Alexander F.M. Kisters

Record of mid-Archaean subduction from metamorphism in the Barberton terrain, South Africa

Although plate tectonics is the central geological process of the modern Earth, its form and existence during the Archaean era (4.0–2.5 Gyr ago) are disputed. The existence of subduction during this time is particularly controversial because characteristic subduction-related mineral assemblages, typically documenting apparent geothermal gradients of 15 °C km-1 or less, have not yet been recorded from in situ Archaean rocks (the lowest recorded apparent geothermal gradients are greater than 25 °C km-1). Despite this absence from the rock record, low Archaean geothermal gradients are suggested by eclogitic nodules in kimberlites and circumstantial evidence for subduction processes, including possible accretion-related structures, has been reported in Archaean terrains. The lack of spatially and temporally well-constrained high-pressure, low-temperature metamorphism continues, however, to cast doubt on the relevance of subduction-driven tectonics during the first 1.5 Gyr of the Earths history. Here we report garnet–albite-bearing mineral assemblages that record pressures of 1.2–1.5 GPa at temperatures of 600–650 °C from supracrustal amphibolites from the mid-Archaean Barberton granitoid-greenstone terrain. These conditions point to apparent geothermal gradients of 12–15 °C—similar to those found in recent subduction zones—that coincided with the main phase of terrane accretion in the structurally overlying Barberton greenstone belt. These high-pressure, low-temperature conditions represent metamorphic evidence for cold and strong lithosphere, as well as subduction-driven tectonic processes, during the evolution of the early Earth.

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.

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.

Lithostratigraphic correlations in the western branch of the Pan-African Saldania belt, South Africa: the Malmesbury Group revisited.

A new lithostratigraphic subdivision is proposed for the low-grade metamorphic, supracrustal rocks of the Pan-African Saldania Belt of the Western Cape Province South Africa, previously referred to as the Malmesbury Group. Two tectonostratigraphic groups can be distinguished that appear to be separated by an unconformity. The lower of these groups is termed the Swartland Group. There are no absolute age constraints for the Swartland Group, but a penetrative D1 deformation in rocks of the Swartland Group is correlated with the onset of oblique crustal convergence along the Pan-African West Coast Belts at ca. 580 to 570 Ma, which also provides an upper age limit of the Swartland Group. The structural evolution and style of deformation suggest a correlation of the Swartland Group with part of both the Gifberg Group of the Vanrhynsdorp Basin and the Port Nolloth Group of the Gariep Belt to the north of the Saldania Belt. The Swartland Group is unconformably overlain by mainly clastic sediments that lack evidence of the early D1 deformation and that are here referred to as the Malmesbury Group. A lower age bracket of the Malmesbury Group is provided by the presumed age of deformation of the underlying Swartland Group ( ca. 575 Ma), while a minimum age of ca. 550 Ma is indicated by the intrusion of the oldest phases of the Cape Granite Suite that cross-cut the lithologies and fabrics of the Malmesbury Group. The lithological evolution and age of the Malmesbury Group suggests a correlation with the late orogenic clastic deposits of the Late-Neoproterozoic to Early-Cambrian Nama Group that formed in the eastern foreland basins of the emerging Pan-African orogenic belts. The new subdivision no longer assumes the presence of three allochthounous terranes in the Saldania Belt and allows for a better correlation of formations with those in Pan-African belts to the north.

Transcurrent shearing, granite sheeting and the incremental construction of the tabular 3.1 Ga Mpuluzi batholith, Barberton granite–greenstone terrane, South Africa

Structural, petrographic and geochronological studies show that the tabular 3.1 Ga Mpuluzi batholith in the Barberton granite–gneiss terrane in South Africa was emplaced via a combination of external and internal processes. External structural controls are indicated by systematic variations in intrusive relationships and strain along the margins of the Mpuluzi batholith and are consistent with an emplacement of the granite in a dilational jog within a NE–ENE-trending system of dextral transcurrent synmagmatic shear zones. Internally, the Mpuluzi batholith is essentially made up of granite sheets. The structurally higher parts of the granite are made up of shallowly dipping sheets that are underlain by an anastomosing network of steeply dipping, variably deformed dykes and sheets. These granite sheets at lower structural levels intruded either into the actively deforming shear zones or into extensional sectors between and along the bounding shear zones. Multiple intrusive relationships and geochronological evidence suggests that granite sheeting and the assembly of the pluton occurred over a period of 3–13 Ma. The spatial and temporal relationship between deformation and magma emplacement reflects episodes of incremental dilation related to deformation along the bounding shear zones and granite sheeting. The transition to the mainly subhorizontal granite sheets at higher structural levels of the tabular Mpuluzi batholith indicates the intrusion of the granites during subhorizontal regional shortening, where the reorientation of the minimum normal stress to vertical attitudes at the shallow levels of emplacement allowed for vertical dilation and subhorizontal emplacement of the granite sheets.

Melt segregation and far-field melt transfer in the mid-crust

Abstract: Migmatites in the Damara Belt in Namibia preserve relationships between anatectic leucosomes and intrusive leucogranite sheets illustrating the consecutive stages of partial melting, local melt segregation and far-field mobilization of melts over hundreds of metres and out of the anatectic region. Initial melting and localized segregation of melts are controlled by gradients in fluid pressure created by pervasively developed, shallowly dipping dilatant fractures. Subsequent melt transfer out of these initial sites of melting and melt storage occurs along subvertical, disc-shaped leucogranite sheets that intersect the leucosomes. The leucogranite sheets propagate as isolated, melt-filled hydrofractures, driven by the pressure differential along the subvertical fractures. The collapse of wall rocks into former melt-bearing fractures, and the presence of residual peritectic phases of the melting reaction trapped in wall rocks testify to the efficient extraction of melt by the mobile hydrofractures during their ascent. This process of melt drainage into and transport by hydrofractures leaves almost no trace of the ascent conduits. This study also shows that melt transport and the stability of melt pathways in strongly layered mid-crustal levels characterized by low deviatoric stresses are determined by the presence of pre-existing anisotropies and the progressively evolving structure in deforming orogens.

Chapter 5.6 TTG Plutons of the Barberton Granitoid-Greenstone Terrain, South Africa

Publisher Summary This chapter describes the tonalites, trondhjemites, and granodiorites (TTG) plutons of the Barberton granitoid-greenstone terrain, South Africa. Plutonic rocks constitute a large part of Archean terranes and occur mostly in the form of variably deformed orthogneisses. A large body of geochemical and experimental data exist for TTGs, and these studies have led to the general conclusion that TTGs are essentially the melts generated by the partial melting of mafic rocks, mostly amphibolites within the garnet stability field. The ca. 3.55–3.50 Ga TTGs, represented by the Steynsdorp pluton, contain a pervasive solid-state gneissosity and occurs mostly as banded gneisses. The protolith of these gneisses is tonalitic, although a granodioritic component, possibly related to the remelting of older tonalites or trondhjemites, is also recorded. Several types of trondhjemites are observed in Barberton TTG plutons. They range from fine- to coarse-grained rocks, with occasional porphyritic varieties and all have similar mineralogy, dominated by plagioclase, quartz, biotite, and microcline. It is found that the ca. 3.45 Ga TTGs are in the east, and younger 3.2 Ga old rocks are in the west, separated from one another by the Inyoni shear zone, which is interpreted to represent a suture zone during ca. 3.25–3.21 Ga orogenesis.

Fingerprints of late Neoproterozoic ridge subduction in the Pan–African Damara belt, Namibia

Subduction of mid-ocean ridges is a common feature in recent convergent margins, but is rarely documented in Proterozoic to Paleozoic orogenic belts. Here we describe evidence for ridge-trench interaction in the deeply eroded late Neoproterozoic Damara orogenic belt, central Namibia. The earliest interaction is indicated by primary intrusive contacts between amphibolite facies mid-ocean ridge metabasalts and trench metasediments. U-Pb zircon ages of 550–540 Ma from syntectonic granites in the forearc indicate the timing of partial melting and mafic underplating of the prism in response to ridge subduction. The thermal peak in the Damara belt, associated widespread granitic and alkalic plutonism, and hydrothermal activity coincide with the waning stages of tectonism at 530–520 Ma and are interpreted to indicate slab window widening and slab delamination. We suggest that the proposed two-stage thermal evolution of the Damara belt, comprising latest Neoproterozoic ridge subduction and early Cambrian slab delamination, represents a fingerprint of ridge subduction in ancient orogens.

Timing and kinematics of the Colenso Fault: The Early Paleozoic shift from collisional to extensional tectonics in the Pan-African Saldania Belt, South Africa

The Colenso fault is a major northwest to southeast trending fault zone in the Pan-African Saldania Belt of the Western Cape Province in South Africa that is spatially closely associated with granitoids of the ~550 to 510 Ma Cape Granite Suite. Most of these granites were previously considered to be largely post-tectonic intrusions, but structural data presented in this study demonstrate the synkinematic emplacement of granitoids into, and along, the Colenso Fault. The kinematic analyses of shear zones and granite fabrics together with previously published and new geochronological data are combined to provide constraints on the complex kinematic history of the fault and the tectonic evolution of the hitherto poorly understood Saldania Belt. Early, strongly gneissose granitoids of the composite Darling batholith (547 ± 6 Ma) were emplaced during sinistral strike-slip movement along the Colenso fault. Both the timing of emplacement and penetrative deformation of the Darling batholith suggest an intrusion of the pluton during the main Pan-African collisional event in the Saldania Belt. The younger Trekoskraal granite intrudes synkinematically into dextral strike-slip faults related to deformation along the Colenso fault. Single-zircon ages from synkinematic aplites constrain the timing of dextral strike-slip shearing to 539 ± 4 Ma. The emplacement of the late-kinematic Cape Columbine granite during dextral strike-slip faulting indicates that dextral strike-slip kinematics along the Colenso fault continued at least until ~520 Ma. These results point to a reversal of strike-slip motion along the Colenso fault at ~540 Ma that coincides with the onset of uplift of rocks of the Saldania Belt. The final exhumation of the belt at ~515 to 520 Ma is marked by the near-surface emplacement of the last phases of the Cape Granite Suite, related subaerial volcanism, sedimentation of the coarse-clastic, fault-bounded Klipheuwel Group, and the overlying fluvial to shallow-marine sequence of the Mid-Cambrian Cape Supergroup. The temporal and spatial overlap between igneous activity and rift-type sedimentation indicates that a substantial part of the Cape Granite Suite was emplaced in an overall transtensional and/or extensional setting. During this time, the voluminous plutonism of the Cape Granite Suite most likely represented a significant heat input that also contributed to a thermal weakening of the crust. In view of the Early Paleozoic extensional setting suggested here, we interpret Ar-Ar mineral ages of ~500 Ma and post-orogenic plutonism that are widely documented from Pan-African belts throughout southwestern Africa to reflect a thermal event related to crustal thinning and associated mantle upwelling that follows the main phase of Pan-African collisional tectonics.

Controls of gold-quartz vein formation during regional folding in amphibolite-facies, marble-dominated metasediments of the Navachab Gold Mine in the Pan-African Damara Belt, Namibia

The Navachab gold mine in central Namibia is situated in amphibolite-facies, marble-dominated metasediments of the Neoproterozoic Damara Sequence in the Pan-African Damara Belt. Gold mineralization is hosted by two main, and at least three minor, sets of auriferous quartz-veins that are developed on the subvertical northwestern limb of the regional-scale, shallow doubly-plunging anticline of the Karibib dome. The orientation, relative timing and progressive deformation of quartz veins indicate that veining occurred during folding and fold amplification of the Karibib dome. This deformation forms part of the main phase of northwest-directed collisional tectonics in the Damara belt at ~540 to 550 Ma. The two main vein sets include (1) bedding-parallel, shallowly-plunging ore lenses and shoots situated at the base of a prominent marble unit, and (2) a laterally extensive swarm of shallowly-dipping quartz veins that truncate the host rocks at high angles. The bulk of the bedding-parallel lenses represent dilational jogs that opened during flexural flow along bedding-parallel slip planes during the amplification of the Karibib dome. Flexural flow and associated bedding-parallel fluid infiltration were concentrated close to the contact between marbles and the underlying siliciclastic formations. This lithological contact, in particular, represented a pronounced rheological contrast resulting in increased slip rates during fold amplification. The highly discordant sets of shallowly-dipping, sheeted quartz veins were emplaced during the fold lock-up stage of the Karibib dome. Quartz veining occurred when the northwestern limb of the dome was rotated to subvertical attitudes, so that bedding-parallel flexural slip ceased to be active. Extensional fracturing and veining was facilitated by the presence of transiently supralithostatic fluid pressures. The high-amplitude fluid pressure cycling associated with extensional fracturing is likely to have triggered gold precipitation in the shallowly-dipping veins. Although bedding-parallel veins and discordant veins are broadly contemporaneous, related to the regional-scale D2 phase of fold-and-thrust tectonics, cross-cutting relationships between the two main vein systems indicate that the two vein sets have represented separate entities and succeeded each other rather than forming an interconnected fracture mesh.