Journal of South American Earth Sciences 12 (1999) 489±500
Geochemical tracers of source rocks in a Cretaceous to Quaternary sedimentary sequence (Eastern Sierras Pampeanas, Argentina) E.L. Piovano a,*, 1, G. RomaÂn Ross b, 1, S. Ribeiro Guevara b, 2, M.A. ArribeÂre b, 2, P.J. Depetris a, 1 a
Facultad de Ciencias Exactas, FõÂsicas y Naturales, Universidad Nacional de CoÂrdoba, Velez Sars®eld, 299, 5000 CoÂrdoba, Argentina Laboratorio de AnaÂlisis por ActivacioÂn NeutroÂnica RA-6, Centro AtoÂmico Bariloche, CNEA, 8400 San Carlos de Bariloche, Argentina
b
Abstract Metamorphic rocks, granitic rocks, and sediments from the Eastern Sierras Pampanas, Argentina, were analyzed for major and trace element concentrations, including rare earth elements (REE). Parental rocks exhibit distinctive REE normalized diagram patterns and elemental ratios, and some elemental ratios reveal signi®cant dierences between rock sources. For example, ratios such as Th/Sc, Cr/Th, and La/Cr have a mean value of 0.7, 8.4 and 0.4 in metamorphic rocks, whereas granitic rocks exhibit means of 1.4, 0.7 and 4.9, respectively. These ratios are also useful in linking detrital materials with the corresponding parental rocks. Metamorphic sources yield sediments with lower Th/Sc and La/Cr, and higher Cr/Th ratios than sediments derived from granitic sources. REE and other elements are enriched in the silt-size fraction, whereas they are diluted by quartz in the sand-size fraction. The size of the Eu/Eu anomaly can be used as a stratigraphical correlation tool in the sedimentary record: Cretaceous rocks show a mean value of 0.9 2 0.1, whereas Tertiary rocks have a mean value of 2.92 0.3. The Eu anomaly in Quaternary and modern sediments ranges from 0.5 to 0.8. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction A major goal in sedimentary studies is to ®nd helpful tools to determine environmental conditions (Folk and Ward, 1957; Passega, 1964; Friedman, 1967), and the provenance of sediments (Suttner, 1974; Argast and Donnelly, 1987; Rooney and Basu, 1994; Cullers, 1995). Source rock types, physicochemical conditions of depositional settings, and diagenetic processes may have a controlling in¯uence on the composition of clastic sedimentary rocks. In analyzing the sedimentary material, geochemical tracers can be used to identify source rocks and weathering processes (Cullers et al., * Corresponding author. E-mail address:
[email protected] (E.L. Piovano). 1 CONICET 2 Instituto Balseiro
1988; Nesbitt, 1979). In combination with traditional methods, which apply facies analysis, geochemical indexes provide an improved and more in-depth analysis of source rock identi®cation. The best geochemical indicators are those that are least aected by weathering processes and represent the composition of the source. Such ``immobile'' elements include the rare earth elements (REE) patterns and ratios of La or Th to Sc, Co or Cr (Taylor and McLennan, 1985; Condie et al., 1995; Liu et al., 1993; Cullers, 1994a,b; 1995). In the present work, we have investigated a terrigenous sedimentary sequence of Cretaceous to Quaternary age derived from a known plutonic±metamorphic complex in order to establish geochemical changes during sediment production. We have also evaluated the relationship between chemical composition and texture to determine the controlling eect of transport processes and grain-size.
0895-9811/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 5 - 9 8 1 1 ( 9 9 ) 0 0 0 3 1 - 0
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Fig. 1. Geological map of the study area (after Gordillo and Lencinas, 1979), and sample locations. Insert at the lower right side shows an enlargement of Quebrada del rõÂ o SuquõÂ a.
2. Geological setting Located in the Eastern Sierras Pampeanas, Central Argentina, the Sierras de CoÂrdoba basement mainly consists of a plutonic±metamorphic complex (Gordillo and Lencinas, 1979) of Lower Paleozoic age which was intruded by granitoids during various times (Rapela et al., 1991, 1998a). The basement mainly includes polymetamorphic rocks, granitoid plutons, and subordinate basic and ultrabasic rocks (Baldo et al., 1996; Rapela et al., 1998b) (Figs. 1 and 2). In the study area, two ranges compose the Eastern Sierras Pampeanas: the Sierra Chica and the Sierra Grande. The Sierra Chica Range, lying towards the eastern portion, is composed mainly of metaigneous and metasedimentary rocks. In the RõÂ o SuquõÂ a area (Gordillo and Lencinas, 1979; Baldo et al., 1996; Rapela et al., 1998b) the basement is composed of Cambrian ortho-gneisses, para-gneisses, schists, migmatites, marbles and amphibolites, and Ordovician trondhjemites and tonalites. West of Sierra Chica, the Achala batholith outcrops in Sierra Grande. It is a major multiphase granitic
complex of Devonian to Carboniferous age (Rapela et al., 1991) characterized by various petrological and geochemical attributes (Lira and Kirschbaum, 1990, Rapela et al., 1991; Demange et al., 1996). The sedimentary sequence studied (Fig. 2) is located on the eastern slope of the Sierra Chica. The sedimentary sequence begins with the red-bedded Early Cretaceous SaldaÂn Formation (250 m thick), which disconformably overlies the basement (Piovano, 1996). This unit consists of conglomerates, sandstones and mudstones, and it has been interpreted as having been deposited in arid alluvial fan environments (Piovano, 1995). The SaldaÂn Formation deposition took place in two sequences separated by an alkaline volcanic event, inferred by the presence of alkali basalt boulders in the uppermost sequence (Piovano, 1996). The Villa Belgrano Formation (Tertiary?) crops out a few kilometers eastward and is composed of redcolored ®ne conglomerates, sandstones, and mudstones accumulated in alluvial fan and braided ¯uvial environments. This formation has an observable thickness of 10 m. Unconformably overlying Tertiary rocks, the clastic
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General Paz Formation. Occurrences of sand and gravel lenses within the ®ne sediments indicate reworking by shallow streams. 3. Samples and methodology
Fig. 2. Stratigraphic scheme for the study area. Collected samples are listed for each Formation.
Quaternary sequence begins with the Estancia Belgrano Formation (Early Pleistocene; Santa Cruz, 1972; Piovano et al., 1992). This Formation (8 m thick) contains stacked ®ning-upward cycles composed of ®ne gravel or sand at the base and mud at the top. These sediments have accumulated in distal areas of alluvial fans. The Pampiano Formation (Middle Pleistocene), unconformably overlies the Estancia Belgrano Formation, and is represented by unstrati®ed clayey-silt or sandy-silt indicating eolian deposition. It has an observable thickness of 7 m. The inter®ngering and overlying RõÂ o Primero Formation (Middle±Late Pleistocene) has a measurable thickness of 3 m. Its texture ranges from gravel to sand, and corresponds to a braided ¯uvial facies. The General Paz Formation (Late Pleistocene±Early Holocene) has a thickness of 4 m and is mainly composed of silt, having isolated gravel/sand lenses. These types of sediments are widely known as loess-like deposits. Eolian processes were not the dominant mechanism in the Pampiano and
The stratigraphic sequence was sampled at the RõÂ o SuquõÂ a Valley, Sierra Chica (CoÂrdoba, Argentina, Fig. 1), where the sedimentary rocks are almost ¯at lying. Sampling took place at the following: (a) Plutonic± metamorphic complex (S1±S7); (b) SaldaÂn Formation, considering the sequences prior to vulcanism (S9±S14), post volcanic (S15±S17) and the basaltic rocks (S8); (c) Villa Belgrano Formation (S18 and S19), and (d) the Quaternary Formations: Estancia Belgrano (S20), Pampiano (S21), RõÂ o Primero (S22) and General Paz (S23). Present-day sediments were also sampled and are represented by bottom sediments taken from the Mal Paso Reservoir (S24) and ¯uvial channel sediments (S25) collected in the Sierra Grande. The chemical composition of sediments was determined on the less than 2-mm grain-size fraction to avoid the bias produced by rock fragments in coarse samples. Grain-size determinations were carried out on the chemically analyzed fraction, by dry sieving and by the pipette methods (McManus, 1988). Mean grainsize, expressed in Phi units (Mz), sorting (S1), skewness (Sk1) and kurtosis (KG) were calculated according to Folk and Ward (1957). Samples were texturally classi®ed following Shepard's scheme (1954). Samples were ground in a tungsten carbide ball-mill and elemental concentrations were determined by instrumental neutron activation analysis (INAA) using the absolute parametric method in the RA-6 Bariloche research reactor. RomaÂn Ross et al. (1995) reported details on the methodology and results obtained in standard reference materials. 4. Results and discussions 4.1. Source rock compositions Lithofacies and paleocurrents in the Cretaceous to Quaternary sequence have revealed that the underlying plutonic±metamorphic complex supplied terrigenous clastic materials (Piovano et al., 1992; Piovano, 1995). The most voluminous units in Sierra Chica are orthogneisses, para-gneisses, schists, migmatites, and the remaining units (i.e., marbles, amphibolites, and Ordovician granitoids) are volumetrically small. Devonian±Carboniferous granitoids are dominant in Sierra Grande. Geochemical features in metamorphic rocks were determined on a gneiss (S1), and a diatexite (S2), both
Table 1 Elemental concentrations and elemental ratios of the samples. Metamorphic rock average does not include marble composition
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a b
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Total sum of REE in ppm. Sedimentary environments are PAF: Proximal alluvial fan, MAF: Middle alluvial fan, DAF: Distal alluvial fan, E: Eolian and F: Fluvial.
with sedimentary protoliths (Baldo et al., 1996; Rapela et al., 1998b), and on an amphibolite (S3) and a marble (S4). Elemental concentrations and elemental ratios are exhibited in Table 1. Three samples analyzed by Rapela et al. (1998b) were also considered: a diatexite (RSU-65), a gneiss (RSU-71) and an ortho-gneiss (RSU-77). Excluding marble, which is dierentiated by a low concentration of REE, the rest of the metamorphic rocks show similar patterns in the REE chondrite-normalized diagram (Fig. 3a), with light REE (LREE) concentrations greater than those of heavy REE (HREE). The La/Yb ratio in metasedimentary rocks ranges from 3.01 to 11.27. Meta-igneous rocks show
large variations in REE patterns (Rapela et al., 1998b) with La/Yb ratios ranging from 5 to 15 in basic and intermediate members, to 55 in the more evolved members. Whether igneous or sediment derived, the metamorphic rocks considered in this paper show a compositional similarity that allows them to be referred to as a single population. In this way, a metamorphic rock mean composition was calculated. The chemical data for marble was not considered to calculate this mean. The average granitic rock composition (Table 1) was calculated using data from a monzogranite, a tonalite, and a pegmatite, all from the Sierra Chica. The samples used were considered to be representative on
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Fig. 3. Chondrite-normalized REE patterns of source rocks and their derived sediments. Chondrite data from Evensen et al., (1978). (a) Metamorphic rocks, (b) Granitic rocks and basalt, (c) SaldaÂn Formation, (d) Tertiary rocks, (e) Quaternary sediments, (f) Present-day sediments.
the basis of previous petrographic knowledge. The REE concentrations and the LREE/HREE ratio in individual samples show marked ¯uctuations (e.g., La/ Yb ranges from 4.03 to 36.99) and the Eu anomaly ranges from almost zero to negative (Fig. 3b). The Achala granitic complex presents a large com-
positional variation in major and trace elements according to the dierent existing facies (Morteani et al., 1995). Distribution patterns of REE concentrations are highly variable with a La/Yb ratio ranging from 10 to 56. The granitic rocks average shows lower REE con-
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495
their respective variances at the 95% con®dence level. Th/Sc, whose variances are not signi®cantly dierent (95% con®dence level) in both rock groups, show signi®cant dierences (95% con®dence level) in their respective means (Student's t test). Metamorphic rocks exhibit lower Th/Sc and La/Cr ratios and higher Cr/ Th values than igneous rocks. The La/Yb ratio does not conclusively separate granitic from metamorphic rocks. Marble is characterized by low elemental concentrations with respect to the remaining source rocks. Hence, its control on the sedimentary composition is easily masked by the in¯uence of metamorphic or igneous rocks. Cretaceous basalts (S8 in Table 1) are of the alkaline type and exhibit the highest REE concentration in the sample set, thus dierentiating volcanic rocks from the remaining source rocks (Fig. 3b). The REE patterns in the normalized diagram are similar to patterns of Cretaceous basalts in the Pampean Range (Kay and Ramos, 1996). 4.2. Chemical composition of sediments
Fig. 4. UCC normalized extended diagrams. (a) Metamorphic and granitic rocks, (b) SaldaÂn Formation, (c) Tertiary rocks, (d) Quaternary sediments, (e) Present-day sediments. Upper Continental Crust (UCC) data from Taylor and McLennan (1985).
centrations than the metamorphic rocks average, and some elemental ratios in both rocks are signi®cantly dierent (Table 1). For example, the F test shows that Cr/Th, La/Cr, and Sm/Nd ratios in metamorphic and granitic rocks exhibit signi®cant dierences between
SaldaÂn Formation samples have similar REE normalized patterns (Fig. 3b), whether they belong to the upper post-volcanic or to the lower pre-volcanic sequences. Samples corresponding to the post-volcanic sedimentary deposition (S15±S17) do not show REE compositional features or elemental ratios that might suggest a signi®cant contribution from volcanic rocks. For example, the Cr/Th ratio, and the SREE dier signi®cantly in basalts (ca 500 and 390, respectively) from sedimentary rocks (ca 6 and 170, respectively). REE patterns of the SaldaÂn Formation are similar to those of metamorphic rocks (Fig. 3). Their mean values for the Eu/Eu and Ce/Ce (0.9 and 1.0 respectively) are slightly higher than those from the plutonic± metamorphic complex. Like the REE normalized patterns, the multi-elemental diagram of average composition of the SaldaÂn Formation (normalized with respect to the upper continental crust, UCC) is similar to that of the metamorphic rock average (Fig. 4a). Multi-elemental diagrams of individual samples also exhibit similar shapes (Fig. 4b) suggesting a uniform metamorphic source during the deposition of the SaldaÂn Formation. With the exception of Sm and Tb values, REE normalized concentrations of the Tertiary Villa Belgrano Formation exhibit the lowest SREE concentration of the sedimentary sequence and depict REE patterns which are closer to the granitic rocks compositional area (Fig. 3c). Although not consistent with a granitic source, the rocks show markedly positive Eu anomalies which are higher than any other of the measured samples (Eu/Eu values of 2.66 and 3.12 in S18 and S19 respectively). Elemental patterns (Fig. 4c) also
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show the lowest concentrations in most of the measured elements, which are closer to a granitic than to a metamorphic rock composition. The mineralogy of Cretaceous and Tertiary sediments can be related to the observed dissimilarity in SREE concentrations. Cretaceous samples consist, in decreasing order of abundance, of altered feldspar, detrital quartz and biotite. Heavy mineral concentrations are low and consist of garnet, amphiboles, pyroxenes and opaques. Tertiary rocks are composed of quartz, microcline, plagioclase and biotite. Accessory heavy minerals are rare and include tourmaline and rutile in quartz grains. Pyroxenes, amphiboles and opaques have not been recorded in the sample set. The abundance of quartz and feldespars and the depletion in ma®c minerals, could explain the low REE concentrations in the Tertiary sedimentary rocks. The REE and Th may be concentrated in monazite, sphene, allanite, apatite, zircon, or garnet, and large variations in the abundance of heavy minerals result in large compositional variation (Cullers et al., 1988). The Eu anomaly value is directly linked to the presence of Ca-plagioclase. The anomalies are the result of fractionation which separates granitic melts from residues containing feldspar, mainly Ca-plagioclase, which is the main host of Eu2+ (Taylor and McLennan, 1988). The low abundance of feldspar (8± 10%) in Tertiary rocks is not consistent with the extreme positive Eu anomaly. As the enrichments or depletions of REE and other soluble major elements could be related to pH (Nesbitt, 1979), the Eu anomalies in Tertiary rocks could be attributed to post-depositional processes. Although both formations are red-colored due to the presence of hematite, the color of the Cretaceous rocks is more intense because it has more ferric pigment than the lighter-colored Tertiary Formation (note %Fe in Table 1). These dierences in color could be due to the leaching of hematite, that could be produced in acidic environments with low E values (Faure, 1992). The low Fe relative abundance is correlated with low REE concentrations, and this is re¯ected by a positive correlation coecient between these variables (SREE vs %Fe; r = 0.74, p < 0.05). Van der Wiejden and Van der Wiejden (1995) noted that the mobility of REE during weathering, under reducing conditions, produces a curious trend in the REE patterns with selective variations in the abundance of REE. They have also shown a case of depletion in Sm and Tb and the immobility of Eu in weathered materials. It is most likely that, as reported by Van der Wiejden and Van der Wiejden (1995), the examined Tertiary rocks probably underwent changes in the redox condition during postdepositional processes which produced the mobilization of iron together with some REE, especially Sm and Tb, thus enhancing the Eu anomaly values.
Fig. 5. Relationships among Th and Nd contents and depositional environments and processes. Grain-size compositions of the samples are presented in Table 1. (5a) SaldaÂn Formation. (5b) Tertiary± Quaternary deposits (including present-day).
Quaternary and recent sediments have REE normalized patterns (Fig. 3d) which are similar to other REE patterns observed in terrigenous sedimentary rocks (e.g., Taylor and McLennan, 1988; Liu et al., 1993). Within Quaternary sediments, alluvial (Estancia Belgrano Formation, S20), and the eolian sediments (Pampiano, S21 and General Paz Formations, S23) show similar REE pattern shapes, that can be related to the pattern of the modern-day sample S24 (Fig. 3e), which was derived from a mixed granitic±metamorphic environment. The sample series (from S20 to S25)
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Fig. 6. Th/Sc and La/Cr variability in sediments and source rocks. Sediments derived from metamorphic rocks show lower Th/Sc and La/Cr ratios than those supplied by a granitic sources. MRA: Metamorphic rocks average, GRA: Granitic rocks average, SFA: SaldaÂn Formation average.
exhibit variable multi-elemental diagrams (Fig. 4d±e). When such diagrams are used to relate sediments to their sources, the conclusion is similar to that obtained when REE normalized patterns are used. Although eolian sediments (S21 and S23) have dierent stratigraphic ages, they contain similar REE normalized patterns, hence suggesting a similar provenance. Even though these eolian deposits had an important input of allochtonous materials including volcanic ash, their resulting REE patterns are close to the metamorphic rock mean pattern. The reworking action of shallow streams on the eolian deposit, with sediments mainly derived from the Sierra Chica (dominantly metamorphic rocks), probably produced a masking eect which hid the allochtonous material signature of the eolian deposits. Isolated gravel lenses in the clay/silty deposits support this assumption. Chemical composition of riverbed samples shows the passage from dominantly granitic terrain (S25), to metamorphic dominance (S24). The shifting is revealed in the REE patterns (Fig. 3e), showing enrichment in the concentration of REE as the relative signi®cance of metamorphic source increases. Sample S22 (RõÂ o Primero Formation of Upper Pleistocene age), which was deposited by the same ¯uvial system, underwent the longest transport over metamorphic terrain and shows the highest REE concentration of the set. We have observed that when metamorphic rocks are present they imprint their own chemical features on the resulting sediments.
4.3. Elemental concentrations, elemental ratios, and the textural eect Table 1 shows chemical compositions and textural data. Positive linear correlations exist between the siltsize percentage and most of the elemental concentrations, indicating a chemical enrichment in this sizefraction. Cs and Rb, for instance, show positive correlation coecients with silt percentage (Cs vs %silt, r = 0.94; Rb vs %silt, r = 0.89; all p < 0.1). Accordingly, both elements are depleted in all samples at higher sand percentages (Cs vs %sand, r=ÿ0.87; Rb vs %sand, r=ÿ0.79; all p < 0.1). Th and Nd concentrations change signi®cantly with silt abundance. As the percentage of silt varies due to the competence of the transporting agent, the SaldaÂn Formation samples are discriminated according to their position within the paleoenvironment (Table 1). In contrast with sediments accumulated in distal zones (Fig. 5a), those sediments deposited in proximal settings (with lesser silt %) have lower Th and Nd values. Tertiary and Quaternary formations depict a similar trend (Fig. 5b). A marked covariance of Hf and Zr (r = 0.94; p < 0.1) suggest an identical source. Hf and Zr positive anomalies, as observed in our study, are usually attributed to the presence of zircon (Condie, 1993). Yb is also concentrated in zircons; large increases in Zr concentrations are associated with smaller increments in Yb concentrations. Such covariance suggests that
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Yb is associated with zircon, a member of the heavy mineral suite which hosts HREE (Sholkovitz, 1990). In agreement with metamorphic source rocks, all sediment samples exhibit high Ta concentrations. Sediments derived entirely from granitic areas (i.e., S25) show Ta concentrations similar to their sources. Positive correlation between SREE, and concentrations of some REE (i.e., Ce, Nd, Eu and Tb) with respect to silt percentages, suggest an enrichment of such elements in this size fraction (e.g., SREE vs %silt, r = 0.42, p < 0.1). The relationship is attributed to the presence of hornblende in controlling light and middle REE in the silt-size fractions. Correlation coef®cients between SREE or individual REE concentrations and clay percentages lacked signi®cance. Low concentrations for most trace-elements in the coarser than 62.5 mm size-fraction is attributed to the higher quartz to other minerals ratios (SREE vs %sand, r=ÿ0.35; p < 0.2). According to the textural classi®cation (Table 1), sandstones and sands are the most depleted in REE. Skewness (Sk1) exhibits a signi®cant and positive correlation coecient with the SREE (r = 0.5; p < 0.1), thus indicating an increase of SREE contents in the positively skewed or ®ner samples. Elemental ratios (Table 1) vary according to dierent sediment grain-sizes. The La/Sc ratio tends to decrease in ®ne grained sediments (La/Sc vs Mz, r=ÿ0.48; p < 0.1) due to the higher Sc concentration in the ®ner classes (e.g., Sc vs Mz, r = 0.44; p < 0.1). The La/Yb ratio is also dierent in dierent grainsizes. Yb concentrations increase with increasing silt relative content (Yb vs %silt, r = 0.4; p < 0.1). Therefore the La/Yb ratio decreases in the ®nest samples (La/Yb vs Mz, r=ÿ0.5; p < 0.1). Neither the Student's t-test nor the F-test allows the use of La/Sc and La/Yb ratios to characterize granitic or metamorphic sources because their variances and means are not signi®cantly dierent. As discussed above, Th/Sc, Cr/Th, La/Cr and Sm/ Nd ratios are signi®cantly dierent (95% con®dence level) in metamorphic and granitic rocks (Table 1). Although these ratios are not transferred unchanged from parent materials to sediments (see above, the in¯uence of silt percentages in Th and Nd concentrations and the relationship between Sc vs Mz), they allow the linking of sediments to parental rocks. The Cr, La, and Sm concentrations do not show correlations with sedimentary size fractions. Some average elemental ratios are useful to the characterization of parental source rocks and their detrital products. Sediments derived from metamorphic sources, for example, can be identi®ed because they show lower values in the Th/Sc and La/Cr ratios, and higher Cr/Th ratios than those produced by a granitic output. Also, Sm/Nd ratios in sediments tend to be
higher from metamorphic rocks sources than from igneous sources. A plot of Th/Sc vs La/Cr illustrates the two groups of sources (Fig. 6). The derived sediments are in intermediate positions between source rock-types. According to these ratios, Cretaceous rocks could be related to a dominant metamorphic source rock (Th/ ScSaldaÂn Fm=0.85 and La/CrSaldaÂn Fm=0.67). Although they were probably aected by leaching processes that changed the original concentrations, Tertiary rocks are also linked to a metamorphic source (Th/ScTertiary and La/CrTertiary rocks=0.67). Both rocks=0.33 Cretaceous and Tertiary rocks show Cr/Th values comparable to metamorphic rocks. The composition of sample S25 (a ``control sample'' taken in a modern-day ¯uvial channel) is representative of sediments derived from a dominantly granitic source. A comparison of S25 to the remaining Quaternary sediments suggests that the latter were derived from dominant metamorphic sources (S21 and S24) and mixed metamorphic±granitic sources (S21, S22 and S23). The Eu anomaly could not be linked with textural parameters. Additionally, high Ce anomalies are correlated with high silt contents (r = 0.52; p < 0.1).
5. Conclusions Trace-element concentrations in the studied sedimentary sequence provide a useful signature which can be used to identify provenance. Granitic rocks show lower REE concentrations than metamorphic rocks, consequently REE normalized and multi-elemental diagrams reveal patterns that can be used in linking sediments to their probable source rocks. Statistical analyses reveal that Cr/Th, La/Cr, Th/Sc and, to a lesser extent, Sm/Nd, exhibit signi®cant dierences in both groups of source rocks (i.e., metamorphics and granitics). Moreover, such ratios appear to be a diagnostic tool which can be used to identify sources. For example, low La/Cr and Th/Sc and high Cr/Th values are a signature of metamorphic provenance. Also, an enrichment in the concentration of REE in ¯uvial sediments is indicative of an increase in the relative signi®cance of the metamorphic source. The longest transport over metamorphic terrain produces the highest REE concentration. Sediment sources can best be discriminated by plotting Th/Sc and La/Cr ratios. For example Cretaceous, Tertiary rocks, and some Quaternary sediments, all with dominant metamorphic source rock, group near the composition of metamorphic rocks. Sediments with mixed sources are separated into a dierent group and a sample representative of sediments derived from a
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dominantly granitic source is markedly dierentiated in the plot. Positive correlations between silt percentages and SREE, or the concentrations of Ce, Nd, Eu and Tb, indicate an enrichment of all these elements in the siltsize fraction. Low concentrations of trace elements at higher sand percentages are attributed to the diluting presence of quartz. In spite of the fact that elemental ratios are aected by the grain-size distribution of sediments, they can be used as geochemical tracers. The Eu anomaly could not be linked with textural parameters. In contrast, Ce anomalies are aected by silt contents. The Eu anomaly can be used in the studied sequence as a stratigraphical correlation tool differentiating Tertiary rocks (mean value of 2.9 2 0.3) from the remaining sediments.
Acknowledgements We thank R. Cullers and C.W. Rapela for their careful revision of an earlier version of this manuscript. We also thank A.J. Kestelman for his interest in this work. The helpful assistance of A. Kirschbaum, D. Gaiero, E. Martinez and B. Theisen is gratefully acknowledged. We are especially grateful to the personnel of the RA-6 for the irradiation of samples. This work has been partially funded by Argentina's CONICET (PIP 4829).
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