Isotopic studies on detrital zircons of Silurian–Devonian siliciclastic sequences from Argentinean North Patagonia and Sierra de la Ventana regions: comparative provenance

Abstract

The Silurian–Devonian siliciclastic sedimentary units known as Sierra Grande Formation and the upper part of the Ventana Group crop out in the eastern area of the North Patagonian Massif and in the Ventania system, toward the Atlantic border of Argentina. Both sequences show similar stratigraphical characteristics and were deposited in a shallow marine platform paleoenvironment. Previous contributions have provided evidence of an allochthonous Patagonia terrane that amalgamate to Gondwana during the Permian–Triassic. However, other lines of research support a crustal continuity southward, where the Pampean and Famatinian events extend into the northern Patagonia. In either case, the detrital input to the Eo–Mesopaleozoic basins generated along the passive margin tectonic setting should reflect the sedimentary sources. In this contribution, new age data on the sedimentary provenance of these units is provided by U–Pb and Lu–Hf isotopic studies on detrital zircons, using LA-ICP-MS and SHRIMP methodologies. The main sedimentary sources of detrital zircons for both regions are of Cambrian–Ordovician and Neoproterozoic age, while a secondary mode is Mesoproterozoic. Zircons from older cratonic sources (Mesoarchean–Paleoproterozoic ages) are scarcely recorded. The sample from the upper section of the Devonian Lolén Formation (Ventana Group) shows an important change in the sedimentary provenance, with a main mode of Mesoproterozoic detrital zircons. Detrital source areas considering the orogenic cycles known for southwest South America (Famatinian, Pampean–Brasiliano, Mesoproterozoic–‘Grenvillian’ and Paleoproterozoic–‘Transamazonian’) are proposed.

Appendices

Appendix 1

Sierra Grande Formation

The unit is predominantly composed of mature quartz-rich sandstones interlayered with shales, subordinated conglomerates and the classic oolithic iron horizons. The maximum thickness of the unit is about 2,130 m and was subdivided in three members known, from base to top, as Polke, San Carlos and Herrada (Zanettini 1981, 1999). The most abundant lithological type is light-colored, quartz and mica-rich sandstone. However, iron-rich sandstones and wackes are also present. Cross-stratification and ripple-marks were recognized. The quartz-rich sandstone layers can easily be followed in the mountain landscape, being therefore useful as key-levels. Sandstone and interbedded shale colors vary between white, gray and brown, often associated with red oolithic iron-mineralized horizons. The presence of mica-rich sandstones and wackes become more conspicuous toward the western outcrops, near the “Yacimiento Sur” and Loma de los Fósiles (Fig. 3). Conglomerates and coarse facies sandstones occur in the base of the unit at the eastern Atlantic side of the region. The unit was developed in an open marine environment, with shallow deep shelf areas dominated by processes of waves and storms. Offshore sedimentation was governed by conditions of good weather and storms represented by the heterolithic facies (Huber-Grünberg 1990; Spalletti et al. 1991; Spalletti 1993). The environmental characteristics of shallow water and low rate of sedimentation, warmer paleoclimatic conditions and a general high sea level stage favoured the iron concentrations. The coastline would have developed with NW–SE direction (Spalletti 1993). After Rossello et al. (1997), the tectonic evolution can be considered as part of a large-scale intracontinental deformation in SW Gondwana inboard of an Andean-type compressive margin. This deformation is characterized by transpression combined with overthrusting to the NE and N–S horizontal contraction. Marine invertebrates (brachiopods, gastropods, trilobites, bivalves and conularids) and certain ichnofossils had been recorded mainly at the Loma de los Fósiles and “Yacimiento Este” areas (Müller 1965; Manceñido and Damborenea 1984; Spalletti et al. 1991). The faunistic record assigned a Middle-Upper Silurian to Lower Devonian age to the Sierra Grande Formation where Wenlockian biozones and endemic components related to the Malvinokaffric realm had been also recognized. Furthermore, the stratigraphical position of the Sierra Grande Formation is constrained by the Ordovician isotopic age of the granitoids from the Punta Sierra Plutonic Complex that intrude the Neoproterozoic–Cambrian basement (Varela et al. 2008).

Appendix 2

Upper Ventana Group

The Providencia Formation shows an increase in the amount of fine-grained rocks; although pink and reddish to whitish gray-colored quartz-rich sandstones is still the dominant lithotype. The layers are mainly massive but cross-stratification is also observable along the 300-m-thick sequence. The Silurian age (Rodríguez 1988) for this unit was stratigraphically constrained. The Lolén Formation with a 450- to 600-m-thick (Harrington 1972, 1980), which comprises a variety of more immature lithologies: quartz-rich sandstones, feldspar–quartz-rich and mica-rich sandstones and wackes (Andreis 1964b; Massabie and Rossello 1984), coarse sandstones and conglomerates to fine pebbles are also present in the unit. Fine-grained rock levels are irregularly distributed throughout the sequence and micaceous black shales are partly metamorphosed into slates. Brachiopod molds identified as Cryptonella sp. cf. baini, Schellwienella sp. and others representative of the Malvinokaffric realm were recorded 100 m above the bottom of the unit (Harrington 1972, 1980), allowing to assign these levels to the Lower Devonian (Andreis 1964a), similarly to other Gondwanan regions (Bokkeveld Group in the Cape Fold Belt or Fox Bay Formation in Malvinas). Cingolani et al. (2002) described the first record of fossil plants in upper reddish levels of the unit, recognizing Haplostigma sp. and Haskinsia cf. H. colophylla, suggesting the Middle Devonian age and a shallowing up (‘continentalization’) sedimentary process.

Appendix 3

Detrital zircon U–Pb and Lu–Hf methodologies

The detrital zircons were obtained after the classical processes of crushing and sieving of about 3–5 kg of each sample. The fractions retained in less than 140-micron mesh were separated using hydraulic processes to obtain heavy minerals pre-concentrates. These pre-concentrates were treated with bromoform (δ = 2.89) to obtain the complete heavy mineral spectra. Methylene iodide (δ = 3.32) was used to obtain a fraction enriched in zircons, followed by an electromagnetic separation with a Frantz Isodynamic equipment when necessary. The final selection of individual crystals was done by handpicking under a binocular microscope. For isotopic dating, all zircon grains were mounted in 2.5-cm-diameter circular epoxy mounts and polished down until the zircons were just revealed. Images of zircons were obtained using the optical microscope (Leica MZ 125) and backscatter electron microscope (JEOL JSM 5800). Zircon grains were dated with a laser ablation microprobe (New Wave UP213) coupled to a MC-ICP-MS (Neptune) at the Laboratorio de Geología Isotópica, Universidade Federal do Río Grande do Sul, Porto Alegre, Brazil. Isotope data were acquired using static mode with spot sizes of 25 and 15 μm. Laser spots for U–Th–Pb selection were guided by internal structures as seen in SEM images of the mounted and polished grains. Laser-induced elemental fractional and instrumental mass discrimination were corrected by the reference zircon GJ-1 (Simon et al. 2004), following the measurement of two GJ-1 analyses to every five-sample zircon spots. The external errors were calculated after propagation error of the GJ-1 mean and the individual sample zircon (or spot). The laser operating conditions were laser output power of 6 J/cm² and a shot repetition rate of 10 Hz. The cup configuration of the MC-ICP-MS Neptune was Faradays ²⁰⁶Pb, ²⁰⁸Pb, ²³²Th, ²³⁸U, MIC’s ²⁰²Hg, ²⁰⁴Hg + ²⁰⁴Pb, ²⁰⁷Pb. The gas input included a coolant flow (Ar) at 15 l/min, an auxiliary flow (Ar) at 0.8 l/min and a carrier flow of 0.75 l/min (Ar) + 0.45 l/min (He); the acquisition was at 50 cycles of 1.048 s. For the interpretation of the detrital zircon ages, only concordant or nearly concordant (less than 10% discordant) data were considered.

U–Pb SHRIMP analyses were undertaken on SHRIMP II and RG of the Research School of Earth Sciences of the Australian National University, Canberra, Australia. Zircon crystals were handpicked, mounted in epoxy resin, ground to half-thickness and polished with 3- and 1-μm diamond paste; a conductive gold-coating was applied just prior to analysis. The grains were photographed in reflected and transmitted lights, and cathodoluminescence (CL) images were produced in a scanning electron microscope in order to investigate the internal structures of the zircon crystals and to characterize different populations as well. SHRIMP analytical procedures followed the methods described in Compston et al. (1984) and Williams (1998). The standard zircon SL13 was used to determinate U concentration, and the U–Pb ratios were referenced to the zircon standard FC1. Raw isotopic data were reduced using the Squid program (Ludwig 2001), and age calculations and Concordia plots were done using both Squid and Isoplot/Ex software (Ludwig 2003). Analyses and ages for individual SHRIMP spots are listed in the data tables and plotted on Concordia diagrams with 1σ uncertainties. Where data are combined to calculate an age, the quoted uncertainties are at 95% confidence level, with uncertainties in the U–Pb standard calibration included in any relevant U–Pb intercept and Concordia age calculations.

The Hf isotope determinations of zircon grains by LA-ICP-MS were performed at the Laboratory for Isotope Geology, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil. A Neptune MC-ICP-MS (Thermo Finnigan) was used to measure Lu, Yb and Hf isotopic signals. The mass spectrometer contains 9 Faraday collectors to detect simultaneously the isotopes required for this methodology. The laser ablation system used in this study was a New Wave Research 213 UV. To obtain further improvements in precision of the Hf isotopic data from zircon material with 40-μm ablation pit size, a N₂ mixing technique was applied (Iizuka and Hirata 2005). In order to optimize the N₂ gas flow rate, the effect of the N₂ gas flow rate on elemental sensitivity was investigated, varying the N₂ gas flow rate using the zircons GJ-1. Signal intensities for three isotopes (¹⁷⁹Hf, ¹⁷⁵Lu and ¹⁷³Yb) obtained from zircon standards increased with N₂ gas flow rate. For the corrections in isobaric interferences of Lu and Yb isotopes on mass 176, the isotopes ¹⁷¹Yb, ¹⁷³Yb and ¹⁷⁵Lu were simultaneously monitored during each analysis. The ¹⁷⁶Lu and ¹⁷⁶Yb were calculated using ¹⁷⁶Lu/¹⁷⁵Lu of 0.026549 and ¹⁷³Yb/¹⁷¹Yb of 1.123456 (Chu et al. 2002; Thirlwall and Walder 1995). The correction for instrumental mass bias used an exponential law and a ¹⁷⁹Hf/¹⁷⁷Hf value of 0.7325 (Patchett et al. 1981) for correction of Hf isotopic ratios. Data were corrected and normalized following the procedure of the laser ablation analyses, in the excel sheet. Each analysis session has the βHf and βYb factors, and in each zircon analysis, we calculate a new βHf and βYb cycle. The mass bias behavior of Lu was assumed to follow that of Yb. It has been noted before that the Yb interference correction is crucial for precise and accurate ¹⁷⁶Hf/¹⁷⁷Hf obtained by laser ablation analysis (Woodhead et al. 2004).