The Chaitenia accretionary orogen of northwest Patagonia: new U-Pb SHRIMP ages of the Foreland Domain

RAPELA, C.W.; HERVE, F; PANKHURST, R.J.; CALDERÓN, M.; FANNING, C.M.; QUEZADA, P.

Puerto Madryn

Congreso; XXI Congreso Geológico Argentino; 2022

Asociación Geológica Argentina

THE CHAITENIA ACCRETIONARY OROGEN OF NORTHWEST PATAGONIA: NEW U-Pb SHRIMP AGES OF THE FORELAND DOMAINCarlos W. Rapela(1), Francisco Hervé(2,3), Robert J. Pankhurst(4), Mauricio Calderón(5),Christopher M. Fanning(6), Paulo Quezada(7)(1)Centro de Investigaciones Geológicas (CIG), CONICET, Universidad Nacional de la Plata, Diagonal 113 Nº 275, 1900 La Plata. E-mail: crapela@cig.museo.unlp.edu.ar(2)Carrera de Geología, Facultad de Ingeniería, Universidad Andrés Bello, Sazie 2119, Santiago, Chile(3)Departamento de Geología, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile(4)Visiting Research Associate, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK(5 Facultad de Ingeniería, Universidad del Desarrollo, Av. Plaza 680, Las Condes, Santiago, Chile)(6)Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia(7)Instituto LAMIR, Universidade Federal do Paraná, Centro Politécnico, Jardim das Américas, Curitiba 81531-980, PR, BrazilThe Andean basement of NW Patagonia between 39º and 43º SL contains remnants of Devonian plutonic, volcanic and metamorphic rocks. While some of them were clearly emplaced in continental crust at the easternmost edge of North Patagonian Massif, in the west of the North Patagonian Cordillera and the coastal region, the igneous rocks were developed in an oceanic environment. Thus, a Foreland Devonian Domain (FDD) and a Pacific Devonian Domain (PDD) are distinguished on either side of the modern Andean Cordillera (Fig. 1). While the Devonian granites of the Foreland Domain can be unambiguously ascribed to subduction in a continental margin (see references in Rapela et al., 2021), the more scattered outcrops of pillow lavas, alkaline trachyte-rhyolite and primitive granites of the PDD required more detailed studies to determine their petrogenesis and tectonic significance. Initial studies suggested that they represented the mixed products of an oceanic island arc that later became accreted to the continent as an allochthonous terrane (the Chaitenia terrane of Hervé et al., 2016). However, whole-rock geochemistry shows that the pillow lavas are dominated by back-arc basalts and that metasandstones interbedded with the pillow lavas contain detrital zircons with U-Pb ages, Hf and O isotope compositions comparable to those of the Foreland Domain granites, i.e., they are of contemporary continental provenance. Thus, they are more likely to represent a Devonian parautochthonous terrane, formed after continental rifting and was laterally accreted during closure of a back-arc basin in an orogen that also encompasses the fore-arc (Rapela et al. 2021), and which we now refer to as the Chaitenia Accretionary Orogen (CAO). Five stages have been recognized in its evolution: (i) a continental arc stage that involved the San Martín arc (c. 405–395 Ma, Emsian); (ii) a forearc rift-ridge transition stage (c. 395–390 Ma, Emsian–Eifelian); (iii) a marginal basin stage that involves the starting of an oceanic arc, the Chaitenia arc (c. 380–390 Ma, Eifelian–Givetian); (iv) a double arc stage (365–380 Ma, Giovetian–Frasnian) and (v) closing of the Chaitenia marginal basin (c. 360–340 Ma, Fammenian–Visean). Three new U-Pb SHRIMP zircon ages on foreland domain samples in the northeastern corner of the North Patagonian Massif provide further insight on the evolution of the CAO (Fig. 1). FO19096 is a foliated hornblende-biotite granodiorite at the side of Route 234, Cuesta de Collón Cura. The Th/U ratios of 22 analysed zircon areas are mostly > 0.3, consistent with crustal igneous zircon. Excluding disparate dates in the range 310-360 Ma, which are clearly influenced by Pb loss, there is a prominent cluster of 12 close to and within uncertainty of Concordia that give a weighted mean of 389 ± 3 Ma (MSWD =1.4). This is considered a best estimate for the time of zoned igneous zircon crystallisation. The calcalkaline granodiorite is coeval within error with the easternmost units of the San Martín arc (such as the Colan Conhué granite, 390–394 Ma) and the start of magmatic activity in the PDD represented by alkaline volcanic and subvolcanic rocks with primitive zircon Hf and O isotopic signatures. FO19098 is a diorite-tonalite orthogneiss from Sañicó. Twenty-seven areas were analysed on 22 zircon grains. Th is generally low, especially in outer rim areas, with Th/U mostly below 0.10, suggesting a metamorphic origin. One analysed core gave a Proterozoic 206Pb/238U age of c. 2040 Ma, another c. 1100 Ma, and two more are apparently Early Paleozoic: all of these are interpreted as inherited. A weighted mean for the 19 ages of the main group yields 363 ± 2 Ma (MSWD = 0.81). About half of these are from areas with low Th/U thought to represent metamorphic growth; the other half are more consistent with igneous growth. This implies igneous emplacement of the diorite-tonalite at essentially the same time as overprinting metamorphism and gneiss formation. This age is consisting with the inferred beginning of marginal basin closure (c. 360–340 Ma), which involved deformation and high-grade metamorphism (Rapela et al., 2021; Renda et al 2021). FO19093 is a porphyritic biotite granite from Cordón de la Piedra Santa. Twenty-three areas were analysed on 22 zircon grains with Th/U ratios of 0.1 to 0.7. Central cavities in several grains indicate rapid crystallisation at a high crustal level; i.e. volcanic to subvolcanic. Length-parallel zoning, in places anastomosing, supports this interpretation. The probability density plot shows a dominant age grouping around 355 Ma tailing to older ages. A weighted mean for 17 analyses has no excess scatter (MSWD =1.07) and gives a 206Pb 238U age of 357 ± 2 Ma, which is considered to date the main period of crystallisation of the granite. As in the case of FO19098, the age indicates that magmatic activity in the FDD lasted until the Lower Mississipian and was coeval with closing of the marginal basin of the CAO. Figure 1. Location of new and published SHRIMP U-Pb zircon ages of igneous and metamorphic rocks in the studied area of northwestern Patagonia. Inset shows the location of the studied area in the generalized terrane map of southern South America, modified from Rapela et al. (2021). Hervé, F., Calderon, M., Fanning, C.M., Pankhurst, R.J., Fuentes, F., Rapela, C.W., Marambio, C., 2016. Devonian magmatism in the accretionary complex of southern Chile. Journal of the Geological Society 173: 587–602.Rapela, C.W., Hervé, F., Pankhurst, R.J., Calderón, M., Fanning, C.M., Quezada ; P., Poblete, P., Palape, C. & Reyes, T. 2021. The Devonian accretionary orogen of the North Patagonian Cordillera. Gondwana Research 96: 1-21.Renda, E.M., González, P.D., Vizán, H., Oriolo, S., Prezzi, C., Ruiz González, V., Schulz, B., Krause, J., Basei, M., 2021. Igneous-metamorphic basement of Taquetrén Range, Patagonia, Argentina: A key locality for the reconstruction of the Paleozoic evolution of Patagonia. Journal of South America Earth Sciences 106, 103045.Research was mainly supported by Fondecyt project 1180457