macla nº 11. septiembre ‘09 revista de la sociedad española de mineralogía 137

Zircon Ion-Probe Dating the Maximum Age of Deposition of the Montemolín Succession (Lower Serie Negra) in the Ossa Morena Zone, / BERTA ORDÓÑEZ CASADO (1,*), DIETER GEBAUER (2), LUÍS EGUÍLUZ (3).

(1) Departamento de Geología, Área de Cristalografía y Mineralogía, Universidad de Oviedo. C/ Jesús Arias de Velasco s/n 33005 Oviedo, (Spain) (2) Department of Earth Sciences, Swiss Federal Institute of Technology, CH-8092 Zürich, (Switzerland) (3) Departamento de Geodinámica. Universidad del País Vasco/EHU, Aptdo. 211 01006 Vitoria, (Spain)

INTRODUCTION. synorogenic successions (Quesada, of the Tentudía formation as < 564 ± 30 1990) according to their relationships Ma ago (SHRIMP-technique, youngest Zircon-dating was proposed to obtain with the tectonic events that the region detrital zircon). On the other hand, the magmatic ages. Zircon is a refractory underwent in late Precambrian times. In overlying volcano-sedimentary mineral which can survive both the present work, we are dealing with sequences (, Bodonal-Cala) weathering and transport processes as the first ones. were considered as Late-Proterozoic in well as high temperature age (Upper Riphean to Upper Vendian; metamorphism and anatexis. The pre-orogenic successions are mainly Liñán and Schmitt, 1980; Liñán et al., the Serie Negra. It is widely represented 1984), and Lower-Cambrian (Ordóñez- Cathodoluminiscence (CL) studies on throughout the OMZ, but it is best known Casado, 1999). polyphased rocks have shown (e.g. in the central area of the Zone, where Ordóñez-Casado, 1999) that the internal Eguíluz (1987) distinguished two ANALYTICAL TECHNIQUES. structures of zircons are often members, a lower one, Montemolín and independent of their morphology. an upper one, Tentudía. Minerals for isotope analyses were Consequently, there is no possibility to separated, after crushing, sieving, heavy establish whether a zircon is a The 1000-2000 m thick, Montemolín liquids as well as a Frantz isodynamic magmatic grain or not without this CL succession consists of graphite-rich separator. Final selection of zircons was study. pelites and quartzwackes, with carried out by hand-picking according to abundant amphibolites towards the top. colour, size and morphology. The zircons Sequential overgrowths can record a Lenticular interbeds of black chert and, selected were mounted in epoxy and history of events provided temperatures locally, marbles are frequent. polished until quasi-central sections are high enough to precipitate new were reached. Cathodoluminescence zircon overgrowths. The microanalysis of The Tentudía succession overlies the (CL) technique was used to identify distinct domains within the same zircon former, apparently with a conformable distinct zircon domains. The same epoxy by SHRIMP (Sensitive High Resolution contact. It consists of: a) an alternating mounts were then used for in situ ion- Ion Microprobe) is considered to be monotonous shale and greywacke suite microprobe analyses (SHRIMP- II). among the most reliable methods to with some intercalations of black obtain radiometric data from magmatic, quartzites and carbonates; and b) GEOCHRONOLOGICAL RESULTS. and metamorphic rocks. Dating the various types of volcano-sedimentary zircons from meta-sedimentary rocks we rocks, (meta)tuffs and greenschist-facies The Montemolín metasedimentary can determine maximum depositional metabasites of variable thickness sequence corresponds to the lower part ages of the sedimentary precursor. (Eguíluz, 1987). of the Serie Negra. A deformed graphite- rich metapelite was collected. This is a GEOLOGICAL SETTING. STRATIGRAPHIC AND RADIOMETRIC biotitic schist with quartz, biotite, CONSTRAINTS. muscovite and plagioclase. Chlorite, The Ossa-Morena Zone (OMZ) is a NW-SE opaques, graphite, zircon, apatite and elongated belt that extends from Acritarchs have been reported in titanite occur as accessory minerals. Portugal to South Spain. One of the different parts of the Serie Negra, most distinctive features within this allowing an imprecise attribution to the The examined zircons are mainly zone is the presence of Precambrian Middle-Upper Riphean (1350-850 Ma) colourless, pink or orange, from rocks first shown by MacPherson (1879) (Arriola et al., 1984; Chacón et al., transparent to turbid, with many and since then this has been a subject 1984). However, these acritarchs have inclusions. They vary from euhedral or of intense debate. been reported (Eguíluz personal com.) to subhedral to rounded. be the product of contamination of the The Precambrian rocks within the OMZ, samples studied. Later studies (Schäfer The CL-images show inner cores, often have been grouped in pre-orogenic and et al., 1993) established the deposition rounded with an oscillatory pattern

palabras clave: Ossa Morena, Montemolín, SHRIMP-zircon, key words: Ossa Morena, Montemolín, SHRIMP-zircon, Depósito. Deposition. resumen SEM 2009 * corresponding author: [email protected]

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surrounded by planar oscillatory 0.14 domains, that in some cases are CL- (1s) 800 white and structureless. There are some CL-dark domains with no visible zoning. 0.12 1.2 700

16.2 ? 592 ± 13 Ma 18A.1 24.2 Th/U 0.67 22.1 600 13.1 25.1 max. deposition age 0.10 Pb / U 18B.2 14.2 12.1 22.2 (grain 18B) 1.1 3.1 206 238 18B.1 6.1 591 ± 11 Ma 18B.2 4.1 500 2.1 0.08207 235 14.3 Pb / U 24.1

400 10.1 14.1 0.06

300 11.1 207Pb / 235 U

18B.1 0.04 0 1 2

590 ± 26 Ma fig 2. a) U-Pb Concordia diagram for zircons from a metasediment of the Montemolín succession. Only ages Th/U 0.68 younger than 800 Ma are shown. Mean age obtained from the comagmatic part of the zircon 18B.

100 mm lead. In general, they have higher Th/U Chacón, J., Fernández, J., Mitrofanov, F.,

fig 1. CL image of zircon 18 section. Circles show ratios than the normal oscillatory Timofeev, B.V. (1984): Primeras the analized area. domains (up to 1.6). dataciones microfitopaleontológicas en el sector – (Anticlinorio de - In terms of SHRIMP-analyses, there is a CONCLUSIONS. positive correlation between U and Th ). Cuad. Lab. Xeol. Laxe, 8, 211- 220. contents. Results from the present study allow Eguíluz, L. (1987): Petrogénesis de rocas pointing out that the age of deposition of ígneas y metamórficas en el antiforme The inner cores reveal the presence of the sedimentary precursor of the Burguillos-Monesterio. Macizo Ibérico distinct provenance ages. The inherited Montemolín succession is Precambrian Meridional. Unpubl. Ph. D. thesis, Univ. País zircons yield concordant U-Pb ages or post Precambrian as the youngest Vasco, 694 pp. around 1.5, 1.8, 2.0 Ga and 2.5 Ga, and detrital grain provides a maximum Liñán, E., Schmitt M. (1980): Microfósiles de las calizas precámbricas de Córdoba a discordant 206Pb/238U age of 2.65 Ga deposition age of about 591 Ma. This 207 206 (España). Temas Geol. Min., 4, 171-194. with a Pb/ Pb age around 2.8 Ga. succession corresponds to an external -----, Palacios, T., Perejón, A. (1984): platform with continental-derived Precambrian–Cambrian boundary and The main age group clustered around sediments containing zircons of various correlation from southwestern and central 600 Ma. The average age for the provenances. On the other hand, all the part of Spain. Geol. Mag., 12, 221-228. youngest detrital grain is 591 ± 11 Ma ages around 600 Ma support a MacPherson, J. (1879): Estudio geológico y (concordia diagram Fig. 2). This age was derivation from Gondwana. petrográfico del norte de la provincia de obtained by two spots in the Sevilla. Bol. Com. Mapa Geol. España, 6, 97-269. comagmatic oscillatory domains that ACKNOWLEDGEMENTS. Ordóñez-Casado, B., (1999): Geochronological surround the inner core of zircon 18B. studies of the Pre-Mesozoic basement of The authors would like to thank W. the Iberian Massif: the Ossa Morena zone The domains analysed have Th/U ratios Compston, I. Williams and M. Fanning and the Allochthonous Complexes within of 0.67-0.68 and U contents of 420-416 for their help at various stages of ion- the Central Iberian zone. Ph.D. Thesis, ppm. Th/U ratios are typically magmatic, probe data collection and evaluation. E.T.H. Zürich 235pp. even in younger domains. Still, we can This study was financed by the Swiss Quesada, C. (1990): Introduction of the Ossa- not exclude some partial Pb loss in National Science Foundation to D.G., Morena Zone (part V). In: Dallmeyer R.D. and Martínez García E. (eds.), Pre-Mesozoic these domains. The age of 591 ± 11 Ma and B.O.C. for her PhD work and Spanish Geology of Iberia. Springer Verlag, Berlin was obtained in two spots in the co- Ministry of Science project CGL2008- Heidelberg, 249-251. magmatic part of the zircon without any 01130/BTE. All this support is gratefully Schäfer, H.J., Gebauer, D., Nägler, T.H., evidence of Pb loss, would represent the acknowledged. Eguíluz, L. (1993): Conventional and ion- maximum age of deposition of the dated microprobe U-Pb dating of detrital zircons Montemolín metasediment. REFERENCES. of the Tentudía Group (Serie Negra, SW Spain): implications for zircon systematics, The second group of zircon domains (CL- Arriola, A., Eguíluz, L., Fernández Carrasco, stratigraphy, tectonics and the Precambrian/Cambrian boundary. Contrib. dark, with high U and Th contents with J.L., Garrote, A. (1984): Individualización de diferentes dominios y unidades en el Mineral. Petrol., 113, 289-299. values up to 2595 ppm and up to 1448 ppm, respectively) yields ages that Anticlinorio Olivenza-Monesterio. Cuad. Lab. Xeol. Laxe, 8, 195-210. represent domains that partially lost