Palaeoproterozoic Rocks Research Articles

Geological evolution of the Antongil Craton, NE Madagascar

The Antongil Craton, along with the Masora and Antananarivo cratons, make up the fundamental Archaean building blocks of the island of Madagascar. They were juxtaposed during the late-Neoproterozoic to early Palaeozoic assembly of Gondwana. In this paper we give a synthesis of the geology of the Antongil Craton and present previously published and new geochemical and U–Pb zircon analyses to provide an event history for its evolution. The oldest rocks in the Antongil Craton form a nucleus of tonalitic gneiss, characteristic of Palaeo-Mesoarchaean cratons globally, including phases dated between 3320 ± 14 Ma to 3231 ± 6 Ma and 3187 ± 2 Ma to 3154 ± 5 Ma. A series of mafic dykes was intruded into the Mesoarchaean tonalites and a sedimentary succession was deposited on the craton prior to pervasive deformation and migmatisation of the region. The age of deposition of the metasediments has been constrained from a volcanic horizon to around 3178 ± 2 Ma and subject to migmatisation at around 2597 ± 49 Ma. A subsequent magmatic episode generated voluminous, weakly foliated granitic rocks, that also included additions from both reworked older crustal material and younger source components. An earlier granodiorite-dominated assemblage, dated between 2570 ± 18 Ma and 2542 ± 5 Ma, is largely exposed in xenoliths and more continuously in the northern part of the craton, while a later monzogranite-dominated phase, dated between 2531 ± 13 Ma and 2513 ± 0.4 Ma is more widely developed. Together these record the stabilisation of the craton, attested to by the intrusion of a younger dyke swarm, the age of which is constrained by a sample of metagabbro dated at 2147 ± 6 Ma, providing the first evidence for Palaeoproterozoic rocks from the Antongil Craton. The youngest events recorded in the isotopic record of the Antongil Craton are reflected in metamorphism, neocrystallisation and Pb-loss at 792 ± 130 Ma to 763 ± 13 Ma and 553 ± 68 Ma. These events are interpreted as being the only manifestation of the Pan-African orogeny seen in the craton, which led to the assembly of the tectonic blocks that comprise the island.

Continuous cratonic crust between the Congo and Tanzania blocks in western Uganda

Western Uganda is a key region for understanding the development of the western branch of the East African rift system and its interaction with pre-existing cratonic lithosphere. It is also the site of the topographically anomalous Rwenzori Mountains, which attain altitudes of >5000 m within the rift. New structural and geochronological data indicate that western Uganda south and east of the Rwenzori Mountains consists of a WSW to ENE trending fold and thrust belt emplaced by thick-skinned tectonics that thrust several slices of Proterozoic and Archaean units onto the craton from the south. The presence of Archaean units within the thrust stack is supported by new Laser-ICP-MS U–Pb age determinations (2637–2584 Ma) on zircons from the Rwenzori foothills. Repetition of the Paleoproterozoic units is confirmed by mapping the internal stratigraphy where a basal quartzite can be used as marker layer, and discrete thrust units show distinct metamorphic grades. The thrust belt is partially unconformably covered by a Neoproterozoic nappe correlated with the Kibaran orogenic belt. Even though conglomerates mark the bottom of the Kibaran unit, intensive brittle fault zones and pseudotachylites disprove an autochthonous position. The composition of volcanics in the Toro-Ankole field of western Uganda can be explained by the persistence of a cratonic lithosphere root beneath the northwardly thrusted Archaean and Palaeoproterozoic rocks of westernmost Uganda. Volcanic geochemistry indicates thinning of the lithosphere from >140 km beneath Toro-Ankole to ca. 80 km beneath the Virunga volcanic field about 150 km to the south. We conclude that the western branch of the East African rift system was initiated in an area of thinner lithosphere with Palaeoproterozoic cover in the Virunga area and has propagated northwards where it now abuts against thick cratonic lithosphere covered by a thrust belt consisting of gneisses, metasediments and metavolcanics of Neoarchaean to Proterozoic age.

Palaeoproterozoic petrified oil field (Shunga Event)

Significant accumulation of autochthonous organic matter preserved in Palaeoproterozoic rocks is rare. One of the largest and most remarkable occurrences is the deposit at Shunga, in Karelia, northwestern Russia. This enormous accumulation of C-rich rocks is part of the world-wide Shunga Event. The integrated data suggest that the OM was most likely derived from planktonic microorganisms. This remarkable accumulation of OM and inferred generation of giant volume of petroleum took place in the aftermath of the ca 2330–2060 Ma Lomagundi—Jatulian Event—the largest positive excursion of δ13Ccarb of sedimentary carbonate in the Earth’s history. The formation of shungite carbon and its diagenetic, catagenetic and metamorphic transformation are related to fundamental problems of biological evolution, global carbon cycle and the earliest oil generation.

Greenland from Archaean to Quaternary. Descriptive text to the 1995 Geological map of Greenland, 1:2 500 000. 2nd edition

The geological development of Greenland spans a period of nearly 4 Ga, from Eoarchaean to the Quaternary. Greenland is the largest island on Earth with a total area of 2 166 000 km2, but only c. 410 000 km2 are exposed bedrock, the remaining part being covered by a major ice sheet (the Inland Ice) reaching over 3 km in thickness. The adjacent offshore areas underlain by continental crust have an area of c. 825 000 km2.
 Greenland is dominated by crystalline rocks of the Precambrian shield, which formed during a succession of Archaean and Palaeoproterozoic orogenic events and stabilised as a part of the Laurentian shield about 1600 Ma ago. The shield area can be divided into three distinct types of basement provinces: (1) Archaean rocks (3200–2600 Ma old, with local older units up to> 3800 Ma) that were almost unaffected by Proterozoic or later orogenic activity; (2) Archaean terrains reworked during the Palaeoproterozoic around 1900–1750 Ma ago; and (3) terrains mainly composed of juvenile Palaeoproterozoic rocks (2000–1750 Ma in age). Subsequent geological developments mainly took place along the margins of the shield. During the Proterozoic and throughout the Phanerozoic major sedimentary basins formed, notably in North and North-East Greenland, in which sedimentary successions locally reaching 18 km in thickness were deposited. Palaeozoic orogenic activity affected parts of these successions in the Ellesmerian fold belt of North Greenland and the East Greenland Caledonides; the latter also incorporates reworked Precambrian crystalline basement complexes.
 Late Palaeozoic and Mesozoic sedimentary basins developed along the continent–ocean margins in North, East and West Greenland and are now preserved both onshore and offshore. Their development was closely related to continental break-up with formation of rift basins. Initial rifting in East Greenland in latest Devonian to earliest Carboniferous time and succeeding phases culminated with the opening of the North Atlantic Ocean in the late Paleocene. Sea-floor spreading was accompanied by extrusion of Palaeogene (early Tertiary) plateau basalts in both central West and central–southern East Greenland.
 During the Quaternary Greenland was almost completely covered by ice, and the present day Inland Ice is a relic from the Pleistocene ice ages. Vast amounts of glacially eroded detritus were deposited on the continental shelves around Greenland.
 Mineral exploitation in Greenland has so far encompassed cryolite, lead-zinc, gold, olivine and coal. Current prospecting activities in Greenland are concentrated on gold, base metals, platinum group elements, molybdenum, iron ore, diamonds and lead-zinc. Hydrocarbon potential is confined to the major Phanerozoic sedimentary basins, notably the large basins offshore North-East and West Greenland. While reserves of oil or gas have yet to be found, geophysical data combined with discoveries of oil seeps onshore have revealed a considerable potential for offshore oil and gas.

Geochemistry of Palaeoproterozoic volcanic rocks of the Iricoumé Group, Pitinga Mining District, Amazonian craton, Brazil

The Iricoumé Group includes 1.88 Ga volcanic units of the Iricoumé–Mapuera volcano-plutonic association, part of the Uatumã magmatic series in the Guyana shield portion of the Amazonian craton. In the Pitinga Mining District, these rocks consist dominantly of felsic trachyte to rhyolite, associated with voluminous ignimbrite and minor ash-fall tuffs and surge deposits. Mafic rocks are present as basaltic clasts within volcanic breccias, and mostly as mafic microgranular enclaves in the associated Mapuera plutonic rocks. The felsic rocks have high contents of SiO2, FeOt, K2O, Rb, and alkalis; low TiO2, CaO, Sr, Ba, Nb, Ta, and Eu; and show metaluminous to weakly peraluminous bulk-rock compositions. They exhibit alkaline geochemical features, expressed by Na2O + K2O averages of 8.8 wt.%, FeOt/(FeOt + MgO) ≥ 0.8, and high Ga/Al ratios, compatible with A-type magmas. The studied samples plot in the field of within-plate or post-collisional rocks in a (Nb + Y) versus Rb diagram. Nb/Y ratios indicate that they are comparable to A2-type rocks which, allied with their high LREE/Nb ratios, suggest that they were produced from mantle sources modified by previous subduction in a post-collisional setting. Two compositional populations of Ca-amphibole, a Mg-rich (actinolite to Mg-hornblende) and a Fe-rich one (Fe-edenite to Fe-pargasite, Fe-hornblende and Fe-actinolite), characterize the Iricoumé Group volcanics. The Fe-rich amphiboles crystallized under lower fO2 and higher pressure conditions compared with the Mg-rich amphiboles, indicating different levels of crystallization or re-equilibration during ascent of the magmas. Zircons from rhyolites show trace-element compositions typical of magmatic crystals with high Th/U ratios, and REE patterns compatible with zircon-melt partition coefficients for silicic magma compositions. Their relatively lower zircon/rock partition coefficients are due to early apatite crystallization. Fractional crystallization mainly of plagioclase-hornblende and biotite-alkali feldspar with minor amounts of apatite explains the geochemical trends observed in the felsic Iricoumé volcanic rocks.

U–Pb and Hf isotopic study of detrital zircons from the Lüliang khondalite, North China Craton, and their tectonic implications

AbstractTwo types of metasedimentary rocks occur in the Trans-North China Orogen of the North China Craton. One type consists of highly metamorphosed supracrustal rocks with protoliths of mature cratonic shale, called khondalites, as found in the Lüliang Complex; rocks of the other type are also highly metamorphosed but less mature, as represented by the Wanzi supracrustal assemblage in the Fuping Complex. U–Pb isotopic data for detrital zircons from khondalites show a provenance dominated by 1.9–2.1 Ga Palaeoproterozoic rocks. These detrital zircons display a wide range of εHfvalues from −16.0 to +9.2 and give Hf isotopic model ages mostly around 2.3 Ga. The high positive εHfvalues approach those for the depleted mantle at 2.1 Ga, highlighting a juvenile crustal growth event in Palaeoproterozoic times. Hf isotopic data also imply thatc.2.6 Ga old crustal material was involved in the Palaeoproterozoic magmatic event. These data are similar to those for the khondalitic rocks from the interior of the Western Block of the North China Craton, suggesting a common provenance. In contrast, other metasedimentary rocks in the Trans-North China Orogen, such as the Wanzi supracrustal assemblage in the Fuping Complex, have a source region with both Palaeoproterozoic and Archaean rocks. Their detrital zircon Hf isotopic data indicate reworking of old crustal material and a lack of significant juvenile Palaeoproterozoic magmatic input. These rocks are similar to the coevally deposited meta-sedimentary rocks in the interior of the Eastern Block. We propose that the Lüliang khondalites were deposited on the eastern margin of the Western Block in a passive continental margin environment and were thrust eastward later during collision with the Eastern Block. Other metasedimentary rocks in the Trans-North China Orogen were deposited on the western margin of the Eastern Block in a continental arc environment. Our data support the eastward subduction model for the Palaeoproterozoic tectonic evolution of the North China Craton.

Petroleum surface oil seeps from a Palaeoproterozoic petrified giant oilfield

AbstractEarly Palaeoproterozoic rocks from the Onega Basin in Russian Fennoscandia contain evidence for substantial accumulation and preservation of organic matter (up to 75 wt% total organic carbon) with an estimated original petroleum potential comparable to a modern supergiant oilfield. The basin contains a uniquely preserved petrified oilfield including evidence of oil traps and oil migration pathways. Here, we report the discovery of the surface expression of a migration pathway, along which petroleum was flowing from the sub‐surface. This surface oil seep, the first occurrence ever reported from the Palaeoproterozoic, appears as original bitumen clasts redeposited in Palaeoproterozoic lacustrine turbidites. The δ13Corg of clastic pyrobitumen ranges between −35.4 and −36.0‰ (n = 14), which is within the range of interbed‐ and vein‐trapped fossil oil (−46 and −24‰), suggesting similar source. Biogenic organic matter, whose isotopic composition was modified during thermal maturation, is the likely source for the migrated hydrocarbon.

Oil-bearing fluid inclusions from the Palaeoproterozoic: A review of biogeochemical results from time-capsules >2.0 Ga old

The observation of oil inclusions trapped prior to 2.0 Ga in Palaeoproterozoic rocks and the ability to obtain detailed molecular geochemical information from them provide a robust way for understanding the early biogeochemical evolution of the Earth. Oil-bearing fluid inclusions (FI) in ca. 2.45 Ga fluvial metaconglomerate of the Matinenda Formation at Elliot Lake, Canada were trapped in quartz and feld-spar during diagenesis and early metamorphism of the host rock, probably before ca. 2.2 Ga. The 2.1 Ga FA Formation sandstone of the Franceville Basin in Gabon that hosts the Oklo natural fission reactors has also been discovered to contain abundant Palaeoproterozoic oil-bearing FIs. This oil occurs within H2O and CO2-dominated inclusions trapped in syntaxial quartz overgrowths and intragranular and transgranular microfractures in detrital quartz, and was most likely trapped 2.1–1.98 Ga. Molecular geochemical analyses of both FI oils reveal a wide range of compounds, including n-alkanes, isoprenoids, monomethylalkanes, aromatic hydrocarbons, and trace amounts of complex multi-ring biomarkers including terpanes, hopanes, methylhopanes, steranes and diasteranes. To ensure a reliable interpretation of oil inclusions, a comprehensive series of outside-rinse blanks and procedural system blanks was analysed by gas chromatography-mass spectrometry; quantitative amounts of the hydrocarbons in these blanks were compared to the FI extracts, so as to provide confidence limits on the experimental integrity of each compound class. Maturity ratios based on reliably detected compound classes show that the FI oils were generated in the oil window, with no evidence of extensive thermal cracking. The presence of biomarkers for cyanobacteria and eukaryotes derived from and trapped in rocks deposited prior to 2.0 Ga is consistent with early evolution of oxygenic photosynthesis and suggests that some aquatic settings had become sufficiently oxygenated for sterol biosynthesis by this time. The extraction of biomarker molecules from Palaeoproterozoic oil-bearing FIs thus establishes a new method, using low detection limits and system blank levels, to trace evolution through Earth’s early history that avoids the potential contamination problems affecting shale-hosted hydrocarbons.

Geodynamic evolution of the 2.25-2.0 Ga Palaeoproterozoic magmatic rocks in the Man-Leo Shield of the West African Craton. A model of subsidence of an oceanic plateau

Abstract The West African Craton is defined by the presence of Archaean and Palaeoproterozoic (Birimian) rocks. Since 1990, researchers have published several papers proposing various regional scale accretionary models for the evolution of the Birimian series and the Eburnean orogeny. The Birimian lithostratigraphic succession starts with a major sequence of tholeiitic pillow basalts with intercalations of sediments, overlain by detrital and carbonate sediments, associated with calc-alkaline volcanics and plutons. A global scheme for the geodynamical evolution can be reasonably constrained using the characteristics of early tholeiitic plutono-volcanic and granitoid rocks. Geochemical, geochronological and structural characteristics of Palaeoproterozoic magmatic rocks of the Man-Leo Shield distinguish three tholeiitic (PTH1, PTH2, PTH3) and two granitic (PAG, PBG) series and date three principal events that have characterized the 2250–2000 Ma period: (1) Event I (2250–2200 Ma) characterized by tholeiitic volcanism with a widespread first tholeiite generation (PTH1), interpreted as the eruption of a mantle plume in an oceanic basin floor; (2) Event II (2200–2150 Ma) marked by second (PTH2) and third (PTH3) tholeiite generations and a calc-alkaline magma setting; high granitization and amphibole-bearing (PAG) granitoid emplacement with greenstone belt deformation. This event has been favoured by a crustal subsidence in a broad synclinorium mega-structure followed by vertical tectonics; (3) Event III (2150–2000 Ma) characterized by main biotite±muscovite (without amphibole) bearing (PBG) granitoid emplacement in a context of transcurrent deformation. There are many similarities between West African Palaeoproterozoic rocks and Archaean formations and their crustal evolution also may be similar.

A three-dimensional model of the Early Precambrian crust under the southeastern Fennoscandian Shield: Karelia craton and Belomorian tectonic province

The Early Precambrian structures of the Archaean Karelia craton and Belomorian tectonic province have been investigated by reflection seismology. Integration of the existing geological database for these two entities and adjacent tectonic units with the seismic data for the crust and upper mantle along the 1-EU geotraverse and cross-traverse 4B totaling c.1300 km allows development of a three-dimensional model of the deep structure of the southeastern Fennoscandian Shield. Interpretation of the seismic data provides evidence that the Early Precambrian crust is characterized by inclined layering. The crustal architecture is a combination of thrust–nappe and thrust–underthrust structural assemblages consisting of Neoarchaean and Palaeoproterozoic rocks that were subsequently further deformed during uplift and emplacement of granite-gneiss domes. The new model substantially modifies previous conceptions of this part of the shield as being constituted of blocks with sub-vertical margins and individual internal layering. It is apparent that the surface traces of gently dipping boundaries are defined by the erosion level and consequently these boundaries are not those of particular “tectonic blocks” in the traditional sense.

Trans-alkaline magmatism in the Serrinha–Pedro Velho Complex, Borborema Province, NE Brazil and its correlations with the magmatism in eastern Nigeria

The Solânea Complex (572 ± 8 Ma) comprises various intrusions that total 200 km 2. It cuts Palaeoproterozoic migmatized orthogneisses of the Serrinha–Pedro Velho Complex, which is part of the São José do Campestre Massif, North Tectonic Domain of the Borborema Province, NE Brazil. It is a post-collisional intrusion constituted of coarse-grained biotite–amphibole monzogranites, fine-grained leucogranites and diorites. Major, trace and rare earth elements data, used to constrain the origin and nature of these rocks, indicate that the monzogranites display chemical features of weakly peraluminous ferro-potassic trans-alkaline granitoids, similar to coeval occurrences described in eastern Nigerian. Sm–Nd data together with the whole rock geochemical data suggest that: (I) the monzogranites were generated by partial melting of tonalitic gneiss from the lower crust, leaving behind a granulitic residue; (II) the diorites originated by partial melting of an enriched lithospheric mantle; (III) the evolution of the Solânea Complex involved magma mixing between the monzogranites and diorites. K-feldspar and amphibole fractionation appear to have occurred during the evolution of the monzogranites. Leucogranites constituting an elliptical shaped intrusion (80 km 2), the Dona Inês Pluton, are intruded to the north of the Solânea Complex. They also occur as small intrusions cross-cutting the border of the Solânea Complex. Geochemical and isotopic data suggest that the leucogranites were generated by melting of crustal Palaeoproterozoic rocks. Rb–Sr data gave an age of 544 ± 16 Ma. Trans-alkaline magmatism is associated with ductile–brittle deformation in the Serrinha–Pedro Velho Complex of the São José do Campestre Massif. This event follows the peak of an extensional-related high-temperature metamorphism (upper amphibolite to granulite facies), with estimated age of 578 to 574 Ma. If a pre-Gondwana reconstruction is considered, this event is seen to extend to the African side of Gondwana in eastern Nigeria.

Geology of the northern Borborema Province, NE Brazil and its correlation with Nigeria, NW Africa

Abstract The Borborema and Benin–Nigeria provinces of NE Brazil and NW Africa, respectively, are key areas in the amalgamation of West Gondwana by continental collision during the Brasiliano/Pan-African orogenies. Both are underlain by complex basement: Nigeria has c. 3.05 Ga Archaean crust but no known Palaeoproterozoic rocks >2.0 Ga; in NE Brazil, 2.6–3.5 Ga Archaean rocks form small cores within Palaeoproterozoic gneiss terrains affected by plutonism at c. 2.17 Ga. Both regions exhibit Late Palaeoproterozoic ( c. 1.8 Ga) rift-related magmatism and metasedimentary sequences overlying the basement. The Seridó Group of NE Brazil (<0.65 Ga) is similar to the Igarra Sequence in SW Nigeria. The Ceará Group, which may date back to c. 0.85 Ga, is a passive margin deposit on crust thinned during initiation of an oceanic domain. In both provinces, basement and sedimentary cover were involved in tangential tectonics that resulted in crust-thickening by nappe-stacking associated with closure of this ocean. Frontal collision between c. 0.66 and 0.60 Ga later evolved to an oblique collision, generating north–south continental strike-slip shear zones at c. 0.59 Ga. In NE Brazil, the main Pan-African suture is probably buried beneath the Parnaíba Basin. The Transbrasiliano Lineament, interpreted as the prolongation of the Kandi–4°50 Lineament in Hoggar, may represent a cryptic suture.

Proterozoic evolution of the Nigeria–Boborema province

Abstract Structural, geochronological, geochemical and mineralization patterns in the Nigeria–Borborema province of western Africa and NE Brazil reflect a complex Proterozoic evolution culminating in the Neoproterozoic Pan-African/Brasiliano orogenesis ( c . 600 Ma). Reworking of the Archaean–early Proterozoic crust produced heterogeneous deformation exemplified by prevalent shears, migmatization, granitization and intrusion of large volumes of granitoids typical of a Himalayan-type thickened crust resulting from continent–continent collision. Dominant north–south to east–west structures, with prominent penetrative fabric and mylonitised wrench faults, refolded, transpressed, or even obliterated older structural trends, which are preserved in nappes of the central Sahara region (NW Africa to Nigeria) and in NE Brazil. Anatexis and recrystallisation were coeval with emplacement of Pan-African granitoids throughout this mobile belt. Bulk chemical modification, especially affecting magmatophile elements and REE patterns, attest to chemical exchange between Archaean basement and Pan-African/Brasiliano rocks. Older crust is present in both regions, including early (3.6–3.5 Ga), mid (3.1 Ga) and late (2.7–2.5 Ga) Archaean, as well as large areas of Palaeoproterozoic rocks reworked by the c . 600 Ma tectono-thermal events. The extent and interpretation of Eburnian/Transamazonian (2.1–2.0 Ga) events have not yet been resolved due to inadequate structural and isotopic data. Litho-structural control of Au, Sn, Nb and Ta mineralization relates to main or late-stage Pan-African deformation.

Explanatory notes to the Geological Map of Greenland, 1:100 000, Ussuit 67 V.2 Nord

The Ussuit map area is situated around the inner Nassuttooq (Nordre Strømfjord) in central West Greenland, in the core of the Palaeoproterozoic Nagssugtoqidian orogen. The orogen largely consists of reworked Archaean gneisses, as well as Palaeoproterozoic ortho- and paragneisses in its central part. Easy access through the branched fjord system and good exposures along its coastlines, combined with less intense reworking compared to neighbouring areas to the west and south, have made the Ussuit map area the most intensely studied and best known part of the orogen. The most recent research and mapping projects were organised by the Danish Lithosphere Centre (1994–1999) and the Geological Survey of Denmark and Greenland (2000–2001).
 The predominant rocks are late Archaean tonalitic and granodioritic orthogneisses, intruded by postkinematic granites. Archaean supracrustal rocks are predominantly of mafic composition, but only few have been recognised. Palaeoproterozoic rocks occur as tectonic sheets (the Ussuit unit) which are always in tectonic contact with the Archaean orthogneisses. The most abundant Palaeoproterozoic rock types are biotite schist and biotite-bearing paragneiss, besides orthogneiss of the Arfersiorfik intrusive suite. The latter rocks are mainly deformed quartz diorites intrusive into the metasedimentary rocks and interpreted as remnants of a magmatic arc above a subduction zone. The Ussuit unit also includes amphibolite, marble and calc-silicate rocks, and lenses of ultrabasic rocks. Small bodies of younger syn- and post-tectonic granites occur throughout the map area. The interleaved Palaeoproterozoic and Archaean rocks form a major anticlinal fold structure between two crustal-scale shear zones, the Nordre Strømfjord shear zone in the north and the Nordre Isortoq steep belt in the south. These shear zones formed during the latest ductiledeformation event (D4), following ductile thrusting (D1) and kilometre-scale folding (D2 and D3). The deformation and high grade metamorphism are the result of collision of two Archaean blocks at c. 1850 Ma, with a presumed strongly deformed suture rooted in the southern Ussuit area.
 No economically feasible mineral occurrences have been discovered to date within the Ussuit map area. Minor sulphide mineralisation related to hydrothermal activity occurs in faults and shear zones, and minor stratabound iron formations have been observed. The most promising industrial mineral deposits are minor diopside occurrences and potential dimension stone in migmatised orthogneiss.

Explanatory notes to the Geological Map of Greenland, 1:100 000, Ussuit 67 V.2 Nord

The Ussuit map area is situated around the inner Nassuttooq (Nordre Strømfjord) in central West Greenland, in the core of the Palaeoproterozoic Nagssugtoqidian orogen. The orogen largely consists of reworked Archaean gneisses, as well as Palaeoproterozoic ortho- and paragneisses in its central part. Easy access through the branched fjord system and good exposures along its coastlines, combined with less intense reworking compared to neighbouring areas to the west and south, have made the Ussuit map area the most intensely studied and best known part of the orogen. The most recent research and mapping projects were organised by the Danish Lithosphere Centre (1994–1999) and the Geological Survey of Denmark and Greenland (2000–2001).
 The predominant rocks are late Archaean tonalitic and granodioritic orthogneisses, intruded by postkinematic granites. Archaean supracrustal rocks are predominantly of mafic composition, but only few have been recognised. Palaeoproterozoic rocks occur as tectonic sheets (the Ussuit unit) which are always in tectonic contact with the Archaean orthogneisses. The most abundant Palaeoproterozoic rock types are biotite schist and biotite-bearing paragneiss, besides orthogneiss of the Arfersiorfik intrusive suite. The latter rocks are mainly deformed quartz diorites intrusive into the metasedimentary rocks and interpreted as remnants of a magmatic arc above a subduction zone. The Ussuit unit also includes amphibolite, marble and calc-silicate rocks, and lenses of ultrabasic rocks. Small bodies of younger syn- and post-tectonic granites occur throughout the map area. The interleaved Palaeoproterozoic and Archaean rocks form a major anticlinal fold structure between two crustal-scale shear zones, the Nordre Strømfjord shear zone in the north and the Nordre Isortoq steep belt in the south. These shear zones formed during the latest ductiledeformation event (D4), following ductile thrusting (D1) and kilometre-scale folding (D2 and D3). The deformation and high grade metamorphism are the result of collision of two Archaean blocks at c. 1850 Ma, with a presumed strongly deformed suture rooted in the southern Ussuit area.
 No economically feasible mineral occurrences have been discovered to date within the Ussuit map area. Minor sulphide mineralisation related to hydrothermal activity occurs in faults and shear zones, and minor stratabound iron formations have been observed. The most promising industrial mineral deposits are minor diopside occurrences and potential dimension stone in migmatised orthogneiss.

No lugar errado, na hora errada: icnofósseis de metazoários no Paleoproterozóico de Minas Gerais?

WRONG TIME, WRONG PLACE: METAZOAN ICHNOFOSSILS IN THE PALAEOPROTEROZOIC OF MINAS GERAIS STATE? Two sedimentary structures comparable to metazoan ichnofossils are described in Palaeoproterozoic rocks of the Cercadinho Formation, Minas Supergroup of the Quadrilatero Ferrifero region, Minas Gerais state, Brazil. The structures are parabolic perturbations in the lamination, perpendicular to the bedding of metasandstones with tabular, tangential and parallel cross-laminations. The first one, similar to an Arenicolites-type burrowing, reaches 1.4 cm in width and about 4.5 cm in high, with sharp walls and disordered internal lamination; the second is comparable to an escape ichnite, up to 1.7 cm in width and 6.7 cm in high. The age of the Cercadinho Formation is well constrained between 2.1 and 2.4 Ga by a series of geochronological and stratigraphical data. A hypothesis that these structures are in fact of metazoan affinities in Palaeoproterozoic rocks is incongruent with the oldest records of metazoan fossils. Another possibility, more likely, is that the structures are not organic-related, and its genesis is so far unknown. Preliminary laboratory experiments, however, advocate the difficulty to generate similar structures by abiotic processes.

<sup>207</sup>Pb-<sup>206</sup>Pb dating of magnetite, monazite and allanite in the central and northern Nagssugtoqidian orogen, West Greenland

Pb-isotopic data for magnetite from amphibolites in the Nagssugtoqidian orogen, central West Greenland, have been used to trace their source characteristics and the timing of metamorphism. Analyses of the magnetite define a Pb-Pb isochron age of 1726 ± 7 Ma. The magnetite is metamorphic in origin, and the 1726 Ma age is interpreted as a cooling age through the closing temperature of magnetite at ~600°C. Some of the amphibolites in this study come from the Naternaq supracrustal rocks in the northern Nagssugtoqidian orogen, which host the Naternaq sulphide deposit and may be part of the Nordre Strømfjord supracrustal suite, which was deposited at around 1950 Ma ago. Pb-isotopic signatures of magnetite from the Arfersiorfik quartz diorite in the central Nagssugtoqidian orogen are compatible with published whole-rock Pb-isotopic data from this suite; previous work has shown that it is a product of subduction-related calc-alkaline magmatism between 1920 and 1870 Ma. Intrusion of pegmatites occurred at around 1800 Ma in both the central and the northern parts of the orogen. Pegmatite ages have been determined by Pb stepwise leaching analyses of allanite and monazite, and source characteristics of Pb point to an origin of the pegmatites by melting of the surrounding late Archaean and Palaeoproterozoic country rocks. Hydrothermal activity took place after pegmatite emplacement and continued below the closure temperature of magnetite at 1800– 1650 Ma. Because of the relatively inert and refractory nature of magnetite, Pb-isotopic measurements from this mineral may be of help to understand the metamorphic evolution of geologically complex terrains.

The Granites gold deposits, Northern Territory, Australia: evidence for an early syn-tectonic ore genesis

The ore deposits of The Granites goldfield are shear-hosted within Palaeoproterozoic amphibolite facies metasedimentary rocks in the Tanami Region, Northern Territory, Australia. The ore bodies are located within a 5- to 35-m thick sequence of steeply dipping unit of metamorphosed iron-rich metasedimentary rocks. Deformation at The Granites was complex and is characterized by five successive deformation phases (D1–5). Shear veins (central and oblique) are the dominant type of vein geometry, with minor development of extensional veins and reverse-fault related veins. Four generations of syn-tectonic veins, corresponding to D1, D3, D4, and D5, have been recognized and are comprised of quartz, quartz-carbonate, calc-silicate, and calcite. In addition, two generations of disseminated sulfide–arsenide mineralization, dominated by pyrrhotite, arsenopyrite, and loellingite, with minor pyrite, chalcopyrite and rare marcasite, formed syn-D1 and syn- to post-D3. Textural and structural evidence indicates deposition of gold was contemporaneous with the syn-D1 veins and sulfide–arsenide mineralization. Four hydrothermal phases are proposed for the formation of the veins and disseminated sulfide–arsenide assemblages. The first phase (D1) was responsible for transport and deposition of the majority of the gold. Minor remobilization and deposition of gold occurred during the D3 and D4 phases. Little is known about the nature of the D1 ore fluid, although a relatively low sulfur content is indicated by the assemblage pyrrhotite–arsenopyrite–loellingite+rare pyrite. The growth of amphibolite facies metamorphic minerals andalusite and almandine garnet during D1 indicates a high temperature for the fluid. The D3 hydrothermal phase coincided with peak metamorphism. D4 fluids were hypersaline, high temperature, CO2-poor, and H2S-poor.

Archaean terranes, Palaeoproterozoic reworking and accretion in the Ukrainian Shield, East European Craton

Abstract The Ukrainian Shield is a large coherent region of exposed Archaean and Proterozoic crust in the southwestern, Sarmatian part of the East European Craton. It is traditionally divided into blocks, or domains, separated by major suture zones. The Azov Domain in the east and the Podolian Domain in the SW are Archaean complexes that have been highly reworked in the Palaeoproterozoic; in contrast, the Archaean (3.2-3.0 Ga) granite-greenstone terrane dominated Middle Dniepr Domain, in the central part of the Shield, is virtually untouched by Proterozoic processes. Palaeoproterozoic rocks dominate the Kirovograd domain in the central Shield. We review previous and recent geochronological results and demonstrate that the Volyn Domain and adjacent parts of the Ros-Tikich Domain in the NW are largely juvenile, c. 2.2-2.0 Ga segments of Palaeoproterozoic crust accreted to the Palaeo- to Mesoarchaean crust in the Podolian Domain. The Podolian Domain includes 3.65 Ga granitoids, with traces of 3.75 Ga material. It has been reworked, at 2.8 Ga and c. 2.0 Ga. Its temporal evolution is thus similar to that of the Azov Domain in the eastern part of the Shield. However, in view of the complex terrane pattern of Sarmatia, this does not necessarily mean that the Podolian and Azov domains were parts of the same continent in the Archaean.

Early Paleoproterozoic–Archean dykes and gneisses in Russian Karelia of the Fennoscandian Shield—New paleomagnetic, isotope age and geochemical investigations

We present here new palaeomagnetic, isotopic age and geochemical data from Archean and Early Palaeoproterozoic rocks in the eastern Fennoscandian Shield. We have studied NE–SW trending gabbronorite dyke sets and their host Archean basement rocks in the Vodlozero block near the 2449 Ma Burakovka layered intrusion in southern Russian Karelia. Both dyke sets are genetically related to the Burakovka intrusion. The other, ca. 25 km long Avdeev dyke, locating a few kilometers south from the Burakovka intrusion, yields a stable single component remanence direction that is in agreement with the direction previously obtained from the Burakovka intrusion. Another NE–SW trending dyke, 0.8 m wide Shalskiy diabase dyke, about 30 km south of the Burakovka intrusion yields a similar remanence direction as the Avdeev dyke. The overall mean remanence direction has a palaeopole at Plat = −12.3°N, Plong = 243.5°E ( A95 = 15.4°, 4 sites, 28 samples). The thin Shalskiy diabase dyke transects a similarly NE–SW trending 500 m wide coarse grained gabbronorite dyke which has now been dated by Sm–Nd method as 2608 ± 56 Ma. Geochemically all the dykes are quite similar showing slight calc-alkaline affinity and low TiO 2 and high SiO 2 with moderate MgO and low Cr and Ni. Furthermore, the dykes are geochemically identical to the 2.45 Ga dyke swarm in the northern Karelian Province. The remanence direction of the thin Shalskiy diabase dyke differs significantly from the high temperature and high coercivity remanence component of the unbaked Archean gabbronorite dyke which yields a palaeopole at Plat = 22.7°N, Plong = 222.1°E (dp = 8.2°, dm = 16.2°, five samples). On the basis of different remanence directions of the diabase dyke and the unbaked Archean gabbronorite dyke, the baked contact test for the diabase dyke is positive. In addition to the high temperature and high coercivity component of the baked and unbaked Archean gabbronorite dyke, in low temperatures and coercivities we isolated a similar component as in the diabase dyke. A comparable remanence component was also obtained from the Archean basement at ca. 8 km from the dykes. We propose that in the studied area, the Archean basement and the Archaean dyke were partly remagnetized due to emplacement and subsequent uplift and cooling of the large Burakovka layered intrusion and related dykes at about 2.40 Ga ago. This interpretation lends support from a new 40Ar/ 39Ar dating of hornblende from another area, Lake Paajarvi area, in northern Karelia. There, a negative baked contact test was previously obtained for the remanence of the dated ca. 2.45 Ga dyke rocks related to the ca. 2.45 Ga Oulanka layered intrusion. The 40Ar/ 39Ar dating of the unbaked Archean basement which yields the same remanence component as the dykes, shows a plateau age of ca. 2.6 Ga, but in addition, it also shows resetting of the basement at ca. 2.4 Ga ago. The dating thus supports reactivation and partial remagnetization of the Archean basement at ca. 2.4 Ga ago. Our new palaeomagnetic results from the Burakovka dykes and the new 40Ar/ 39Ar dating from the Lake Paajarvi area give support to our previous interpretation that at Lake Paajarvi area the remanence component suggested to be 2.4 Ga, despite to negative baked contact test, is indeed of this age. Therefore, it is implied that the results can be used for continental reconstructions.