Significance of the Early Jurassic Garamilla Formation in the Western Nordpatagonian Massif

Accepted Manuscript Significance of the early Jurassic Garamilla formation in the Western Nordpatagonian Massif Leonardo Benedini, Daniel Gregori PII: S0895-9811(13)00053-9 DOI: 10.1016/j.jsames.2013.03.016 Reference: SAMES 1170 To appear in: Journal of South American Earth Sciences Received Date: 26 October 2012 Accepted Date: 21 March 2013 Please cite this article as: Benedini, L., Gregori, D., Significance of the early Jurassic Garamilla formation in the Western Nordpatagonian Massif, Journal of South American Earth Sciences (2013), doi: 10.1016/j.jsames.2013.03.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT MANUSCRIPT ACCEPTED Graphical Abstract (for review) *Highlights (for review) Research highlights We study the volcanites of Garamilla Formation in western Nordpatagonian Massif. It is composed by three units with andesitic, dacitic and rhyolitic compositions. Radiometric dating indicates Early JurassicA CCEPTEDtime for development MANUSCRIPT of the volcanic system. Field evidences show a half graben and trapdoor structure related to eruption. It is comparable with equivalent units in the Neuquén Basin and Nordpatagonian Massif. MANUSCRIPT ACCEPTED *Manuscript Click here to view linked References 1 2 3 4 5 6 7 8 9 10 SIGNIFICANCE OF THE EARLY JURASSIC GARAMILLA 11 12 13 FORMATION IN THE WESTERN NORDPATAGONIAN MASSIF 14 15 ACCEPTED MANUSCRIPT 16 17 18 19 20 21 22 23 24 25 Leonardo Benedini* and Daniel Gregori 26 27 28 29 30 31 32 33 INGEOSUR, Cátedra de Geología Argentina, Departamento de Geología, 34 35 Universidad Nacional del Sur, San Juan 670, 8000 Bahía Blanca, Argentina. 36 37 38 39 40 [email protected],[email protected] 41 MANUSCRIPT 42 43 44 45 46 47 48 49 50 *Corresponding author: tel: +54-291-4595101 ext: 3031. [email protected] 51 52 53 54 55 56 ACCEPTED 57 58 59 60 61 62 1 63 64 65 Abstract 1 2 By means of facial, stratigraphic, petrographic, geochemical and 3 4 5 geochronological studies we characterize the Garamilla Formation, cropping out 6 7 in the western Nordpatagonian Massif. 8 9 10 The studies of these volcanic rocks reveal an Early Jurassic volcanic episode 11 12 formed by three volcanic units that change from normal calc-alkaline to high-K 13 14 calc-alkaline series. Other geochemical features reveal a progressive change 15 ACCEPTED MANUSCRIPT 16 17 from an initial subduction-related volcanism to one intraplate-related volcanism. 18 19 This volcanic episode is temporally and geochemically equivalent to those 20 21 22 volcanic units located in half-grabens in several areas of the Neuquén Basin. 23 24 The volcanic units were erupted into different structural designs. A portion of its 25 26 27 depocenter was interpreted as a transtensional half-graben, whereas the other 28 29 exhibits a trapdoor structure. The lineament trends that bound the volcanic 30 31 system were also recognized in western Nordpatagonian Massif, and were 32 33 34 assigned to the Gondwanide Orogeny. 35 36 37 38 39 40 41 MANUSCRIPT 42 43 44 45 46 47 48 49 50 51 52 53 Key words: Volcanism, Early Jurassic, Garamilla Formation, Western 54 55 56 Nordpatagonian Massif, Argentina. ACCEPTED 57 58 59 60 61 62 2 63 64 65 1. Introduction 1 2 The Early Mesozoic tectonic history of the southern South America region is 3 4 5 closely linked to the rupture and separation of the Gondwana Supercontinent, 6 7 which began in the Triassic with the formation of intracontinental rifts. Later, in 8 9 10 the Lower Jurassic, the western margin of South America was affected by the 11 12 development of subduction systems. As a result, oblique batholiths (Pankhurst 13 14 et al., 1992; Rapela and Pankhurst, 1992;Rapela et al., 2005) appear in the 15 ACCEPTED MANUSCRIPT 16 17 western margin of Patagonia, due to convergent conditions during the Early 18 19 Jurassic. Therefore, the major crustal extension in the Andean margin was 20 21 22 related to a trench roll-back, which would be responsible for the associated 23 24 subduction extension that characterizes most of this period (Mpodozis and 25 26 27 Ramos, 2008). 28 29 The extra-Andean Mesozoic volcanism in Patagonia is related to large-scale 30 31 half-grabens, mainly oriented NNW–SSE as suggested by Gust et al. (1985), 32 33 34 Homovc et al. (1996) and Feraud et al. (1999). 35 36 Specifically, the western Nordpatagonian Massif was subject to major 37 38 39 extensional stress, related to the break-up of Gondwana during Triassic times, 40 41 with the development of continental rifts. MANUSCRIPT 42 43 44 During the Early Jurassic, ENE–WSW oriented extensional conditions prevailed 45 46 in this area, promoting the development of extensive volcanism and the opening 47 48 49 of the marine Neuquén Basin located immediately west of the Nordpatagonian 50 51 Massif. In the eastern Nordpatagonian Massif, the Jurassic volcanic pile is 52 53 composed of extensive ignimbrite deposits of the Marifil Complex, which are 54 55 56 chemically, petrologically and isotopicallyACCEPTED uniform. The overall activity persisted 57 58 over some 20 Ma. Pankhurst and Rapela (1995) suggest that rifting and break- 59 60 61 62 3 63 64 65 up of the Gondwana supercontinent produced extensive melting of the lower 1 2 crustal, necessary for the development of this huge magmatism. 3 4 5 In the western Nordpatagonian Massif, the Jurassic units outcrop between 6 7 Piedra del Aguila, Comallo, and the La Esperanza localities, and are 8 9 10 represented by volcanic and subvolcanic rocks. Due to the absence of detailed 11 12 mapping, profiling, radiometric dating, and geochemical data, these rocks were 13 14 compared with apparently similar volcanic sequence outcrops more than 200 15 ACCEPTED MANUSCRIPT 16 17 km south. Therefore they were assigned to the Garamilla Formation by Cucchi 18 19 et al. (1998). 20 21 22 For the first time, this paper documents facies characteristics, stratigraphy and 23 24 the geochemistry of the volcanic and subvolcanic sequences erupted in a key 25 26 27 area represented by the boundary between the western Nordpatagonian Massif 28 29 and the Neuquén Basin. The study focuses on analysis of the volcanic rocks 30 31 outcropping in the area east of the Limay River between 69°47´–70° 00´ W and 32 33 34 40° 04´–40° 10´ S. This paper also reports geochronological studies, which 35 36 allow comparison with coeval sequences recognized in the Neuquén Basin and 37 38 39 the eastern Nordpatagonian Massif. In addition, we aim to better understand the 40 41 geochemical and petrologic evolution of the volcanism, its tectonicMANUSCRIPT setting and 42 43 44 the interrelationship between volcanism and the regional lineaments. Ultimately, 45 46 we attempt to correlate equivalent volcanic events of the central area of the 47 48 49 Neuquén Basin and central Nordpatagonian Massif. 50 51 52 53 2. Local geology 54 55 56 The study area is located in the westernACCEPTED sector of the Nordpatagonian Massif 57 58 that mostly occupies northern Patagonia (Fig.1) where three tectonomagmatic 59 60 61 62 4 63 64 65 cycles can be recognized. The older one belongs to the Pampean Orogeny 1 2 (Vendian-Middle Cambrian) and is represented by low-degree micaceous 3 4 5 schists, quartzites and phyllites of the Cushamen Formation (Volkheimer, 6 7 1964). However, new dating of detrital zircons in equivalent rocks outcropping in 8 9 10 the area of Río Chico by Hervé et al. (2008) demonstrated a Carboniferous age 11 12 for this metasedimentary sequence. 13 14 The low-degree metamorphic rocks were intruded by tonalite, granodiorite and 15 ACCEPTED MANUSCRIPT 16 17 granite of the Mamil Choique Formation (Volkheimer, 1964; Sesana, 1968; 18 19 Ravazolli and Sesana, 1977; Nullo, 1979; Núñez and Cucchi, 1990 and 1997). 20 21 22 This unit represents the Famatinian Orogeny and forms near all the pre-Triassic 23 24 outcrops in the Limay River area in the western Nordpatagonian Massif. 25 26 27 Radiometric dating, using Rb-Sr isochrones (Linares et al., 1988; Cingolani et 28 29 al., 1991; Dalla Salda et al., 1994), indicates a Silurian-Devonian age for the 30 31 type of area. Later studies by López de Luchi et al. (1999, 2000), Cerredo et 32 33 34 al.(2000) and Pankhurst et al. (2006) show that this magmatic event can be 35 36 related to the Gondwana Orogeny. Neither the Pampean nor the Famatinian 37 38 39 rocks are displayed in Figure 1 in order to avoid confusion. 40 41 Covering in non-concordance the above described units appearsMANUSCRIPT the Triassic 42 43 44 Los Menucos Formation, which represents a continental rift. Above all the 45 46 described units appears a set of rhyolitic and dacitic ignimbrites, rhyolitic lava- 47 48 49 flows, and tuffs, which were assigned by Nullo (1978) to the Garamilla 50 51 Formation, and originally correlated with the Los Menucos Formation. 52 53 This unit outcrops mainly in the area between the Limay River, La Esperanza 54 55 56 and Laguna Blanca and has been regionallyACCEPTED studied by Volkheimer (1964), 57 58 Coira (1979), Núñez and Cucchi (1990, 1997), and Benedini and Gregori 59 60 61 62 5 63 64 65 (2012).

Load more