Low-grade Greenstone Research Articles

Hydrated komatiites as a source of water for TTG formation in the Archean

The preserved Archean continental crust is dominantly comprised of tonalite-trondhjemite-granodiorite (TTG) suites associated with less abundant low-grade greenstone belts. The exact processes that form TTGs, as well as the source rock they are derived from, are difficult to constrain from the sparse Archean geological record. However, studies show that the water-present partial melting of metamorphosed basalt at temperatures of 750–950°C is required to produce large volumes of partial melt with TTG compositions. In this contribution, we investigate if hydrated komatiites — a constituent of greenstone belts — played a vital role in TTG genesis. Using petrology, mineral chemistry and phase equilibria modelling of representative komatiite samples, combined with analysis of a global geochemical dataset of komatiites and basaltic komatiites, we show that during metamorphism hydrated komatiites can release at least 6 weight % mineral-bound water. Up to 5 weight % of this water is released by breakdown of chlorite and tremolite at temperatures between 680 and 800°C, regardless of the P–T path (i.e., tectonic scenario) experienced by the komatiitic rocks. As the temperatures of komatiite dehydration are above the water-saturated basalt solidus, the released water can trigger voluminous partial melting of basalt to ultimately create TTG batholiths. This considerable hydration potential of komatiites is due to their high XMg (XMg = molar Mg/[Mg+Fe]), which stabilises water-rich minerals during oceanic alteration on the seafloor, but also extends the stability of Mg-rich chlorite to high temperatures. During prograde metamorphism, the XMg, CaO and Al2O3 content of the reactive rock composition determines the proportion of chlorite vs amphibole, and therefore the volume of water which can be transported to temperatures of >750°C. Despite the low abundance of komatiites in greenstone belts, they potentially played a vital role in crustal formation and the Earth's early water cycle.

Role of Volcano-Sedimentary Basins in the Formation of Greenstone-Granitoid Belts in the West African Craton: A Numerical Model

Greenstone belts in the West African Craton (WAC) are separated by several generations of granitoids intruded at ca. 2.18–1.98 Ga. Simultaneous folding and exhumation play an important role in the formation of greenstone-granitoid belts. However, the overall tectonic regime and origin of granitoids remain controversial. In this study, we present the estimates of the mantle potential temperature (Tp) for the WAC, which yields values of about 1500–1600 °C, pressure estimates of initial and final melting yield values of about 3.7–5.2 GPa and 1–1.3 GPa, respectively. Subsequently, 2D thermo-mechanical models have been constructed to explore the width of volcano-sedimentary basin on spatial-temporal evolution of diapirs that emplaced in the lower-middle crust during compression. The models show that the width of the volcano-sediment layer plays an important role in the formation mechanisms of greenstone-granitoid belts. The lower crust beneath sedimentary sequences is deformed into a buckle fold during the first compressional stage, through which relief uplifts slowly. Subsequently, the buckle fold is further deformed into several individual folds. Diapirs made of lower crust rocks ascend and emplace in the middle-upper crust resulting from instability. Benefitting from the mantle temperature, the pressure estimates and the numerical modelling results, a new geodynamic model was constructed. This model indicates that a series of sheet-like granitoids possibly derived from either subducted mélanges, lower crust and/or mantle melting that are accumulated at depths of the subcontinental mantle would channel along diapirs before feeding the upper crust. When the granitoids arrive at the solidified lids of the diapirs, they would favour migrating horizontally and intrude into the upper crust through weakening zones between the diapirs. Our geodynamic model also suggests an asymmetry of structures between the upper and middle-lower crust, with the dome-like granitoids overlying high-grade sedimentary synforms and high-grade diapirs underlying low-grade greenstone belts.

3-D numerical modelling of the influence of pre-existing faults and boundary conditions on the distribution of deformation: Example of North-Western Ghana

High-strain zones bound and separate the high-grade tectono-metamorphic terranes from low-grade greenstone belts in the North-Western parts of Ghana. These belts are bounded by granitoid domains characterized by two main episodic pulses of magmatic intrusive events, which occurred between 2213Ma and 2086Ma. High-strain zones are thought to play a significant role in creating fluid pathways, particularly for partially molten material from lower crustal sources to the upper crust. In this study, a three-dimensional thermo-mechanical model has been used to explore the evolution of high-strain zones and relief under compressional and simple shear boundary conditions. Different orientations of a system of branched strike-slip faults were tested. The effects of the frictional angle and density contrast on the evolution of relief were also compared in this study. The resulting model indicates domains of tensile vs. compressional strain as well as shear zones. This shows that the internal fault zones as well as the host rock in between the faults behave as relatively weaker domains than the external regions. Under both applied compressive and simple shear boundary conditions, these weakened domains constitute preferential zones of tensile and shear strain accommodations in the upper crust, which may favour infilling by deeper partially molten rocks. This processes is suggested by the authors as the most likely processes to have occurred in pre-existing branched shear zones systems in North-Western Ghana during the Eburnean orogeny (around 2.20–2.10Ga).The orientations of faults in these models play an important role in controlling the evolution of relief and localized deformation. In particular, greatest the largest relief is obtained when faults dip parallel to each other and when they are inclined at depth, as they thus facilitate strain rotation and material transfer from depth. The host rock density does not play a primary role in producing relief compared to variations in friction angle at crustal scale of our model. Relief increases by 200–300m when the host rock density is increased by 200kg/m3, whereas relief reduces by about 1200m when decreasing the host rock friction from φ=20° to 10°. This study suggests a model for interpreting the evolution and locus of exhumation of partially molten rocks in North-Western Ghana.

An ocean-floor carbonaceous sedimentary sequence in the 3.2-Ga Dixon Island Formation, coastal Pilbara terrane, Western Australia

The Dixon Island Formation of the Pilbara terrane, Western Australia, extends from Cleaverville Beach to the Dixon Island coast, and is the only example worldwide of a coastal outcrop of a 3.2–3.1Ga low-grade greenstone belt. The Dixon Island Formation was situated in an immature island arc setting and comprises siliceous, carbonaceous deep-water sediments that contain evidence for hydrothermal and microbial activity. The extensive outcrop along the coastline makes it possible to examine in detail the characteristics of Mesoarchean sedimentation in a hydrothermal environment. This study focuses on a continuously exposed carbonaceous, black chert succession in the central part of the northern coastline outcrop on Dixon Island. At this site, a 20-m-thick, carbonaceous, black chert sequence conformably overlies basement rocks of highly altered komatiite–rhyolite tuffs. The black chert sequence formed well-bedded black chert with carbonaceous peloidal matter and fragmented grains, and the sequence is homogeneous and finely laminated. In this sequence, evidence of low-temperature hydrothermal fluid, sediments and alteration stractures are well preserved in the lowermost section, indicating that high levels of hydrothermal activity occurred on the ocean floor during deposition. In particular, swarms of black chert veins provide evidence of post-volcanic hydrothermal activity that released organic matter and silica to the ocean. The carbonaceous peloidal textures and δ13Corg values of sediments located just above the basement, which hosts the vein swarms, suggest that the veins were the conduits for hydrothermal fluid which contained organic-rich silica material and that flowed onto the seafloor to form the homogeneous carbonaceous cherts along with hydrothermal-related sediment. The δ13Corg values of organic matter in the black cherts range from −42‰ to −22‰ (average=−31.9‰; n=313). Lighter δ13C values (−35‰ to −42‰) characterize carbonaceous laminated black chert located ∼5m above the basement, where biogenic structures (e.g. biomat bed, microfossil structures) are found. The lighter δ13C values might be related to methanotrophic micro-organic activity within the sediments during hydrothermal activity. In summary, we reconstructed the sedimentary environment upon a Mesoarchean hydrothermal ocean floor that was a site of microbial activity and local methanogenesis.

High- to ultrahigh-temperature metasomatism related to brine infiltration in the Neoarchean Limpopo Complex: Petrology and phase equilibrium modeling

Fluids associated with granulite-facies metamorphism are considered to have controlled various lower crustal processes such as dehydration/hydration reactions, partial melting, material circulation, and high-temperature metasomatism. Here, we report new petrological data obtained from enderbite and its metasomatized equivalents from the Southern Marginal Zone (SMZ) of the Neoarchean Limpopo Complex, South Africa, and evaluate the alteration process caused by infiltration of high-salinity aqueous fluids during granulite-facies metamorphism. At the study area on Farm Commissiedraai, enderbite (plagioclase+quartz+K-feldspar+orthopyroxene) is partly metasomatized with the formation of continuous alteration zones comprising plagioclase+quartz+K-feldspar+orthopyroxene+garnet, plagioclase+quartz+K-feldspar+garnet, and finally mesoperthite/antiperthite+quartz+garnet+sillimanite. Bulk K2O increases, and CaO, MgO, FeO, and TiO2 decreases continuously from enderbite to the most intensely metasomatized rock. The occurrence of aqueous fluid inclusions with final melting temperatures of −29.2°C to −21.6°C trapped as a primary phase within quartz in the highly altered sample indicates that a high-salinity brine (probably NaCl rich) with additional components such as MgCl2 and CaCl2 controlled metasomatism. We employed a phase equilibrium modeling technique to evaluate the metasomatic process in the system NCKFMASH using THERMOCALC 3.3, and discovered that addition of K to and removal of Ca and Mg from the enderbite gave rise to the metasomatized mineral assemblages. Modeling also indicated that metasomatic alteration occurred at ca. 900°C, which is consistent with post-peak metamorphic conditions reached during the initial stage of exhumation in the SMZ, and with temperature calculations based on ternary-feldspar geothermometry of antiperthite in the highly metasomatized sample. The brine fluid that accompanied metasomatism infiltrated along the high-temperature Petronella Shear Zone, and was probably derived from devolatilization of low-grade greenstone materials of the Kaapvaal Craton as the result of thrusting hot granulite over cool granite-greenstone belts. Our results therefore suggest that metasomatism at this locality occurred closely after the peak metamorphism having been reached at ∼1000°C in the root zone of the Limpopo orogeny at ∼2.72Ga.

Low-pressure metamorphism during Archaean crustal growth: a low-strain zone in the northern Nagssugtoqidian orogen, West Greenland

One of the first detailed accounts of Precambrian supracrustal rocks in central West Greenland came from a small group of islands and skerries a few kilometres north-east of Aasiaat (Fig. 1). In 1948, K. Ellitsgaard-Rasmussen spent a few days on the islands and published a metamorphic study of their low-grade greenstones and aluminous clastic rocks (Ellitsgaard-Rasmussen 1954). He observed a striking dissimilarity between these supracrustal rocks and the grey gneisses found in most of the Aasiaat region, although the latter were at that time also assumed to be of supracrustal origin. He furthermore noted that the regional significance of the islands should be pursued, and that the island of Maniitsoq 4 km west of the small islands might hold a key to their interpretation. More than 50 years were to elapse before the islands were surveyed again in July 2003, during field work for the Ikamiut map sheet in the northern Nagssugtoqidian orogen (van Gool et al. 2002). The collision of two Archaean continents during the c. 1850 Ma Nagssugtoqidian orogeny caused intensive structural and thermal reworking at up to granulite facies grade in most of central West Greenland; see Connelly et al. (2000) and van Gool et al. (2002). The small islands north-east of Aasiaat are indeed regionally important, because they document a previously unrecognised low-grade, low-strain domain of presumed Archaean age that has largely escaped the Nagssugtoqidian orogeny, and as predicted by Ellitsgaard-Rasmussen (1954) a clue to their significance was found on Maniitsoq.

Structural-metamorphic evolution of the Southern Yenisey Range of Eastern Siberia: implications for the emplacement of the Kanskiy granulite Complex

¶The Southern Yenisey Range of Eastern Siberia consists of the granulite facies Kanskiy Complex bordered in the west by the lower-grade Yeniseyskiy and Yukseevskiy Complexes. Three deformational events were recognized in each of the three complexes along the Yenisey River cross-section: a D1 fabric forming event, a D2 shear and folding event, and a D3 shear event. Thrust kinematics across the Southern Yenisey Range suggest that during the D2 event the Kanskiy Complex was thrusted along a regional ductile shear zone onto the lower-grade complexes. This resulted in shearing and folding as well as the development of a dynamic metamorphic zonation. In the low-grade greenstone belt part of the cross section (Yukseevskiy complex) D2 shearing is associated with peak prograde (T ∼ 660 °C and P ∼ 5.8 kbar) metamorphism. The retrograde P-T path of the Yukseevskiy Complex coincides with minimum T of the near-isobaric cooling P-T paths for the adjacent granulites of the Kanskiy Complex (Perchuk et al., 1989). The metamorphism can therefore be attributed to deformation and heat transfer caused by exhumation of the Kanskiy Complex in the time period 2000–1800 Ma which also defines the most significant tectono-thermal event in the Southern Yenisey Range. The tectono-metamorphic pattern and evolution of the low- to high-grade metamorphic complexes of the Southern Yenisey Range is very similar to that described for the ∼ 2600 Ma Limpopo Complex of Southern Africa and the ∼ 1900 Ma Lapland Complex of the Kola Peninsula. Similar geodynamic processes were therefore possibly responsible for the formation of these high-grade terrains suggesting that their formation is linked to a general geodynamic model.

TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK, SUPERIOR PROVINCE, QUEBEC, CANADA

Previously considered one of Earth’s largest granulite terranes, the Minto block, in northern Quebec, consists dominantly of granitic rocks with sparse supracrustal remnants that record low-pressure, high-temperature metamorphism to the amphibolite and granulite facies at ca. 2.7 Ga. Much of the region is underlain by plutonic rocks, including igneous charnockitic bodies, that provide little evidence of metamorphism. In contrast, pelitic assemblages in low-grade greenstone belts and their higher-grade shoulders define four metamorphic zones: 1) sub-staurolite-grade slates, found only in the Duquet belt, 2) staurolite – andalusite – sillimanite – garnet-zone schists, present in all belts, 3) marginal garnet – sillimanite ± kyanite migmatites, and 4) garnet – sillimanite ± cordierite and garnet – orthopyroxene granulites, as enclaves within plutonic units. A prograde low-pressure, hightemperature regional facies-series is defined by andalusite – sillimanite assemblages, thermobarometric analyses that indicate peak conditions ranging from 3 kbar, 560°C to 8.4 kbar, 840°C, and short segments of anticlockwise P–T trajectories preserved in two of the lowest-grade areas. Kyanite occurs in migmatites in the vicinity of a ca. 2.81 Ga shear zone bounding two distinct supracrustal assemblages and marks an earlier Barrovian event. All belts are polydeformed, with a common, broadly synmetamorphic, NNW-striking, steep foliation and down-dip lineation related to regional-scale F2 folds. Distributed late-metamorphic dip-slip shear appears responsible in some areas for cross-strike juxtaposition of low- and high-grade metamorphic zones. Older, generally poorly preserved D1 deformation, recognized in several belts, may represent accretionary structures. Variably developed younger deformation includes NNW dextral transcurrent brittle–ductile shear zones (S3), variably developed east-trending F4 folds, and WNW-striking pseudotachylite zones. U–Pb monazite ages of 2702 Ma date high-grade metamorphism of a pelitic granulite, crystallization of two diatexites, and staurolite-zone metamorphism in one schist. Together with similar zircon ages of crust-derived granites, these data point to a major collisional orogeny at this time. Older titanite (2748–2765 Ma) indicates areas that escaped >650°C metamorphism during the 2.7 Ga event. Younger monazite occurs in a migmatite-grade rock (2688, 2672 Ma) and staurolite-zone schists (2679–2642, 2648, 2637, and 2642–2628 Ma), similar to titanite ages in greenstones (2643–2550 Ma) and 40 Ar/ 39 Ar hornblende ages (2.64–2.56 Ga). These post-peak metamorphic ages in the greenstone belts based on monazite and titanite are attributed to hydrothermal growth from fluids channeled along permeable zones; they may reflect late reheating events observed throughout the Superior Province as leucogranitic magmatism, metamorphism in the deep crust, and mineral growth in shear zones.

Extensional emplacement of a high-grade granite gneiss complex into low-grade greenstones, Eastern Goldfields, Yilgarn Craton, Western Australia

A high-grade granite gneiss complex was emplaced into low-grade greenstones after an early episode of Late Archaean deformation (D1) in the Eastern Goldfields of Western Australia. The components of the granitic gneiss complex range from migmatitic granitic gneiss to foliated and massive granite. The complex is surrounded by a marginal zone of high-grade (amphibolite facies) greenstones. The oldest phases of the complex are foliated and gneissic granite with igneous emplacement ages of c. 2675 Ma, that postdate the bulk of greenstone volcanism (c. 2720-2675 Ma). The complex includes voluminous and largely undeformed monzogranite that is correlated with widespread granite plutonism at c. 2660 Ma. Fabrics in marginal high-strain zones between the granite gneiss complex and greenstones, and in faults separating high-grade from low-grade greenstones, indicate that emplacement, or uplift, of the complex was partly accommodated by normal faulting. Final emplacement occurred during the c. 2660 Ma granite event, and overlapped with regional east-west shortening (D2–D3) in the adjacent greenstones. Uplift of the granitic gneiss complex to higher crustal levels may have resulted from preferential injection of granite magma into gentle domes of the basal greenstone contacts. These domes formed during earlier regional extension of the greenstones.

Terrane accretion in the southwestern Yilgarn Craton: evidence from a deep seismic crustal profile

Interpretation of a seismic, gravity and magnetic traverse across the western margin of the Yilgarn Craton, undertaken approximately 110 km north of Perth, together with new RbSr and UPb geochronological data from the southwestern portion of the craton, have led to a re-evaluation of the geological relationships and to the development of a terrane accretion model for the region. The geophysical data established the three-layered nature of the crust, with E-dipping reflectors present throughout its thickness of 32–40 km. Reflectors in the upper crust correlate with shear zones mapped at the surface and are interpreted as major thrusts that have transported crustal slices from the east. These ‘sole-out’ at a depth of about 10 km along a well-defined décollement and indicate ‘thin-skinned’ tectonism. The shear zones define the boundaries of three terranes which, from west to east, are herein defined as the Balingup, Boddington and Lake Grace terranes. The Balingup Terrane is composed of trough facies metasediments, believed to have been deposited at ∼ 2.8 Ga and subsequently metamorphosed under medium pressure amphibolite facies conditions. This terrane contains some of the youngest granitoids in the Yilgarn Craton (2612 and 2527 Ma) and these have been extensively sheared by late Archaean movement along the Darling Fault Zone. The boundary with the Boddington Terrane to the east is not precisely defined, partly due to lack of exposure. It is marked in the south by a narrow, 100 km long strip of tectonised quartz monzonite and further north by a change in the RbSr biotite ages of granitoids; from 400–500 Ma in the west to 2300–2600 Ma in the east. The Boddington Terrane contains a sequence of Archaean shelf facies metasediments (part of the Jimperding Metamorphic Belt), believed to have been deposited at ∼ 3.1 Ga, and metamorphosed up to amphibolite-granulite transition facies under low-pressure conditions. These sequences occur mainly in flat-lying nappes, which may have been tectonically transported from the east during assembly with the Lake Grace Terrane. The Boddington Terrane also contains two low-grade greenstone belts, formed between 2650 and 2670 Ma and intruded by 2640 Ma granitoids. The eastern boundary of the Boddington Terrane is marked by a major change in gravity, which also corresponds with a series of shear zones at the surface, a change in structural style and a major change in the nature of the granitoids. East of this boundary in the Lake Grace Terrane, granitoids of two ages have been recognised: charnockites formed at 2640 Ma and post-tectonic granodiorites formed at ∼ 2580 Ma. Metasedimentary rocks, metamorphosed to granulite facies, occur as rafts and lenses with steep upright folds and are enclosed in leucocratic, garnetiferous granitic gneiss. Greenstones, also metamorphosed to granulite facies and recording ages of around 2790 Ma, are also enclosed within the charnockites and are thus substantially older than the low-grade greenstones in the Boddington Terrane to the west. Assembly of the Balingup and Boddington terranes appears to have been completed prior to the emplacement of the Logue Brook Granite at 2612 Ma. The Boddington and Lake Grace terranes came together after the 2640 Ma granulite facies metamorphic event but the precise timing of assembly is not currently constrained. It was certainly completed by 2411 Ma, the time of emplacement of a major set of E-W-trending mafic dykes (the Widgemooltha Dyke Suite) which extend for over 500 km across the Yilgarn Craton. Although the Widgemooltha Dyke Suite appears to be absent from the Balingup Terrane, this is possibly related to extensive NW-shearing during the Proterozoic, which also led to extensional re-activation of the décollement that marks the base of the upper crust in the region.

Record of emergent continental crust ∼3.5 billion years ago in the Pilbara craton of Australia

ISOTOPIC data for the Earth's oldest rocks1–7 imply that a considerable volume of continental crust existed during the early Archaean aeon (>3.0 Gyr ago), but it is not known when this crust first began to form emergent landmasses. Sedimentary geochemistry suggests8,9that the area of exposed continent was negligible until late in the Archaean10, a contention supported by the fact that, until now, all greenstone supracrustal volcanic and sedimentary successions shown to have been deposited on eroded continental basement have yielded ages of ≲3.0 Gyr. Here we report the discovery of an angular unconformity (an ancient erosion surface) beneath rocks of the 3.46-Gyr Warrawoona Group in the Pilbara craton of Australia, currently the oldest known well-preserved greenstone succession. Below the unconformity, low-grade greenstones older than 3.5 Gyr were intruded by voluminous granitoids before erosion. As the overlying Warrawoona rocks are only mildly metamorphosed, slightly deformed and were deposited near sea level, we infer that they accumulated on crust that was already rigid, cool and buoyant. Thus, by 3.46 Gyr ago, the sub-Warrawoona rocks formed an emergent block of continental crust, the most ancient known.