The purpose of this contribution is to
present preliminary results regarding
the kinematics and deformation conditions
of the Indiavaí-Lucialva Shear
Zone, based on the analysis of the
texture and microfabrics of related
quartzites.conferenc

Almeida, Harrizon Lima de

Gudmundsson, Agust

Leiss, Bernd

Philipp, Sonja

Ruiz, Salina

Tanner, David

Vollbrecht, Axel

English

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Microfabrics and textures of quartzites of the Indiavaí- Lucialva shear zone, SW Amazonian Craton: preliminary conclusions

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TSK 11 Göttingen 2006 Almeida et al.Microfabrics and textures ofquartzites of the Indiavaí-Lucialva shear zone, SWAmazonian Craton: prelimi-nary conclusions PosterHarrizon Lima de Almeida1 AmarildoSalina Ruiz1 Axel Vollbrecht2IntroductionThe purpose of this contribution is topresent preliminary results regardingthe kinematics and deformation con-ditions of the Indiavaí-Lucialva ShearZone, based on the analysis of thetexture and microfabrics of relatedquartzites.Geological settingThe Amazonian Craton consists of olderArchean cores related to a larger ac-cretionary event represented by the for-mation of several province belts (Teix-eira et al. 1989; Tassinari et al.2000). The tectonic evolution of theSW Amazonian Craton, in particular,has been attributed the formation ofan accretionary complex, Proterozoicin age (Rodonia-San Ignacio Orogeny).Although that division was based ongeochronologic data, lineaments havebeen proposed as structural limits be-tween several terranes. Saes (1999) pro-posed that the Santa Helena and Jauru(Alto Jauru, Geraldes et al., 2001) ter-ranes were separated by the Indiavai-Lucialva shear zone (ILSZ). The ILSZconstitutes an important mega struc-ture which deformed units of the Jauruterrain and parts of the limit east of theSanta Helena terrain (Fig. 1). Based1 Dep. Geologia Geral, Universidade Fed-eral de Mato Grosso-ICET/DGG 2 Geowis-senschafliches Zentrum der Georg-August-Uni-versität Göttingen (GZG)on field studies and Ar-Ar muscoviteages, Ruiz (2005) and Ruiz et al. (2005)have demonstrated that the ILSZ wasformed during an extensional tectonicevent, possibly associated with the re-arrangement of collided blocks relatedto the Sunsás Orogeny (ca. 900Ma).The Indiavaí–Lucialva Shear ZoneThe Indiavaí–Lucialva Shear Zone(ILSZ) strikes NW-SE over a dis-tance of about 100 km separatingmagmatic rocks from a metavulcanic-metasedimentary sequence. In thestudied area (Fig. 1) rocks whichdisplay a distinct mylonitic fabric fre-quently contain feldspar porphyroclastsand garnet. In addition, quartz-richtectonites, former quartzites, developedalong the ILSZ. The mylonitic foliation(smyl) of these mylonitic quartizites,marked by aligned muscovite flakesand strongly flattened small quartzgrains, is subparallel to the margins ofthe shear zone which strikes 315–330°and dips with angles between 60° and70° to the east-northeast. Shear sensecriteria are usually abundant mainly ingneisses which have retained evidenceof mylonitization at mesoscale. More-over, the rocks show a well-developedstretching lineation which is definedmainly by elongated grains of quartzand muscovite laths. The dip anglesof this lineation typically vary betweenabout 50° and 70°. The ILSZ clearlyexhibits a normal fault kinematic,i.e. a top-to-the-NE movement, jux-taposing the Santa Helena Batholithagainst metavulcanic-metasedimentarysequences and orthogneisses of thebasement.1Almeida et al. TSK 11 Göttingen 2006Figure 1: Geological sketch map of the SW Amazonian Craton. The stereoplots displaythe poles to the mylonitic foliation smyl (A) and stretching lineation L (B), measuredwithin the ILSZ; modified after Ruiz (2005).MicrofabricsIn all the analyzed quartzites, quartzforms polycrystalline aggregates con-sisting of inequigranular grains withvery irregular interlobate grain bound-aries. The average quartz grainsize scatters between 0.3 and 1.4mm(Fig. 2A). The occurrence of dissec-tion microstructures (Urai et al. 1986)with the consequent formation of iso-lated ‘island’ grains, has also beenobserved in the several thin sections(Fig. 2A). Such microstructures are in-terpreted as resulting from fast mi-gration of grain boundaries, correlatedwith the regime 3 microstructure ofHirth & Tullis (1992) which indicateshigh-temperature (HT) conditions dur-ing mylonitisation. In addition, otherfeatures pointing to grain boundary mi-gration like dragging and pinning mi-crostructures (Jessel, 1987) have beenobserved. Microstructures represent-ing a subsequent intracrystalline defor-mation include deformation bands andirregular subgrains. Frequently, twosets of deformation bands are developed(‘chessboard patterns’; Fig. 2B) indicat-ing HT crystal plasticity (e.g., Kruhl,1996). Muscovite occurs as solid inclu-sions in quartz or along the quartz grainboundaries. In the first case, muscoviteconsists of boudinaged single crystalselongated parallel to the mylonitic fo-liation (Fig. 2C). It also displays defor-mation (kink) bands, particulary withinlarger grains. Kinematic indicators, e.g.mica and tourmaline displaying distinctasymmetric fish-structures, point to atop-to-the-ENE (normal) sense of shear(Fig. 2D). Altogether, grain boundarymigration has to be assumed here as themain mechanism of dynamic recrystal-lization.Quartz texturesAs a start, quartz c-axis textures(crystallographic-preferred orienta-2TSK 11 Göttingen 2006 Almeida et al.Figure 2: Deformation microstructuresin mylonitic quartzite A. Quartz grainswith irregular shape and lobate boundaries.The arrow points to an isolated ‘island’grain. B. Two sets of deformation bandsin quartz (‘chessboard patterns’). C. Tour-maline (top) and muscovite grains (middle)showing boudinage. D. Tourmaline microfish (arrow) documenting a top-to-the-eastsense of shear when reoriented to the field.tions) have been measured for threeselected quartzite mylonite samples(Fig. 3). The quartz c-axis patterns arecomplex with a more or less pronouncedasymmetry with respect to the referencedirections (smyl and L) indicating a pre-dominance of non-coaxial deformation.The c-axis maxima close to smyl suggestthat prism slip in <a> or [c], which isgenerally attributed to HT deformationregimes (e.g. Mainprice et al. 1986), hassignificantly contributed to the texturedevelopment. Only sample C shows, inaddition, a significant concentration of<c>-axes sub-normal to smyl pointingto basal slip at lower temperatures.Intermediate positions of maxima areprobably related to combined prism andbasal slip in connection with complexdeformation geometries (e.g. Garbutt& Teyssier, 1991). According to Stippet al. (2002) complex pole figure of thisFigure 3: Quartz c-axis textures of threeselected quartzite mylonites (A, B, C) fromthe ILSZ; horizontal line-trace of smyl; dots— direction of L; trace of smyl is ori-ented with respect to geographic directions;Schmidt net, lower hemisphere.kind are typical for HT deformationof quartz polycrystals. As related tothe field, the obliquity of the patternsindicates a top-to-the-NE sense ofshear.ConclusionsBased on both characteristic quartz mi-crostructures and quartz c-axis texturesthe knowledge about the deformationconditions within the ILSZ could beimproved. The microfabrics and in-tracrystalline structures of quartz in-dicate HT deformation with dominantgrain boundary migration recrystalliza-tion and subsequent weak plastic defor-mation producing two sets of deforma-tion bands in many grains. Deformationin a HT deformation regime is also con-firmed by typical quartz c-axes textureswhich, in addition, reflect a strong com-ponent of non-coaxial strain.Acknowledgements The researchwas supported by CNPQ Grant350565/2004-0 and FAPEMAT Grant3.2.2.33/02-2004.3Almeida et al. TSK 11 Göttingen 2006ReferencesGarbutt JM & Teyssier C (1991) Prism <c>-slip in quartzites of the Oakhurst MyloniteBelt, California. Journal Structural Geology13: 657–666Geraldes MC, Van Schmus W, Condie KC, BellS, Teixeira W & Babinski M (2001) Pro-terozoic geologic evolution of the SW part ofthe Amazonian Craton in Mato Grosso state,Brazil. Precambrian Research 11: 91–128Hirth G & Tullis J (1992) Dislocation creepregime in quartz aggregates. Journal Struc-tural Geololy 2: 145–159.Jessel MW (1987) Grain-boundary migra-tion microstructure in a naturally deformedquartzite. Journal Structural Geology 9:1007–1014Kruhl JH (1996) Prism- and basal-plane par-allel subgrain boundaries in quartz: a mi-crostructural geothermobarometer. JournalMetamorphic Geolology 14: 581–589Mainprice D, Bouchez JL, Blumenfeld P &Tubia JM (1986) Dominant c-slip in natu-rally deformed quartz: implications for dra-matic plastic softening at high temperature.Geology 14: 819–822Ruiz AS (2005) Evolução geológica do sud-este do Cráton Amazônico: região limítrofeBrasil-Bolívia. PHD Thesis. Unesp, RioClaro, 250ppRuiz AS, Simões LSA, Almeida HL, Misson,AG & Manzano, JC (2005) Análise estru-tural do batólito Santa Helena: implicaçõessobre a evolução tectônica do SW do CrátonAmazônico durante as orogenias San Ignácio-Rondoniano e Sunsás-Aguapeí. X SimpósioNacional de Estudos Tectônicos. Curitiba,Brasil, 411–414Saes G (1999) Evolução tectônica e pale-ogeográfica do aulacógeno Aguapeí (1.2–1.0Ga) e dos terrenos do seu embasamentona porção sul do Cráton Amazônico. PHDThesis. Usp, São Paulo, 135 ppStipp M, Stünitz H, Heilbronner R, Schmid S(2002) The eastern Tonale fault zone: a ‘nat-ural laboratory’ for crystal plastic deforma-tion of quartz over a temperature range from250 to 700°C. Journal Structural Geology 24:1861–1884Tassinari CG, Bettencourt JS, Geraldes MC,Macambira MB & Lufon JM (2000) TheAmazonian Craton. In: Cordani U, MilaniEJ, Thomaz Filho A & Campos DA (ed) Tec-tonic Evolution of South América, I Inter-national Geological, Rio de Janeiro, Brazil,41–95Teixeira W, Tassinari CG, Cordani UG,Kawashita K (1989) A review of thegeochronology of the Amazonian Craton:tectonic implications. Precambrian Research42: 213–227Urai JL, Means WS & Lister G (1986) Dynamicrecrystallization of minerals. GeophysicalMonographs 36: 161–1994

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Microfabrics and textures of quartzites of the Indiavaí- Lucialva shear zone, SW Amazonian Craton: preliminary conclusions

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