Nucleation and growth of new grains in recrystallized quartz vein: An example from banded iron formation in Iron Quadrangle, Brazil

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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 the progressive deformation. A combination of light and EBSD (electron backscatter diffraction) 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 the direction of one of the rhombohedral planes of the host crystals. It is suggested that new grains nucleated by rotation of broken fragments from the host grains. c-axes the of host are distributed in a small-circle close to the foliation plane while the c-axes of the new grains in microcracks are more scattered when compared with the host orientations. New grains grew with their c-axes approximately perpendicular to the shortening direction.

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 the 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 the 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 Egydio-Silva, 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 of other processes during the progressive deformation in the microfracture zones, once these aggregates were deformed at relatively low temperature (∼300 °C) with participation of aqueous fluid (Hippertt and Egydio-Silva, 1996, Lagoeiro, 1998).

Section snippets

Geological setting and sample description

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 São Francisco Craton (Almeida, 1977). The IQ comprises metavolcanic and metasedimentary sequences surrounded by gneissic–granitic–migmatitic domes (Alkmim and Marshak, 1998). The sequences are folded and a regional foliation (S1/Sb) developed parallel to the axial planes of the

Methods

All investigated samples (BIFQV01, BIFQV02 and BIFQV03) were cut perpendicular to the boundary quartz-iron oxide and parallel to the long axis of elongate quartz grains. These planes and directions were used to orient the samples. The X-axis was taken parallel to the maximum elongation of quartz grains, the Z-axis perpendicular to the interface quartz-iron oxide and the Y-axis perpendicular to the XZ plane. Initially, a series of optical micrographs of the veins were taken with crossed

Results

The quartz veins consist of large fragments of single crystals (millimeters in diameter) separated by aggregates of smaller quartz grains (Fig. 2, Fig. 3). These grains occur along narrow rows (two or three grains wide) of slightly elongated crystals as well as larger grains filling wide gaps between fragments of single crystals that were broken off from the host grains.

Discussions

In the classical model for dynamically recrystallized grains by subgrain rotation, subgrains are expected to gradually increase the misorientation to the parent grain. Eventually, subgrain walls become high angle boundaries (>10°) and then become new grains. The misorientation between parent and new grains is expected to be low (<20°). The new grains should also have sizes compatible with those of subgrains. However, in our samples there is a discrepancy between size of subgrains and the new

Conclusions

Our microstructural and crystallographic data allow us to draw the following conclusions:

  • (1)

    Microfractures developed preferentially at 35–65° to the c-axis of the host crystals. After little rotation, these microcracks might have healed and they can be now traced in the host crystals as trails of fluid and solid inclusions. Further microcracking led to the development of planar microfractures parallel to the trace of one of the rhombohedral planes in the flanking host domains.

  • (2)

    New grains may have

Acknowledgements

We thank to Luiz Morales and Issamu Endo for stimulating discussions and suggestions. L. Lagoeiro is grateful for financial support by CNPq project 200968/2005-0 and FAPEMIG CRA APQ-3166-5.02/07.

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