Zircon typologies and internal structures as petrogenetic indicators in contrasting variscan biotite-rich granite plutons from Northern Portugal

Abstract

The aim of the present morphological zircon study is to characterize the sources reservoirs involved in the generation of the different type of granites, testing the classic ‘Pupin method’ and zircon geochemistry against the petrogenetic indications given by geochemical and isotopic data. In northern Portugal large volumes of granitoids were emplaced during the last stage (D3) of the Variscan orogeny and display a wide range of petrological signatures. We studied the morphologies, internal structures and geochemistry of zircons from (1) Syn-D3 biotite granitoids: Ucanha–Vilar, Lamego, Felgueiras, Sameiro and Refoios do Lima plutons (2) Late- D3 biotite-dominant granitoids: Vieira do Minho pluton and (3)post-D3 biotite granitoids: Vila Pouca de Aguiar pluton. The typological evolutionary trends suggest a crustal or dominantly crustal origin for the syn-D3 Refois do Lima granite and for the late- D3 pluton whereas an hibridisation process is proposed for the Ucanha-Vilar, Lamego and Sameiro granites. The zircon population from the post- D3 granites define a typological evolutionary trend betweeen calc-alkaline and subalkaline granites suggesting a under crustal or mantle source. The petrogenetic model proposed by zircon typological and geochemical study in all plutons is in accordance with geochemical and isotopic data. In fact the Syn-D3 biotite granitoids display Sri ratios and εNd values varying in the range 0.7072 - 0.7116 and - 4.4 to - 6.3, and εNd = - 5.6 to - 5.7 and finally post- D3 biotite granitoids respectively; the late-D3 pluton present Sri= 0.7089-0.7090 present weakly evolved isotopic compositions, Sri = 0.7044– 0.7077 and εNd= -2.0 to -2.6.