Rb-Sr AND SINGLE-ZIRCON GRAIN 207 Pbf 206 Pb CHRONOLOGY OF THE MONESTERIO GRANODIORITE AND RELATED MIGMATITES . EVIDENCE OF A LATE CAMBRIAN MELTING EVENT IN THE OSSA-MORENA ZONE , IBERIAN MASSIF

The Monesterio granodiorite, a small granodioritic body emplaced in a migmatitic complex in the SW of the Olivenza-Monesterio antiform, is a key plutonic body to understanding the relationships among the magmatism, metamorphism, and deformation in the Ossa-Morena Zone, SW Iberian Massif. We dated the granodiorite with the singlezircon stepwise-evaporation 207Pbp06Pb method, and the related migmatization event with the Rb-Sr method on leucosomes. Our results indicate that the Monesterio granodiorite crystallised at 510 ± 7 Ma and its protolith had a component with Upper Proterozoic zircons with a minimum a~e of 1696 Ma. Leucosomes give a Rb-Sr age of 511 ± 40 Ma (MSWD = 1,7) with initial 7Srj86Sr =0.70914 ± 0.00048. The lower initial 87srj86Sr of the granodiorite and its calc-alkaline chemistry precludes it from having derived from the same protolith as the migmatites. The existence of different magmatic bodies in the Ossa-Morena Zone with ages clustering around 500-510 Ma reveals the existence of a significant melting event during the Late Cambrian that involved protoliths with very different geochemical and isotopic signatures.


Introduction
The tectonometamorphic history of the Ossa-Morena Zone, SW Iberian Massif, is highly controversial due to the superposition of the Cadomian and Variscan deformations (Quesada, 1990, Dallmeyer and Quesada, 1992, Azor et al., 1995) and large discrepancies in available geochronological data.A good example is the Monesterio granodiorite, a small granodioritic body emplaced in a migmatitic complexo Although crucial for understanding the relationships among magmatism, metamorphism and deformation in this zone, the complex has been dated by various methods from "" 400 to 550 Ma (see below).
Radiometric dating of the Monesterio granodiorite anc,l associated migmatites is certainly complexo The granodiorite is affected by a strong shear zone, has little petrographic or chemical variation, and its primary mineralogy is partially affected by hydrothermal alteration, making it very complicated to obtain reliable data with Rb-Sr or K-Ar methods.In addition, we found that a significant fraction of zircon crystals have inherited cores, so that conventional U-Pb ages of this mineral would give unrealistic mixed ages; besides, monazite and xenotime are often metamict (see Ochsner, 1993).Dating the migmatization event of the surrounding metapelites is also problematic.Isotopic disequilibrium during partial melting (Bea, 1996;Knesel and Davidson, 1996;Tommasini and Davies, 1997) makes it extremely difficult to use coupled mesosome-leucosomemelanosome samples for Rb-Sr isochrons.The low solubility of zircon in low-temperature highly silicic leucosomes (Watson and Harrison, 1983;Watson, 1996), on the other hand, means it is unreliable to use zircons for dating the leucosome segregation.
In this paper we present the results of a study aimed at obtaining the very best ages for the crystallization of the Monesterio granodiorite and the migmatization event that affected its host rock.To do so, we dated the gra.nodiorite with the single-zircon stepwise-evaporation 207Pbp06Pb method (Kober, 1986(Kober, , 1987)).This technique, as it is less sensitive to secondary processes than U -Pb dating, can yield accurate crystallization ages on sorne zircon populations that display complex discordant patterns, and is still capable of dating different concentric parts of a single zircon grain (e.g.Doughertypage and Foden, 1996;Karabinos, 1997).Due to the abovmentioned lack of solubility of zircon in leucosornes, we dated the migmatization event with the Rb-Sr method.P. MONTERO, K. SALMAN, T. ZINGER, F. BEA

Geological background and petrography
The Monesterio granodiorite is a small elongated body of '" 70 km 2 , located in the S of the Olivenza-Monesterio antiform (fig.1).This structure is a NW-SE km-scale SW-vergent fold, where the basal part of the Ossa-Morena stratigrafic sequence crops out (Azor, 1994).The lowermost formation, called Serie Negra, is composed of a thick sequence of metapelites and metagreywackes with intercalations of amphibolites, black quartzites and rare marbles (Carbalhosa, 1965;Eguiluz, 1987).Its base, where the central part the Monesterio granodiorite is located, is affected by low-P high-T metamorphism that locally produces metatexitic migmatites (Eguíluz et al., 1983;Eguíluz and Abalos, 1992).The contact between the granodiorite and wall-rock metapelitic migmatites is usually sharp and crossed by abundant aplitic dikes, thus suggesting the granodiorite is intrusive.However, when deformation is intense, the contact seems to be more gradual, suggesting the granodiorite might be a subautochthonous body.
Migmatites consist of metapelitic metatexites.Leucosomes appear as discordant lens-shaped veins seldom larger than 100 x 50 x 30 cm.They are composed of quartz, K-feldspar, albite-oligoclase (An2-AnlS), muscovite, and rare cordierite in large prismatic crystals.Biotite is rare, appearing as thin selvages located mostly at the contacts of leucosome veins and therefore interpreted as restitic.Accessory phases are limited to a few grains of ilmenite, apatite, monazite and zircon.After dissolution in HF, leucosomes always leave a residuum (0.05-0.2 wt.%) comprising very minute particles of graphite, suggesting they derived from partial melting of Serie Negra metapelites.

Previous chronological data
Previous attempts to determine the age of the Monesterio granodiorite are detailed below: (1) Quesada (1990)   (Simplified from Eguíluz and Abalos, 1992).
(2) Dallmeyer and Quesada (1992) determined 4oAr/ 39 Ar ages on three mineral concentrates.The first was a homblende concentrate from a mafic xenolith inside the granodiorite and recorded an age of 553.1 ± 6.3 Ma.The other two were muscovite concentrates from mylonitic samples in shear zones within the granodiorite, which gave apparent ages of 458.9 ± I Ma and 412.8 ± 1.2 Ma respectively.
(4) Ordoñez-Casado el al. (1997) dated the outermost parts of zircons from the surrounding leucosomes with SHRIMP, obtaining an average age of 524 ± 7 Ma.

Samples
Zircons for chronology were collected from the least deformed and unaltered outcrop of the granodiorite, located in the southem part of the pluton, near the village of Monesterio.It is a biotite granodiorite with abundant large zircon crystals that were concentrated using conventional techniques of heavy liquids and magnetic separation.Zircon crystals appear either as long or short prisms with poorly developed pyramidal faces that, in most cases, correspond to PI and P2 morphotypes (Pupin, 1980).They are usual1y pale yel10w to brownish, turbid, translucent or opaque, and sorne of them have smal1 dark inclusions.Selected zircons had a size about 250 x 150 pm.Cathodoluminescence studies showed that crystals with a rounded anhedral core are relatively common.We selected six unbroken idiomorphic zircon grains for analysis.
Rb-Sr dating of leucosomes was performed on four specimens.Three are whole-rock samples col1ected from a large, coarse-grained, almost pegmatitic leucosome vein.From the fourth sample, which is an adjacent medium-grained small vein, whole-rock and three mineral concentrate -plagioclase, K-feldspar and muscovite-were analysed separately.

Analytical procedure
Single-zircon 207Pbp06Pb stepwise-evaporation and Rb-Sr isotope analyses were done at the University of Granada with a SEM-RPQ multicollector Finnigan MAT 262 Mass Spectrometer with a double-filament ion source arrangement.
Zircon grains were mounted on canoe-shaped Re filaments and heated until the Pb beam intensity was sufficient and common Pb emission (monitored by 204Pb signal) low enough.Then Pb was col1ected on the ionization filament for 20-30 min and afterwards analysed in 5 blocks with 7 scans per block.Data were acquired by peak hopping with the 206-204-206-207-208 mass sequence, using a secondary electron multiplier (SEM) as detector.The 204/206 mass-ratio was monitored to detect and, if necessary, correct for common Pb.Once the analysis was finished, a new analytical cycle (hereafter cal1ed step) started by heating the zircon on the evaporation filament to a higher temperature than in the previous step and analysing on the ionization filament as before.The procedure continued until all the Pb is exausted from the zircon.The number of steps depends on the size and Pb content of each zircon.Measurements with 204Pb/ 206 Pb higher than 0.00 I or Standard Errors (SE) on 207Pbp06Pb higher than 0.8 % at the 2 (J level were rejected.Factors for common Pb correction were calculated by iteration from the 204Pbp06Pb and 204PbP07Pb ratios provided by the model of Stacey and Kramers (1975) at the calculated age until convergence to a constant value.Mass fractionation was corrected by multiplying by -V(207/206).Standard Errors for each step were calculated according to the formula: --.SE = 2*0'/..Jn.However, the 95 % confidence int<;rval for the final age is given by (X -t(0.025)0'/..Jn, X + t(0.025pNn),where X and O' are the average and the standard deviatlOn of measured steps, n the number of steps, and t(0.025) is the upper (0.025) point of the t-distribution for n-l degrees of freedom (see Johnson and Bhattacharyya, 1984).87srj86 analyses were done after separation by ion-exchange resins using conventional methods.External precision (20') measured in 10 replicates of the standard WS-E (Govindaraju et al., 1994) was about ± 0.003 % re!. for 87Srj86Sr.87Rbj86Sr ratios were measured directly by ICP-MS (Montero and Bea, 1998), with an external precision better than ± 1.2 % re!. (20').
Crystallization age of Monesterio granodiorite zone, gave 1112 ± 12 Ma, and step 3 provided 1696 ± 23 Ma.The age of the first step is virtually identical to that of coreless zircons and is likewise supposed to represent the age of magmatic crystallization.The third step represents a minimum estimate of the core age, and the second step probably represents a mixing value.The short prism, Mok-2.l1,gave one step with 1315 ± 7 Ma, obviously a mixing value between the ages of the old core and the younger rimo We therefore conclude that the Monesterio granodiorite crystallizedat circa 510 Ma and its protolith had a component with Upper Proterozoic zircons with a minimum age of 1696 Ma.These values correspond roughly to the lower and upper intercepts on concordia given by U-Pb data on zircon concentrates (Ochsner, 1993).
Isotopic data from stepwise Pb evaporation in single-zircons from the Monesterio granodiorite are summarized in table l.
Under the cathodoluminiscence microscope, four coreless zircons were selected.They were idiomorphic and had oscillatory zoning,.so we assumed they were truly magmatic.Three of them yielded several steps with a uniform age from rim to core close to 510 Ma.The other zircon gave only one step with nearly the same value.AH the steps in these four crystals (fig.2) gave a mean age of 510 ± 4 Ma (at 95 % confidence), which we consider the best estimation of the time of crystallization.
We also analysed two crystals that had a anhedral coreo One of them, the long prism Mok-2.4,yielded three steps with different ages.Step 1, representing the outer rim, yielded 493 ± 39 Ma, with high common Pb content; step 2, representing the intermediate

Age of Migmatization
In the 87Rbj86Sr vs. 87Srj8 6 Sr diagram (fig.3), the four whole-rock samples and the K-feldspar and plagioclase concentrates plot on an isochron at 511 ± 40 Ma with initial 87Srj8bSr = 0.70914 ± 0.00048.The goodnes-of-fit is excellent, as revealed by a MSWD value of 1.7.Data are closely spread in a narrow range of 87Rbj86sr, which accounts for the high error in the age estimateso The muscovite concentrate does not plot on the same isochron but is considerably younger (table 2) so that the muscovite -whole-rock age is circa 438 Mao This effect is probably due to the partial loss of radiogenic 87Sr, highly incompatible in micas, either as a consequence of later thermal events (see discussion in Azor et al., 1995) or simply caused by hydrothermal alteration.Mob-l, Mob-2, Mob-3 and Sok6-wr are whole-rock samples.Sok6-pl, Sok6-kfsp, and Sok6-ms are plagioclase, K-feldspar and muscovite concentrates respectively.The muscovite concentrate has not been used for the isochron.

Discussion and conclusions
The crystallization age of Monesterio granodiorite and the migmatization event in the surrounding migmatites is the same, about 510 Ma.However, the much lower initial 87Sr/86Sr, the calc-alkaline chemistry, and lack of graphite precludes this granodiorite from having been derived from the same protolith, suggesting instead that granodioritic melts: (1) originated from a feldspar-rich deeper source with lower 87Sr/86Sr than the metapelites, and (2) were later intruded into a ductile migmatitic core, as indicated by the above-described field relationships.
Fig. 2.-Mean-age and confidence interval representation of eight measurements in four different zircon grains from the granodiorite of Monesterio.Lines connect individual measurements on a single zircon grain.

Table 1 .
-Isotopic data from single-zircon evaporation of the Monesterio granodiorite.
* Not used for calculation of crystallization age.204Pbfw6Pb corrected for mass fractionation and common lead, see text.SE =Standard Error.Crystallization age 20' level =510 ± 4 Ma.

Table 2 .
-Rb-Sr isotopic data of minerals and whole-rock samples from migmatite leucosomes surrounding the Monesterio granodiorite.