Modern-style plate subduction preserved in the Palaeoproterozoic West African craton (original) (raw)
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Precambrian Research, 2006
Whole-rock geochemistry and Sr-Nd isotopic data for granitoids and volcanic rocks of four main different igneous phases, the Usagaran phase (2.05-1.93 Ga), the Ubendian phase (1.88-1.85 Ga), the Lukamfwa phase (1.65-1.55 Ga) and the Irumide phase (1.05-0.95 Ga), recognised along the southern margin of the Congo Craton in the Bangweulu Block and Irumide Belt of Zambia, demonstrate a long history of crustal recycling of a cryptic Archaean basement complex. The isotopic record indicates that a largely similar crustal source can be assigned to all these magmatic phases, with subtle differences in isotopic and geochemical record reflecting varying distance to orogenic activity during each of these episodes, and varying, but always minor amounts of juvenile mantle input. T DM model ages, ranging between 3.3 and 2.8 Ga for the granitoids, and between 2.9 and 2.4 Ga for volcanic rocks, indicate preponderant Archaean crust within the Bangweulu Block and within the Irumide Belt. The corresponding initial ε Nd values vary between −6 and −15 for the granitoids, and between −2 and −7 for the felsic volcanic rocks. Only some of the ca. 1.85 Ga mafic volcanic units record a more juvenile character (ε Nd (T) between −5 and 0 and T DM model ages between 3.2 and 2.4 Ga) but still with a significant old crust input.
Precambrian Research, 2018
The question of whether high-grade metamorphism and crustal melting in the early Archaean were associated with modern-style plate tectonics is a major issue in unravelling early Earth crustal evolution, and the eastern Kaapvaal craton has featured prominently in this debate. We discuss a major ca. 3.2 Ga tectono-magmatic-metamorphic event in the Ancient Gneiss Complex (AGC) of Swaziland, a multiply deformed medium-to high-grade terrane in the eastern Kaapvaal craton consisting of 3.66-3.20 Ga granitoid gneisses and infolded greenstone remnants, metasedimentary assemblages and mafic dykes. We report on a 3.2 Ga granulite-facies assemblage in a metagabbro of the AGC of central Swaziland and relate this to a major thermo-magmatic event that not only affected the AGC but also the neighbouring Barberton granitoid-greenstone terrane. Some previous models have related the 3.2 Ga event in the eastern Kaapvaal craton to subduction processes, but we see no evidence for long, narrow belts and metamorphic facies changes reflecting lithospheric suture zones, and there is no unidirectional asymmetry in the thermal structure across the entire region from Swaziland to the southern Barberton granitegreenstone terrane as is typical of Phanerozoic and Proterozoic belts. Instead, we consider an 2 underplating event at ca. 3.2 Ga, giving rise to melting in the lower crust and mixing with mantle-derived under-and intraplated mafic magma to generate the voluminous granitoid assemblages now observed in the AGC and the southern Barberton terrane. This is compatible with large-scale crustal reworking during a major thermo-magmatic event and the apparent lack of a mafic lower crust in the Kaapvaal craton as shown by seismic data.
2018
This thesis investigates the Palaeoproterozoic crust of the West African Craton in southwest Ghana, providing insight into its controversial geodynamic and tectonic evolution. Rocks of the study area comprise greenschist- to amphibolite facies, mafic to felsic volcanic and volcaniclastic rocks, high-grade paragneisses and low-grade volcano-sedimentary packages, all of which are extensively intruded by multiple generations of granitoids. New lithological, metamorphic and structural maps are constructed using integrated field mapping and interpretation of regional airborne geophysical datasets. This approach is used to constrain the deformation history of the sparsely exposed rocks of the NE- to NNE-striking Sefwi Greenstone Belt and the adjacent volcano-sedimentary domains deformed during the Eburnean Orogeny (2150-2070 Ma). Combined geochemical and geochronological analysis of the magmatic suites of the Sefwi Greenstone Belt reveal calc-alkaline, volcanic arc affinities, as well as ...
Record of mid-Archaean subduction from metamorphism in the Barberton terrain, South Africa
Nature, 2006
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 1,2 . The existence of subduction during this time is particularly controversial because characteristic subduction-related mineral assemblages, typically documenting apparent geothermal gradients of 15 8C km 21 or less 3 , have not yet been recorded from in situ Archaean rocks (the lowest recorded apparent geothermal gradients 4 are greater than 25 8C km 21 ). Despite this absence from the rock record, low Archaean geothermal gradients are suggested by eclogitic nodules in kimberlites 5,6 and circumstantial evidence for subduction processes, including possible accretion-related structures 2 , 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 Earth's history 7 . Here we report garnet-albite-bearing mineral assemblages that record pressures of 1.2-1.5 GPa at temperatures of 600-650 8C from supracrustal amphibolites from the mid-Archaean Barberton granitoid-greenstone terrain. These conditions point to apparent geothermal gradients of 12-15 8C-similar to those found in recent subduction zones-that coincided with the main phase of terrane accretion in the structurally overlying Barberton greenstone belt 8 . 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.
Gondwana Research, 2003
The geochemistry of the Leisure Bay Formation, Natal Metamorphic Province suggests that its protoliths were greywackes, pelites and arkoses that were deposited in an oceanic island arc environment. These rocks contain the mineral assemblage biotite + hypersthene + cordierite (with hercynite inclusions) + garnet + quartz + feldspar. Numerous generations of garnet genesis are evident from which a long history of metamorphism can be interpreted MI involved syn-D, high tempcrature/low pressure metamorphism (-4kb and > 8 5 0~) and dehydration melting to produce essentially anhydrous assemblages particularly in the vicinity of, and probably related to the intrusion of the Munster Suite sills. The inclusions of hercynite in cordierite and the garnet + quartz symplectites after hypersthene + plagioclase (-550°C and-5kb) suggests isobaric cooling after MI. This indicates an anticlockwise P-T loop related to the early intrusion of subduction related calc alkaline magmatic rocks. M, involved syn-D, dehydration melting of hydrous assemblages possibly related to the emplacement of many A-type rapakivi charnockite granitoids, which provided heat and loading. The D, tectonism postdated all lithologies in the region, except for synto late-D, granitoid plutons, and is interpreted as a transpressional tectonotherinal reworking of pre-existing (Proterozoic) crust at-1030Ma.
The Maluwe basin, north-adjacent to the Sunyani basin, is the northernmost of the northeast-trending Eburnean volcaniclastic depositories in Ghana. These basins are separated from one another by remnants of Eoeburnean crust, all formed during the evolution of an arc-backarc basins complex in a Palaeoprote-rozoic intraoceanic environment. The Bolé–Nangodi belt terrane to the northwest, of mostly Eoeburnean crust is fault bounded with the Maluwe basin along the northeast-trending Bolé–Navrongo fault zone. The stratigraphic sequence, which was the key to unravelling the structural evolution of the study area, was established by means of field observations aided by precision SHRIMP geochronology. The quartzitic, pelitic, quartzofeldspathic and granitic gneisses of the Eoeburnean crust (>2150 Ma) experienced complex metamorphic mineral growth and migmatitization, mostly under static crustal conditions and were subjected to several deformation episodes. The foliated mafic and metasedimentary enclaves within the Ifanteyire granite establish deformation to have taken place prior to 2195 Ma, while the tectonically emplaced Kuri amphibolites within the 2187-Ma gneissic Gondo granite indicate a stage of rifting followed by collision. Deformation of granite dykes in the Gondo granites at 2150 Ma concluded the development of the Eoeburnean orogenic cycle (D EE). The Sawla Suite, contemporaneous with the deposition of the Maluwe Group, intruded the tectonic exhumed Bolé–Nangodi terrane during extension between 2137 and 2125 Ma. The rifting separated the Abulembire fragment from the Bolé–Nangodi terrane. During subsequent northwestward subduction of young back-arc basin oceanic crust the volcaniclastic strata of the Maluwe Group and Sawla granitoids were deformed (D E1) under chlorite/sericite greenschist-grade conditions. The NE-trending folds had sub-horizontal axes and subvertical axial planes. Simultaneous to the D E1 orogenesis the molasses of the Banda Group was deposited concordantly on the submerged Sunyani strata after a hiatus of 20 million years. After cessation of the NW–SE-directed compression the early Tanina Suite intruded as batholiths, dykes and sheets and produced garnet, staurolite, sillimanite and kyanite in their thermal aureoles. Docking of the Sunyani basin produced the D E2 thrust related folding and stacking in the deformed and gran-itoid invaded Maluwe basin as well as the single stage sin-and anticlinoria in the Sunyani and Banda Groups. In the Maluwe basin the Abulembire fragment acted as a resistor and the approaching front rotated anticlockwise and clockwise around the barrier to form west-and north-directed piggy-back thrust-stacking and deformation of the Tanina Suite granitoids. Due to the low metamorphic conditions the D E2 fabric is limited to crenulation cleavages in the more psammitic and pelitic units. The fold axes are double plunging (N–S and E–W) up to 60° with the axial planar fabric subvertical. Post-D2 tectonic relaxation has allowed the emplacement of the last Tanina Suite calc-alkaline melts and was succeeded by N–S extension fracturing (DE 3) along which mantle derived Wakawaka gabbroids and syenite intruded. The D E1 folding occurred between 2125 and 2122 Ma and D E2 before 2119 Ma. The tectonic relaxation occurred at 2118 Ma. Around 2100 Ma, NE–SW directed strike-slip shearing (D E4), fractured the Bolé–Nangodi terrane and enhanced the basin-belt boundary. Along the boundary, the displacement was dextral along vertical faults but, southward, it became more east-over-west thrust related. Associated tension gashes are filled with vein quartz and pegmatite and typical of the brittle sector of 1464-343X/$-see front matter j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / j a f r e a r s c i the crust. Tectonism in this part of the intraoceanic accretionary arc back-arc complex was concluded by limited, right-lateral strike-slip (D E5) movement which formed some breccias.
Precambrian Research, 2014
The Awsard-Tichla zone, in the Moroccan sector of the Reguibat Rise, comprises three lithodemic units, the TTG Aghaylas Suite, the Tichla greenstone belt, and the intrusive feldspathoidal syenites of the Awsard-Lechuaf group. The Aghaylas suite consists of Mesoarchean tonalites and trondhjemites with minor granodiorites and granites that are locally migmatised. These rocks are notably depleted in HREE, Y, Nb, Ta, and U, with elevated Th/U and K/Rb, 87 Sr/ 86 Sr (t) from 0.7003 to 0.7030, Nd (t) from +2 to +5, and Nd model ages in the range T CR = 3.03-2.92 Ga. Eight TTG gneisses yielded SHRIMP U-Pb zircon crystallisation ages between 3.04 Ga and 2.92 Ga. One TTG leucocratic gneiss from the neighbouring Oulad Dlim sector has a T CR = 3.09 and a zircon crystallisation age of 2.94 Ga but also contains a 3.11 Ga population of pre-magmatic zircons. The Tichla greenstone belt, on the other hand, was formed between 3.03 Ga and 3.01 Ga, though Nd model ages suggest the contribution of an older crust. Field evidence and the chemical and isotopic compositions of the TTGs are consistent with crustal recycling of slightly younger juvenile TTG. After 2.92 Ga there is no trace of magmatic activity until the intrusion of the Awsard feldspathoidal (kalsilite and nepheline) syenites at 2.46 Ga. This reveals that, for reasons that are not yet well understood, the Archean crust in this sector of the Reguibat Rise stabilised long before than any other surrounding cratonic area. Furthermore, the crust stayed stable until the Archean-Proterozoic transition that, here, was marked by a peculiar high-K silica-subsaturated magmatism more extreme in composition than in any other known Archean terrane.