1995) 177-194

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~*~Nd. 1.86 Ga and 665 Ma reference lines in the Sr plot are from Bossi et al. (1993) and Rivalenti et al. (1994). The 1.9 Ga line in the Nd plot refers to the Treinta y Tres data. The dykes of the Nico Perez region are probably of BC age, but both R b - S r and S m - N d isotopic systems do not provide firm age indications. From the data reported in Fig. 2, Rivalenti et al. (1994) obtained a best fitting line to an age of 665 + 203 Ma. One K / A r determination on biotite from a granite at a dyke contact gave 581___13 Ma, supporting a BC age of dyke emplacement. Fig. 2 shows that the R b - S r isotopic system does not provide an isochron for the Treinta y Tres dykes. K / Ar determinations on hornblende and biotite have given ages of 743 4- 12 and 550-t- 14, respectively. S m - N d isotopes define a regression line corresponding to an age of 1.9 + 0.5 Ga. This age does not represent the age of the dykes, which intrude BC granites and it might indicate the age of the source. In the following, it will be assumed that the dykes of the Florida region have an age of ~ 1.86 Ga and those of Nico Perez and Treinta y Tres are coeval, with an age of ~ 750 Ma. The last assumption merely proposes

The dykes of the Florida and Nico Perez regions (Fig. 3) consist of basalts, andesitic basalts and andesites with tholeiitic affinity (Bossi et al., 1993; Rivalenti et al., 1994). Most Florida dykes have chilled, aphyric or microcrystalline margins and none is metamorphosed. They have subophitic texture and are composed of plagioclase (An67~,6), augite (Wo26_33, FS2o_ 55) and minor magnetite and ilmenite. Pigeonite (WOlo~15 , FS35_67 ) is common in the andesitic basalts and rare in the andesites, where augite is often mantled by a thin rim of hastingsitic hornblende. Quartz-feldspar intergrowths are common. Biotite and apatite are accessory. The dykes of the Nico Perez region have aphyric margins, but their inner parts are porphyritic. The phenocrysts are mainly plagioclase (Ans5 60), minor

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114. Mazzucchelli et al. / Precambrian Research 74 (1995) 177-194

a u g i t e (W024_36 , Fszl-44) and rare altered olivine. The

matrix is ophitic and contains augite, plagioclase, accessory pigeonite (Wo8, Fs24) and oxides (mainly Ti-magnetite). Most samples investigated contain interstitial glassy blebs of rhyolitic composition (Rivalenti et al., 1994). The dykes of the Treinta y Tres region are transitional basalts and alkali basalts (Fig. 3), with up to 2.3% of CIPW normative Ne. The rocks have ophitic texture, with primary plagioclase (An76~7), clinopyroxene ( W o 4 3 ~ 6 , FS 12-16), rare altered olivine, ilmenite and subordinate Ti-magnetite. Biotite, apatite and rare zircon are accessories. The primary mineral assemblage is variably transformed to amphibole, chlorite, and sometimes sphene, epidote and carbonate. This mineral assemblage is not associated with deformation and the primary ophitic texture is preserved. Girardi et al. (1994) considered that the related metamorphism was due to a post-magmatic, deuteric process.

5. Geochemistry Representative analyses of the three dyke swarms are in Table 1. Whole data set and analytical methods are in Bossi et al. (1993), Rivalenti et al. (1994) and Girardi et al. (1994). Average values of selected trace element ratios are in Table 2. Major and trace element variations of the three dyke swarms, relative to mg [molar ratio MgO/ ( M g O + F e O ) , assuming Fe203/FeO=0.15], and their incompatible trace element variations vs. Zr are compared in Figs. 4 and 5, respectively. On the basis of mass balance calculations, the variation trends of Figs. 4 and 5 were explained by crystallization and removal of a gabbro assemblage consisting of olivine, pigeonite, augite and plagioclase in the Florida swarm (Bossi et al., 1993), olivine, clinopyroxene and augite in the Nico Perez swarm (Rivalenti et al., 1994) and clinopyroxene and plagioclase in the Treinta y Tres swarm (Girardi et al., 1994). The authors inferred that fractional crystallization occurred in the magma chambers before dyke intrusion, since no important intra-dyke chemical variation was observed. Mass balance calculation reproduced also the total variation range of trace elements observed in each swarm, but did not account for the variation in Rb, K and incompatible element ratios in Fig. 5. These were

181

inferred to depend on slightly different parental magma compositions. Previous studies did not discuss boundary layer (in-situ) crystallization (Langmuir, 1989; Nielsen and DeLong, 1992), which could produce variable ratios of incompatible elements differing only slightly in their partition coefficients and generate trends markedly deviating from those expected from homogeneous (Rayleigh) crystallization. This process, however, was probably not important because trace element ratios vary randomly and independently on element concentration (Fig. 5), whereas in-situ crystallization would result in correlations between element ratios and element concentration (Nielsen and DeLong, 1992). Indeed, a correlation between element concentration and ratios is observed only when the ratio involves elements with different incompatibility (e.g., Y / Z r and Ti/Y which decrease when Zr increases, Ti and Y being less incompatible than Zr). The Florida and Nico Perez swarms have essentially similar geochemical characteristics. Their mg ranges are 0.26-0.55 and 0.36~3.54, respectively, and their trace element patterns show enrichment in LILE (K, Rb and Ba) and LREE with respect to HREE and HFSE, depletion of Nb with respect to K and La and of Ti with respect to Zr (Table 2; Fig. 6). In both swarms Rb is enriched with respect to other LILE (e.g., K/Rb = 229 _ 13 and 180 _ 24, and Ba/Rb = 7.6 _+2.4 and 7 . 0 _ 2.7, in Florida and Nico Perez, respectively; Table 2) and also with respect to Sr ( R b / Sr=0.26_0.05 and 0.21 ___0.07, respectively). Although fractionation may increase Rb/Sr, Bossi et al. (1993) and Rivalenti et al. (1994) calculated that a high Rb/Sr was already present in the parent melts, where Rb/Sr was estimated within the range 0.08-0.11 and 0.07-0.12 in Florida and Nico Perez, respectively. Minor geochemical differences of the Florida and Nico Perez swarms are, for a given mg, a higher concentration of SiO2 and K and lower A1203, FeO, CaO and Zr concentrations in the former. For a given Zr, the Florida dykes are also systematically higher in LILE, Ce, Nb and P and lower in Y and Sc and have higher K/Rb and lower La/Nb. With respect to the Florida and Nico Perez dykes, the Treinta y Tres swarm yielded a lower mg range (0.57~0.67, except for one sample, Fig. 4) and, for a given Zr is markedly depleted in LILE, Ce, Y and Sc and enriched in Sr, P and Ti.

M. Mazzucchelli et al. I Precambrian Researclz 74 (1995) 177-194

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The incompatible trace element pattern of the Treinta y Tres swarm is notable for its marked depletion in Rb, resulting in a positive Ba anomaly (Fig. 6) and low Rb/Sr ratio averaging 0.02_+0.01 (Table 2), a less pronounced Nb negative anomaly (average La/ Nb = 1.3_+0.3, compared with L a / N b = 2 . 1 _+0.2 and 2.6 + 0.3 for Florida and Nico Perez, respectively), and a steeper REE pattern [average ( L a / Y b ) , = 8.0+ 2.5 and (Sm/Yb)n = 3.6 4- 0.8; Table 2]. Plots of the ratios of highly incompatible elements (Fig. 7) provide a further demonstration of the different geochemical behaviour of the three dyke swarms. They form arrays radiating from an enriched MORB, or OIB, composition and plot on the same trend of increasing B a / N b at increasing La/Nb. However, the Zr/Nb ratio increases at different rates with increasing La/Nb in Florida, Nico Perez and Treinta y Tres although the Zr/Nb variation range is similar (Table 2).

Further significant differences arise from the Sr and Nd isotope characteristics of the three dyke swarms. In the ENJ--Esrdiagram (Fig. 8) the Florida dykes plot in the enriched quadrant and trend from a composition close to Bulk Earth toward mantle components with low ENd and virtually constant Est. These components could be of EM1 type (Hart, 1988). In contrast, the trend defined by the geochemically similar Nico Perez basalts, which also plot in the enriched quadrant, requires mantle components with high Esr and relatively low ENd- In the latter case, components of EM2 type (Hart, 1988) may have been involved. The Florida and Nico Perez arrays tend to merge in the field of the Treinta y Tres dykes, which, with the exception of one sample, have both positive ENd (2.0--3.7) and Esr (7.023.6).

185

M. Mazzucchelli et al. / Precambrian Research 74 (1995) 177-194

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M. Mazzucchelli et al. / Precambrian Research 74 (1995) 177-194

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6. Petrogenetic aspects The different geochemical characteristics of the three dyke swarms could be accounted for either by assuming different melting degrees from a similar mantle source, variable crustal contamination during dyke intrusion,

or different compositions and processes of their parental mantle source(s). A lower melting degree of a mantle similar to that which generated the Nico Perez swarm is suggested for the Florida swarm, the latter being enriched in all the elements more incompatible than Zr and depleted in Y

M. Mazzucchelli et al. / Precambrian Research 74 (1995) 177-194

Variable melting degree cannot produce variations in incompatible element ratios as large as those observed between Treinta y Tres and the other dyke swarms (Table 2). Furthermore, even assuming that the Treinta y Tres dykes derived from a lower mantle melting degree, as suggested by the presence of alkali basalts, they are enriched in Ti, P and Sr, but are depleted in LILE. This makes variable melting of similar mantle sources inconsistent. Because lithophile elements are typically enriched with respect to Nb in the continental crust (Taylor and McLennan, 1985), geochemical characteristics similar to those of the Florida and Nico Perez dykes are often ascribed to crustal contamination during dyke emplacement [Piccirillo et al. (1989), Brandon et al. (1993) and references therein]. In the present case, however, the evidence does not favour significant crustal contamination. Bossi et al. (1993) showed that crustal compositions compatible with the Uruguay basement would have resulted in a much larger variation of initial 875r/86Sr in the parental melts of the Florida dykes. Furthermore, crustal contamination would generate mixing lines between the variations of parameters (e.g., isotopes, K/Rb, Ba/Rb, La/Nb, Ba/Nb, Ti/Zr) that

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primitive mantle, Hofmann, 1988). The fields of MORBs, high- and low-Ti Paranfi basahs and Gough Island basalts are reported for comparison. Data source for Paranfi and Gough as in Fig. 6. MORB data from Le Roex et al. (1983, 1992).

and Sc (less incompatible than Zr). This process, however, does not explain their different isotopic characteristics.

Gough Island F i g . 8. ~N,I VS. ~sr at the assumed age of 7 5 0 M a for Nico Perez and

Treinta y Tres and of 1.86 Ga for Florida; Bulk Earth (BE) presentday values are: S7Sr/S6Sr=0.7047; 1 4 3 N d / 144Nd=0.51264. Generalized fields of Paran~i basalts (data source as in Fig. 6), Gough basalts (Le Roex et al., 1992), Etendeka (Hawkesworth et al., 1987 ) and Western Australia (Hergt et al., 1991 ) are reported for comparison.

188

M. Mazzucchelli et al. / Precambrian Research 74 (1995) 177-194

120 100

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(La/Nb)n Fig. 9. Variation of Esrand Er~ vs. (La/Nb),. are different in mantle and crustal systems. As exemplified by Fig. 9, a rough correlation suggestive of crustal contamination exists between La/Nb and esr, but only if all the swarms are considered together. Correlations are absent within each dyke swarm, therefore Bossi et al. (1993) and Rivalenti et al. (1994) concluded that crustal contamination was not an important factor in the Florida and Nico Perez basalt genesis. As for the Treinta y Tres swarm, the lack of significant crustal imprint in the basalts is supported by their low Rb/Sr (0.02_0.01), high K/Rb (563 +214) and Sro < 0.7054. The geochemical characteristics of the dyke swarms depend, therefore, on those of their mantle source(s) and related processes. These are considered further below. 6.1. The Florida and Nico Perez swarms

A comparison of the Florida and Nico Perez trace element patterns with models of fractionated melt derived from 5-20% melting of a garnet-facies primitive mantle (Fig. 6) shows that the trace-element enrichment of the two swarms decreases from LILE

(and from Rb to K) to LREE, whereas, besides a lower Nb, the concentration ofNd, Sr, Zr and Ti (and HREE; Bossi et ai., 1993; Rivalenti et al., 1994) is very similar to that corresponding to 5-10% melt models. According to Tatsumi et al. (1986) elemental solubility in hydrous fluids is controlled by their ionic potential and radius. Since in both swarms element enrichment with respect to melt models decreases with increasing ionic potential and decreasing ionic radius [see Weaver (1991) for a list of ionic radius and potentials], the trace element patterns are consistent with hydrous fluid participation in basalt genesis. The Nb negative anomalies of the Florida and Nico Perez trace element patterns may have been caused either by the low solubility of HFSE with respect to LILE and LREE in hydrous fluids (McCulloch and Gamble, 1991 ), or by the presence of a residual titanate phase (rutile, sphene, ilmenite, perovskite), which may have been stabilized under H20-rich conditions (Hellman and Green, 1979; Saunders et al., 1980) and may have fractionated Nb and Ti relative to Zr. The positive correlations of La/Nb with Zr/Nb, and the low Ti/Zr ratio of Florida (average 52__+11) and Nico Perez (average 5 1 + 5 ) with respect to Treinta y Tres ( 7 0 + 9 ) and to P-MORB (72) and N-MORB (98) (Table 2) supports the presence of the above phase. Although mantle metasomatism promoted by hydrous fluids is typically related to dehydration of subducting plates at convergent plate margins (Saunders et al., 1980; Gill, 1981; Thorpe, 1982; McCulloch and Gamble, 1991), features similar to those of the Florida and Nico Perez swarms are common in several ensialic dyke and flood basalt occurrences (Sheraton and Black, 1981; Ellam and Cox, 1989; Piccirillo et al., 1989; Hergt et al., 1991, Carlson, 1991; see the comparison with the Gondwana basalts). There is little geological evidence for subduction contemporaneous with the Florida and Nico Perez swarm events. Cordani et al. (1988) indicate that at 1.86 Ga the Rio de La Plata craton was stable, implying an ensialic environment for the Florida swarm emplacement. The Nico Perez swarm, if related with subduction, may perhaps represent mantle melting in a back arc environment at least 500 km from a trench zone (see Section 2). Furthermore, both swarms have tholeiitic, and not calc-alkaline, composition and are not as HFSE-depleted as are typical arc magmas (Saunders et al., 1980; Oliveira and Tarney, 1990, fig. 8; Mc-

M. Mazzucchelli et al. / Precambrian Research 74 (1995) 177-194

Culloch and Gamble, 1991). On the contrary, their HFSE concentration, except for Nb, is within the range of 5-10% model melts of primitive mantle (Fig. 6). In ensialic environments geochemical characteristics similar to those of Florida and Nico Perez swarms have been variably explained by the presence of a recycled sedimentary component in the mantle source introduced during previous subductions (e.g., Weaver et al., 1987; Weaver, 1991; Hergt et al., 1991; Zhao and McCulloch, 1993), non-subduction-related metasomatism caused by mantle fluids or melts from deeper mantle (Sheraton and Black, 1981; Ellam and Cox, 1989) and melting of the sub-continental lithospheric mantle assisted by the dehydration of hydrous phases in underplated oceanic crust under the thermal effect of a rising or sinking plume (Tarney, 1992). The results of these processes are virtually indistinguishable on the basis of the present data. In order to account for the relative enrichment in Rb with respect to K, the participation of a terrigenous component may be required, because calculations of the fluid composition in equilibrium with altered MORB show much lower enrichment in Rb with respect to K ( K / R b > 300; Weaver, 1991). This may be a component of the underplated oceanic crust or a recycled component in the asthenosphere depending on the model. There is no correlation between variable degree of Rb enrichment and esr values in the Florida and Nico Perez swarms (Bossi et al., 1993; Rivalenti et al., 1994), indicating that mantle metasomatism (i.e., the addition of Rb-enriched hydrous fluids) was more or less contemporaneous with melting. In the Florida swarm the EM2-type geochemical characteristics (enrichment of Rb with respect to K and Ba; Weaver, 1991) do not correspond with the EMl-type isotopic characteristics, whereas Nico Perez has EM2-type geochemical and isotopic features. Furthermore, in both cases, the variation trends of incompatible trace element ratios (Fig. 7) suggest that hydrous fluids metasomatized an enriched or OIB-type mantle, which may be one of the end members if the trends of Fig. 7 are interpreted as mixing lines. We may, therefore, speculate that the lithospheric mantle in the Palaeoproterozoic had EM I geochemical and isotopic characteristics, whatever the process which generated this composition, e.g., slightly metasomatized primitive mantle (Hart, 1988) or presence of subducted pelagic sediments (Weaver, 1991). This mantle suf-

.728

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Fig. 10. Model relationship showing the Rb/Sr and Sm/Nd values required in a mantle source at 1.86 Ga and having isotope compositions observed in the Florida swarm, in order to evolve time-integrated isotope values observed in Treinta y Tres and Nico Perez. BE = Bulk Earth.

fered underplating of oceanic crust (Tarney, 1992), or received fluid inputs from deeper mantle, close in time to the melting responsible for the Florida event. Thermal perturbation responsible for the Florida dyke generation caused breakdown of hydrous phases and the fluids released favoured melting of large lithospheric mantle segments, resulting in EM2-type characteristics of the melts, which record the EMl-type isotopic features. Isotopic decay in the lithospheric mantle which suffered enrichment processes related with the Palaeoproterozoic Florida event, but escaped melting, may have isotopic characteristics like those observed in the Neoproterozoic Nico Perez dyke swarm. Fig. 10 shows that the initial Sr and Nd isotopic values of Nico Perez could be derived from initial values for Florida, assuming

190

M. Mazzucchelli et al. / Precambrian Research 74 (1995) 177-194

mantle R b / S r = 0 . 0 6 4 ) . 0 9 and S m / N d = 0 . 3 3 - 0 . 4 1 (ranges estimated for the primary mantle melts at Florida; Bossi et al., 1993). Mantle melting processes during the Nico Perez dyke generation were similar to those related with Florida, but affected a lithosphere which had evolved to EM2-type characteristics.

does not account for the low Rb of the Treinta y Tres basalts. Ancient mantle metasomatism followed by ancient depletion is supported by the 1.9 Ga " d a t e " obtained from the Nd isotopic array, which is broadly coincident with that for the Florida metasomatism and melting event. We speculate, therefore, that the geochemical

6.2. T h e T r e i n t a y T r e s s w a r m 0.08

The variation trends of trace element ratios (Fig. 7) suggest that the composition of the Treinta y Tres basalts may be explained by mixing E-MORB, or OIB, with a N-MORB component. Specifically, the enriched component which better corresponds with the Treinta y Tres geochemical characteristics are the Gough OIB (Table 2; Fig. 7). The Treinta y Tres basalts differ from these for being depleted in the most incompatible elements and slightly enriched in Y and Sc (Fig. 6). The ratios of incompatible elements with similar partition coefficients are comparable in Treinta y Tres and Gough (Table 2 and, for La/Nb, B a / N b and K / R b see also Fig. 7), whereas the ratios of elements having the more incompatible element at the denominator are higher in Treinta y Tres (e.g., T i / N b and Zr/Nb; Table 2; Fig. 7). These features suggest that Treinta y Tres derives from a higher melting degree of a Gough-type (or E-MORB-type) source. However, assuming the source composition to be constant, it is difficult to explain incompatible element ratio variations as large as those observed between Treinta y Tres and Gough (e.g., Ti/Nb, Zr/Nb, Rb/Sr; Table 2). This is shown graphically in Fig. 11. The Treinta y Tres values would be consistent with melting of a (Gough-type) enriched source only if the latter suffered previous depletion episodes. Alternatively, if the mantle enrichment process which produced OIB was that proposed by Le Roex (1985), the addition of the metasomatic component was smaller for the Treinta y Tres mantle than for Gough. The fact that the Treinta y Tres basalts are particularly depleted in elements like Rb, which have extremely low concentrations in depleted sources, and which are, therefore, particularly suitable for recording even small-degree metasomatism, does not favour the latter process. Also, the geochemical characteristics of the Treinta y Tres dykes are poorly explained by mixing between asthenospheric ( M O R B ) magmas and lithospheric lamproitic melts (Ellam and Cox, 1991), which, although explaining the high Sr concentration,

i

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F Fig. 11. Comparison of Rb/Sr, Ti/Nb and Zr/Nb variations and average ( • ) of the Treintay Tres basalts with the melt compositional variation (©) of a Gough Basalt source (0). The latter has been estimated by invertingthe non-modalbatch meltingequation for the garnet-bearing mantle facies and using modal source composition, melting proportionsand partition coefficientsin Bossi et al. (1993). The melt composition (average Gough basalts, Le Roex, 1985; mg = 0.61 ) correspondsto F = 5%. The figuredemonstratesthat even very high melting degrees cannot produce the values observed in Treinta y Tres.

M. Mazzucchelli et al. / Precambrian Research 74 (1995) 177-194

and isotopic characteristics of the Treinta y Tres dykes may reflect a Palaeoproterozoic feature of their lithospheric mantle source. The mantle heterogeneity implied by the geochemical differences with the coeval Nico Perez swarm may result from several factors, e.g., in the case of Treinta y Tres, lack of terrigenous sediments in an underplated oceanic crust, lack of the underplated oceanic crust, depletion of the lithospheric mantle keel, possibly containing ancient underplated crust or affected by deep mantle metasomatism, by previous melting (at ~ 1.8 Ga). The latter possibility is favoured by the isotopic composition of the Treinta y Tres basalts, which are consistent with the isotopic ranges for Florida by assuming Rb/Sr = 0.02--0.03 and S m / N d = 0 . 3 7 - 0 . 4 3 (Fig. 10). Alternatively, these values were characteristic of the Palaeoproterozoic mantle which escaped metasomatism, but suffered isotopic resetting during the Florida thermal event.

7. Comparison with Mesozoic Gondwana flood basalts The Mesozoic break-up of Gondwana is associated with large volumes of mantle-derived magmas and it is debated whether these melts inherited their characteristics from the subcontinental lithosphere or asthenosphere and the possible role of mantle plumes. Since crustal contamination is shown to have played a negligible role, the Uruguay basic dykes provide a valuable insight into the characteristics of the Proterozoic subcontinental mantle sources, which may further constrain the extent to which the geochemistry of Mesozoic Gondwana basalts reflect pre-existing mantle features. The trace element patterns of Florida and Nico Perez resemble those of low-Ti basalts of Gondwana and of southern Paranfi (Fig. 6), although the latter have large geochemical variations (Fig. 7). The high-Ti basalt from Brazil and the alkali basalts from western Victoria have trace element patterns (Fig. 6) and element ratios (Table 2; Fig. 7) similar to those of the Treinta y Tres swarm. Isotopically, the low-Ti Gondwana basalts exhibit variation trends similar to those found in Nico Perez, i.e., large esr variation accompanied by comparatively minor variation of eNd (Fig. 8). The few data on the high-Ti basalts suggest their possible trend toward an EM 1-type composition, like that of the Florida swarm.

191

These similarities suggest that the Mesozoic mantle inherited its geochemical and isotope features from processes that occurred during the Proterozoic, in agreement with Mantovani et al. (1985, 1987) and Hawkesworth et al. (1988), who concluded that the Paranfi basalts derived from an incompatible-elementenriched, lithospheric mantle source which was established by 1.8 Ga. Similar results were obtained by Oliveira and Tarney (1990), who found that the geochemical characteristics of the Neoproterozoic gabbros and volcanics of the Sergipe fold belt (Brazil) were similar to those of the Paranfi basalts.

8. Summary and conclusions The three Uruguayan dyke swarms range in age from 1.86 Ga (Florida region) to ~ 0.7 Ga (Nico Perez and Treinta y Tres regions). The oldest Florida dykes and the Nico Perez swarm have similar geochemical features characterized by LILE and LREE enrichment and negative Nb anomaly, but differ in their isotopic characteristics, the Florida swarm being dominated by large eNj variations (EM1 type) and the Nico Perez swarm by large eSr variations (EM2 type). The Treinta y Tres swarm has geochemical characteristics comparable with those of OIB's, specifically the Gough Island basalts, and has slightly positive end and est. Crustal contamination during basalt intrusion had little influence on the composition of the Florida and Nico Perez swarms, and even smaller on that of the Treinta y Tres swarm. Hence, these dykes most likely reflect some of the trace element characteristics and processes of their upper mantle sources. The trace element patterns of the Florida and Nico Perez swarms, specifically the LILE enrichment and negative Nb anomaly, suggest their derivation from a trace-elementenriched, lithospheric mantle. Melting was favoured by metasomatism of hydrous fluids, which may have derived from the breakdown of hydrous phases contained in an underplated oceanic crust (Tarney, 1992) or formed by deeper mantle fluid injections in the lithosphere, as suggested by some mantle xenoliths (Erlank et al., 1987; Demarchi et al., 1988). The EM 1-type isotopic characteristics of the Florida dykes reflect those of the parent mantle, whereas their EM2-type geochemistry points to the hydrous fluid

192

M. Mazzuc chelli et al. / Precambrian Research 74 (1995) 177-194

effects. The E M 2 - t y p e isotopic characteristics o f the N i c o Perez s w a r m are consistent with the isotopic decay o f a mantle source which u n d e r w e n t the Palaeoproterozoic e n r i c h m e n t processes g e o c h e m i c a l l y recorded in Florida. The isotopic characteristics o f the Treinta y Tres basalts are also consistent with isotopic decay of a mantle with the isotopic range o b s e r v e d for Florida, but their g e o c h e m i c a l characteristics require either that the mantle source did not suffer the e n r i c h m e n t events inferred during the Florida episode, or, if it did, that it suffered d e p l e t i o n during the Florida mantle melting. In both cases the R b - S r and S m - N d isotopic systems were reset in P a l a e o p r o t e r o z o i c times. Alternative hypotheses, e.g., m i x i n g o f asthenospheric and iithospheric melts, are not supported. The petrogenetic processes p r o p o s e d implicitly assume that the thermal perturbation which induced mantle m e l t i n g occurred in an ensialic environment, possibly related with a p l u m e thermal effect. The alternative possibility o f a genetic link with a subduction e n v i r o n m e n t w o u l d be consistent with the proposed interaction b e t w e e n hydrous fluids and mantle, but is not supported, albeit not definitely excluded, by the tholeiitic c o m p o s i t i o n o f the N i c o Perez and Florida swarms and their l o w e r L I L E / H F S E ratio values with respect to subduction-related m a g m a s . The poor or absent geological e v i d e n c e for subduction coeval with the dyke intrusion seems significant. The h e t e r o g e n e o u s g e o c h e m i c a l and isotopic features o f the P r o t e r o z o i c U r u g u a y a n dykes suggest significant h e t e r o g e n e i t y o f the subcontinental mantle beneath G o n d w a n a (e.g., C o i l e r s o n and Sheraton, 1986). T h e i r g e o c h e m i c a l and isotope characteristics are similar to those o f the M e s o z o i c basalts related with the G o n d w a n a break-up. As p r o p o s e d by M a n t o v a n i et al. ( 1985, 1987) and H a w k e s w o r t h et al. ( 1 9 8 8 ) , it is, therefore, possible that the M e s o z o i c basalt generation affected a lithospheric mantle source which recorded Proterozoic heterogeneity.

Acknowledgements W e a c k n o w l e d g e the thorough revisions of J.W. Sheraton and J. Tarney and the critical reading, correction o f the English and discussion of A. Cundari. This research was financially supported by Italian A g e n c i e s ,

C N R and M U R S T , F A P E S P and C N P q .

and

by

Brazilian

Agencies,

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