Geocronologia U-Pb-Hf dos granitos sintranscorrentes e relações estruturais ao longo da zona de cisalhamento Sobral, noroeste do Ceará

Abstract

The Transbrasiliano Lineament (LTB) is an intercontinental structure that was consolidated at the end of the Neoproterozoic. It is approximately 4,000 km long, extending from Paraguay and Argentina, cutting through the center-west and northeast of Brazil, extending to the African Continent. In the northwest portion of the State of Ceará, this lineament is called the Sobral Shear Zone (ZCS), presenting, mainly, features of dextral movement and mylonitic foliation in the NE-SW direction, separating the Northwest of Ceará (DNC) and Central Ceará domains (DCC). The studies carried out at the mesoscopic scale allowed identifying syntranscurrent granites along the ZCS with different mylonitization intensities; primary foliations completely transposed by dynamic processes related to the transcurrent movement of the ZCS, which generated mylonitic foliations with high dip (vertical/subvertical) mainly in the NE-SW direction; in addition to horizontal mineral stretch lineation; as well as predominantly dextral, rarely sinistral kynematics. On the microscopic scale, the syntranscurrent granites were classified petrographically and subdivided into 2 groups: G1 (mylonitic syenogranites and monzogranites) and G2 (mylonitic granodiorites); and through microstructural analysis it was possible to interpret the deformation conditions that affected the mineral components of these rocks. The microstructures are characteristics of ductile deformation that recrystallized quartz and plagioclase crystals by high temperature SGR (Subgrain Rotation) and GBM (High-Temperature Grain Boundary Migration) mechanisms, generating fabric s-c structures, migration at the crystal boundary, subgrains, window structure, core-mantle and feldspar fish. The micaceous minerals present fish ductile features indicating dextral deformation, isolatedly sinistral. The similarity of the mesoscopic, macroscopic and microscopic structures allowus to state that the mylonitization imposed by the occurrence of the ZCS is recorded and consistent at all scales. From the geochemical data it is possible to notice that these granites have an acid character (SiO2 > 67%) and peraluminous; they are predominantly from syncolisional environments; and that the G2 is poorly fractionated (Rb/Sr < 0.5) with a Rb/Sr ratio between 0.1 and 0.3. On the other hand, the G1 presents a characteristic of a moderate to high fractional suite (Rb/Sr > 0.5 and < 20.5) with a Rb/Sr ratio between 1.0 and 2.4. Only one G1 sample has a low degree of fractionation (Rb/Sr = 0.2). There is enrichment of lithophile elements (LILE) which include Ba and Rb are relatively more enriched compared to ETRL such as La, Ce and Nd, and in relation to elements with high field strength (HFS) such as Th, Ti, Zr, Nb and Ta. There is also enrichment in LREE relative to HREE, and this is confirmed by the slight slope of the REE diagram. Samples of two mylonitic granodiorites (MH-19-05 and MH-19-09) were analyzed using the U-Pb-Hf method and the concord ages of crystallization were obtained at 589±6 Ma (Ediacaran) with MSWD=1.5 and of upper inherited Paleoproterozoic intercept of 2171±21 Ma (Rhyacian) with MSWD=0.81 in sample 09. In sample 05 a Paleoproterozoic upper intercept age of 2094±37 Ma (Late Rhyacian) with MSWD=1.6 was obtained. The Hf-T DM C ages in sample 09 were calculated at 0.59 Ga and range from 2.21 to 2.33 Ga and the εHf (0.59 Ga) values are strongly negative ranging from –11.5 to – 13.4 (Rhyacian-Siderian sources from crustal reworking); and at 2.17 Ga and range from 2.50 to 3.16 Ga and their εHf (2.17 Ga) values range from –6.9 to +3.7 (mixture of Neoarchean-Mesoarchean crustal and mantle sources). In sample 05, the Hf-TDM C ages were calculated at 2.09 Ga Ma and range from 2.56 to 2.63 Ga with positive values of εHf (2.09 Ga) ranging from +0.7 to +1.8 (Neoarchean mantle source).