Elsevier

Gondwana Research

Volume 96, August 2021, Pages 1-21
Gondwana Research

The Devonian accretionary orogen of the North Patagonian cordillera

https://doi.org/10.1016/j.gr.2021.04.004Get rights and content

Highlights

Abstract

The Andean basement of NW Patagonia contains remnants of a Devonian accretionary orogen that lasted for about 50 Ma. We present and review U-Pb geochronology, major and trace element geochemistry and O- and Hf-isotope analyses of zircon for Devonian igneous and metasedimentary rocks from the western edge of the North Patagonian Massif to the Pacific coast. Two contrasting magmatic domains are characterised. The Foreland Devonian Domain (FDD) of the North Patagonian Massif is dominated by subduction-related calcalkaline granite, granodiorite and tonalite emplaced in the interval c. 405–395 Ma and here named the San Martín arc. They are generally enriched in alkalis, light rare earth and Large Ion Lithophile elements, and have igneous zircon with δ18O mostly in the range +6 to +9‰ and ƐHft typically +3 to −5, indicating magmas that incorporated continental crustal material, possibly Mesoproterozoic. A younger (374 ± 3 Ma) body in this group has lower δ18O (down to +5‰) and Hf-isotope characteristics at the primitive end of this range. The Pacific Devonian Domain (PDD) lies on the western flank of the Andes and has uniformly primitive character. Granitic rocks here represent a more extended magmatic history (c. 395–355 Ma) and are mostly subalkaline tonalites with δ18O (<+6‰) that fall within or below the normal mantle field: with one exception their ɛHft values are > +4 and closer to Devonian depleted mantle composition. The PDD also has metabasalts of both N-MORB and E-MORB affinities, interbedded with Devonian turbidite metasandstones in which detrital zircon O- and Hf-isotope compositions and morphology indicate coetaneous derivation from the FDD arc. The results lead to a tectonic model involving continental arc, forearc rifting, Chaitenia oceanic arc and marginal basin formation, double arc subduction, and marginal basin closure in Mississippian times. This is recognisable as part of a more extensive Devonian accretion scenario along the SW Gondwana margin.

Introduction

Devonian magmatism in southern South America records key information on geological and paleogeographic evolution during the mature stage of SW Gondwana, i.e., from the final aggregation of the supercontinent during the Pampean orogeny (550–520 Ma, Casquet et al., 2018) up to the beginning of break-up in Triassic–Early Jurassic times.

We are concerned here with the area encompassing the western edge of the North Patagonian Massif in Argentina westwards across the Andes into Chile at the same latitude. Devonian magmatism in the eastern sector was demonstrated by Varela et al., 2005, Pankhurst et al., 2006. In the Chilean sector Devonian igneous rocks were first identified by Duhart (2001) and further substantiated by Quezada et al., 2015, Hervé et al., 2016, Hervé et al., 2018), who proposed a hypothesis of two Devonian magmatic belts: one emplaced in the continental crust of Patagonia and one generated in an oceanic environment and then accreted as the Chaitenia arc terrane.

Most previous studies of the Devonian basement at the western edge of the North Patagonian Massif agree in ascribing the igneous and metamorphic rocks to an active continental margin (Varela et al., 2005, Varela et al., 2015, Pankhurst et al., 2006; Serra-Varela et al., 2019, Serra-Varela et al., 2020; Oriolo et al., 2019, Marcos et al., 2020, Renda et al., 2019, Renda et al., 2021). However Martínez et al. (2012) related high-grade metamorphism and deformation in the Bariloche area to collision of Chilenia terrane with Gondwana. In the coastal region at 41°S, 40Ar-39Ar dating of eclogite–amphibolite assemblages in blueschist boulders suggested oceanic lithosphere subduction on the paleo-Pacific margin prior to 361 Ma (Kato et al., 2008).

Our aim is to look in more detail at the timing and generation of Devonian magmatic rocks of the North Patagonian Andes at 39–44°S (Fig. 1), where variation of age, chemical and isotopic composition can be studied through a 220 km E–W cross section. We include new U-Pb SHRIMP crystallization ages, geochemical analyses and Hf-O isotope determinations in zircon of representative samples. The evidence obtained provides new insight into the geological, paleogeographic and tectonic evolution of a large sector of the proto-Pacific edge of SW Gondwana over 50 Ma, resulting in the accretion of an island-arc/back-arc terrane.

Section snippets

Regional context

A notable feature of Devonian magmatism in NW Patagonia is its location. At 39°–42°S it is spatially coincident with the Mesozoic–Recent Andean orogen (Fig. 1). The next magmatic event in the Andean sector was the emplacement of early Carboniferous cordilleran granites, also occurring in a roughly N–S belt, although at 42°S Devonian and Carboniferous plutonic rocks spread south-eastwards, eventually disappearing beneath Mesozoic sediments of the San Jorge basin. Isolated and widely scattered

Sampling and analytical methods

Samples from the main FDD outcrop areas listed in Section 2.2 were used in this study. Apart from one new sample of San Martín tonalite (FO1957), these were selected from those previously dated by our research group for which zircon separates were available. Basement outcrops in the PDD sector of the North Patagonian Andes are sparse and often difficult of access due to dense forest. Therefore most PDD samples were taken from lake shores, creeks, road cuts, glacial valleys, and fjord coasts:

Igneous rock geochemistry

Representative chemical analyses of igneous rocks from both Devonian domains are shown in Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, with selected data summarised data in Table 1. The FDD igneous rocks are all granitoids with 56% to 75% SiO2 (Fig. 5a), predominantly hornblende-biotite granodiorite and biotite monzogranite enriched in normative orthoclase compared to the PDD equivalents (Fig. 5b). It should be noted that several of the rocks classified as granodiorite in the normative

Geochronology

Three new Devonian igneous crystallization ages have been obtained for intrusive rocks: in the Lago Yelcho and the Caleta Gonzalo–Río Blanco areas of the PDD (samples FO1970 and FO1977, Fig. 4) and in the San Martín de los Andes area of the FDD (sample FO1957, Fig. 2). The U-Pb data for these are displayed in Fig. 11. A detrital zircon provenance analysis for metasandstone schist at Fiordo Reñihue also yielded a probable Devonian age for deposition (Fig. 12). Results are included in Table 2

Zircon Hf-O isotopes and U-Pb age relationships

The determined Hf and O compositions in zircon are summarised in Table 3. Values of δ18O and ƐHft for igneous samples are plotted versus the U-Pb crystallization age of their host rock in Fig. 13 together with relevant data reported previously from these Devonian domains (Hervé et al., 2016, 2018). Igneous activity lasted for about 50 Ma, from the Early Devonian and until latest Devonian–earliest Mississippian, and it is clear that with very few exceptions samples from the two domains have

Origin and tectonic setting of the Devonian magmas

This study of Devonian magmatism in northwestern Patagonia, reveals a protracted, episodic history of about 50 Ma. Contrasting lithological, geochemical and isotopic characteristics allow the distinction of two different sectors: the FDD to the east, and the PDD to the west (Fig. 1). Apart from this separation, the outcrops are effectively coincident with the modern Andean chain at 39°–42°S, which has remained in approximately the same position relative to the continent since the emplacement of

Conclusions

After the Cambrian and Ordovician events that lead to the aggregation of SW Gondwana, represented by outcrops on the Atlantic side of the North Patagonian Massif, the main site of magmatism jumped ~ 500 km to the proto-Pacific margin, at the western and southwestern edge of the North Patagonian Massif continental crust. This Early Devonian event was the first of a series of protracted episodic convergence of the Andean accretionary orogen, which has continued to the present day.

Combined U-Pb

CRediT authorship contribution statement

Carlos W. Rapela: Writing - original draft, Conceptualization, Methodology, Formal analysis, Investigation, Resources, Data curation, Visualization, Funding acquisition. Francisco Hervé: Conceptualization, Funding acquisition, Writing - review & editing, Supervision, Investigation, Resources, Project administration. Robert John Pankhurst: Writing - review & editing, Validation, Formal analysis, Investigation, Data curation, Visualization. Mauricio Calderon: Methodology, Investigation,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was funded by Consejo Nacional de Investigación Científica y Tecnológica (presently ANID) through Project Fondecyt N°1180457 to FH, CONICET grant PUE 2290160100083 to CWR and additional research funds from CMF. Most zircon separation carried out by Mr. Juan Vargas. Geologist Aníbal Soto helped with fieldwork near Chaitén and Gonzalo and Elizabeth (Geology students from Universidad Católica de Temuco) provided three rock samples collected under miserable weather conditions near

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