To Link and Cite This Article: Doi: 10.5710/AMGH.17.01.2020.3270
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Submitted: June 18th, 2019 – Accepted: January 17th, 2020 – Published online: February 12th, 2020 To link and cite this article: doi: 10.5710/AMGH.17.01.2020.3270 1 THE CONTINENTAL CRUST OF NORTHEASTERN PATAGONIA 2 3 CARLOS W. RAPELA(1), ROBERT J. PANKHURST(2) 4 5 (1) Centro de Investigaciones Geológicas (CIG), CONICET, Universidad Nacional de la 6 Plata, Diagonal 113 Nº 275, 1900 La Plata, Argentina. E-mail: 7 [email protected] 8 (2) Visiting Research Associate, British Geological Survey, Keyworth, Nottingham NG12 9 5GG, UK. E-mail: [email protected] 10 11 Total number of pages (text + references): 34, illustrations: 9, Figure captions: 2, tables: 2, 12 supplementary data: 1 13 14 Proposed header: THE CONTINENTAL CRUST OF NORTHEASTERN PATAGONIA 15 16 Corresponding author: Carlos W. Rapela 17 18 19 20 1 21 1- Abstract 22 The basement of northeastern Patagonia is characterized by early Paleozoic igneous and 23 metamorphic rocks that do not crop out in the central, western and Andean sectors of the 24 North Patagonian Massif. A review of U-Pb geochronology, geochemistry, and the Nd, Sr, 25 Hf and O isotope signature of the early Cambrian and Early Ordovician magmatic rocks 26 supports the hypothesis that the continental crust of northeastern Patagonia was essentially 27 continuous with that of the Eastern Sierras Pampeanas in early Cambrian times. 28 Mesoproterozoic lower crust is also inferred for this sector. New zircon Hf and O analyses 29 of early Cambrian (Pampean) granites in the Sierras Pampeanas are indistinguishable from 30 those of Cambrian granite in NE Patagonia, indicating an important crustal component in 31 the source. The detrital zircon age patterns of the inferred basement are also similar in the 32 two regions, strongly suggesting a southern Kalahari provenance. A modified hypothesis to 33 explain the continuity of NE Patagonia with the Pampean belt of the Sierras Pampeanas 34 during early Cambrian times, as well as their SW Gondwana geological affinity, is to 35 consider this entire belt as an outboard sector of the mid-Cambrian rifting observed along 36 the South America–South Africa–Weddell Sea margin. The detached sector would then 37 have become juxtaposed against the Río de la Plata craton across the right-lateral Córdoba 38 fault in late Cambrian times. 39 2- Keywords 40 Northeastern Patagonia, Continental crust, Gondwana, early Paleozoic, U-Pb provenance, 41 Hf-O isotopes 42 3- Resumen 43 LA CORTEZA CONTINENTAL DEL NORESTE DE LA PATAGONIA. El basamento 44 del noreste de la Patagonia es un sector caracterizado por rocas ígneas y metamórficas de 45 edad paleozoica inferior, que no afloran en los sectores central, occidental y andino del 2 46 Macizo Norpatagónico. Una revisión de estudios previos de geocronología U-Pb, 47 geoquímica, e isótopos de Nd, Sr, Hf y O en las rocas magmáticas cámbricas y ordovícicas 48 sostiene la hipótesis de que la corteza continental del noreste de la Patagonia fue continua 49 con la correspondiente a las Sierras Pampeanas Orientales durante el Cámbrico inferior. Se 50 infiere también para este sector, una corteza inferior mesoproterozoica. Nuevos análisis de 51 Hf y O en circón en granitos cámbricos inferiores (pampeanos) de las Sierras Pampeanas 52 son indistinguibles de los granitos de Cámbrico inferior del NE de la Patagonia, e indica un 53 fuerte componente cortical en la fuente de los mismos. El patrón de edades detríticas en el 54 basamento, es también similar en ambas regiones, lo que sugiere una fuerte proveniencia 55 del sur del Kalahari. Otra hipótesis para explicar la continuidad del NE de la Patagonia con 56 el Cinturón Pampeano de las Sierras Pampeanas, así como también su afinidad geológica 57 con el SO de Gondwana, es considerar que el cinturón completo fue un sector externo 58 (outboard) del rifting Cámbrico medio que se observa a lo largo del margen de Sudamérica 59 (Sierra de la Ventana)–Sudáfrica–Mar de Weddell. El sector separado fue luego 60 yuxtapuesto contra el cratón del Río de la Plata por la falla dextral Córdoba, en el 61 Cámbrico superior. 62 4- Palabras clave 63 Noreste de Patagonia, Corteza continental, Gondwana, Paleozoico inferior, Proveniencia 64 U-Pb, Isótopos de Hf y O 65 66 3 1 DESPITE the many geological studies carried out during the last 35 years and the important 2 technological and theoretical advances in almost all geological disciplines during that time, 3 the origin of the continental crust of Patagonia is still being debated. Controversies 4 continue largely because over most of the exposed surface of Patagonia, the early 5 Paleozoic rocks and the allegedly Precambrian lower crust are covered by younger units. 6 The thick cover includes extensive Late Paleozoic–Early Mesozoic plutonic-volcanic 7 complexes, Mesozoic sedimentary basins associated with the opening of the South Atlantic 8 Ocean, and Tertiary plateau basalts. 9 The hypothesis of Patagonia as single exotic continental block that collided with 10 Gondwana during the late Paleozoic (Ramos, 1984) has been challenged and modified. 11 Autochthonous and para-autochthonous models that recognize the assembly of several 12 blocks derived from Gondwana have been proposed (e.g. Pankhurst et al., 2006, 2014; 13 Ramos, 2008; Ramos and Naipauer, 2014; Gregori et al., 2008; Rapalini et al., 2010, 2013; 14 González et al., 2011, 2018; see González et al., 2018, for a more detailed description of 15 the different hypotheses). The aim of the present contribution is to review and provide 16 further evidence for the age, chemical and isotopic composition of the oldest rocks of 17 northeastern Patagonia in order to further constrain any hypothesis regarding their origin. 18 The scattered basement outcrops of northern Patagonia, from Río Colorado in the north 19 to the southern limit of the North Patagonian Massif (NPM) (Fig. 1), reveal differences in 20 the evolution of the upper crust of the eastern and western areas. The northeastern sector, 21 discussed in more detail in this paper (Fig. 2), is dominated by Cambrian low-grade 22 igneous and metamorphic basement and Early Ordovician granites with sparse occurrence 23 of early Cambrian granites. Early Paleozoic magmatism has not been so far found in the 24 central sector of the North Patagonian Massif, which is covered by plutonic-volcanic 25 complexes of a large Permo-Triassic province (Luppo et al., 2018 and references therein) 4 26 and Tertiary plateau basalts (Kay et al., 2007). In the southwestern North Patagonian 27 Massif rather imprecise ages of 420–425 Ma and 360–380 Ma for metamorphic and 28 detrital zircon suggest an unrelated geological history (Pankhurst et al., 2006; Martínez 29 Dopico et al., 2011). Recent studies in the basement of the North Patagonian cordillera 30 show that Devonian magmatism occurred in two contemporaneous belts: one emplaced 31 immediately east of the North Patagonian Massif, the other located west of the Andes in 32 Chile and interpreted as an oceanic island arc represented by pillow lavas, metabasalts, 33 primitive granites and deep-marine fossiliferous slates (Hervé et al., 2016, 2018). 34 The northeastern corner of Patagonia is the only sector where direct comparison of 35 composition and age can be made with the Early Paleozoic crust of the Sierras Pampeanas 36 to the north (Fig. 3), where an extensive dataset of U-Pb ages, geochemistry and Rb, Sr, 37 Nd, Hf and O isotopes is available (Pankhurst et al., 2014; Rapela et al., 2018 and 38 references therein). Here we review the evidence provided by detrital zircon age patterns in 39 the basement rocks of both areas, extended by new data for a metasedimentary enclave in 40 Ordovician granite from the Río Colorado area. Recent studies of Hf and O isotopes in 41 zircons from Early Paleozoic granites in the Sierras Pampeanas and Patagonia are reviewed 42 and equivalent new data for Cambrian granites in the Sierras Pampeanas are reported here 43 for comparison. This combined approach provides solid evidence regarding the 44 composition and age of the northeastern Patagonian crust and new constraints on the 45 provenance of this sector. Alternative para-autochthonous hypotheses that encompass the 46 Eastern Sierras Pampeanas as well are also discussed. 47 GEOLOGICAL BACKGROUND 48 From north to south, the relevant areas of northeastern Patagonia are found around Río 49 Colorado (Sierra de Pichi Mahuida), Valcheta–Aguada Cecilio and Sierra Grande (Fig. 5 50 2.1). The description below is restricted to the igneous-metamorphic basement and the 51 Early Paleozoic granite bodies. 52 Río Colorado area 53 To the south of the modern Andean flat-slab sector at 27–33° S., sparse and scattered 54 outcrops indicate that the Sierras Pampeanas belts extended southwards towards the 55 geographic limit of Patagonia at Río Colorado (Fig. 1) (Chernicoff et al., 1996, 2010, 56 2012). In this latter area, discontinuous and low-relief outcrops located along the river and 57 its tributaries were long ago considered to be a southern extension of the Sierras 58 Pampeanas (Fig. 2.1) (Linares et al., 1980). 59 The first comprehensive description of the lithology and radiometric age of the rocks at 60 Río Colorado was made by Tickyj et al. (1999a, b), who also considered that they were 61 formed during the Famatinian episode of the Sierras Pampeanas. The basement here is a 62 low-grade metasedimentary complex of slates, schists and metasandstones with a WNW 63 foliation considered to have resulted from syn-metamorphic deformation: a second event 64 produced decimetric folding (Tickyj et al., 1999a). The age of metamorphism was poorly 65 defined by rather imprecise K-Ar and Rb-Sr ages between 640 and 340 Ma (Linares et al., 66 1980).