A corundum and sapphirine paragenesis from the Limpopo Mobile Belt, southern Africa

An assemblage consisting of corundum, sapphirine, spinel, cordierite, garnet, biotite and bronzite is described from the Messina area of the Limpopo Mobile Belt, and consideration given to its petrogenesis. Various geothermometers and geobarometers have been applied in an attempt to determine the temperatures and pressures of metamorphism.
A former coexistence of garnet and corundum is suggested to have developed during the earliest high pressure phase of the metamorphism, where temperatures exceeded 800°C and pressures as high as 10kbar may have been experienced. Subsequently, continuous retrograding reactions from medium pressure granulite facies at about 800°C and 8kbar towards amphibolite facies generated spinel, cordierite, sapphirine and possibly also bronzite. The most notable reaction was probably of the form: garnet + corundum = cordierite + sapphirine + spinel.     

RbSr chemical and isotopic response of gneisses in late Archean shear zones of the Limpopo mobile belt,southern Africa

This note reports on the Rb, Sr and K chemistry and Sr isotope composition of sheared gneisses from the Limpopo mobile belt, an Archean high-grade terrain in southern Africa. Retrogression during shearing appears to have shifted the chemical and isotopic composition of the gneisses away from those of the surrounding unsheared granulites towards compositions characteristic of certain late Archean granites within the mobile belt and on the adjacent Zimbabwe craton. These results bear on the issues of RbSr dating of high-strain events and of granite genesis in the Limpopo belt.     

High-magnesium staurolite in a sapphirine-garnet rock from the Limpopo Belt,Southern Africa

Unusually Mg-rich staurolite with F=Fe/(Fe+ Mg) = 0.51 occurs in a peraluminous silica-deficient sapphirine-garnet-gedrite-spinel-corundum-phlogopite rock as relic inclusions within pyrope-almandine garnet (F=0.43–0.48), from which it is generally separated by sapphirine (F=0.19). Gedrite has F=0.24–0.27, phlogopite 0.19, and spinel 0.43. The gradual disappearance of staurolite is due to a shift of the 3-phase AFM assemblage staurolite-garnetsapphirine to more Fe-rich compositions according to probably prograde reactions between staurolite and garnet to form sapphirine together with either quartz or gedrite. Thus still more Mg-rich staurolite seems to have existed in the rock at earlier stages of metamorphism characterized by lower temperatures and perhaps higher pressures. The local equilibria now frozen in indicate minimum pressures of 7–8 kbar and temperatures in excess of about 800° C.Staurolite is free from Zn, very low in Si, and richer in Ti than any other staurolite found thus far. Most notable is the unique Mg/Fe fraction between staurolite and garnet with staurolite being the more Fe-rich phase (K_D=1.32).     

Fluids in granulites of the Southern Marginal Zone of the Limpopo Belt, South Africa

The Southern Marginal Zone of the Limpopo Belt in South Africa is characterised by a granulite and retrograde hydrated granulite terrane. The Southern Marginal Zone is, therefore, perfectly suitable to study fluids during and after granulite facies metamorphism by means of fluid inclusions and equilibrium calculations. Isolated and clustered high-salinity aqueous and CO₂(-CH₄) fluid inclusions within quartz inclusions in garnet in metapelites demonstrate that these immiscible low H₂O activity fluids were present under peak metamorphic conditions (800-850 °C, 7.5-8.5 kbar). The absence of widespread high-temperature metasomatic alteration indicates that the brine fluid was probably only locally present in small quantities. Thermocalc calculations demonstrate that the peak metamorphic mineral assemblage in mafic granulites was in equilibrium with a fluid with a low H₂O activity (0.2-0.3). The absence of water in CO₂-rich fluid inclusions is due to either observation difficulties or selective water leakage. The density of CO₂ inclusions in trails suggests a retrograde P-T path dominated by decompression at T<600 °C. Re-evaluation of previously published data demonstrates that retrograde hydration of the granulites at 600 °C occurred in the presence of H₂O and CO₂-rich fluids under P-T conditions of 5-6 kbar and ~600 °C. The different compositions of the hydrating fluid suggest more than one fluid source.     

Petrochemistry and mineralogy of early Precambrian anorthositic rocks of the Limpopo belt, southern Africa

Two sections of the anorthosite ‘complex’ were examined at Messina, South Africa and at Pikwe, Botswana. Thirty XRF whole-rock analyses of samples in stratigraphic order show that alkalies at Messina increase upwards from leuco-gabbros to anorthosites, but no such correlation was found at Pikwe. Electron probe analyses of plagioclases in 33 samples indicate extensive normal zoning and variation (An₃₁-An₈₀) especially at Pikwe. The highest An-contents tend to vary monotonically with stratigraphic position, whereas the lowest values are erratic. Twenty-two amphibole analyses indicate that SiO₂, TiO₂ and MgO/FeO increase, decrease and increase, respectively, with height at Messina, but 22 analyses from Pikwe show no such trends. Their compositions are similar to those from the Fiskenaesset complex. Aluminous chromites are Fe-rich, also like the Fiskenaesset ones.

The Limpopo anorthositic rocks belong to a layered igneous body that was recrystallized by regional metamorphism and subjected to erratic metasomatism. The original Limpopo and Fiskenaesset bodies strongly resemble gabbroic and peridotitic relics in the Peninsular Ranges Batholith in S. California.     

The tectonic generation of the Limpopo Mobile Belt,and a definition of its western extremity

A new mechanism is suggested for the generation of the interference fold pattern which characterizes the Limpopo Mobile Belt. The mechanism is directly related to shear movement along the Tuli-Sabi Shear Zone, renamed the Tuli-Sabi Straightening Zone. The mobile belt is regarded as a taphrogenic lineament (McConnell, 1974) and its generation is compatible with the tectonic environment active in Proterozoic times according to Sutton and Watson (1974). Field evidence shows that the Tuli-Sabi Straightening Zone dies out in Botswana at Moshakabela, and it is reasoned that the mobile belt as a whole also disappears in this vicinity. It does not extend into central and western Botswana beneath the Karroo and Kalahari cover. Detailed examination of ERTS-1 imagery of northeastern Botswana strengthens these deductions. The Tuli-Sabi Straightening Zone and the characteristic fold patterns of the mobile belt can be seen quite clearly on the satellite imagery. Furthermore, the Tuli-Sabi Straightening Zone appears to be displaced southwards at the international boundary between Botswana and Rhodesia. The existence of a fold belt trending about N150° superimposed on the Limpopo Mobile Belt in the west of the area is postulated which is not the Shashe Mobile Belt (Crockett, 1967).     

SHRIMP dating of titanite from metasyenites in the Central Zone of the Limpopo Belt,South Africa

SHRIMP dating of titanite from metasyenites in the Central Zone of the Limpopo Belt yields a mean ²⁰⁷Pb/²⁰⁶Pb age of 2010.3 ±4.5 Ma calculated from 23 analyses. This age, combined with petrographic and field observations, suggests the metamorphism in the syenites occurred during Palaeoproterozoic event.     

Evolution of the Bhandara-Balaghat granulite belt along the southern margin of the Sausar Mobile Belt of central India

The Bhandara-Balaghat granulite (BBG) belt occurs as a 190 km long, detached narrow, linear, NE-SW to ENE-WSW trending belt that is in tectonic contact on its northern margin with the Sausar Group of rocks and is bordered by the Sakoli fold belt in the south. The Bhandara part of the BBG belt is quite restricted, comprising a medium to coarse grained two-pyroxene granulite body that is of gabbroic composition and preserves relic igneous fabric. The main part of the belt in Arjuni-Balaghat section includes metasedimentary (quartzite, BIF, Al- and Mg-Al metapelites) and metaigneous (metaultramafic, amphibolite and two-pyroxene granulite) protoliths interbanded with charnockite and charnockitic gneiss. These rocks, occurring as small bands and enclaves within migmatitic and granitic gneisses, show polyphase deformation and metamorphism. Geochemically, basic compositions show tholeiitic trend without Fe-enrichment, non-komatitic nature, continental affinity and show evolved nature. Mineral parageneses and reaction textures in different rock compositions indicate early prograde, dehydration melt forming reactions followed by orthopyroxene stability with or without melt. Coronitic and symplectitic garnets have formed over earlier minerals indicating onset of retrograde IBC path. Evidences for high temperature ductile shearing are preserved at places. Retrogressive hydration events clearly post-date the above paths. The present study has shown that the BBG belt may form a part of the Bastar Craton and does not represent exhumed oceanic crust of the Bundelkhand Craton. It is further shown that rocks of the BBG belt have undergone an earlier high-grade granulite metamorphism at 2672 ± 54 Ma (Sm-Nd age) and a post-peak granulite metamorphism at 1416 ± 59Ma (Sm-Nd age, 1380 ± 28Ma Rb-Sr age). These events were followed by deposition of the Sausar supracrustals and Neoproterozoic Sausar orogeny between 973 ± 63Ma and 800 ± 16Ma (Rb-Sr ages).     

Natural boron-free kornerupine and its breakdown products in a sapphirine rock of the Limpopo Belt,Southern Africa

Large crystals of boron-free kornerupine occurring in MgAl-rich inclusions within meta-anorthosites are partially replaced by symplectitic pseudomorphs consisting essentially of the assemblage sapphirine-cordieritegedrite. The highly magnesian, hydrous kornerupines (F= 0.10–0.14) have compositions close to the oxide ratio (Mg, Fe) O· Al₂O₃· SiO₂. Sapphirines (F=0.09) show decreasing Al-contents with continued grain growth and equilibration. Gedrite (F=0.15) contains sodium in amounts near the limit of solid solution, although the kornerupine starting material is free of this element and it is very rare in the enclosing rock. Cordierite (F=0.05) is also free from sodium.For conditions of P _H2O = P _tot the appearance of boron-free kornerupine requires relatively high temperatures (> 700 °C) and a minimum pressure near 4 kb within this zone of the Limpopo Belt. The subsequent replacement reaction occurred nearly isochemically except for Na and probably H₂O, which were introduced into the symplectite. Textural features suggest that the breakdown reaction of kornerupine is actually governed by the magnitude of sodium activity: Relatively low values would favor the appearance of boron-free kornerupine, whereas higher values lead to the more common assemblage sapphirine-cordierite-gedrite.     

Rapid evolution from sediment to anatectic granulite in an Archean continental collision zone: the example of the Bandelierkop Formation metapelites,South Marginal Zone,Limpopo Belt,South Africa

The metamorphic history of the Southern Marginal Zone (SMZ) of the Limpopo Belt, South Africa, possibly provides insight into one of the oldest preserved continental collision zones. The SMZ consists of granitoid gneisses (the Baviaanskloof Gneiss) and subordinate, infolded metasedimentary, metamafic and meta‐ultramafic lithologies (the Bandelierkop Formation) and is regarded as the c. 2700 Ma granulite facies reworked equivalent of the Kaapvaal craton basement. The granulite facies metamorphism is proposed to have occurred in response to collision between the Kaapvaal and Zimbabwe cratons. Previous studies have proposed a wide variety of P–T loops for the granulites, with considerable discrepancy in both the shapes of the retrograde paths and the magnitude of the peak P–T conditions. To date, the form of the prograde path and the timing of the onset of metamorphism remain unknown. This study has used a range of different metasedimentary rocks from a large migmatitic quarry outcrop to better constrain the metamorphic history and the timing of metamorphism in the SMZ. Detrital zircon ages reveal that the protoliths to the metasedimentary rocks were deposited subsequent to 2733 ± 13 Ma. Peak metamorphic conditions of 852.5 ± 7.5 °C and 11.1 ± 1.3 kbar were attained at 2713 ± 8 Ma. The clockwise P–T path is characterized by heating in the sillimanite field along a P–T trajectory which approximately parallels the kyanite to sillimanite transition, followed by near‐isothermal decompression at peak temperature and near‐isobaric cooling at ~6.0 kbar. These results support several important conclusions. First, the sedimentary rocks from the Bandelierkop Formation are not the equivalent of any of the greenstone belt sedimentary successions on the Kaapvaal craton, as has been previously proposed. Rather, they post‐date the formation of the Dominion and Witwatersrand successions on the Kaapvaal craton. From the age distribution of detrital zircon, they appear to have received significant input from various origins. Consequently, at c. 2730 Ma, the Baviaanskloof Gneiss most likely acted as basement onto which the sedimentary succession represented by the Bandelierkop Formation metapelites was deposited. Second, the rocks of the SMZ underwent rapid evolution from sediment to granulite facies anatexis, with a burial rate of ~0.17 cm yr^?1. Peak metamorphism was followed by an isothermal decompression to 787.5 ± 32.5 °C and 6.7 ± 0.5 kbar and isobaric cooling to amphibolite facies conditions, below 640 °C prior to 2680 ± 6 Ma. This age for the end of the high‐grade metamorphic event is marked by the intrusion of crosscutting, undeformed pegmatites that are within error the same age as the crosscutting Matok intrusion (2686 ± 7 Ma). Collectively, the burial rate of the sedimentary rocks, the shape of the P–T path, the burial of the rocks to in excess of 30 km depth and the post‐peak metamorphic rapid decompression argue strongly that the SMZ contains sediments deposited along an active margin during lateral convergence, and that the SMZ was metamorphosed as a consequence of continental collision along the northern margin of the Kaapvaal craton at c. 2700 Ma.     

Melting, carbonic fluids and water recycling in the deep crust: an example from the Limpopo Belt, South Africa

The proposed retrograde orthoamphibole isograd in the Southern Marginal Zone of the Limpopo Belt separates hydrated, amphibolite grade metapelites from their granulite grade precursors and provides an intriguing geological dilemma. Widespread rehydration of metapelitic granulites under conditions of 660–600 °C and ≥0.6 GPa, and CO₂-dominated fluid-inclusion populations appear to suggest thorough flushing of the high-grade crust with an externally derived carbonic fluid. However, past studies of the carbon and oxygen isotope geochemistry of the hydrated rocks have not demonstrated the involvement of any voluminous out of equilibrium’ fluid in the evolution of the rocks. This contribution proposes a model wherein the hydrating fluids are derived from crystallizing anatectic leucosomes, generated by in situ fluid-absent biotite melting along the prograde path. Model equilibrium fluid compositions suggest that reaction between this melt-derived H₂O and biogenic graphite produced CO₂-rich fluid compositions and potentially high fluid:rock ratios at the wet granite solidus. Declining temperature resulted in fluid compositions shifting to higher XH₂O, with the precipitation of graphite essentially at the sites of initial fluid generation, thereby preserving original (pre-metamorphic) isotopic heterogeneities. The hydration pattern of the Southern Marginal Zone appears to be a function of melt migration. In the hydrated zone, leucosomes generally approximate minimum melt compositions and in this zone H₂O was effectively recycled between the prograde and retrograde assemblages. In contrast, leucosomes in the granulite grade portion of the terrane have lost a K₂O- and H₂O-rich melt fraction, and although some hydration has occurred in this zone, orthopyroxene is generally preserved in metapelites. In a general context, in situ crystallization of graphitic partially melted source rocks has the potential to produce high fluid-rock ratios at temperatures close to the wet granite solidus. This single process holds the potential for widespread retrogression of formerly high-grade assemblages, at a variety of aH₂O values, without external fluid input.     

Reaction history of garnet-sapphirine granulites and conditions of Archaean high-pressure granulite-facies metamorphism in the Central Limpopo Mobile Belt, Zimbabwe

ABSTRACT Sequential reaction textures in Archaean garnet-corundum-sapphirine granulites from the Central Zone of the Limpopo Belt document a progression from early, coarse-grained, high-pressure (P > 9.5 kbar) granulite-facies assemblages (M1) to late, low-pressure (P <6 kbar) granulite-facies sub-assemblages (M2). The stable M1 assemblage was garnet (57% pyrope; Mg/(Mg + Fe) = 62) + sapphirine + corundum + gedrite + phlogopite + rutile. Late-M1 boron-free kornerupine grew at the expense of garnet and corundum, and coexisted with garnet, sapphirine and gedrite. Partial or complete breakdown of coarse garnet and kornerupine during M2 resulted in the development of pseudomorphs and coronas consisting of fine-grained symplectic intergrowths of cordierite, gedrite and sapphirine (later, spinel). The majority of reaction textures can be explained in terms of a stable reaction sequence, and a model time-sequence of mineral facies can be constructed. When compared with a qualitative petrogenetic grid of (Fe, Mg)-discontinuous reactions in the FMASH multisystem sapphirine-garnet-corundum-spinel-cordierite-gedrite-kornerupine, the facies-sequence indicates decompression at essentially constant T assuming constant a(H₂O). Exhumation of M1 corundum inclusions during M2 breakdown of kornerupine resulted in production of metastable spinel by a disequilibrium reaction with gedrite. A second disequilibrium reaction of the spinel with cordierite produced sapphirine. The operation of such reaction while pressure was decreasing (the opposite dP from that implied by the texture if assumed to be the product of an equilibrium reaction) has serious implications for the use of reaction textures in the construction of P-T vectors. Garnet-biotite thermometry on garnet interiors and phlogopite inclusions in corundum yields temperatures of ca. 850°C for the M1 stage. A minimum late-M1 pressure of ca. 7 kbar is indicated by the former association of kornerupine and corundum. Relict M1 kyanites reported by other workers indicate a minumum early-M1 pressure of 9.5 kbar, implying metamorphism at depths of at least 33 km (probably 38km). The high-pressure granulite-facies metamorphism was followed by an almost isothermal pressure decrease of > 5 kbar, indicative of rapid uplift. The P-T path is interpreted as the product of a single metamorphic cycle which probably took place in response to tectonic thickening of the crust. Such a process contrasts with the extensional origin recently proposed for isobarically cooled granulite-facies terranes.     

Corundum + quartz and Mg-staurolite bearing granulite from the Limpopo Belt, southern Africa: Implications for a P–T path

A new occurrence of the rare corundum + quartz assemblage and magnesian staurolite has been found in a gedrite–garnet rock from the Central Zone of the Neoarchean Limpopo Belt in Zimbabwe. Poikiloblastic garnet in the sample contains numerous inclusions of corundum + quartz ± sillimanite, magnesian staurolite + sapphirine ± orthopyroxene, and sapphirine + sillimanite assemblages, as well as monophase inclusions. Corundum, often containing subhedral to rounded quartz, occurs as subhedral to euhedral inclusions in the garnet. Quartz and corundum occur in direct grain contact with no evidence of a reaction texture. The textures and Fe–Mg ratios of staurolite inclusions and the host garnet suggest a prograde dehydration reaction of St → Grt + Crn + Qtz + H₂O to give the corundum + quartz assemblage. Peak conditions of 890–930 °C at 9–10 kbar are obtained from orthopyroxene + sapphirine and garnet + staurolite assemblages. A clockwise P–T path is inferred, with peak conditions being followed by retrograde conditions of 4–6 kbar and 500–570 °C. The presence of unusually magnesian staurolite (Mg / [Fe + Mg] = 0.47–0.53) and corundum + garnet assemblages provides evidence for early high-pressure metamorphism in the Central Zone, possibly close to eclogite facies. The prograde high-pressure event followed by high- to ultrahigh-temperature metamorphism and rapid uplifting of the Limpopo Belt could have occurred as a result of Neoarchean collisional orogeny involving the Zimbabwe and Kaapvaal Cratons.     

Shear zone evolution and timing of deformation in the Neoproterozoic transpressional Dom Feliciano Belt,Uruguay

New structural, microstructural and geochronological (U-Pb LA-ICP-MS, Ar/Ar, K-Ar, Rb-Sr) data were obtained for the Dom Feliciano Belt in Uruguay. The main phase of crustal shortening, metamorphism and associated exhumation is recorded between 630 and 600 Ma. This stage is related to the collision of the Río de la Plata and Congo cratons at ca. 630 Ma, which also involved crustal reworking of minor crustal blocks such as the Nico Pérez Terrane and voluminous post-collisional magmatism. Subsequent orogen-parallel sinistral shearing gave rise to further deformation up to ca. 584 Ma and resulted from the onset of the convergence of the Kalahari Craton and the Río de la Plata-Congo cratons. Sinistral shear zones underwent progressive strain localization and retrograde conditions of deformation during crustal exhumation. Dextral ENE-striking shear zones were subsequently active at ca. 550 Ma, coeval with further sinistral shearing along N- to NNE-striking shear zones. The tectonothermal evolution of the Dom Feliciano Belt thus recorded the collision of the Río de la Plata and Congo cratons, which comprised one of the first amalgamated nuclei of Gondwana, and the subsequent incorporation of the Kalahari Craton into Western Gondwana.     

The processes that control leucosome compositions in metasedimentary granulites: perspectives from the Southern Marginal Zone migmatites,Limpopo Belt,South Africa

Anatexis of metapelitic rocks at the Bandelierkop Quarry (BQ) locality in the Southern Marginal Zone of the Limpopo Belt occurred via muscovite and biotite breakdown reactions which, in order of increasing temperature, can be modelled as: (1) Muscovite + quartz + plagioclase = sillimanite + melt; (2) Biotite + sillimanite + quartz + plagioclase = garnet + melt; (3) Biotite + quartz + plagioclase = orthopyroxene ± cordierite ± garnet + melt. Reactions 1 and 2 produced stromatic leucosomes, which underwent solid‐state deformation before the formation of undeformed nebulitic leucosomes by reaction 3. The zircon U–Pb ages for both leucosomes are within error identical. Thus, the melt or magma formed by the first two reactions segregated and formed mechanically solid stromatic veins whilst temperature was increasing. As might be predicted from the deformational history and sequence of melting reactions, the compositions of the stromatic leucosomes depart markedly from those of melts from metapelitic sources. Despite having similar Si contents to melts, the leucosomes are strongly K‐depleted, have Ca:Na ratios similar to the residua from which their magmas segregated and are characterized by a strong positive Eu anomaly, whilst the associated residua has no pronounced Eu anomaly. In addition, within the leucosomes and their wall rocks, peritectic garnet and orthopyroxene are very well preserved. This collective evidence suggests that melt loss from the stromatic leucosome structures whilst the rocks were still undergoing heating is the dominant process that shaped the chemistry of these leucosomes and produced solid leucosomes. Two alternative scenarios are evaluated as generalized petrogenetic models for producing Si‐rich, yet markedly K‐depleted and Ca‐enriched leucosomes from metapelitic sources. The first process involves the mechanical concentration of entrained peritectic plagioclase and garnet in the leucosomes. In this scenario, the volume of quartz in the leucosome must reflect the remaining melt fraction with resultant positive correlation between Si and K in the leucosomes. No such correlation exists in the BQ leucosomes and in similar leucosomes from elsewhere. Consequently, we suggest disequilibrium congruent melting of plagioclase in the source and consequential crystallization of peritectic plagioclase in the melt transfer and accumulation structures rather than at the sites of biotite melting. This induces co‐precipitation of quartz in the structures by increasing SiO₂ content of the melt. This process is characterized by an absence of plagioclase‐induced fractionation of Eu on melting, and the formation of Eu‐enriched, quartz + plagioclase + garnet leucosomes. From these findings, we argue that melt leaves the source rapidly and that the leucosomes form incrementally as melt or magma leaving the source dumps its disequilibrium Ca load, as well as quartz and entrained ferromagnesian peritectic minerals, in sites of magma accumulation and escape. This is consistent with evidence from S‐type granites suggesting rapid magma transfer from source to high level plutons. These findings also suggest that leucosomes of this type should be regarded as constituting part of the residuum from partial melting.     

Structural analysis and implicit 3D modelling of high-grade host rocks to the Venetia kimberlite diatremes,Central Zone,Limpopo Belt,South Africa

The Beit Bridge Complex of the Central Zone (CZ) of the Limpopo Belt hosts the 519 ± 6 Ma Venetia kimberlite diatremes. Deformed shelf- or platform-type supracrustal sequences include the Mount Dowe, Malala Drift and Gumbu Groups, comprising quartzofeldspathic units, biotite-bearing gneiss, quartzite, metapelite, metacalcsilicate and ortho- and para-amphibolite. Previous studies define tectonometamorphic events at 3.3–3.1 Ga, 2.7–2.5 Ga and 2.04 Ga. Detailed structural mapping over 10 years highlights four deformation events at Venetia. Rules-based implicit 3D modelling in Leapfrog Geo™ provides an unprecedented insight into CZ ductile deformation and sheath folding. D₁ juxtaposed gneisses against metasediments. D₂ produced a pervasive axial planar foliation (S₂) to isoclinal F₂ folds. Sheared lithological contacts and S₂ were refolded into regional, open, predominantly southward-verging, E–W trending F₃ folds. Intrusion of a hornblendite protolith occurred at high angles to incipient S₂. Constrictional-prolate D₄ shows moderately NE-plunging azimuths defined by elongated hornblendite lenses, andalusite crystals in metapelite, crenulations in fuchsitic quartzite and sheath folding. D₄ overlaps with a: 1) 2.03–2.01 Ga regional M₃ metamorphic overprint; b) transpressional deformation at 2.2–1.9 Ga and c) 2.03 Ga transpressional, dextral shearing and thrusting around the CZ and d) formation of the Avoca, Bellavue and Baklykraal sheath folds and parallel lineations.     

Kinematic analysis of sinistral cataclastic shear zones along the northern margin of the Mino Belt, central Japan

In this paper, cataclastic shear zones along the northern margin of the Mino Belt, central Japan are described, and the significance of the shearing in the tectonic evolution of SW Japan is examined. The Mino Belt in SW Japan is composed of accretionary complexes of Jurassic to Early Cretaceous age. Field investigation revealed that remarkable cataclastic shear zones trending east to northeast run along the northern margin of the Mino Belt. Closely spaced cleavage is developed in these shear zones. Lineation on the cleavage plunges at shallow to moderate angles. Deformation structures (e.g. composite planar fabric and asymmetric structure of clasts) in the sheared rocks clearly indicate a sinistral sense of shear. The shearing ceased by latest Cretaceous time, because the sheared rocks are overlain by unsheared Upper Cretaceous volcanic rocks. The sinistral shearing may be closely related to Cretaceous sinistral movement along the eastern margin of Asia. Sinistral shearing along the northern margin of the Mino Belt can be considered as a key for re-examining the tectonic development of SW Japan.     

Geochronological problems related to polymetamorphism in the Limpopo Complex, South Africa

The integration of new and published geochronologic data with structural, magmatic/anatectic and pressure–temperature (P–T) process information allow the recognition of high-grade polymetamorphic granulites and associated high-grade shear zones in the Central Zone (CZ) of the Limpopo high-grade terrain in South Africa. Together, these two important features reflect a major high-grade D₃/M₃ event at ~ 2.02 Ga that overprinted the > 2.63 Ga high-grade Neoarchaean D₂/M₂ event, characterized by SW-plunging sheath folds. These major D₂/M₂ folds developed before ~ 2.63 Ga based on U–Pb zircon age data for precursors to leucocratic anatectic gneisses that cut the high-grade gneissic fabric. The D₃/M₃ shear event is accurately dated by U–Pb monazite (2017.1 ± 2.8 Ma) and PbSL garnet (2023 ± 11 Ma) age data obtained from syntectonic anatectic material, and from sheared metapelitic gneisses that were completely reworked during the high-grade shear event. The shear event was preceded by isobaric heating (P = ~ 6 kbar and T = ~ 670–780 °C), which resulted in the widespread formation of polymetamorphic granulites. Many efforts to date high-grade gneisses from the CZ using PbSL garnet dating resulted in a large spread of ages (~ 2.0–2.6 Ga) that reflect the polymetamorphic nature of these complexly deformed high-grade rocks.