Isotopic geochronology of granitic rocks from the Central Iberian Zone : comparison of methodologies

Five granitic rocks, concentrically disposed from core to rim, were distinguished in the Castelo Branco pluton. U-Pb-Th electron microprobe monazite ages from granitic rocks are similar and ranging between 297-303 Ma. The granitic rocks from Castelo Branco pluton are 310 ± 1 Ma old, obtained by U-Pb (ID-TIMS) in separated zircon and monazite crystals, indicating a similar emplacement age for all granitic rocks of the pluton. Initial 87Sr/86Sr isotopic ratios and εNd310 and δ18O values suggest three distinct pulses of granitic magma and that they are derived from partial melting of heterogeneous metasedimentary materials. The other granitic rocks are related by magmatic differentiation and show small variations in (Sr/Sr)310, εNd310 and δ18O. The granitic pluton of Castelo Branco shows a rare reverse zoning.


Introduction
The emplacement age of granitic rocks can be obtained using recent analytical techniques of high precision.This allows the identification of important geologic episodes and constitutes an important component in the study of the evolution of orogenic processes and crustal materials.U-Pb-Th age determination from electron microprobe geochemical analyses of monazites is a relatively recent techni-que with relevant application on the geochronology of magmatic episodes (Montel et al., 1996).This is a fast and cost-effective methodology, with good resolution, being able to date crystals with 5 µm diameter and allowing the study of heterogeneities inside of the same crystal.Minerals are not destructed and the textural relationship of the sample is preserved.The main disadvantage can be related to the lesser precision and minor sensitivity presented by this methodology.Therefore, U-Pb-Th monazite datation can be considered as a first approach to relative information on the age of a rock.
Associated to the necessity of a more accurate and precise analytical techniques, the U-Pb isotopic data of zircon and monazite crystals by ID-TIMS (Isotope Dilution Thermal Ionization Mass Spectrometry) became more usual.In this methodology, the radiogenic Pb ( 207 Pb and 206 Pb), produced by U isotopic decay, allows to get two independent ages and a third one, not independent, from the isotopic 207 Pb/ 235 U and 206 Pb/ 238 U ratios, and corresponding to the 207 Pb/ 206 Pb ratio (Rollinson, 1993).These three different ages obtained using the same mineral, confers a high validity to the U-Pb system.In U-enriched and Pb-poor minerals, like zircon and monazite, the amount of initial common Pb is insignificant relatively to the radiogenic Pb, which supports the application to these minerals.
Strontium and Nd occur in different amounts in the rocks, occurring as trace elements, and dispersed in SiO 2 -enriched rocks.Rb-Sr datation method is applied in the geochronology of granitic rocks.Rb-Sr and Sm-Nd isotopic data are important geochronological indicators.Initial 87 Sr/ 86 Sr and εNd T isotopic magma ratios are characteristic of the magma sources and will remain constant during fractional crystallization processes (Rollinson, 1993).
Whole-rock oxygen isotopic data (δ 18 O) provides important information relative to the conditions of origin and emplacement of rocks.These data have been particularly used together with geochemical and petrogenetic studies on the magmatic identification and characterization processes.
The Castelo Branco pluton consists of five concentrically zoned peraluminous granitic rocks.The aim of this work is to present and to interpret the different isotopic data (U-Pb-Th; U-Pb; Rb-Sr, Sm-Nd and δ 18 O) obtained in the granitic rocks of Castelo Branco pluton.These data will be used to infer the emplacement age of the pluton as well as information on its source.

Geological setting
The Central Iberian Zone (CIZ) is located at the central part of the Iberian Massif.This zone contains Pre-Cambrian to Lower Palaeozoic rocks and two different domains can be identified: The Ollo de Sapo formation and the Schist-Metagraywacke Complex (CXG) (Martínez Catalán et al., 2004).The Castelo Branco pluton intruded the Cambrian schist-metagraywacke complex, which consists of alternating metapelites and metagraywackes with metaconglomerate and marble intercalations.
The Castelo Branco pluton is exposed over an area of 390 km 2 and consists of five concentric latetectonic Variscan granitic rocks, with an age of 310 ± 1 Ma and presenting synchronism of installation (Fig. 1).
porphyritic biotite-muscovite granite (GrD), which contains equal amounts of biotite and muscovite (Fig. 1).A coarse-grained muscovite > biotite granite (GrE) occupies the periphery of the pluton in the sector from N to E (Fig. 1).The contact between granites GrD and GrE is also sharp, but the two granites are usually altered at this contact.The pluton produced mica schists in the outer zone and a pelitic hornfels in the inner zone of the contact metamorphic aureole, particularly on the southern and eastern zones of the pluton.

Analytical methods
The U-Pb-Th monazite individual ages are calculated from the U, Pb and Th monazite concentrations obtained by electron microprobe on thin sections.This age can be considered valid if, at the time of crystallization, the Pb is practically inexistent and losses of Pb, Th and U had not occurred.The U-Pb-Th determinations on monazites were performed on a Cameca SX100 electron microprobe at the Laboratoire Magmas et Volcans of the University of Blaise Pascal, Clermont-Ferrand (France).Analytical conditions included an accelerating voltage of 15 kV and a beam current of 150 nA.Standards used included UO 2 (U Mα), ThO 2 (Th Mα), apatite (Ca Kα, P Kα), zircon (Si Kα) and synthetic phosphates (Y Lα, La Lα, Ce Lα, Pr Lα, Nd Lα, Sm Lα, Gd Lα).The theoretical basis and associated statistical treatment of data follow the analytical procedure detailed by Montel et al. (1996).A least-square modelling approach was applied to the ages of the crystals and its quality is assessed from the mean square of weighted deviates (MSWD).
U-Pb isotopic data were determined using zircon and monazite separated crystals from selected representative granitic rocks of Castelo Branco pluton.Zircon and monazite separation was carried out by a combination of magnetic separation and flotation in heavy liquids.The U-Pb isotopic results for zircon and monazite were obtained by isotope dilution thermal ionization mass spectrometry (TIMS) using a Finnigan-Mat 262 spectrometer at the Department of Geosciences, University of Oslo, Norway, following the standard procedure of Krogh (1973) with the adaptations described by Corfu (2004).
The Sr and Nd isotope analyses were obtained at the Centro de Instrumentación Científica of the University of Granada, Spain.Samples were diges-ted using ultra-clean reagents and analyzed by TIMS in a Finnigan Mat 262 spectrometer, after chromatographic separation with ion-exchange resins.Normalization values were 86 Sr/ 88 Sr = 0.1194 and 146 Nd/ 144 Nd = 0.7219 and the blanks were 0.6 and 0.09 nanograms for Sr and Nd, respectively.The standards yields values of 87 Sr/ 86 Sr = 0.710252 ± 0.000021 (2σ) NBS 987 and 143 Nd/ 144 Nd = 0.511844 ± 0.000010 (2σ) La Jolla. 87Rb/ 86 Sr and 147 Sm/ 144 Nd were determined by ICP-MS following the method developed by Montero & Bea (1998), with a precision better than ± 1.2% and 0.9% (2σ), respectively.
Oxygen isotope analyses of whole-rock samples were carried out at the Department of Earth Sciences, University of Western Ontario, Canada, using a conventional extraction line and employing chlorine trifluoride as reagent.This methodology has a precision of ±0.2‰ and the used standards include quartz and laboratory CO 2 .

Isotope geochemistry
To compare different isotopic data and associated emplacement ages for the granitic rocks of Castelo Branco pluton, three different geochronological techniques were used.It will be also presented the whole-rock Rb-Sr, Sm-Nd and δ 18 O isotopic ratios from granitic rocks to characterize the magmatic sources.

U-Th-Pb on monazite
Electron microprobe U-Th-Pb isotopic data on monazite, considers that monazite contains small amounts of common Pb, resulting from Th and U isotopic decay.Th and U enrichment of monazite promotes a radiogenic Pb accumulation on the mineral structure, remaining in precise amounts during less than 100 Ma (Montel et al., 1996).
Monazites in all granitic rocks of the Castelo Branco pluton were dated by EPMA (Table 1).Those in samples GrA, GrdB, GrdC and GrE yield average U-Th-Pb ages of 303 Ma to 301 Ma.U-Pb-Th monazite ages from granite GrD is somewhat lower at 297 ± 3 Ma (Table 1).However, the granitic rocks obtained ages are similar and consistently within the analytical error (Table 1).
The highest MSWD values obtained for GrD and GrE granites can be associated with the number of points and some scatter, which may reflect some initial Pb associated with unresolved geological complexity or due to alteration processes.

U-Pb on zircon and monazite
U-Pb isotopic analyses were carried out on separated zircon and monazite crystals from three samples representing the GrA, GrdB and GrE of Castelo Branco pluton, using the ID-TIMS method (Table 2).Granodiorite GrdC and granite GrD were not considered because field relationships and geochemical characteristics suggest that they are related to granodiorite GrdB by a fractional crystallization process (Antunes et al., 2008).U-Pb zircon and monazite ages inside the granitic rocks are consistent and similar (Table 2).
The zircon crystals from granite GrA are concordant, defining a good alignment (MSWD = 1.11) and yielding an emplacement age of 309.9 ± 1.1 Ma.The more concordant zircon yields a 207 Pb/ 206 Pb age of 309.9 ± 1 Ma with a discordance of 1.1% (Table 2).
The monazite crystals plot slightly reversely discordant, a fact commonly linked to Palaeozoic geological processes.The 207 Pb/ 235 U ratio is not affected by this disequilibrium effect and can be used as the closest estimate for the age of the monazite (Table 2).
The zircon analyses for a sample of granodiorite GrdB plot on or close to the Concordia curve.However, one zircon crystal suggests minor inheritance and the age of 310.1 ± 0.8 Ma from the more concordant zircon should be considered (Table 2).The monazite and zircon crystals yields a similar age (Table 2).
The zircon analyses from granite GrE defines a discordia line which yields a lower intersect age of 309.1 ± 0.6 Ma and an upper intersect age of about 1,000 Ma.The emplacement age is similar to the 207 Pb/ 206 Pb age for the more concordant zircon crystal (Table 2).The monazite analyses scatter towards the Concordia curve, however the more concordant monazite yields and age of 309.7 ± 0.4 Ma, similar to the concordant zircon ages (Table 2).
Considering the U-Pb zircon and monazite isotopic data of the granitic rocks from Castelo Branco pluton and according to the age uncertainties associated to the method, it could indicate a 310 Ma emplacement for these granitic rocks.This suggests that the granitic rocks from Castelo Branco pluton are contemporaneous (Table 2).