The Lonco Trapial Formation in the Sierra De Mamil Choique, Rıo

Journal of South American Earth Sciences 15 (2002) 625–641 www.elsevier.com/locate/jsames Middle Jurassic dyke swarms in the North Patagonian Massif: the Lonco Trapial Formation in the Sierra de Mamil Choique, Rıó Negro province, Argentina Mońica G. Lo´pez de Luchia,*, Augusto E. Rapalinib aInstituto de Geocronologıá y Geologıá Isoto´pica, Pabellon INGEIS, Ciudad Universitaria, 1428 Buenos Aires, Argentina bLaboratorio de Paleomagnetismo, ‘D.A. Valencio’, Departamento de Cs. Geolo´gicas, Univ. de Buenos Aires, Pabelloń II, Ciudad Universitaria, 1428 Buenos Aires, Argentina Received 1 February 2002; accepted 1 June 2002 Abstract Middle Jurassic volcanism is a major magmatic event in the evolution of Patagonia. In the southwestern corner of the North Patagonian Massif at Sierra de Mamil Choique, basic to intermediate, ca. 170 Ma dykes are widespread. In this article, new chemical data from the dykes are presented, analyzed, and compared with regional information about Jurassic volcanism. The dyke swarms are composed of an alkaline and a subalkaline series. Fractionating phases that controlled the magmatic evolution were plagioclase (andesine) þ clinopyroxene with clinoamphibole (for compositions of SiO2 . 60%) in the subalkaline series and clinoamphibole þ plagioclase (oligoclase) ^ magnetite in the alkaline series. The source of both series could be an enriched lithospheric mantle or lower crust. Although crustal contamination or enrichment by subduction-derived fluids cannot be demonstrated from the available data, a subduction-enrichment process of the magma source is suggested for the subalkaline rocks. Different volatile composition, water content and perhaps temperature at the melting site would control the higher Zr content of the alkaline series and would allow low melt fractions to leave their sources. Absolute values for Ba/Nb (60–160), La/Nb (4–5), and K/Nb (.2000) in both series indicate a significantly LIL-enriched crustal component. Nd/Th of approximately 4 for the alkaline rocks argues against a primitive mantle, supported by the low MgO, Cr, and Ni and high LILE contents. In contrast, Nd/Th between 15 and 27 in the subalkaline rocks is similar to a mantle source. Crustal components are indicated by the higher La/Yb and Na2O and lower CaO in the alkaline rocks. Regional, structural, and, to a certain extent, petrological evidence for the Middle Jurassic dyke swarms of the Sierra de Mamil Choique point to an extensional intracontinental tectonic setting in which older structures controlled the development of the volcanism. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Dyke swarms; Petrology of basic to intermediate volcanism; Middle Jurassic; Patagonia 1. Introduction Chon-Aike Province (Kay et al., 1989; Pankhurst et al., 1998), a general designation that includes the extra-Andean Jurassic volcanism represents a major magmatic event in Patagonia, the Marifil, Chon-Aike, Lonco Trapial, and Bajo the evolution of Patagonia (Lesta and Ferello, 1972; Page Pobre Formations. In central and eastern Patagonia, two and Page, 1993; Pankhurst and Rapela, 1995; Pankhurst units of silicic rocks have been distinguished: the (185–168/ et al., 1998, 2000; Bertrand et al., 1999). The volcanic rocks 175 Ma) Marifil Formation in the eastern sector of the North are predominantly rhyolitic and form one of the world’s Patagonian Massif (NPM) (Rapela and Pankhurst, 1993; most voluminous silicic provinces (Pankhurst and Rapela, Pankhurst and Rapela, 1995; Alric et al., 1996) and the 1995), which extends from the Atlantic margin to Chile (167–165 Ma) Chon-Aike Formation in the eastern sector (Fig. 1). As a whole, the rocks have been designated the of the Deseado Massif (DM) (Pankhurst et al., 1993). Basic to intermediate volcanism, or the Central Volcanic Belt of Page and Page (1993), develops to the west of these areas * þ Corresponding author. Tel.: 54-11-4-783-3021; fax: 54-11-4-783- and includes the 175–159? Ma Lonco Trapial Formation in 3024. E-mail addresses: [email protected] (M.G. Lo´pez de Luchi), the NPM, a general term that refers to the previously [email protected] (A.E. Rapalini). designated Lonco Trapial, Canãdon Asfalto, and Taquetren 0895-9811/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0895-9811(02)00083-4 626 M.G. Lo´pez de Luchi, A.E. Rapalini / Journal of South American Earth Sciences 15 (2002) 625–641 Fig. 1. Regional distribution of the Chon-Aike province (Kay et al., 1989; Pankhurst et al., 1998) redrawn from Pankhurst and Rapela (1995). Star: SMC dyke swarms (Lo´pez de Luchi and Rapalini, 1997, 1999). NPM: North Patagonian Massif; SJB: San Jorge Basin; DM: Deseado Massif. Formations (Pankhurst et al., 1998). It also includes the Bajo involvement of subduction activity along the Pacific margin Pobre Formation in the DM, previously regarded as of Gondwana (Fig. 2). contemporaneous with the Chon-Aike Formation (Alric Basic to intermediate dykes are widespread in the NPM, et al., 1996) or younger (Pankhurst et al., 2000). Aragoń but their geochemistry, petrology, and ages are poorly et al. (2000) describe the 161.4 ^ 7.3 Ma alkaline Alvar constrained. In the southwestern corner of the NPM, andesites as lava flows of a sodic alkaline series located in between Sierra de Mamil Choique (SMC; SW Rio Negro the Piedra Parada-Paso del Sapo area in Chubut. Province) and Sierra del Medio (NW Chubut Province), the North–south migration of acidic volcanism associated dykes appear as two parallel WNW belts of dyke swarms. with a western migration of volcanic activity with younger A preliminary geochemical characterization, together with more basic compositions has been proposed (Pankhurst and K/Ar ages, an AMS study, and paleomagnetism, has been Rapela, 1995). New shrimp data of zircons from Jurassic provided only for the dyke swarms of the SMC that make up silicic rocks enabled Pankhurst et al. (1999, 2000) to the northern belt and have an almost 40 km along-strike recognize three distinct magmatic events: V1, V2, and V3. extension (Lo´pez de Luchi and Rapalini, 1997, 1999; V1 is dated at 184 ^ 2 Ma and considered related to a Rapalini and Lo´pez de Luchi, 2000). predominantly extensional period, whereas V2 On the basis of the available information (Lo´pez de (168 ^ 3 Ma) and V3 (153 Ma) reflect the increasing Luchi and Rapalini, 1997, 1999), dyke swarms belong to the M.G. Lo´pez de Luchi, A.E. Rapalini / Journal of South American Earth Sciences 15 (2002) 625–641 627 Fig. 2. Synthesis of the different ages assigned to the formations that have been included within the Chon-Aike Province. Lonco Trapial Formation, which comprises a sequence of Previous regional studies of volcanic rocks from the Lonco lavas, volcaniclastic rocks, and sedimentary intercalations Trapial Formation recognize a subalkaline group for (cf. Canãdoń Asfalto Formation) that extends along the the basic lithologies (Page and Page, 1993) and a sodic western and southern margin of the NPM (Fig. 1). alkaline series for the lava flows that make up the Alvar Intermediate dykes associated with small zones of carbo- andesites (Aragoń et al., 2000). A similar geochemical nitic and siliceous hydrothermal alteration have been noted distinction in the two series was defined for the rhyolites of in Sierra de Taquetreń in Chubut (Pankhurst et al., 1998), the Marifil Formation near Las Plumas (Demichelis et al., and trachyandesites–trachybasalts have been described as 1996). small domes in Las Plumas (Haller, 1981). The Alvar Country rock mainly consists of the intrusive Early andesites are andesites–trachyandesites and trachytes that Carboniferous Mamil Choique granitoids (MCG) (Lo´pez de made up the lava flows in the Paso del Sapo area. This Luchi et al., 1999) that are ductile deformed biotite ^ article presents a petrological study of the SMC dyke hornblende granitoids with a predominant WNW–NW swarms (Fig. 3) to characterize their chemistry, magmatic foliation. The pre-MCG evolution is related to medium- evolution, source, and tectonic setting in relation to the pressure regional metamorphism of a siliciclastic sequence, evolution of northern Patagonia in the Middle Jurassic. with some acidic and basic volcanic thin interlayers. The MCG were emplaced after regional metamorphism. Regional relationships among metamorphism, deformation, 2. Geology and magmatism correspond to a collisional setting; peak metamorphic conditions are synchronous or shortly after the 2.1. General description second deformation, and magmatism postdates regional metamorphism (Cerredo and Lo´pez de Luchi, 1998). The The Middle Jurassic dyke swarms of the SMC (418400 – MCG were intruded by undeformed Permian granitoids 418550S and 708–708330W) are located in the northernmost (Cerredo and Lo´pez de Luchi, 1998). sector of to the central volcanic belt (Page and Page, 1993) There is no stratigraphic control to define a precise in extra-Andean Patagonia and included in the Lonco temporal location in the Sierra because younger Permian Trapial Formation (Lo´pez de Luchi and Rapalini, 1999). granitoids (Cerredo and Lo´pez de Luchi, 1998)are They are exceptionally well exposed along a WNW–NW restricted outcrops separated from the area covered by the belt that cross-cuts the Sierra from its NW corner to Pto. dyke swarms. Antinau and continues toward the SE and in the smaller K/Ar data of 168.4 ^ 3.5 and 172.7 ^ 4.5 Ma for the satellite hills located at the northern border of the range (e.g. dykes indicate a Bajocian-Bathonian magmatic event Loma Guacha hill) (Fig. 3). Dykes are made up of dark coeval with the Lonco Trapial Formation (Lo´pez de Luchi green to greenish gray basaltic and andesitic rocks and some and Rapalini, 1997). The paleomagnetic pole for these reddish gray dacites. According to their mineralogy and dykes is coincident with the paleomagnetic pole of the geochemistry, the dykes constitute two series: subalkaline Chon-Aike Formation in Estancia La Reconquista, Santa and alkaline (Lo´pez de Luchi and Rapalini, 1997, 1999).

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