Dome emplacement and formation of kilometre-scale ...directory.umm.ac.id/Data Elmu/jurnal/P/Precambrian...microtextures, c-axis fabrics analysis and strain analysis with outcrop-scale

Precambrian Research 102 (2000) 99–121

Dome emplacement and formation of kilometre-scalesynclines in a granite–greenstone terrain (Quadrilatero

Ferrıfero, southeastern Brazil)

J. Hippertt a,*, B. Davis b

a Departamento de Geologia, Uni6ersidade Federal de Ouro Preto, 35400-000 Ouro Preto, MG, Brazilb Delta Gold NL, P.O. Box 152, Kalgoorlie, Western Australia 6430, Australia

Received 11 February 1999; accepted 7 January 2000

Abstract

The Quadrilatero Ferrıfero is a portion of the Brazilian Precambrian shield containing a granite–greenstone terraincharacterized by granite–gneiss domes encircled by kilometre-scale synclines. Integration of porphyroblast-matrixmicrotextures, c-axis fabrics analysis and strain analysis with outcrop-scale structural relationships indicates that themain phase of domes and synclines evolution occurred during the westerly directed 0.8–0.6 Ga Pan-African-Brasil-iano regional shortening. This tectonic event has produced the single pervasive foliation observed in the foldedsupracrustal sequences of the Quadrilatero Ferrıfero. Dip-slip ductile shear zones developed at the dome–supracrustalinterface are synchronous with dome evolution during the Pan-African-Brasiliano deformation, as indicated bycontemporaneous growth of aureole porphyroblasts in the shear planes. These shear zones facilitated steeply directeddownward flow of country rock material around the granite–gneiss domes. The marked density contrast betweengranite–gneiss material and the iron-rich supracrustals provided a gravitationally favourable environment fordownward wallrock flow. Variations in shear zone kinematics away from the domes, combined with kilometer-scaleviscous drag, contributed to produce kilometre-scale synclines in the supracrustal rocks constrained between upwardmoving domes. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Kilometre-scale synclines; Granite–greenstone terrain; Dome emplacement

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1. Introduction

A number of kilometre-scale, generally sub-cir-cular or elliptical granite–gneiss massifs displaydevelopment of kilometre-scale synclines in theirsurrounding country rocks. Such structures in-clude intrusive granite plutons (e.g. Brun et al.,1990; Veenhof and Stel, 1991), mantled gneissic

* Corresponding author. Present address: Department ofEarth and Planetary Systems Science, Hiroshima University,739 Higashi-Hiroshima, Japan.

E-mail address: [email protected](J. Hippertt)

0301-9268/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

PII: S 0301 -9268 (00 )00061 -9

J. Hippertt, B. Da6is / Precambrian Research 102 (2000) 99–121100

domes (e.g. Jelmsa et al., 1993), and core complexstructures (e.g. Holm and Lux, 1996). The forma-tion of kilometre-scale synclines is commonly in-terpreted to result from the emplacement ofdiapiric plutons. Integral to this interpretation is amodel involving return flow of the material sur-rounding and overlying the diapir during its as-cent and emplacement within the upper level ofthe crust. Such processes have been invoked toexplain the geometry of the surrounding synclines,which are commonly interpreted to be a reflectionof mass-transfer processes that operate to provideroom for the intruding pluton (see reviews byPaterson and Fowler, 1993; Paterson et al., 1996).Consequently, formation of the synclines in thesemodels is proposed to occur synchronously withemplacement and can be independent of any con-gruous regional deformation.

Alternatively, similar geometries comprisinggranitic plutons surrounded by synclines have alsobeen proposed as having formed by subsequentdeformation of country rocks around the plutonsduring contractional deformation (e.g. Davis,1993). However, the axial surfaces and fold axesof the synclines in these cases commonly anasto-mose around the plutons and taper off in direc-tions at a high angle to the maximumcompression direction of regional deformation.Commonly, the plutons are considered to haveacted as relatively rigid objects within the regionalstress field, thereby causing strain accumulationagainst their margins that results in enhanceddeformation in the vicinity of the contacts (e.g.Meneilly, 1983; Davis, 1993).

However, it is difficult to tell which model hasoperated in cases where the synclines do notdefine continuous encircling geometries aroundthe plutons. Contention as to which model is mostapplicable has been prevalent recently in the liter-ature, particularly for plutons that were consid-ered to be classic examples of ballooninggranitoids (e.g. Ramsay, 1989). Crucial to deter-mining which model is most appropriate is anintegration of the deformation history in aureolecountry rocks with that recognized regionally, asthe porphyroblasts grown in the aureoles canpreserve foliations that have been destroyed in thematrix during subsequent deformation.

The Quadrilatero Ferrıfero is dominated bygranite–gneiss domes with diameters in the orderof a few to several tens of kilometres that aretypically surrounded by kilometre-scale synclines.These structures are well exposed, including theircontact with the granite–gneiss domes, and theyshow development of metamorphic minerals inthe contact metamorphic aureoles. In this paper,microstructural and field-based information areused to interpret the deformation processes re-sponsible for syncline development associatedwith dome emplacement, using as an example thedome-and-keel structure of the Quadrilatero Fer-rıfero. A model is then proposed for dome em-placement and syncline development thatintegrates field information with microstructuraldata from spatially oriented thin sections.

2. Geologic and structural setting

The Quadrilatero Ferrıfero, one of the mostimportant regions of the Brazilian Precambrianshield for its reserves of Fe, Mn and Au, is agranite–greenstone terrain (Schorscher, 1978) lo-cated in the southern portion of the Sao Fran-cisco craton in southeastern Brazil (Fig. 1). Itcomprises a supracrustal sequence of Archaeanmetavolcanic rocks (the Rio das Velhas green-stone unit, 2.9–2.7 Ga) and Lower Palaeoprotero-zoic metasedimentary rocks (the Minassupergroup, 2.1–2.5 Ga), which have been region-ally metamorphosed to greenschist to lower am-phibolite facies (Herz, 1978) during theTransamazonian (2.1–2.0 Ga) and Pan-African-Brasiliano (0.8–0.6 Ga) orogenies. Available U–Pb zircon dating (Machado and Carneiro, 1992)indicates that the Rio das Velhas greenstone unitwas deposited on an Archaean granitic–gneissicbasement. Reactivation of this basement in subse-quent orogenic events (cf. Marshak et al., 1997)has produced the dome-and-keel structure of theQuadrilatero Ferrıfero.

The greenstone unit consists mainly of chloriteschists produced during low-grade metamorphismof basic and ultrabasic volcanic rocks. Locally,the primary igneous microstructure of these rocksis preserved in the form of spinifex textures

J. Hippertt, B. Da6is / Precambrian Research 102 (2000) 99–121 101

(Schorscher, 1978). Mica schists, carbonaticrocks, quartzites and banded iron formations(BIFs) also occur in variable proportions as aresult of low-grade metamorphism of chemicaland clastic sediments, as well as volcanic rocks ofintermediate-acid composition.

The overlying Palaeoproterozoic metasedimen-tary sequence, which is separated from the green-stone sequence by an erosional unconformity, ismainly composed of quartzites, mica schists, phyl-lites and carbonatic rocks. These lithologies areinterpreted to represent infill of a platformal basinwith a basal rift sequence overlain by marinesediments (Chemale et al., 1994). This sequencealso includes a several hundred metres-thick unitof BIFs (Itabira Group) composed of quartziteswith variable proportion of carbonates and Feoxide (hematite and magnetite), which host the Fereserves of the Quadrilatero Ferrıfero.

The supracrustal sequences have been foldedwithin interconnected kilometre-scale synclinesand anticlines that surround the granitic domes ofthe Quadrilatero Ferrıfero (Fig. 2). These domesgenerally have a complex internal structure (e.g.Hippertt, 1994a) and comprise a number of plu-tonic granitoids of granitic, granodioritic andtrondjemitic composition, as well as gneisses andmigmatites. Available U/Pb zircon dating indi-cates an Archaean age (2.7–3.2 Ga) of crystalliza-tion for these granitic rocks (e.g. Machado andCarneiro, 1992; Machado et al., 1992). However,the significance of these results is still uncertain assome of these ages were obtained in zircon cores,which may represent detrital zircons that do notnecessarily reflect the crystallization age of thehost granitoid (Hippertt, 1996). The contacts be-tween domes and supracrustals are generallymarked by low to medium metamorphic gradeshear zones with variable kinematics.

Fig. 1. Location and simplified geologic map of the Quadrilatero Ferrıfero in southeastern Brazil.


Fig. 2. Map showing the kilometer-scale synclinal-homoclinal keel and shear zones in the dome/supracrustals contacts. Locations ofcross-sections shown in Figs. 3–5 are indicated. Stereograms show orientation of mylonitic foliation (Sm) and stretching lineation(L) in different domains of the contact shear zones (circled numbers).


The ascent and emplacement mechanism of thegranitic domes of the Quadrilatero Ferrıfero is acontroversial topic that has already been the sub-ject of discussion in the literature (Chemale et al.,1996; Hippertt, 1996). Classical views generallyfavor a model involving emplacement in the mag-matic state (e.g. Herz, 1970), whereas emplace-ment of dome material in a predominantly plasticstate was proposed by Hippertt (1994a). Otherrecent papers (e.g. Marshak et al., 1992), however,claim that solid state uplift of the domes occurredduring a regional extensional tectonic event dur-ing the Transamazonian orogeny (2.1–2.0 Ga).Emplacement of the granitic–migmatitic domesinto the relatively cooler supracrustal sequenceshas induced the development of porphyroblasticthermal aureoles around the domes (Marshak etal., 1992). These aureoles, firstly recognized andmapped by Herz (1978), are a crucial feature toelucidate the development of the dome-synclinearchitecture of the Quadrilatero Ferrıfero. Meta-morphic porphyroblasts within these aureolescontain preserved foliations which have enabledstudy of the interrelationships between deforma-tion and granite emplacement via microstructuralanalysis of inclusion trails and porphyroblast-ma-trix relationships (discussed below).

Recent studies (e.g. Marshak et al., 1992; Mar-shak et al. 1997; Chemale et al., 1994; Alkmimand Marshak, 1998) have summarized the tectonicevolution of the Quadrilatero Ferrıfero into twomain tectonic cycles. The first event (2.1–2.0 GaTransamazonian orogeny) has been interpreted asa period of predominant crustal extension, withsome authors (e.g. Marshak et al., 1992) invokingsynchronous dome emplacement and developmentof kilometre-scale synclines. The second majorevent, which has been attributed to the Pan-African-Brasiliano orogeny (0.8–0.6 Ga), wascontractional and produced west-verging trusts inthe east margin of the Quadrilatero Ferrıfero.Alternatively, Chauvet et al. (1994) interpreted allthe main structures and metamorphic paragenesisof the Quadrilatero Ferrıfero region as related tothe Pan-African-Brasiliano deformation andmetamorphism.

2.1. Kilometre-scale synclines

The axial traces of the kilometre-scale synclinesanastomose around the granitic domes (Fig. 2),forming a discontinuous keel that is approxi-mately 240 km in length and 20 km in width. Inthe eastern portion of the Quadrilatero, severalsegments of this keel appear to be isolated at thepresent erosional level, but were probably origi-nally connected (Chemale et al., 1994), as sug-gested by their identical stratigraphic sequences.

Both the Archaean greenstone unit and theoverlying Palaeoproterozoic metasedimentary se-quences are folded within this keel, forming tightsynclinoria or, locally, overturned homoclines.The geometry of the interconnected kilometre-scale synclines varies greatly from one point toanother. For example, the Moeda Syncline, westof the Bacao complex, has its east limb progres-sively overturned towards the south where theBacao complex lies on the overturnedsupracrustal sequences (Fig. 3). In contrast, theDom Bosco syncline on the southern margin ofthe Bacao complex (Fig. 4), which was stronglyaffected by westward tectonic transport duringthe Pan-African-Brasiliano orogeny, has sufferedsuperimposition of a strike-slip component. Northof the Bonfim Complex, the surrounding keel is atruncated, overturned syncline that would be bet-ter described as a steeply dipping homocline (Fig.5). In the following section, the structural/mi-crostructural framework is described as observedin representative profiles across some sectors ofthe synclinal-homoclinal keel.

3. Structures and microstructures

3.1. Shear zones

Several tens of metres-thick, dip-slip shearzones, which generally dip at a high angle out-ward from the domes, typically occur at the con-tact between the domes and supracrustals in thewestern portion of the Quadrilatero, which wasrelatively more protected from the pervasive Pan-African-Brasiliano deformation. The sense ofshear in these zones is consistent with a relative


Fig. 3. Sketch showing distribution of S–C fabrics and XZ finite strain ellipses in a profile across the overturned Moeda syncline(see Fig. 2 for location). Numbers between parenthesis indicate average angle between S- and C-foliations in each domain. The axialrate (Rxz) is indicated for each ellipse. In the eastern portion of the profile, ‘a’ and ‘b’ represent local variations in the orientationof the contact shear zone. Note that a higher finite strain was produced in the west limb which is not overturned (see text).

upward movement of the domes. The downwardmovement of encompassing supracrustal materialappears to have been steeply directed, rather thanaccommodated by strike-slip movements, as indi-cated by stretching lineations that generally pitchsteeply (70–90°) in the shear planes (Fig. 2).Thus, these marginal shear zones are extensional(normal) where the syncline limbs are not over-turned, but record a reverse sense of movementwhere the contact plane dips towards the domes(Fig. 3). Strike-slip and thrust components havealso been detected at other contacts (e.g. aroundthe Bacao and Caetes domes, respectively; Fig. 2).These variations in the kinematics of the granite–supracrustals contact have been previously inter-preted either as post-emplacement tectonic effects

(e.g. Chemale et al., 1994) or aureole effects dur-ing dome ascent (Hippertt, 1994a).

Metamorphic assemblages in these shear zones(e.g. muscovite–chlorite–epidote; muscovite–bi-otite–garnet) are compatible with deformationunder greenschist/lower amphibolite facies condi-tions, suggesting that these shear zones were ac-tive during imposition of regional low to mediumgrade metamorphism that affected most of theQuadrilatero Ferrıfero region during the 0.8–0.6Ga Pan-African-Brasiliano orogeny (Chemale etal., 1994). The shear zones affect bothsupracrustals and granitic rocks and were thelocus of intense fluid activity that induced volumeloss via dissolution of quartz and carbonates andconsequent phyllonitization (Hippertt, 1994b). In


the granitic rocks, deformation was principallyaccommodated via crystal-plasticity and solution-transfer processes in quartz and retrograde soften-ing reactions in feldspar (Hippertt, 1998). In thesupracrustal rocks, crystal plasticity of carbonatesand slip on basal planes of mica were also impor-tant, and accommodated much of the shearing

component of the deformation (Davis, 1993,1995).

Bedding-parallel ductile shear zones occurthroughout the synclines. These shear zones varyin width from millimetres to a few decimetres inwidth, and generally occur at the interface be-tween beds of contrasting composition and rheol-

Fig. 4. S–C fabrics and XZ finite strain ellipses across the normal Dom Bosco syncline (see Fig. 2 for location). The southern limbwas strongly affected by a later strike-slip deformation that partially obliterated the S–C fabrics and the strain ellipses related tothe syncline development. Note shear sense reversal in the northern limb (see text).

Fig. 5. S–C fabrics and XZ finite strain ellipses across the Serra do Curral homocline (see Fig. 2 for location). Fold traces andcleavage traces are indicated.


ogy. The sense of shear in these interlayer shearzones is in most cases consistent with a downwardmovement of the older beds. Thus, the kinematicsof this internal shear zones is opposite to thecontact shear zones, which has commonly pro-duced an abrupt reversal in sense of shear a fewtens of metres away from the dome–supracrustalsinterface, as observed in the west margin of theMoeda Syncline (Fig. 3).

3.2. Clea6ages, S–C fabrics, oblique fabrics andstrain ellipses

There is a single pervasive cleavage developedin the supracrustal rocks all over the synclinalkeel of the Quadrilatero Ferrıfero. This cleavageis generally parallel to, or maintains a low angleobliquity to bedding (B15–20°), suggesting con-tinued bedding-parallel shear during folding. Inthe hinge zones of the kilometre-scale synclines,however, the plane axial character of this cleavageis revealed in some outcrop scale folds.

S–C fabrics and oblique grain shape fabrics arealso common in the folded supracrustal se-quences. S–C fabrics of microscopic, hand speci-men and outcrop scales are dominant in both theinternal and marginal shear zones (Fig. 6), andpossess unambiguous asymmetries consistent withthe shear sense inferred from independent criteriasuch as shear bands and displacement of quartzveins. Oblique fabrics, generally defined by a dis-continuous mica foliation or grain shape fabricsin quartz aggregates are widespread in domainsbetween consecutive shear zones. The asymmetryof these shape fabrics provides reliable shear senseindicators (Simpson and Schmid, 1983). Theshape fabrics show opposite geometries on differ-ent synclinal limbs with reversals in asymmetryoccurring generally close to the syncline axis.

XZ finite strain ellipses were determined by theFry (1979) method in oriented thin sections ofporphyroclastic quartzose horizons with low de-

grees of recrystallization in all the main strati-graphic units across the profiles shown in Figs.3–5. The ellipses generally show opposite obliqui-ties in the different syncline limbs, consistent withthe variation in the asymmetry of S–C fabrics.The most elongate finite strain ellipses were deter-mined in the contact shear zones between domesand supracrustals. In addition, inverted synclinelimbs, such as the east limb of the Moeda syn-cline, tend to record smaller degrees of finitestrain than the normal limbs (Fig. 3).

3.3. Quartz c-axis fabrics

Quartz c-axis fabrics in sheared granitoids andsupracrustals have been investigated with variabledegrees of finite strain throughout the profile thatcrosscuts the Moeda syncline (Fig. 7). Both mar-ginal and internal shear zones show predominantasymmetric single girdle c-axis fabrics, which typ-ically form during progressive simple shear(Schmid and Casey, 1986). The asymmetry of thisfabric, however, is not always coincident with thatindicated by S–C fabrics, oblique fabrics andother independent criteria. This is a commonproblem in shear zones, which generally preventsutilization of c-axis fabrics as a reliable shearsense indicator (Hanmer and Passchier, 1991).The important inference, however, is that thesesingle girdle fabrics reflect a predominant non-coaxial deformation in both the contact zone andbetween the sedimentary beds in the interior ofthe synclines. In contrast, the less deformed do-mains between pairs of consecutive shear zones,i.e. the interior of the sedimentary beds, showweaker asymmetric cross girdle fabrics (Fig. 8),which are currently interpreted to be a product ofgeneral strain regimes with components of bothpure shear and simple shear (Lister and Hobbs,1980). Occasionally, near-symmetric cross girdlefabrics have also formed in these domains, reflect-ing a local strain regime close to ideal pure shear.

Fig. 6. (a) Field view of the contact between Bonfim dome (Bo) and Moeda quartzites (M) in the west limb of the Moeda syncline(looking south). Width of view is 2 km. (b) Outcrop scale S–C-like structure in the Moeda Quartzite, west limb of the MoedaSyncline. Width of view is 1.8 m. (c) Centimetre scale S–C structure in mylonitic granitoid (margin of the Bonfim dome). The pencover is 5 cm-long. (d) Intrafolial, isoclinal fold in banded iron formation (BIF) (Dom Bosco syncline). Width of view is 3.5 m. (e)Margin of the contact shear zone affecting a granite in the west margin of the Caetes dome. Width of view is approximately 50 cm.


Fig. 6.


These variations in c-axis fabrics are also de-tectable at the thin-section scale in metasedimentsthat are finely laminated. In these rocks, domainsthat are only a few millimetres thick in the limit-

ing zone between two sedimentary beds show anasymmetric single girdle fabric, while the quart-zose matrix inside the beds shows a more symmet-ric fabric. These crystallographic fabrics are

Fig. 7. Quartz c-axis fabrics determined across the overturned Moeda syncline. Numbers between parenthesis indicate number ofgrains measured in each diagram. The two larger diagrams represent overall c-axis fabrics for each syncline limb. Density contoursare 1.25 and 2.75% per 1% area.

Fig. 8. (a) Outcrop view of Moeda quartzites in the east limb of the Moeda syncline. The fractures correspond to millimetre-scaleshear zones, that developed on relatively more micaceous horizons between the decimetric quartzose strata. The approximatelocation of the thin sections show in ‘b’ and ‘c’ are indicated. Width of view is 1.6 m. (b) Representative microstructure andcorresponding c-axis fabric of a quartzose layer. An important component of coaxial deformation is reflected in the low asymmetryof the c-axis fabric skeleton. Strain is accommodated through pressure solution in grain boundaries that face the maximumshortening direction (Ps) plus opening of tension fractures (Te) at high angle to the maximum extension direction. Large dimensionof photo is 1.2 mm. Density contours are 1.25 and 2.5% per 1% area. N=123. (c) Microstructure and c-axis fabric of the shearedmicaceous horizons. The c-axis fabric skeleton is an asymmetric single girdle indicative of non-coaxial deformation. Width of viewis 550 mm. Density contours are 1.25, 2 and 3% per 1% area. N=200.


Fig. 8.


consistent with the overall distribution of shearzones and S–C fabrics within the synclines andreflect well-organized strain partitioning betweeninterlayer and intralayer domains during synclinedevelopment (discussed below).

3.4. Folds

Numerous parasitic folds are represented on thelimbs of the first-order folds that define the kilo-metre-scale synclines, with fold geometries evidentdown to the thin-section scale. Metasedimentarylithologies with different rheologic propertiesshow contrasting folding patterns. The hematite-rich layers in the BIF unit show the most complexand tight fold geometries (Fig. 6d), whereas foldsin the quartz-rich units are much more open andregular. Folds in the hematite-rich layers, and alsoin some phyllitic horizons, are intrafolial andcommonly grade into kinks (see below), theirasymmetry being generally opposite to the bed-ding-parallel shear inferred from S–C fabrics andother criteria.

Outcrop scale, open to moderately tight foldsshowing a plane axial cleavage are also commonin the hinge zones of the kilometre-scale synclines,principally in the more pelitic lithologies. Thegeometry and orientation of the majority of thesefolds, with fold axis generally subparallel to thesyncline keel axis, suggest they are also smallerscale folds related to development of the kilome-tre-scale synclines. However, a minor proportionof tight folds with fold axis oblique or perpendic-ular to the syncline axis are observed locally insome sectors of the folded supracrustal sequences(cf. Chemale et al., 1994), and may in part repre-sent preserved folds related to the Palaeoprotero-zoic Transamazonian orogeny (Alkmim andMarshak, 1998).

3.5. Kinks

Kinks are well developed in the hematite-richlayers of the BIFs because of comparatively lessercompetency of this lithology relative to the sur-rounding quartz-rich layers (Fig. 9). Where thehematite content is relatively less, kinks com-monly display a smooth transition into asymmet-

ric intrafolial folds (drag folds). The orientationsof the kink planes are variable but generallydefine an average oblique angle with lithologicallayering of 30–50°, dipping inward into the syn-clines. Hence, dip directions of kink planes reverseon opposite syncline limbs. The kink pattern iscommonly very complex, with the appearance ofsecondary kinks at a large range of scales. Fig. 10summarizes the orientation of kink planes ob-tained in the studied sectors of the synclinal keel.Brecciated zones with variable magnitudes andorientations are intimately associated with themost intensely kinked BIFs (Fig. 9b–c), the thick-est breccias generally occurring adjacent to thedisplaced younger strata. There appears to be adirect relationship between the dip angle of thehematite-rich layer and the complexity of the kinkpattern, subvertical and overturned layers gener-ally showing the most intense kinking. The inten-sity of kinking is also directly related to theoverall Fe oxide content (hematite and magnetite)of the BIF layers. Layers with 80–90% of he-matite tend to show the most complex kinkingpatterns. These relationships suggest that thiscomplex kinking pattern may be the result ofintrafolial gravitational collapse of the high-den-sity hematite-bearing layers during syncline for-mation (discussed below).

3.6. Porphyroblasts

Porphyroblast growth represented by muscoviteand, more locally, kyanite porphyroblasts arecommon throughout the metasedimentary se-quences of Quadrilatero Ferrıfero. In domainsadjacent to the granitic domes, however, a meta-morphic aureole, whose width varies between afew to several hundred metres, is generally presentand is characterized by the presence of stauroliteand biotite porphyroblasts. These aureoles reflecta higher temperature of metamorphism in do-mains adjacent to domes. In this section, porphy-roblast textures observed in these aureoles aredescribed.

Rocks from the aureole of the Bonfim and BeloHorizonte domes locally exhibit well-developedstaurolite/biotite porphyroblast growth, particu-larly within pelitic horizons. Best exposures of


Fig. 9. (a) Kink band in a hematite-rich banded iron formation (BIF) layer (He). Note how the kink band does not propagate intoadjacent quartzite layer (Q), indicating the intrafolial character of this structure. Width of view is 65 cm. (b) Overturned andintensely kinked BIF strata in the west end of the Serra do Curral homocline. Note relative downward movement of younger strataand the brecciated zone adjacent to it (see text). The main kink plane is also indicated (dotted line). (c) Complex pattern of kinksand fractures in BIF layers of the west limb of the Moeda Syncline. Note that some kink bands modify into breccia zones (Br). Smallarrows indicate the relative movements on individual kink planes. Width of view is 1.6 m. (d) Detail of a brecciated zone in BIFlayers. The white matrix is carbonate-rich. Width of view is 25 cm.

these rocks occur on the northern end of theBonfim dome near the townships of Brumadinhoand Ibirite (see Jordt-Evangelista et al., 1992;Marshak et al., 1992). Porphyroblasts at theselocalities are several millimetres across. A perva-sive schistosity is evident in outcrop and is sub-parallel to lithological layering (bedding), which iscommonly defined by porphyroblast-poor quart-zose layers and porphyroblast-rich pelitic hori-zons. Elongate porphyroblasts, in particular

biotite, commonly define a well-developed down-dip stretching lineation.

Spatially oriented thin sections from the aureoleof the Bonfim dome commonly show preservationof bedding in staurolite and, more rarely, biotiteporphyroblasts (Fig. 11a,b). Bedding preserved inporphyroblasts is continuous with bedding in thematrix, and evidence for porphyroblast growthduring the main fabric-forming deformation eventis clear. This deformation has produced the single


pervasive foliation evident in outcrop all over theQuadrilatero Ferrıfero and defined by the shape-preferred orientation of mica and quartz grains.This foliation is generally subparallel to beddingbut attains angles of approximately 10° to it inzones where the percentage of quartz dominatesover that of biotite. Inclusion trails defined byquartz and opaques (ilmenite and magnetite) arecontinuous with the dominant matrix foliation,and porphyroblasts commonly preserve an angu-lar relationship between bedding and this fabricthat is greater than that in the matrix (Fig. 10b).This is suggestive of continued shearing in anorientation subparallel to bedding, which oc-

curred after growth and preservation of originalfoliation/bedding asymmetries.

A more local deformation is expressed as crenu-lations, which are locally differentiated and gener-ally restricted to mica-rich pelitic horizons. Thebest development of crenulations in the pelitichorizons is largely due to the presence of porphy-roblasts, which have acted as rigid objects againstwhich accumulation of shortening strain has oc-curred. This strain intensification has producedlocal zones of phyllosilicate concentration on theporphyroblast margins (Fig. 10c).

Both staurolite and biotite porphyroblasts haveovergrown the pervasive matrix foliation associ-

Fig. 10. Frequency diagram showing orientation of kink planes relative to lithological layering in different sectors of the synclinalkeel. The two sketches in the bottom illustrate the different relationships (positive and negative obliquities) between kink plane andbedding. Note that most kinks are 30–50° oblique, and in a orientation consistent with a downward movement of the upper strata(positive obliquity).


Fig. 11. Photomicrographs of aureole porphyroblasts from the Quadrilatero Ferrıfero. (a) Biotite porphyroblast bordered bystaurolite porphyroblasts, both of which include bedding as inclusion trails parallel to the long edge of the photo. Note thatinclusion trails in the biotite display more pronounced curvature than those in staurolite, which is consistent with progressivelow-medium grade metamorphism (i.e. biotite after staurolite) concurrent with progressive deformation. Sample is from northernend of Bonfim dome. Long edge of photo is 4.0 mm. (b) Staurolite porphyroblast that has grown along a phyllosilicate-rich layerdefining bedding. Opaques and quartz inclusions define inclusion trails with an angular relationship with bedding, which arecontinuous with the matrix foliation. Sample is from northern end of Bonfim dome. Long edge of photo is 1.0 mm. (c)Intensification of cleavage on the margin of the staurolite porphyroblast where strain accumulation has occurred. Matrix comprisedof muscovite and quartz. Sample is from northern end of Bonfim dome. Long edge of photo is 2.0 mm. (d) Garnet porphyroblastincluding a small biotite porphyroblast and displaying sigmoidal inclusion trails that are continuous with a weakly curved foliationin the matrix. Progressive deformation has caused strain accumulation against the porphyroblast margin resulting in phyllosilicateconcentration. Matrix is comprised of muscovite and quartz. Sample is from location adjacent to contact of the Bacao complex.Long edge of photo is 2.0 mm. Polars partly crossed in all photos.

ated with the main deformation event. The greaterdegree of curvature preserved in biotite relative tothat in staurolite porphyroblasts (e.g. Fig. 11a),combined with further intensification of matrixcrenulations against the porphyroblast margins,suggests growth of the biotite during the maindeformation but after staurolite. Relative over-growth textures confirm these microstructuralrelationships.

Similar porphyroblast–matrix relationships oc-cur between biotite and staurolite porphyroblasts

from the aureole of the southern margin of theBacao complex (e.g. Gomes, 1989). Samples col-lected from country rocks adjacent to this contactcontain a pervasive crenulation cleavage and gar-net porphyroblasts with well-developed sigmoidalinclusion trails and including biotite and stauro-lite (Fig. 11d). This relationship is consistent withgrowth of the garnets during the main deforma-tion event but after staurolite and biotite, asexpected to occur during progressive low tomedium grade metamorphism.


4. Interpretations and discussion

4.1. Gra6itational structures

The complex kink pattern that typically occursin the BIF layers of the Quadrilatero Ferrıferohas been interpreted by previous workers (e.g.Chemale et al., 1994) to result from a polydefor-mational tectonic evolution that could have oc-curred in this region. Kink distributions andgeometries have been investigated in different sec-tors of the syncline keel (Fig. 10), and the conclu-sion is that their geometry is extremely variableand is not consistent with those present in otherlithologies. The critical point in understanding theorigin of these kinks is that they are more abun-dant in syncline sectors where the BIF layers aresteeply dipping (\60°). In addition, BIF layers ininverted syncline limbs typically host the mostcomplex kink patterns. The intensity of kinking isalso directly related to the local hematite/mag-netite content in the individual BIF layers. Theserelationships, combined with the observation thatthe kinks are nearly always in an orientation thatreflects slip downwards of the overlying youngerstrata over the older ones (even when the strataare inverted as in Fig. 9b), is herein interpreted asbeing indicative of gravitational collapse of denserBIF layers [density (D) varies between 3.4 and4.8, depending on the Fe oxide content] relative tothe adjacent, less dense lithologies (e.g. quartzite,phyllites, carbonatic rocks; with DB2.9), asshown schematically in Fig. 12.

4.2. Kinematic framework during synclinede6elopment

The single pervasive foliation present in thefolded supracrustal sequences is subparallel or hasa low angle obliquity to bedding all over thesynclinal-homoclinal keel. The opposite sense ofshear in each syncline limb, as indicated by allmeso- and microstructural shear sense indicators,and the pronounced strain partitioning betweenintralayer domains (with predominant coaxial de-formation) and interlayer domains (with predomi-nant simple shear), as indicated by c-axispatterns, are all consistent with a general kine-

matic framework with local and regional straincomponents. This is particularly clear in invertedsynclines such as the Moeda syncline (Fig. 3). Theexistence of opposite senses of layer-parallel shearin different limbs is interpreted to reflect the oper-ation of flexural slip/flow (Tanner, 1989) duringsyncline development at low/medium metamor-phic grade conditions. This is corroborated by thefact that most of the slip is accommodated be-tween the sedimentary strata (where a strong rhe-ologic contrast occurs) via bedding parallel,interlayer shear. In contrast, the interior of thestrata have been important for localizing the pureshear component (indicated by the existence ofcross-girdle c-axis fabrics; Fig. 8), which reflectsthe bulk shortening direction of finite strain dur-ing the syncline closure. Evidence for brittle de-formation of quartz (Fig. 8b) indicates thatdeformation occurred at temperatures below 300–350°C and/or at relatively fast strain rates (Pass-chier and Trouw, 1996), therefore preventingpervasive ductile flow in the supracrustal rocks.At these rheologic conditions, flexural slip/flowappears to be a suitable mechanism to accommo-date folding (Tanner, 1989).

The overall kinematic pattern has been largelyinfluenced by the regional tectonic transport fromeast to west, which has determined the progressiveclosure and ultimate inversion of some synclinesectors. Thus, two distinct situations have beenproduced, which are functions of the relative rolesof local (derived from flexural slip/flow) and re-gional strain components (ultimately responsiblefor syncline closure and inversion). These rela-tionships are sketched in Fig. 13. During theinitial stages of syncline development (or in caseswhere no syncline limb inversion has occurred)with two ascending domes symmetrically disposedrelative to the syncline axis, no difference in themagnitude and distribution of finite strain acrossthe syncline is expected to occur (Fig. 13a). Ge-ometries in syncline sectors without limb inver-sions are consistent with this (e.g. the westernportion of the Dom Bosco syncline; Fig. 4). Incontrast, a totally different situation should havebeen produced in inverted synclines (e.g. Serra doCurral and Moeda synclines), where the regionaltectonic transport responsible by the limb inver-


sion induced a non-coaxial strain component thatinteracted in different ways in each limb with thenon-coaxial component derived from flexural slip(Fig. 13b). Thus, in overturned limbs, the regionalsimple shear component derived from the domemovement that caused the syncline inversion op-posed the simple shear derived from flexural slip,

so that a smaller finite strain was produced inthese domains. In contrast, in the normal, non-in-verted limbs these two non-coaxial componentswere synthetic, leading to a higher finite strain inthese domains. This scenario is consistent with allmicrostructural and kinematic information col-lected across the asymmetric Moeda syncline.

Fig. 12. Sketch illustrating kink development due to intrafolial gravitational collapse of denser hematite-rich layers duringprogressive syncline closure. More complex kink patterns and intense brecciation along layer boundaries are produced in locationswhere banded iron formation (BIF) layers are steeply dipping (see text).


Fig. 13. Suggested kinematic frameworks during evolution of the dome/syncline structure of the Quadrilatero Ferrıfero in acompressive regional stress field. (a) Symmetric syncline closure associated with vertical dome ascent. There is a pronouncedstrain/deformation partitioning between intralayer domains, where coaxial deformation is predominant, and interlayer domains,where flexural slip induces opposite shears in each limb. Note that the shear sense in the contacts with domes is opposite to theflexural shear in the corresponding syncline limb. (b) Syncline closure with overturning. Rs is the regional shear componentassociated with the horizontal dome transport that induced the syncline inversion. Note that Rs is synthetic to flexural shear in thenormal limb, but it is opposite to it in the overturned limb. As a consequence, finite strain produced in the overturned limb shouldbe smaller than in the normal limb, as occurs in the overturned Moeda syncline.


4.3. Granite emplacement, syncline de6elopmentand aureole structures

Explanation of processes involved in the de-velopment of the different types of dome–supracrustal structures such as intrusive plutonsand core complexes invokes the necessity to con-sider mass transfer processes that must occur tomake space available, as deformation in thestructural aureoles only provide accommodationof 15–35% of the country rock material thathad to be displaced to allow granitic plutons tobe emplaced (Paterson and Fowler, 1993). As aconsequence, mass transfer processes necessarilyinvolved a downward movement of the countryrock. Similarly, published studies of Archaeangranite–greenstone terrains have invoked theprocess of diapirism, with upwelling of relativelyless dense granitic material through surroundinggreenstones (e.g. Anhaeusser, 1984; Hickman,1984; Bouhallier et al., 1993; Jelmsa et al., 1993;Pons et al., 1995; Chardon et al., 1998). Al-though these examples involve emplacement ofigneous plutons, the same space problems andmass transfer considerations must be taken intoaccount during development of dome–supracrustal structures by solid-state processesduring basement reactivation, such as occurs inthe Quadrilatero Ferrıfero dome-and-keel struc-ture and in other mantled gneisses and corecomplexes (Marshak et al. 1997).

The pervasive development of shear zones atthe dome–supracrustal contacts, which generallydisplay steep down-dip lineations and dome-side-up senses of shear, is consistent with downwardflow of supracrustal sequence. This would ap-pear to be a mechanically viable process becauseno work need be done against gravity (e.g. Pa-terson and Miller, 1998) during downward trans-fer of the relatively more dense supracrustals.The gravitational collapse of the denser BIF lay-ers and the solution transfer mechanisms in themarginal shear zones (e.g. Hippertt, 1994b),along with attenuation of syncline limbs closestto the domes, provide examples of such masstransfer processes in the Quadrilatero Ferrıferodome-syncline architecture.

Kilometre-scale synclines display close spatialrelationships to the domes, and pronouncedstaurolite/biotite porphyroblast growth is spa-tially restricted to the margins of the domes.This suggests that dome formation is linkedboth to syncline development and porphyroblastgrowth. This contention is further supported bythe steep down-dip preferred orientations ofmetamorphic minerals in the shear zones, whichrepresent stretching lineations on the foliationthat has accommodated the shearing strain.

Previous authors (e.g. Marshak and Alkmin,1989; Chemale et al., 1994) have suggested thatthe earliest deformation in the Quadrilatero Fer-rıfero was represented by a Palaeoproterozoicextensional event that was also responsible forformation of the granite–gneiss domes and nu-cleation of the regional synclines. Alternatively,Chauvet et al. (1994) have attributed the devel-opment of these structures to the Pan-African-Brasiliano contractional deformation. Theintensification of foliation development adjacentto porphyroblasts during the event in which theygrew is also more suggestive of contractional de-formation (e.g. Bell and Cuff, 1989; Vernon,1989; Davis, 1993, 1995), the critical criterionfor syntectonic growth being that inclusion trailsare continuous with the matrix foliation (e.g.Johnson and Vernon, 1995). In addition, por-phyroblast growth (staurolite, biotite, garnet) re-stricted to the margins of the domes andoccurred during the main fabric-forming defor-mation expressed by pervasive foliation develop-ment in the country rock of both the Bacao andBonfim domes. In summary, the above observa-tions are found to be consistent with interpreta-tions such as those of Chauvet et al. (1994), whosuggested that the main structures of theQuadrilatero Ferrıfero are related to contrac-tional deformation during the Pan-African-Brasiliano orogeny. Therefore, formation of thesynclines was interpreted to have occurred syn-chronous with emplacement of the domes andstaurolite/biotite porphyroblast growth in theaureoles during the Pan-African-Brasilianoorogeny.


4.4. Model for dome emplacement and synclinede6elopment

Given the above relationships and interpreta-tions, a model for formation of the domes wassuggested (Fig. 14) which is compatible with thegeometry of the surrounding synclines, timing ofporphyroblast growth in the aureoles, kinematicsof the shear zones, and structures at all scales.Initiation of dome formation could have occurredearly during the Palaeoproterozoic extensionalevent, with development of basement heterogenei-ties that acted as dome precursors as suggested byMarshak et al. (1992) and Chemale et al. (1994).

This extensional tectonics was synchronous withpronounced anatexis and granite formation (e.g.Machado et al., 1992) probably due to stressrelease and associated water access into the Ar-chaean basement. However, there is no evidencethat this Palaeoproterozoic event has producedany pervasive cleavage, there being a single perva-sive foliation defined by muscovite–quartz–(chlo-rite) all over the Quadrilatero Ferrıfero regionwhich is most likely 0.8–0.6 Ga old (Chauvet etal., 1994). Subsequent to this earliest extensionaldeformation, there was a change in deformationregime to one of west-directed compression dur-ing the 0.8–0.6 Ga Pan-African-Brasiliano

Fig. 14. Suggested model for the development of the dome/syncline architecture of the Quadrilatero Ferrıfero. Stage 1 representsoriginal basement/supracrustals structure prior to Proterozoic orogenies. Stage 2 illustrates the 2.1–2.0 Ga Transamazonianextensional tectonic event (cf. Marshak and Alkmin, 1989), when normal faults and dome/syncline precursors may have formed.Downward H2O flow through normal faults favoured partial melting and granite magmatism in this period. Stage 3 represents thewesterly-directed 0.8–0.6 Ga Pan-African-Brasiliano compression which is synchronous with low to medium grade regionalmetamorphism, basement reactivation and return flow in the supracrustal rocks. Shear zones are developed on dome–supracrustalcontacts. The ascending ‘hot’ domes induce development of thermal aureoles characterized by staurolite/biotite porphyroblasts in thesurrounding supracrustals (black objects). Thrusts are developed in the eastern portion of the Quadrilatero. Progressive shorteningtightens the synclines and enhances the dome/syncline architecture. A moderate to steeply dipping foliation is produced in thesupracrustal rocks.


orogeny, which accentuated the kilometre-scalebasement heterogeneities and caused nucleationand amplification of synclinal structures in paral-lel with basement reactivation and dome ascent(Hippertt, 1994a). The enhanced development ofcleavage synchronous with porphyroblast growthin the structural aureoles around the domes sug-gests that dome-induced deformation processesmay have been important and operated in parallelwith this regional compression. Alternatively, thiscan be explained as an effect of strain localizationaround the developing domes, which would havebehaved as comparatively rigid objects relative tothe country rocks in a compressive regional stressfield.

Progressive shortening induced upward move-ment and amplification of domal structuresthrough solid state deformation (Hippertt, 1994a),with mass transfer processes causing return flowof supracrustal material on the dome margins,thereby accentuating synclinal geometries. Similarprocesses have been invoked by Bloem et al.(1997) who suggested that kilometre-scale struc-tures in the granite–greenstone terrain of theBullfinch–Parker Dome area, Western Australia,occurred due to regional shortening broadlysimultaneous with upward movement of grani-toids by solid-state diapirism and/or ballooningplutonism.

Synclinal development in the Quadrilatero Fer-rıfero was enhanced on the eastern and westernmargins of the domes due to the concentration ofstrain resulting from WNW-directed shorteningand synchronous downward movement ofsupracrustal rocks. Porphyroblast growth associ-ated with dome development occurred early in thecompressional stages of the 0.8–0.6 Ga Pan-African-Brasiliano deformation. Continued pro-gressive deformation caused strain accumulationon porphyroblast margins, resulting in zones ofenhanced phyllosilicate concentration and localcleavage differentiation. Shear zones marginal tothe domes formed progressively during this defor-mation and aided movement of the supracrustalmaterial around the flanks of the dome, develop-ing syn-kinematic metamorphic assemblages com-patible with the greenschist/lower amphibolitefacies metamorphism responsible for porphyrob-

last growth in the structural aureoles. These shearzones were operative at this stage and accommo-dated downward flow of supracrustal material intandem with relative upward movement of thedomes, as indicated by shear zone kinematics andthe preferred growth of metamorphic mineralsalong the shear planes. Progressive shorteningwould have enhanced cleavage and shear zonedevelopment, with further tightening of thesynclines.

5. Conclusions

1. The main processes for dome emplacementand syncline formation were operative during dePan-African-Brasiliano crustal contraction. Im-portant processes were steeply directedsupracrustal wall-rock flow around the domes,accommodated largely by synchronous develop-ment of high-strain shear zones coincident withthe dome–supracrustal contacts and spaced bed-ding-parallel shear related to flexural slip/flow.The relative movements of supracrustals anddome material, and the changes in shear zonekinematics away from the domes, caused kilome-tre-scale variations in viscous drag of the countryrocks, producing kilometre-scale synclines. Heatsupplied by the ascending granite–gneiss domes,as they came from deeper (and heater) crustallevels due to basement reactivation (cf. Marshaket al., 1992), induced development of thermalmetamorphic aureoles with staurolite and biotiteporphyroblasts. Strain accumulation at thesupracrustal–dome interface, the density contrastbetween supracrustals and granite–gneisses, andcontractional deformation during westerly-di-rected Pan-African-Brasiliano shortening all aidedin formation of the synclines, with the downwardflow of denser supracrustals being a gravitation-ally favourable process.

2. The integration of microscale textural andkinematic relationships with meso- and kilometre-scale documentation of tectonites and large-scalegeologic structures play a crucial role in the devel-opment of tectonic models. Syn-kinematic biotiteand staurolite porphyroblasts in the contact meta-morphic aureoles around the domes have revealed


the pervasive nature of the Pan-African Brasilianofoliation and providing structural timing criteriafor emplacement of the domes, contact-parallelshear zones and kilometre-scale synclines.

3. Kilometre-scale structures such this domes-synclines architecture in the Quadrilatero Ferrıf-ero are a good natural laboratory for studyinghow local and regional stress–strain componentsmay interact and influence the resulting structuralgeometries. Kilometre-scale folding of very het-erogeneous sedimentary sequences containing lay-ers with a higher density than the surroundinglithologies (such as the BIFs in the syncline keel)may be an ideal situation for gravitational col-lapse of the denser lithologies, giving rise to com-plex structures and geometries that do notnecessarily reflect the regional tectonic stress field.Such gravity-impelled processes contribute to theglobal mass transfer during syncline developmentin low to medium grade metamorphic ambients,but should exert a decreasing role downward inthe crust where more cohesive deformation pro-cesses such as dislocation- and diffusion-assisteddeformation gain importance.

Acknowledgements

Hippertt thanks for financial support fromFAPEMIG (process CEX 117/95) and CNPq re-search grants (processes 452943/97-3 e 521847/95-8). Davis conducted this research while holding anAustralian Postdoctoral Fellowship. Vitalinoaided in field logistics and location of key out-crops. Reginaldo Silva contributed with discus-sions and photos for Fig. 6(b) and 9(b). Journalreviews were made by R. Caby and an anony-mous reviewer.

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