Anisotropy of magnetic susceptibility study in two classical localities of the Gastre Fault System, central Patagonia

Journal of South American Earth Sciences 30 (2010) 151e166

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Anisotropy of magnetic susceptibility study in two classical localities of the Gastre Fault System, central Patagonia C.B. Zaffarana a, *, M.G. López de luchi b, c, R. Somoza a, b, R. Mercader b, d, R. Giacosa e, R.D. Martino b, f a

Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina c Instituto de Geocronología y Geología Isotópica (INGEIS), Argentina d Departamento de Física, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Argentina e Servicio Geológico Minero Argentino (SEGEMAR), Argentina f Departamento de Geología Básica, Facultad de Ciencias Exactas y Naturales, Universidad de Nacional de Córdoba, Argentina b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 October 2009 Accepted 22 October 2010

The Central Patagonian Batholith is a suite of acid and meso-silicic rocks cropping out in central Patagonia. The emplacement of these rocks has been proposed to be related to the activity of a system of dextral transcurrent faults, the NW-SE Gastre Fault System. This fault system has been ascribed a transcontinental magnitude and a w500 km dextral displacement during Gondwana dismembering in Jurassic times. However, the timing, kinematics and amount of displacement of the Gastre Fault System are still controversial. In this work we have visited two localities which were subject of controversial observations, in order to perform petrographical, microstructural and anisotropy of the magnetic susceptibility studies to contribute to the ongoing discussion. The results mostly agree with the findings von Gosen and Loske (2004) in that rocks spatially and temporally associated to the Gastre Fault System do not show evidence supporting the existence of a major dextral fault system active during Jurassic times. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Gastre Patagonia West Gondwana breakup Transcontinental fault

1. Introduction The Central Patagonian Batholith (Fig. 1) comprises a Late Paleozoic to lower Mesozoic suite of predominantly acid to mesosilicic plutons defining a >100 km, NW-SE trending corridor in the southern part of the North Patagonian Massif (Rapela et al., 1991, 1992; Rapela and Pankhurst, 1992). The Gastre Fault System, originally defined by Coira et al. (1975), was described as a set of NW-SE fractures locally associated to mylonites and cataclasites affecting the Central Patagonian Batholith (Rapela et al., 1991). The kinematics of the Gastre Fault System and its relationship with the granitoids of the Central Patagonian Batholith is subject of debate. Coira et al. (1975) described the regional structure as a conjugate fault system made up by a dominantly NW-SE set of oblique-sinistral faults and a subordinate NE/SW set of extensional faults. Later, Rapela et al. (1991, 1992) suggested that the kinematics in the fault zone indicates NW-SE dextral strike-slip displacement without evidence of regional extension. In contrast, Franzese and Martino (1998) suggested an oblique reverse displacement with

* Corresponding author. E-mail address: [email protected] (C.B. Zaffarana). 0895-9811/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2010.10.003

a sinistral component after studying the same outcrops that of Rapela et al. (1991). Recently, von Gosen and Loske (2004) reported heterogeneous deformation, with rocks in the western sector of the Gastre Fault System recording penetrative shortening with dip-slip displacement. Another aspect that led to opposite interpretations is the relationship between the fault system and the granitoids. Rapela et al. (1991, 1992) considered that the emplacement of the Central Patagonian Batholith is related to the activity of the Gastre Fault System whereas Franzese and Martino (1998) considered that the Gastre Fault System affected previously emplaced epizonal plutons. Different interpretations were given on the nature of the protolith of low-grade metamorphic rocks that host granitoids of the Central Patagonian Batholith at two locations. Proserpio (1978) grouped these rocks in the Paleozoic Calcatapul Formation (Fig. 1), describing this units as composed of low-grade acid to intermediate pyroclastic and volcanic rocks recording dynamic metamorphism and intruded by the Central Patagonian Batholith. In contrast, Rapela et al. (1991) described these outcrops as a complex system of mylonites and ultramylonites developed in Mesozoic, meso-silicic subvolcanic dikes intruding the plutons of the Central Patagonian Batholith. More recently, von Gosen and Loske (2004) revived the Proserpio (1978) scheme considering

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Fig. 1. Geologic map of the Central Patagonina Batholith in the Gastre area, western sector of the North Patagonian Massif, extracted from Zaffarana, C.B. (Ph.D. in prep). Formal name of units compiled from Proserpio (1978) and Rapela et al. (1992). The dashed line encloses the area where the deformation of Gastre Fault System was previously described (Rapela et al., 1991; Rapela and Pankhurst, 1992).

the metavolcanic rocks as an independent unit intruded by granitoid rocks. Thus, there are various aspects of the geology and tectonics in the zone that are subject to debate. Contributing to unravel any of them should be of great interest for models of western Gondwana breakup, because a widely invoked hypothesis considers that hundred kilometers of dextral strike-slip motion along the Gastre Fault System in Early Jurassic times accommodated dispersal of Patagonia and an associated w90
clockwise rotation in the Malvinas-Falkland platform (e.g. Marshall, 1994; Storey et al., 1999; MacDonald et al., 2003; Martin, 2007; Torsvik et al., 2009). In this contribution we present new data from two classical areas of the Central Patagonian Batholith, Estancia (Ea.) Yancamil and Puesto Jaramillo (Fig. 1), which were previously studied by Rapela et al. (1991), Franzese and Martino (1998) and von Gosen and Loske (2004), and all of these three studies reaching contrasting interpretations on the same set of rocks, as mentioned above. With the aim to increase the information from rocks in this critical areas we applied a combination of field and microstructural observations as a diagnostic feature to extract information on temperature and timing of deformation in relation with crystallization, and low field anisotropy of the magnetic susceptibility measurements to determine the internal structure of all of the outcropping rocks. For better interpretation of the latter, hysteresis, isothermal remanent magnetization, backfield and thermomagnetic curves were performed in order to estimate which minerals are contributing to, and eventually controlling the bulk magnetic susceptibility of the rocks. Our results agree with the findings of Proserpio (1978) and von Gosen and Loske (2004) with respect to the presence of the Calcatapul Formation as an individual unit having a volcano-sedimentary

protolith and hosting plutons of the Central Patagonian Batholith. We also agree with von Gosen and Loske (2004) in that outcrops in the observed localities from the Gastre area do not support the presence of a major, dextral strike-slip zone active during Jurassic times. The latter is in conflict with the kinematics hypothesized for the Gastre Fault System in some models of western Gondwana breakup.

2. The Central Patagonian Batholith 2.1. Host rock One of the difficulties when studying the Central Patagonian Batholith in the Gastre area is the almost lack of host rock outcrops. The only rocks intruded by the Central Patagonian Batholith correspond to the Calcatapul Formation (Proserpio, 1978; Nullo, 1978) which is present in just two localities of the western sector of the Sierra de Calcatapul (Fig. 1): Ea. Yancamil (Fig. 1; 3) and Puesto Uribe, the latter being a small outcrop located 20 km NW of Ea. Yancamil. According to Proserpio (1978) and Nullo (1978), the Calcatapul Formation mostly consists of low-grade acid to intermediate pyroclastic and volcanic rocks that were affected by dynamic metamorphism. These authors assigned the unit to the middle Paleozoic. On the contrary, Rapela et al. (1991) did not consider these rocks as the host of the Central Patagonian Batholith but described them as a complex system of dextral mylonites and ultramylonites affecting a system of Mesozoic acid-mesosilicic subvolcanic dikes. They further suggested that the kinematics of deformation observed in these rocks is present in the whole Central Patagonian Batholith.

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More recently, von Gosen and Loske (2004) described the metavolcanic rocks as constituting a unit independent from the granitoids because they observed that it is intruded by them. According to these authors, the Calcatapul Formation in the area of Ea. Yancamil (Fig. 1) is made up by lapilli (mm to cm scale lensshaped, feldspar-rich aggregates), rhyolite and conglomerate layers. The latter contain mm to dm sized pebbles of mudstones, pyroclastic rocks and rhyolites. Fine-grained varieties contain angular rock fragments of mm to cm size (see also Proserpio, 1978). In the southwestern outcrops, thick layers of metavolcanic rocks, originally lava flows, are interlayered with several meters thick, dark phyllitic mud/siltstone. In addition, dm thick lenses of sandstones with quartz-conglomerates occur. 2.2. Late Paleozoic-early Mesozoic magmatism The Late Paleozoic-early Mesozoic magmatism in the Gastre area is represented by granitoids that have been differently grouped. Proserpio (1978) regarded most of the granitoids near Gastre and in the Sierra de Calcatapul as part of the Late Paleozoic Mamil Choique Granitoids (Proserpio, 1978; Nullo, 1978). On the other hand, the pink granites, aplites, microgranites and granite porphyries were considered as part of the Permian or Permo-Triassic Lipetrén Formation (e.g. Proserpio, 1978; Nullo, 1978, 1979; Volkheimer and Lage, 1981; Cucchi, 1993). Later, Rapela et al. (1991, 1992) used geochemistry and Rb-Sr ages to divide the Mamil Choique Granitoids of Proserpio (1978) into the Late Paleozoic Mamil Choique Granitoids and the Late Triassic-Early Jurassic Central Patagonian Batholith. Rapela et al. (1991) defined the latter in the Sierra de Calcatapul, dividing the granodiorites and granites in two suites: the 220  3 Ma (Rb-Sr, whole-rock) “Gastre Suite” (or “superunit”), and the 208  1 Ma (Rb-Sr, whole-rock) “Lipetrén Suite” (or “superunit”), the emplacement of both these suites being interpreted as controlled by the Gastre Fault System. Recently, von Gosen and Loske (2004) got a U-Pb on zircon minimum age of 261  17 Ma (Permian) for a porphyritic biotititic granite which they named “Yancamil granite”. This pluton was previously mapped as part of the Late Triassic Lipetren Superunit by Rapela et al. (1991). The above age-discrepancies point to strong uncertainties about how many plutonic events could be amalgamated in the Central Patagonian Batholith. For this paper we use the names of units proposed by Rapela et al. (1991). However, for mapping we apply the criterion of C. B. Zaffarana (PhD thesis, in preparation) where the Gastre suite is characterized by widespread evidence for magma mingling, a process that is not observed in the Lipetren suite (Fig. 1). These different criteria led to some conflicts when mapping of the Central Patagonian Batholith, as may be illustrated regarding outcrops in Estancia Yancamil (Fig. 3). Rapela et al. (1991) assigned all of the plutonic outcrops in this locality to their w208 Ma (Rb-Sr) Lipetren Superunit. In contrast, von Gosen and Loske separated these outcrops into the foliated, w260 Ma (U-Pb) Yancamil granite and “undeformed” granites that they assigned to the Triassic Lipetren Superunit. We see no compositional differences between the foliated and “undeformed” granites, so all of them are mapped into a single unit in this paper (Figs. 1 and 3), in agreement with early work of Proserpio (1978). In fact, available data are ambiguous in resolving both how many magmatic pulses formed the Gastre suite of Rapela et al. (1991) and which the ages of these possible magmatic events are. Further isotopic dating would be necessary to asses the timing of formation of the Central Patagonian Batholith. 3. Methodology In order to evaluate the history of deformation of the studied localities, field and microstructural observations were performed

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on a collection of 51 thin sections, with those from outcrops having visible tectonic foliation being orientated according to planar and, whatever possible, linear structures observed in the field. Temperature of acquisition of both fabrics and microstructures was estimated according to the extensive literature available on the subject (see references within Passchier and Trouw, 2005 and Vernon, 2004). Anisotropy of the magnetic susceptibility measurements were performed in order to further explore the internal structure of the rocks. Anisotropy of the magnetic susceptibility is a widely accepted technique and a number of studies have investigated the relationship between the anisotropy of the magnetic susceptibility and the strain ellipsoids (e.g. Kligfield et al., 1981; Borradaile and Mothershill, 1984; Rochette et al., 1992; Hrouda, 1993; Borradaile and Henry, 1997). Anisotropy of the magnetic susceptibility is today a standard technique for petrofabric studies in granitoids (e.g.Archanjo et al., 1995, 2002; Ferré and Améglio, 2000; McNulty et al., 2000; Neves et al.,  et al., 2005; D’Eramo et al., 2006; Stevenson et al., 2007; 2003; Zák and many others), having outstanding relevance in determination of magmatic lineations, which are hard to measure in the field or in the laboratory using the microscope (Bouchez et al., 1981). The three principal axes of the anisotropy of the magnetic susceptibility ellipsoid (K1  K2  K3) define, when K1 > K2, a magnetic lineation parallel to K1, and a magnetic foliation (when K2 > K3) as the plane containing K1 and K2, with K3 being the pole to foliation. The common relationship between the anisotropy of the magnetic susceptibility ellipsoid and petrofabric shows the magnetic lineation parallel to the structural lineation (stretching or flow) and the magnetic foliation parallel to the structural foliation (flattening or flow). Sometimes this relationship may be obliterated by superposition of fabrics or by a particular mineralogy (e.g. Rochette et al., 1992, 1999; Borradaile and Henry, 1997). Hysteresis, isothermal remanent magnetization and backfield curves were used to investigate the magnetic mineralogy and to estimate the relative contribution of the paramagnetic and ferromagnetic phases in representative samples. Hysteresis parameters such as coercive force Hc, saturation magnetization Ms and remanent magnetization Mr, together with the remanent coercive force Hcr taken from backfield measurements were used to estimate the domain state of magnetite following the relationships of Day et al. (1977). Temperature variation of bulk magnetic susceptibility (thermomagnetic curves) was also applied to further investigate the magnetic properties of rock minerals (e.g. Hrouda, 2003, 2010, among others). This study reports anisotropy of the magnetic susceptibility results from 11 sites (Table 1), each of them comprising several samples distributed in 100e1000 m2 area of outcrops of a certain lithology. Four sites belong to Puesto Jaramillo, from which one represents widespread (this locality) biotite-hornblende granitoids and the remainder three represent mafic microdioritic dikes. Seven sites arise from Ea. Yancamil area, three of them representing the Calcatapul Formation and four of them representing the porphyritic biotitic granite and its intruding leucocratic dikes (Fig. 3). Whenever possible, either or both planar and linear structures were determined in the field in order to compare with the magnetic fabric. Anisotropy of the magnetic susceptibility measurements on 75 cylindrical specimens were performed by using a MFK1-B Kappabridge susceptibilimeter. Anisotropy of the magnetic susceptibility susceptibility ellipsoids (with principal axes K1 > K2 > K3) were calculated from a minimum of five specimens per site using matrix averaging routines (Jelinek, 1978) with the programs ANISOFT 4.2 (written by M. Chadima and V. Jelinek, 2008; www.agico.com). Hysteresis curves and isothermal remanent magnetization e backfield analyses were performed in representative specimens using a Lakeshore 7404 Vibrating Sample Magnetometer (VSM)

Fig. 2. Puesto Jaramillo outcrops and representative thin sections. A) Magmatic foliation in the biotite-hornblende granitoids of the Central Patagonian Batholith having NW-SE strike and subvertical inclination defined by parallelism of plagioclase, biotite and hornblende and by elonged mafic microgranular enclaves; B) Magmatic texture in thin section shown by subhedral plagioclase surrounded by quartz and moderately chloritized mafic minerals; C) High-temperature deformation in the biotite-hornblende granitoids given by chessboard texture in quartz; D) NW-SE subvertical narrow milonitic belts developed in the biotite-hornblende granitoids E) Incipient ribbon-structure defined by elongate quartz subgrains in contact with plagioclase; F) Microdioritic mafic dike of NE-SW strike (site “Dike 1” see Table 1) intruding the biotite-hornblende granitoids. Note the magmatic NW-SE foliation in the granitoid cutting the dike wall. Qtz ¼ quartz, Pl ¼ plagioclase, Chl ¼ chlorite.

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Fig. 3. Inbox showing the visited outcrops in the Ea. Yancamil area, numbered from 1 to 5. The anisotropy of the magnetic susceptibility stereograms from Ea. Yancamil sites (lower hemisphere equal area projections) are shown here to have an idea of the areal distribution of magnetic fabrics (stereograms A, B, C and D belong to outcrop 1, E belongs to outcrop 4 and F to outcrop 5). A) Metavolcanites of Calcatapul Formation; B) Low-temperature deformed porphyritic biotitic granite; C) Leucocratic porphyritic dikes intruding the porphyritic biotitic granite; D) Undeformed porphyritic biotitic granite; E) Undeformed porphyritic biotitic granite further southeast; F) Metavolcanites of Calcatapul Formation further SE.

with an applied field up to 1.8 T. Thermomagnetic curves were obtained from the same representative specimens with a CS4 High Temperature Furnace Apparatus and a CSL Low Temperature Cryostat Apparatus in connection with the MFK1-B Kappabridge susceptibilimeter. The program CUREVAL 8 (Chadima and Jelinek, 2008; www.agico.com) was used for data processing and graphical representation of the thermomagnetic curves. 4. Results 4.1. Petrography and microstructures 4.1.1. Puesto Jaramillo locality Undated biotite-hornblende granitoids cropping out in Puesto Jaramillo (Fig. 1) were first mapped as the Late Paleozoic Mamil Choique granitoids (Proserpio, 1978) and later were included in the Triassic Gastre Superunit of Rapela et al. (1991, 1992). The local suite is composed of biotite-hornblende granitoids (mostly granodiorites

with subordinate granites and monzodiorites) with mafic microgranular enclaves and intruding microdioritic dikes (Fig. 2). The biotite-hornblende granitoids are mostly medium-grained granodiorites (and also granites and monzodiorites) composed of plagioclase (40e60%), quartz (15e35%), microcline (15e20%), hornblende (5e8%), biotite (2e5%), primary sphene (1e2%) and opaque minerals (Fig. 2A). Subvertical, NW-SE trending magmatic foliation (Fig. 2A,F) is defined by parallel alignment of plagioclase, biotite and amphibole crystals and also by elongated mafic microgranular enclaves. Plagioclase of these biotite-hornblende granitoids is mostly represented by An14 to An30 oligoclase (optical determinations, Fig. 2B,C). Some plagioclase individuals may form nearly 3 cm long, tabular megacrystals showing very sharp boundaries between the more-altered, calcic core and the acidic fresh rim. The smaller plagioclase individuals also have the same zonation pattern, having myrmekite lobules when in contact with microcline. Biotite and hornblende inclusions are common too. Microcline has its typical

Table 1 Anisotropy of the magnetic susceptibility results. Lithology: B-H G is biotite-hornblende granitoids; MD NE is microdiorite dike trending NE, MD NW is microdiorite dike trending NW; CF mv is Calcatapul Formation metavolcanites (followed by site number, Fig. 3); D P-BtG is “deformed” porphyritic biotite granite; U P-BtG is “undeformed” porphyritic biotite granite, LD is leucocratic dikes. Km (St.Dev) is mean magnetic susceptibility (standard deviation), L is mean lineation, F is mean foliation, P’ is corrected anisotropy degree (Jelinek, 1981), T is shape parameter (Jelinek, 1981) where 0 < T < 1 indicates oblateness and 1 < T < 0 indicates prolateness, Dec and Inc are declination and inclination of downward direction. Site

JARA 1 D1 D2 D3 CALCA 1 CALCA 2 CALCA 3 YANCA 1 YANCA 2 YANCA 3 YANCA 4

Lithology

B-H G MD NE MD NE MD NW CF mv O1 CF mv O1 CF mv O5 D P-BtG O1 U P-BtG O1 LD O1 U P-BtG O4

Km E3SI

21.0 23.2 20.8 7.2 1.4 5.2 1.4 0.9 0.6 0.5 0.7

St.Dev. E3SI

11.4 10.4 14.5 3.4 2.0 6.7 0.5 1.8 0.2 0.6 0.5

L

1.090 1.034 1.034 1.026 1.057 1.038 1.021 1.021 1.026 1.013 1.022

F

1.057 1.119 1.130 1.090 1.051 1.099 1.056 1.043 1.080 1.044 1.013

P’

1.153 1.165 1.179 1.125 1.111 1.145 1.081 1.066 1.113 1.061 1.035

T

0.215 0.547 0.566 0.544 0.049 0.429 0.440 0.343 0.507 0.530 0.256

Autovalues

K1

K1

K3

Decl

Incl

Decl

K3 Incl

1.076 1.059 1.063 1.046 1.054 1.056 1.032 1.028 1.043 1.023 1.019

0.938 0.916 0.909 0.935 0.949 0.926 0.957 0.965 0.941 0.967 0.984

326.7 310.3 311.9 330.6 5.7 207.4 338.9 299.0 358.4 267.1 48.5

12.0 39.9 38.4 17.6 85.7 83.5 9.1 57.0 73.0 65.7 20.0

230.1 207.8 210.8 231.2 225.5 56.3 71.1 55.0 223.2 55.1 183.4

28.5 14.5 13.7 27.2 3.3 5.7 13.6 15.0 12.3 21.0 62.7

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cross-hatched twinning and is anhedral. It contains plagioclase and quartz inclusions. Flame perthites are common, as well as moderate alteration. Quartz is interstitial. Mafic and accessory minerals occur in clots, and are represented by hornblende, biotite, sphene, opaque minerals, zircon and apatite. Hornblende is fresh and pleochroic from light to dark green. Biotite is pleochroic from light to dark brown; it is strongly altered to chlorites and secondary irregular grains of sphene. Primary sphene usually occurs in euhedral crystals that have an acute rhombic cross section. Opaque minerals are present in two different size fractions: a coarse-grained fraction as independent crystals and a finer-grained fraction formed from hornblende alteration. The mafic microgranular enclaves have dioritic composition. Euhedral and zonal plagioclase (60%) and hornblende (20%) define the enclave mineral network whose interstices are filled by the late crystallization of quartz (10%) and orthoclase (5%). There is also a 5% made up by apatite, biotite altered to chlorite, allanite (pleochroic from red to incolore) and opaque minerals. Hornblende prisms are subparallel to the enclave borders. In general, foliation within the enclaves is parallel to magmatic foliation of the host rock, suggesting that different rock types had similar rheology at the time of fabric acquisition. The magmatic fabric of the biotite-hornblende granitoids is overprinted by high-temperature subsolidus deformation as indicated by chessboard subgrains in quartz (Fig. 2C), and hightemperature grain-boundary migration in quartz and feldspars. Locally, a superimposed low-temperature deformation phase is responsible for the development of discrete, NW-SE subvertical mylonitic belts of almost 1 m width (Fig. 2D). The mylonite development was associated to lower greenschist facies conditions leading to dynamic recrystallization by bulging in quartz (Fig. 2E), micro-kinking in plagioclase, alteration and retrogradation of microcline-plagioclase to white micas and of biotite to chloriteopaque minerals. The biotite-hornblende granitoids are intruded by four w2 m width microdioritic dikes (Fig. 2F). Three of them have a NE/SW (wN30
E) trend, whereas the fourth dike’s strike is NW-SE (and so is parallel to the magmatic and subsolidus foliation in the host rock). The dikes have mostly aphyric to porphyric texture (phenocrysts, when present, are represented by plagioclase and hornblende) and have a general composition given by hornblende (45%), plagioclase (40%), opaque minerals (10%) and quartz (5%). Plagioclase is the main primary mineral that is present in the groundmass. The dikes are affected by an intense alteration to abundant secondary chlorite, green amphibole and sphene, which involves both the phenocrysts and the groundmass. Interstitial quartz shows bulging recrystallization and has undulose extinction. These characteristics and anisotropy of the magnetic susceptibility data (see below) indicate that these dikes were affected by weak solid-state deformation responsible for the development of a mineral association compatible with lower greenschist facies. 4.1.2. Ea. Yancamil locality Ea. Yancamil is located in the southern slope of the Sierra de Calcatapul, and is one of the two places where the Calcatapul Formation crops out (Proserpio, 1978; Fig. 1). In this area there are intrusive rocks in sharp contact with the metavolcanic rocks of Calcatapul Formation. The intrusives consist of an equigranular granodiorite and a porphyritic biotitic granite intruded by leucocratic porphyritic dikes (Fig. 3). von Gosen and Loske (2004) recognized two deformations in the area: a penetrative D1 deformation at lower greenschist facies metamorphism that affected Calcatapul Formation and the porphyritic biotitic granite (“Yancamil Granite” for the mentioned authors) but which did not affect the Triassic Lipetren suite, and a later D2 brittle deformation which

affected all the granitoids in the area. In this work we confirm many of the observations of von Gosen and Loske (2004) adding more field, microstructural and anisotropy of the magnetic susceptibility data from these and other two additional outcrops in the area (Fig. 3). These two additional outcrops consist of a sector where the porphyritic biotitic granite (Yancamil granite of von Gosen and Loske, 2004) looks undeformed and an outcrop of a small slice of an equigranular granodiorite intruding Calcatapul Formation metavolcanites NE of Ea. Yancamil. 4.1.2.1. Calcatapul Formation. The Calcatapul Formation at Estancia Yancamil consists of a w700 m thick succession of alternating acidmesosilicic volcanic, pyroclastic and sedimentary protoliths metamorphosed to lower greenschist facies. As described by von Gosen and Loske (2004), the unit shows NW-SE subvertical planar fabric (S1) developed at very low- to negligible angle with respect to S0, the latter as defined by the interlayering of volcanic and sedimentary protoliths. Subordinately with respect to the dominant parallelism between the primary (S0) and the tectonic (S1) foliation planes, cross-cutting relationships between them may be observed in some places (e.g. Fig. 4A). Apparently there is no tectonic repetition of layers. As von Gosen and Loske (2004) pointed out, determination of stratigraphy in this vertical succession is difficult because of no preservation of primary sedimentary structures. However, close to the contact with the Yancamil biotite granite (see below) we observed a particularly low strained lava characterized by the occurrence of minute (half cm scale), randomly oriented plagioclase prisms. Farther to the north across the section we observed a conglomerate carrying volcanic pebbles identical to that peculiar lava (Fig. 4B). This isolated observation suggests that the Calcatapul Formation at Estancia Yancamil may be a top-to-the-north volcano-sedimentary succession. The metapyroclastic rocks (Fig. 4D) preserve lapilli fragments which consist of small violet lenses composed of recrystallized quartz and feldspar. These rocks alternate with more strongly foliated and lineated light-gray rocks, which resemble low-grade metapelites (phyllites). Their metamorphic foliation is remarkable parallel, locally anastomosing and describing microfolds. Metavolcanic rocks, probably metarhyolites and/or metadacites, exhibit a banded appearance and show quartz and alkaline feldspar porphyroclasts (sometimes up to 1 cm long) which stick out in the aphanitic groundmass. Some dark metalavas intercalated in the section show an almost undeformed appearance. Metaconglomerates bear sometimes rounded, sometimes angular boulders of dominantly volcanic rocks of different compositions and sizes (Fig. 4B,C). In thin section, most of the acid to meso-silicic metavolcanic and metapyroclastic rocks show penetrative deformation that is characterized by microstructures dominated by bulging and subgrainrotation recrystallization (Fig. 5). Phenocrysts are represented by plagioclase, orthoclase and quartz in variable proportions (Fig. 5). Euhedral prismatic feldspar crystals are fresh, unfractured or slightly fragmented; plagioclase is normally zoned and has polysynthetic twinning (Fig. 5AeC), whereas orthoclase is perthitic. In some samples, feldspar phenocrysts are altered to fine-grained aggregates composed of sericite and clays, and have indented and rounded borders due to recrystallization. Quartz phenocrysts have undulose extinction and sometimes preserve sharp borders and show embayments, forming “quartz eyes”, which are common in deformed-recrystallized volcanic sequences (Fig. 5E, Vernon, 2004). Phenocrysts rotate within the dynamically recrystallized matrix and are partially rimmed by strain shadows (Fig. 5A,B,D and G), sometimes composed of fibrous aggregates of quartz and sericite. Groundmass is composed of quartz, sericite and chlorite.

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Fig. 4. Outcrops in Ea. Yancamil area. A) In spite of widespread parallelism between S0 and S1 in the Calcatapul Formation, cross-cutting relationship is observed in some places. In the photo, metavolcanites with the plane of outcrop roughly perpendicular to both S0 and S1. View to the SE; B) and C) Metavolconglomerates within Calcatapul Formation with clasts of volcanic origin and different sizes; D) Metapyroclastic rocks.

Fig. 5. Calcatapul Formation in thin section. A) Zonal plagioclase porphyroclasts immersed in recrystallized matrix. Notice the concentration of mica (forming local P domain) against the strong porphyroclast; B) Plagioclase porphyroclast surrounded by strain shadows show a sigma shape which points to NE-block-up sense of shear. The section is the plane perpendicular to the foliation that contains the lineation; C) Basic metavolcanite with phenocrysts of plagioclase; D) Altered feldspar porphyroclast (d-object) in finer-grained sericite and quartz recrystallized matrix; E) Feldspar and quartz porphyroclast in quartz-sericite recrystallized matrix; F) epidote and quartz (top) replacing a former mafic mineral and larger-grained quartz lense (fiamme?); G) Imbricated plagioclase porphyroclast (domino texture) in metavolcanite. Pl ¼ plagioclase, Qtz ¼ quartz.

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Groundmass quartz shows bulging recrystallization in their intergranular surfaces. Heterogeneously developed planar fabric in the groundmass is defined by insoluble opaque minerals that concentrate in foliation planes, by dynamically recrystallized quartz and by plagioclase phenocrystals fractured in “bookshelf” manner (constituting asymmetric boudins) and altered to epidote (Fig. 5G). Boudin necks are filled by fibrous quartz. In the metapyroclastic rocks, yards and pumice fragments can still be recognized in the massive groundmass. Lenticular aggregates mainly formed by quartz stick out in the matrix because of their coarser grain and could be interpreted as recrystallized fiammes (Fig. 5F) defining a weak primary foliation plane (S0). The only textural change observed in the metapyroclastic rocks was the recrystallization of their matrix, which probably was former glass. The metavolcanites show evidences of bulging recrystallization in response to variable dislocation concentration which forms small, strain-free grains; subgrain-rotation recrystallization in quartz; incipient bulging recrystallization in feldspars; and “bookshelf” microfracturing in feldspars. These processes occur at relatively low to medium temperature conditions (300e500
C) and/or fast strain rates (Passchier and Trouw, 2005; Vernon, 2004). The studied rocks possibly record conditions compatible with the lower bound in the above mentioned temperature range. Feldspar s-objects in the metavolcanites indicate an uplift of the northeastern bock parallel to the steep lineation (Fig. 5B), in agreement with the kinematic observations made by von Gosen and Loske (2004). 4.1.2.2. Magmatic rocks. The Calcatapul Formation at Ea. Yancamil is intruded by both a medium to coarse-grained, slightly porphyritic biotitic granite (the “Yancamil granite” of von Gosen and Loske, 2004, see Fig. 6) and an equigranular granodiorite. The porphyritic biotitic granite is broadly composed of quartz (35e40%), up to 2 cm orthoclase megacrystals partially inverted to microcline (35e40%), plagioclase (25e15%) and a 5% compositional fraction shared by biotite, opaque minerals and apatite. Variably trending leucocratic porphyritic dykes cut across the porphyritic biotitic granite intrusion (Fig. 6A) and the steeply dipping succession of the Calcatapul Formation. Structural observations come out from four outcrops (Fig. 3). The porphyritic biotitic granite in outcrop 1 (the main outcrop from where von Gosen and Loske’s observations come out) shows a NWSE subvertical magmatic foliation (Fig. 6A) defined by shape preferred orientation of mostly subhedral K-feldspar megacrystals and by chlorite and biotite flakes. Locally, particularly near the contact with the host rock, the granite is strongly sheared, with development of NW-SE, subvertical to moderately dipping shear planes (Fig. 6B) with moderate (w40
) to steep lineations. The deformed porphyritic biotitic granite of outcrop 1 (Fig. 3) is characterized by subhedral microcline which may form perthitic megacrysts (Fig. 6C, D) with inclusions of biotite and of rounded, undeformed quartz. There are also anhedral quartz individuals with undulose to chessboard extinction. Sometimes quartz may be flattened and forms an incipient ribbon-structure (Fig. 6D). Microcline tends to form incipient subgrains. Biotite is pleochroic from light to dark brown and is subhedral. Subhedral plagioclase is polysynthetically twinned and normally zoned. Myrmekite lobules project onto microcline from the mostly unzoned rims (Fig. 6E). Plagioclase is fractured and kink bands develop too. Epidote masses are common and might come from the alteration of a former mafic mineral. The porphyritic biotitic granite is intruded by fine-grained, 5e50 cm thick, possibly late-magmatic leucocratic dikes which usually show pegmatoid miaroles. The dikes are composed of quartz (40%), microcline (35%), plagioclase (20%) and a 5% shared by biotite, apatite and opaque minerals. Quartz is strongly deformed,

showing large flattened grains with undulose extinction which are traversed by bands of smaller quartz subgrains formed by subgrainrotation recrystallization (Fig. 6F). Microcline is slightly megacrystic and has flame perthites. It tends to form small incipient subgrains and is intensely altered to sericite. Myrmekite lobules are present in the contact between plagioclase and microcline. Plagioclase has polysynthetic twinning and brittly fractured showing domino texture (Fig. 6F). Biotite is almost completely altered to sericite and opaque minerals. Northeast of Ea. Yancamil (outcrop 2, Fig. 3) a small slice of an equigranular granodiorite intrudes the Calcatapul Formation. The rock is composed of plagioclase (40%), quartz (40%), microcline (15%) and a subordinate fraction (5%) formed by opaque minerals, epidote and apatite. This small outcrop registers more intense low-temperature deformation than the porphyritic biotitic granite of outcrop 1. Quartz subgrains having internal deformation form elongated subgrains, and feldspars with undulose extinction form porphyroclasts immersed in a fine-grained matrix composed of quartz and micas (Fig. 7)A. Porphyroclasts are traversed by bands of bulging recrystallized quartz subgrains. Plagioclase porphyroclasts exhibit domino texture and microkinks (Fig. 7B), whereas alkaline feldspar ones have undulose extinction and are partially microclinized. In outcrop 3 (w800 m south of Ea, Yancamil, see Fig. 3), the porphyritic biotitic granite shows a S1 plane dipping 45
towards the N10
E and a stretching lineation dipping 37
towards the NW, whereas the adjacent outcrop of the Calcatapul Formation shows a S1 plane dipping 82
towards the N50
E and a subvertical stretching lineation (Fig. 8A). The leucocratic dikes close to this outcrop show the same NW-SE planar fabric as the porphyritic biotitic granite, and in some cases they are sinistrally displaced along the NW-SE foliation planes of the granite (Fig. 8B). This is a different relationship than the observed between the granite and its dikes in outcrop 1. Insofar, we have described the porphyritic biotitic granite with magmatic and subsolidus high- and low-temperature deformation. However, this granite may appear tectonically undeformed in outcrops 1 and 4 (Fig. 3). In these outcrops, the perthitic orthoclase can be subhedral (when in megacrystals) or anhedral, enclosing other minerals. Plagioclase exhibits polysinthetic twinning and quartz is anhedral and has clear extinction. The rock has suffered an intense alteration process, responsible for the alteration of mafic minerals (biotite retrogradation to chlorite, epidote and opaque minerals) and feldspars (orthose altered to clays and plagioclase altered to sericite and clays). 4.2. Magnetic mineralogy and anisotropy of the magnetic susceptibility results 4.2.1. Puesto Jaramillo locality Rock magnetic data indicate that the magnetic susceptibility in both the biotite-hornblende granitoids and the microdioritic dikes is controlled by magnetite with a very subordinate participation of a paramagnetic fraction (Fig. 9AeE). The Day plot (Day et al., 1977, see Fig. 10) points out that the magnetite size in the biotite-hornblende granitoids and in the microdioritic dikes is multidomain and pseudo-single domain, respectively, the latter corresponding to very small multidomain grains. No evidence for single domain magnetite was found. The thermomagnetic curves of the dikes and the biotitehornblende granitoids show a sharp decrease in susceptibility near 575
C revealing the presence of Ti poor magnetite (Fig. 9FeI). The sample from Dike 3 shows a clear Hopkinson peak (Fig. 9H), which together with the absence of the Verwey transition be a characteristic of either or both single domain and/or pseudo-single domain magnetite, the latter is preferred regarding the results in

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159

Fig. 6. Outcrops and thin sections of the magmatic rocks of outcrop 1 (see Fig. 3) in Ea. Yancamil area. A) Yancamil biotite granite intruded by leucocratic dike. Penetrative NW-SE subvertical foliation traverses both lithologies; B) Localized subvertical shear zone of NW-SE strike traversing Yancamil biotite granite; C) Fractured microcline megacrystal surrounded by recrystallized quartz interspersed with sericite; D) Flattened quartz forming “ribbon microstructure” against more resistant microcline individuals; E) Myrmekite in contact with microcline in the porphyritic biotitic granite; F) Strongly deformed leucocratic porphyritic dike showing domino texture in plagioclase and quartz subgrains. Mc ¼ microcline, Qtz ¼ quartz, Pl ¼ plagioclase.

Fig. 7. Thin section of the granodiorite intruding Calcatapul Formation in outcrop 2 (see Fig. 3). A) Highly-deformed granitoid with foliation planes defined by white mica and quartz and more resistant elongate quartz subgrains; B) Fractured plagioclase and deformed quartz. Qtz ¼ quartz, Pl ¼ plagioclase.

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Fig. 8. Observations made in outcrop 3 in Ea. Yancamil area (see Fig. 3). A) Heterogeneous deformation in the contact with Calcatapul Formation showing a foliation plane strike/ dip ¼ 320
E/82
NE and a lineation plunging 82
to the 50
E, whereas the porphyritic biotitic granite dippinwith in Calcatapul Formation and the porphyritic biotitic granite shows a foliation plane strike/dip ¼ 280
E/45
NNE and a lineation plunging 37
to the 326
E; B) Dike intruding porphyritic biotitic granite offset by a sinistral component of displacement in the S1 foliation.

the Day et al. (1977) plot. The other two dikes do not show a clear Hopkinson peak (even though magnetic susceptibility rises a little before the decrease at the Curie temperature), and show marked Verwey transitions at low-temperatures, suggesting that multidomain magnetite can be the main ferromagnetic phase in these rocks (Fig. 9F,G). The complete absence of the Hopkinson peak and the presence of a Verwey transition in the biotite-honblende granitoid thermomagnetic curve clearly indicate that multidomain magnetite is the main ferromagnetic phase in these rocks either (Fig. 9I).

In addition, the heating curves of the four studied samples (especially the sample from Dike 3, Fig. 9H) display a clear bump at 270e300
C which is followed by a distinct drop in the magnetic susceptibility. This behaviour can be interpreted as two different possibilities: first, as the superposition of the Hopkinson peaks of fine-grained Ti richer magnetite with smaller Curie temperature because of their higher titanium content, or secondly, as the thermally-induced conversion of metastable cubic maghemite (g-Fe2O3) to weakly magnetic rombohedral magnetite (a-Fe2O3) (Dunlop and Ozdemir, 1997; Zhu et al., 1999; Deng et al., 2000).

Fig. 9. Rock magnetism analyses of Puesto Jaramillo area. A) Hysteresis curve carried out on in the biotite-hornblende granitoids; B) Hysteresis curve carried out on dike 1; C) Hysteresis curve carried out on dike 2; D) Hysteresis curve carried out on dike 3; E) Representative normalized curves of acquisition of isothermal remanent magnetization of the samples of Puesto Jaramillo and Ea. Yancamil; F) Thermomagnetic curve of dike 1; G) Thermomagnetic curve of dike 2; H) Thermomagnetic curve of dike 3; I) Thermomagnetic curve of the biotite-hornblende granitoids.

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Fig. 10. Day plot magnetic grain-size (Day et al., 1977) for the representative samples from Puesto Jaramillo and Ea. Yancamil. SD ¼ single domain, PSD ¼ pseudo-single domain, MD ¼ multidomain. Pseudo-single domain (PSD) grains are those above the 70e100 nm threshold size (Williams and Wright, 1998), and are characterized by showing a hard (SD like) paleomagnetic behaviour and by providing normal magnetic ellipsoid (like multidomain grains).

161

Averaged bulk magnetic susceptibility (Km) from the biotitehornblende granitoids (site JARA 1) varies from 38  103 to 0.56  103 SI, the lower values corresponding to the rocks deformed at lower greenschist facies. There is a direct relationship between Km and the corrected anisotropy degree (P’, Jelinek, 1981) indicating that a higher magnetic anisotropy is related with higher magnetite content (Fig. 11A). Magnetic lineation is slightly more developed than magnetic foliation because the linear component in magnetic ellipsoids is the better defined in both sample and site level (T < 0, see Table 1 and Fig. 11B). Nevertheless, a good planar component is observed because the site-averaged magnetic ellipsoid is largely triaxial (Fig. 11C). The magnetic fabric of the biotite-hornblende granitoids of Puesto Jaramillo is characterized by NW-SE magnetic foliation plane (parallel to the magmatic foliations observed in the field) and subhorizontal magnetic lineations (Fig. 11C). Samples from the NE trending dikes (sites D1 and D2) provide Km of w20  103 SI, whereas those from the NW trending dike (site D3) yield 7  103 SI (see Table 1). There is not evident relationship between Km and P’ in these dikes (Fig. 11A). Anisotropy of the magnetic susceptibility ellipsoids from the three dikes are dominated by the planar component at both sample and site level (Fig. 11B and Table 1), although a well defined lineation produces triaxial ellipsoids sites (Fig. 11D, E). The magnetic fabric from all of the studied dikes is almost parallel to the one observed in the hosting biotite-hornblende granitoids (Fig. 11CeE). It is noticeable that the magnetic foliation from the NESW trending dikes is strongly discordant with respect to the dike walls (Fig. 2F), contrasting with the parallel-to-the-dike-wall

Fig. 11. Anisotropy of the magnetic susceptibility of Puesto Jaramillo sites (scalar and vectorial data, anisotropy of the magnetic susceptibility stereograms are lower hemisphere equal area projections). A) Medium susceptibility versus Jelinek’s mean anisotropy degree diagram (Km vs. P’); B) Jelinek’s corrected anisotropy degree (P0 ) versus shape factor (T) parameter diagram; C) Stereograms of biotite-hornblende granitoids; D) Stereogram of dike 1 showing the trend of the dike and the magmatic foliation in the host rock (see Fig. 2F); E) Stereogram of dike 3; F) Primary titanomagnetite in mafic dikes showing incipient hematite-ilmenite oxi-exolution lamellae.

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magnetic foliation that usually characterizes magmatic fabric in dikes (Knight and Walker, 1988; Rochette et al., 1992, 1999; Borradaile and Gauthier, 2003; Chadima et al., 2009). Rock magnetism (see above) indicates that this feature can not be attributed to inverse fabric related to the presence SD magnetite. Petrographic observations of polished sections reveal that magnetite in these dikes occurs as: 1) primary, euhedral titanomagnetite grains with variable development of hematite-ilmenite oxi-exolution lamellae (Fig. 11F), and 2) minute crystals disseminated in the rocks mass. The origin of the magnetic fabric in the NE trending dikes is discussed below. 4.2.2. Estancia Yancamil locality 4.2.2.1. Calcatapul Formation. Three sites were studied from Calcatapul Fm., two of them (CALCA 1 and CALCA 2) belong to outcrop 1 and the third (CALCA 3) belongs to outcrop 5 (Fig. 3 and Table 1). Bulk magnetic susceptibility of samples from this unit varies from 20  103 to 0.1  103 SI for the basic and acid metavolcanites, respectively (see Table 1). A combination of composition and strain determines the degree of anisotropy in site CALCA 2, where strongly deformed metalapilli and metapiroclastites having Km w3  103 SI show much higher P’ values (20e30%) than those shown by basic metalavas without observable cleavage having Km between 5 and 20  103 SI and P’ w 12%; whereas metapiroclastites with the lower values of susceptibility (w0.15  103 SI) show the lower values of P’ (between 2 and 5%). Magnetic ellipsoids from Calcatapul sites are triaxial, so both magnetic foliation and lineation are well defined (Fig. 3A,F). In spite of the almost undeformed appearance of the mafic metalavas (e.g. see boulder in metaconglomerate in Fig. 4B) their anisotropy of the magnetic susceptibility results show a well defined magnetofabric with their axes parallel one to one to those seen in the magnetic ellipsoids from the strongly foliated metapyroclastic rocks. The NW-SE, subvertical magnetic foliation (Fig. 3A) corresponds very well with the S1 measured in the field, indicating that the magnetic fabric has relation with the strain ellipsoid. Therefore the observed magnetic lineation indicates the stretching direction in these rocks. As was observed by von Gosen and Loske (2004), the lineation is subvertical in samples near the porphyritic biotitic granite in outcrop 1 (Fig. 3A). However, we found suhhorizontal lineation in samples of outcrop 5 sited farther SE (in site CALCA 3, Fig. 3F), confirming poorly defined subhorizontal lineation observed from barely aligned sericite and pressure shadows in clasts. 4.2.2.2. Magmatic rocks. The anisotropy of the magnetic susceptibility of the porphyritic biotitic granite and its leucocratic dikes was studied in outcrop 1 (sites YANCA 1 and 2) and in outcrop 4 (site YANCA 4) (Figure INBOX YANCA JARA). Typical values of Km from the porphyritic biotitic granite range from w0.1 to w1.2  103 (Table 1), so from these values could be inferred to that the anisotropy of the magnetic susceptibility is likely controlled by both ferromagnetic and paramagnetic minerals (eg. Hrouda, 2010). Magnetic hysteresis, isothermal remanent magnetization and thermomagnetic curves were performed on a representative sample from the deformed porphyritic granite of outcrop 1. In the hysteresis loop it can be seen that the paramagnetic fraction (likely dominated by biotite according to petrography) is very important in controlling the induced magnetization (Fig. 12C). The fast saturation before 200 mT in the isothermal remanent magnetization curve indicates that the apparently subordinate ferromagnetic fraction is magnetite (Fig. 9E). In the Day plot (Day et al., 1977; Fig. 10) it can be observed that it is multidomain magnetite, allowing us to discard the possibility of inverse fabric for this mineral fraction, so both biotite and magnetite may have roughly coaxial ellipsoids if they are cogenetic.

The thermomagnetic curve also show the superimposed effects of both magnetite and paramagnetic minerals, with the Curie temperature of w580
C indicating the presence of Ti poor magnetite, and the hyperbolic variation of susceptibility with temperature being typical of paramagnetic minerals (Fig. 12D). Averaged bulk magnetic susceptibility varies between 0.9 and 0.6  103 SI for the porphyritic biotitic granite and is around 0.5  103 SI for the leucocratic dikes (see Table 1). There are apparent differences in the relationship between Km and P’ from sites of the porphyritic biotitic granite (Fig. 12E), with site YANCA 1 showing a rather uniform Km (excepting one sample) and variable P’, and site YANCA 2 showing the opposite, a rather uniform P’ and variable Km, whereas a slight direct relation between Km and P’ is observed in site YANCA 4. This may suggest that degree of anisotropy could reflect strain in the site YANCA 1 whereas it would be mainly compositionally controlled in the site YANCA 4. Magnetic ellipsoids show magnetic foliations parallel to the magmatic and tectonic foliations observed in the pluton in outcrop 1 (Fig. 3BeD). The magnetic lineations, plunging moderately to steeply, are interpreted as parallel to the petrofabric lineation as well. The NW-SE magnetic foliation observed in the fine-grained leucocratic dike (site YANCA 3, Fig. 3C) is deviated about 50
with respect to the strike of the dike, which in turn would be the expected strike of magmatic foliation. Indeed, the magnetic foliation in the leucocratic dike parallels the planar fabric widely observed in the field, which fit the regional foliation, pointing out a tectonic origin for this magnetofabric. Samples from the “undeformed” porphyritic biotitic granite of outcrop 4 show a subhorizontal, NE trending magnetic lineation associated to a gentle dipping magnetic foliation (Fig. 3E). Although principal axes from individual samples are more scattered than those from other sites, the subhorizontal magnetic foliation from this “undeformed” outcrop strongly contrasts with the widespread, subvertical NW-SE foliation that characterized the visibly deformed rocks in the area, which in turn constitutes the structural feature that led to the definition of the Gastre Fault System as a major tectonic element. 5. Discussion 5.1. Origin of the magnetic fabric in the microdioritic dikes of Puesto Jaramillo The magnetic foliation in the NE-SW dikes of Puesto Jaramillo (dikes 1 and 2) is roughly orthogonal to dike plane (e.g. Fig. 11D). This geometric relationship is not the expected for a foliation associated to magmatic flow, which would tend to be from lowangle to parallel with respect to dike walls (e.g. Knight and Walker, 1988; Rochette et al., 1999). The fact that anisotropy of the magnetic susceptibility in these dikes is dominated by multidomain magnetite grains (Fig. 9) rules out the possibility that the observed orthogonally between the dike plane and the magnetic foliation were related to a control of single domain magnetite particles on the anisotropy of the magnetic susceptibility ellipsoid (e.g. Potter and Stephenson, 1988; Rochette et al., 1992, 1999; Chadima et al., 2009). The magnetic ellipsoid from the NE trending dikes is almost coaxial with the magnetic ellipsoid associated to magmatic and tectonic magnetofabrics in other rocks of the locality, strongly suggesting that the magnetic fabric in the NE-SW trending dikes is related to a secondary process that have erased the flow fabric. Petrographic observations reveal that magnetite occurs as primary, euhedral titanomagnetite grains and as minute crystals disseminated in the rocks mass. Anisotropy of the magnetic susceptibility

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163

Fig. 12. Rock magnetism and anisotropy of the magnetic susceptibility scalar data of the rocks of Ea. Yancamil area. A) Km vs. P0 diagram for the three sites of Calcatapul Fm; B) P0 vs. T diagram for the three sites of Calcatapul Fm.; C) Hysteresis curve of the sample of the porphyritic biotitic granite; D) Thermomagnetic curve of the sample of the porphyritic biotitic granite; E) Km vs. P’ diagram of the porphyritic biotitic granite and its dikes; F) P0 vs. T diagram of the porphyritic biotitic granite and its dikes.

fabric could be related to grain shape orientation of the minute magnetite grains if they grew during the weak solid-state deformation described above. Alternatively, in the case of deuteric or earlier growth of the minute magnetite grains, the observed anisotropy of the magnetic susceptibility signal could have been produced by intragranular processes during the solid-state deformation, for instance stress alignment of domain walls in the titanomagnetites (e.g. Borradaile and Kehlenbeck, 1996). In any case, the magnetofabric of the NE trending dikes do not represent magmatic strain but tectonic strain. We can not unambiguously determine if the origin of the magnetofabric in the NW trending dike is magmatic of tectonic because the observed NW-SE magnetic foliations are compatible with both of these origins.

5.2. Timing of deformation in Puesto Jaramillo Petrographic observations in rocks from Puesto Jaramillo reveal the predominance of magmatic and high-temperature deformation in the granitoids, and the record of lower greenschist facies deformation in the mylonites and, with lower intensity, in the microdiorite dikes. It is worth noting that these dikes and their granitoid host represent products of coexisting mafic and felsic magmas (C.B. Zaffarana, PhD thesis, in prep.). Anisotropy of the magnetic susceptibility results from the locality show that all of the rocks have a magnetic ellipsoid characterized by NW-SE, subvertical foliations and shallow, W-plunging magnetic lineations (Fig. 11). The anisotropy of the magnetic

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susceptibility foliation is parallel to the magmatic foliation in the biotite-hornblende granitoids, defined by alignment of plagioclase, biotite, amphibole and elongated mafic microgranular enclaves. The discrete, small mylonite strips (Fig. 2D) and the anisotropy of the magnetic susceptibility from the NE trending microdiorite dikes are the only direct evidence of low-temperature deformation in the area. However, low-temperature deformation in the dikes implies that their biotite-hornblende granitoid host must have also undergone some degree of low-temperature deformation because both the granitoids and the microdiorites are roughly coeval (C.B. Zaffarana, PhD thesis, in prep.). Any low-temperature deformation in the granitoids would be however undetected because anisotropy of the magnetic susceptibility alone can not discriminate between two superimposed, coaxial ellipsoids of different origin (magmatic one of them and tectonic the other one). It is unlikely that magmatic, high-temperature and lowtemperature coaxial deformations detected in the area represent different, time-separated events. Instead, available data suggest that the observed finite deformation progressively developed during the cooling path of the pluton under almost invariant conditions. The parallelism of all of the observed fabrics (Fig. 11) with the regional NW-SE trend of the Gastre Fault System further suggests that the latter exerted an important control on magma emplacement and subsequent deformation.

5.3. Timing and style of deformation in Ea. Yancamil Microstructures in both the porphyritic biotitic granite and its associated leucocratic dikes in outcrop 1 indicate high-temperature (>500
C), deformation recorded by chessboard texture in quartz (above 700
C, Blumfeld et al., 1986; Mainprice et al., 1986), microcline instead of orthoclase, myrmekite and incipient subgrain formation (evidence of dynamic recrystallization) in microcline, which indicates temperatures between 450 and 600
C (Passchier and Trouw, 2005). Lower-temperature deformation textures and minerals are superimposed, indicating greenschist metamorphic facies conditions (Regimen 1 from Hirth and Tullis, 1992; Tullis et al., 2000). Regimen 1 is characterized by recrystallization involving progressive subgrain rotation and it is characterized by bulging and dynamic recrystallization in quartz, fracturing of feldspars, and micro-kinking in plagioclase (kink bands occur at 400e500
C, Pryer, 1993; Ji, 1998a,b). Biotite retrogradation to chlorite and feldspar alteration to clays and sericite could be related with the low-temperature deformation. Likewise, ductil deformation in quartz and brittle deformation in feldspar broadly indicate greenschist facies deformation conditions (400e500
C) in the equigranular granodiorite either. We interpret these 700e400
C solid-state microtextures as the record of deformation occurred during the cooling of a syntectonic pluton. Our data from Ea. Yancamil area agree with, and expands the findings of von Gosen and Loske (2004), with their D1 deformation affecting the whole Calcatapul Formation although just in part the Permian porphyritic biotitic granite. The coexistence of outcrops showing highly variable strain intensity led to von Gosen and Loske (2004) to emphasize the heterogeneous character of their D1 deformation. Our results depict a more complex scenario, where the porphyritic biotitic granite appears visually undeformed at outcrop 4 (Fig. 3) and the stretching lineations (L1) shows variable plunge in both the granite and the host, from subvertical (as found by von Gosen and Loske, 2004) to subhorizontal. The observed association of subvertical NW-SE trending foliation (S1) with subvertical to subhorizontal L1 suggests that the style of the penetrative deformation in the area could be the product of either

partitioned transpression or different evolutionary stages of highly oblique transpression (e.g. Tikoff and Greene, 1997). The small area where host-rock crops out (Figs. 1 and 3) limits the study of host-pluton relationships in the Central Patagonian Batholith. In particular, we did not unambiguously observe structural or rheological heterogeneities capable to partially or fully accommodate strain partitioning. The different ellipsoids described a few meters apart from each other in the contact zone (Fig. 3E) may be related to a rheological heterogeneity (granite-host) since it is difficult to explain why a younger event (would be in the intrusive) would not obliterate the record of the older event located a few meters distant in the adjacent host rock if both rock types had similar rheology at times of the young event. The possible factors that led to development of vertical or horizontal stretching lineation within the Calcatapul Formation are even more obscure. In any case, vertical lineation appears close to the contact with the deformed porphyritic biotitic granite, whereas horizontal lineation of outcrop 5 appears close to the contact with the “undeformed” granite of outcrop 4. An undetected structure could have been responsible for preferential accommodation of horizontal shear in the metavolcanic rocks close to the zone where the “undeformed” granite was emplaced, whereas dominantly horizontal shortening was accommodated close to the porphyritic biotitic granite. As a simple scheme, we speculate that magma batches of granite were intruding the Calcatapul Formation during its last D1 deformation stages and after D1 deformation ended, so the granite would mainly be a late syntectonic intrusion. Finally, and albeit further data are necessary to unambiguously asses the kinematics of horizontal shear component, it may be admitted that the agreement between the left-lateral displacements shown by late-magmatic dikes intruding the porphyritic biotitic granite (Fig. 8B) and the overall sinistral oblique kinematics reported by Franzese and Martino (1998) argues against the dextral shear claimed for some paleogeographic models involving transcontinental character of the Gastre Fault System (e.g. Rapela and Pankhurst, 1992; Marshall, 1994; Storey et al., 1999; MacDonald et al., 2003; Martin, 2007; Torsvik et al., 2009). 6. Implications for models of western gondwana breakup Some models of Western Gondwana breakup consider the Gastre Fault System as a major strike-slip fault zone that accommodated large amounts of dextral displacement between Patagonia and cratonic South America during the Jurassic (Rapela and Pankhurst, 1992; Ben-Avraham et al., 1993; Marshall, 1994; Curtis and Hyam, 1998; Thomson, 1998; Barker, 1999; Storey et al., 1999; MacDonald et al., 2003; Martin, 2007; Torsvik et al., 2009). These models further claim that this tectonic evolution is associated with the separation of the Malvinas Platform from eastern South Africa and its subsequent w90
clockwise rotation. The latter must have occurred sometime later than 190 Ma, which is the age of the dolerites in the Gran Malvina Island that provided the paleomagnetic evidence of rotation (Mitchell et al., 1986; Taylor and Shaw, 1989). Parallelism of magmatic and subsolidus fabrics in the studied localities and concordance of solid-state fabrics with structural trend observed at the Calcatapul Formation fit widely accepted evidence for synkinematic pluton emplacement. Considering this, the U-Pb ages from the Porphyritic biotitic granite favor a Permian age for the lower greenschist facies deformation at Ea. Yancamil. The age of intrusives at Puesto Jaramillo is unconstrained; they could be Late Triassic if correlatable with the rocks of the Gastre Superunit with Rb-Sr data (Rapela et al., 1991), although an older age can not be rule out regarding uncertainty about the actual age of these outcrops (available ages for the Central Patagonian Batholith are 260, 220 and 210 Ma, see above Section 2). In

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any case, deformation at Puesto Jaramillo seems to be associated with the intrusive event (see discussion above) and then its age is likely pre-Jurassic. On the other hand, the younger strain recorded in the plutons of the Central Patagonian Batholith is manifested by NW-SE stripes of brittle deformation which were first described by Rapela et al. (1991). However, and with caution because our observations are a really limited, we did not see such a NW-SE trending brittle deformation affecting the dominantly Jurassic, and possibly Triassic in part (Franzese et al., 2002), lavas and ignimbrites overlying the Central Patagonian Batholith. The strain ellipsoids in Ea. Yancamil may be reconciled with sinistral (?), partitioned transpression, whereas those of Puesto Jaramillo seems to be dominated by strike-slip motion (Fig. 3A,C) with probable dextral kinematics (von Gosen and Loske, 2004). Some kilometers farther SE, outcrops of moderately dipping N-S mylonites in the Gastre village (Fig. 1) show oblique-sinistral kinematics (von Gosen and Loske, 2004). Thus, as claimed by von Gosen and Loske (2004), available kinematic data reveal heterogeneities that strongly diminish the hypothesis considering the Gastre Fault System as a major dextral strike-slip zone. Overall, our results argue against both the dextral transcurrent style and the Jurassic activity attributed to the Gastre Fault System in many evolutionary models of western Gondwana. By inference, the Gastre Fault System can not be invoked to accommodate the post 185 Ma rotation of the Falkland-Malvinas platform. Moreover, recent plate kinematics analysis indicates that the western Gondwana paleogeography may be reconstructed without appealing to a microplate-like behaviour for the Malvinas/Falkland Islands (Eagles and Vaughan, 2009). These observations favor the alternative scenario where the clockwise tectonic rotation found in the island is part of a widespread rotational deformation in southern South America during the supercontinent breakup, as it is supported by pre-mid-Cretaceous clockwise rotations found in several Patagonian areas south of the Gastre zone (Geuna et al., 2000; Somoza et al., 2008). 7. Conclusions In the introduction we have pointed out several controversial aspects related to the Gastre Fault System. Our results related to these subjects point that: 1) It is confirmed the existence of Calcatapul Formation as an individual unit affected by lower greenschist facies metamorphism. The unit is intruded by a Permian porphyritic biotitic granite showing similar strain ellipsoid in some outcrops (see also von Gosen and Loske, 2004) and appearing visually undeformed on others. We did not find, in the area shown in Fig. 1, the heterogeneously distributed mylonitic outcrops of up to 3 km wide and 7 km mentioned by Rapela and Pankhurst (1992). 2) We agree with von Gosen and Loske (2004) in that the outcrops in the Gastre region do not provide supportive evidence for the existence of a dextral Gastre Fault System of transcontinental magnitude. 3) Relationships between magmatism and strain strongly suggest that the main deformation affecting rocks of the Central Patagonian Batholith is dominantly pre-Jurassic in age. The above considerations strongly diminish the proposed role of the Gastre Fault System as a major tectonic element playing an important role in accommodating translations and rotations of western Gondwana’s microplates during the Jurassic. In particular, the results argue against any relationship between the large clockwise rotation paleomagnetically determined from studies in

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