The composition of zircon in Variscan granites from Northern Portugal

H C.B Martins,J Abreu

doi:10.3989/egeol.41729.318

Abstract

A group of slightly peraluminous Variscan plutons in Northern Portugal were selected from the study of zircon composition. The selected plutons are: the Vila Pouca de Aguiar and the Lavadores-Madalena plutons with I-type affinities and the Vieira do Minho pluton, an l-S transitional type. Zircon occurs as euhedral to subhedral crystals and exhibit finely concentric oscillatory magmatic zoning mainly related to variations of Hf, Y, U and Th concentrations. Most zircon crystals show the dominant “xenotime” substitution. The zircon crystals have Zr/Hf ratio in the range of 21 to 52, with no significant differences between the different granites. These values are in the same range of other peraluminous granites and are in accordance with a crustal signature of zircon. Moreover, the range of Zr/Hf values in zircon crystals overlaps with that of crustal sources and consequently to the potential protoliths proposed in the genesis of the Vieira do Minho and the Vila Pouca de Aguiar plutons, namely meta-igneous crustal sources at different levels. Although zircon from the Lavadores-Madalena pluton has a compositional range similar to the other plutons, an origin by hibridisation has been proposed. However, similar zircon chemistry between this pluton and Vila Pouca de Aguiar and Vieira do Minho plutons could also suggest a similar crustal source.

Highlights

Zircon is an invaluable tool in countless geological studies due is wide distribution in a spectrum of rock types and in granites
The abundance of zircon is low, it may strongly affect the behaviour of many trace elements during the crystallization of magmas, and understanding its compositional variation is important for investigating the evolution of magmatic silicic systems (e.g. Hoskin et al 2000; Belousova et al, 2002 and 2006; Claiborne et al, 2010)
We study the composition of zircon from a group of late- to post- Variscan plutons in NW of Portugal which present variable typology (Martins et al, 2007, 2009 and 2013; Silva, 2010)

Summary

Introduction

Zircon is an invaluable tool in countless geological studies due is wide distribution in a spectrum of rock types and in granites. The granitic plutons selected for this study are located the Central Iberian Zone and one of them on the western border of the same zone, NW Portugal, being the Vila Pouca de Aguiar pluton, the Vieira do Minho pluton and the Lavadores-Madalena pluton, respectively (Fig. 1) Their emplacement were controlled by tectonic regional structures during the late stages of the Variscan orogeny, mainly from 310 to 299 Ma (Martins et al, 2007, 2009, 2011, 2013). The Vieira do Minho pluton is composed by the porphyrytic coarse-grained Vieira do Minho granite (VMG) and the medium-grained Moreira de Rei granite (MRG), both are biotite bearing granitic rocks of monzonitic-granodioritic composition (Almeida et al 2002; Martins et al 2013) They are peraluminous with A/CNK ranging from 1.04 to 1.24, and with a I-S transitional type affinity.

Analytical methods

Findings

Discussion and conclusions