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_Journal of Structural Geology 32 (2010) 595e604_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg_ _Nucleation and growth of new grains in recrystallized quartz vein: An example from banded iron formation in Iron Quadrangle, Brazil_ _Leonardo Lagoeiro*, Paola Barbosa_ _Universidade Federal de Ouro Preto, Departamento de Geologia, Ouro Preto, MG 35400-000, Brazil_ _article info_ _Article history: Received 28 August 2008 Received in revised form 8 March 2010 Accepted 12 March 2010 Available online 18 March 2010_ _Keywords: Crystallographic orientation Universal stage Electron backscatter diffraction (EBSD) Recrystallization Quartz_ _abstract_ _Intracrystalline microcracks developed in quartz single crystals deformed in greenschist metamorphic conditions. A detailed study of samples collected in tabular to lens shape quartz vein was carried out to investigate how the microcracks initiated and how the microstructures evolved with progressive deformation. A combination of light and EBSD techniques was used to analyze the microstructures and determine the crystallographic orientation of quartz grains. The crystallographic orientations of microcracks indicate that they might have initiated parallel to one of the rhombohedral planes of host crystals. It is suggested that new grains nucleated by rotation of broken fragments from host grains. c-axes of host are distributed in a small circle close to foliation plane while the c-axes of new grains in microcracks are more scattered when compared with host orientations. New grains grew with their c-axes approximately perpendicular to shortening direction._ _© 2010 Published by Elsevier Ltd._ _1. Introduction_ _Small new grains found in zones of localized deformation are thought to be produced by dynamic recrystallization. The strong crystallographic preferred orientation (CPO) and the microstructures of dynamically recrystallized aggregates are taken as evidence for crystal-plastic origin of these zones. Dynamic recrystallization is considered a process that accompanies dislocation creep and involves formation and migration of high angle grain boundaries in response to deformation, in the same mineral (Vernon, 1981; Urai et al., 1986). Recrystallization does not produce new minerals during deformation, a process better referred to as neocrystallization (Urai et al., 1986). Models to describe dynamic recrystallization are based on two main processes: (1) subgrain rotation recrystallization (Urai et al., 1986; White, 1977) and (2) grain-boundary migration recrystallization (Urai et al., 1986; Gordon and Vandermeer, 1966)._ _Models based on dislocation glide have been used to explain the crystallographic preferred orientations (CPO) determined in quartz (Jessell, 1988; Jessell and Lister, 1990). Experimental studies reported CPOs in host grains not entirely recrystallized developed by mechanical reorientation as a result of intracrystalline slip (Gleason et al., 1993). Nonetheless most of the best CPOs in rocks were determined in recrystallized quartz grains._ _However, investigation of dynamically recrystallized grains in both experimentally (den Brok, 1992; Vernooij et al., 2006) and naturally deformed aggregates (van Daalen et al., 1999; Bestmann and Prior, 2003) have shown that the misorientation between parent and recrystallized grains cannot be easily explained in terms of subgrain and grain boundary recrystallization. Therefore, other processes may exert an important control on the misorientation between host and recrystallized grains. Three models have been proposed to explain the large misorientation angle observed between host and recrystallized grains: (1) recrystallized grains once formed may be deformed by grain-boundary sliding assisted by diffusive mass transfer (Bestmann and Prior, 2003; Lagoeiro and Fueten, 2008); (2) new grains precipitated out of solution in voids and microcracks (den Brok and Spiers, 1991; Hippertt and EgydioSilva, 1996) and (3) new grains are fragments that were rotated and separated from the host grain during sliding (den Brok 1994; van Daalen et al., 1999; Vernooij, et al., 2006)._ _The samples studied in this paper have features similar to those predicted in those three models above. Intracrystalline microfractures developed in quartz single crystals found in tabular to lens shape quartz veins. However, microstructures and crystallographic orientations differ slightly from those described in the previous studies. In our studied quartz veins, we aim to understand how the intracrystalline microcracks nucleate and further evolve to an aggregate of recrystallized quartz grains. We also investigate in detail the role of dynamic recrystallization as well as other processes during progressive deformation in the microfracture zones, once these aggregates were deformed at relatively low temperature (w300 x0eC) with participation of aqueous fluid (Hippertt and Egydio-Silva, 1996; Lagoeiro, 1998)._ _596_ _L. Lagoeiro, P. Barbosa Journal of Structural Geology 32 (2010) 595e604_ _The quartz vein studied in this paper came from banded iron formations from the Iron Quadrangle (IQ) in the Southeast of Brazil (Fig. 1). The IQ is an Archean Proterozoic terrane located at the southern boundary of the SГЈo Francisco Craton (Almeida, 1977). The IQ comprises metavolcanic and metasedimentary sequences surrounded by gneissicegraniticemigmatitic domes (Alkmim and Marshak, 1998). The sequences are folded and a regional foliation (S1 Sb) developed parallel to the axial planes of major synclines and anticlines. In the iron formation rocks, the regional foliation (S1) is expressed as an alternately compositional banding (Sb) of quartz and iron oxides (magnetite and hematite) parallel to the S1 foliation. Ductile shear zones developed mainly parallel to this compositional banding and are related to flexural slip occurring during folding process. The samples of quartz veins came from one of these shear zones located in the overturned limb of the Serra do Curral Syncline. It consists of a steeply SE-dipping isoclinal syncline trending NEeSW. The axial foliation banding is moderately to steeply SE-dipping and the stretching lineation, associated with top-to-the-SE sense of tectonic transport, trends NWeSE._ _The mineral assemblage (Pires, 1995) and microstructures (Hippertt, 1994) found in metapelitic and quartizitc (quartz and sericite) country rocks indicate that the deformation occurred under greenschist facies conditions (w300e350 x0eC) and involved widespread participation of water-rich fluids (Herz, 1978). The long axes of quartz and hematite grains define down-dip lineation used to orient samples in the field._ _The analyzed quartz veins are embedded in a matrix of iron oxide (magnetite and hematite) and quartz. They are oriented parallel to foliation compositional banding. Compositional banding consists of an alternation of varied proportions of iron oxide and quartz layers. Most quartz veins are made of fragments of single crystals (host grains) of few millimeters wide and several millimeters long (Fig. 2) connected by polycrystalline aggregates of granular quartz crystals (Lagoeiro and Fueten, 2008). Quartz host grains are elongated approximately parallel to lineation direction. Some quartz veins are composed only by aggregates of granular quartz of lens shape described as pure quartz layers. In this study only quartz with both types of grains, i.e., single crystals and granular aggregates of quartz, was selected for microstructural and crystallographic preferred orientation (textural) analyzes. Although several vein samples have been sectioned for microscopic observations only three more representative examples (BIFQV01, BIFQV02 and BIFQV03) were taken for detailed microstructural and crystallographic orientation analyses._ _3. Methods_ _All investigated samples (BIFQV01, BIFQV02 and BIFQV03) were cut perpendicular to boundary quartz-iron oxide and parallel to long axis of elongate quartz grains. These planes and directions were used to orient the samples. The X-axis was taken parallel maximum elongation of quartz grains, Z-axis perpendicular interface quartz-iron oxide and Y-axis perpendicular_ _Fig. 1. Map of SГЈo Francisco Craton (SFC) showing location QuadrilГЎtero FerrГ xadfero (QF). Sampling location is circled labeled BIFQV. The banded iron formation (BIF) in the QuadrilГЎtero FerrГ xadfero geological map painted black (after Dorr, 1969 and Alkmim Marshak, 1998)._ _L. Lagoeiro, P. Barbosa Journal of Structural Geology 32 (2010) 595e604_ _Fig. 2. A mosaic along XZ section one analyzed quartz veins (BIFQV01). (a) Optical micrographs shown with accessory plate (l Вј 550 nm) inserted. (b) The analyzed microstructures presented as outline grain and fractured boundaries. Each microstructural domain separated by white shades gray marked roman numerals. c-axis pole figures of quartz grains measured each domain are shown. I) corresponds host domains; II) the new crystals narrow rows; III) the new grains wide voids IV) the isolated new grains._ Ключевые слова: e, r, o