The Sierra de Pie de Palo (SPP) is one of the Western Sierras Pampeanas (WSP) of Argentina which resulted from Cenozoic uplift in response to shortening of the Andean foreland ( Jordan and Allmendiger, 1986 ; Isacks, 1988 ). The WSP mainly consists of middle to late Mesoproterozoic basement and a late Neoproterozoic (Ediacaran) sedimentary cover ( McDonough et al., 1993 ; Casquet et al., 2001 ; Galindo et al., 2004 ; Rapela et al., 2005 ). Basement and cover were overprinted by metamorphism and penetrative deformation during the widespread early to middle Ordovician accretionary Famatinian orogeny ( Casquet et al., 2001 ; Mulcahy et al., 2011 ; van Staal et al., 2011 ). The metasedimentary succession was recognized and named by Baldo et al. (1998) as the Difunta Correa metasedimentary sequence (DCMS) and represents a poorly known Neoproterozoic basin. Likely, the DCMS also crops out in other ranges of the WSP, namely Maz, Espinal and Umango ( Figure 1a ) where a mesoproterozoic basement has also been recognized ( Casquet et al., 2008 ; Varela et al., 2011 ). However, the original relationships between the basement and the metasedimentary cover are always oscured by shearing and faulting.

The DCMS constitutes a key entity to the paleogeography of the WSP during the late Neoproterozoic, before its accretion to southwestern Gondwana in the early Cambrian ( Casquet et al., 2012 ; Rapela et al., 2015 ). These authors suggest that the WSP Mesoproterozoic basement developed after the MARA Paleoproterozoic craton (MARA, acronym of Maz, Arequipa, Río Apa). This craton was accreted to Laurentia -and consequently reworked- in the middle to late Mesoproterozoic orogenies between ca. 1.3 and 1.0 Ga. MARA drifted away from Laurentia in the late Neoproterozoic, resulting in the opening of the Iapetus Ocean. This basement was further involved in the early Cambrian Pampean orogeny of the Eastern Sierras Pampeanas by collision with other southwestern Gondwana cratons (such as Kalahari). This collision resulted in the closure of the hypothetical Clymene Ocean ( Trindade et al., 2006 ), and its southeastern extension (present position), i.e., Puncoviscana Ocean.

Conventional petrological provenance studies of detrital rocks (sandstones; Dickinson and Suczek, 1979 ; Zuffa, 1985 ; Jonhsson, 1 993 ; Arribas et al., 2007 ) are hindered if the rocks underwent significant metamorphism and deformation. Primary components of the sedimentary rocks are deeply modified by processes such as recrystallization, formation of new minerals and development of a

Figure 1. Figure 1. a) Geological setting of the Sierras Pampeanas showing the location of the Sierra de Pie de Palo (SPP); b) Geological map of the SPP; c) Schematic eastwest cross-section of the SPP showing the main geological units and structural boundaries. DCMS: Difunta Correa metasedimentary sequence.

new tectonic fabric (foliation). In such cases, methods based on the chemical composition of the metasedimentary rocks help to define the source areas and to establish the tectonic setting of sedimentary basins ( Bhatia and Crook, 1986 ; McLennan et al., 1990 , 2003 ; Zimmermann et al., 2011 ). However, chemical data alone can lead to erroneous interpretation of the tectonic setting, especially in ancient basins ( Armstrong-Altrin and Verma, 2005 ; Verdecchia and Baldo, 2010 ). Therefore, they are supported here by complementary evidence from regional geological knowledge and detrital zircon data ( Rapela et al., 2015 ).

Some major and trace elements (e.g., Si, Al, Fe, Mg, Mn, Ca, K, Ba, Sr, Rb and Cs) can be removed from sediments during weathering, diagenesis and low- to medium- grade metamorphism, and therefore they are not appropriate for constraining source areas and/or tectonic settings. On the other hand, elements such as Ti, La, Ce, Nd, Y, Th, Zr, Hf, Nb, and Sc within resistant detrital minerals preserve the initial inter-elemental ratios, and, in consequence, they are useful for provenance analyses ( Bhatia and Crook, 1986 ; Floyd and Leveridge, 1987 ; McLennan, 1989 ; McLennan et al., 1990 ; Zimmermann and Bahlburg, 2003 ).

The aim of this research is to infer source areas and the tectonic setting of the DCMS through regional, chemical, and isotope evidences of significant rock types, and to discuss the paleogeographic implications. Moreover, we present here the first mention of phosphatic clasts in the Sierras Pampeanas and evaluate their paleoclimatic and paleogeographic significance.

The SPP consists of five lithological assemblages bounded by east-dipping shear zones and thrusts. From west to the east they are: the Caucete Group, the Pie de Palo Complex, the Central Complex, the DCMS, and the Nikizanga Group ( Figure 1 ). All these units are overprinted by metamorphism and intruded by igneous rocks, both assigned to the Ordovician Famatinian orogeny ( Pankhurst and Rapela, 1998 ; Casquet et al., 2001 ; Mulcahy et al., 2011 ; Baldo et al., 2012 ).

The Caucete Group ( Borrello, 1969 ; Vujovich, 2003 ; Galindo et al., 2004 ; Naipauer et al., 2010 ; van Staal et al., 2011 ) is exposed along the western margin of the SPP, in the footwall of the major Pirquitas thrust ( Figure 1b ), and mainly consists of low- to medium-grade metamorphic rocks such as metasandstones and marbles, and to a lesser extent, metavolcaniclastic rocks. A late Neoproterozoic–early Cambrian depositional age was established for this group based on Sr-isotope composition of marbles ( Galindo et al., 2004 ) and U-Pb zircon ages from metasandstones ( Naipauer et al., 2010 ).

The Pie de Palo Complex (sensu Mulcahy et al., 2011 ) lies between the Pirquitas and the Duraznos thrusts ( Figure 1b and 1c ) and is mainly composed of ultramafic and mafic rocks, largely metagabbros and garnet-amphibolites. Mesoproterozoic (ca. 1,067–1,204 Ma) U-Pb zircon crystallization ages were obtained for this complex ( Vujovich et al., 2004 ; Morata et al., 2010 ; Rapela et al., 2010 ; Mulcahy et al., 2011 ). The Pie de Palo Complex is an igneous sequence formed in a back-arc setting ( Vujovich and Kay, 1998 ).

The Caucete Group and the Pie de Palo Complex have been correlated with the cover and the basement, respectively, of the worldwide known Precordillera terrane. This correlation was based on Sr-isotope composition of marbles ( Galindo et al., 2004 ) and detrital zircon ages ( Naipauer et al., 2010 ) of Caucete Group, and on common Pb composition and geochronology of the ultramafic and mafic rocks of the Pie de Palo Complex ( Abbruzzi et al., 1993 ; Kay et al., 1996 ). Many authors consider that the Precordillera is an exotic terrane of Laurentian affinity that accreted to the proto-Andean margin during the Famatinian orogeny ( Thomas and Astini, 2003 ; Ramos, 2004 ).

The Central Complex is defined here as a large block between the Duraznos and the Morales thrusts. The latter ductile thrust was recognized in the Morales creek ( Figure 1b ) and we were able to track it northward using satellite images. The boundary between the Central Complex and the DCMS is poorly known due to access difficulties. The Central Complex consists of schists, quartzites, marbles, migmatites, amphibolites, metavolcanic rocks and orthogneisses ( Casquet et al., 2001 ; Mulcahy et al., 2011 ). One orthogneis is an A-type granitoid of ca. 774 Ma attributed to the break-up of Rodinia ( Baldo et al., 2006 ). Other orthogneisses range from ca. 1,280 to ca. 1,027 Ma ( McDonough et al., 1993 ; Rapela et al., 2010 ; Garber et al., 2014 ), and therefore, the depositional age of the metasedimentary rocks of this complex has to be older than those of the igneous intrusions. In consequence, the Central Complex probably constitutes a mesoproterozoic basement.

The DCMS was described by Baldo et al. (1998) as a sequence made up of metapelites, Ca-pelitic schists, quartzites, quartz-feldspar metasandstones, marbles and para-amphibolites. The DCMS is widespread along the central and eastern parts of the SPP. A late Neoproterozoic depositional age for the DCMS was proposed by Galindo et al. (2004) on the basis that the Sr-isotope composition of the calcite marbles corresponds to sea-water composition at that time. The age of, at least, the lower and the middle part of the DCMS probably is ca. 600 Ma according to the more recent sea-water compilations and U-Pb detrital zircon ages ( Rapela et al., 2005 , 2015 ; Murra et al., 2014 ). We recognize four units in the DCMS; from bottom to top: La Loma, Flores, Vallecito, and La Bomba ( Figure 2 ).

Figure 2. Figure 2. Main units of the Sierra de Pie de Palo (SPP) and structural relationships. Thicknesses are not to scale.

The Nikizanga Group crops out in the southeastern portion of the SPP and consists of low- to medium- grade metamorphic rocks (quartzites, marbles, graphitic schists and, to a lesser extent, amphibolites; Figure 1b ). An early Cambrian depositional age was obtained for this group based on Sr-isotope composition of marbles (Filo del Grafito marbles; Galindo et al., 2004 ), which was latter supported by U-Pb detrital zircon ages from a quartzite ( Ramacciotti et al., 2014 ).

The DCMS was affected by numerous shear zones, thrusts and brittle Andean faults that complicate the overall picture ( Figure 3 ). Sedimentary structures and textures of the DCMS were largely obliterated and replaced by metamorphic textures as a consequence of strong deformation, recrystallization, and formation of new minerals during the Ordovician Famatinian metamorphism ( Casquet et al., 2001 ). Only the metaconglomerates preserve some primary structures (e.g., graded-bedding and cross-bedding) and relict minerals in weakly deformed areas. Thus, the classification of protoliths was mainly based on geochemical data and not on mineral modes. The metasedimentary units are described below, from bottom to top ( Figure 4 ):

La Loma unit consists almost exclusively of black quartz schists and widely outcrops in southeastern SPP ( Figure 3 ). Schists are massive or banded and show a NE-SW striking, east-dipping, penetrative tectonic foliation ( Figure 5a ). They were affected by numerous shear zones, and often show chevron folds ( Figure 5b ). The mineral assemblage mainly consists of quartz, with minor amounts of biotite, muscovite, plagioclase and zircon, with a fine-grained (< 1 mm) granoblastic texture ( Figure 5c ).

The Flores unit consists mainly of metaconglomerates, and less abundant Ca-pelitic schists. The former are matrix-supported with monocrystalline clasts of quartz and feldspar (2–15 mm), and locally, large (5–20 cm) phosphatic clasts which are described below. The metaconglomerates are lens-shaped and often grade in short distances into arenites ( Figure 5d ). The metamorphic mineral assemblage of the matrix is Qtz-Pl-Bt-Ms±Grt-Ep-Amph-Zrn-Ap-Op (abreviations after Kretz, 1983 ; Figure 5e and 5f ). Phosphatic clasts within the metaconglomerates are between 2 and 20 cm long and were flattened parallel to the main foliation. They consist of Ap-Qtz-Bt-Ilm (identified by electron microprobe) with microgranular texture ( Figure 6a and 6b ). Internally, the clasts show an alternation of phosphate-rich rock and thin, curved, quartz-rich veinlets (white arrows in Figure 6b ). Sometimes an outer zone is found between the phosphatic clasts and the host conglomerate, containing abundant disseminated apatite, probably due to the exchange reaction during metamorphism (zone I in Figure 6a ; see Discussion section). The mineral mode of phosphate rock was estimated using the software imageJ ( Abràmoff et al., 2004 ). Sample SPP-22007D1 yielded: 45.5 % Ap, 41.6 % Qtz, 10.2 % Bt, 2.7 % Ilm ( Figure 7 ). Ca-pelitic schists consist of Ms-Hbl-Grt-Bt-Ep-

Figure 3. Figure 3. Geological map of the southern SPP showing the four units of the Difunta Correa metasedimentary sequence (DCMS).

Chl-Pl±Zrn-Op. They are characterized by the presence of Ca-minerals (amphibole and/or epidote) in association with typical metapelite minerals (e.g., muscovite, biotite, garnet). Distinctive textural features consist of large, needle-shaped porphyroblasts of hornblende lying in the foliation plane with a radial arrangement ( Figure 8a and 8b ), and of garnet porphyroblasts discordant to the foliation ( Figure 8b ). Both textures suggest that minerals grew up late relative to foliation development. The textures and mineral composition of the DCMS Ca-pelitic schists are remarkably similar to garben-schists or garbenschiefer in the eastern Alps ( Selverstone et al., 1984 ).

The Vallecito unit, with a maximum apparent thickness of 3000 m, is mainly composed of calcite-rich marbles interbedded with calcsilicate rocks and quartzites. An abundant type of calc-silicate rock is para-amphibolite, i.e., amphibolite formed after a sedimentary protolith ( Rapela et al., 2005 ). Marbles are often intruded by Ordovician granitoids ( Baldo et al., 2012 ) ( Figure 8c ) and exhibit a granoblastic texture with the minerals Cal±Dol-Ms-Bt-Chl-Pl-Qtz-Ttn-Zrn-Ap ( Figure 8d ).

La Bomba unit is composed of a succession of metapelites and metasandstones of variable composition ( Figure 9a ). The main rock types are: staurolite-garnet schists, garnet schists, hornblende-garnet metasandstones and calcite-rich metasandstones. The staurolite-garnet schists have a mineral assemblage of St-Grt-Bt-Ms-Pl-Qtz±Ap-Zrn and are characterized by the abundance of staurolite and garnet porphyroblasts set in a foliated mica-rich matrix ( Figure 9b and c ). The garnet schists consist of Ms-Grt-Bt-Qtz-Pl also with garnet as porphyroblasts and a lepidoblastic to granoblastic matrix ( Figure 9d ). The hornblende-garnet metasandstones bear many similarities to Capelitic schists (garben-schists) of the Flores unit, whit the exception that they are more psammitic ( Figure 10a-10c ; compare with Figure 8a ). The calcite-rich metasandstones are mainly composed of quartz and calcite, whit minor amounts of muscovite, and show a granoblastic texture ( Figure 10d ).

Representative samples from the three meta-detrital units that form the DCMS were chosen for chemical and isotope analyses. Details of each sample are given in Table 1 .

Chemical analyses were carried out on eleven samples ( Table 2 ). Six samples were analyzed at Activation Laboratories Ltd. (Actlabs, Ontario, Canada) following the 4 Litho-research routine. Major elements were determinate by ICP-AES, whereas minor and trace elements were determinate by ICP-MS. The remaining five samples were analyzed by X-ray fluorescence (XRF) at the Laboratorio of the Instituto de Geología y Minería, Universidad Nacional de Jujuy, Argentina on a Rigaku FX2000 spectrometer with Rh tube ( Table 2 ). Ground and homogenized samples were fused with lithium tetraborate for major element analyses. Trace element determinations were performed on rock powder pellets mixed with methyl methacrylate, and pressed at 20 t. Operating conditions were 50 kV and 45 mA.

Figure 4. Figure 4. Schematic lithostratigraphic column of the Difunta Correa metasedimentary sequence. The youngest zircon age is from Rapela et al. (2005, 2015).

Figure 5. Figure 5. Field and petrographic characteristics of La Loma unit (a-c) and the Flores unit (d-f). a) Evenly foliated, black, quartz schists; b) Banded and folded (chevron folds) quartz schists; c) Fine-grained granoblastic texture mainly composed of quartz and, to a lesser extent, of oriented micas which define the foliation of the rock; d) Normal graded bedding in metaconglomerates; e) and f) PPL and crossed polars photomicrograph of metaconglomerates showing relict minerals (clasts) and recrystallized matrix.

Figure 6. Figure 6. Phosphatic clasts within metaconglomerates of the Flores unit. a) Thin section of one phosphatic clast showing an outer zone of apatite-enriched host conglomerate (zone I); b) Crossed polars photomicrograph of a phosphatic clast with a truncated alternation of phosphate-rich rocks and thin veinlets of quartz (white arrow) and a cross-cutting quartz vein.

Rb-Sr and Sm-Nd isotope analyses were carried out on four detrital rocks. Additionally, Rb-Sr isotope data was collected from two phosphatic clasts within the metaconglomerates of the Flores unit ( Table 3 ). Analytical work was carried out at the Centro de Geocronología y Geoquímica de Isótopos of the Universidad Complutense de Madrid on a PHOENIX© automated multicollector mass-spectrometer. Whole rocks (siliciclastic rocks) were first decomposed in 4 ml HF Mercksuprapure and 2 ml HNO3 Merck-suprapure in Teflon digestion bombs for 48 hours at 120 ºC, and finally in 6M HCl. Concentrations of Rb and Sr, as well as Rb/Sr atomic ratios, were determined by X-ray fluorescence spectrometry at the X-Ray Centro de Difracción de la Universidad Complutense following the methods of Pankhurst and O`Nions (1973) . Sr and REE were separated using Bio-Rad AG50 x 12 cation exchange resin. The phosphatic clasts were decomposed in 6M HCl in Nalgene beakers and, after evaporation, dissolved with 3M HNO3; Sr was separated using an extraction chromatographic SrResinTM. Errors are quoted throughout as two standard deviations (2σ) from measured or calculated values. The decay constant used in the calculations is λ87Rb = 1.42×10-11 year-1 recommended by the IUGS Subcommision for Geochronology ( Steiger and Jäger, 1977 ). Bulk Earth Sr-isotope composition was calculated from Allègre et al. (1983) and Taylor and McLennan (1985) . Analytical uncertainties are estimated to be 0.01 % for 87Sr/86Sr and 1 % for 87Rb/86Sr ratios. Epsilon-Sr (εSr) values were calculated relative to a Bulk Earth present-day 87Sr/86Sr value of 0.7045 and 87Rb/86Sr of 0.0827 ( DePaolo, 1988 ). Replicate analyses of the NBS-987 Sr-isotope standard yielded an average 87Sr/86Sr ratio of 0.710245 ± 0.00003 (n = 14), (accepted value: 0.71025 ± 0.00005; Faure, 2001 )

Samarium and neodymium were determined by isotope dilution using spikes enriched in 149Sm and 150Nd and were separated from the REE group using Bio-beads coated with 10 % HDEHP. Errors are quoted throughout as two standard deviations (2 σ) from measured or calculated values. The decay constants used in the calculations are the values λ147Sm = 6.54×10-12 year-1 recommended by the IUGS Subcommision for Geochronology ( Steiger and Jäger, 1977 ). Analytical uncertainties are estimated to be 0.006 % for 143Nd/144Nd ratios and 0.1 % for 147Sm/144Nd ratios. Epsilon-Nd (εNd) values were calculated relative to a present-day chondrite 143Nd/144Nd value of 0.512638 and

Figure 7. Figure 7. a) Secondary electron (SEM) image showing the slightly orientated microgranular texture of a phosphatic clast core (SPP-22007D1). The black areas are holes which were not considered for modal analysis, and are merged with quartz in b); b) Image a) processed with ImageJ software.

Figure 8. Figure 8. a) and b) Ca-pelitic schists (garben-schists) from the Flores unit. c) and d) Marbles from the Vallecito unit. c) Deformed marble with folded felsic intrusive, d) Crossed polars photomicrograph of marbles showing a granoblastic texture.

Figure 9. Figure 9. La Bomba unit. a) Succession of metapelites and metasandstones; b) Staurolite-garnet schist; c) PPL photomicrograph of a staurolite-garnet schist; d) Crossed polars photomicrograph of a garnet schist.

Figure 10 Figure 10 La Bomba unit. a) Hornblende-garnet metasandstone with non-oriented hornblende prisms, locally displaying a radial arrangement; b) and c) PPL and crossed polars photomicrograph of a hornblende-garnet metasandstone; d) Crossed polars photomicrograph of a calcite-rich metasandstone.

147Sm/144Nd of 0.1967 ( Jacobsen and Wasserburg, 1980 ; Goldstein et al., 1984 ). Eight analyses of La Jolla Nd-standard measured during sample analysis gave a mean 143Nd/144Nd ratio of 0.511845 ± 0.00001 (accepted value: 0.511858 ± 0.00007; Lugmair and Carlson, 1978 ). Single stage model ages (TDM) was calculated according to DePaolo (1988) .

The composition of phosphate minerals was determined in one phosphatic clast (SPP-22007D1; Table 4 ) using a JEOL JXA8230 Superprobe at the Universidad Nacional de Córdoba, Argentina. Carbon-coated polished sections were analyzed at 15 kV using 20 nA beam and a range of natural and synthetic standards.

Detrital rocks geochemistry

Geochemical data from the DCMS siliciclastic rocks are shown in Table 2 . According to Herron´s (1988) plot ( Figure 11 ) the samples are classified as pelites (shales and Fe-Shale) and sandstones (wackes, sub-litharenites, litharenites and Fe-sandstones). The Ca-pelitic schist from the Flores unit shows the lowest concentration of SiO2 (40.3–46.4 %) and the highest contents of Al2O3 (26.2–28.9 %) and K2O (6.6–7.2 %). Metapelites from La Bomba unit (staurolite-garnet

Table 1 Table 1 Location and description of the analyzed samples. Mineral abbreviations after Kretz (1983).

Table 2. Table 2. Chemical analyses. *Samples analyzed at Actlabs. The remaining samples were analyzed at the Laboratory of the Instituto de Geología y Minería, Universidad Nacional de Jujuy, Argentina. Total Fe content is expressed as Fe2O3.

Table 3. Table 3. Isotope composition of samples from the Difunta Correa metasedimentary sequence.

schists) show similar values compared to the Ca-pelitic schist, whereas quartz schists (La Loma U.), metaconglomerates (Flores U.) and metasandstones (La Bomba U.) exhibit higher concentrations of SiO2 (63.8–75.6 %) and the lowest contents of Al2O3 and K2O (4.2–14.6 % and 0.7–3.4 % respectively; Table 2 ).

Geochemical variations among samples ( Figure 12 ) reflect changes in their mineralogical composition. Modal variations of phyllosilicates (pelitic component) relative to quartz-feldspar (psammitic component) are supported by: 1) negative correlations of SiO2 against TiO2 (R2 = 0.66; p < 0.01), Al2O3 (R2 = 0.98; p < 0.001), Fe2O3 (R2 = 0.71; p = < 0.01), MgO (R2 = 0.58; p < 0.05), and K2O (R2 = 0.77; p < 0.001; Figure 12a-12e) and 2) positive correlations of TiO2 against Fe2O3 (R2 = 0.62; p < 0.01), Al2O3 (R2 = 0.56; p < 0.05), and K2O (R2 = 0.65; p < 0.01), and of Al2O3 with K2O (R2 = 0.85; p < 0.001; Figure 13 ). No clear patterns between CaO, Na2O, MnO and P2O5 are recognized probably because they are mobile elements ( Figure 12f-12i ).

Relationships between major and trace elements are shown in Figure 14 . The effect of the pelitic component is reflected by a positive correlation between K2O against Ba (R2 = 0.90; p < 0.001) and Rb (R2 = 0.92; p < 0.001) ( Figure 14a and 14b ). The highest concentrations of Zr and SiO2 are found in metasandstones (La Bomba U.), quartz schist (La Loma U.) and metaconglomerates (Flores U.).

The REE concentration of metasedimentary rocks is strongly influenced by the grain size and mineral composition. In this sense, Cullers et al. (1987) found that the REE concentration is about 20 %

Table 4. Table 4. Mineral chemistry of phosphate minerals as weight% (sample SPP- 22007D1). Total Fe content as FeO.

higher in shales than in sandstones. Heavy minerals, such as zircon and garnet, contribute significantly to the increase in the heavy rare earth element (HREE) content ( Morton, 1991 ). The chondritenormalized ( Sun and McDonough, 1989 ) REE diagram of the DCMS samples is shown in Figure 15 . The model average composition of PCS (Proterozoic Cratonic Sandstones; Condie, 1993 ) and PAAS (Post- Archean Australian Shales; Taylor and McLennan, 1985 ) are included for comparison. The REE content in metapelites from La Bomba unit and Ca-pelitic schist from the Flores unit ranges between 170.7 and 279.9 ppm, slightly enriched with respect to PAAS. Metasandstones (La Bomba U.), metaconglomerates (Flores U.) and quartz schist (La Loma U.) show higher REE values (65.7–177.7 ppm) than PCS. All samples are characterized by a light rare earth elements (LREE) enrichment (LaN/EuN = 2.87–7.30), and a flat array (except the sample SPP-424) to the HREE (TbN/LuN = 0.57–1.17). In general, all samples of the DCMS exhibit a negative Eu anomaly (EuN/EuN* = 0.61–0.74), which is inherited from the sediment source.

Sr-Nd isotope composition of detrital rocks

The Sr-isotope composition of four siliciclastic samples is shown in Table 3 . Siliclastic rocks yielded 87Sr/86Sr ratios recalculated to the time of sedimentation (ca. 600 Ma) between 0.71150 and 0.71930. The corresponding εSr600 values range from 107 to 218, i.e., well above Bulk Earth.

The 143Nd/144Nd600 values of the four detrital rocks range from 0.51156 to 0.51176. The εNd600 values range from -6.04 to -2.14 and Nd depleted mantle model ages (TDM) are between 1.33 and 1.64 Ga.

Phosphatic clasts

Four grains of phosphate minerals were analyzed in one phosphatic clast from metaconglomerates of the Flores unit ( Table 4 ). The composition (weight %) is very homogenous with 1.97 to 2.47 wt % F and minor contents of FeO, MnO and Cl. They are classified as apatite-(CaF), i.e., fluor-apatite ( Pasero et al., 2010 ). The 87Sr/86Sr ratio measured in two phosphatic clasts yielded values of 0.72163 and 0.72290 ( Table 3 ).

Elemental and isotope geochemistry, as well as geological evidence suggest that the Ediacaran DCMS was probably deposited on a felsic to intermediate continental basement at a continental passive margin. The main source of the DCMS was probably the Mesoproterozoic basement that crops out in the sierras de Maz, Espinal, Umango and in the Central Complex of SPP. This basement underlaid the DCMS at the time of sedimentation. Furthermore, rocks with Nd isotope composition similar to the DCMS are also exposed in southeastern Laurentia (present position), in the Grenville and Granite-Rhyolite provinces ( Figures 17 and 18 ). These Laurentian sources have also been invoked –particularly the second- to explain the detrital zircon patterns with no likely sources in the WSP Mesoproterozoic basement ( Rapela et al., 2015 ).

The relationships between the DCMS and the mafic-ultramafic Pie de Palo Complex remain unknown because both are separated by the Duraznos thrust that underwent displacement during the Ordovician and the Silurian ( Mulcahy et al., 2011 ) ( Figure 2 ). Moreover, the juvenile Pie de Palo Complex - and the Caucete Group - probably was part of the Precordillera terrane, exotic or simply allocthonous, which reached a position close to the present in the Middle to late Ordovician ( Ramos et al. 1998 ; Thomas and Astini, 2003 ; Astini and Dávila, 2004 ).

Our data are compatible with the idea that the WSP basement was part of the MARA continental block ( Casquet et al., 2012 ). According to this hypothesis the southeastern passive margin of Laurentia in the Neoproterozoic was not the Appalachian margin but the eastern (present position) MARA margin, where the original sediments of the DCMS were laid down in the Puncoviscana/Clymene Ocean ( Figure 18 ).

Metasedimentary rocks from the DCMS were classified as shales, Fe-shale and immature sandstones (wacke, sub-litharenite, litharenite and Fe-sandstone) based on chemical parameters. Modal variations of phyllosilicates (mainly pelitic component) and quartz-feldspar (psammitic component) are supported by negative correlations of TiO2, Al2O3, Fe2O3, MgO and K2O with SiO2 and positive correlations of Fe2O3, Al2O3, and K2O against TiO2, and of Al2O3 with K2O. Almost all samples exhibit a negative Eu anomaly (EuN/EuN* = 0.61–0.74), which indicates a typical continental crust composition.

At least part of the DCMS is of marine origin and the detrital sediments came from a felsic to intermediate continental source and were deposited on the continental passive margin of the MARA block

Figure 18. Figure 18. Paleogeographical reconstruction of Laurentia, Amazonia, Río de la Plata (RP) and Kalahari (K) showing the guessed position of MARA (acronym of Maz, Arequipa, Rio Apa) craton at ca. 620 Ma. Ages of Laurentian Precambrian orogenic belts and cratons according to Goodge et al. (2004) and Tohver et al. (2004) (Laurentia in its present position). Outcrops of basement with Mesoproterozoic ages in MARA have been included: AM, Arequipa Massif (Peru); RA, Rio Apa (Brazil, Paraguay); WSP, Western Sierras Pampeanas (Argentina). Laurentia: TH, Trans-Hudson and related mobile belts; P, Penokean orogen; Y, Yavapai orogen; M, Mazatzal orogen; G-R, Granite-Rhyolite province. Arrows show the hypothetical direction of sediment transport from the probable source areas of the Difunta Correa metasedimentary sequence (DCMS).

in the Puncoviscana/Clymene Ocean. This source probably laid in the Mesoproterozoic basement of the WSP and in the Grenville and Granite-Rhyolite provinces of southeastern Laurentia. The phosphatic clasts from the Flores unit probably correspond to reworking of phosphorites formed after one of the worldwide Neoproterozoic glaciations. Because the age of the lower tract of the DCMS can be constrained to be younger than ca. 630 Ma we suggest that this glaciation was the Marinoan, although glaciogenic sediments have not been firmly recognized yet in this part of the Sierras Pampeanas.