Quartz crystallographic and morphologic fabrics during folding/transposition in mylonites

Quartz grain shape fabrics (GSF) and c-axis lattice preferred orientation (LPO) were investigated in folded quartz ribbons with variable fold geometry in low metamorphic-grade granitic mylonites from northwestern Argentina. The folds, which vary from open to isoclinal, show an asymmetric short limb–long limb geometry developed simultaneously with the quartz fabrics during progressive simple shear. GSF shows a consistent obliquity relative to the bulk shear plane throughout the folded ribbons. GSF inclination varies between 25° and 45° oblique to the shear plane, indicating that relatively small finite strain increments (γ∼1.2) are attained before the GSF regeneration. In contrast, the c-axis preferred orientation varies throughout the folded ribbons. Unfolded ribbons and long limbs of folds show a typical c-axis maximum near the shear plane pole, while short limbs of tight and isoclinal folds show a c-axis maximum nearly orthogonal to that of long limbs. This pattern of domainal LPO is maintained in thickened ribbons derived from folding and transposition. This indicates that the GSF regeneration is comparatively much faster than a cycle of folding/transposition. Conversely, the LPO regeneration is much slower. We conclude that folding/transposition is a mechanism for development of domainal quartz LPO in kinematic frameworks with dominant simple shear.Fil: Hongn, Fernando Daniel. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Instituto Geonorte; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Hippertt, J. F. Universidade Federal de Ouro Preto; Brasi

The Archean–Paleoproterozoic evolution of the Quadrilátero Ferrífero (Brasil): Current models and open questions

The Quadrilátero Ferrífero is a metallogenic district (Au, Fe, Mn) located at the southernmost end of the São Francisco craton in eastern Brazil. In this region, a supracrustal assemblage composed of Archean greenstone and overlying Neoarchean–Paleoproterozoic sedimentary rocks occur in elongated keels bordering domal bodies of Archean gneisses and granites. The tectonomagmatic evolution of the Quadrilàtero Ferrífero began in the Paleoarchean with the formation of continental crust between 3500 and 3200 Ma. Although this crust is today poorly preserved, its existence is attested to by the occurrence of detrital zircon crystals with Paleoarchean age in the supracrustal rocks. Most of the crystalline basement, which is composed of banded gneisses intruded by leucogranitic dikes and weakly foliated granites, formed during three major magmatic events: Rio das Velhas I (2920–2850 Ma), Rio das Velhas II (2800–2760 Ma) and Mamona (2760–2680 Ma). The Rio das Velhas II and Mamona events represent a subduction-collision cycle, probably marking the appearance of a modern-style plate tectonic regime in the Quadrilátero Ferrífero. Granitic rocks emplaced during the Rio das Velhas I and II events formed by mixing between a magma generated by partial melting of metamafic rocks with an end member derived by recycling gneissic rocks of older continental crust. After deformation and regional metamorphism at ca. 2770 Ma, a change in the composition of the granitic magmas occurred and large volumes of high-K granitoids were generated. The ca. 6000 m-thick Minas Supergroup tracks the opening and closure of a basin during the Neoarchean–Paleoproterozoic, between 2600 and 2000 Ma. The basal sequence involves continental to marine sediments deposited in a passive margin basin and contain as a marker bed the Lake Superior-type Cauê Banded Iron Formation. The overlying sediments of the Sabará Group mark the inversion of the basin during the Rhyacian Minas accretionary orogeny. This orogeny results from the collision between the nuclei of the present-day São Francisco and Congo cratons, generated the fold-and thrust belt structure of the Quadrilátero Ferrífero. Afterwards, the post- orogenic collapse resulted in the deposition of the Itacolomi Group and in the genesis of the dome-and-keel structure. In this paper, we review current knowledge about the 1500 Ma long-lasting tectonomagmatic and structural evolution of the Quadrilátero Ferrífero identifying the most compelling open questions and future challenges.Fil: Farina, F.. Universidade Federal de Ouro Preto; BrasilFil: Capucine, A.. Universidade Federal de Ouro Preto; BrasilFil: Martínez Dopico, Carmen Irene. Universidade Federal de Ouro Preto; Brasil. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Aguilar Gil, C.. Universidade Federal de Ouro Preto; BrasilFil: Moreira, H.. Universidade Federal de Ouro Preto; BrasilFil: Hippertt, J. P.. Universidade Federal de Ouro Preto; BrasilFil: Cutts, K.. Universidade Federal de Ouro Preto; BrasilFil: Alkmim, F.. Universidade Federal de Ouro Preto; BrasilFil: Lana, C.. Universidade Federal de Ouro Preto; Brasi

Ar-40/Ar-39 ages (600-570 Ma) of the Serra do Azeite transtensional shear zone: evidence for syncontractional extension in the Cajati area, southern Ribeira belt

This paper presents 40Ar/39Ar ages of the rocks from the Serra do Azeite transtensional shear zone in the southern part of the Ribeira belt, between the States of São Paulo and Paraná, and also discusses the regional correlations and the tectonic implications for other parts of the belt. The geochronological data suggest that transtensional deformation was active between 600 and 580 Ma (hornblende and muscovite apparent ages, respectively). This time span is considerably older than previous proposals for the period of activity of these structures (520-480 Ma) in the northern segment of this belt and in the Araçuaí belt. Kinematic analysis of the dated mylonites shows extensional structures with top-down movement to ESE compatible with structures found in other tectonic segments in the eastern portion of the Quadrilátero Ferrífero and in the Rio Doce Valley region. Our ages are situated in the same time interval defined for the alkaline magmatism of the Serra do Mar suite. We suggest that the regional tectonic framework was developed during continental-scale extension. This process has been coeval with convergent strain in the adjacent Neoproterozoic shear zones of the Apiaí/Ribeira and Araçuaí belts, which make up significant segments of these belts. The available data show that these structures may not be simply related to post-orogenic gravitational collapse, but must involve a more complex process probably related to dynamic balance between crustal thickening and thinning during tectonic convergence, basin formation and exhumation processes