Cristiano Lana

Archean crustal structure of the Kaapvaal craton, South Africa – evidence from the Vredefort dome

Abstract Crystalline Archean basement rocks in the core of the Vredefort dome present a profile through a substantial part of the middle and lower crust of the Kaapvaal craton. Previously, this profile has been subdivided into two terranes with allegedly distinct lithologies and tectonometamorphic histories that were juxtaposed along a crustal-scale Late Archean brittle–ductile thrust zone. Lithological and structural mapping across the dome indicates, however, that the basement lithologies share a common polyphase tectonic history culminating in high-grade metamorphism and melting at ∼3.1 Ga. No evidence was found of the postulated tectonic terrane boundary, but the alleged boundary does coincide with a 1–2 km wide transition zone between upper amphibolite facies migmatitic gneisses and more restitic granulite facies gneisses. The implications of these results for Archean regional tectonic models for the Kaapvaal craton are discussed.

Quartz plastic segregation and ribbon development in high-grade striped gneisses

Quartz microstructures and c-axis fabrics formed during development of polycrystalline quartz ribbons in striped gneisses from the high-grade Alem Paraiba shear zone, in southeastern Brazil, are documented. Cluster analysis of quartz grains in samples exhibiting different degrees of shear strain revealed that formation of ribbons was a mass conservative process, where isolated quartz grains became plastically segregated and then coalesced to form polycrystalline ribbons. These ribbons are separated by feldspar-rich domains devoid of quartz. The stage at which individual, stretched quartz grains start to contact each other and initiate ribbon development represents a crucial microstructural change from single grain to polycrystalline ribbon deformation mode, which is reflected by an abrupt increase in the smoothness of the ribbon boundaries. This change is interpreted to represent a strain-softening kink in the stress-strain-time path. Progressive ribboning is accompanied by strengthening of the c-axis fabric Z-maximum, indicative of continued plastic flow by basal glide. Operation of basal glide at these high-temperature conditions (680–700°C) is interpreted to be a consequence of relatively dry deformation conditions. A model is then proposed for development of straight quartz ribbons in high-grade striped gneisses, where scattered quartz grains are continuously stretched and segregated by crystal–plastic processes. The small angle misorientation of the contacting grains enables subsequent coalescence and resulting grain size enlargement. Pervasive grain boundary migration accounts for the straight grain boundaries and rectangular grain shapes within the ribbons.

Nature of the archean midcrust in the core of the Vredefort dome, Central Kaapvaal Craton, South Africa

Extreme uplift associated with the formation of the 2.02 Ga Vredefort dome has exposed a substantial cross section through the crystalline early Archean basement complex rocks of the Kaapvaal craton. The rocks comprise polydeformed high-grade tonalite-trondhjemite-granodiorite (TTG) gneisses, migmatites and late-tectonic intrusive granitoids that straddle the upper amphibolite-to granulite-facies transition. Field, petrographic and geochemical data indicate that compositional heterogeneity occurs on a local scale and reflects the migmatitic character of the rocks rather than crustal-scale layering as has been previously proposed. No evidence has been found to support exposure of either a melt-depleted, refractory, lower crust or an upper crustal batholithic granite layer; however, the immense volume of granitic leucosome in the rocks suggests that the exposed section represents an intermediate level between these two zones. Granitic leucosomes in the upper amphibolite-facies migmatites appear to be intrusive into the predominantly trondhjemitic host rocks, rather than of in situ derivation. Leucosome compositions in the granulite-facies migmatites are more variable, ranging from granitic and charnockitic to enderbitic, probably reflecting at least some local derivation. Leucosomes and small granitoid bodies show local-scale geochemical variation that can be explained in terms of variable amounts of melt segregation and migration, and fractionation of minerals such as K-feldspar within the melts.

Exhumation of Mesoarchean TTG gneisses from the middle crust; insights from the Steynsdorp core complex, Barberton granitoid-greenstone terrain, South Africa

The processes leading to the assembly of supracrustal greenstone sequences and plutonic rocks of the trondhjemite-tonalite-granodiorite (TTG) suite, the two main building blocks of all Archean cratons, form a central aspect of our understanding of early continental growth. Key metamorphic and structural data are presented from granitoid-greenstone contacts in the Steynsdorp dome, the oldest and one of the best preserved parts of the Mesoarchean Barberton granitoid-greenstone in South Africa. Pressure and temperature estimates of P = 10–13 kbar and T = 640–660 °C from supracrustal rocks of the Steynsdorp dome indicate the burial of these rocks to depths >30–40 km during the main phase of collisional (D2) tectonics recorded in the terrane. Peak metamorphic assemblages define flattening-type fabrics, indicating crustal stacking of cool, rigid crust. The subsequent retrogression of rocks is associated with constrictional fabrics that are interpreted to have formed in response to the orogen-parallel extrusion and exhumation of rocks. The progressive retrogression of rocks and associated fabric development, unidirectional lineation and fold plunges, and consistent granitoid-up, greenstone-down kinematic indicators point to the exhumation of the TTG gneisses along an extensional detachment below the low-grade Barberton greenstone belt. The results are consistent with findings along the granitoid-greenstone contacts some 40 km west of the Steynsdorp area, and they indicate the regional extent of this metamorphic core complex and the allochthonous nature of the high-pressure, low-temperature terrane with respect to the rest of the greenstone belt. This also implies that the 3.45 Ga suite of TTG rocks south of the Barberton greenstone belt is unlikely to have represented the source of similar-aged volcanic rocks in the belt. More voluminous and larger extent of 3.45 Ga TTG plutonism represents a significant early crust-forming event in the Barberton terrain, the evidence of which has been obliterated during later episodes of tectonomagmatic recycling. The existence of the high-pressure, low-temperature terrane in the southern Barberton granitoid-greenstone terrain indicates the presence of a cool and rigid continental nucleus in the Mesoarchean around which plate-tectonic processes could initiate.

Deformation partitioning during folding of banded iron formation

Abstract Domainal electron and optical microscopy and c -axis fabric analysis are utilized to document microstructures and the associated deformation partitioning between crystal plastic, brittle and fluid-assisted deformation mechanisms during folding of Proterozoic banded iron formation (a hematite–quartz–calcite multilayer) from the Quadrilatero Ferrifero granite–greenstone terrain (southeastern Brazil). The operation of different mechanisms was partially determined by the contrasting rheologic response of these three minerals at greenschist facies metamorphic conditions. Fracturing was the main deformation process in hematite, while solution-transfer accounted for part of the deformation in both calcite and quartz. Notably, there is strong interaction among the different deformation mechanisms, which have influenced each other in various ways. Fracturing of hematite in hinge zones of folds caused opening of gaps, which were sealed by direct precipitation of silica from the fluid phase. As a consequence, a chemical potential gradient between crystals and fluid phase was produced, and quartz was dissolved to restore the thermodynamical crystal–fluid equilibrium. Thus, brittle deformation of hematite partially controlled solution-precipitation creep in quartz. Heterogeneous access of fluid into the deforming medium also affected the deformation processes. Inhomogeneous deformation in the quartz–calcite aggregates generated intergranular porosity and increased fluid access, with solution-transfer becoming dominant in these domains. In contrast, the relatively more homogeneous deformation in the pure quartz aggregates served to maintain well-adjusted grain boundaries and reduced fluid access into the intergranular space, such that these domains deformed uniformly by crystal plastic processes, at relatively dry conditions.

Successive midcrustal, high-grade metamorphic events provide insight into Mid-Archean mountain-building along the SE margin of the proto-Kaapvaal craton

In this study, we investigate the evolution of continental crust at the Paleoarchean to Mesoarchean boundary by documenting the deposition, deep burial, and partial melting of metasedimentary gneisses along the SE margin of the proto–Kaapvaal craton. Successive high-grade metamorphic events recorded by the gneisses between ca. 3.23 and 3.07 Ga not only coincided with the timing of subduction-accretion in the adjacent Barberton greenstone belt at ca. 3.23–3.22 Ga, but also with discrete pulses of potassic granitic magmatism during differentiation and consolidation of the newly assembled lithosphere. Mineral equilibria modeling demonstrates that the granulites evolved along pressure-temperature ( P - T ) paths similar to those documented for metamorphism in modern collisional orogens. High-temperature deformation at ca. 3.11–3.07 Ga during uplift of the terrane was coaxial with the main NE-SW–trending structural grain of the Barberton greenstone belt and reflects the regional NW-SE shortening and NE-SW orogen-parallel extension exhibited by the younger potassic granites. We reconcile these features with Mesoarchean terrane assembly in Barberton via two NW-dipping subduction zones. The trace of the first is represented by the main terrane boundary within the Barberton greenstone belt; the second resulted in accretion, burial, and high-temperature metamorphism along the SE margin of the Kaapvaal craton, with the granulites providing valuable insight into the mid- to lower-crustal response to what appears to have been a protracted accretionary orogenic event.

Deformation partitioning during folding and transposition of quartz layers

Abstract Banded iron formation (BIF) from the Quadrilatero Ferrifero (southeastern Brazil) shows a compositional layering with alternating iron-rich and quartz-rich layers. This layering was intensively folded and transposed at a centimeter/millimeter scale through a component of bedding-parallel shear related to flexural slip at middle to high greenschist facies conditions (400–450 °C). The microstructure and c-axis fabrics of normal limbs, inverted limb and hinge zones of a selected isoclinal fold were analyzed combining optical and scanning electron microscopy (SEM) and digital image analysis. In the normal limbs, recrystallized quartz grains show undulose extinction, relatively dry grain boundaries, c-axes at high angle to foliation and a pervasive grain shape fabric (GSF) indicating operation of crystal-plastic processes. In the inverted limb, quartz grains show more serrated and porous (“wet”) grain boundaries; the GSF is similar to that of the normal limb, but c-axes are oriented at 90° to those of the normal limb. We interpreted these characteristics as reflecting operation of solution-precipitation deformation in inverted limbs, as a consequence of grains having been rotated to an orientation that was hard to basal 〈a〉 glide, but easy to dissolution-precipitation creep. This deformation partitioning between crystal-plasticity and solution-transfer during folding/transposition of quartz may explain the common occurrence of layered quartz rocks, where individual layers show alternating c-axis fabrics with opposite asymmetries but a consistent GSF orientation. Such characteristics may reflect an earlier event of pervasive folding/transposition of a preexisting layering.

In situ LA-ICPMS U-Pb dating of detrital zircons from the Cercadinho Formation, Minas Supergroup.

In this paper, new geochronological data obtained for the detrital rocks of the Cercadinho Formation, basal unit of the Piracicaba Group, Minas Supergroup, are presented. U-Pb Laser Ablation Induced Coupled Mass Spectrometer (LA-ICPMS) analysis in detrital zircon from quartzite samples provided a new maximum depositional age for the Cercadinho Formation. The youngest zircon population, among the dated samples, provided an average age of 2680 ± 24 Ma. These rocks have an expressive contribution of Meso- to Neoarchean zircons, with ages between 2812 ± 19 and 2909 ± 19 Ma, and older populations between 3212 ± 18 and 3272 ± 16 Ma, which occur mainly in the core of younger zircon grains. Comparing the U-Pb ages obtained in this work with previously published geochronological data for the basal units of the Minas Supergroup (Moeda Formation) an aging of the source for the detrital rocks of Cercadinho Formation can be observed, with a major contribution of zircons from TTG rocks crystallized between the Meso- and Neoarchean.

Geoelectric evidence for centripetal resurge of impact melt and breccias over central uplift of Araguainha impact structure

We present five profiles from electrical resistivity tomography (ERT), with surface constraints and gravity data, in the central uplift of the Araguainha impact structure in central Brazil. The central uplift, the overlying polymict breccias, and decameter-scale impact melt rocks are characterized by contrasting ranges of electrical resistivity. Our resistivity model provides empirical evidence that supports the existing model in which impact melt and breccias resurged toward the crater center in the final stages of the cratering process. On the basis of our results from the first use of ERT in impact cratering studies, we conclude that the deposition and flow of impact melt and breccias over the central uplift were influenced by the geometry of the lithologic boundaries in the central uplift.

Geology and geochemistry of a granite-greenstone association in the southeastern Vredefort dome, South Africa

A sequence of mafic to ultramafic chlorite-actinolite-hornblende-talc rocks with komatiitic and komatiitic basalt compositions has been mapped in the southeastern Vredefort dome. A volcanic origin for the rocks is indicated by pillow structures, flow banding, variolites and spinifex textures in low-strain domains. They are intercalated with quartz-sericite-biotite schists. The rocks are metamorphosed to mid-greenschist-facies (~400°C) and are heterogeneously deformed, with a dominant subvertical, northwest-trending schistosity (S3) that developed under peak metamorphic conditions. To the northwest, the greenstones are bounded by a kilometre-wide dip-slip shear zone, which includes poorly-exposed high-grade (~700°C, 5 kbar) amphibolites and migmatitic trondhjemitic gneisses, and pegmatitic granite, and which separates the low-grade rocks from the high-grade gneissic core of the dome. The variation in metamorphic grade across the mapped area is attributed primarily to the displacement across the shear zone (southeast side down), but it may also reflect, in part, deeper levels of exhumation towards the centre of the dome. In contrast to the komatiitic greenstones, the amphibolites within the shear zone have a tholeiitic composition; however, they also show features consistent with volcanic precursors and are interleaved wirh banded ironstones and quartz-sericite-biotite schists. The mafic and ultramafic metavolcanics display close similarities to the komatiites and komatiitic basalts in the Johannesburg dome, but are relatively depleted in Fe2O3, TiO2 and CaO and enriched in Al2O3 relative to the Barberton greenstones. They are compositionally distinct from the dismembered granulite-facies greenstone remnants in the central parts of the dome.