Granite-greenstone Belt Research Articles

Sadiola Hill: A World-Class Carbonate-Hosted Gold Deposit in Mali, West Africa

The ~8-Moz Sadiola Hill gold deposit is located in the Kedougou-Kenieba inlier, a window of deformed ca. 2200 to 2050 Ma rocks that crop out in eastern Senegal and western Mali. The geology of the inlier differs from other Paleoproterozoic granite-greenstone belts and sedimentary basins by the abundance of carbonate rocks. The Sadiola Hill gold deposit occurs within 3 km of the Senegal-Mali shear zone, and country rocks in the region have undergone polycyclic deformation. The lithostratigraphy of the open pit consists of impure limestones overlain by younger sequences of detrital sedimentary rocks, which include arenite, wacke, and siltstone. The sedimentary rocks have been subjected to regional greenschist-facies metamorphism and have been intruded by multiple generations of synkinematic, calc-alkaline stocks and dikes. The bulk of the ore is hosted within a N-S- to NNW-trending, 10- to 50-m-wide, brittle-ductile dilational shear zone defined as the Sadiola shear zone. Gold lodes also occur along an array of steep NNE-trending shear zones. Geometric and kinematic analyses indicate that the ore-hosting structures were undergoing sinistral displacement at the time of mineralization, locally defined as the D3s NNW-SSE-shortening event. Hydrothermal alteration is polyphase and includes an early high-temperature calc-silicate phase (i.e., porphyroblastic growth of actinolite-tremolite) followed by a potassic phase (i.e., biotite-calcite-quartz ± K-feldspar-tourmaline-actinolite) that was synchronous with ore mineral deposition. Paragenetic studies reveal a multistage ore development that includes an As-rich sulfide stage, followed by an Au-Sb stage. The ore is associated with a metal enrichment suite of Au-As-Sb ± Cu-Fe-W-Mo-Ag-Bi-Zn-Pb-Te. The Sadiola Hill deposit shares a similar relative timing and structural setting to that of other world-class orogenic gold systems in the West African craton (e.g., Ashanti, Loulo), with gold deposition occurring during a period of transcurrent tectonics, after the cessation of region-wide compressional deformation. The high-temperature paragenesis at Sadiola Hill is atypical of gold mineralization in the craton and indicates that the late Eburnean tectono-magmatic activity between ca. 2080 and 2060 Ma played an important role in the dynamics of hydrothermal fluid circulation along the Senegal-Mali shear zone.

Initial gold enrichment within a Neoarchean granite-greenstone belt: Evidence from ore-bearing and ore-barren samples in the Jiapigou deposits, NE China

The Jiapigou gold mineralization hosted in a Neoarchean granite-greenstone belt in the southern Jilin Province, at the easternmost part of the northern margin of the North China Craton (NCC), is one of the important gold deposits in China. However, its evolution, in particular the initial Au source and enrichment, have not been well constrained. The initial gold enrichment defined by Au concentrations of Neoarchean unaltered gneisses of this region, range from <0.005ppm to 0.001ppm, increasing to 0.006–0.031ppm Au for the ore-hosting Neoarchean altered mylonite gneisses. A combined study of zircon trace element, UPb dating, Hf isotope compositions, as well as whole rock chemical compositions was performed for dioritic gneisses from both ore-bearing (close to the ore body) and ore-barren zones (within the nearby rock body) with a view to constrain the gold initial enrichment. The dioritic gneisses have varied SiO2 contents (54.19–61.34wt.%), Al2O3 (13.06–16.75wt.%) with moderate to high K2O/Na2O ratios (0.7–1.3) and MgO (2.20–3.77%). Based on their chemical compositions, positive zircon εHf(t) values, and their formation ages of 2512±9Ma, we infer that these dioritic rocks were generated by partial melting of a juvenile source, most likely thickened mafic lower continental crust (LCC) with residual plagioclase associated with minor amphibole. The ore-bearing sample (H5) from a ductile shear zone has been extensively altered by fluids and is the sample containing zircons with blurred zoning and reverse discordance. Zircons from H5 have much higher radiogenic Pb but lower U and Pb contents than those in the ore-barren sample (H3), which contains zircons with oscillatory zoning and represents the basement rock. Based on the available petrological, geochronological and geochemical constraints, we propose the following model for Au initial enrichment within the Neoarchean granite-greenstone belt. At ca. 2.54Ga, associated with the accretionary processes in the proto-NCC, subduction-related fluids infiltrated the metasomatised subcontinental lithospheric mantle (SCLM), scavenging Au and Pb (mainly radiogenic Pb, henceforth Pb*) and triggering partial melting. The resulting basaltic magmas underplated the LCC and were re-melted at 2512±9Ma, when the onset of an extensional regime permitted the advection of heat and melts/fluids into the LCC. This generated voluminous dioritic magmas with juvenile Hf isotope signatures with little evidence of interaction with the mantle. Subsequent ca. 2505Ma re-melting of some granodioritic rocks led to the formation of potassic granite and residual fluids further enriched in Au and Pb*, both focused into ductile shear zones and other lithospheric weak zones. The fluids with anomalous UPb fractionation resulted in the rarely reported zircon reverse discordance, alteration and also enrichment in Au, as observed in sample H5 and H4, representing the initial stage of Au enrichment within the Neoarchean granite-greenstone belt.

Neoarchean Andean-type active continental margin in the northeastern North China Craton: Geochemical and geochronological evidence from metavolcanic rocks in the Jiapigou granite-greenstone belt, Southern Jilin Province

The Jiapigou granite-greenstone belt of Southern Jilin Province is located at the northeastern margin of the North China Craton (NCC). The Neoarchean metavolcanic rocks of this region provide strong lines of evidence in petrology and geochemistry for Neoarchean crust–mantle geodynamic evolution and significant crustal growth events. The metavolcanic rock assemblage is dominated by amphibolite, hornblende/garnet biotite plagioclase gneiss, and minor felsic gneiss. Zircon LA–ICP–MS U–Pb isotopic analyses reveal that these supracrustal rocks may be formed by two episodes of magmatism. The first magmatic episode occurred at ∼2688Ma and was recorded by minor felsic gneiss. The second episode occurred at ∼2588–2536Ma and was marked by a major lithological assemblage of metavolcanic rocks in this area. All the supracrustal rocks experienced amphibolite facies metamorphism at ∼2510–2480Ma.Except for the felsic gneiss, seventeen metavolcanic rock samples can be geochemically classified into three groups. Group #1 has tholeiitic basaltic chemical composition, and is characterized by slightly LREE-enriched chondrite-normalized REE pattern with low (La/Yb)N ratio and weakly negative Nb anomalies, similar to those of primitive arc tholeiite. Their magmatic precursor was most likely generated by partial melting of upper sub-arc mantle wedge slightly metasomatized by slab-derived fluids or melts in a subduction initiation environment. Group #2 with basaltic composition is akin to typical calc-alkaline basalt (CAB), showing fractionated chondrite-normalized REE patterns and evidently negative Nb, Ta and Ti anomalies. The magmatic precursor of the Group #2 was derived from partial melting of a sub-arc mantle wedge at relatively deeper levels, significantly metasomatized by slab-derived fluids or melts. Group #3 show chemical composition of andesite to dacite, and is distinguished by wide variations in SiO2, MgO and Mg# values, strongly fractionated chondrite-normalized REE patterns with higher (La/Yb)N ratios, low Yb and Y contents, and negative Nb, Ta, Ti and P anomalies, analogous to the Phanerozoic adakites. They might be derived from magma mixing between the mantle wedge melts metasomatized by slab-derived melts with perhaps an ancient crustal contribution.The felsic gneiss shows positive zircon εHf(t2) values (+2.2 to +4.5), together with ∼2.7Ga metamorphic volcanic rocks in the neighboring area, indicating a ∼2.7Ga crustal growth episode. Most of the metavolcanic rocks display positive εHf(t2) values of +0.4 to +7.4, the highest of which is close to the value of contemporaneous depleted mantle, suggesting a significant crustal growth event at ∼2.59–2.54Ga. Combined with numerous ∼2.83–2.65Ga inherited zircons and ∼2688Ma felsic gneiss, the Jiapigou granite-greenstone belt evolved at an Andean-type active continental margin, recording the transition from primitive arc to mature arc.

A 2.5 Ga fore-arc subduction-accretion complex in the Dengfeng Granite-Greenstone Belt, Southern North China Craton

The Dengfeng granite-greenstone belt (DGGB), located in the southern segment of the Central Orogenic Belt (COB) of the North China Craton (NCC), consists of a volcano-sedimentary assemblage, intruded by tonalite, trondhjemite, granodiorite (TTG suite), diorite, granites and late mafic dikes. The volcano-sedimentary assemblage in the DGGB mainly consists of tectonically imbricated basaltic amphibolites, meta-gabbroic rocks with minor ultramafic rocks, and metagreywacke, marble and quartzite, consistent with characterisitics of typical Phanerozoic subduction-accretion complexes. The basaltic amphibolites yield a metamorphic zircon 207Pb/206Pb age of 2507±26 Ma, interpreted to represent the peak age of amphibolite facies metamorphism that took place during subduction/accretion of the basaltic protolith. The basaltic amphibolites are characterized by a tholeiitic affinity, and flat LREE patterns with minor negative Nb and Zr anomalies. Based on mixed MORB- and arc-affinities, the basaltic amphibolites in the DGGB are interpreted to have formed in a fore-arc tectonic setting. One late potassic granite dike cutting across the fabrics of the volcano-sedimentary assemblage yields an intrusion age of 2492±35 Ma, constraining the minimum deformation age for tectonic assembly of the package. The TTG gneisses and diorites intruded the western margin and the center of the subduction-accretion complex, respectively. One TTG sample and one diorite sample yield igneous zircon 207Pb/206Pb ages of 2514±26 Ma and 2518±36 Ma, respectively, constraining their intrusion ages. The TTG gneisses display high ratios of (La/Yb)cn and Sr/Y, and depletion in HFSE with negative Nb, Ta and Ti anomalies, consistent with those of typical Archean TTGs. The TTG gneisses are therefore considered to be generated from partial melting of a shallowly subducting oceanic slab and/or accreted arc amphibolites. The diorites have high concentrations of MgO (2.89–6.05wt.%), Ni (148–178ppm) and Cr (85.7–120.6ppm), and highly fractionated REE patterns and are depleted in HFSEs with negative Nb, Ta, Zr, Hf and Ti anomalies. We suggest that the high-Mg diorites in the DGGB may have been derived from a hydrated mantle wedge which was previously metasomatized by subduction-derived melts and/or fluids. Collectively, a Neoarchean subduction-accretion-collision event is therefore proposed to have generated the DGGB. The volcano-sedimentary assemblage in the DGGB represents a fore-arc subduction-accretion complex, which is interpreted to be related to the suture zone of ca. 2.5 Ga arc-continent collision between a TTG-dominated arc terrane in the COB and the Eastern Block of the NCC. We further propose a long N-S striking Neoarchean suture zone occurring in the eastern margin of the COB mainly consists of the ca. 2.5 Ga subduction-accretion complex in the DGGB to the south, the ca. 2.5 Ga mélange belt in the Zanhuang Complex in the central, and the ca. 2.5 Ga ophiolitic mélange belt in the Zunhua-Dongwanzi structural belt to the north, which separate an arc terrane in the COB from the Eastern Block of the NCC.

Zircon U–Pb–Hf isotope systematics and geochemistry of Helong granite-greenstone belt in Southern Jilin Province, China: Implications for Neoarchean crustal evolution of the northeastern margin of North China Craton

As one of the major Neoarchean granite-greenstone belts in China, the Helong terrane is located at the northeastern margin (Southern Jilin Province) of the North China Craton (NCC). It is dominated by metamorphic supracrustal rock assemblage and granitoid gneiss. The former contains amphibolite, hornblende plagioclase gneiss, biotite plagioclase gneiss and banded iron formation (BIF), whereas the latter is dominated by the Tonalite–Trondhjemite–Granodiorite (TTG) gneisses. Zircon LA–ICP–MS U–Pb isotopic analyses indicate that the supracrustal rock assemblage was formed at 2683–2654Ma and underwent regional metamorphism (lower amphibolite facies) at 2614–2553Ma before the widespread emplacement of plutonic granitoids at 2551–2532Ma. Both greenstone sequence and plutonic granitoid rocks experienced the subsequent regional metamorphism (greenschist to lower amphibolite facies) at 2508–2400Ma. The supracrustal rocks can be divided into three groups based on their lithological and geochemical characteristics. Detailed petrogenetic and geochemical studies suggest that the Group #1 amphibolite samples were derived from partial melting of depleted upper mantle that had been metasomatized by slab-derived fluids; Group #2 amphibolite samples were derived by partial melting of a depleted mantle wedge that was strongly metasomatized by subduction-derived fluids or melts; and Group #3 hornblende/biotite plagioclase gneiss samples were derived from partial melting of juvenile basalt and minor amounts of older sediments in the lower crust level. The granitoid gneisses in the Helong granite-greenstone belt can be divided into relatively high magnesium group (HMG) and low magnesium group (LMG). The HMG samples were derived from parental magmas generated by partial melting of juvenile basalt and minor amounts of older sediments within the lower crust level, whereas magmatic precursors of the LMG samples were generated by partial melting of a metapelite-dominated source under lower temperature melting conditions.U–Pb dating of inherited or captured zircons, combined with zircon Lu–Hf isotope data, indicates a significant ∼2.9–2.7Ga crustal growth episode, whereas the ∼2.55–2.53Ga magmatism involved crustal recycling in the Helong terrane. Consequently, the Helong terrane preserves a record of Neoarchean tectonic environment of Andean-type active continental margin.

The Yatela gold deposit in Mali, West Africa: The final product of a long-lived history of hydrothermal alteration and weathering

The Yatela gold deposit is located in the Kédougou-Kénieba inlier (KKI), a window of ca. 2200-2050 Ma rocks that are exposed in eastern Senegal and western Mali. The geology of the KKI differs from other Paleoproterozoic granite-greenstone belts and sedimentary basins by the abundance of carbonate rocks. The Yatela deposit occurs within 8 km of the regional-scale Senegal–Mali Shear Zone. Country rocks in the Yatela region have been subjected to polycyclic deformation and regional greenschist-facies metamorphism. A syn-kinematic diorite stock has intruded the metasedimentary sequences in the open pit and is associated with a hornblende-hornfels contact aureole. Field relationships and micro-textural data indicate that the primary gold mineralisation is shear-hosted. The similar relative timing and structural setting between the Yatela primary gold mineralisation and other world-class deposits in the region (e.g., Loulo, Lawrence et al., 2013a; Massawa, Treloar et al., 2014; Sadiola Hill, Masurel et al., in press) suggest that regional orogenic gold mineralisation occurred during a period of transcurrent tectonics, after the cessation of regional compressional deformation. The primary gold mineralisation at Yatela, however, is low-grade and sub-economic. It is hosted by marbles and, to a lesser extent, diorite. The primary ore is pyrite-rich, with abundant chalcopyrite, minor arsenopyrite and accessory Zn–Pb–Sb–Fe–Ag–Co–Ni-bearing mineral species. Post-Birimian surficial dissolution of hydrothermally altered and mineralised host marbles resulted in the creation of troughs, which were draped and infilled with a ferruginous dissolution residue enriched in gold. This auriferous residuum formed the economic resource mined at Yatela until decommissioning in 2013. The Yatela gold deposit is unique with respect to mineralisation types encountered in West Africa because an auriferous residuum of economic interest (>1 Moz) derives from an underlying sub-economic Birimian occurrence. This study provides a new appreciation for the economic interest of Yatela-style orebodies and their prospectivity in other gold provinces of the West African Craton where carbonates are present, such as in the Siguiri basin in Guinea and the Ity region in western Côte d’Ivoire.

Extraterrestrial factors and their role in the Earth’s tectonic evolution in the Early Precambrian

The paper is focused on the fundamental problem of influence of extraterrestrial factors on the Earth’s geologic and tectonic evolution. Extraterrestrial factors played a decisive role in the Earth’s genesis, the formation of the first Hadean continental crust, and the beginning of the Archean era. Their significant influence persisted in the later epochs: Even in the Phanerozoic, extraterrestrial factors might have had a considerable influence on the environment. The sialic cores of protocontinental crust (4.4-3.9 Ga) with first-generation greenstone zones (3.8-3.2 Ga) and the global system of granite-greenstone belts (3.1-2.7 Ga) formed in the rotation-plume regime, mainly in the subequatorial hot belt. The formation of these global structures was, to a large extent, influenced by asteroid impacts, which caused the impact-triggered genesis of mantle plumes. Dramatic changes in the subsequent geologic history began at 2.7-2.0 Ga; at 2.0 Ga they terminated with the Moon’s transition to an orbit similar to the present-day one (50 ± 3 Earth’s radii), accompanied by the abrupt slowdown of the Earth’s axial rotation, the termination of formation of the layer D", and the start of recent plate tectonics, which is accompanied by the plume tectonics.

Timing and origin of Mesozoic magmatism and metallogeny in the Wutai-Hengshan region: Implications for destruction of the North China Craton

The Wutai-Hengshan region in the northwestern domain of Taihang Mountains (TM) occurs along the Trans-North China Orogen (TNCO) that amalgamates the Western and Eastern Blocks of the North China Craton (NCC). The metamorphic basement in the region is composed of ca. 2.5Ga granite-greenstone belts within which the Mesozoic magmatic suites are widely emplaced, associated with polymetallic mineralization. The timing, origin and evolution of the magmatic rocks in the Wutai-Hengshan region provide important clues to evaluate the timing, mechanism and process of the destruction of the NCC. In this study, we report new zircon U–Pb and in-situ Hf isotopic data on some of the Mesozoic intrusions associated with Au–Ag–Cu–Mo mineralization from this region. The results show that magmatism and metallogeny in the Yixingzhai and Boqiang-Gengzhuang mining regions occurred during 142–130Ma, starting with felsic granite porphyry with Mo–Cu mineralization, and intermediate magma associated with gold mineralization. The emplacement of magma and mineralization in Diaoquan mining region occurred about 132–130Ma, with the intermediate magma emplaced earlier than felsic magma. The in-situ Hf isotopic data indicate that most of the felsic magma was produced by partial melting of the lower crust, and that the intermediate magma witnessed intrusion of mafic magma derived from mantle with magma mixing in different proportion. The process of crystallization differentiation played an important role in the formation of Diaoquan complex intrusion.The magmatism and metallogeny in the Wutai-Hengshan region were coeval with the craton destruction of the NCC. The Mesozoic magmatism and metallogeny in this region during 142–130Ma contributed to the destruction of the stable cratonic architecture of the NCC. The mafic lower crust was modified by magma derived from mantle during this process. The regional geodynamics associated with these processes are correlated to the subduction of the paleo-Pacific oceanic lithosphere.

Paleo- to Mesoarchean polymetamorphism in the Barberton Granite-Greenstone Belt, South Africa: Constraints from U-Pb monazite and Lu-Hf garnet geochronology on the tectonic processes that shaped the belt: Discussion

[Cutts et al. (2014)][1] combined thermodynamic modeling of compositional change in metamorphic garnet with metamorphic peak and retrograde pressure-temperature ( P - T ) data derived from quantitative P - T pseudosections to determine clockwise P - T paths for several samples from the Tjakastad

Reply to “Paleo- to Mesoarchean polymetamorphism in the Barberton granite-greenstone belt, South Africa: Constraints from U-Pb monazite and Lu-Hf garnet geochronology on the tectonic processes that shaped the belt: Discussion” by M. Brown

Were modern-type subduction-accretion–like processes already operative in the Archean, or did a hotter Archean Earth with consequently weaker lithosphere prevent this and favor gravitationally driven vertical tectonics? Brown’s comment on [Cutts et al. (2014)][1] weighs in on this central

Chapter 11 Palaeoproterozoic supracrustals of the Bastar Craton: Dongargarh Supergroup and Sausar Group

Abstract The Bastar Craton of India is composed of Archaean nuclei of tonalite–trondhjemite–granodiorite gneisses, enveloped by an older granite–greenstone belt (&gt;3000 Ma) with banded iron formation (BIF), and an auriferous younger granite–greenstone belt with BIF. Available geological, geochemical and geochronological data indicate multiple episodes of orogeny with high-grade metamorphism at 3200–3300, 2600–2700, 2100–2200, 1900–2000, 1800–1850, 1500–1600 and 1400–1450 Ma, and continental rifting and basin development marked by emplacement of mafic dyke swarms at c. 2900 (subalkaline mafic dykes; BD-1A), 2480 (high-Mg mafic dykes; BD-1B), 2100 (Fe–tholeiite dykes; BD-2A), 1880 (Fe–tholeiites dykes; BD-2B), 1776 and 1422 Ma. Associations of extensive bimodal volcanics and riftogenic sediments are found in the Neoarchaean and Palaeoproterozoic basins of the craton. Evidence of Palaeoproterozoic (Huronian) glaciation and associated ‘cap carbonate’ followed by deposition of fine clastics with manganese ore is found in the Palaeoproterozoic Sausar Group. The lithological association of the Sausar Group is comparable to the carbonate–tillite association of the Huronian Supergroup, Snowy Pass Supergroup, Transvaal Supergroup and Turee Creek Group. The geological evolution of the Bastar Craton matches that of Western Australia and South Africa. Such similarities can be analysed to develop a unified Palaeoproterozoic assembly for these provinces.

Tectonic juxtaposition of crust and continental growth during orogenesis: Example from the Rengali Province, eastern India

The southern boundary of the Singhbhum Craton witnessed multiple orogenies that juxtaposed thin slice of granulite suite of the Rengali Province against the low-grade granite-greenstone belt of the craton along the E–W trending Sukinda Thrust. The strong southerly dipping mylonitic foliation within the granulites along with the prominent down-dip mineral lineation, suggest a northerly-verging thrusting. Mylonitized charnockite at the contact zone contains enclaves of mafic and ultramafic granulite, whereas granitoid gneiss contains enclaves of pelitic granulite. Mafic granulite enclaves preserve an early (S1M) foliation that formed during D1M deformation. This rock, along with the host charnockite, were intensely deformed by the D2M thrusting event and resulting S2M foliation development in both rock suites. Geothermobarometric and pseudosection analyses show that the garnet-clinopyroxene-plagioclase-orthopyroxene-ilmenite-quartz assemblage in mafic granulite was stabilized at high-pressure and temperature conditions (10−12 kbar, 860 °C) and was overprinted by a fine-grained assemblage of clinopyroxene-plagioclase ± hornblende that developed during decompression (down to 5.5–7.5 kbar). Matrix hornblende shows incipient breakdown to garnet-clinopyroxene-quartz intergrowth due to a granulite facies reworking. A contrasting P-T history is preserved in the pelitic granulite. The peak assemblage garnet-orthopyroxene-cordierite-quartz-rutile was stabilized at ∼6.0 kbar, 730 °C which resulted from heating of the mid crust magma during the D2M thrusting. The contrasting P-T histories could result from the tectonic juxtaposition of lower- and mid-crustal section during the D2M event. Evidences of an early orogenic imprint within the mafic granulite imply involvement of deep continental crust during southward growth of the Singhbhum Craton.

Application of Lidar to resolving tectonic and glacial fabrics in glaciated terrain: An example from an Archean granite-greenstone belt, northeastern Minnesota

In this study we examine an Archean granite-greenstone terrane in northeastern Minnesota (USA) to illustrate the application of high-resolution Lidar (light detection and ranging) altimetry to mapping regional tectonic fabrics in forested glaciated areas. We describe the recognition of lineaments and distinguish between tectonic and glacial lineament fabrics. We use a 1-m Lidar derived bare earth digital elevation model to construct multiple shaded-relief images for lineament mapping with sun elevation of 45° and varying sun azimuth in 45° intervals. Two suites of lineaments are apparent. Suite A has a unimodal orientation, mean trend of 035, and consists of short (

Late Neoarchean crustal evolution of the eastern North China Craton: A study on the provenance and metamorphism of paragneiss from the Western Shandong Province

Metamorphosed clastic rocks provide important constrains on the composition of upper continental crust. Here we report and characterize a sequence of Neoarchean paragneiss in association with migmatite from the Qixingtai area in Western Shandong Province (WSP) in the North China Craton (NCC), that represent metamorphosed moderate- to fine-grained clayey clastic rocks.The garnet from the paragneiss is characterized by nil or weakly compositional zoning. The biotite shows low TiO2 content (<2wt.%) and high Fe/[Fe+Mg] molar ratios (0.45–0.48). The P–T estimates suggest that the paragneiss sequence was metamorphosed at 5.5–5.7kbar and 678–679°C. The detrital zircons from the sillimanite garnet gneiss yield concordant 207Pb/206Pb ages from 2.78 to 2.53Ga (discordance ≤15%), suggesting that the protolith sediments were deposited after 2.53Ga. The overgrowth rims on the detrital zircons of the paragneiss and those from the leucosome of the migmatite show similar low Th/U values, moderate Th and high U and common lead contents, indicating that the metamorphic and anatectic zircons formed under fluid-rich conditions. The intrusive contact between the massive monzogranite and the banded migmatite suggests that the regional metamorphic/anatectic event occurred before the emplacement of the monzogranite. However, the upper intercept ages, defined by the overgrowth rims of the pre-existing zircons from the leucosome, are broadly similar to those of the crystallization of the magmatic zircons from the monzogranite within errors at ∼2.50Ga.The paragneiss exhibits SiO2 contents in the range of ∼58wt.% (garnet gneiss)–∼70wt.% (sillimanite gneiss), and negative correlations with Al2O3 (23.4–15.4wt.%), TiO2 (0.90–0.35wt.%), MgO (4.5–1.1wt.%), FeOt (8.3–2.6wt.%) and Cr (382–131ppm). The REE patterns also show a systematic change from garnet gneiss to sillimanite gneiss. In trace element discrimination diagrams, the paragneiss plots between the end-members of komatiite/basalt and TTG/granite, suggesting that the sediments sourced from a mixture of ultramafic–mafic and felsic protoliths. Combined with the geochronological data, we suggest that the 2.75–2.71Ga komatiite–tholeiite sequence of the Taishan association and TTGs in the central terrane and the 2.56–2.53Ga TTGs in the southwestern part of the WSP as the potential provenance for the clastics.The protolith of the paragneiss in the Qixingtai area belong to the sedimentary sequence of the Taishan association. Spatially associated coeval arc-like assemblage and other geological evidence suggest that, (1) the metasediments were likely deposited in a back arc basin, (2) the upper amphibolites-facies metamorphism, constrained by the mineral assemblages in the paragneiss, might be related to the closure of the basin, and (3) the generation of large volumes of matured K-rick continental crust occurred not earlier than 2.53Ga in the WSP granite–greenstone belt.

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.

A geological synthesis of the Precambrian shield in Madagascar

Available U–Pb geochronology of the Precambrian shield of Madagascar is summarized and integrated into a synthesis of the region’s geological history. The shield is described in terms of six geodynamic domains, from northeast to southwest, the Bemarivo, Antongil–Masora, Antananarivo, Ikalamavony, Androyan–Anosyan, and Vohibory domains. Each domain is defined by distinctive suites of metaigneous rocks and metasedimentary groups, and a unique history of Archean (∼2.5Ga) and Proterozoic (∼1.0Ga, ∼0.80Ga, and ∼0.55Ga) reworking. Superimposed within and across these domains are scores of Neoproterozoic granitic stocks and batholiths as well as kilometer long zones of steeply dipping, highly strained rocks that record the effects of Gondwana’s amalgamation and shortening in latest Neoproterozoic time (0.560–0.520Ga).The present-day shield of Madagascar is best viewed as part of the Greater Dharwar Craton, of Archean age, to which three exotic terranes were added in Proterozoic time. The domains in Madagascar representing the Greater Dharwar Craton include the Antongil–Masora domain, a fragment of the Western Dharwar of India, and the Neoarchean Antananarivo domain (with its Tsaratanana Complex) which is broadly analogous to the Eastern Dharwar of India. In its reconstructed position, the Greater Dharwar Craton consists of a central nucleus of Paleo-Mesoarchean age (>3.1Ga), the combined Western Dharwar and Antongil–Masora domain, flanked by mostly juvenile “granite–greenstone belts” of Neoarchean age (2.70–2.56Ga). The age of the accretionary event that formed this craton is approximately 2.5–2.45Ga. The three domains in Madagascar exotic to the Greater Dharwar Craton are the Androyan–Anosyan, Vohibory, and Bemarivo. The basement to the Androyan–Anosyan domain is a continental terrane of Paleoproterozoic age (2.0–1.78Ga) that was accreted to the southern margin (present-day direction) of the Greater Dharwar Craton in pre-Stratherian time (>1.6Ga), and rejuvenated at 1.03–0.93Ga with the creation of the Ikalamavony domain. The Vohibory domain, an oceanic terrane of Neoproterozoic age was accreted to the Androyan–Anosyan domain in Cryogenian time (∼0.63–0.60Ga). The Bemarivo domain of north Madagascar is a terrane of Cryogenian igneous rocks, with a cryptic Paleoproterozoic basement, that was accreted to the Greater Dharwar Craton in latest Ediacaran to earliest Cambrian time (0.53–0.51Ga).

Crustal evolution in the central part of Eastern NCC: Zircon U−Pb ages from multiple magmatic pulses in the Luxi area and implications for gold mineralization

The Luxi area in the central part of Eastern North China Craton (NCC) is located in a relatively stable intraplate environment and preserves some of the large granite–greenstone belts in the North China Craton (NCC). The intensity and extent of multiple magmatic pluses associated with the destruction of the NCC during Mesozoic were comparatively less in this region. However, the nature and timing of the magmatic pulses in the Luxi area are critical to the evaluation of the differential destruction of the NCC's lithosphere as well as in understanding the origin and extent of the associated gold mineralization.In this study, we report new zircon U−Pb data on the basement rocks in the Luxi area which show that the protoliths of the TTG (tonalite–trondhjemite–granodiorite) gneisses, granitoids and amphibolites formed at 2572.2±8.8Ma, 2531±12Ma, and 2572±32Ma respectively. The Luxi area experienced two major crust accretions at 2.7−2.75Ga and ca. 2.5Ga. The Mesozoic Tongshi complex formed at 175.6±1.7−178.4±2.1Ma associated with fluid modification at the later stage of magma crystallization. The alteration age (162.2±3.2−165.4±2.8Ma) may represent the mineralization age of the Guilaizhuang gold deposit. The diorite porphyry of Tongjing complex was emplaced at 128.0±5.4Ma. The inherited zircon and Lu−Hf isotope data suggest extensive recycling of basement rocks in the Luxi area. The sequence of Mesozoic magmatic intrusions shows a clear variation from intermediate (alkaline) through intermediate (cal-alkaline), and felsic–mafic, and the main intrusive ages can be divided into distinct phases at 170–200Ma, 100–150Ma (with peak at 120–130Ma), and 80–90Ma respectively. The Tongshi complex formed during the compression–extension stage after the collision of the Yangtze Craton and North China Craton, whereas the Tongjing complex and other alkaline, felsic and mafic complexes in the Luxi area formed within the geodynamic setting of Pacific subduction. The early Cretaceous magmatism also simultaneously occurred in the Taihang Mountain area (far from the plate margin) in the central part of the North China Craton, with related gold mineralization. The Mesozoic gold mineralization in Jiaodong and Luxi in the eastern, and those in the central parts of the North China Craton appear to be linked to extensive crustal recycling in an extensional tectonic regime.

Paleo- to Mesoarchean polymetamorphism in the Barberton Granite-Greenstone Belt, South Africa: Constraints from U-Pb monazite and Lu-Hf garnet geochronology on the tectonic processes that shaped the belt

The Barberton Granite-Greenstone Belt (BGGB) of South Africa is an exceptionally well preserved Meso-Paleoarchean metamorphic supracrustal belt, one of only a few in the world. Studies of metamorphism in the BGGB have considerable potential to advance our understanding of tectonic processes in the Archean crust. Two current hypotheses persist to explain the origin of amphibolite-facies metamorphism in the southern BGGB. The first interprets these rocks to be the consequence of accretionary tectonics, while the second proposes a “dome-and-keel” vertical tectonic process driven by sinking of greenstone layers and the doming of the underlying granitoid crust. In this study, metamorphic pressure-temperature ( P-T ) analysis has been combined with garnet Lu-Hf and monazite U-Pb geochronology to directly date the amphibolite-facies metamorphism within the Stolzburg terrane of the BGGB. A garnet-biotite-chlorite–bearing sample yields a Lu-Hf garnet age of 3233 ± 17 Ma and a garnet-staurolite-kyanite–bearing sample produces a U-Pb monazite age of 3191 ± 9 Ma, whereas an andalusite-kyanite–bearing sample produces a U-Pb monazite age of 3436 ± 18 Ma. Phase diagrams and garnet compositional modeling produce a clockwise P-T evolution, with rocks reaching peak P-T conditions of 8.5 kbar and 640 °C for the ca. 3200 Ma event and minimum peak P-T conditions of ∼4.5 kbar and 550 °C for the ca. 3435 Ma event. The duration of metamorphism for the ca. 3200 Ma event is estimated to be ∼50–20 m.y. based on differences in age between U-Pb and Lu-Hf systems and durations needed to fit models of diffusionally modified garnet chemical zoning. Similarly shaped P-T paths over the Stolzburg terrane indicate that the metamorphism occurred in response to crustal thickening due to an accretionary tectonic process. Thus, the Stolzburg terrane constitutes an orogenic core, exhumed along the Komati fault.

Isotopic geochronological evidence for the Paleoproterozoic age of gold mineralization in Archean greenstone belts of Karelia, the Baltic Shield

The Rb-Sr age of metasomatic rocks from four gold deposits and occurrences localized in Archean granite-greenstone belts of the western, central, and southern Karelian Craton of the Baltic Shield has been determined. At the Pedrolampi deposit in central Karelia, the dated Au-bearing beresite and quartz-carbonate veins are located in the shear zone and replace Mesoarchean (∼2.9 Ga) mafic and felsic metavolcanic rocks of the Koikar-Kobozero greenstone belt. At the Taloveis ore occurrence in the Kostomuksha greenstone belt of western Karelia, the dated beresite replaces Neoarchean (∼2.7 Ga) granitoids and is conjugated with quartz veins in the shear zone. At the Faddeinkelja occurrence of southern Karelia, Aubearing beresite in the large tectonic zone, which transects Archean granite and Paleoproterozoic mafic dikes, has been studied. At the Hatunoja occurrence in the Jalonvaara greenstone belt of southwestern Karelia, the studied quartz veins and related gold mineralization are localized in Archean granitoids. The Rb-Sr isochrons based on whole-rock samples and minerals from ore-bearing and metasomatic wall rocks and veins yielded ∼1.7 Ga for all studied objects. This age is interpreted as the time of development of ore-bearing tectonic zones and ore-forming hydrothermal metasomatic alteration. New isotopic data in combination with the results obtained by our precursors allow us to recognize the Paleoproterozoic stage of gold mineralization in the Karelian Craton. This stage was unrelated to the Archean crust formation in the Karelian Block and is a repercussion of the Paleoproterozoic (2.0–1.7 Ga) crust-forming tectonic cycle, which gave rise to the formation of the Svecofennian and Lapland-Kola foldbelts in the framework of the Karelain Craton. The oreforming capability of Paleoproterozoic tectonics in the Archean complexes of the Karelian Craton was probably not great, and its main role consisted in reworking of the Archean gold mineralization of various genetic types, including the inferred orogenic mesothermal gold concentrations.

Hydrothermal Fluid Processes and Evolution of the Giant Serra Norte Jaspilite-Hosted Iron Ore Deposits, Carajas Mineral Province, Brazil

Hydrothermal Fluid Processes and Evolution of the Giant Serra Norte Jaspilite-Hosted Iron Ore Deposits, Carajas Mineral Province, Brazil