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Internal Structure of the Late Triassic Central Patagonian Batholith at Gastre, Southern Argentina: Implications for Pluton Emplacement GEOSPHERE, V

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Internal Structure of the Late Triassic Central Patagonian Batholith at Gastre, Southern Argentina: Implications for Pluton Emplacement GEOSPHERE, V Research Paper GEOSPHERE Internal structure of the Late Triassic Central Patagonian batholith at Gastre, southern Argentina: Implications for pluton emplacement GEOSPHERE, v. 13, no. 6, p. XXX–XXX and the Gastre fault system doi:10.1130/GES01493.1 Claudia B. Zaffarana1, Rubén Somoza2,†, Darío L. Orts1, Roberto Mercader3, Bárbara Boltshauser2, Víctor Ruiz González2, and 9 figures; 1 supplemental file Carla Puigdomenech2 1Instituto de Investigación en Paleobiología y Geología (IIPG), Consejo de Investigaciones Científicas y Técnicas (CONICET). Av. Julio A. Roca 1242 General Roca (8332), Provincia de Río Negro CORRESPONDENCE: [email protected] 8332, Argentina 2Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires (I.G.E.B.A.), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Intendente Güiraldes 2160, Pabellón II, Piso 1, Ciudad Universitaria, Buenos Aires 1428, Argentina CITATION: Zaffarana, C.B., Somoza, R., Orts, D.L., 3Departamento de Física, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Buenos Aires 1900, Argentina Mercader, R., Boltshauser, B., González, V.R., and Puigdomenech, C., 2017, Internal structure of the Late Triassic Central Patagonian batholith at Gastre, southern Argentina: Implications for pluton em- ABSTRACT resents a key element in paleogeographic models of pre-Gondwana breakup, placement and the Gastre fault system: Geosphere, v. 13, no. 6, p. 1973–1992, doi:10.1130/GES01493.1. as it was emplaced during the transition between the Gondwanide (late Paleo- The Central Patagonian batholith (CPB) comprises two Late Triassic calcalka- zoic) and the Andean (Jurassic to present) tectonic cycles in South America. Received 30 December 2016 line plutonic suites (the Gastre superunit of 221 ± 2 Ma and the Lipetrén supe- The emplacement of the CPB has been inferred as syntectonic with the activity Revision received 25 July 2017 runit of 215 ± 1 Ma) which have been interpreted as a record of major dextral mo- of the Gastre fault system, a controversial NW-SE subvertical structure that Accepted 12 September 2017 tion along the transcontinental NW-SE–striking subvertical Gastre fault system has been conceived as a major dextral fault cross-cutting Patagonia (Fig. 1A; Published online 27 October 2017 in Jurassic times. We performed a detailed study of the internal structure of the Rapela et al., 1991, 1992; Rapela and Pankhurst, 1992; Marshall, 1994; König CPB through structural and anisotropy of magnetic susceptibility (AMS) tech- and Jokat, 2006; Martin, 2007; Riley et al., 2016). The Gastre fault system was niques. The Gastre superunit reveals a very consistent pattern of NW-SE–striking widely invoked to restore Patagonia to achieve a tectono-stratigraphic correla- steeply dipping magmatic foliations. Tectonic fabrics within the CPB are scarce tion between the Paleozoic successions from the Malvinas-Falkland islands and generally parallel to the magmatic fabrics. The magmatic and solid-state and the Cape fold belt of South Africa (Marshall, 1994; Storey et al., 1999; lineations within the CPB are steeply, intermediately, or shallowly plunging. Macdonald et al., 2003; Hole et al., 2016). This correlation also requires ~100° The combination of flattened magmatic and solid-state fabrics with subvertical clockwise rotation of the Malvinas-Falkland islands, which in turn has been orientations and with steep to shallow lineations, together with the kinematic independently supported by paleomagnetic results from NE-SW–trending Ju- indicators in two mylonite belts with suspected CPB protoliths, suggests that rassic dikes cropping out in the islands (Taylor and Shaw, 1989). According the Gastre superunit was emplaced within a sinistral transpressive regime. The to this model, almost all the dextral motion through the Gastre fault system shallower stocks of the Lipetrén superunit are more isotropic and have mag- must be younger than 190–180 Ma, which is the age of the paleomagnetically matic fabrics associated with magma chamber dynamics. The deformation of studied dikes (Mussett and Taylor, 1994; Stone et al., 2008, 2009; Hole et al., the CPB is coaxial with the late Paleozoic deformation in the hosting Calcata- 2016). Figure 1A shows the inferred trace of the Gastre fault system passing pul Formation. The late Paleozoic deformation in the North Patagonian Massif along the outcrops of the Late Triassic CPB. Recent paleomagnetic data from generated widespread NW-SE subvertical fractures which could have aided the the overlying Lonco Trapial Formation (Fig. 1B) indicate that no clockwise tec- emplacement of the CPB. The internal structure of the CPB does not support a tonic rotations took place in the Jurassic through the Gastre district (Zaffarana model of dextral strike-slip movements on major Jurassic faults. and Somoza, 2012). Furthermore, local mesoscale geological observations in the type locality of the Gastre fault system argue against the idea that the CPB has been af- INTRODUCTION fected by continental-scale Jurassic dextral shearing (von Gosen and Loske, 2004; Zaffarana et al., 2010, 2014). However, a comprehensive structural The Central Patagonian batholith (CPB; Fig. 1A; Rapela et al., 1991, 1992; characterization of the CPB has remained a pending task. In this contribu- Zaffarana et al., 2014) is a suite of Late Triassic calcalkaline plutons that rep- tion, we present the results of a study of the internal structure of the plutons For permission to copy, contact Copyright that compose this batholith. In many sectors of the batholith, the determi- Permissions, GSA, or [email protected]. †Deceased nation of foliation and lineation is difficult because of the preponderance of © 2017 Geological Society of America GEOSPHERE | Volume 13 | Number 6 Zaffarana et al. | Internal structure of the Late Triassic Central Patagonian batholith at Gastre, southern Argentina Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/6/1973/3990756/1973.pdf 1973 by Universidad Nacional de La Plata user on 04 September 2019 Research Paper 69°50'0"W 69°40'0"W 69°30'0"W 69°20'0"W 69°10'0"W B 4 2 ° 2 0 ' 0 " S Puesto Uribe 74°0'0"W 72°0'0"W 70°0'0"W 68°0'0"W 66°0'0"W Earliest Toarcian S Si " EarlyJurassic err 0 a ' PuestoYancamil d 0 Late Triassic e ° Ca 8 lca 3 EarlytoMiddle Triassic Jaramillo ta pu Permian-Triassic l S 4 " 2 0 ' ° 1 0 0 ° Puesto Jaramillo ' 0 Puesto 0 4 " Loma Rechene S S Alta " 0 La Horqueta ' Cordón 0 de la ° 2 Figure1B Cruz 4 Gastre F system S El Molle " 0 ' ault 0 ? ° 4 Pampa de 4 AgniaBasin Cañadón Gastre Asfalto S " Basin 10 km 0 ' Southern 4 0 2 ° San Jorge ° 6 Atlantic 0 4 Basin ' Ocean 0 " S S " Deseado 0 ' ? Undifferentiated granites REFERENCES OF PANEL B 0 Massif ° 8 4 HorquetaGranodiorite Lipetrén AMSsamplingsite A Central Biotitic granites superunit Patagonian Paleozoic REFERENCES OF PANEL A batholith Hornblende quartz-diorites (LateTriassic) Late Paleozoic Holocene Early-MiddleJurassic Equigranular biotitic monzogranites Gastre granites Sediments Lonco Trapial, Puesto superunit and basalts Lizarralde,Los Tobianos and Porphyriticbiotite-hornblende monzogranites Calcatapul Las LeonerasFormations Formation Equigranular hornblende-biotite granodiorites Figure 1. (A) Regional map of southern South America showing the inferred trace of Gastre Fault System. The figure shows that it does not deflect the late Pliensbachian-Toarcian deposits of the Pampa de Agnia Basin (Lesta et al., 1980; Uliana and Biddle, 1978; Uliana and Legarreta, 1999; Vicente, 2005), and that it traverses the San Jorge basin without any offset. The outcrops of the Late Triassic Central Patagonian batholith are located along its trace. (B) Regional map of the Central Patagonian batholith with the main distribution of sampling sites and with inset of the area studied in detail. GFS—Gastre fault system; AMS—anisotropy of magnetic susceptibility. Based on von Gosen and Loske (2004), Zaffarana et al. (2014), and Cábana et al. (2017). GEOSPHERE | Volume 13 | Number 6 Zaffarana et al. | Internal structure of the Late Triassic Central Patagonian batholith at Gastre, southern Argentina Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/6/1973/3990756/1973.pdf 1974 by Universidad Nacional de La Plata user on 04 September 2019 Research Paper magmatic fabrics and because of the common presence of equigranular and composed of small crystals of quartz and feldspar. These rocks could signal in some cases fine-grained plutonic rocks. To solve this, we performed an- the highest structural level exposed in the batholithic system. The only clearly isotropy of magnetic susceptibility (AMS) measurements in systematically extrusive rock that could be associated with the magmatic system of the CPB distributed sites (as much as possible) over an area of ~400 km2 around corresponds to a meter-scale outcrop (42°01.169′S, 69°23.198′W) of a gray ig- the Gastre locality. The integration of the AMS results with our more local nimbrite with eutaxitic texture and a rhyolitic to dacitic composition. meso- and microscale structural observations results in a suitable picture of The Gastre superunit yields a Rb-Sr isochron age of 222 ± 3 Ma (data from the internal structure of the plutonic rocks (Fig. 1B), which has implications Rapela et al. [1992] reinterpreted by Zaffarana et al. [2014]) and a U-Pb zircon to magmatic processes. age of 221 ± 2 Ma that was mentioned (without providing analytical data) by Rapela et al. (2005). The Gastre superunit also yields an 40Ar/39Ar stepwise bi- otite cooling age of 213 ± 5 Ma (Zaffarana et al., 2014). With respect to the Li- GEOLOGICAL SETTING petrén superunit, a recently reported U-Pb SHRIMP (sensitive high-resolution ion microprobe) age of 215 ± 1 Ma for its monzogranites (Lagorio et al., 2015) The CPB is composed of Late Triassic (time scale of Cohen et al., 2016) cal- suggests that the previous 40Ar/39Ar stepwise biotite cooling age of 206 ± 4 Ma calkaline plutons (Rapela et al., 1991, 1992; Rapela and Pankhurst, 1992).
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