In this contribution we present new U-Pb SHRIMP ages and in
situ LA-ICP-MS trace element geochemistry of zircon crystals from
the Río de los Sauces anatectic granite, Córdoba, Argentina. Notable
difference in texture and composition allowed us to identify two
zircon populations in a single granite sample that are interpreted
as reflecting metamorphic and igneous origins. Zircons regarded
as restitic crystals entrained during the melt segregation yielded a
slightly older concordia age of 537.1 ± 4.8 Ma (2σ) than those interpreted
as igneous, dated at 529 ± 6 (2σ) Ma. Inherited metamorphic
zircons are interpreted to represent solid-state growth during high
temperature metamorphism of the Pampean orogeny at the onset
of the anatexis or metamorphic peak. By contrast, igneous zircons
would record the crystallization age of Zr within the Río de los Sauces
granite. The textural, compositional and geochronological data of
both zircon populations suggest that the inception of the anatexis,
the melt segregation and crystallization occurred during a short period
of time of 8 myEn este trabajo se aportan nuevos datos de edades U-Pb SHRIMP
y análisis in situ LA-ICP-MS de elementos traza de circones provenientes
del granito Río de los Sauces, Córdoba, Argentina. A partir
de marcadas diferencias texturales y composicionales se pudieron
identificar dos poblaciones de circones en una misma muestra del
granito, las cuales sugieren orígenes metamórficos e ígneos. Las
edades concordia obtenidas en los circones metamórficos e ígneos
fueron de 537,1 ± 4,8 Ma (2σ) y 529 ± 6 (2σ) Ma, respectivamente. Se
interpreta que los circones metamórficos representan el crecimiento
en estado sólido durante el metamorfismo de alta temperatura de
la orogenia Pampeana, durante el inicio o el clímax de la anatexia.
Por su parte, las edades de los circones ígneos representan la edad
de cristalización del granito Río de los Sauces. Los datos texturales,
composicionales y geocronológicos de ambas poblaciones de circones
sugieren que el inicio de la anatexia, la segregación del fundido
y la cristalización ocurrieron durante un periodo breve de tiempo de
8 ma

Aragón, Eugenio

Coniglio, Jorge E.

Demartis, Manuel

D’Eramo, Fernando J.

Esteban Guzmán, José Julián

Pinotti, Lucio P.

English

Arias Montano: Institutional Repository of the University of Huelva

GEOGACETA, 68, 2020Copyright© 2020 Sociedad Geológica de España / www.geogaceta.com 27Anatectic granites of the Sierras Pampeanas from Córdoba (Argentina): U-Pb SHRIMP and zircon trace element LA-ICP-MS study of diachronic metamorphism and crystallizationGranitos anatécticos de las Sierras Pampeanas de Córdoba (Argentina): edades U-Pb SHRIMP y estudio LA-ICP-MS de elementos traza en circón de metamorfismo y cristalización diacrónicosFernando Javier D’Eramo1, José Julián Esteban Guzmán2, Manuel Demartis1, Jorge Enrique Coniglio1, Eugenio Aragón3 and Lucio Pedro Pinotti11 Instituto de Ciencias de la Tierra, Biodiversidad y Ambiente (ICBIA) (UNRC-CONICET), Departamento de Geología, Universidad Nacional de Río Cuarto, Ruta Nacional 36 Km 601, (X5804BYA) Río Cuarto, Córdoba, Argentina. E-mail: fjderamo@exa.unrc.edu.ar; mdemartis@exa.unrc.edu.ar; jconiglio@exa.unrc.edu.ar; lpinotti@exa.unrc.edu.ar 2 Departamento de Geodinámica, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV/EHU, apartado 644, 48080 Bilbao, Spain. E-mail: jj.esteban@ehu.es 3 Universidad Nacional de la Plata, CIG-CONICET. Diagonal 113 64, 1900. La Plata, Argentina. E-mail: earagon@cig.museo.unlp.edu.ar ABSTRACTIn this contribution we present new U-Pb SHRIMP ages and in situ LA-ICP-MS trace element geochemistry of zircon crystals from the Río de los Sauces anatectic granite, Córdoba, Argentina. Nota-ble difference in texture and composition allowed us to identify two zircon populations in a single granite sample that are interpreted as reflecting metamorphic and igneous origins. Zircons regarded as restitic crystals entrained during the melt segregation yielded a slightly older concordia age of 537.1 ± 4.8 Ma (2σ) than those inter-preted as igneous, dated at 529 ± 6 (2σ) Ma. Inherited metamorphic zircons are interpreted to represent solid-state growth during high temperature metamorphism of the Pampean orogeny at the onset of the anatexis or metamorphic peak. By contrast, igneous zircons would record the crystallization age of Zr within the Río de los Sauces granite. The textural, compositional and geochronological data of both zircon populations suggest that the inception of the anatexis, the melt segregation and crystallization occurred during a short pe-riod of time of 8 my.Key-words: Pampean orogeny, Pampean metamorphism, Río de Los Sauces granite, U-Pb SHRIMP dating, LA-ICP-MS RESUMENEn este trabajo se aportan nuevos datos de edades U-Pb SHRIMP y análisis in situ LA-ICP-MS de elementos traza de circones prove-nientes del granito Río de los Sauces, Córdoba, Argentina. A partir de marcadas diferencias texturales y composicionales se pudieron identificar dos poblaciones de circones en una misma muestra del granito, las cuales sugieren orígenes metamórficos e ígneos. Las edades concordia obtenidas en los circones metamórficos e ígneos fueron de 537,1 ± 4,8 Ma (2σ) y 529 ± 6 (2σ) Ma, respectivamente. Se interpreta que los circones metamórficos representan el crecimiento en estado sólido durante el metamorfismo de alta temperatura de la orogenia Pampeana, durante el inicio o el clímax de la anatexia. Por su parte, las edades de los circones ígneos representan la edad de cristalización del granito Río de los Sauces. Los datos texturales, composicionales y geocronológicos de ambas poblaciones de circo-nes sugieren que el inicio de la anatexia, la segregación del fundido y la cristalización ocurrieron durante un periodo breve de tiempo de 8 ma.Palabras clave: orogenia Pampeana, metamorfismo pampeano, granito Río de Los Sauces, datación U-Pb SHRIMP, LA-ICP-MSIntroduction The Sierras Pampeanas geological province, in central Argentina, comprises several mountain ranges constituted by an igneous and metamorphic basement that was mostly generated during two main orogenic events: the Pampean and the Famatinian orogenies (Casquet et al., 2018; Rapela et al., 2018; Weinberg et al., 2018). The Pampean orogeny is widely re-presented in the Sierras de Córdoba (Fig. 1), the southernmost part of the Sierras Pampeanas geological province, in which the main tectono-thermal event led to an upper amphibolite to granulite facies metamorphism and anatexis (Rapela et al., 1998; Guereschi and Martino, 2014). Gneisses and schists evolved into meta-texites and diatexites at P-T conditions that reached 8–9 kbar and more than 800 °C (Rapela et al., 1998; Otamendi et al., 2004; Weinberg et al., 2018), generating melts that were segregated to form gra-nitic bodies. Available ages suggest that this HT metamorphic event has lasted for at least 35 Ma, between 550 and 515 Ma (Rapela et al., 1998; Weinberg et al., 2018). The studies addressing the peralu-minous magmatism associated with this metamorphic event are concentrated in specific parts of the Sierras de Córdoba, and have dated magmatism at 522 Ma, synchronous with metamorphism and anatexis (Rapela et al., 1998, 2002; Tibaldi et al., 2008). However, anatexis and as-sociated magmatism are not necessarily coeval and the time gap between them may differ in each single case (Vander-haeghe et al., 1999; Keay et al., 2001; Es-teban et al., 2015). In this contribution we present LA- ICP-MS trace element data and U-Pb SHRIMP ages of zircon crystals sampled from a single site of the Río de los Sau-ces granite, Sierra de Comechingones (Argentina), in which both metamorphic and igneous events have been recorded. We report here for the first time in the Pampean orogeny the diachronic cha-racter of metamorphism and associated granitic magmatism.The Río de los Sauces graniteThe Río de los Sauces granite is a 1200 x 150 m tabular body located two kilome-Geogaceta, 68 (2020), 27-30 Fecha de recepción: 29/01/2020ISSN (versión impresa): 0213-683X      Fecha de revisión: 23/04/2020ISSN (Internet): 2173-6545    Fecha de aceptación: 29/05/2020GEOGACETA, 68, 2020 F. J. D’Eramo, J.J. Esteban Guzmán, M. Demartis, J.E. Coniglio, E. Aragón and L.P. Pinotti28 Petrología y Geoquímica/ Petrology and GeochemistryLA-ICP-MS geochemistrySixty-eight zircon crystals from both populations of the same sample, after SHRIMP acquisition, were analysed by Laser Ablation Quadrupole Inductively Coupled Plasma Mass Spectrometry (LA-Q-ICP-MS) at the University of the Basque Country to obtain trace and rare earth element (REE) concentrations.As it was stated above, one of the most prominent geochemical diffe-rences is the Th/U ratio, being lower in the ZP1 and higher in the ZP2. This ratio has been used to discriminate between magmatic (Th/U > 0.1) and metamor-phic (Th/U < 0.1) zircons (e.g., Hoskin and Schaltegger, 2003), although some discrepancies have been stated (Harley et al., 2007). In our study, the differences in Th/U ratios between both populations are even greater (Fig. 4A), suggesting clear contrasting origins (metamorphic vs. igneous).Other geochemical ratio, such as Nb/Ta, also suggests differences in their ori-gin, being almost invariably higher in ZP2 when compared with ZP1 (Fig. 4B).Regarding the rare earth elements (REE), both zircon populations show po-sitive patterns of normalized values with high overlap between them. Heavy REE contents, however, are somewhat diffe-rent. For example, the (Lu/Dy)N ratios are less variable in zircons from ZP1 (1.3-2.4) than those from ZP2 (0.8-3.6).The release of U during dehydration reactions in partial melting processes leads to an increase in the U/Ce ratio of zircons with metamorphic origin (Casti-ñeiras et al., 2011). Zircons from the ZP1 display U/Ce values that range between 21 and 936, whereas those from the ZP2 vary only from 6-54 (Fig. 4C). The Ti-in-zircon geothermometer (Ferry and Wat-Two types of zircons were identified from CL images (Fig. 2): zircon popula-tion 1 and 2 (ZP1 and ZP2). The first one (ZP1) is represented by complex zoned patterns, with dark rims at CL images su-rrounding inherited xenomorphic cores, dark luminescent idiomorphic zircons with weak oscillatory zoning and Th/U ratios lower than 0.02. Zircons from the second population (ZP2) are prismatic and bipyramidal with concentric undis-turbed oscillatory growth zoning.Nine and eight spot analyses in zir-con crystals from ZP1 and ZP2 yielded 206Pb/238U concordia ages of 537.1 ± 4.8 (2σ) (Fig. 3A) and 529 ± 6 (2σ) Ma (Fig. 3B), respectively. Although these two ages partially overlap each other, they could indicate that both zircon popula-tions are not coeval.ters northwards from the Río de los Sau-ces village (Fig. 1). It intrudes garnet-rich gneisses and metatexites, the last ones showing stromatic structures. Leucoso-mes frequently coalesce and rise along axial-plane foliations forming sheet-sha-ped or even pluton-like pegmatite-leu-cogranite bodies. The Río de los Sauces granite is one of these examples in which the coales-cence of several leucogranite sheets led to the formation of a tabular-shaped pluton intruded concordantly with the foliation of the enclosing gneisses, close to the gneiss-metatexite contact (Fig. 1). A weakly foliated, coarse-grained leuco-granite (microcline + quartz + plagio-clase + biotite + muscovite + garnet ± sillimanite, and minor accessories, such as zircon, sphene and apatite) is the main igneous facies. Pegmatitic textures are distributed as small patches within the Río de los Sauces pluton.U-Pb SHRIMP zircon agesThe main leucogranite facies of the Río de los Sauces pluton was sam-pled in only one station (32º30´37.7´´S / 64º34´18.8´W). Zircon crystals were se-parated and mounted by conventional methods. Further imaging by cathodo-luminiscense and analyses on a SHRIMP-IIe/MC were performed at the Centro de Pesquisas Geocronologicas, University of São Paulo (CPGeo-IGc-USP; Brazil).Fig. 1.- Geological map and cross-section  of the sheet-shaped, Río de los Sauces granite. Inset shows its location in the Sierras de Córdoba, central Argentina.Fig. 1.- Mapa y corte geológicos del granito Río de los Sauces, con morfología laminar. El recua-dro en la esquina superior izquierda muestra la localización en las Sierras de Córdoba, centro de Argentina.Fig. 2.- Cathodoluminescence images of the analyzed zircons from the Río de los Sauces granite. Labels indicate corresponding 206Pb/238U ages and Th/U ratios (in parentheses).Fig. 2.- Imágenes de catodoluminiscencia de los circones analizados del granito Río de los Sau-ces. Los rótulos indican la correspondiente edad 206Pb/238U y la relación Th/U (entre paréntesis).GEOGACETA, 68, 2020Anatectic granites of the Sierras Pampeanas from Córdoba (Argentina): U-Pb SHRIMP and zircon...29Petrología y Geoquímica/ Petrology and Geochemistrystrongly correlated with other ages re-ported by several authors for the Pam-pean metamorphism and magmatism in the Sierras de Córdoba (Guereschi and Martino, 2014; and references therein). The results here presented imply that crystallization of melt segregations after partial melting of metasedimentary roc-ks took place around 8 ma later than the HT regional metamorphism.A time lag between syntectonic gra-nites and regional metamorphism was previously reported by some authors in other orogenic domains. Keay et al. (2001), for example, proposed that Mio-cene peak metamorphism and related magmatism in the Naxos Island, Greece, were separated more than 5 ma each other. Esteban et al. (2015) also establi-shed a time span of 7 ma between the metamorphic climax of the high-grade Variscan HT/LP metamorphism (Late Car-boniferous) and the final emplacement of the Lys-Caillouas pluton at middle crustal levels.The presence of metamorphic zircons in the Río de los Sauces granite suggests that zircon crystals derived from the metamorphic protoliths have the Sierras de Córdoba. Therefore, the age of 537.1 Ma obtained in dark CL rims of metamorphic zircons from ZP1 are deemed to represent the age of the HT Pampean metamorphism. This meta-morphic event might be the responsible for the solid-state growth of the zircon rims. The zircon crystals would have been further transported within the anatectic magma until it was emplaced and crystallized at 529 Ma. The obtai-ned ages in both zircon populations are son, 2007) was applied in this study, ob-taining mean temperatures of 735 ± 17 and 750 ± 22 ºC for the ZP1 (4.5 ppm Ti) and ZP2 (5.5 ppm Ti) zircons, respec-tively. Although temperatures obtained from both populations are similar within uncertainties, slightly higher values can be identified in zircons from ZP2 with res-pect to ZP1 (Fig. 4D).DiscussionTextural and geochemical evidence were used to discriminate the two zir-con populations of the Río de los Sauces granite. Zircons from the first population (ZP1) have textural (zoned patterns, dark rims and inherited cores) and geochemi-cal (low Th/U and Nb/Ta, and high U/Ce) features consistent with metamorphic origin. By contrast, long prismatic and bypiramidal crystals from the ZP2, with significantly high Th/U and Nb/Ta and low U/Ce ratios, are here interpreted to be of igneous origin. The above stated textural and geo-chemical differences among both zir-con types are consistent with the U-Pb SHRIMP ages since the metamorphic zircons from ZP1 are ~8 ma older than igneous zircons from ZP2, as it can be ob-served from concordia ages. According to field relationships, tex-tural and geochemical evidence, the age of 529 Ma obtained in zircons from ZP2 is here interpreted as the crystallization age of the Río de los Sauces granite. Field relationships, i.e., the intrusion of the sheet-like Río de los Sauces pluton into gneisses after leucosome coales-cence, also point to the emplacement and crystallization of this igneous body during the waning of the HT metamor-phic event of the Pampean orogeny in Fig. 3.- Conventional Concordia plots of (A) ZP1 (metamorphic low Th/U zircons), (B) ZP2 (igneous high Th/U zircons).Fig. 3.- Gráficos de Concordia convencional de (A) la población de circones 1 (circones metamór-ficos con relaciones Th/U bajas), (B) la población de circones 2 (circones ígneos con relaciones Th/U altas).Fig. 4.- Bivariate diagrams of both zircon populations (metamorphic-ZP1 and igneous-ZP2): (A) Th vs. U; (B) Nb vs. Ta; (C) U/Ce vs. Th; (D) Th/U vs. temperature. See color figure in the web.Fig. 4.- Diagramas bivariantes en ambas poblaciones de circones (metamórficas ZP1 e ígneas ZP2): (A) Th vs. U; (B) Nb vs. Ta; (C) U/Ce vs. Th; (D) Th/U vs. temperatura. Ver figura en color en la web.GEOGACETA, 68, 2020 F. J. D’Eramo, J.J. Esteban Guzmán, M. Demartis, J.E. Coniglio, E. Aragón and L.P. Pinotti30 Petrología y Geoquímica/ Petrology and GeochemistryHarley, S.L., Kelly, N.M. and Möller, A. (2007). Elements 3, 25-30.Hoskin, P.W.O. and Schaltegger, U. (2003). Reviews in Mineralogy and Geochemis-try 53(1), 27–55.Keay, S., Lister, G. and Buick, I. (2001). Tec-tonophysics 342, 275–312.Otamendi, J.E., Castellarini, P., Fagiano, M., Demichelis, A. and Tibaldi, A. (2004). Gondwana Research 7(4), 1143-1155.Rapela, C.W., Pankhurst R.J., Casquet, C., Baldo, E., Saavedra, J., Galindo, C. and Fanning, C. (1998). En: The Pro-to-Andean Margin of Gondwana (R.J. Pankhurst and C.W. Rapela, Eds.) Geo-logical Society of London, Special Pu-blication 142, 181-217.Rapela, C.W., Baldo, E.G., Pankhurst, R.J. and Saavedra, J. (2002). Journal of Pe-trology 43(6), 1003-1028.Rapela, C., Pankhurst R.J., Casquet C., Dahlquist, J.A., Fanning, C.M., Baldo, E.G., Galindo, C., Alasino, P.H., Ramac-ciotti, C., Verdecchia, S.O., Murra, J.A. and Basei, M.A.S. (2018). Earth-Science Reviews 187, 259-285.Tibaldi, A.M., Otamendi J.E., Gromet, L.P. and Demichelis, A.H. (2008). Journal of South American Earth Sciences 26, 172-187.Vanderhaeghe, O., Teyssier, C. and Wy-soczanski, R. (1999). Canadian Journal of Earth Sciences 36, 917– 943.Weinberg R.F., Becchio R., Farías P., Susaño N. and Sola A. (2018). Earth-Sci-ence Reviews 187, 219-247.AcknowledgmentsThis work has been supported by grants PICT1754/16, PIP688 CONICET, PPI-UNRC 18/C456 and GIU17/033 of “Grupos de Investigación” of the Univer-sity of the Basque Country. The authors wish to thank Juan Diaz Alvarado and one anonymous reviewer for positive comments and suggestions for helpful discussions of the manuscript. ReferencesCasquet, C., Dahlquist, J., Verdecchia, S.O., Baldo, E.G., Galindo, C., Rapela, C., Pankhurst, R.J., Morales, M.M., Murra, J.A. and Fanning, C.M. (2018). Ear-th-Science Reviews 177, 209–225. Castiñeiras, P., Navidad, M., Casas, J.M., Liesa, M. and Carreras, J. (2011). Journal of Geology 119, 521–34.Esteban, J.J., Aranguren, A., Cuevas, J., Hilario, A., Tubía, J.M., Larionov, A. and Sergeev, S. (2015). Geological Magazine 152, 935–941. Ferry, J.M. and Watson, E.B. (2007). Con-tributions to Mineralogy and Petrology 154, 429–437.Guereschi, A. and Martino, R. (2014). En: Relatorio del XIX Congreso Geológico Argentino: geología y recursos naturales de la provincia de Córdoba (R.D. Marti-no and A.B. Guereschi, Eds.) Asociación Geológica Argentina, 125-148.been incompletely dissolved and then included as restitic crystals thus allowing to study the timing of the metamorphism prior to the emplacement of the pluton. The mean temperatures of 735 and 750 ºC in metamorphic and igneous zircons respectively are coherent with calculated metamorphic conditions in Sierras de Córdoba (Rapela et al., 1998; Otamendi et al., 2004). Since the crystallization temperatures calculated for both zircon populations are within the uncertainty of each other, a further increase in the regional temperature during anatexis cannot be inferred.ConclusionsThe anatectic Río de los Sauces gra-nite represents the coalescence, expe-lling, emplacement and crystallization of leucosomes from the surrounding mig-matites. Zircons analyzed from a single sample of the Río de los Sauces pluton were grouped into two populations who-se textural, compositional and geochro-nological features point to a contrasting origin: near peak high-temperature me-tamorphic solid-state growth vs. magma crystallization. These two events were se-parated in time by ~8 ma (537.1 vs. 529 Ma). Our contribution is the first constra-int of this time lag for the Pampean oro-geny in the Sierras de Córdoba. 

Granitos anatécticos de las Sierras Pampeanas de Córdoba (Argentina): edades U-Pb SHRIMP y estudio LAICP-
MS de elementos traza en circón de metamorfismo y cristalización diacrónicos

En este trabajo se aportan nuevos datos de edades U-Pb SHRIMP y análisis in situ LA-ICP-MS de elementos traza de circones provenientes del granito Río de los Sauces, Córdoba, Argentina. A partir de marcadas diferencias texturales y composicionales se pudieron identificar dos poblaciones de circones en una misma muestra del granito, las cuales sugieren orígenes metamórficos e ígneos. Las edades concordia obtenidas en los circones metamórficos e ígneos fueron de 537,1 ± 4,8 Ma (2σ) y 529 ± 6 (2σ) Ma, respectivamente. Se interpreta que los circones metamórficos representan el crecimiento en estado sólido durante el metamorfismo de alta temperatura de la orogenia Pampeana, durante el inicio o el clímax de la anatexia. Por su parte, las edades de los circones ígneos representan la edad de cristalización del granito Río de los Sauces. Los datos texturales, composicionales y geocronológicos de ambas poblaciones de circones sugieren que el inicio de la anatexia, la segregación del fundido y la cristalización ocurrieron durante un periodo breve de tiempo de 8 my.In this contribution we present new U-Pb SHRIMP ages and in situ LA-ICP-MS trace element geochemistry of zircon crystals from the Río de los Sauces anatectic granite, Córdoba, Argentina. Notable difference in texture and composition allowed us to identify two zircon populations in a single granite sample that are interpreted as reflecting metamorphic and igneous origins. Zircons regarded as restitic crystals entrained during the melt segregation yielded a slightly older concordia age of 537.1 ± 4.8 Ma (2σ) than those interpreted as igneous, dated at 529 ± 6 (2σ) Ma. Inherited metamorphic zircons are interpreted to represent solid-state growth during high temperature metamorphism of the Pampean Orogeny at the onset of the anatexis or metamorphic peak. Meanwhile, igneous zircons would record the crystallization age of Zr within the Río de los Sauces granite. The textural, compositional and geochronological data of both zircon populations suggest that the inception of the anatexis, the melt segregation and crystallization occurred during a short period of time of 8 my.Fil: D'eramo, Fernando Javier. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales. Instituto de Ciencias de la Tierra, Biodiversidad y Ambiente - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Ciencias de la Tierra, Biodiversidad y Ambiente; ArgentinaFil: Esteban Guzmán, José Julián. Universidad del País Vasco; EspañaFil: Demartis, Manuel. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales. Instituto de Ciencias de la Tierra, Biodiversidad y Ambiente - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Ciencias de la Tierra, Biodiversidad y Ambiente; ArgentinaFil: Coniglio, Jorge Enrique. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales. Instituto de Ciencias de la Tierra, Biodiversidad y Ambiente - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Ciencias de la Tierra, Biodiversidad y Ambiente; ArgentinaFil: Aragon, Eugenio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Pinotti, Lucio Pedro. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales. Instituto de Ciencias de la Tierra, Biodiversidad y Ambiente - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Ciencias de la Tierra, Biodiversidad y Ambiente; Argentin

D'eramo, Fernando Javier

Coniglio, Jorge Enrique

Aragon, Eugenio

Pinotti, Lucio Pedro

CONICET Digital

GEOGACETA, 68, 2020Copyright© 2020 Sociedad Geológica de España / www.geogaceta.com 95Anatectic granites of the Sierras Pampeanas from Córdoba (Argentina): U-Pb SHRIMP and zircon trace element LA-ICP-MS study of diachronic metamorphism and crystallizationGranitos anatécticos de las Sierras Pampeanas de Córdoba (Argentina): edades U-Pb SHRIMP y estudio LA-ICP-MS de elementos traza en circón de metamorfismo y cristalización diacrónicosFernando Javier D’Eramo1, José Julián Esteban Guzmán2, Manuel Demartis1, Jorge Enrique Coniglio1, Eugenio Aragón3 and Lucio Pedro Pinotti11 Instituto de Ciencias de la Tierra, Biodiversidad y Ambiente (ICBIA) (UNRC-CONICET), Departamento de Geología, Universidad Nacional de Río Cuarto, Ruta Nacional 36 Km 601, (X5804BYA) Río Cuarto, Córdoba, Argentina. E-mail: fjderamo@exa.unrc.edu.ar; mdemartis@exa.unrc.edu.ar; jconiglio@exa.unrc.edu.ar; lpinotti@exa.unrc.edu.ar 2 Departamento de Geodinámica, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV/EHU, apartado 644, 48080 Bilbao, Spain. E-mail: jj.esteban@ehu.es 3 Universidad Nacional de la Plata, CIG-CONICET. Diagonal 113 64, 1900. La Plata, Argentina. E-mail: earagon@cig.museo.unlp.edu.ar ABSTRACTIn this contribution we present new U-Pb SHRIMP ages and in situ LA-ICP-MS trace element geochemistry of zircon crystals from the Río de los Sauces anatectic granite, Córdoba, Argentina. Nota-ble difference in texture and composition allowed us to identify two zircon populations in a single granite sample that are interpreted as reflecting metamorphic and igneous origins. Zircons regarded as restitic crystals entrained during the melt segregation yielded a slightly older concordia age of 537.1 ± 4.8 Ma (2σ) than those inter-preted as igneous, dated at 529 ± 6 (2σ) Ma. Inherited metamorphic zircons are interpreted to represent solid-state growth during high temperature metamorphism of the Pampean Orogeny at the onset of the anatexis or metamorphic peak. Meanwhile, igneous zircons would record the crystallization age of Zr within the Río de los Sauces granite. The textural, compositional and geochronological data of both zircon populations suggest that the inception of the anatexis, the melt segregation and crystallization occurred during a short pe-riod of time of 8 my.Key-words: Pampean orogeny, Pampean metamorphism, Río de Los Sauces granite, U-Pb SHRIMP dating, LA-ICP-MS RESUMENEn este trabajo se aportan nuevos datos de edades U-Pb SHRIMP y análisis in situ LA-ICP-MS de elementos traza de circones prove-nientes del granito Río de los Sauces, Córdoba, Argentina. A partir de marcadas diferencias texturales y composicionales se pudieron identificar dos poblaciones de circones en una misma muestra del granito, las cuales sugieren orígenes metamórficos e ígneos. Las edades concordia obtenidas en los circones metamórficos e ígneos fueron de 537,1 ± 4,8 Ma (2σ) y 529 ± 6 (2σ) Ma, respectivamente. Se interpreta que los circones metamórficos representan el crecimiento en estado sólido durante el metamorfismo de alta temperatura de la orogenia Pampeana, durante el inicio o el clímax de la anatexia. Por su parte, las edades de los circones ígneos representan la edad de cristalización del granito Río de los Sauces. Los datos texturales, composicionales y geocronológicos de ambas poblaciones de circo-nes sugieren que el inicio de la anatexia, la segregación del fundido y la cristalización ocurrieron durante un periodo breve de tiempo de 8 my.Palabras clave: orogenia pampeana, metamorfismo pampeano, granito Río de Los Sauces, datación U-Pb SHRIMP, LA-ICP-MSIntroduction The Sierras Pampeanas geological province, in central Argentina, comprises several mountain ranges constituted by an igneous and metamorphic basement that was mostly generated during two main orogenic events: the Pampean and the Famatinian orogenies (Casquet et al., 2018; Rapela et al., 2018; Weinberg et al., 2018). The Pampean orogeny is widely re-presented in the Sierras de Córdoba (Fig. 1), the southernmost part of the Sierras Pampeanas geological province, in which the main tectono-thermal event led to an upper amphibolite to granulite facies metamorphism and anatexis (Rapela et al., 1998; Guereschi and Martino, 2014). Gneisses and schists evolved into meta-texites and diatexites at P-T conditions that reached 8–9 kbar and more than 800 °C (Rapela et al., 1998; Otamendi et al., 2004; Weinberg et al., 2018), genera-ting melts that segregate to form granitic bodies. Available ages suggest that this HT metamorphic event has lasted for at least 35 Ma, between 550 and 515 Ma (Rapela et al., 1998; Weinberg et al., 2018). The studies addressing the peralu-minous magmatism associated with this metamorphic event are concentrated in specific parts of the Sierras de Córdoba, and have dated magmatism at 522 Ma, synchronous with metamorphism and anatexis (Rapela et al., 1998, 2002; Tibaldi et al., 2008). However, anatexis and as-sociated magmatism are not necessarily coeval and the time gap between them may differ in each single case (Vander-haeghe et al., 1999; Keay et al., 2001; Es-teban et al., 2015). In this contribution we present LA- ICP-MS trace element data and U-Pb SHRIMP ages of zircon crystals sampled from a single site of the Río de los Sau-ces granite, Sierra de Comechingones (Argentina), in which both metamorphic and igneous events have been recorded. We report here for the first time in the Pampean orogeny the diachronic cha-racter of metamorphism and associated granitic magmatism.The Río de los Sauces graniteThe Río de los Sauces granite is a 1200 x 150 m tabular body located two kilome-Geogaceta, 68 (2020), 27-30 Fecha de recepción: 29/01/2020ISSN (versión impresa): 0213-683X      Fecha de revisión: 23/04/2020ISSN (Internet): 2173-6545    Fecha de aceptación: 29/05/2020GEOGACETA, 68, 2020 F. J. D’Eramo, J.J. Esteban Guzmán, M. Demartis, J.E. Coniglio, E. Aragón and L.P. Pinotti96 Petrología y Geoquímica/ Petrology and GeochemistryLA-ICP-MS geochemistrySixty-eight zircon crystals from both populations of the same sample, after SHRIMP acquisition, were analysed by Laser Ablation Quadrupole Inductively Coupled Plasma Mass Spectrometry (LA-Q-ICP-MS) at the University of the Basque Country to obtain trace and rare earth element (REE) concentrations.As it was stated above, one of the most prominent geochemical diffe-rences is the Th/U ratio, being lower in the ZP1 and higher in the ZP2. This ratio has been used to discriminate between magmatic (Th/U > 0.1) and metamor-phic (Th/U < 0.1) zircons (e.g., Hoskin and Schaltegger, 2003), although some discrepancies have been stated (Harley et al., 2007). In our study, the differences in Th/U ratios between both populations are even greater (Fig. 4A), suggesting clear contrasting origins (metamorphic vs. igneous).Other geochemical ratio, such as Nb/Ta, also suggests differences in their ori-gin, being almost invariably higher in ZP2 when compared with ZP1 (Fig. 4B).Regarding the rare earth elements (REE), both zircon populations show po-sitive patterns of normalized values with high overlap between them. Heavy REE contents, however, are somewhat diffe-rent. For example, the (Lu/Dy)N ratios are less variable in zircons from ZP1 (1.3-2.4) than those from ZP2 (0.8-3.6).The release of U during dehydration reactions in partial melting processes leads to an increase in the U/Ce ratio of zircons with metamorphic origin (Casti-ñeiras et al., 2011). Zircons from the ZP1 display U/Ce values that range between 21 and 936, whereas those from the ZP2 vary only from 6-54 (Fig. 4C). The Ti-in-zircon geothermometer (Ferry and Wat-Two types of zircons were identified from CL images (Fig. 2): zircon popula-tion 1 and 2 (ZP1 and ZP2). The first one (ZP1) is represented by complex zoned patterns, with dark rims at CL images su-rrounding inherited xenomorphic cores, dark luminescent idiomorphic zircons with weak oscillatory zoning and Th/U ratios lower than 0.02. Zircons from the second population (ZP2) are prismatic and bipyramidal with concentric undis-turbed oscillatory growth zoning.Nine and eight spot analyses in zir-con crystals from ZP1 and ZP2 yielded 206Pb/238U concordia ages of 537.1 ± 4.8 (2σ) (Fig. 3A) and 529 ± 6 (2σ) Ma (Fig. 3B), respectively. Although these two ages partially overlap each other, they could indicate that both zircon popula-tions are not coeval.ters northwards from the Río de los Sau-ces village (Fig. 1). It intrudes garnet-rich gneisses and metatexites, the last ones showing stromatic structures. Leucoso-mes frequently coalesce and rise along axial-plane foliations forming sheet-sha-ped or even pluton-like pegmatite-leu-cogranite bodies. The Río de los Sauces granite is one of these examples in which the coales-cence of several leucogranite sheets led to the formation of a tabular-shaped pluton intruded concordantly with the foliation of the enclosing gneisses, close to the gneiss-metatexite contact (Fig. 1). A weakly foliated, coarse-grained leuco-granite (microcline + quartz + plagio-clase + biotite + muscovite + garnet ± sillimanite, and minor accessories, such as zircon, sphene and apatite) is the main igneous facies. Pegmatitic textures are distributed as small patches within the Río de los Sauces pluton.U-Pb SHRIMP zircon agesThe main leucogranite facies of the Río de los Sauces pluton was sam-pled in only one station (32º30´37.7´´S / 64º34´18.8´W). Zircon crystals were se-parated and mounted by conventional methods. Further imaging by cathodo-luminiscense and analyses on a SHRIMP-IIe/MC were performed at the Centro de Pesquisas Geocronologicas, University of São Paulo (CPGeo-IGc-USP; Brazil).Fig. 1.- Geological map and cross-section  of the sheet-shaped, Río de los Sauces granite. Inset shows its location in the Sierras de Córdoba, central Argentina.Fig. 1.- Mapa y corte geológicos del granito Río de los Sauces, con morfología laminar. El recua-dro en la esquina superior izquierda muestra la localización en las Sierras de Córdoba, centro de Argentina.Fig. 2.- Cathodoluminescence images of the analyzed zircons from the Río de los Sauces granite. Labels indicate corresponding 206Pb/238U ages and Th/U ratios (in parentheses).Fig. 2.- Imágenes de catodoluminiscencia de los circones analizados del granito Río de los Sau-ces. Los rótulos indican la correspondiente edad 206Pb/238U y la relación Th/U (entre paréntesis).GEOGACETA, 68, 2020Anatectic granites of the Sierras Pampeanas from Córdoba (Argentina): U-Pb SHRIMP and zircon...97Petrología y Geoquímica/ Petrology and Geochemistrystrongly correlated with other ages re-ported by several authors for the Pam-pean metamorphism and magmatism in the Sierras de Córdoba (Guereschi and Martino, 2014; and references therein). The results here presented imply that crystallization of melt segregations after partial melting of metasedimentary roc-ks took place around 8 ma later than the HT regional metamorphism.A time lag between syntectonic gra-nites and regional metamorphism was previously reported by some authors in other orogenic domains. Keay et al. (2001), for example, proposed that Mio-cene peak metamorphism and related magmatism in the Naxos Island, Greece, were separated more than 5 ma each other. Esteban et al. (2015) also establi-shed a time span of 7 ma between the metamorphic climax of the high-grade Variscan HT/LP metamorphism (Late Car-boniferous) and the final emplacement of the Lys-Caillouas pluton at middle crustal levels.The presence of metamorphic zircons in the Río de los Sauces granite suggests that zircon crystals derived from the metamorphic protoliths have the Sierras de Córdoba. Therefore, the age of 537.1 Ma obtained in dark CL rims of metamorphic zircons from ZP1 are deemed to represent the age of the HT Pampean metamorphism. This meta-morphic event might be the responsible for the solid-state growth of the zircon rims. The zircon crystals would have been further transported within the anatectic magma until it was emplaced and crystallized at 529 Ma. The obtai-ned ages in both zircon populations are son, 2007) was applied in this study, ob-taining mean temperatures of 735 ± 17 and 750 ± 22 ºC for the ZP1 (4.5 ppm Ti) and ZP2 (5.5 ppm Ti) zircons, respec-tively. Although temperatures obtained from both populations are similar within uncertainties, slightly higher values can be identified in zircons from ZP2 with res-pect to ZP1 (Fig. 4D).DiscussionTextural and geochemical evidence were used to discriminate the two zir-con populations of the Río de los Sauces granite. Zircons from the first population (ZP1) have textural (zoned patterns, dark rims and inherited cores) and geochemi-cal (low Th/U and Nb/Ta, and high U/Ce) features consistent with metamorphic origin. By contrast, long prismatic and bypiramidal crystals from the ZP2, with significantly high Th/U and Nb/Ta and low U/Ce ratios, are here interpreted to be of igneous origin. The above stated textural and geo-chemical differences among both zir-con types are consistent with the U-Pb SHRIMP ages since the metamorphic zircons from ZP1 are ~8 ma older than igneous zircons from ZP2, as it can be ob-served from concordia ages. According to field relationships, tex-tural and geochemical evidence, the age of 529 Ma obtained in zircons from ZP2 is here interpreted as the crystallization age of the Río de los Sauces granite. Field relationships, i.e., the intrusion of the sheet-like Río de los Sauces pluton into gneisses after leucosome coales-cence, also point to the emplacement and crystallization of this igneous body during the waning of the HT metamor-phic event of the Pampean orogeny in Fig. 3.- Conventional Concordia plots of (A) ZP1 (metamorphic low Th/U zircons), (B) ZP2 (igneous high Th/U zircons).Fig. 3.- Gráficos de Concordia convencional de (A) la población de circones 1 (circones metamór-ficos con relaciones Th/U bajas), (B) la población de circones 2 (circones ígneos con relaciones Th/U altas).Fig. 4.- Bivariate diagrams of both zircon populations (metamorphic-ZP1 and igneous-ZP2): (A) Th vs. U; (B) Nb vs. Ta; (C) U/Ce vs. Th; (D) Th/U vs. temperature. See color figure in the web.Fig. 4.- Diagramas bivariantes en ambas poblaciones de circones (metamórficas ZP1 e ígneas ZP2): (A) Th vs. U; (B) Nb vs. Ta; (C) U/Ce vs. Th; (D) Th/U vs. temperatura. Ver figura en color en la web.GEOGACETA, 68, 2020 F. J. D’Eramo, J.J. Esteban Guzmán, M. Demartis, J.E. Coniglio, E. Aragón and L.P. Pinotti98 Petrología y Geoquímica/ Petrology and GeochemistryHarley, S.L., Kelly, N.M. and Möller, A. (2007). Elements 3, 25-30.Hoskin, P.W.O. and Schaltegger, U. (2003). Reviews in Mineralogy and Geochemis-try 53(1), 27–55.Keay, S., Lister, G. and Buick, I. (2001). Tec-tonophysics 342, 275–312.Otamendi, J.E., Castellarini, P., Fagiano, M., Demichelis, A. and Tibaldi, A. (2004). Gondwana Research 7(4), 1143-1155.Rapela, C.W., Pankhurst R.J., Casquet, C., Baldo, E., Saavedra, J., Galindo, C. and Fanning, C. (1998). En: The Pro-to-Andean Margin of Gondwana (R.J. Pankhurst and C.W. Rapela, Eds.) Geo-logical Society of London, Special Pu-blication, 142, 181-217.Rapela, C.W., Baldo, E.G., Pankhurst, R.J. and Saavedra, J. (2002). Journal of Pe-trology 43(6), 1003-1028.Rapela, C., Pankhurst R.J., Casquet C., Dahlquist, J.A., Fanning, C.M., Baldo, E.G., Galindo, C., Alasino, P.H., Ramac-ciotti, C., Verdecchia, S.O., Murra, J.A. and Basei, M.A.S. (2018). Earth-Science Reviews 187, 259-285.Tibaldi, A.M., Otamendi J.E., Gromet, L.P. and Demichelis, A.H. (2008). Journal of South American Earth Sciences 26, 172-187.Vanderhaeghe, O., Teyssier, C. and Wy-soczanski, R. (1999). Canadian Journal of Earth Sciences 36, 917– 943.Weinberg R.F., Becchio R., Farías P., Susaño N. and Sola A. (2018). Earth-Sci-ence Reviews 187, 219-247.AcknowledgmentsThis work has been supported by grants PICT1754/16, PIP688 CONICET, PPI-UNRC 18/C456 and GIU17/033 of “Grupos de Investigación” of the Univer-sity of the Basque Country. The authors wish to thank Juan Diaz Alvarado and one anonymous reviewer for positive comments and suggestions for helpful discussions of the manuscript. ReferencesCasquet, C., Dahlquist, J., Verdecchia, S.O., Baldo, E.G., Galindo, C., Rapela, C., Pankhurst, R.J., Morales, M.M., Murra, J.A. and Fanning, C.M. (2018). Ear-th-Science Reviews 177, 209–225. Castiñeiras, P., Navidad, M., Casas, J.M., Liesa, M. and Carreras, J. (2011). Journal of Geology 119, 521–34.Esteban, J.J., Aranguren, A., Cuevas, J., Hilario, A., Tubía, J.M., Larionov, A. and Sergeev, S. (2015). Geological Magazine 152, 935–941. Ferry, J.M. and Watson, E.B. (2007). Con-tributions to Mineralogy and Petrology 154, 429–437.Guereschi, A. and Martino, R. (2014). En: Relatorio del XIX Congreso Geológico Argentino: geología y recursos naturales de la provincia de Córdoba (R.D. Marti-no and A.B. Guereschi, Eds.) Asociación Geológica Argentina, 125-148.been incompletely dissolved and then included as restitic crystals thus allowing to study the timing of the metamorphism prior to the emplacement of the pluton. The mean temperatures of 735 and 750 ºC in metamorphic and igneous zircons respectively are coherent with calculated metamorphic conditions in Sierras de Córdoba (Rapela et al., 1998; Otamendi et al., 2004). Since the crystallization temperatures calculated for both zircon populations are within the uncertainty of each other, a further increase in the regional temperature during anatexis cannot be inferred.ConclusionsThe anatectic Río de los Sauces gra-nite represents the coalescence, expe-lling, emplacement and crystallization of leucosomes from the surrounding mig-matites. Zircons analyzed from a single sample of the Río de los Sauces pluton were grouped into two populations who-se textural, compositional and geochro-nological features point to a contrasting origin: near peak high-temperature me-tamorphic solid-state growth vs. magma crystallization. These two events were se-parated in time by ~8 ma (537.1 vs. 529 Ma). Our contribution is the first constra-int of this time lag for the Pampean oro-geny in the Sierras de Córdoba. 

Granitos anatécticos de las Sierras Pampeanas de Córdoba (Argentina): edades U-Pb SHRIMP y estudio LAICP-MS de elementos traza en circón de metamorfismo y cristalización diacrónicos

LAReferencia - Red Federada de Repositorios Institucionales de Publicaciones Científicas Latinoamericanas

http://rabida.uhu.es/dspace/bitstream/10272/19235/2/Anatectic.pdf

Granitos anatécticos de las Sierras Pampeanas de Córdoba (Argentina): edades U-Pb SHRIMP y estudio LAICP- MS de elementos traza en circón de metamorfismo y cristalización diacrónicos

Abstract

Similar works

Full text

Available Versions

Arias Montano: Institutional Repository of the University of Huelva

CONICET Digital

LAReferencia - Red Federada de Repositorios Institucionales de Publicaciones Científicas Latinoamericanas