U-Pb zircon and monazite geochronology of post-collisional Hercynian granitoids from the Central Iberian Zone (Northern Portugal)

In the Central Iberian Zone (CIZ) of the Iberian Massif large volumes of granitoids were emplaced during the post-collisional stage of the Hercynian orogeny (syn- to post-D3, the last ductile deformation phase). Twelve granitic units and a quartz monzodiorite were selected for a U-Pb zircon and monazite,geochronological study. They represent successive stages of the D3 event. The Ucanha-Vilar, Lamego, Sameiro and Refoios do Lima plutons are coeval (313 +/- 2 Ma, 319 +/- 4 Ma, 316 +/- 2 Ma and 314 +/- 2 Ma, respectively) and belong to the earliest stage. Later on the Braga massif was emplaced, its different units yielding the same age: 309 +/- 3 Ma for the Braga granite, 309 +/- 1 Ma for the Gonca granite and 311 +/- 5 Ma for a related quartz monzodiorite. The Braga massif is subcontemporaneous with the Agrela and Celeiros plutons (307 +/- 3.5 Ma and 306 +/- 2 Ma, respectively), in agreement with field data. The Briteiros granite is younger (300 +/- 1 Ma), followed by the emplacement of the Peneda-Geres massif (Geres, Paufito, Illa and Canis granites). The Geres granite, emplaced at 296 +/- 2 Ma, seems to represent a first magmatic pulse immediately followed by the intrusion of the Paufito granite at 290 +/- 2.5 Ma. For the Carris granite a minimum emplacement age of 280 +/- 5 Ma was obtained. Based on these results the following chronology is proposed: (1) syn-D3 biotite granitoids, 313-319 Ma; (2) late-D3 biotite-dominant granitoids, 306-311 Ma; (3) late- to post-D3 granitoids, ca. 300 Ma; (4) post-D3 granitoids, 290-296 Ma. These chronological data indicate that successive granitic intrusions were emplaced in the CIZ during a short time span of about 30 Ma that corresponds to the latest stages of the Hercynian orogeny. A rapid and drastic change occurred at about 300 Ma, between a compressive ductile tectonic regime (D3, ca. 300-320 Ma) associated to calc-alkaline, monzonitic and aluminopotassic plutonism and a fragile phase of deformation (D-l) which controlled the emplacement of the subalkaline ferro-potassic plutonism at 290-296 Ma. (C) 1998 Elsevier Science B.V. All rights reserved.