The Precambrian/Lower Cambrian pluton from Vila Nova (Central Portugal) El plutón Precámbrico-Cámbrico Inferior de Vila Nova (Centro de Portugal)

The Vila Nova pluton is a small, Pre-Variscan granitic body that intruded rocks of the Central Iberian Zone near the contact with the Ossa Morena Zone and is affected by several shear zones and faults. Its contact metamorphic aureole is constituted by micaschist with porphyroblasts in the outer zone and hornfels in the inner zone. Small metasedimentar xenoliths are dispersed all over the body. The pluton has a great mineralogical heterogeneity with pronounced variations in muscovite/biotite and plagioclase/microcline contents and is classified as granite, granodiorite or tonalite. It is a leucogranite, highly peraluminous (A/CNK = 1.31 – 1.64), magnesian and calc-alkaline to alkaline-calcic. The variation diagrams show curvilinear trends with silica. Eu/Eu* = 0.47 – 0.77 and there is a slight enrichment in LREE relative to HREE. The normalized diagrams indicated dominantly crustal granite, related to subduction. U-Pb isotopic data of zircon and monazite gives 540-542 Ma age.


Introduction
One of the main purposes of modern petrology is the characterization of the process of renovation and accretion which control crustal evolution.Granitoids, well represented in orogenic domains, are the most important crust constituents and their origin is still an important objective in geochemistry investigations.Recent studies show that granitoids are important indicators of the mechanisms previously referred, due to their variable composition and geodynamic setting.
The age of granitoid rocks can be obtained by modern and highly accurate techniques and its The Precambrian/Lower Cambrian pluton from Vila Nova (Central Portugal)

ABSTRACT
The Vila Nova pluton is a small, Pre-Variscan granitic body that intruded rocks of the Central Iberian Zone near the contact with the Ossa Morena Zone and is affected by several shear zones and faults.Its contact metamorphic aureole is constituted by micaschist with porphyroblasts in the outer zone and hornfels in the inner zone.Small metasedimentar xenoliths are dispersed all over the body.The pluton has a great mineralogical heterogeneity with pronounced variations in muscovite/biotite and plagioclase/microcline contents and is classified as granite, granodiorite or tonalite.It is a leucogranite, highly peraluminous (A/CNK = 1.31 -1.64), magnesian and calc-alkaline to alkaline-calcic.The variation diagrams show curvilinear trends with silica.Eu/Eu* = 0.47 -0.77 and there is a slight enrichment in LREE relative to HREE.The normalized diagrams indicated dominantly crustal granite, related to subduction.U-Pb isotopic data of zircon and monazite gives 540-542 Ma age.
determination allows the identification of important geologic episodes being a fundamental component in the study of the evolution of crustal orogenic processes (Carrigan et al., 2005).
This paper is focused on a mineralogical, geochemical and isotopic study (U-Pb), of the granitic pluton of Vila Nova, which intruded the fragility zone imposed by the Cadomian suture between the Central Iberian Zone and the Ossa Morena Zone of the Iberian Massif.

Geological setting
The pre-variscan Vila Nova granitic pluton, outcrops in the CIZ and intrudes the Neoproterozoic/ Cambrian Schist-Metagraywacke Complex, in Central Portugal.It has a slightly elliptical shape of approximately 15 km 2 , aligned in the N-S direction.To the West, the igneous body contacts with Proterozoic, Neoproterozoic and Ordovician rocks by a complex system of faults related to the Blastomilonitic Porto-Coimbra-Tomar shear zone (Fig 1).
To the East, the igneous body produced a metamorphic contact aureole with micaschist with porphyroblasts in the outer zone and hornfels in the inner zone.To the Northwest, the granitic rock contacts with sedimentary clastic rocks of Cretaceous age by a thrust (Fig. 1).The pluton is affected by several shear zones and faults, especially near the Blastomilonitic shear zone, and shows an intensive weathering, which results on strong and penetrative arenization.
Small metasedimentary xenoliths of centimetric dimensions are hosted by the granitic rock.They are surmicaceous and some have prismatic andalusite.Quartz ocellis (up to 10 cm long) occur dispersed all over the granite and small and thin (milimetric) quartz veins with chlorite are also found.

Analytical methods
Seven samples were collected for petrographic, geochemical and isotopic studies in the Vila Nova pluton.Sampling was conditioned by the limitations imposed by the alteration and outcrop scarcity of the area.
Samples were prepared in Earth Sciences Department, University of Coimbra (Portugal).Thin sections were made for modal analysis and petrographic studies.The samples were crushed in an agate mortar to < 75 µm.Major, trace and rare earth elements were determined by ICP-MS in Actlabs, Canada.FeO was determined by titration with potassium permanganate with a precision of 1%, at the Earth Sciences Department, University of Coimbra.
The less altered granitic sample was chosen for U-Pb ID-TIMS isotopic studies.It was grinded on a jaw crusher.The fraction inferior to 180 µm was separated by heavy liquids and magnetic separation to obtain a concentrate of zircon and monazite.
A representative fraction of zircon was selected avoiding crystals with external disturbed domains, inclusions or zonation.After chemical treatment isotopic ratios were obtained by isotope dilution and thermal ionization mass spectrometry (ID-TIMS), using a Finningan Mat 62 mass spectrometer at the Geosciences Department, University of Oslo (Norway), following Krogh (1982) methodology with the adaptations of Corfu (2004).The analysis accuracy and respective ages were obtained with 2α of significance degree.Results were plotted in Concordia diagrams (Krogh, 1982;Davidson & Van Breemen, 1988).
The granitoid rock is a leucogranite, peraluminous, with A/CNK ranging from 1.31 to 1.69 and can be classified as a S-type granite.It is also classified as a magnesian, calc-alkaline to alkaline-calcic granite (Frost et al., 2001) or as a volcanic arc granite (Fig. 3a, b, c) (Pearce et al., 1984).The Ocean Ridge Granite (ORG) and the Primordial Mantle normalized patterns indicates that is a crustal granite (Fig. 3d), related to subduction (Fig. 3e), with positive anomalies for Rb, Th, Ce and Sm and negative anomalies for Ba, Ta, Nb, Sr and Ti (Fig. 3d, e).
Variation diagrams (not presented here) show regular curvilinear trends with decrease in TiO 2 , Al 2 O 3 , FeO, MgO, Ba, Sc, V, Cr, Ni, Nb, Zr, Hf and ∑REE with SiO 2 increase, K 2 O and Rb show segmented trends, while CaO, Sr and Cs don't show any trend, which can be due to the hydrothermal alteration (Reis, 2009).

Isotope geochemistry
The least altered sample from Vila Nova pluton was selected for U-Pb isotopic data determination.In this sample five fractions of zircon and two fractions of monazite were analysed.The selected crystals have prismatic habit and are colourless.They do not present inclusions, fractures or disturbed domains.
On the concordia diagram, 206 Pb/ 238 U versus 207 Pb/ 235 U, three zircon fractions are plotted on the Concordia, while two zircon fractions are discordant and define an inverse discordia curve (Fig. 5).The age defined by the three concordant zircon fractions is 541.2 ± 0.8 Ma (MSWD = 1.3).The inverse discordia curve intercepts the concordia curve in the lower part, with a minimum emplacement age of 540.4 ± 0.8 Ma and an upper interception age of 1887 ± 17 Ma and a MSWD of 0.65.(Pearce et al., 1984).Geochemical normalized pattern to ORG (Ocean Ridge Granite; Pearce et al., 1984) (d).Geochemical normalized pattern to primordial mantle (McDonough et al. 1992) (e).
The more concordant zircon indicates a 207 Pb/ 206 Pb age of 540.5 ± 0 Ma, with a discordance of 0.1%, identical to the other zircon fraction ages obtained.
The monazite fractions are concordant, although they tend to plot far from zircon, because they are easily affected by disturbances associated to the geologic episodes (Kalt & Corfu, 2000), showing a 206 Pb excess due to the 230 Th incorporated during monazite crystallization (Schärer, 1984).The mon-azite crystal that plots close to the zircon has a 207 Pb/ 235 U age of 542.7 ± 1.0 Ma.
The magmatic emplacement age is of 541.2 ± 0.8, according to the three concordant zircon fractions.The range 540-542 Ma is the possible period of installation to the Vila Nova Pluton, with an Precambrian-Cambrian age.

Conclusions
The Vila Nova Pluton is a heterogeneous peraluminous granitic rock.This pluton has a crustal origin, subduction related, that intruded near the suture between the CIZ and the OMZ in the Precambrian-Cambrian transition.The U-Pb age, obtained with concordant zircon and monazite age is 541.2 ± 0.8 Ma.This result defines a range of 540-542 Ma for the pluton emplacement.
The Vila Nova Pluton and probably the other plutons of the area are indicative of an igneous activity in the CIZ, related to Cadomian Orogeny and strongly affected by the Variscan orogeny.The Vila Nova pluton is an evidence of a crustal renewal episode during the Precambrian-Cambrian transition in the CIZ, also suggested by Neves et al. (2001) for other granitc plutons in the same geotectonic setting.