A long-lived active margin revealed by zircon U–Pb–Hf data from the Rio Apa Terrane (Brazil): New insights into the Paleoproterozoic evolution of the Amazonian Craton

We present the first regional in-situ zircon U–Pb–Hf isotopic data from metaigneous and metasedimentary rocks from the Paleo- to Mesoproterozoic Rio Apa Terrane (RAT), a crustal fragment outcropping in the central-western Brazil and north-eastern Paraguay. These new ages and Hf isotopic data delineate three magmatic events, which record the construction of the temporally and isotopically distinct Western and Eastern Terranes of the RAT. The Western Terrane comprises the 2100–1940 Ma Porto Murtinho Complex and the 1900–1840 Ma Amoguijá Belt, which both define a crustal reworking array in εHfT-time space evolving from a precursor source with Hf TDM age of ca. 2700 Ma. The 1800–1720 Ma Caracol Belt constitutes the Eastern Terrane and yields suprachondritic εHfT signatures up to +7.1, indicating significant juvenile input. The metasedimentary Amolar Group and Rio Naitaca Formation in the Western Terrane have maximum depositional ages of 1850–1800 Ma and subchondritic εHfT signatures down to −5.7, similar to the underlying basement of the Amoguijá Belt. In the Eastern Terrane, the Alto Tererê Formation has a maximum depositional age of 1750 Ma and mostly suprachondritic εHfT signatures, similar to magmatic rocks of the underlying Caracol Belt. Together, the new igneous and detrital zircon age and Hf isotopic data record a temporal and spatial transition from 2100 to 1840 Ma crustal reworking in the west to more juvenile magmatism at 1800–1720 Ma in the east. This transition is interpreted to reflect convergent margin magmatism associated with periods of subduction zone advance and retreat in an accretionary orogenic setting. Comparison of the εHfT-time signature of the RAT with the Amazonian Craton suggest penecontemporaneous development, with the Western and Eastern Terranes of the RAT being correlative with the Ventuari-Tapajós and Rio Negro-Juruena Province of the Amazonian Craton, respectively. Our new data also reveal that the εHfT signatures of the RAT are distinct from the Maz terrane, which refutes the MARA Block hypothesis

Using apatite to resolve the age and protoliths of mid-crustal shear zones: A case study from the Taxaquara Shear Zone, SE Brazil

Shear zones accommodate strain and facilitate migration of hydrothermal fluid and magma through the crust. Unravelling the deformation history of shear zones requires correspondence between the closure temperature of mineral geochronometers and the temperature of deformation. Here, we adopt apatite U–Pb-trace element analysis as a tool for dating deformation and tracing the protoliths of mid-crustal shear zones through a case study of the Taxaquara Shear Zone (TSZ), a major transpressional shear zone in the southern Ribeira Belt of SE Brazil. Apatite from mylonites in the TSZ yield U–Pb ages of 558–536 Ma, considering uncertiainties, which slightly overlap with 40Ar/39Ar ages of 538 ± 2 Ma from muscovite in the lower limit. The closure temperature of apatite is estimated at 500–460 °C, which is slightly higher than that estimated for syn-kinematic muscovite (445–420 °C). Apatite from shear zone mylonites has Sr/Y and LREE systematics typical of apatite from S- and I-type granitoids, suggesting the adjacent and undeformed Pilar do Sul and Piedade granites are the likely protoliths of the mylonites. This interpretation is supported by new U–Pb ages of ca. 605 Ma from pre-kinematic zircon and titanite from mylonites, which corresponds closely with new U–Pb apatite ages and previously published U–Pb monazites ages from the Pilar do Sul Granite. We suggest the U–Pb system of apatite in the TSZ was reset via volume diffusion during rapid cooling given that it preserves the igneous geochemical signatures. Moreover, this interpretation is consistent with the lower apatite closure temperature (500–460 °C) relatively to the temperature of deformation (530–480 °C). The revised ~560–535 Ma age for the TSZ demonstrates that it post-dates the collisional phase of the Ribeira Belt (620–595 Ma and 595–565 Ma), indicating protracted strain accommodation during the Brasiliano–Pan African orogeny, and supports correlation with the 600–550 Ma and 570–550 Ma transpressional Dom Feliciano and Kaoko Belts. This study demonstrates that apatite is a powerful tool for unravelling the history of mid-crustal shear zones as it is stable in a wide range of lithotypes, has trace element compositions that are sensitive to the environment of formation, and Pb closure temperatures typical of mid-crust conditions. U–Pb-trace element analysis of apatite provides a robust means to date shear zones that can be complimentary to, or independent of, more traditional 40Ar/39Ar analysis of mica or amphibole