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Gondwana Research 19 (2011) 275–290

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Gondwana Research

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Provenance of Ordovician clastic sequences of the San Rafael Block (Central Argentina), with emphasis on the Ponón Trehué Formation

P. Abre a,⁎, C. Cingolani b, U. Zimmermann a,d, B. Cairncross a, F. Chemale Jr. c a Department of Geology, University of Johannesburg, PO Box 524, Auckland Park 2006, Johannesburg, South Africa b Centro de Investigaciones Geológicas, CONICET-Universidad Nacional de La Plata, Calle 1 no. 644, B1900TAC La Plata, Argentina c Núcleo de Geociencias, Universidade Federal do Sergipe, Brazil d Present address: Department of Petroleum Engineering, University of Stavanger, 4036 Stavanger, Norway article info abstract

Article history: The Ordovician Ponón Trehué Formation is the only early Palaeozoic sedimentary sequence known to record Received 23 May 2009 a primary contact with the Grenvillian-age basement of the Argentinean Cuyania terrane, in its southwards Received in revised form 26 April 2010 extension named the San Rafael block. Petrographic and geochemical data indicate contributions from a Accepted 23 May 2010 dominantly upper continental crustal component and a subordinated depleted component. Nd isotopes Available online 4 June 2010 indicate εNd of −4.6, ƒSm/Nd −0.36 and TDM 1.47 Ga in average. Pb-isotope ratios display average values for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb of 19.15, 15.69 and 38.94 respectively. U–Pb detrital zircon ages Keywords: Cuyania terrane from the Ponón Trehué Formation cluster around values of 1.2 Ga, indicating a main derivation from a local Provenance basement source (Cerro La Ventana Formation). The Upper Ordovician Pavón Formation records a younger Geochemistry episode of clastic sedimentation within the San Rafael block, and it shows a more complex detrital zircon age Isotope geochemistry population (peaks at 1.1 and 1.4 Ga as well as Palaeoproterozoic and Neoproterozoic detrital grains). Detrital zircon dating Detailed comparison between the two Ordovician clastic units indicates a shift with time in provenance from Ordovician Ponón Trehué and Pavón localized basement to more regional sources. Middle to early Upper Ordovician age is inferred for accretion Formations of the Cuyania terrane to the proto-Andean margin of Gondwana. © 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction The evolution of the Cuyania terrane concept could be summarized as follows: The term ‘Precordillera’ was ﬁrst used for a physiographic The Cuyania terrane (Fig. 1) in central Argentina is characterized province within the deformed Andean foreland of western Argentina. by a Mesoproterozoic (Grenvillian-age) basement with depleted Pb Mainly Palaeozoic sedimentary rocks are exposed in a Cenozoic thin- isotopic signatures and Mesoproterozoic Nd model ages resembling skinned fold-thrust belt (Fig. 1) generated by ﬂat slab subduction of the basement rocks of the same age from Laurentia (Ramos, 2004; Sato Nazca Plate (28° to 33° SL). This geological province is characterized by et al., 2004 and references therein). Several authors have proposed the extent of its fossil-rich Cambrian–Middle Ordovician carbonate para-autochthonous (Aceñolaza et al., 2002; Finney et al., 2005) platform, unique in South America and was called the Precordillera s.st. versus allochthonous (e. g. Ramos et al., 1986; Dalla Salda et al., 1992; The Precordillera terrane concept was coined later on (Ramos et al., Cingolani et al., 1992; Dalziel et al., 1994; Astini et al., 1995; Thomas 1986; Ramos, 1988) to name an early Palaeozoic Laurentian derived and Astini, 1996; Keller, 1999) geotectonic models for the early exotic block, with a Grenvillian-age basement (registered in xenoliths Palaeozoic evolution of the Cuyania terrane. No consensus has been in Tertiary volcanic rocks), that was accreted to the pre-Andean reached since all the stratigraphical, palaeontological, palaeomag- Gondwana margin (Pampia terrane). Strong evidence that the netic, and isotopic data available thought to support the palaeogeo- Grenvillian-age basement and the early Palaeozoic carbonate and graphic proximity of this terrane to Laurentia (e.g. Benedetto, 1993; siliciclastic cover extended further to the East and South of the original Lehnert and Keller, 1993; Buggisch et al., 1993; Ramos et al., 1998; proposed terrane, lead Ramos et al. (1998) to introduce the concept of a Thomas et al., 2001; Rapalini and Cingolani, 2004), can also be greater Cuyania composite terrane. This terrane incorporates the early interpreted as indicating a Gondwanan signature (Aceñolaza et al., Palaeozoic Precordillera s.st. as well as its southern extension into the 2002; Finney, 2007). San Rafael and Las Matras blocks, along with adjacent parts of the Western Pampeanas Ranges (e.g. Pie de Palo Range) that comprise a Grenvillian-age basement (Ramos, 2004). However, some authors ⁎ Corresponding author. Tel.: +27 727195504; fax: +54 221 4215677. ‘ E-mail addresses: [email protected] (P. Abre), as Finney, (2007) described the Cuyania terrane as the greater [email protected] (C. Cingolani), [email protected] (F. Chemale). Precordillera’ or ‘Precordillera terrane’. Whether minor ranges from

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Fig. 1. Satellite image based map showing Cuyania terrane boundaries as dashed lines and blocks boundaries as continuum lines (based on Ramos et al., 2000; Astini and Dávila, 2004; Porcher et al., 2004). All the entities forming the Cuyania terrane develop a Grenvillian-age basement characterized by Nd, Sr and Pb depleted isotopic signatures (Ramos, 2004; Sato et al., 2004). Righter inlet: location of neighbouring terranes. the Western Pampeanas Ranges form as well part of this crustal block is (Ramos et al., 1996; Fig. 1). Despite the disagreement regarding the still under debate (e.g. Umango Range; Porcher et al., 2004). geotectonic evolution of the Cuyania terrane, provenance analyses of The western boundary of the Cuyania terrane coincides with the its early Palaeozoic sedimentary record using modern techniques are western boundaries of the Precordillera s.st., and the San Rafael scarce (e.g. Loske, 1994; Cingolani et al., 2003; Naipauer, 2007; and the Las Matras blocks, being delimited by the Chilenia terrane Gleason et al., 2007; Naipauer et al., 2010). Author's personal copy

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Two Ordovician units are found within the San Rafael block, two members, which comprise Conodont Biozones (Heredia, 2006). named the Ponón Trehué (Darriwilian to Sandbian) and Pavón The lower member (Peletay) is composed of conglomerates and (Sandbian) Formations. The main purpose of this paper is to examine conglomeratic arkoses, limestones, quartz–arenites and black shales. newly obtained data from the Ponón Trehué Formation using The upper member (Los Leones) is composed of mudstones, petrography, whole-rock geochemistry and isotope geochemistry siltstones, arenites and conglomeratic arenites. Blocks of limestones, (Sm–Nd and Pb–Pb systematics). U–Pb detrital zircon laser ablation granitoids, gneisses and amphibolites are common in the upper part dating from the Ponón Trehué and Pavón Formations is also of the younger member (Bordonaro et al., 1996). The limestone presented. Comparison of the provenance indicators of the Ponón blocks bearing invertebrate fossils resemble the Lower/Middle Trehué Formation, which is the only unit that directly contacts the Ordovician La Silla and San Juan Formations of the Precordilleran Cerro La Ventana Formation (basement of the San Rafael block), with platform (Heredia, 2006), while the crystalline clast and block the Pavón Formation, reveals important information of the sources for compositions resemble Cerro La Ventana Formation basement, the clastic deposition during the proposed Ordovician accretion of the indicating substantial reworking of local lithologies. The continental Cuyania terrane. sedimentary Carboniferous arkosic sandstones overlie the Ponón Trehué Formation through either an unconformity or a fault contact 2. Geological setting of the Ponón Trehué Formation (Fig. 3). The Ponón Trehué Formation is exposed in three areas (Fig. 3). The The Darriwilian to Sandbian Ponón Trehué Formation is a fossil- outcrops located to the south of the Ponón Trehué Creek (Fig. 3, areas rich, carbonate-siliciclastic sequence unconformably overlying 2 and 3) were synchronously named as Ponón Trehué Formation Mesoproterozoic basement of the Cerro La Ventana Formation (Criado Roqué and Ibáñez, 1979) and as Lindero Formation (Núñez, (Heredia, 1996; Cingolani and Varela, 1999; Beresi and Heredia, 1979). Bordonaro et al. (1996) deﬁned the Ponón Trehué Formation as 2003; Cingolani et al., 2005; Heredia, 2006). It outcrops near the to the outcrops located on the southern edge of the Ponón Trehué southern end of the San Rafael block (Cuyania terrane), Mendoza Creek as well as the outcrop located 1.5 km to the north of the Province, central-western Argentina (Fig. 2). It was subdivided into mentioned creek (Fig. 3, areas 1 and 2). The name Lindero Formation

Fig. 2. Geological sketch map of the San Rafael Block (simpliﬁed from Dessanti, 1956; González Díaz, 1972; Núñez, 1979). See the location of the main outcrops of the Pavón and Ponón Trehué Formations. On the righter side, the integrated lithostratigraphic column chart of the San Rafael Block is shown. CLV: Cerro La Ventana Formation; PT: Ponón–Trehué Formation (Darriwilian–Sandbian); P: Pavón Formation (Sandbian). PT is the only unit in contact with the basement of the Cuyania terrane. Author's personal copy

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Fig. 3. Detailed geology of the Ponón Trehué area, with outcrops of the Ponón Trehué Formation in the central-northern part (areas named 1, 2 and 3). On the righter side, the lithostratigraphic section of the Ponón Trehué Formation at the La Tortuga section is shown. Samples used for the present study (numbered to the right) were taken at the lower part of the section where the platform deposits are represented; the upper part of the section is composed by olistostromic-type deposits. Modiﬁed from Núñez (1979), Astini (2002) and Heredia (2006).

was retained for the outcrops to the south of the Ponón Trehué 3. Sampling and analytical techniques Creek (also south of the Cerro Lindero; Fig. 3, area 3). Subsequent studies allowed Heredia (1996, 2006) to establish that all the Sampling for all analyses of the Ponón Trehué Formation was done abovementioned outcrops belong to an olistostromic sequence and at La Tortuga section (Fig. 3 and area 3; 35° 10′ 53″ S–68° 18′ 13″ W). the term Ponón Trehué Formation has since been used and is here Detrital zircons of the Pavón Formation were obtained from a applied. The olistostromes are related to extensional tectonics subfeldspathic–arenite taken from outcrops located on the eastern interpreted as a response to ﬂexural subsidence of the Precordillera side of the Cerro Bola region (Fig. 2). Samples for geochemical analyses carbonate platform starting in late middle Ordovician (Darriwilian) were prepared and milled at CIG (La Plata, Argentina). Major and and may indicate the time of accretion of Cuyania to the proto-Andean selected trace elements (Ni, V, Cu, Ga, Sr, Y, Zr, Zn, Nb, Rb, Ba and Pb) margin of Gondwana (Astini, 2002). were measured on fusion beads (using a 50/50 lithium metaborate/ Author's personal copy

P. Abre et al. / Gondwana Research 19 (2011) 275–290 279 lithium tetraborate as ﬂux) and on pressed powder tablets (using 8:4 g GJ-1 analyses to every ﬁve sample zircon spots. The external errors ratio between sample and Herzog binder pellets), using a Phillips were calculated after propagation error of the GJ-1 mean and the wavelength-dispersive X-Ray Fluorescence spectrometer at SPECTRAU individual sample zircon (or spot). The laser operating conditions (Central Analytical Facilities of the University of Johannesburg, were: laser output power of 6 J/cm2 and a shot repetition rate of

South Africa). Detection limits using the XRF are Si2O=250 ppm, 10 Hz. The cup configuration of the MC-ICP-MS Neptune was: 206 208 232 238 202 204 204 207 Al2O3 =145 ppm, CaO=40 ppm, MgO and Na2O=65 ppm, Faradays Pb, Pb, Th, U, MIC's Hg, Hg+ Pb, Pb. K2O=47 ppm, MnO=15 ppm, TiO2 =20 ppm, Fe2O3 =150 ppm, The gas input included a coolant flow (Ar) at 15 l/min, an auxiliary P2O5 =9 ppm, Ni, Rb, Sr, Cu, Y and Zr=1 ppm, V=3 ppm, Nb and flow (Ar) at 0.8 l/min and a carrier flow of 0.75 l/min (Ar)+0.45 l/min Pb=0.5, Ba=13 ppm. The loss on ignition was calculated by weight (He); the acquisition was at 50 cycles of 1.048 s. difference prior and after heating 1 g of sample powder for 2 h at 1100 °C in an electric oven. Other trace elements (Sc, Cr, Co, Cs, Hf, Ta, 4. Petrography and geochemistry W, Th, U, As and Sb) and rare earth elements (La, Ce, Nd, Sm, Eu, Tb, Yb and Lu) were obtained by INAA (Instrumental Neutron Activation The samples studied from the Ponón Trehué Formation are Analysis) at ACME Laboratories (Canada). Detection limits for elements claystones, siltstones and fine-grained sublith- and subfeldspathic- analyzed by INNA are Sc, Sm and Sb=0.1 ppm, Cr and Nd=5 ppm, Co, arenites (Dott, 1964). The sandstones are moderately sorted and Cs, Hf and W=1 ppm, Ta, U, As, La and Tb=0.5 ppm, Th, Eu and comprise a small amount of clay-rich matrix. Monocrystalline quartz Yb=0.2 ppm, Ce=3 ppm, and Lu=0.05 ppm. Errors are at 1-sigma. is subrounded to subangular. Polycrystalline quartz, also subangular For the isotopic determinations of Sm–Nd, whole-rock powders to subrounded, with no sutured boundaries is sometimes present. K- were spiked with mixed 149Sm–150Nd tracer and dissolved in Teflon feldspar is usually totally replaced by chlorite or clay minerals and vial using an HF–HNO3 mixture and 6 N HCl until complete material subrounded. Sedimentary lithoclasts derived from siltstones, carbo- dissolution. Column procedures used cationic AG-50W-X8 (200– nates, mudstones and chert are present. Calcite cement is present as 400 mesh) resin in order to separate REE, followed by Sm and Nd well as very scarce detrital muscovite lamellae. Zircon, apatite, separation using anionic politeflon HDEHP LN-B50-A (100–200 µm) chromian spinel, tourmaline, rutile, Fe-oxides (including hematite) resin according to Patchett and Ruiz (1987). Each sample was dried to and other opaque minerals comprise the heavy minerals fraction of a solid and then loaded with 0.25 N H3PO4 on appropriated filament the Ponón Trehué Formation. In the QFL ternary diagram, samples (single Ta for Sm and triple Ta–Re–Ta for Nd). Isotopic ratios were from the Ponón Trehué Formation plot in the recycled orogen field measured in static mode with a VG Sector 54 multi-collector mass (Fig. 4). The alteration of feldspars to clay minerals (point-counted as spectrometer at the Laboratório de Geologia Isotópica of Universidade matrix) resulted in a displacement towards the Q apex, when plotting Federal do Rio Grande do Sul (LGI-UFRGS, Brazil). 100–120 ratios with the samples analyzed. Therefore, interpretations of tectonic setting a 0.5 to 1-volt 144Nd beam were normally collected. Nd ratios were using the QFL diagram needs to be taken with caution. normalized to 146Nd/144Nd=0.72190. All analyses were adjusted for Elements concentrated in mafic (Sc, Cr, and Co) and in silicic (La, variations instrumental bias due to periodic adjustment of collector Th, and REE) rocks, REE (rare earth elements) patterns and the positions as monitored by measurements of our internal standards. character of the Eu-anomaly have been used for provenance and Measurements for the Spex 143Nd/144Nd=0.511130±0.000010. tectonic setting determinations (Taylor and McLennan, 1985; McLen- Total blanks average were b150 pg for Sm and b500 pg for Nd. nan and Taylor, 1991), being particularly useful in cases were Correction for blank was insignificant for Nd isotopic compositions petrographical data are not conclusive. The signature of the source and generally insignificant for Sm/Nd ratios. rock may be modified by weathering, hydraulic sorting and diagenesis For the Pb isotopic measurements, an aliquot of 1 ml from (Nesbitt and Young, 1982; Cullers et al., 1987, Nesbitt et al., 1996). dissolved whole-rock samples used for Sm–Nd analysis was taken. Therefore, the determination of the effects that these factors had on Pb was extracted with ion exchange techniques, with AG-1 X 8, 200– the geochemical composition of sedimentary rocks provides evidence 400 mesh, anion resin. Each sample was dried to a solid and added a for the correct interpretation of the provenance (e.g. Cox and Lowe, solution of HNO3 with 50 ppb Tl in order to correct the Pb fractionation during the analyses (Tanimizu and Ishikawa, 2006). Isotopic Pb compositions were obtained at LGI-UFRGS with a Neptune MC-ICP-MS in static mode, with collecting of 60 ratios of Pb isotopes. The obtained values of NBS 981 common Pb standard during the analyses were in agreement with the NIST values. Sandstones were crushed and sieved to less than 100 µm to obtain zircon crystals. Through hydraulic processes the heaviest fraction was separated, which was treated with bromoform (δ =2.89) and methylene iodide (δ=3.32) to obtain a fraction enriched in zircons, followed by an electromagnetic separation with a Frantz Isodynamic Separator. The final selection of individual crystals was done by hand- picking under a binocular microscope. For U–Pb dating all zircons were mounted in epoxy in 2.5 cm-diameter circular grain mounts and polished down until the zircons were revealed. Cathodoluminescence images of each individual zircon grains were obtained with a SEM- BSE-EDS (JEOL JSM-5600 with a tungsten filament and EDS analyses were done using a Noran X-ray detector and Noran Vantage software; the system was set at 15 keV, a working distance of 20 mm and a live time of 60 s per spot). Zircon grains were dated with a laser ablation microprobe (New Wave UP213) coupled to a MC-ICP-MS (Neptune) at LGI-UFRGS. Isotope data were acquired using static mode with spot Fig. 4. Ternary diagram (Dickinson and Suczek, 1979; Dickinson et al., 1983) showing a provenance from recycled orogen for the Ponón Trehué Formation. Qt = total quartz; sizes of 25 and 15 µm. Laser-induced elemental fractional and F = K-feldspar+plagioclase; L = total lithoclasts (Lm+Lv+Ls), and their range of instrumental mass discrimination were corrected by the reference variation is: Qt=79–83%, F=8–11% and L=6–12%. The shadow area represents the zircon GJ-1 (Simon et al., 2004), following the measurement of two data spread of the Pavón Formation (data from Cingolani et al., 2003). Author's personal copy

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1995). The chemical composition of all samples from the Ponón tion during diagenesis cannot be used in this case (e.g. Roser and Trehué Formation analyzed is shown in Table 1. Korsch, 1986). Weathering effects on sedimentary rocks can be quantitatively The trace elements are considered useful for provenance deter- assessed using the Chemical Index of Alteration (CIA; Nesbitt and mination, because they tend to reflect source rock compositions. In Young, 1982). The CIA values for sandstones range from 69 to 79 and particular, the concentrations of high field strength elements such as those for mudstones are between 74 and 76 (Table 1). The Zr, Nb, Hf, Y, Th and U are the most useful (Taylor and McLennan, distribution of samples within the A–CN–K space (Fig. 5a) cannot be 1985). The REE represent reliable provenance indicators, as they easily explained by a normal weathering path (the field of vertical would reflect the average REE composition of the source material lines in Fig. 5a indicates the predicted weathering trend for the (Taylor and McLennan, 1985; McLennan, 1989). average upper crustal composition; Bock et al., 1998). Diagenetic During weathering, there is a tendency for an elevation in the ratio processes or mixing of sources should therefore explain the behaviour between Th and U to above upper crustal igneous values, due to the of samples from the Ponón Trehué Formation. Thus, certain oxidation of U4+ to the more soluble U6+ (McLennan, 1989). Most of provenance discriminators based on elements subject to remobiliza- the samples from the Ponón Trehué Formation display Th/U ratios

Table 1 Chemical analyses of the Ponón Trehué Formation. Major elements are in % whereas trace and REE are in ppm. N denotes normalized to chondrite values. Aver: average; SD: standard deviation; b.d.l.: below detection limits. Eu/Eu*=EuN/(0.67SmN+0.33TbN) and CeN/Ce*=CeN/(0.67 LaN +0.33 NdN), where subscript N denotes values normalized to chondrite. UCC: upper continental crust; data from McLennan et al. (2006).

Mudstones Sandstones

Sample CT3 CT7 CT8 CT6 CT1 CT2 CT4 CT5 Aver SD UCC

SiO2 74.36 59.11 60.30 70.61 78.99 83.85 75.32 77.67 72.53 8.22 66

TiO2 1.42 1.54 1.46 1.40 0.42 0.19 1.39 1.32 1.14 0.49 0.5

Al2O3 13.00 20.28 18.32 13.23 7.77 3.59 10.86 8.67 11.96 5.15 15.2

Fe2O3 3.62 5.76 8.09 6.03 3.09 1.82 4.31 4.83 4.69 1.83 4.5 MnO 0.01 0.04 0.04 0.08 0.02 0.06 0.04 0.03 0.04 0.02 0.06 MgO 1.04 1.44 1.42 1.23 0.52 0.23 0.91 0.81 0.95 0.40 2.2 CaO 0.27 0.21 0.26 0.16 0.55 2.67 0.17 0.21 0.56 0.80 4.2

Na2O 0.62 0.64 1.29 0.85 0.48 1.02 0.78 0.37 0.76 0.28 3.9

K2O 2.93 4.75 3.81 2.78 2.40 1.04 2.34 1.50 2.69 1.11 3.4

P2O5 0.17 0.07 0.11 0.08 0.40 0.17 0.09 0.11 0.15 0.10 0.16 LOI 3.26 4.67 5.00 3.33 3.52 3.54 2.79 3.74 3.73 0.69 Σ 100.70 98.52 100.09 99.78 98.15 98.16 98.98 99.26 99.20 0.87 100.1 CIA 75 76 74 74 69 74 79 74.4 2.6 50 Sc 14.3 20.1 19.3 14.3 10.0 4.1 12.9 11.1 13.26 4.81 13.6 Cr 240 150 150 150 80 140 150 170 153.75 40.91 83 Ni 47 49 75 45 87 23 31 25 49.56 20.92 44 Cs 3 5 5 3 3 b.d.l. 3 2 3.43 1.05 4.6 Rb 110 178 153 107 83 36 89 61 102.1 43.3 112 Ba 1109 1145 763 756 4969 3824 964 575 1763.1 1557.2 550 Hf 10 7 7 9 3 1 9 12 7.25 3.42 5.8 Th 11 14 14 11 7.4 3.7 11 120 10.51 3.23 10.7 U 2.7 1.5 4.3 4.0 3.2 2.0 3.1 3.0 2.98 0.87 2.8 V 123 158 148 119 193 83 117 115 132 31.26 107 Sr 45 66 85 45 70 41 38 36 53.25 16.84 350 Y 39.6 38.6 38.2 25.9 23.4 2.4 25.7 30.3 28.02 11.41 22 Zr 319 253 272 299 110 55 295 402 250.68 106.30 190 Nb 25 27 25 24 10 7 23 22 20.39 7.21 12 Pb 11.3 11.9 19.9 14.0 39.6 15.6 10.2 13.7 17.03 9.00 17 Zn 65 86 117 84 154 55 69 60 85.88 31.92 71 La 39 43 42 31 30 14 31 31 32.63 8.64 30 Ce 95 85 83 70 59 28 71 75 70.75 19.12 64 Nd 41 34 36 31 20 12 25 22 27.63 8.96 26 Sm 10.7 8.1 8.6 6.4 5.6 2.7 6.7 7.2 7.00 2.19 4.5 Eu 1.9 1.3 1.5 1.1 1 0.5 1.2 1.2 1.21 0.38 0.9 Tb 1.6 1.2 1.0 1.1 1 b.d.l. 1.2 1.3 1.20 0.19 0.64 Yb 5.0 5.3 5.2 4.3 2.6 1.1 4.6 4.8 4.11 1.39 2.2 Lu 0.89 0.88 0.84 0.8 0.4 0.2 0.79 0.83 0.70 0.24 0.32 ΣREE 195.09 178.78 178.14 145.7 119.6 58.5 141.49 143.33 145.08 40.14 128.54 Eu/Eu* 0.54 0.49 0.56 0.51 0.52 0.53 0.49 0.52 0.02 0.63 Ce/Ce* 1.1 0.94 0.93 1.03 0.96 0.94 1.09 1.17 1.02 0.09 1.07 K/Cs 8107 7879 6322 7698 6627 6461 6234 7047 751 6136 Th/Sc 0.77 0.70 0.73 0.77 0.74 0.90 0.85 1.08 0.82 0.12 0.79 Zr/Sc 22.31 12.59 14.09 20.91 11.00 13.41 22.87 36.22 19.17 7.81 14 Th/U 4.07 9.33 3.26 2.75 2.31 1.85 3.55 4.00 3.89 2.18 3.8 La/Sc 2.73 2.14 2.18 2.17 3.00 3.41 2.40 2.79 2.60 0.43 2.2 La/Th 3.55 3.07 3.00 2.82 4.05 3.78 2.82 2.58 3.21 0.49 2.8 Cr/V 1.95 0.95 1.01 1.26 0.41 1.69 1.28 1.48 1.25 0.44 0.77 Y/Ni 0.84 0.79 0.51 0.58 0.27 0.10 0.83 1.21 0.64 0.33 0.5 Ti/Nb 343 337.4 342.9 353.1 263.2 165.9 358.1 359.9 315.46 63.57 LaN/YbN 5.27 5.48 5.46 4.87 7.80 8.60 4.55 4.36 5.80 1.45 9.3 LaN/SmN 2.29 3.34 3.07 3.05 3.37 3.26 2.91 2.71 3.00 0.34 4.2 TbN/YbN 1.37 0.97 0.82 1.09 1.64 1.12 1.16 1.17 0.25 1.24 Cr/Th 21.82 10.71 10.71 13.64 10.81 37.84 13.64 14.17 16.67 8.69 7.76 Ti/Zr 26.63 36.50 32.07 28.11 22.98 20.71 28.29 19.67 26.86 5.36 12.9 Author's personal copy

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Fig. 5. Geochemical data of the Ponón Trehué Formation (PT) were plotted on several diagrams. Data from Pavón Formation are from Cingolani et al. (2003).a)A–CN–K diagram based on molecular proportions. The CIA scale is shown on the left. This index uses molecular proportions as follows: CIA=(Al2O3 /(Al2O3 +CaO*+Na2O+K2O))×100, where CaO* refers to the calcium associated with silicate minerals. The average upper continental crust according to Taylor and McLennan (1985), as well as idealized mineral compositions. Field of vertical lines indicates the predicted weathering trend for the average upper crustal composition. b) Plot of Th/U versus Th based on McLennan et al. (1993). c) Th/Sc versus Zr/Sc diagram after McLennan et al. (2003). d) Chondrite normalized REE patterns for the Ponón Trehué Formation. PAAS = post-Archaean Australian shales pattern (Nance and Taylor, 1976) is draw for comparison. Continuous grey lines (QC1 and QR1) correspond to the samples with the highest and the lowest sum of REE from the Pavón Formation (data from Cingolani et al., 2003). Chondrite normalization factors are those listed by Taylor and McLennan (1985).

typical for upper crustal derived rocks (Fig. 5b). Loss of U due to The chondrite normalized REE patterns for mudstones and sand- weathering is evident for sample CT7 (U concentration of 1.5 ppm; stones of the Ponón Trehué Formation (Fig. 5d) show a moderately Table 1). Low Th/U ratios due to U gain are also observed (Fig. 5b). enrichment in LREE, a negative Eu-anomaly and a suspected rather The Zr/Sc ratio reflects reworking because Zr is strongly enriched flat HREE. The samples are enriched in HREE compared with the PAAS, in zircon whereas Sc is not, and zircon can be easily recycled which is considered in turn to reflect the average composition of the (McLennan et al., 1993). The Th/Sc ratio indicates the degree of post-Archaean upper crust. Sample CT2 shows the effects of dilution igneous differentiation processes since Th is preferentially partitioned in the REE concentration due to high silica (Table 1). into melts during crystallization, and as a result, it is enriched in felsic rather than mafic sources, whereas Sc is concentrated in mafic 5. Isotope geochemistry igneous rocks (Feng and Kerrich, 1990; McLennan et al., 1993). In Fig. 5c, the Ponón Trehué Formation shows a cluster of data indicating 5.1. Sm–Nd that processes of recycling were not important and that the source had dominantly a typical upper crustal composition. However, the The Sm/Nd ratio is primarily modified during processes of mantle– high Sc content of certain samples indicates a subordinated mafic crust differentiation allowing the estimation of the model age or the input. time at which the initial magma was separated from the upper mantle The input of a mafic source could be discriminated using the Y/Ni (Nelson and DePaolo, 1988). In the case of sediments, it can only and Cr/V ratios (McLennan et al., 1993). Cr/V ratios higher, and Y/Ni approximate the average crustal residence age of the protoliths. ratios lower than the UCC, along with enrichment in elements such as Results from the five selected samples from the Ponón Trehué Sc, Cr and V compared with the UCC values might indicate the Formation analyzed for the present provenance study are shown in influence of a source with a composition less evolved than the average Table 2. The εNd (t) values where t=462 Ma (depositional age) are of UCC for the Ponón Trehué Formation (Table 1). An ophiolitic source −4.47±0.39 in average; ƒSm/Nd has an average value of −0.37±0.02 can be neglected. and the average TDM age is 1.44±0.078 Ga. Author's personal copy

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Table 2 143 144 Sm–Nd and Pb–Pb isotopes for the Ponón Trehué Formation. TDM calculated according to DePaolo (1981). n.d.: no data. The εNd indicates the deviation of the Nd/ Nd value of the 147 144 143 sample from that of CHUR (DePaolo and Wasserburg 1976), whereas the ƒSm/Nd is the fractional deviation of the sample Sm/ Nd from a chondritic reference. εNd (0)={[( Nd/ 144 143 144 143 144 147 144 147 144 Nd)sample (t=0)/0.512638]−1}*10,000. εNd (t)={[( Nd/ Nd)sample (t)/( Nd/ Nd)CHUR (t)]−1}*10,000. ƒSm/Nd=( Sm/ Nd)sample/( Sm/ Nd)CHUR−1. 143 144 143 144 143 144 147 144 εNd (t)={[( Nd/ Nd)sample (t)/( Nd/ Nd)CHUR (t)]−1} * 10,000. Nd/ NdCHUR=0.512638. Sm/ NdCHUR=0.1967. Pb data are corrected for mass fractionation.

Sample CT1 CT3 CT4 CT5 CT6 CT8

Age (Ma) 462 462 462 462 462 462 Sm (ppm) 4.29 6.22 4.50 4.54 n.d. 5.47 Nd (ppm) 21.88 28.87 21.56 21.68 n.d. 27.32 147Sm/144Nd 0.11858 0.13016 0.12613 0.12676 n.d. 0.12102 143Nd/144Nd 0.512230 0.512203 0.512173 0.512217 n.d. 0.512161 Error (ppm) 18 13 36 14 n.d. 14

εNd (0) −7.96 −8.48 −9.07 −8.20 n.d. −9.31

εNd (t) −3.95 −4.56 −4.91 −4.08 n.d. −4.84 TDM (Ma) 1298 1522 1504 1438 n.d. 1442 ƒSm/Nd −0.40 −0.34 −0.36 −0.36 n.d. −0.38 206Pb/204Pb n.d. 19.303141 19.173025 19.160300 19.100324 19.027669 Error (ppm) n.d. 14 22 31 16 5 207Pb/204Pb n.d. 15.706133 15.703028 15.703960 15.689129 15.666211 Error (ppm) n.d. 10 23 38 19 5 208Pb/204Pb n.d. 38.989437 38.970501 38.975361 38.833531 38.948940 Error (ppm) n.d. 16 23 40 17 14 208Pb/206Pb n.d. 2.0198499 2.0325439 2.0341752 2.0331038 2.0469290 Error (ppm) n.d. 17 15 11 8 23 207Pb/206Pb n.d. 0.81364848 0.81902006 0.81960700 0.82139061 0.82333189 error (ppm) n.d. 14 16 3 9 34

The TDM ages are comparable to TDM ages for Mesoproterozoic and second stage Pb evolution curve for average crust. They overlap the supracrustal younger rocks of the Cuyania terrane (Kay et al., 1996; ﬁeld of Proterozoic rocks of the Southern and Central domains of the Cingolani et al., 2003; Sato et al., 2004; Cingolani et al., 2005; Gleason Arequipa–Antofalla Basement (Aitcheson et al., 1995; Tosdal, 1996; et al., 2007). They are also within the range of variation of the TDM ages Loewy et al., 2004). A similar behaviour is observed for the samples on of Neoproterozoic to Palaeozoic sequences in northwestern a thorogenic (208Pb/204Pb) versus 206Pb/204Pb present-day composi- Argentina, including the igneous rocks of the Pampia terrane (Rapela tion (Fig. 8b). In both diagrams, it is also evident that the Pb system of et al., 1998; Bock et al., 2000; Lucassen et al., 2000). Similar data are the Ponón Trehué Formation differs consistently from the Grenvillian known from the basement of Chilenia (Bahlburg et al., 2001), from xenoliths of the inferred basement of the Cuyania terrane (Kay et al., Mesoproterozoic rocks from Antarctica, Falklands/Malvinas plateau 1996), Proterozoic rocks from Eastern North America, rocks from the and Natal–Namaqua Metamorphic belt (Wareham et al., 1998)and Northern domain of the Arequipa–Antofalla Basement (Tosdal, 1996; the Western Pampeanas Ranges (Vujovich et al., 2005). Loewy et al., 2004) and from Mesoproterozoic rocks of the Natal–

Fig. 6a shows the relationship between εNd (t) and Th/Sc ratio of Namaqua Metamorphic belt, Falkland/Malvinas Microplate and samples from the Ponón Trehué Formation where it is seen that the Antarctica (Wareham et al., 1998). Compared with supracrustal

εNd (t) values obtained are neither typical of UCC nor of a juvenile rocks of the Cuyania terrane (Gleason et al., 2007) the data here input and the Th/Sc ratio is indicative of a source less fractionated than presented have higher 208Pb/204Pb and lower 206Pb/204Pb ratios the UCC but not clearly maﬁc in composition. The same can be (Fig. 8). deduced using the plot of ƒSm/Nd versus εNd (see Fig. 6b), where the ƒSm/Nd values could be assigned to an old upper crust or an arc component but the εNd (t) values are between the two ﬁelds. 6. Comparison with the Pavón Formation εNd (t) values for the Ordovician units of the San Rafael block are similar to those from other Ordovician sedimentary rocks from the The early Upper Ordovician (Sandbian) Pavón Formation crops out Cuyania terrane (Abre, 2007; Gleason et al., 2007), as well as from in the central area of the San Rafael block (Fig. 2). The unit appears in rocks of the Famatinian arc (Pankhurst et al., 1998; Fig. 7). εNd values isolated outcrops dismembered by Tertiary tectonism; it is neither in are within the range of variation of the Laurentian Grenville crust contact with the Grenvillian-age basement nor with the Ponón Trehué (Patchett and Ruiz, 1989) and the Central and Southern domains of Formation and it is covered by Upper Palaeozoic volcaniclastic rocks. It the Arequipa–Antofalla Basement (sensu Loewy et al., 2004). Nd data is a sandy marine turbidite sequence composed of arenites, wackes presented by Cingolani et al. (2005) for the Cerro La Ventana and pelites and bearing Climacograptus bicornis Biozone (Cuerda and

Formation show εNd values in the range of variation of data calculated Cingolani, 1998; Manassero et al., 1999). Geological details regarding to the time of deposition of the Ponón Trehué Formation (Fig. 7). the Pavón Formation can be found in Cingolani et al. (2003). The Pavón Formation comprises feldspathic- and quartz–wackes and subordinated subfeldspathic- and sublith-arenites. The sandstone 5.2. Pb–Pb composition of the Pavón Formation indicates in QFL diagrams of Dickinson et al. (1983), a provenance from recycled orogen and The use of Pb isotopes is an established tool for evaluating the continental block (Fig. 4; Cingolani et al., 2003). Noteworthy is the provenance of clastic sedimentary rocks (Hemming and McLennan, presence of detrital chromian spinels derived from host rocks 2001). Pb isotopes from ﬁve selected samples from the Ponón Trehué emplaced within mid-ocean ridge and intraplate environments Formation were analyzed for the present provenance study. The (Abre, 2007; Abre et al., 2009). Geochemical data for the Pavón 206Pb/204Pb ratio ranges from 19.028 to 19.303, the 208Pb/204Pb ratio Formation can be found in Cingolani et al. (2003). The CIA values for ranges from 38.83 to 38.99, whereas the radiogenic 207Pb/204Pb ratio this unit indicate intermediate to advanced weathering conditions, is in between 15.66 and 15.71 (Table 2). but samples follow a general weathering trend parallel to the A–CN On an uranogenic-Pb diagram (Fig. 8a), samples from the Ponón boundary (Fig. 5a). Th and U concentrations and Th/U and Zr/Sc ratios Trehué Formation plot slightly above the Stacey and Kramers (1975) are similarly variable compared with the Ponón Trehué Formation Author's personal copy

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chromian spinels conﬁrm inputs from a depleted source of an uncertain age. Detrital zircon ages also suggest that the source of the Pavón Formation is not as restricted as that of the Ponón Trehué Formation (see below). Sources that could have contributed to this

less negative εNd value are found within the Central and Southern domains of the Arequipa–Antofalla Basement (Fig. 7; Loewy et al., 2004). Sm–Nd data from the Western Pampeanas Ranges are scarce and ages of the analyzed rocks are not available (Porcher et al., 2004; Vujovich et al., 2005), being therefore not possible to calculate their

εNd at the time of deposition of the Pavón Formation. However, the positive εNd values found within the Umango Range (εNd(t = 1.1Ga) between +1.4 and +4.2; Porcher et al., 2004) are worth noting as probable contributors of detritus (Fig. 7).

7. Detrital zircon dating of the Ponón Trehué and Pavón Formations

U–Pb dating of detrital zircons is a powerful tool that allows the identification of mainly felsic to intermediate provenance compo- nents in clastic sedimentary rocks (e.g., Fedo et al., 2003; Veevers and Saeed, 2009; Kuznetsov et al., 2010). Zircons were obtained for the Ponón Trehué (sample CT1; Fig. 3) and the Pavón Formations (sample QMOTO1; map on Cingolani et al., 2003). Detrital zircons with Th/U ratios indicative of a magmatic origin (Th/U ratio more than 0.2; Vavra et al., 1999; Hoskin and Schaltegger, 2003) were observed for both units, with only a very few exceptions of metamorphic derived zircon grains (Tables 3 and 4). Cathodoluminescence images from both units show that most of the zircon grains are subhedral and display oscillatory magmatic zoning, whereas only a few have patchy metamorphic zoning (e.g. the zircon C-III-85 from Fig. 9a). Detrital zircon ages of the Ponón Trehué Formation (n=38) display a main probability peak at 1213.4 Ma, and clusters at 1164 and 1066 Ma; only one discordant grain has a younger age of 834 Ma (Fig. 9a). The very narrow range of detrital zircon ages implies a restricted provenance. The low Zr/Sc ratio of that specific sample, which is the less recycled sandstone from this unit, implies a local source. The detrital zircon dating of the Pavón Formation (n=51) indicates a main probability peak at 1106.3 Ma and clusters at 1058.8 and 1369 Ma, whereas two grains are Neoproterozoic (634 and 615 Ma) and one grain has a Palaeoproterozoic age of 1652 Ma (Fig. 9b). These detrital zircon ages indicate sources of wider age Fig. 6. a) εNd (t) versus Th/Sc ratios and b) ƒSm/Nd versus εNd (t). In both diagrams it is shown that the Ponón Trehué Formation shows a similar distribution compared with range compared to the Ponón Trehué Formation source. the Pavón Formation (data from Cingolani et al., 2003), although one sample from the Upper Ordovician sandstones of the Cuyania terrane showed a ε − latter unit has an Nd (t) of 0.4 and a low Th/Sc ratio (0.43) which is indicating the broader U–Pb detrital zircon spectrum with dominant peaks between ƒ − – input from a juvenile source. A Sm/Nd of 0.34 tends to indicate that the Sm Nd system 1.0 and 1.5 Ga, although the 1.2 Ga peak present in the Ponón Trehué from this sample is not fractionated. Formation is not evident (Gleason et al., 2007). Noteworthy is the 1.4 Ga peak found by these authors in the Estancia San Isidro (Fig. 5b and c), whereas the Th/Sc ratios include also lower values. Cr/ Formation (Middle Ordovician; Precordillera s.st.) which matches a V and Y/Ni ratios suggest the influence of a mafic source. For both similar cluster observed in the Pavón Formation. The 1.4 Ga peak is units the depleted component could be represented by the detrital also found in the Cambrian La Laja and Cerro Totora Formations spinels (Abre, 2007; Abre et al., 2009). REE patterns of the Pavón (Cuyania terrane), but these units completely lacks of 1.0 to 1.2 Ga Formation are also similar to those from the Ponón Trehué Formation zircons (Thomas et al., 2004; Finney et al., 2005). Minor population of (Fig. 5d). 600–700 Ma detrital zircons is also referred by Finney (2007) and

The Pavón Formation shows similar εNd (t), ƒSm/Nd and TDM values Gleason et al. (2007) in Upper Ordovician siliciclastics of the Cuyania (Cingolani et al., 2003) compared with the Ponón Trehué Formation terrane. The detrital zircon population could comprise zircon grains

(Fig. 6), besides a juvenile input represented by a εNd value of −0.4 provided by recycling of older sedimentary sequences of the Cuyania (Fig. 6a). Pb data from the Pavón Formation are not available. terrane, particularly when detritus had been deposited by turbidite The Pavón Formation foreland basin resulted from the accretion of currents. the Cuyania terrane to Gondwana, and received a main sedimentary Mesoproterozoic rocks that could have contributed to the more input from the local Grenvillian basement known as the Cerro La important cluster of detrital zircons are present in several neighbour- Ventana Formation (Cingolani et al., 2003). The range of variation of ing areas. Mesoproterozoic ages within the basement of the Cuyania

εNd values of the Cerro La Ventana Formation are in the range of terrane are found at: the Cerro La Ventana Formation, San Rafael block variation of data calculated to the time of deposition of the Pavón (1.1–1.2 Ga; Cingolani and Varela, 1999; Cingolani et al., 2005), the Formation (Fig. 7). The exemption is the −0.4 value (Fig. 7), implying Pie de Palo Range (1.0–1.2 Ga; McDonough et al., 1993) and the contributions from other sources besides the Cerro La Ventana Umango, Maz and Espinal Ranges (1.0–1.2 Ga; Varela and Dalla Salda, Formation. Geochemistry (Cr/V and Y/Ni) and the presence of detrital 1992; Varela et al., 1996; Casquet et al., 2006; Rapela et al., 2010). The Author's personal copy

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Fig. 7. εNd versus age of the Ponón Trehué and Pavón (data from Cingolani et al., 2003) Formations and of areas evaluated as probable sources. CHUR: Chondritic Uniform Reservoir;

U: Umango Range. A: granulitic and amphibolitic, and G: acidic-gneissic (εNd (t)=+7), xenoliths from the inferred basement of the Cuyania terrane (Kay et al., 1996).

Arequipa–Antofalla Basement shows ages between 0.97 and 1.6 Ga (Bahlburg and Hervé, 1997 and references therein). The Amazon craton (1.0–1.55 Ga; Loewy et al., 2004; Schwartz and Gromet 2004 and references therein), the Grenville Province of Laurentia (0.9– 1.3 Ga; summary from Carrigan et al., 2003) and the basement rocks of the Chilenia terrane (Ramos and Basei, 1997) also display Mesoproterozoic ages. Palaeoproterozoic rocks are also found in the Arequipa–Antofalla Basement (1.9–2.0 Ga; Bahlburg and Hervé, 1997 and references therein), the Amazon craton (1.8–1.95 Ga; see Loewy et al., 2004; Schwartz and Gromet 2004 and references therein), within Laurentia (1.6–1.8 Ga; summary from Carrigan et al., 2003) but also at certain areas of the Western Pampeanas Ranges (1.8–1.9 Ga; Maz Range; Casquet et al., 2006). Some of these areas can be ruled out as sources based on Sm–Nd, Pb–Pb and palaeocurrents data as explained in the discussion. The Neoproterozoic zircons could be linked mainly to the Pampean/Brazilian Orogen. However, zircons in the 600–700 Ma age range are recorded in Cuyania Palaeozoic sedimentary units (Finney, 2007; Gleason et al., 2007). Noteworthy is the absence of detrital zircons of the Famatinian cycle (Upper Cambrian–Lower Devonian). The Mesoproterozoic detrital zircon ages found are consistent with recycled portions of the Pie de Palo basement (Finney, 2007; Gleason et al., 2007 and references therein), thought to represent exposed basement of the Cuyania terrane north of the San Rafael block (Fig. 1).

8. Discussion

8.1. Provenance indicators and source rocks

Petrographical, geochemical and isotopic analyses show that the source rocks for the Ordovician sedimentary sequences of the San Rafael block have a dominant unrecycled UCC composition, but an input from a less fractionated component is also evident. The main geochemical differences between the Ponón Trehué and the Pavón Formations are the general wider variability of geochemical proxies for the later (Fig. 5), indicating a broader range of provenance Fig. 8. a) 207Pb/204Pb versus 206Pb/204Pb present-day ratios and b) 208Pb/204Pb versus 206Pb/204Pb present-day ratios. Solid circles represent samples from the Ponón Trehué composition. Zircon age spectra for both units (Fig. 9) constrain the Formation; SK: Stacey and Kramers reference line. age of the main sources to the Mesoproterozoic. However, the Pavón Author's personal copy

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Table 3 Detrital zircon dating of the Ponón Trehué Formation. Isotopic ratios corrected for fractionation by comparison with standard GJ-1. Concordant ages calculated using Isoplot/Ex (Ludwig, 2001); in italics, zircon spots for which the 207Pb/206Pb was used. Errors are in 1-sigma for ratios (in %) and ages (absolute).

Isotopic ratios Age (Ma) Concordant age

Sample 232Th/238U 207Pb/235U± 206Pb/238U ± % disc. ƒ206 206Pb/238U±207Pb/235U± 207Pb/206Pb ± Ma

A-I-01 0.49 1.99744 2.52 0.17479 2.20 18 0.001 1038 23 1115 28 1266 15 1266±48 A-I-01b 0.45 2.19197 1.55 0.19149 1.01 11 0.001 1129 11 1178 18 1270 15 1270±45 A-I-03 0.48 2.13908 1.97 0.19915 1.39 −2 0.001 1171 16 1162 23 1144 16 1165 ±13 A-I-02a 0.33 1.34998 4.87 0.13708 2.64 15 0.102 828 22 868 42 970 40 834±20 A-I-03 0.16 2.05176 1.64 0.18954 1.44 4 0.006 1119 16 1133 19 1160 9 1138 ±11 A-I-05 0.50 2.26971 1.04 0.20442 0.43 1 0.001 1199 5 1203 12 1210 11 1199 ±4.6 A-06 0.41 2.17768 2.00 0.19888 1.60 1 0.001 1169 19 1174 23 1183 14 1174 ±14 A-I-10 0.31 2.33728 2.38 0.21279 2.30 −5 0.000 1244 29 1224 29 1189 7 1199 ±11 A-I-12 0.16 2.27266 2.60 0.20182 1.92 4 0.003 1185 23 1204 31 1238 22 1199 ±18 A-I-15 0.16 2.29091 2.33 0.20389 1.86 3 0.003 1196 22 1209 28 1233 17 1209 ±16 A-I-19 0.48 2.33586 1.90 0.21036 1.63 −2 0.001 1231 20 1223 23 1210 12 1221 ±13 B-II-25 0.26 2.06161 2.12 0.19312 1.85 −1 0.000 1138 21 1136 24 1132 12 1135 ±14 B-II-29 0.49 2.33317 2.16 0.20909 0.60 0 0.005 1224 7 1222 26 1220 25 1224 ±7 B-II-33 0.40 2.49116 1.20 0.22019 0.73 −3 0.012 1283 9 1269 15 1247 12 1277 ±8 B-II-34 0.47 2.28317 2.09 0.20436 1.79 2 0.000 1199 21 1207 25 1222 13 1209 ±15 B-II-40 0.42 2.24650 1.84 0.20312 1.63 1 0.000 1192 19 1196 22 1202 10 1197 ±12 B-II-43 0.49 2.32042 1.84 0.20716 1.31 1 0.014 1214 16 1219 22 1227 16 1217 ±13 B-II-48 0.23 2.29776 1.70 0.20492 1.06 2 0.009 1202 13 1212 21 1229 16 1206 ±11 B-II-54 0.47 2.20956 1.06 0.19828 0.82 4 0.001 1166 10 1184 13 1217 8 1217±27 B-II-55 0.27 2.25312 1.45 0.20223 0.97 2 0.001 1187 12 1198 17 1217 13 1193 ±9.5 B-II-56 0.34 2.19581 1.51 0.19473 0.46 8 0.000 1147 5 1180 18 1240 18 1149 ±100 C-III-63 0.15 2.11401 3.82 0.18815 2.35 10 0.009 1111 26 1153 44 1233 37 1126 ±23 B-II-61 0.17 2.17676 2.36 0.19250 2.28 9 0.002 1135 26 1174 28 1246 8 1246±25 B-II-65 0.37 1.97031 1.90 0.18117 1.25 8 0.008 1073 13 1105 21 1169 17 1169±59 C-III-66 0.49 2.10575 1.44 0.19080 1.15 6 0.001 1126 13 1151 17 1198 10 1200±17 C-III-69 0.25 1.92456 2.61 0.16934 2.27 20 0.011 1008 23 1090 28 1256 16 1256±59 B-II-75 0.34 2.21532 4.84 0.20399 4.59 −3 0.001 1197 55 1186 57 1166 18 1175 ±26 C-III-73 0.37 2.21646 4.21 0.20352 4.02 −2 0.001 1194 48 1186 50 1172 15 1177 ±22 B-II-85 0.36 2.16012 1.69 0.19643 1.03 3 0.003 1156 12 1168 20 1191 16 1161 ±10 D-IV-109a 0.45 1.82395 5.42 0.16150 4.22 22 0.024 965 41 1054 57 1243 42 1243±100 D-IV-109b 0.47 2.05928 1.98 0.18466 1.66 10 0.007 1092 18 1135 23 1219 13 1241±44 D-IV-112 0.32 2.07703 3.32 0.18123 2.32 16 0.024 1074 25 1141 38 1272 30 1272±94 D-IV-116 0.53 1.87917 4.81 0.17799 1.22 5 0.058 1056 13 1074 52 1110 52 1065 ±12 D-IV-118 0.34 2.11591 2.07 0.20177 1.43 −8 0.000 1185 17 1154 24 1097 16 1097±60 D-IV-120 0.37 1.52118 3.84 0.14503 3.47 20 0.007 873 30 939 36 1097 18 1097±69 D-IV-126 0.58 2.14267 1.66 0.20534 1.03 −11 0.000 1204 12 1163 19 1087 14 1170 ±100 D-IV-131 0.76 2.17900 1.83 0.20721 1.10 −10 0.003 1214 13 1174 21 1102 16 1184 ±100 D-IV-115 0.43 2.12890 2.51 0.19237 2.28 6 0.000 1134 26 1158 29 1203 13 1203±46

Formation shows two peaks, one at 1.1 Ga and another at 1.4 Ga (with Pavón Formations. The scarcity of Famatinian aged detrital zircons minor contributions from Palaeoproterozoic and Neoproterozoic within the Ordovician record of the Cuyania terrane could indicate sources), whereas the Ponón Trehué Formation shows a main peak that the Famatinian magmatic arc was not related to the docking of at 1.2 Ga. the Cuyania terrane (Finney et al., 2005). However, the presence of a Further constraints are provided by sedimentologic characteristics positive area acting as a barrier would have been enough to prevent which indicates for the Ponón Trehué Formation a dominant those detritus to reach the Ordovician basin (Abre, 2007). provenance from the underlying Mesoproterozoic Cerro La Ventana As it is shown on Figs. 7 and 8 rocks from Antarctica, Falklands/ Formation (Heredia, 2006), while palaeocurrents point to an eastern Malvinas Microplate and the Natal–Namaqua Metamorphic belt provenance for the detritus of the Pavón Formation invalidating accomplish for Mesoproterozoic ages and have comparable TDM western sources such as the Chilenia terrane (Manassero et al., 1999; ages, but have different Pb-isotope compositions (Wareham et al., Cingolani et al., 2003). 1998). These differences allow discarding such areas (named SAFRAN Several areas should be evaluated as sources for the Ponón Trehué in the para-autochthonous model; Aceñolaza et al. 2002; Finney et al., and Pavón Formations in relation to the models proposed to explain 2005) as sources. the tectonic evolution of the Cuyania terrane: the Grenville Province Several models assigned a Laurentian (Southern Appalachian) of Laurentia, the Western Pampeanas Ranges, the basement of the origin for the Cuyania terrane (e.g. Ramos et al., 1986; Dalla Salda Cuyania terrane, the Famatinian arc, the Arequipa–Antofalla Base- et al., 1992; Dalziel et al., 1994; Astini et al., 1995; Thomas and Astini, ment (Central Andes), Antarctica, Falklands/Malvinas Microplate and 1996; Keller, 1999). The time of detaching from Laurentia varies the Natal–Namaqua Metamorphic belt (e.g. Ramos et al., 1986; Dalla according to different authors, but a link to Laurentia is considered Salda et al., 1992; Dalziel et al., 1994; Astini et al., 1995; Aceñolaza even until the end of the Ordovician (e.g. Thomas and Astini, 2003). If et al. 2002; Finney et al., 2005). the Cuyania terrane was attached to Laurentia during the Ordovician, The subduction towards east of the Iapetus crust beneath then a Laurentian signature could be recorded within (and/or Gondwana resulted in developing the Famatinian magmatic arc reworked into) the sedimentary sequences, as suggested for the (530–460 Ma; Pankhurst et al., 1998) and the docking of the Cuyania Cambrian clastic record (Naipauer, 2007; Naipauer et al., 2010). The terrane. As documented in foreland basins elsewhere, a provenance Appalachian belt comprises Grenvillian-age igneous-metamorphic from the arc should be expected (Miall, 2000). The lack of detrital rocks (and minor Palaeoproterozoic and Neoproterozoic rocks), and zircons within the age range of the Famatinian magmatic arc (Fig. 9) since it was elevated during most of the Palaeozoic, it acted as a major indicates that this area was not a source for the Ponón Trehué and source (Boghossian et al., 1996). Even though these rocks could Author's personal copy

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Table 4 Detrital zircon dating from the Pavón Formation. Isotopic ratios corrected for fractionation by comparison with standard GJ-1. Concordant ages calculated using Isoplot/Ex (Ludwig, 2001); in italics, zircon spots for which the 207Pb/206Pb were used. Errors are in 1-sigma for ratios (in %) and ages (absolute).

Isotopic ratios Age (Ma) Concordant age

Sample 232Th/238U 207Pb/235UU ± 206Pb/238 ± % disc. ƒ206 206Pb/238U± 207Pb/235U± 207Pb/206Pb ± Ma

G-VII-458 0.58 1.81152 2.31 0.17440 1.88 4 0.0003 1036 19 1050 24 1077 15 1049 ±15 G-VII-456 0.66 1.83079 3.89 0.17622 1.74 3 0.0046 1046 18 1057 41 1078 37 1048 ±16 G-VII-454 0.44 1.98432 1.66 0.18713 1.17 1 0.0013 1106 13 1110 18 1119 13 1108 ±11 G-VII-449 0.78 2.86412 2.09 0.23598 1.38 1 0.0005 1366 19 1373 29 1383 22 1375 ±12 G-VII-446 0.44 2.90877 2.44 0.24167 1.95 −2 0.0003 1395 27 1384 34 1367 20 1375 ±12 G-VII-439 0.67 1.96370 1.52 0.18602 1.12 1 0.0000 1100 12 1103 17 1110 11 1090 ±7.7 G-VII-437 0.86 2.09490 2.97 0.19529 1.92 −1 0.0008 1150 22 1147 34 1142 26 1149 ±18 G-VII-436a 0.26 2.02791 2.74 0.19107 1.84 −1 0.0013 1127 21 1125 31 1121 23 1126 ±17 G-VII-436b 0.50 2.03482 1.95 0.19228 0.89 −2 0.0009 1134 10 1127 22 1115 19 1132 ±8.9 G-VII-435 0.90 2.57362 2.77 0.21294 1.71 9 0.0009 1244 21 1293 36 1375 30 1356±34 G-VII-434 0.68 1.78238 4.06 0.17078 2.72 6 0.0015 1016 28 1039 42 1087 33 1087±40 G-VII-432 0.47 3.04117 1.70 0.25658 1.37 −10 0.0012 1472 20 1418 24 1337 13 1356±34 G-VII-431 0.61 1.87367 2.57 0.18004 1.73 1 0.0007 1067 18 1072 28 1081 20 1070 ±16 G-VII-429 0.51 2.79883 2.18 0.23342 1.31 1 0.0004 1352 18 1355 30 1360 24 1354 ±14 G-VII-428 0.29 1.82389 1.65 0.17585 1.16 3 0.0005 1044 12 1054 17 1075 13 1050 ±10 G-VII-426 0.89 1.98214 3.10 0.16918 0.84 23 0.0123 1008 8 1109 34 1315 39 1361±66 G-VII-424 0.45 2.90639 2.81 0.24215 1.47 −3 0.0008 1398 21 1384 39 1362 33 1393 ±17 G-VII-423 0.72 2.96160 2.52 0.24593 1.39 −4 0.0009 1417 20 1398 35 1368 29 1409 ±16 G-VII-417 0.40 1.87287 2.61 0.17763 1.46 5 0.0008 1054 15 1072 28 1107 24 1059 ±14 G-VII-415 0.44 2.73001 2.63 0.22594 1.76 4 0.0005 1313 23 1337 35 1374 27 1327 ±19 G-VII-414 0.43 1.95331 1.99 0.18756 1.11 −2 0.0004 1108 12 1100 22 1083 18 1105 ±11 G-VII-413 0.40 2.45500 2.61 0.21043 1.51 6 0.0009 1231 19 1259 33 1307 28 1304±45 G-VII-412 0.71 2.35359 1.64 0.20218 0.76 9 0.0001 1187 9 1229 20 1302 19 1304±45 G-VII-411 0.69 1.90578 1.96 0.17675 1.19 9 0.0026 1049 12 1083 21 1152 18 1167±35 G-VII-409 0.41 2.16132 2.70 0.20232 1.56 −5 0.0012 1188 19 1169 32 1134 25 1180 ±16 G-VII-404 0.45 1.83630 2.58 0.17594 1.40 4 0.0009 1045 15 1059 27 1087 24 1048 ±13 G-VII-408 0.96 2.07574 2.36 0.19212 1.55 2 0.0005 1133 18 1141 27 1156 21 1137 ±14 F-VI-402 0.59 4.08749 1.89 0.29270 1.35 0 0.0016 1655 22 1652 31 1648 22 1652 ±15 F-VI-401 1.46 1.87702 2.48 0.17151 1.99 14 0.0014 1020 20 1073 27 1181 17 1167±35 F-VI-400 0.87 2.00281 2.60 0.18453 2.17 6 0.0014 1092 24 1116 29 1165 17 1167±35 F-VI-397 0.51 1.90289 2.04 0.18074 1.67 3 0.0001 1071 18 1082 22 1105 13 1082 ±14 F-VI-396 0.49 1.98944 2.14 0.18645 1.27 3 0.0016 1102 14 1112 24 1131 20 1106 ±12 F-VI-395 0.46 1.92244 2.48 0.18328 1.24 1 0.0007 1085 13 1089 27 1097 24 1086 ±12 F-VI-393 0.55 1.94213 2.60 0.18404 0.93 2 0.0015 1089 10 1096 28 1109 27 1090 ±9.2 F-VI-391 0.32 1.95343 2.64 0.18551 1.19 1 0.0003 1097 13 1100 29 1105 26 1097 ±12 F-VI-390 0.56 2.37360 3.01 0.21921 2.51 −10 0.0009 1278 32 1235 37 1160 19 1126±32 F-VI-389 0.42 1.82514 1.93 0.17191 1.17 9 0.0005 1023 12 1055 20 1121 17 1126±32 F-VI-387 0.66 1.83003 1.75 0.17121 1.25 10 0.0029 1019 13 1056 18 1135 14 1126±32 F-VI-385 0.55 2.00671 2.54 0.19225 1.73 −4 0.0012 1134 20 1118 28 1087 20 1124 ±16 F-VI-384 0.49 1.99576 2.23 0.18650 1.30 3 0.0015 1102 14 1114 25 1137 21 1106 ±12 F-VI-382 0.51 0.87311 2.50 0.10328 1.45 3 0.0003 634 9 637 16 650 13 634±8.6 F-VI-379 0.44 2.02631 1.53 0.19214 1.11 −2 0.0003 1133 13 1124 17 1108 12 1127 ±10 F-VI-376 0.46 1.93860 2.50 0.18484 1.41 0 0.0005 1093 15 1095 27 1097 23 1094 ±13 F-VI-375 0.67 2.94807 1.62 0.23155 1.38 9 0.0004 1343 19 1394 23 1474 13 1474±32 F-VI-372 0.97 1.87010 3.22 0.18052 1.25 0 0.0014 1070 13 1071 34 1072 32 1070 ±12 F-VI-368 0.36 1.94540 1.82 0.18206 1.17 5 0.0011 1078 13 1097 20 1134 16 1086 ±11 F-VI-363 0.50 2.73140 2.34 0.23087 1.37 0 0.0006 1339 18 1337 31 1334 25 1338 ±15 F-VI-359 0.02 0.82952 2.59 0.10018 1.73 −2 0.0006 615 11 613 16 605 12 615±9.9 F-VI-358 0.52 1.95880 2.52 0.18562 1.77 1 0.0008 1098 19 1101 28 1109 20 1100 ±16 F-VI-356 0.39 1.93119 3.42 0.18232 1.89 3 0.0010 1080 20 1092 37 1117 32 1083 ±18 F-VI-355 0.29 1.87360 2.94 0.18019 2.05 1 0.0007 1068 22 1072 32 1080 23 1070 ±18 account for the detrital zircon ages and the Sm–Nd signature (Patchett Trehué Formation (Fig. 7; Cingolani et al., 2005). Unfortunately, the Pb and Ruiz, 1989) of the clastic rocks here studied, the Pb isotopes do composition of the Cerro La Ventana Formation remains unknown but not show a Laurentian signature (Fig. 8; Kay et al., 1996). samples are in progress.

Since the basement known as the Cerro La Ventana Formation was The relative wider range of detrital zircon ages and εNd values of already uplifted by the Darriwilian (Heredia, 2006) such a provenance the Pavón Formation might indicate that although the Cerro La is very likely, particularly when analyzing the Ponón Trehué Ventana Formation could have been an important source, another Formation. The Cerro La Ventana Formation consists of maﬁcto source was also providing detritus to the basin. Source areas towards intermediate gneisses grading to amphibolites and migmatites, as the east are likely (Manassero et al., 1999; Cingolani et al., 2003), but well as acidic to intermediate granitoids (Cingolani et al., 2005). Sm– no outcrops are known. Instead, similar Mesoproterozoic basement Nd, Rb–Sr and U–Pb on zircons indicate Mesoproterozoic ages (1.1 to rocks are recorded in the northern outcrops of the Western 1.2 Ga) and geochemical and isotopic characteristics of mantle- Pampeanas Ranges (e.g. Pie de Palo, Varela and Dalla Salda, 1992; derived rocks (Cingolani and Varela, 1999; Thomas et al., 2001; McDonough et al., 1993; Varela et al., 1996; Casquet et al., 2006). Cingolani et al., 2005). Although only some of all the basement rock Zircon age dating from the clastic record agrees with the ages found types had been studied until today (diorites and tonalites), they within the Western Pampeanas Ranges. Sm–Nd data available from provide the best ﬁt regarding sedimentology (Heredia, 2006), Nd the Western Pampeanas Ranges are scarce, but particularly notewor- signature and zircon ages. Nd data show εNd values in the range of thy are the positive εNd values found within the Umango Range (Fig. 7; variation of data calculated to the time of deposition of the Ponón Porcher et al., 2004, Vujovich et al., 2005). The TDM ages of the Pavón Author's personal copy

P. Abre et al. / Gondwana Research 19 (2011) 275–290 287

Fig. 9. U–Pb distribution of analyzed detrital zircons with probability curves and concordia plot diagrams for the a) Ponón Trehué (n=38) and b) Pavón Formations (n=51). Histogram bars represent time intervals of 40 Ma. Isoplot/Ex (Ludwig, 2001) was used for age calculations. Representative CL microphotographs of selected zircon grains used for detrital dating show the predominance of magmatic internal textures. Bar length is 100 µm.

Formation are comparable to TDM ages for Mesoproterozoic xenoliths part of the Cuyania basement, other areas (Maz and Espinal ranges) are interpreted as the basement of the Cuyania terrane (Kay et al., 1996). interpreted as representing the active Gondwana margin during the The Western Pampeanas Ranges and the Arequipa–Antofalla Lower Palaeozoic (Porcher et al., 2004). Moreover, the Pie de Palo Basement might constitute a single crustal block para-autochthonous Range is considered autochthonous to Gondwana prior to the Lower with respect to the pre-Famatinian margin of Gondwana (Casquet Palaeozoic (Casquet et al., 2006). In this scenario, the depositional et al., 2006), then a minor contribution from the Central and Southern basins resulted from the extensional regime (Astini, 2002; Cingolani Domains can also be invoked to explain the Sm–Nd signature of the et al., 2003) that followed the accretion during the Middle Ordovician of Pavón Formation (Fig. 7). However, a northern provenance is not the Cuyania terrane to Gondwana as depicted on Fig. 10. supported by palaeocurrents, and immaturity of the sedimentary A para-autochthonous or allochthonous origin for the Cuyania rocks including the subhedral character of the zircons tends to dismiss terrane cannot be resolved, until more data from the probable sources long transports. are added. The isotopic signatures from source rocks proposed in the para-autochthonous to Gondwana models are not imprinted within 8.2. Tectonic implications the sedimentary successions here studied. Because a Laurentian derivation for the Neoproterozoic to Cambrian successions of the The evidence here presented undoubtedly link the Mesoproterozoic Cuyania terrane was deduced (Astini et al., 1995; Naipauer, 2007; Cerro La Ventana Formation as a provenance component to the Naipauer et al., 2010), it could be expected that the same rocks could Ordovician sedimentary deposition within the San Rafael block. have provided detritus to the Ordovician record. Typical Laurentian However, extensive petrographical, geochemical and isotopic studies signatures however are not shown by the provenance indicators of all the rock types comprised in the Cerro La Ventana Formation are applied to the Ordovician successions of the San Rafael block, needed to further support this. Considering the current information particularly concerning Pb–Pb composition (Figs. 7 and 8). available, another eastern source such as the Western Pampeanas Considering that the Cerro La Ventana Formation and the Western Ranges has to be invoked to understand the provenance of the Pavón Pampeanas Ranges are the most probable source areas (Fig. 10), they Formation. These differences might indicate that even though the Cerro would have been uplifted since at least the Darriwilian in order to La Ventana Formation was a source since the Darriwilian providing provide detritus to the Ponón Trehué and Pavón Formations. If the detritus to a restricted extensional basin, towards younger ages Western Pampeanas Ranges were para-autochthonous to Gondwana (Sandbian) other sources such as the Western Pampeanas Ranges (Casquetetal.,2006), then the Cuyania terrane might have collided at were available to provide detritus to the Pavón Formation foreland least immediately before the beginning of the Ordovician clastic basin (Fig. 10). Although certain parts of the Western Pampeanas deposition (Darriwilian; Fig. 10). In such geotectonic scenario, the Ranges (Umango Range) have a Laurentian signature and are therefore Western Pampeanas Ranges not only provided detritus to the basin Author's personal copy

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Fig. 10. Interpretative schematic cross sections showing the geotectonic evolution of the Cuyania terrane. a) Sedimentary record of Cambrian–Early Ordovician age is not recorded within the San Rafael block; the Cuyania terrane was approaching Gondwana as a result of the closure of the Iapetus Ocean. The Famatinian arc was active and volcaniclastic sediments were deposited (Pankhurst et al., 1998). b) The Ponón Trehué Formation received an input restricted to the Cerro La Ventana Formation; it is the only sedimentary unit from the Cuyania terrane in contact with the basement. c) Progressive subsidence of the basin; the uplift of the Western Pampeanas Ranges provided other eastern source for the Pavón Formation (besides the Cerro La Ventana Fm.) and prevented input of detrital material from the Famatinian arc. The basins were generated as a response of the extension that followed the accretion of the Cuyania terrane to Gondwana (Astini, 2002; Cingolani et al., 2003; Heredia, 2006). The Ponón Trehué and the Pavón Formations are not in contact.

(Pavón Formation), but acting as a positive area prevented that detrital Mesoproterozoic Cerro La Ventana Formation, with which it is in material from the Famatinian arc reached the basin. contact. 2) The Pavón Formation shows similar but wider ranges of variation 9. Conclusions of all the provenance indicators, implying that it received more regional sediment dispersal. Particularly, Sm–Nd isotopes and 1) Petrography, geochemistry and isotope geochemistry indicate that detrital zircon dating indicate external to the San Rafael block the Ponón Trehué Formation had a dominantly local, basement- sources (besides the Cerro La Ventana Formation), most probably derived provenance that is consistent with all that is known of the from the nearby Western Pampeanas Ranges. Author's personal copy

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3) The lack of Famatinian detrital zircons observed for both units Bordonaro, O., Keller, M., Lehnert, O., 1996. El Ordovícico de Ponón Trehué en la provincia de Mendoza (Argentina): redefiniciones estratigráficas. XIII Congreso is explained by the presence of a positive area that not only Geológico Argentino y III Congreso de Exploración de Hidrocarburos, Actas I, pp. prevented the arrival of arc-derived detritus but has also acted as 541–550. source (Western Pampeanas Ranges). Buggisch, W., von Gosen, W., Henjes-Kunst, F., Krumm, S., 1993. The age of Early Paleozoic deformation and metamorphism in the Argentine Precordillera — evidence from K– 4) The Cuyania terrane might have accreted to Gondwana before the Ar data. Zentralblatt Geologie und Palaontologie T I, H1/2, 275–286. deposition of the units here studied (Darriwilian), in order to Carrigan, C.W., Miller, C.F., Fullagar, P.D., Bream, B.R., Hatcher Jr., R.D., Coath, C.D., 2003. receive the input from the Western Pampeanas Ranges. Ion microprobe age and geochemistry of southern appalachian basement, with implications for Proterozoic and Paleozoic reconstructions. Precambrian Research 120, 1–36. Casquet, C., Pankhurst, R.J., Fanning, C.M., Baldo, E., Galindo, C., Rapela, C.W., González- Acknowledgements Casado, J.M., Dahlquist, J.A., 2006. U–Pb SHRIMP zircon dating of Grenvillian metamorphism in Western Sierras Pampeanas (Argentina): correlation with the Arequipa–Antofalla craton and constraints on the extent of the Precordillera P. Abre thanks the Faculty of Sciences (University of Johannesburg) terrane. Gondwana Research 9, 524–529. for the financial support and G. Blanco for extensive discussions. Cingolani, C., Varela, R., 1999. The San Rafael block, Mendoza (Argentina). Rb–Sr Fieldwork was financed by CONICET and ANPCYT (PICT 07-10829, isotopic age of basement rocks. II South American Symposium on Isotope Geology, – – – Anales, pp. 23 26. Argentina). Sm Nd and Pb Pb data were possible thanks to the Post- Cingolani, C.A., Cuerda, A.J., Aceñolaza, F.G., 1992. El Paleozoico Inferior sedimentario de graduate students grant system of the International Association of Argentina y Chile. In: Gutiérrez Marco, J.G., Saavedra, J., Rábano, I. (Eds.), Paleozoico Sedimentologists and the staff of the LGI-UFRGS (Brazil), particularly Inferior de Ibero-América. Universidad de Extremadura, pp. 255–277. fi Cingolani, C., Manassero, M., Abre, P., 2003. Composition, provenance and tectonic Prof. Kawashita, K. and Prof. Dussin, I. Zircon dating was nanced by setting of Ordovician siliciclastic rocks in the San Rafael Block: Southern extension the National Research Foundation (NRF), South Africa. 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