Sequence Of Quartzites Research Articles

Selective metasomatism of ultramafic cumulates within Archean supracrustal sequences

The Neoarchean Storø Supracrustal Belt in SW Greenland comprises a sequence of mature quartzite, metapelite, amphibolite, and ultramafic rocks that underwent amphibolite facies metamorphism during the amalgamation of the Mesoarchean Akia Terrane and the Eoarchean Færingehavn Terrane. In this belt, tourmaline is found in a transition zone between ultramafic and metapelitic rocks, but also occurs as orbicules within the ultramafic rocks. These tourmaline orbicules hosted by ultramafic rocks are reported for the first time in the North Atlantic craton, thus indicating a unique formation mechanism. We conducted a comprehensive examination of the petrology, whole-rock and mineral chemistry, and oxygen isotope compositions from borehole samples in the Storø Supracrustal Belt, to elucidate the metasomatic events associated with the formation of the orbicular tourmalines. The Storø ultramafic rocks have high MgO, Cr, and Ni contents, with low abundances of REE and HFSE, and preserve a typical cumulate texture. These features are similar to those of ultramafic cumulates found in other Archean supracrustal belts, suggesting a cumulate origin for the Storø ultramafic rocks. Furthermore, the morphology and composition of the tourmaline orbicules within these cumulates indicate that they originated from melts with high boron and high water concentrations that infiltrated the ultramafic rocks. The main factor influencing the morphology of the tourmaline orbicules is the silicification of the ultramafic rocks, driven by their lower chemical potential of SiO2 compared to the surrounding rocks. This silicification process, in combination with compositional variations of cumulates during fractional crystallization, has contributed to the geochemical diversity observed in Archean ultramafic rocks. Thus, it is crucial to understand the effects of such selective metasomatism on Archean ultramafic rocks, as this will facilitate the extraction of original information preserved in the early rock record.

Integrated subsurface investigation for magmatic sulfide mineralization in Betul Fold Belt, central India

Magnetotelluric (MT), Electrical Resistivity Tomography (ERT), Time-Domain Induced Polarization (TDIP), Geochemical and Geological studies followed by drilling and down-hole logging were undertaken with in the Betul Fold Belt (BFB) in Central India, to demarcate zones of magmatic Ni-Cu-PGE sulfide mineralization. The BFB is predominantly composed of circular to elongate gabbro bodies of the Padhar Mafic-Ultramafic Complex, intruded into a sequence of bimodal volcanic rocks and quartzites. Near-surface samples of ultramafic rocks were subjected to precise geochemical analysis and scanned by an electron microscopy with an energy dispersive spectrometer (SEM-EDS). This work indicated the presence of pyrite, pyrrhotite, chalcopyrite, pentlandite, and minor amounts of W-Cd bearing boweiite and palladenite assemblage. These minerals are regarded as favorable to the occurrence of Ni-Cu-PGE sulfide mineralization. MT data derived from two profiles were analyzed and modeled using 2D and 3D inversion algorithms. The robust conductivity anomalies obtained from the MT model have been interpreted coupled with electrical tomography, geology, and geochemistry data. The near-surface shallow depth conductors observed in the ERT sections are interpreted as the sulfide mineralized zones. They corroborate the MT results. These conductive zones reflect the occurrence of the magmatic Ni-Cu-PGE bearing sulfide mineralization associated with rocks of the mantle-derived Padhar Mafic Ultramafic Complex. This geophysical data, in conjunction with petrological and geochemical analysis of drill core samples have allowed the identification of the origin and paragenesis of sulphide mineralization in the study area. Geochemical studies suggest that the parental magma was generated from a subduction modified, metasomatized and an enriched mantle source that was subsequently emplaced in a magmatic continental arc setting. The interpreted conductors, observed at shallow depths (∼200-300 m), have been generated by secondary hydrothermal fluid circulation leading to vein formation in the host Padhar Mafic-Ultramafic Complex. The MT and electrical tomography models delineate the geological boundaries of the sulfide-bearing mineralized deposits in the BFB.

Role of fluids on deformation in mid-crustal shear zones, Raft River Mountains, Utah

AbstractFluids are commonly invoked as the primary cause for weakening of detachment shear zones. However, fluid-related mechanisms such as pressure-solution, reaction-enhanced ductility, reaction softening and precipitation of phyllosilicates are not fully understood. Fluid-facilitated reaction and mass transport cause rheological weakening and strain localization, eventually leading to departure from failure laws derived in laboratory experiments. This study focuses on the Miocene Raft River detachment shear zone in northwestern Utah. The shear zone is localized in the Proterozoic Elba Quartzite, which unconformably overlies the Archaean basement, and consists of an alternating sequence of quartzite and muscovite-quartzite schist. In this study, we characterize fluid-related microstructures to constrain conditions that promoted brittle failure in a plastically deforming shear zone. Thin-section analyses reveal the presence of healed microcracks, transgranular fluid inclusion planes and grain boundary fluid inclusion clusters. Healed microcracks occur in three sets, one sub-perpendicular to the mylonitic foliation, and a set of two conjugate microcracks oriented at ∼40–60° to the mylonitic foliation. Healed microfractures are filled with quartz, which has a distinct fabric, suggesting that microcracks healed while the shear zone was still at conditions favourable for quartz crystal plasticity. Transgranular fluid inclusion planes also occur in three sets, similar in orientation to the healed microfractures. Fluid inclusions commonly decorate grain and subgrain boundaries as inter- and intragranular clusters. Our results document ductile overprint of brittle microstructures, suggesting that, during exhumation, the Raft River detachment shear zone crossed the brittle–ductile transition repeatedly, providing pathways for fluids to permeate through this shear zone.

Metal recycling tracked by mercury and helium isotopes in platinum–palladium nuggets from Córrego Bom Sucesso, Brazil

The enigmatic botryoidal nuggets of platinum (Pt) and palladium (Pd) from Córrego Bom Sucesso in the southern Serra do Espinhaço, Minas Gerais, Brazil, are considered to have formed during supergene alteration of placer deposits. This requires hitherto unidentified precious-metal enrichment processes and is inconsistent with Pt–Os age of 180 Ma that entails formation at depth, as recently proposed. Here we report the first mercury (Hg) and helium (He) isotopic determinations of Pt–Pd nuggets. Mercury isotopic compositions have a mass-independent fractionation (MIF) signature with an odd-mass deficit (Δ199Hg = − 0.22 ± 0.04; 1SD, n = 15), which requires aqueous photochemical reduction of Hg (II). Extremely low 3He/4He (<0.001 Ra) and extremely high concentrations of He (up to 1.9 × 1017 at/g) are indicative of nugget formation from He-enriched fluids within the quartzite sequence of the Espinhaço basin, not from meteoric surface water. The data are consistent with a nugget-forming setting in the deep biosphere, as a result of groundwater interaction with Pt–Pd–Hg minerals in Pan-African-Brasiliano post-orogenic veins. We propose that the negative Hg-MIF signature was inherited from the vein minerals that originally acquired their Hg from Earth's surface during the intracratonic sedimentation of the Proterozoic Espinhaço basin.

Comparative geology and metamorphic evolution of the Luswishi Dome, Copperbelt, Zambia: Implications for exploration targeting

The Luswishi Dome forms part of the Neoproterozoic Lufilian Arc which hosts copper-cobalt ores of the Central African Copperbelt. We report new data from a study that combines observations from drill core logging, petrographic and chemical analyses, geothermobarometry and phase equilibria modelling. Drill core logging identifies lithostratigraphic units which are partially correlated with the Katanga Supergroup based on the occurrence of regional markers. The Nguba Group constrained by the presence of glacial tillite includes variably deformed, carbonaceous shales, and carbonates. An underlying sequence of quartzites, metapelites, calc-silicates, meta-sandstones, shales, dolomites and gabbros is partially correlated with the Roan Group. Overprinting relationships of structures in drill core provide evidence for possible polyphase deformation. Early deformation generated folds which were overprinted by faults and later by brecciation caused by hydrofracturing during later deformation, and subsequently metasomatism. Petrographic examinations reveal porphyroblast-inclusion-matrix relationships which indicate that the area experienced at least two penetrative ductile deformation events, D1 and D2, which are in turn overprinted by brittle structures at D3 and later phases. Thermobarometry calculations identify peak metamorphic conditions of 580–600 °C with maximum pressures of ~7 ± 1 kbar. Integrating thermobarometry with pseudosection modelling allows reconstruction of clockwise P-T-d paths consistent with consumption of epidote, chlorite, muscovite, quartz and low Ca-plagioclase to form biotite, garnet, high Ca-plagioclase and aluminosilicates. These observations suggest Barrovian intermediate P/T-type metamorphism which can be attributed to a tectonic scenario of burial under geothermal gradients of ~20–25 °C/km prior to uplift and erosion. These data complement previous models of regional metamorphism by crustal thickening during the Lufilian orogeny.

Geochemistry of the Palaeoproterozoic quartzites of Lower Cuddapah Supergroup, South India: Implications for the palaeoweathering, provenance, and crustal evolution

The lower part of the Proterozoic Cuddapah Basin exposes three quartzite sequences, namely, Gulcheru, Pulivendla, and Gandikota formations. Gulcheru Quartzites show angular to subangular texture, whereas Pulivendla and Gandikota quartzites contain rounded to subrounded detrital grains. Mineralogically, all quartzites are predominantly composed of quartz with subordinate amount of feldspar, mica, and heavy minerals. Clay and silt are present as a matrix while silica and carbonate act as a cementing material. Based on detrital modes and major element geochemistry, these sediments are classified as arkose to subarkose. The petrological and geochemical attributes of the basal formations of the Cuddapah quartzite indicate cyclic weathering under variable humid subtropical climate. The CIA values of these quartzites suggest moderate to intense chemical weathering for the Gulcheru and Gandikota quartzites and low to moderate chemical weathering for Pulivendla Quartzites. The major element ratios (SiO2/Al2O, K2O/Na2O), trace element ratios, chondrite‐normalised REE patterns and UCC normalised multi element diagrams suggest that these quartzites were predominantly derived from the felsic sources, with subordinate contribution from mafic sources. Provenance studies and mixing models, in conjunction with palaeocurrent data suggest that these quartzites of the Lower Cuddapah Supergroup received sediments from various sources including Eastern Dharwar Craton as well as from more juvenile crust.

Geochronology of Hydrothermal Processes Leading to the Formation of the Au–U Mineralization at the Rompas Prospect, Peräpohja Belt, Northern Finland: Application of Paired U–Pb Dating of Uraninite and Re–Os Dating of Molybdenite to the Identification of Multiple Hydrothermal Events in a Metamorphic Terrane

The Peräpohja belt comprises a greenschist to amphibolite facies; multiply-folded supracrustal sequence of quartzites; mafic volcanics; carbonate rocks; black shales; mica schists and greywackes deposited from ca. 2.44 Ga to 1.92 Ga; during protracted rifting of the Archaean basement. Metamorphism and multiple folding of the basin fill occurred during the Svecofennian orogeny (1.92–1.80 Ga). The Rompas Au–U mineralization is hosted within deformed and metamorphosed calcsilicate veins in mafic volcanics. Textural evidence suggests that deposition and periods of uraninite re-mobilization were followed by localized hydrocarbon-bearing fluid flow which produced pyrobitumen crusts around grains of uraninite. Gold precipitated during the latest hydrothermal event at around 1.75 Ga. In situ U–Pb dating of uraninite by laser ablation inductively coupled mass spectroscopy (LA-ICP-MS), and Re–Os dating of molybdenite, indicate that primary hydrothermal uranium mineralization forms two age clusters; about 2.03–2.01 and 1.95–1.94 Ga. Resetting of the U–Pb system and precipitation of new generations of uraninite are associated with major deformation and metamorphic stages of the Svecofennian orogeny at 1.91–1.89 Ga, 1.85 Ga, and 1.80 Ga. Gold deposition was synchronous with the emplacement of the 1.75–1.78 Ga late/post-orogenic granitoids. The gold-producing hydrothermal event is also recorded by Re–Os dating of molybdenite from the gold-bearing Mg-metasomatized metasedimentary and metavolcanic units at the Palokas prospect; a few kilometres from Rompas. Results of this study confirm that some domains in the structure of uraninite may preserve the original crystallization age, despite an overprinting amphibolite facies metamorphic and other hydrothermal events. The study supports the utility of in situ U–Pb dating of uraninite and the ability of Re–Os dating to assist in sorting out different hydrothermal events in areas with complex tectonic; magmatic and metamorphic histories.

Lithostratigraphy of the Mesoproterozoic Wilgenhoutsdrif Group

The Mesoproterozoic metamorphic formations of the Wilgenhoutsdrif Group underlie a large area around the Orange River east of Upington. The quartzitic Grootdrink Formation occurs at the base of the sequence and contacts with older units are either unconformable or along thrusts. The Zonderhuis Formation is a sequence of low-grade pelitic phyllites and impure quartzites with subordinate lenses of metavolcanic rocks and serpentinite. The Leerkrans Formation is thought to follow conformably and consists of deformed basaltic and quartz-porphyritic lavas and volcanogenic metasediments. The entire group is isoclinally folded and foliated, with greenschist facies metamorphic parageneses, and is altered by carbonate veins and epidotisation. The age of felsic volcanic rocks is reliably established at 1289 or 1293 Ma with 2σ errors of 9 Ma. The Wilgenhoutsdrif Group now forms part of the Kaaien Terrane of the Namaqua-Natal Province. It may have formed in a continental back-arc basin at about the same time as the arc-related volcanic rocks of the Areachap Group formed by subduction of an ocean basin to the west, prior to ~1.2 Ga Namaqua terrane assembly by collisions with the Kaapvaal-Rehoboth Craton.

Origin and evolution of the ∼1.9 Ga Richtersveld Magmatic Arc, SW Africa

The results of a study of the Paleoproterozoic Richtersveld Subprovince/Magmatic Arc (RMA) in southern Namibia are integrated with previous work in adjacent NW South Africa to produce a new, unified model for the evolution of the RMA. The RMA was variably affected by the ∼1Ga Namaqua. In the west, low-grade metamorphic rocks (Vioolsdrif Domain) record a limited Namaqua imprint whereas in the east, the equivalent rocks were converted to amphibolite-facies gneisses (Pella Domain). Tectonic slivers of RMA rocks are also found in an imbricate thrust zone that forms the base of the over-riding Namaqua-age Kakamas Domain further east. The volcanic and equivalent plutonic rocks (Orange River Group (ORG) and Vioolsdrif Suite respectively) of the RMA form a coeval calc-alkaline magma series with characteristic island-arc geochemical signatures. The entire magmatic history took place between 1910 and 1865Ma, contrary to the much longer time-spans previously suggested. A large inclusion of migmatite (Bankwasser Migmatite) gave Archaean detrital zircon ages. It is interpreted as a possible fragment of Archaean Kaapvaal Craton basement that was incorporated into the RMA prior to ∼1885Ma. ∼2Ga detrital zircon ages in the ORG and ∼2Ga magmatic zircon inheritance and xenoliths entrained in the Vioolsdrif Suite reveal a hitherto unrecognised pre-RMA history. Recent work has shown that a pre-RMA arc (Sperrgebiet Arc) had developed to the west at ∼2020Ma in an upfaulted block within the Neoproterozoic Gariep Orogen. Consequently, a two-arc model is proposed for the early evolution of western Namaqualand with the early Sperrgebiet Arc evolving at ∼2020Ma which, possibly by processes of subduction migration, was cannibalised by the RMA at ∼1885Ma. Our data also suggests that certain quartzite sequences, traditionally placed within the ORG, were deposited after cessation of RMA activity and are thus unrelated to it.

Optimizing a Knowledge-driven Prospectivity Model for Gold Deposits Within Peräpohja Belt, Northern Finland

This paper combines knowledge- and data-driven prospectivity mapping approaches by using the receiver operating characteristics (ROC) spatial statistical technique to optimize the process of rescaling input datasets and the process of data integration when using a fuzzy logic prospectivity mapping method. The methodology is tested in an active mineral exploration terrain within the Paleoproterozoic Perapohja Belt (PB) in the Northern Fennoscandian Shield, Finland. The PB comprises a greenschist to amphibolite facies, complexly deformed supracrustal sequence of variable quartzites, mafic volcanic rocks and volcaniclastic rocks, carbonate rocks, black shales, mica schists and graywackes. These formations were deposited on Archean basement and 2.44 Ga layered intrusions, during the multiple rifting of the Archean basement (2.44–1.92 Ga). Younger intrusive units in the PB comprise 2.20–2.13 Ga gabbroic sills or dikes and 1.98 Ga A-type granites. Metamorphism and complex deformation of the PB took place during the Svecofennian orogeny (1.9–1.8 Ga) and were followed by intrusions of post-orogenic granitoids (1.81–1.77 Ga). The recent mineral exploration activities have indicated several gold-bearing mineral occurrences within the PB. The Rompas Au-U mineralization is hosted within deformed and metamorphosed calc-silicate veins enclosed within mafic volcanic rocks and contains uranium-bearing zones without gold and very high-grade (>10,000 g/t Au) gold pockets with uraninite and uraninite-pyrobitumen nodules. In the vicinity of the Rompas, a magnesium skarn hosted disseminated-stockwork gold mineralization was also recognized at the Palokas-Rajapalot prospect. The exploration criteria translated into a fuzzy logic prospectivity model included data derived from regional till geochemistry (Fe, Cu, Co, Ni, Au, Te, K), high-resolution airborne geophysics (magnetic field total intensity, electromagnetic, gamma radiation), ground gravity and regional bedrock map (structures). The current exploration licenses and exploration drilling sites for gold were used to validate the knowledge-driven mineral prospectivity model.

Association of gold with uraninite and pyrobitumen in the metavolcanic rock hosted hydrothermal Au-U mineralisation at Rompas, Peräpohja Schist Belt, northern Finland

The Perapohja Schist Belt comprises a supracrustal sequence of quartzites, mafic volcanics and volcaniclastics, carbonate rocks, black shales, mica schists and greywackes which were deposited from ca. 2.44 to ~1.91 Ga, during the rifting of the Archaean basement in the eastern part of the Fennoscandian shield. Metamorphism and multiple folding of the basin fill took place during the Svecofennian orogeny (1.9–1.8 Ga) followed by intrusions of late-orogenic (1.84–1.80 Ga) and post-orogenic granitoids (1.79–1.76 Ga). The Rompas Au-U mineralisation is hosted by deformed calcsilicate veins in mafic volcanic rocks and locally contains very high grade (>10,000 g/t Au) gold pockets with strict spatial association of gold minerals to uraninite and pyrobitumen. Chemical ages from the unaltered domains in the structure of uraninite indicate a 1.95–1.90 Ga age for the deposition of the primary, high temperature (e.g. U/Th < 100 in uraninite) hydrothermal uranium mineralisation. These data are in agreement with the results of previous U-Pb dating of uraninite by SIMS. Textural evidence suggests that metamorphic recrystallisation of the uraninite-bearing quartz-dolomite veins into calcsilicate mineral assemblages during the Svecofennian orogeny (1.9–1.8 Ga) was followed by a hydrocarbon-bearing fluid flow event and radiolytic polymerisation of hydrocarbons around grains of uraninite. Gold precipitated during a subsequent hydrothermal process in the fractures of uraninite, as well as in the cracks and on the botryoidal surfaces of uraninite-pyrobitumen nodules. Remobilisation and redeposition of uranium by these hydrothermal events produced secondary uraninite grains with chemical ages between 1.85 and 1.65 Ga. Native gold is associated with galena, altaite, hunchunite, nickeline and rare cobaltite, Pb-bearing maldonite, pyrite, pyrrhotite, chalcopyrite, molybdenite and titanite. Raman spectra show disordered structure of undeformed pyrobitumen nodules in contrast with the well-ordered graphite in calcsilicate veins. Mean random reflectance data for pyrobitumen indicate 270–340 °C maximum temperature of thermal maturation—this temperature range is also considered as the temperature of gold deposition. Results of multiple sulphur isotope analyses of organic material-, pyrite- and acid-volatile-bound sulphur show distinct ranges of δ34S values for SORG and SCRS in uraninite-pyrobitumen (from −6.99 to −3.55‰ and from −10.02 to −4.41‰, respectively) and uraninite-pyrobitumen-native gold mineral associations (from +1.36 to +6.87‰ and from +0.42 to +9.7‰, respectively). Δ33S data indicate local occurrence of nonmass-dependent sulphur isotope fractionation owing to interaction of fluids with organic material. Concentration of lead in uraninite is depleted along the gold mineral filled fractures whereas the uranogenic lead isotope contents of galena, altaite and hunchuite deposited in the same fractures are extremely high, suggesting that the dominant source of lead for the crystallisation of these minerals was the radiogenic lead content of uraninite. Taking into account this source of radiogenic lead, the calculated Pb-Pb model ages for the lead minerals are between 1.75 and 1.70 Ga. Sulphur and tellurium removal from the fluid by reaction with radiogenic lead released by uraninite appears to be an important mechanism in the strongly localised deposition of gold minerals. Scavenging of sulphur by pyrobitumen nodules from gold transporting fluids was an additional process triggering precipitation of gold. Carbon particles and organic functional groups in pyrobitumen probably acted as nucleation and adsorption centres for gold minerals.

The rational mode of nut coke charging into the blast furnace by compact trough-type charging device

The laboratory research on the 1:5 scale physical model of a trough-type bell-less charging device (BLCD) resulted in the rational mode of nut coke charging into the BLCD hopper together with sinter and pellets. This rational mode provides the highest uniformity coefficient of its entering the furnace throat area. The amount of nut coke in the BLCD hopper was at rates of 100, 550 and 1000 kg per 16 tons batch for an actual furnace. In this study, a three-factor Box-Behnken experimental design was used. The following factors have been considered: the sequence of nut coke charging into the hopper, the degree of charge gate opening, and nut coke consumption per feeding. The mathematical description of the output parameter is given by a second-degree polynomial. The significance of each coefficient of the equation resulted from experimental data processing was determined by Student’s t test. The position of nut coke in the centre of the raw material mass in the BLCD hopper resulted in the highest uniformity coefficient and reached 0.85. We investigated the influence of the charging sequence of sinter, quartzite, ore, pellets and nuts into the BLCD hopper on the furnace performance and blast furnace processing during three periods of experienced heats lasting 8–9 days in one of the blast furnaces with a compact trough-type BLCD at Magnitogorsk Iron and Steel Works OJSC (MMK). We evaluated the drainage ability of the hearth on several factors: the amount of slag remaining in the furnace after discharging, the time interval from the start of discharging until the slag occurrence and slag viscosity. In the first period, when nut coke was positioned on the bottom of the BLCD hopper, the amount of slag, remaining in the furnace, was 0.38 of the ladle. When nut coke was charged into the BLCD hopper last (period 2), the amount of slag increased to 0.44 showing the deterioration in the drainage property of the hearth. In the third period, when nut coke was positioned in the centre of the raw material mass, the amount of slag decreased to 0.31 of the ladle that resulted in the improved filtration of melting liquid products through the coke packed bed. The deterioration in the filtering ability of the coke packing in the second period, compared to the first one, was followed by decreased coefficient of the replacement rate of coke by nut coke from 0.64 to 0.59 kg/kg. In the third period, improved penetrability of the coke packing was followed by an increased coefficient of the replacement of coke by nut coke to 0.85 kg/kg. Indicators of the furnace performance intensity according to their types, the drag coefficient of charge by zones and the reduction degree with reducing agents corresponded to the results obtained.

The structure and metamorphism of the Red Point Metamorphic Complex—A newly discovered high-pressure metamorphic complex from the south coast of Tasmania

This study presents new data on the deformational and metamorphic history of previously unstudied Cambrian high-pressure metamorphic rocks exposed on the remote south coast of Tasmania. The Red Point Metamorphic Complex consists of two blocks of high-pressure, medium-grade metamorphic rocks including pelitic schist and minor garnet-bearing amphibolite, which are faulted against a sequence of low-grade phyllite and quartzite. The Red Point Metamorphic Complex records five phases of deformation, all of which except the first are expressed at a mesoscopic scale in both the medium- and low-grade rocks. Peak metamorphic conditions in the high-pressure epidote–amphibolite facies is recorded by medium-grade schist and amphibolite and was synchronous with the second major deformation event, which produced a pervasive schistosity and mesoscale isoclinal folds. The juxtaposition of the low- and medium-grade rocks is interpreted to have first occurred prior to the development of upright, opening folding associated with the third deformation. However, the present contacts between the two contrasting metamorphic sequences formed during widespread faulting and ductile-shear zone development associated with the fourth and fifth deformation events. The new data from the Red Point Metamorphic Complex provide insights into the structural and metamorphic history experienced by the medium-grade rocks of Tasmania during the Cambrian Tyennan Orogeny. This study demonstrates that Cambrian medium-grade metamorphic rocks are more widespread throughout Tasmania than previously realised, which represents an important step towards understanding the large-scale architecture of the Tyennan Orogen.

Syn- and post-tectonic granite plutonism in the Sausar Fold Belt, central India: Age constraints and tectonic implications

Sausar Fold Belt (SFB) in central India forms the southern part of the Central Indian Tectonic Zone (CITZ) – a crustal scale Proterozoic mobile belt dissecting the Indian craton, whose tectonothermal history and age is important for understanding the Proterozoic crustal history of the Indian craton. SFB comprises a gneissic basement (TBG: Tirodi Biotite Gneiss) overlain by a supracrustal sequence of quartzite–pelite–carbonate (SSG: Sausar Group). SSG and TBG are deformed and metamorphosed in greenschist to amphibolite facies. Two phases of granite intrusion are observed in the SSG – a syntectonic foliated granite and a post-tectonic massive granite, with clear structural relationship with the host rocks. Monazite chemical dating (U–Th–total Pb) of the foliated and massive granites yield Neoproterozoic (ca. 945–928Ma) ages that contradict many earlier geochronological interpretations. Foliated granites and the immediately adjacent TBG show monazite grains with ca. 945Ma mean age, interpreted as the timing of D2 deformation and amphibolite facies metamorphism of SSG. The post tectonic granites intruded these rocks around 928Ma, and were largely undeformed. A terminal thermal overprint is found in some monazite grain rims at ca. 785Ma age. The younger Sausar tectonothermal events have overprinted the adjacent high-grade granulites of Ramakona–Katangi Granulite (RKG) belt, and should not be considered as parts of the same tectonothermal event representing different depth sections only.

Provenance of the Passo Feio complex, Dom Feliciano Belt: Implications for the age of supracrustal rocks of the São Gabriel Arc, southern Brazil

The Passo Feio complex (PFC) is a sequence of metapelite, amphibolite, metavolcanic/metavolcanoclastic rocks, marble, calc-silicate rocks, quartzite and magnesium schist. It is part of the São Gabriel Terrane, a Neoproterozoic juvenile arc formed during the early stages of evolution of the Neoproterozoic Dom Feliciano Belt (DFB), in southern Brazil. This belt corresponds to the southernmost portion of the Mantiqueira Province, an important Neoproterozoic orogenic system exposed in the NNE direction along the southeastern coast of Brazil. The geotectonic significance of the original Passo Feio basin in the tectonic evolution of São Gabriel Terrene is not well understood. It has been considered as part of a passive margin sequence or as a back-arc sequence. Geochronological and isotopic data are very scarse for the Passo Feio rocks and this has hampered the better understanding of its significance in the Neoproterozoic tectonic evolution of southern Brazil. In the present study the age and significance of metasedimentary rocks of the PFC were investigated. The provenance study was carried out in four metapelite samples from the southern and northern Passo Feio complex, using U-Pb dating of detrital zircon. The results showed varied provenance patterns and zircon ages range from 3637 to 803 Ma. Paleo- and Mesoproterozoic detrital zircon grains are present in all samples, but an important Neoproterozoic population has been identified in one of them. 3.5 Ga old zircon grains form the oldest population of detrital zircon ever reported in southern Brazil. Paleoproterozoic/Archean terranes within the Rio de la Plata Craton may represent the main source of detrital sediments and this suggests that part of the Passo Feio complex might have been a passive margin sequence, developed along the northeastern margin of that paleocontinent. However, the presence of a Neoproterozoic zircon population is not consistent with derivation solely from the craton and indicates contribution from younger sources, such as the Neoproterozoic São Gabriel Arc itself. This Neoproterozoic zircon population with ages ranging from 948 to 803 Ma, suggests that the onset of the magmatic/tectonic evolution of the São Gabriel Arc might have started as early ca. 0.95 Ma.

Depositional age and sediment source of the auriferous Moeda Formation, Quadrilátero Ferrífero of Minas Gerais, Brazil: New constraints from U–Pb–Hf isotopes in zircon and xenotime

The Moeda Formation is a metasedimentary clastic sequence of Witwatersrand-like metaconglomerate and quartzite at the base of the Minas Supergroup, Quadrilátero Ferrífero of Minas Gerais, Brazil, which is also known for its world-class iron-ore deposits. The Moeda Formation can be subdivided into three units (Units I to III from bottom to top) with each unit consisting of metaconglomerates at their base, mostly auriferous, grading into quartzite and locally into phyllite. This study presents the first combined U–Pb and Lu–Hf isotope data from detrital zircons and xenotimes of auriferous metaconglomerates and quartzites from Units II and III. These data indicate that deposition of sediments occurred ≤2.68Ga for Unit II and ≤2.62Ga for Unit III, as supported by the youngest magmatic zircon grains. The data further provide evidence that the source area of sediments was affected by crust re-working during two major magmatic-metamorphic events at 2.89–2.87 and 2.72–2.69Ga, and minor events between 3.3 and 3.0 and at 2.80 and 2.65Ga. This interpretation is supported by mainly subchondritic ɛHft between −10 and +1.4, and large ɛHft variations including all magmatic events. A few superchondritic ɛHft obtained for the magmatic events at 3.3, 2.9, and 2.7Ga, point to limited juvenile crust addition. The new dataset indicates that the clastic sediments of the auriferous Moeda Formation were mainly derived from an evolved continental crust in contrast to primitive magmatic arc sources, which are suggested for the Mesoarchaean auriferous sediments of the Witwatersrand Basin of South Africa, and those of the eastern Goldfield province of Australia.

Neoarchean continental crust formation and Paleoproterozoic deformation of the central Rae craton, Committee Bay belt, Nunavut

Integrated mapping, structural analysis, and U–Pb geochronology of the Committee Bay area, Nunavut, establish a record of Neoarchean crustal growth followed by penetrative Paleoproterozoic deformation. Supracrustal rocks include a lower ca. 2.73 Ga mafic–ultramafic volcanic-dominated sequence, a middle, economically significant 2.71 Ga intermediate volcanic-bearing sequence with intercalated sulphidized, gold-bearing iron formation, and an upper &lt;2.69 Ga clastic ± komatiite–quartzite sequence. Following an 80 million year hiatus, this succession was intruded by voluminous ca. 2.61–2.57 Ga granodiorite–tonalite–granite ± diorite, which do not appear to have thermally or tectonically affected the supracrustal belt. Instead, three generations of structures record polyphase Paleoproterozoic deformation of the region. D1 structures are consistent with a doubly vergent structural fan developed at ca. 2.35 Ga in response to the Arrowsmith orogeny that affected the western Rae margin. Penetrative D2 structures dominate the map pattern and include northeast-trending, southeast-dipping folds and fabrics within which gold is localized. The general southeast dips of S2 and inclined, northwest-vergent attitude of F2 reflect northwest-directed shortening at 1.84–1.82 Ga. The absence of syn-D2 plutonic rocks in the west and central Committee Bay belt support amphibolite-facies metamorphism as a response to crustal thickening, which, in turn, led to syn-D2 crustal melting in the east. Regionally extensive upright to northwest-vergent D2 structures and associated ca. 1.85–1.82 Ga tectonometamorphism across the Rae craton are attributed to an early stage of the Hudsonian orogeny involving microcontinent collision(s) with its southeastern margin. D3 folds and dextral shearing at ca. 1780 Ma accommodated localized, late-stage compressional strain during final amalgamation of Laurentia.

Early Ordovician to Silurian evolution of exotic terranes in the Scandinavian Caledonides of the Ofoten–Troms area – terrane characterization and correlation based on new U–Pb zircon ages and Lu–Hf isotopic data

Abstract Isotope dilution thermal ionization mass spectrometry U–Pb dating and coupled Lu–Hf solution inductively coupled plasma mass spectrometry analyses of zircon were acquired from magmatic rocks along two transects across the Scandinavian Caledonides in the Troms–Ofoten region of Norway to explore possible correlations and gain insight into the evolution of far-travelled nappes within the Upper and Uppermost Allochthons. One pulse of magmatic activity was recorded at c. 489 Ma in the Tromsø Nappe. In the underlying Nakkedal Nappe, a magmatic pulse was recorded at c. 450 Ma, being contemporaneous with eclogite facies metamorphism in the area. Tonalites in the structurally underlying Lyngen and Gratangseidet ophiolitic complexes, both forming the substratum to carbonate–schist–quartzite sequences (Balsfjord and Evenes groups, respectively), yielded ages of 481 and 474 Ma. Obtained ε Hf(t) values are, however, distinctly different and indicate a juvenile origin for the Gratangseidet tonalite (ε Hf(474) =+9.57) and the presence of Palaeoproterozoic source material for the Lyngen tonalite (ε Hf(481) =−16.8 to −2.3). The 474 Ma age of the Snaufjellet granite intruding the Bogen Group structurally above the Evenes Group requires a thrust between the two units. An age of 435 Ma on the Heia gabbro of the Nordmannvik Nappe is comparable to that of the Råna norite (437 Ma) in the Narvik area, supporting a correlation with the Narvik Nappe Complex. The youngest intrusion dated to 425 Ma is a leucogranite that intruded the Balsfjord Group. The Lu–Hf data indicate a common source for several of the magmatic rocks in the Uppermost Allochthon, as well as Laurentian arc granitoids from East Greenland and possibly from Newfoundland and related areas.

Ultrahigh‐pressure metamorphism at the Lanterman Range (northern Victoria Land, Antarctica)

AbstractNorthern Victoria Land is a key area for the Ross Orogen—a Palaeozoic fold belt formed at the palaeo‐Pacific margin of Gondwana and also known as the Delamerian and Tyennan Orogeny in southern Australia, and Tasmania, respectively. A narrow and discontinuous high‐ to ultrahigh‐pressure (HP/UHP) belt, consisting of mafic and ultramafic rocks (including garnet‐bearing types) within a metasedimentary sequence of gneisses and quartzites, is exposed at the Lanterman Range (northern Victoria Land), at the boundary between two main lithotectonic terranes (the Wilson and Bowers terranes). Mesostructural and microstructural relations between eclogitic boudins and country gneisses are, in some areas, characterized by interlayering with sharp contacts on a cm scale. Geological, petrological and geochronological studies indicate that mafic, ultramafic and felsic host rocks underwent a common metamorphic evolution with an UHP eclogite–facies stage ≈ 500 Ma ago at temperatures of up to ≈ 850°C and pressures up to 3.3 GPa. The retrograde P–T path is similar and, indeed, it overlaps in every lithology; it is a nearly isothermal path from UHP conditions up to deep crustal levels, and becomes a cooling–unloading path from intermediate to shallow levels. The discovery of UHP rocks at the Lanterman Range yields eclogite–facies rocks of this area among the rare UHP rocks of the world and represents the only known locality in Antarctica and along the entire palaeo‐Pacific margin of Gondwana during the Ross–Delamerian–Tyennan Orogen. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Satellite based observations on the deformation pattern in parts of Delhi fold belt, Jaipur, Rajasthan

rocks. This has led to events which were associated withdifferent types of deformation and polyphase metamorphismof the rock sequences (Sinha-Roy et al. 1998).Thecompressional tectonic regime operated in the southern partof Bundelkhand granite gneissic complex (BGC) andAravalli fold belt was complemented by a tensional regimeresponsible in creation of linear basin in Proterozoic period,known as Delhi Basin. The fold belt developed during theclosing of Delhi Basin shows spatial variation in geologiccontent, tectonic pattern and probably also in the timing ofdevelopment. Delhi fold belt therefore can be subdividedinto two parts – North Delhi fold belt (NDFB) and SouthDelhi fold belt (SDFB) (Sinha-Roy et al. 1998). The studyarea belong to Bayna-Lalsot sub-basin, located 40 kmsouthwest of Bharatpur, forms the eastern limit of theoutcrops of the NDFB which is characterized by severalfossil grabens, horsts, distributed broadly in three mainsedimentary sub-basins (Singh,1984). From east to west,these are the Bayna-Lalsot, the Alwar, and the Khetrisub-basin.The rocks belong to the Bayna-Lalsot sub-basin aremostly metasediments such as quartzite, phyllite etc. A threetier classification of Delhi Supergroup of rocks in theBayna-Lalsot Basin is proposed (Singh, 1982). Pre-Delhirocks of this sub basin is exposed at the southern mar gin ofthe basin and are mainly schist, phyllite, altered basic rocks.The metasedimentary rocks of Alwar and Ajabgarh Groupsare exposed within the study area. Alwar Group constitutesthe sequence of conglomerate, quartzite and minor shalewhereas Ajabgarh rocks are mainly carbonaceous shale,phyllite and ferruginous quartzite.