SHRIMP U-Pb zircon ages for the synkinematic magmatism in the Dorsal de Cangucu Transcurrent Shear Zone, Dom Feliciano Belt (Brazil): Tectonic implications

摘要

The Dorsal de Cangucu Transcurrent Shear Zone (DCTSZ) is the main Neoproterozoic structure of the Dom Feliciano Belt in the Rio Grande do Sul sector (Brazil), developed during the late stages of the Brasilano/Pan-African orogenic cycle. This structure, together with its northern - Major Gercino Transcurrent Shear Zone (Santa Catarina sector, Brazil) - and southern extension - Sierra Ballena Transcurrent Shear Zone (Uruguay), constitute a 1000 km long discontinuous lineament. Synkinematic granitoids are widespread along the DCTSZ. The main units are represented by the porphyritic, calc-alkaline high-K Quiteria granite, and the peraluminous Arroio Francisquinho and Cordilheira granites (Cordilheira Suite). In order to better constrain the time frame of the synkinematic magmatism associated with the DCTSZ, we present in this paper new zircon SHRIMP (Sensitive High Resolution Ion Micro Probe) U-Pb ages for this magmatism. The Quiteria granite yielded a concordia U-Pb age of 618.9 +/- 4 Ma, interpreted as the magmatic crystallization age. The Arroio Francisquinho and Cordilheira granites yielded concordia U-Pb ages of, respectively, 615.2 +/- 13 Ma and 607.3 +/- 12 Ma, interpreted as the magmatic crystallization ages. The latter ages constrain the main phase of synkinematic magmatism in the DCTSZ. Zircon rims of the studied samples yielded U-Pb ages between 585 and 540 Ma might represent the influence of post-magmatic fluids and deformation caused by the transcurrent activity of the shear zone. Zircon crystals interpreted as inherited yielded U-Pb ages ranging from 1800 to 650 Ma for the three studied rocks. These inherited ages are similar to those found in detrital zircon from the regional gneissic basement. The new ages for the earliest synkinematic magmatism in the DCTSZ match those of the Major Gercino and Sierra Ballena Shear Zone, strengthen previous correlation between the shear zones.