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Plastic Deformation and Development of Clinopyroxene Lattice Preferred Orientations in Eclogites

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Plastic Deformation and Development of Clinopyroxene Lattice Preferred Orientations in Eclogites Journal of Structural Geology 24 *2002) 1357±1368 www.elsevier.com/locate/jstrugeo Plastic deformation and development of clinopyroxene lattice preferred orientations in eclogites JeÂroÃme Bascoua,b,*, AndreÂa Tommasia, David Mainpricea aLaboratoire de Tectonophysique, UMR 5568 Universite de Montpellier II and CNRS, Place E. Bataillon, F-34095 Montpellier cedex 5, France bInstituto de GeocieÃncias, Universidade de SaÄo Paulo, Rua do Lago, 562, BR-05508-080 SaÄo Paulo, Brazil Received 20 February 2001; revised 9 August 2001; accepted 9 October 2001 Abstract We use an anisotropic viscoplastic self-consistent *VPSC) model to simulate the development of omphacite lattice preferred orientations *LPOs) in response to deformation by dislocation glide. In these simulations, we consider slip systems identi®ed either in naturally deformed omphacite or in experimentally deformed diopside. Simulated LPOs reproduce very well the characteristic omphacite LPO pattern in naturally deformed eclogites: a strong concentration of [001]-axes sub-parallel to the lineation and of *010)-poles sub-perpendicular to the foliation. These models reconcile the interpretation of omphacite LPOs in eclogites and TEM observations of naturally deformed omphacite, since they show that omphacite LPOs in naturally deformed eclogites may develop by dislocation glide on 1/2k110l{11Å0}, [001]{110} and [001]*100). We also investigate the effect of the strain regime on the omphacite LPO development. The simulations show that changes in deformation regime lead to second-order variations in omphacite LPO patterns similar to those observed in eclogites, such as an asymmetry of the LPO relative to the structural frame, a stronger concentration of the [001]-axes relative to the *010)-pole concentration, or a dispersion of [001]-axes in the foliation plane. This suggests that omphacite LPO patterns may carry information on the deformation regime *simple shear, transtension¼) active during the high-pressure events. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Lattice preferred orientations; Deformation; Dislocation glide 1. Introduction [100]*010) for olivine at high-temperature conditions), the preferred orientation of the slip direction and the slip plane Clinopyroxene is a major mineralogical constituent of tend to coincide with the ¯ow direction and the ¯ow plane, the oceanic crust and upper mantle. Analysis of lower respectively *Nicolas and Poirier, 1976; Mainprice and crustal sections *e.g. the Ivrea Zone, Northern Italy; Nicolas, 1989). Thus information on the ¯ow regime can Fountain and Salisbury, 1981; Pin and Sills, 1986) and be deduced from the orientation of the slip systems relative xenoliths *Halliday et al., 1993) shows that clinopyroxene to the structural frame. Particularly, in the case of a non- is also concentrated in ma®c granulites and gabbroic coaxial deformation, the obliquity between the LPO and the intrusions of the lowermost crust. Such ma®c rocks are X, Y and Z structural axes *X lineation and Z normal to transformed in eclogites by high- *or ultra-high) pressure the foliation) allows the determination of sense of shear. The metamorphism in subduction and/or collision zones. Clino- majority of LPO measurements for the clinopyroxene group pyroxene omphacite as the major and most deformable in the literature have been made on omphacites from constituent of eclogites may then be a key mineral to eclogites. The characteristic LPO of omphacite is marked decipher the tectonic processes active during the high- by a strong concentration of *010)-poles normal to the pressure event. foliation and of [001]-axes parallel to the lineation *e.g. An important effect of the plastic deformation of rocks is Boundy et al., 1992; Godard and Van Roermund, 1995; the development of a lattice preferred orientation *LPO) of Abalos, 1997; Bascou et al., 2001; Mauler et al., 2001; their constituent minerals *Nicolas and Poirier, 1976). When Pipenbreier and StoÈckert, 2001). If one assumes that LPO one slip system is predominant during deformation *e.g. development is controlled by glide on a single dominant slip system, this LPO suggests a dominant activation of the [001]*010) slip system. However, dislocations associated * Corresponding author. Tel.: 133-4671-43602; fax: 133-4671-43603. with this slip system are not observed in naturally deformed E-mail address: [email protected] *J. Bascou). omphacite. In fact, TEM observations of dislocations in 0191-8141/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0191-8141*01)00137-7 1358 J. Bascou et al. / Journal of Structural Geology 24 >2002) 1357±1368 omphacites from naturally deformed eclogites *Van melting point of sodic clinopyroxenes *jadeite, omphacite) Roermund and Boland, 1981; Van Roermund, 1983; Buatier compared with diopside *Pipenbreier and StoÈckert, 2001). et al., 1991; Godard and Van Roermund, 1995) show that In order to investigate the deformation mechanisms that dislocation creep results in dominant activation of the could produce the observed clinopyroxene LPOs, we use a 1/2k110l{11Å0}, [001]{110} and [001]*100) slip systems. viscoplastic self-consistent model *VPSC) to simulate the Based on microstructural observations, Godard and Van omphacite LPO development in response to deformation by Roermund *1995) and Mauler et al. *2001) have proposed dislocation glide. In these models, we consider that that activation of other deformation mechanisms, such as omphacite deforms by slip on those systems identi®ed in grain boundary migration and diffusive mass transfer or naturally deformed eclogites or in experimentally deformed diffusion creep with concomitant anisotropic growth, may diopside. To investigate the effect of strain regime on the reconcile the apparent discrepancy between LPO data and LPO pattern, we modeled omphacite LPO development in TEM observations. simple shear, axial compression, transpression and trans- Deformation experiments also contribute to our under- tension. Modeled LPO patterns are then compared with standing of the deformation of clinopyroxenes. Such experi- omphacite LPO patterns of eclogites sampled in various ments have been mainly performed on diopside single orogenic domains. crystals *Ave Lallemant, 1978; Ingrin et al., 1992; Raterron et al., 1994) or polycrystalline aggregates *Boland and Tullis, 1986; Lavie, 1998; Mauler et al., 2000; Bystricky 2. Naturally deformed omphacite LPO and Mackwell, 2001). These experiments show that plastic ¯ow of diopside is essentially accommodated by three 2.1. Previous data on clinopyroxene LPOs deformation mechanisms. At low temperature, high strain rate, and high stress, diopside deforms by mechanical twin- Clinopyroxene LPOs have been, up to now, essentially ning on *100) and *001) planes *e.g. Ave Lallemant, 1978). measured on 5-axes universal stage *e.g. Helmstaedt et al., At higher temperature, plastic deformation occurs by dis- 1972; Van Roermund, 1983; Boundy et al., 1992; Barruol location creep or diffusion creep depending on grain size and Mainprice, 1993; Godard and Van Roermund, 1995; and ¯ow stresses *Lavie, 1998; Bystricky and Mackwell, Abalos, 1997) and, more recently, using the electron back- 2001). Dislocation creep accommodated by an association scattered diffraction *EBSD) technique *Bascou et al., 2001; of dislocation glide and dynamic recrystallization by Mauler et al., 2001; Pipenbreier and StoÈckert, 2001). The subgrain rotation and grain boundary migration is dominant most common omphacite LPO is the `*010) fabric' that is in both natural clinopyroxenites *Bystricky and Mackwell, characterized by a concentration of *010)-poles and [001]- 2001) and synthetic polycrystals deformed by torsion axes perpendicular to the foliation and parallel to the *Mauler et al., 2000). In addition, single crystal deformation lineation, respectively. Variations on the relative concentra- experiments at moderate and high temperatures show that tion of *010)-poles and [001]-axes in this characteristic LPO plastic deformation of diopside results in dominant activa- have been traditionally interpreted in terms of strain tion of {110} and *100) slip systems. Between 800 and symmetry during high-pressure deformation *Helmstaedt 9008C, [001]*100) is the easiest system with a ¯ow stress et al., 1972; Godard and Van Roermund, 1995; Abalos, on the order of 60 MPa at laboratory strain rates, [100]*010) 1997). Two end-members are described: the `L-type or and 1/2k110l{11Å0} are less easily activated *¯ow stresses constriction fabric', characterized by *010)-poles in a girdle are on the order of 200 MPa), and [001]{110} systems are perpendicular to the lineation and [001]-axes forming a hard to activate *Ingrin et al., 1992). At higher temperatures single maximum strongly concentrated in the lineation *above 10008C), the activation of 1/2k110l{11Å0} slip and the `S-type or ¯attening fabric', characterized by a systems is favored *Ingrin et al., 1991; Raterron et al., strong *010)-poles concentration perpendicular to the 1994). foliation and [001]-axes lying in the foliation plane. These Slip systems observed in omphacite and diopside are end-member fabrics correspond to coaxial-deformation similar. However, rheological laws obtained for diopside regimes. However, omphacite LPO patterns asymmetric to aggregates suggest that unreasonable high stresses and/or the X, Y and Z structural axes have also been observed in low strain rates are needed to deform clinopyroxene-rich naturally deformed
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