Retrowedge-related Carboniferous units and coeval magmatism in the northwestern Neuquén province, Argentina

Abstract

The studied Carboniferous units comprise metasedimentary (Guaraco Norte Formation), pyroclastic (Arroyo del Torreón Formation), and sedimentary (Huaraco Formation) rocks that crop out in the northwestern Neuquén province, Argentina. They form part of the basement of the Neuquén Basin and are mostly coeval with the Late Paleozoic accretionary prism complex of the Coastal Cordillera, south-central Chile. U–Pb SHRIMP dating of detrital zircon yielded a maximum depositional age of 374 Ma (Upper Devonian) for the Guaraco Norte Formation and 389 Ma for the Arroyo del Torreón Formation. Detrital magmatic zircon from the Guaraco Norte Formation are grouped into two main populations of Devonian and Ordovician (Famatinian) ages. In the Arroyo del Torreón Formation, zircon populations are also of Devonian and Ordovician (Famatinian), as well as of Late Neoproterozoic and Mesoproterozoic ages. In both units, there is a conspicuous population of Devonian magmatic zircon grains (from 406 ± 4 Ma to 369 ± 5 Ma), indicative of active magmatism at that time range. The ε_Hf values of this population range between −2.84 and −0.7, and the TDM-(Hf) are mostly Mesoproterozoic, suggesting that the primary sources of the Devonian magmatism contained small amounts of Mesoproterozoic recycled crustal components. The chemical composition of the Guaraco Norte Formation corresponds to recycled, mature polycyclic sediment of mature continental provenance, pointing to a passive margin with minor inputs from continental margin magmatic rocks. The chemical signature of the Huaraco Formation indicates that a magmatic arc was the main provenance for sediments of this unit, which is consistent with the occurrence of tuff—mostly in the Arroyo del Torreón Formation and very scarcely in the Huaraco Formation—with a volcanic-arc signature, jointly indicating the occurrence of a Carboniferous active arc magmatism during the deposition of the two units. The Guaraco Norte Formation is interpreted to represent passive margin deposits of mostly Lower Carboniferous age (younger than 374 Ma and older than 326 Ma) that precede the onset of the accretionary prism in Chile and extend into the earliest stage of the accretion, in a retrowedge position. The Arroyo del Torreón and Huaraco formations are considered to be retrowedge basin deposits to the early frontal accretionary prism (Eastern Series) of Chile. The presence of volcanism with arc signature in the units provides evidence of a Mississippian magmatic arc that can be correlated with limited exposures of the same age in the Frontal Cordillera (Argentina). The arc would have migrated to the West (Coastal Batholith) during Pennsylvanian–Permian times (coevally with the later basal accretionary prism/Western Series). The source of a conspicuous population of Devonian detrital zircon interpreted to be of magmatic origin in the studied units is discussed in various possible geotectonic scenarios, the preferred model being a magmatic arc developed in the Chilenia block, related to a west-dipping subduction beneath Chilenia before and shortly after its collision against Cuyania/Gondwana, at around 390 Ma and not linked to the independent, Devonian–Mississippian arc, developed to the south, in Patagonia.

Appendix: Description of methods

Chemical analysis of unaltered and homogeneous samples includes the following: major elements were analyzed by XRF and trace element determinations were performed by ICP/MS, both at Activation Laboratories Ltd. of Canada. For high LOI values, quantitative analyses were made by thermogravimetry, using a TA instrument model SDT Q600 at SEGEMAR laboratories.

U–Pb analyses were carried out at Curtin University of Technology, Perth, and the Hf isotopic analyses at Macquarie University, Sydney.

Samples ZD62-946 (Guaraco Norte Formation, meta-sandstone collected at point 1, Fig. 1) and ZD58-67a (Arroyo del Torreón Formation, meta-tuff collected at point 2, Fig. 1) have been crushed, milled, sieved, and washed to remove very fine material (clay and silt sizes). The 60–250 mesh fractions were treated with heavy liquids (to remove light minerals) and magnetic separator (to concentrate the less magnetic minerals such as zircon). Zircon was handpicked and organized in an epoxy mount, which was polished and carbon-coated for SEM (Scanning Electron Microscope) study. Back-scattered images (BSE) were taken using a JEOL6400 SEM at the Center for Microscopy and Microanalyses at University of Western Australia. Images of zircon are critical for identifying internal features such as core and rims and to help avoiding areas with high common lead content (inclusions, fractures, and metamict areas). Epoxy mount (UWA 05–85) was gold coated for SHRIMP analyses. The SHRIMP analytical spot was about 25 μm in diameter and four or five LA scans were used for each spot-analysis. Both ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²⁰⁶Pb ages are presented in Table 2 but ²⁰⁶Pb/²³⁸U ages are used for the ages in cumulative plots and for individual grains except for Mesoproterozoic grains c.1–1., d.1–1, and e.12–2. The uncertainties of individual ages are quoted at 1σ, whereas the final ages and those used in the plots are calculated at 2σ level (about 95 % confidence). SHRIMP data were reduced using SQUID software (Ludwig 2001) and plots are prepared using ISOPLOT/Ex (Ludwig 2003).

Hf-isotope analyses reported here were carried out in situ using a New Wave Research LUV213 laser-ablation microprobe, attached to a Nu Plasma multicollector ICPMS at GEMOC Key Center, Macquarie University, Sydney. Most analyses are carried out with a beam diameter of about 40 μm, a 10 Hz repetition rate, and energies of 0.6–1.3 mJ/pulse. Typical ablation times are 30–120 s, resulting in pits 20–40 μm deep. The analytical spots of Hf-isotope analyses were located in the same site of the previous U–Pb SHRIMP analyses. Isobaric interferences of ¹⁷⁶Lu and ¹⁷⁶Yb on ¹⁷⁶Hf were corrected by the Nu Plasma because the mass bias of the instrument is independent of mass over the mass range considered. Interference of ¹⁷⁶Lu on ¹⁷⁶Hf is corrected by measuring the intensity of the interference-free ¹⁷⁵Lu isotope and using ¹⁷⁶Lu/¹⁷⁵Lu = 0.02669 to calculate the intensity of ¹⁷⁶Lu. Similarly, the interference of ¹⁷⁶Yb on ¹⁷⁶Hf is corrected by measuring the interference-free ¹⁷²Yb isotope and using ¹⁷⁶Yb/¹⁷²Yb to calculate the intensity of ¹⁷⁶Yb. The spiking of JMC475 Hf standard is used to determine the value of ¹⁷⁶Yb/¹⁷²Yb (0.5865) required to yield the value of ¹⁷⁶Hf/¹⁷⁷Hf obtained on the pure Hf solution.

The ¹⁷⁶Lu decay constant used to calculate initial ¹⁷⁶Hf/¹⁷⁷Hf, ε_Hf values, and model age is 1.983 × 10−11 (Bizzarro et al. 2003). Typical uncertainties on single ¹⁷⁶Lu/¹⁷⁷Hf analyses are about 1 epsilon unit (±0.001–0.002 %) incorporating both spatial variation of Lu/Hf and analytical uncertainties. Hf data are given in Table 3. ε_Hf values, also summarized in Table 3, were calculated at the ²⁰⁶Pb/²³⁸U age of each grain (T). For grains c.1-2 and e.12-2, the ε_Hf is calculated at the ²⁰⁷Pb/²⁰⁶Pb age.