Late-Variscan emplacement and genesis of the Vieira do Minho composite pluton, Central Iberian Zone: Constraints from U–Pb zircon geochronology, AMS data and Sr–Nd–O isotope geochemistry

Abstract

In the Central Iberian Zone synorogenic (syn-and late-D 3 , the last deformation phase) composite batholiths of biotite-rich granitods with peraluminous character are well represented. We report here U-Pb zircon and monazite ages, geochemical, Sr-Nd-O isotopic data and Anisotropy of Magnetic Susceptibility (AMS) studies for the late-D 3 Vieira do Minho pluton, in northern Portugal. AMS reveals the paramagnetic behaviour of these granites and the magnetic fabric enhances the major role of NW-SE regional anisotropies on controlling the ascent and emplacement of the magmas. Magnetic anisotropy and medium temperature microstructures also point out the evolution of regional tectonics during crystallization of the magma. U-Pb zircon and monazite analyses yield consistent ages of 310 ± 2 Ma and 312 ± 2 Ma, interpreted as emplacement age. The Vieira do Minho pluton consists of two peraluminous monzogranites, the VMG and the MRG. Both have similar ε Nd values (VMG= −5.2 to −5.7; MRG =−4.98 to −5.96) but the VMG is slightly enriched in 87 Sr/ 86 i = 0.7087-0.7098 as well as in δ 18 O in the range of 10.6‰ to 11.0‰ than the MRG with 87 Sr/ 86 i =0.7064-0.7075 and δ 18 O=9.9‰-10.5‰. These granites are associated with coeval scarce grabroic intrusions and/or mafic microgranular enclaves which are not considered as mafic precursors of the associated granitic magmatism. Instead, a lower metaigneous crustal source, at different levels, is proposed based on the available data set.

Zircon crystal morphology and internal structures as a tool for constraining magma sources: Examples from northern Portugal Variscan biotite-rich granite plutons

“…On the other hand, both granitoids have similar e Nd values (VMG = À5.2 to À5.7; MRG = À4.98 to À5.96), but the VM granite is slightly more enriched in ( 87 Sr/ 86 Sr) i = 0.7087-0.7098 as well as in d 18 O in the range from 10.6% to 11.0% than the MR granite with ( 87 Sr/ 86 Sr) i = 0.7064-0.7075 and d 18 O = 9.9%-10.5% (Martins et al, 2013).…”

“…The gradational contacts between the Vieira do Minho and Moreira de Rei granite indicate an almost synchronous emplacement. The U-Pb isotopic analyses carried out on zircon and monazite fractions from the Vieira do Minho granite indicate a crystallization age of 310 AE 2 Ma (Martins et al, 2013). The Moreira de Rei granite has been dated by Dias et al (2002) yielding an age of 308 AE 4 Ma.…”

“…This was the main period of successive generation of granites (Ferreira et al, 1987), which exhibit large compositional variability (Dias et al, 1998;Martins et al, 2009Martins et al, , 2013. The classification of granitoids is related to this third phase of deformation (Dias et al, 1998;Ferreira et al, 1987;Martins et al, 2009Martins et al, , 2011Martins et al, , 2013 divided into: syn-D 3 granitoids, 313-319 Ma (peraluminous biotite granodiorites to monzogranites and highly peraluminous two-mica leucogranites); late-D3 biotite-dominant granitoids, 306-311 Ma (mainly as composite massifs displaying a wide compositional range from gabbroic to granitic, being metaluminous to peraluminous); late-to post-D3 granitoids, ca. 300 Ma (highly peraluminous, two-mica leucogranites); post-D3 granitoids, 290-296 Ma (slightly metaluminous to peraluminous monzogranites occurring as zoned plutons).…”

“…This pluton is composite and consists of two different granite units (Almeida et al, 2002;Martins et al, 2013): the Vieira do Minho granite (VM) is a coarse-grained monzogranite and the Moreira de Rei (MR) granite a medium-grained monzogranite.…”

“…With the present morphological zircon study, we aimed to characterize the source reservoirs involved in the generation of sin-, late-and post-D 3 biotite-rich granitoids from northern Portugal, a suite of geochemically wellcharacterized plutons, which have been the subject of intensive whole-rock and mineral-scale studies (Almeida et al, 2002;Dias et al, 1998Dias et al, , 2002Martins et al, 2007Martins et al, , 2009Martins et al, , 2013Simões, 2000) testing the classic ''Pupin method'' against the petrogenetic indications given by geochemical and isotopic data.…”