Fernández-Suárez, Javier

Gartner, Andreas

Gutiérrez-Alonso, Gabriel

Hofmann, Mandy

Jablonski, Jerónimo

López Carmona, Alicia

English

Gestion del Repositorio Documental de la Universidad de Salamanca

GEOGACETA, 64, 2018163Copyright© 2018 Sociedad Geológica de España / www.geogaceta.comRecepción: 25 de enero de 2018Revisión: 12 de abril de 2018Aceptación: 25 de abril de 2018Geogaceta, 64 (2018), 163-166ISSN (versión impresa): 0213-683XISSN (Internet): 2173-6545ABSTRACTThis paper reports a study of the crystallization age of the deformedJuzbado Granite through LA-ICP-MS U-Pb geochronology. Seven zircongrains were dated providing ages ranging from 340 to 2500 Ma. The U-Pbage data supply a maximum crystallization age of 340 Ma and insights intothe nature of potential source rocks. The ages of the inherited zircon grainsor xenocrysts are in agreement with the zircon contents found in thepotential source rocks. Given that the intrusion is deformed by the fabricrelated to the Juzbado-Penalva do Castelo shear zone, its minimum age mustbe ca. 308 Ma. Based on the petrological similarities with other granitebodies in the surrounding area (Tormes anatectic dome) its age is likely tobe around 320 Ma.Key-words: : Juzbado Granite, shear zone, geochronology, TormesDome, Variscan.RESUMENEste trabajo se centra en el estudio de la edad de cristalización del granitodeformado de Juzbado mediante geocronología U-Pb a través de ablación láser. Sedataron 7 granos de circón, obteniéndose edades comprendidas entre 340 y 2500Ma. Los datos obtenidos proporcionan una edad máxima de cristalización de 340Ma y aportan datos sobre la naturaleza de los potenciales materiales fuente. Lasedades de circones heredados existentes en el granito son coincidentes con lasencontradas en las posibles rocas fuente. Dado que esta intrusión está deformadaintensamente por la fábrica relacionada con la Zona de Cizalla de Juzbado-Penalvado Castelo, la edad mínima de este cuerpo es de 308 Ma. La comparación de lascaracterísticas petrológicas de este granito con las de otros en los alrededores (en elDomo del Tormes) permite asignarle una edad en torno a 320 Ma.Palabras clave: granito de Juzbado, zona de cizalla, geocronología, Domodel Tormes, Orógeno Varisco.IntroductionThe determination of crystallization agesof granitoid intrusions is paramount in un-derstanding the complex patterns of mag-matic evolution of orogenic belts. One of themost complex magmatic areas in the Varis-can belt of the Iberian Peninsula is the Tor-mes anatectic dome (TAD), located in theCentral Iberian Zone, where complex intru-sive relations have been described anddated (see López-Moro et al., 2018, Pereiraet al., 2018). Most crystallization ages ob-tained for granitoids in this domain are com-prised between ca. 325 and 318 Ma. Thesegranites are interpreted to have been origi-nated in the course of the orogenic exten-sion that caused extensive decompressionand partial melting of the crust.In this work, we attempted to date oneof the granite bodies from the TAD throughLA-ICP-MS U-Pb (Laser Ablation–InductivelyCoupled Plasma–Mass Spectrometry Ura-nium-Lead) geochronology using zirconcrystals. Given the paucity of zircon grainsrecovered from the studied sample (7) it wasnot possible to obtain a reliable crystalliza-tion age but only a maximum crystallizationage. In addition, the obtained U-Pb zirconages provide insights on the source rocks ofthe Juzbado pluton when compared withother intrusive bodies found in the TAD.Geological settingWithin the TAD, at its southeasternboundary, there is a distinct granitoid bodyknown as the deformed Juzbado Granite"e(or Traguntia-Juzbado orthogneiss; Garcíade Figuerola and Parga, 1968). This intru-sion crops out as an elongated body (ca. 18× 0.5 km) trending N70ºE (Fig. 1), compo-sed of muscovite leucogranite with sparsebiotite and occasional tourmaline. The Juz-bado Granite shows a ubiquitous macros-copic S-C fabric. At microscopic scale, themain evidence of deformation is wides-pread quartz recrystallization and the pre-sence of “mica-fish” structures in the CU-Pb geochronolgy of the deformed Juzbado Granite(Salamanca, NW Spain)Geocronología U-Pb del granito deformado de Juzbado (Salamanca, NO de España)Gabriel Gutiérrez-Alonso1,2, Alicia López-Carmona1,2, Javier Fernández-Suárez3, Jerónimo Jablonski4, Mandy Hofmann5 and Andreas Gärtner51 Departamento de Geología, University of Salamanca, 37008 Salamanca, Spain. gabi@usal.es, alioli@usal.es2 Geology and Geography Department, Tomsk State University, Lenin Street 36, Tomsk 634050, Russian Federation.3 Departamento de Petrología y Geoquímica, Universidad Complutense and IGEO, CSIC, 28040 Madrid, Spain. jfsuarez@geo.ucm.es 4 Museo de la Falla de Juzbado, C/Consistorial s/n, Juzbado, 37115 Salamanca, Spain. museodelafalla@gmail.com5 Senckenberg Naturhistorische Sammlungen Dresden, Museum für Mineralogie und Geologie, Sektion Geochronologie, GeoPlasma Lab, Königsbrücker Landstraße 159, 01109Dresden, Germany. mandy.hofmann@senckenberg.de, andreas.gaertner@senckenberg.deplanes (Fig. 2). The Juzbado Granite cropsout within the sub vertical, left-lateral Tra-guntia-Juzbado-Penalva do Castelo ShearZone (Iglesias and Ribeiro, 1981; Jiménezde Ontiveros and Hernández Enrile, 1983;Fig. 1), which imparts the strong macrosco-pic fabric. The dated sample was collectedin a fresh outcrop within the village of Juz-bado (coordinates 41°4'36.63"N -5°51'39.17"W).The Juzbado Granite marks the sou-thern boundary of the TAD separating anorthern domain characterized by the pre-sence of migmatites, two mica granites,and coarse gneisses from another lowgrade domain to the south with tourma-line-rich granites (López-Plaza and López-Moro, 2008).GeochronologyZircon grains were separated at the Uni-versity of Salamanca (Spain). Approximately10 kg of sample were crushed in a jaw crus-her and sieved for the fraction 63-400 µm.Concentrates were obtained using Wilfleytable, Frantz isodynamic magnet separatorand heavy liquids (diiodomethane). Zirconcrystals mounted in resin blocks and polis-hed to approximately half their thicknesswere analyzed for U and Pb isotopes at theMuseum für Mineralogie und Geologie(Senckenberg Naturhistorische Sammlun-gen, Dresden). Details of the analytical me-thod and protocols can be found inGutiérrez-Alonso et al. (2018).The studied sample yielded only sevenzircon grains and a total of 18 analyseswere performed on them. Figure 3 showsthe cathodoluminiscence images of the zir-con grains and the location of laser spotanalyses within each zircon. Table I reportsthe U-Pb results of the 15 analyses whosediscordance is less than 10%.The following is a description of U-Pbage results keyed to the analyzed crystalsshown in figure 3.Analysis a5 (Fig. 3) and analyses a9 anda10 (zircon image not shown in Fig. 3) aremore than 50% discordant and are thus notreported in Table I.Analyses a6, a7 and a8 (grain 3, fromleft to right in Fig. 3) yield concordant andoverlapping analyses with a concordia ageof 340.1 ± 2.6 Ma (Visean) (Fig. 4C). Thiszircon grain is homogeneous, showing veryweak zoning, in contrast to the othercrystals with sharper CL domains.Analyses a4, a11, a12, a13, a14 and a15were obtained on three different grains (Fig. 3)and they yielded early Cambrian ages. Analysisa4 on grain 2 (Fig. 3) yielded a concordia ageof 530 ± 8 Ma. Analyses a11, a12, a13 ongrain 4 yielded a pooled concordia age of539.4 ± 8.2 Ma (MSWD= 0.77). Analyses a14and a15 (grain 5) yielded a pooled concordiaage of 517 ± 5 Ma (MSWD= 0.006).Analyses a16, a17 and a18 (grain 6) didnot yield overlapping analyses but all threeare concordant and their 206Pb/238U ages arecomprised between 950 and 1008 Ma (Ste-nian-Tonian).Finally, analyses a1, a2 and a3 (grain 1)yielded concordant to slightly reverse-con-cordant 206Pb/238U ages comprised between2460 and 2528 Ma (latest Neoarchean-ear-liest Siderian).Discussion and ConclusionsThe above data indicate that the age ofthe Juzbado granite is not older than Visean(ca. 340 Ma). However, given the scarcity ofzircon in this sample it cannot be ruled outthat the age of the intrusion is younger.López Moro et al. (2018) have shown thatthere are syntectonic granites whose crysta-G. Gutiérrez-Alonso, A. López-Carmona, J. Fernández-Suárez, J. Jablonski, M. Hofmann and A. Gärtner164 Geocronología / GeochronologyGEOGACETA, 64, 2018Fig. 2.- Thin section image (under crossed pola-rized light) of the studied sample. Note thequartz recrystallization and the presence ofhighly strained domains corresponding to C pla-nes. mu-muscovite; pl-plagioclase; kfs-potassicfeldspar; q-quartz. See color figure in the web.Fig. 2.- Microfotografía de la muestra estudiada (luzpolarizada, nícoles cruzados). Obsérvese la presen-cia de cuarzo recristalizado y de dominios con altadeformación correspondientes a planos de tipo C.mu moscovita; pl-plagioclasa; kfs-feldespato potá-sico; q-cuarzo. Ver figura en color en la web.Fig. 1.- Schematic geological map of the deformed Juzbado Granite and its surrounding area (Villar Alonso et al., GEODE, 2018). Shaded pattern highlights the de-formation zone linked to the Juzbado-Penalva do Castelo shear zone. See color figure in the web.Fig. 1.- Mapa geológico esquemático del granito deformado de Juzbado y su entorno (GEODE, Villar Alonso et al., 2018). El área sombreada se corresponde con loslugares en los que se puede observar la deformación relacionada con la zona de cizalla de Juzbado-Penalva do Castelo. Ver figura en color en la web.GEOGACETA, 64, 2018165Geocronología / GeochronologyU-Pb geochronolgy of the deformed Juzbado Granite (Salamanca, NW Spain)llization age is ca. 320 Ma and they containvaried amounts of ca. 340 Ma inherited zir-con crystals as well as other pre-Variscanpopulations. On the other hand, the defor-mation that affects this intrusion has beenrecently dated at ca. 308 Ma (Gutiérrez-Alonso et al., 2015; Díez Fernández and Pe-reira, 2017) and makes it unlikely that thisleucogranite belongs to the post-tectonicgranitoid suite (cf. Gutiérrez-Alonso et al.,2011). Therefore, with the data presentedherein, the age of the studied granite is ei-ther Visean, as indicated by the youngestanalyzed zircon (Fig. 4C), in which case itwould belong to the early Carboniferoussuite of NW Iberia (Gutiérrez-Alonso et al.,2018) or younger, in which case the gra-nite would most likely belong to thesyntectonic leucogranite suite (e.g. LópezMoro et al., 2018).The older than 340 Ma zircon grains in-dicate that the source rocks involved in thegenesis of the Juzbado granite containedearly Cambrian, Stenian-Tonian and latestNeoarchean zircon grains. These inheritedzircon grains could in turn derive from pri-mary igneous sources or could have beenrecycled into pre-Variscan sedimentaryrocks or a combination of both.In conclusion, the deformed JuzbadoGranite has a Variscan age that cannot befully constrained due to the paucity of zircongrains present in it. Its maximum age is340.1 ± 2.6 Ma as shown by the youngestzircon grain in the sample (Fig. 4C). Its mi-nimum age is constrained by the age of thedeformation caused by the Juzbado-Penalvado Castelo Shear Zone. Therefore, withoutfurther research it is not possible to assignthe Juzbado deformed granite to either theVisean “Early Carboniferous Suite” (ECS, re-cently proposed by Gutiérrez-Alonso et al.,2018 and roughly coincident with the“Older Granodiorites” Capdevila et al.,1973) or the “Granodioritas Precoces” (DePablo Maciá, 1981) nor to the ca. 320 Masyntectonic suite that makes most of themagmatic bodies found in the TAD (López-Moro et al., 2018).If we take into account the petrologi-cal differences between the ECS and thesyntectonic suite, our sample is more likelyto correspond to the latter, as the ECS bo-dies are mostly biotite-rich granites andgranodiorites. From this point of view, weinterpret that the deformed Juzbado Gra-nite is a distinct body within the TAD thatshares a common origin with the syntec-tonic suite.Regarding the older zircon grains foundin the deformed Juzbado Granite, their agescorrespond to populations/events found inthe Central Iberian Zone: the “Ollo deSapo” event and the Cadomian, Stenian-To-nian and Neoarchean-Siderian zircon grainsfound as detrital components in the sedi-mentary rocks in the region. Small differen-ces of inherited ages with respect to thosefound in the TAD could be related to the pe-ripheral position of the Juzbado Granite, in-volving a slight variation in protholiths. Inaddition, a lower partial melting degree (ora more differentiated granite melt) could ac-count for certain compositional differencesof the granite itself, containing abundantmuscovite and occasional tourmaline incontrast to tourmaline-lacking two-micagranites from the TAD.AknowledgementsThis work has been funded by the Spa-nish Ministry of Economy and Competitive-ness under the project ODRE III-Oroclines &Delamination: Relations & Effects(CGL2013-46061-P) and Происхожде-ние, металлогения, климатическиеэффекты и цикличность КрупныхИзверженных Провинций (КИП)(Origin, metallogeny, climatic effects, and cycli-cal large igneous provinces; 14.Y26.31.0012;Russian Federation) to GGA and ALC andFig. 3.- Cathodoluminiscence (CL) images of six of the seven analyzed zircon grains with the position of the ablated spots. Black spot represents a discordant mea-surement. Ages of the labeled spots are posted in table IFig. 3.- Imágenes de catodoluminiscencia(CL) de seis de los siete granos de circón analizados indicando la posición de los puntos ablacionados. El punto negro re-presenta un análisis discordante. Las edades de los resultados en cada punto se encuentran en la tabla I.Fig. 4.- Concordia diagrams with the ages ob-tained in this study. A) Age of all the concor-dant analyses shown in Table I. B) Concordantanalyses with ages between 300 and 1100 Ma.C) Concordia plot and calculated age of theanalyses performed on the youngest zircongrain (Concordia Age: 340.1 ± 2.6 Ma; MSWDof concordance: 0.013; probability of concor-dance: 0.91).  See color figure in the web.Fig. 4.- Diagramas de concordia mostrando lasedades obtenidas en este estudio (Tabla I). A)Edades obtenidas en todos los análisis realiza-dos. B) Detalle de las edades concordantes entre300 y 1100 Ma. C) Diagrama de concordia y edadcalculada de los análisis realizados en el circónmás joven (Edad: 340.1 ± 2.6 Ma; MSWD de laconcordancia: 0.013; probabilidad de concordan-cia: 0.91). Ver figura en color en la web.GEOGACETA, 64, 2018 G. Gutiérrez-Alonso, A. López-Carmona, J. Fernández-Suárez, J. Jablonski, M. Hofmann and A. Gärtner166 Geocronología / GeochronologyCGL-2016-76438-P to JFS. ALC also wasfunded by a “Juan de la Cierva” grant(FJCI-2014-20740). JFS acknowledgessupport from research project CGL-2016-76438-P (Spanish Ministry of Economyand Competitiveness). Assistance by U.Linnemann and R. Krause at the Sencken-berg Naturhistorische Sammlungen Dres-den is greatly appreciated. Elena NúñezGuerrero is thanked for assistance in zir-con processing and X. Arroyo for his helpwith CL imaging. Insightful reviews by M.López-Plaza and A. Rubio-Ordóñez aregreatly acknowledged.ReferencesCapdevila, R., Corretgé, L.G. and Floor, P. (1973).Bulletin de la Société Géologique de FranceXV (3-4), 209-228.De Pablo Maciá, J.G. (1981). Cadernos do La-boratotio Xeolóxico de Laxe 2, 51-60.Díez Fernández, R. and Pereira, M.F. (2017). Li-thosphere 9(5), 726-744.García de Figuerola, L. and Parga, R. (1968).Acta geológica hispánica 3(3), 69-72.Gutiérrez-Alonso, G., Fernández-Suárez, J., Jeffries,T.E., Johnston, S.T., Pastor-Galán, D., Murphy,J.B., Franco, M.P. and Gonzalo, J.C. (2011).Tectonics 30(5),TC5008. Gutiérrez-Alonso, G., Collins, A.S., Fernández-Suárez, J., Pastor-Galán, D., González-Clavijo,E., Jourdan, F., Weil, A.B. and Johnston, S.T.(2015). Tectonophysics 643, 44-54.Gutiérrez-Alonso G., Fernández-Suárez, J.,López-Carmona, A., Gartner, A. (2018). Li-thosphere 10, 194-216.Iglesias, M. and Ribeiro, A. (1981). Comunicacionesdel Servicio Geológico de Portugal67(1), 89-93.Jimenez de Ontiveros, P. and Hernández Enrile,J.L. (1983). Stvdia Geologica Salmanticensia18, 139-158.López-Moro, F.J., López-Plaza, M., Gutiérrez-Alonso, G., Fernández-Suárez, J., López-Carmona, A., Hofmann, M. and Romer, R.L.(2018). International Journal of EarthSciences 107, 985-1004.López-Plaza M. and López-Moro F.J. (2008).Eurogranites in Western Castilla y León,part II. Salamanca, 1–192.Pereira, M.F., Díez Fernández, R., Gama, C.,Hofmann, M., Gärtner, A. and Linnemann,U. (2018). International Journal of EarthSciences 107, 251-267.Stacey, J.S. and Kramers, J.D. (1975). Earth andPlanetary Science Letters 26, 207-221.Villar Alonso, P., Portero Urroz, G., GonzálezCuadra, P., García Crespo, J., Nieto García,A.B., Rubio Pascual, F.J., Gómez Fernández,F. and Jiménez Benayas, S. (2018). In:GEODE. Mapa Geológico Digital Continuode España (online) (Accessed 10/01/2018)http://info.igme.es/cartografiadigital/geo-logica/geodezona.aspx?Id=Z1300. Table. I.- LA-ICP-MS U-Pb results.Tabla. I.- Resultados de los análisis de U-Pb por LA-ICP-MS.Sample Isotopic ratios and 2σ (%) errors Ages and 2σ absolute errors (Ma)JuzbadoAnal. # 206Pb/238Ua ±2s 207Pb/235Ua ±2s 207Pb/206Pba ±2s 206Pb/238U ±2s 207Pb/235U ±2s 207Pb/206Pb ±2s disc%a7 0.0541 1.49 0.3954 5.03 0.0531 4.80 339 5 338 15 331 109 -2.5a6 0.0539 1.35 0.3985 3.13 0.0535 2.82 338 5 340 9 353 63 4.3a8 0.0545 1.29 0.3993 3.21 0.0531 2.94 342 4 341 9 335 67 -2.3a15 0.0830 1.14 0.6646 17.64 0.0581 17.60 514 6 517 74 532 386 3.5a14 0.0848 1.91 0.6760 9.65 0.0578 9.46 525 10 524 40 522 208 -0.7a4 0.0857 1.54 0.6976 6.49 0.0591 6.31 530 8 537 27 570 137 7.0a13 0.0870 2.48 0.6967 17.31 0.0581 17.14 538 13 537 75 534 375 -0.7a11 0.0870 2.39 0.6965 11.68 0.0581 11.44 538 12 537 50 532 250 -1.1a12 0.0887 4.13 0.7281 5.92 0.0595 4.25 548 22 555 26 587 92 6.7a16 0.1587 1.83 1.5377 4.70 0.0703 4.33 950 16 946 29 936 89 -1.4a18 0.1633 1.22 1.6269 3.32 0.0723 3.09 975 11 981 21 994 63 1.9a17 0.1694 1.45 1.6541 3.65 0.0708 3.35 1009 14 991 23 953 69 -5.9a2 0.4646 1.34 10.3800 2.97 0.1620 2.65 2460 28 2469 28 2477 45 0.7a3 0.4749 2.31 9.7536 3.49 0.1489 2.62 2505 48 2412 33 2334 45 -7.3a1 0.4801 1.53 11.0028 2.22 0.1662 1.61 2528 32 2523 21 2520 27 -0.3disc%= percent discordance calculated from 207Pb/206Pb and 206Pb/238U ages (negative values: reversely discordant analyses)a corrected for background, mass bias, laser induced U-Pb fractionation and common Pb using Stacey and Kramers (1975) model Pb composition. 207Pb/235U calcu-lated using 207Pb/206Pb/(238U/206Pb × 1/137,88). Errors are propagated by quadratic addition of within-run errors (2σ) and the reproducibility of GJ-1 (2σ).

U-Pb geochronolgy of the deformed Juzbado Granite(Salamanca, NW Spain) - Geocronología U-Pb del granito deformado de Juzbado (Salamanca, NO de España)

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