Palaeoproterozoic Times Research Articles

An extended Neoarchaean to Neoproterozoic history of the Sandmata Complex (Aravalli Craton, northwestern India): Insights from metamorphic evolution and zircon-monazite geochronology of high-grade quartzofeldspathic gneisses

The Sandmata Complex in the Aravalli Craton of northwestern India experienced syn- to post-collisional high-grade metamorphism and migmatization related to the 1.9–1.7 Ga Aravalli orogeny. Garnet-bearing quartzofeldspathic gneisses are an important rock type in the Sandmata Complex. Their prograde assemblage is represented by garnet cores and inclusion minerals Bt + Pl + Qz, whereas the assemblage Pl + Kfs + Qz + Btmatrix + Sil + Ilm, along with chemically homogeneous garnet porphyroblasts, represents peak metamorphism. Post-peak retrograde metamorphism is reflected in the growth of muscovite, chlorite and biotite, and diffusion adjustments in garnet rims. Mineral paragenesis, mineral chemistry, conventional thermobarometry, and phase equilibrium modelling indicate P–T conditions of 6.7–10.2 kbar at 725–830 °C for peak metamorphism, and 4.8–6.9 kbar at 450–610 °C for post-peak retrogression. The P–T conditions determined from the quartzofeldspathic gneisses define a near-isobaric cooling P–T path. In contrast, previously studied high-grade pelitic gneisses and metagranitoids from the Sandmata Complex define an isothermal decompression P–T path. U–Pb dating of magmatic zircon cores in the quartzofeldspathic gneisses provides the crystallization age of their felsic protoliths as ∼ 2.52 Ga. Zircon and monazite geochronology show that peak metamorphism occurred at ∼ 1.90–1.78 Ga and thus shortly preceded the emplacement of the Gyangarh-Asind igneous suite and Anjana granite, previously dated at ∼ 1.72–1.64 Ga. A model of oceanic crust subduction followed by continent–continent collision during the Aravalli orogeny can explain granulite facies metamorphism in the Sandmata Complex. The combined results of this and previous studies support the operation of modern-style plate tectonic processes involving subduction and continental collision during Palaeoproterozoic time. We also identify a younger (∼0.90–0.78 Ga) metamorphic episode, preserved mostly in recrystallized monazite grains elongated parallel to the final deformation fabric, likely related to the South Delhi orogeny.

Petrogenesis and Geodynamic Significance of Xenolithic Eclogites

Kimberlite-borne xenolithic eclogites, typically occurring in or near cratons, have long been recognized as remnants of Precambrian subducted oceanic crust that have undergone partial melting to yield granitoids similar to the Archean continental crust. While some eclogitized oceanic crust was emplaced into cratonic lithospheres, the majority was deeply subducted to form lithologic and geochemical heterogeneities in the convecting mantle. If we accept that most xenolithic eclogites originally formed at Earth's surface, then their geodynamic significance encompasses four tectonic environments: (a) spreading ridges, where precursors formed by partial melting of convecting mantle and subsequent melt differentiation; (b) subduction zones, where oceanic crust was metamorphosed and interacted with other slab lithologies; (c) the cratonic mantle lithosphere, where the eclogite source was variably modified subsequent to emplacement in Mesoarchean to Paleoproterozoic time; and (d) the convecting mantle, into which the vast majority of subduction-modified oceanic crust not captured in the cratonic lithosphere was recycled. ▪Xenolithic eclogites are fragments of ca. 3.0–1.8 Ga oceanic crust and signal robust subduction tectonics from the Mesoarchean.▪Multiple constraints indicate an origin as variably differentiated oceanic crust, followed by subduction metamorphism, and prolonged mantle residence. ▪Xenolithic eclogites thus permit investigation of deep geochemical cycles related to recycling of Precambrian oceanic crust.▪They help constrain asthenosphere thermal plus redox evolution and contribute to cratonic physical properties and mineral endowments.

Timing of shear deformation in the Singhbhum Shear Zone, India: implications for shear zone-hosted polymetallic mineralization

AbstractThe Singhbhum Shear Zone in eastern India hosts several Fe oxide–Cu–Au (IOCG)-type polymetallic deposits, mined primarily for U, Cu and apatite, with elevated concentrations of rare earth elements, Ni, Co, Mo, Te and Au in association with low-Ti magnetite. Although the main stages of hydrothermal U, Cu and rare earth element mineralization are known to be Palaeoproterozoic in age, the age of shear deformation in the host shear zone has hitherto not been constrained. Here, we report Re–Os ages of syn-shearing massive molybdenite occurring along shear surfaces transecting the uranium ores in the Jaduguda uranium deposit. Integrating the obtained Re–Os age of c. 1.64–1.59 Ga of molybdenite, the known ages of mineralization and the known tectonothermal events in the adjoining Proterozoic Mobile Belt, we propose that the main stages of polymetallic hydrothermal mineralization pre-dated the pervasive shear deformation event in the Singhbhum Shear Zone. We further suggest that the shear zone was not the principal foci of the hydrothermal mineralization of the main stages. Instead, the shear zone was localized during the Palaeoproterozoic to Mesoproterozoic transition (c. 1.64–1.59 Ga) along pre-existing crustal-scale extensional faults which had earlier been the foci of hydrothermal alteration and mineralization in Palaeoproterozoic time (c. 1.9–1.8 Ga). Shear deformation and metamorphism have reconstituted/redistributed existing mineral/metal inventories with/without neo-mineralization.

Lead Isotopes and the Sources of Granitic Magmas: The Sveconorwegian Granite and Pegmatite Province of Southern Norway

Lead isotope analyses of K-feldspar from late Sveconorwegian (900–1000 Ma) granitic pegmatites and A-type, ferroan granitic intrusions in four different areas of southern Norway analyzed by laser-ablation inductively coupled plasma source mass spectrometry (LA-ICPMS) give compositions in the range 206Pb/204Pb = 16.637 to 17.555, 207Pb/204Pb = 15.445 to 15.534, 208Pb/204Pb = 36.317 to 37.459. These compositions broadly overlap with the initial compositions estimated from previously published solution TIMS whole-rock and feldspar Pb isotope analyses of late Sveconorwegian granitic plutons across the region, suggesting that magmas forming A-type granite plutons and granitic pegmatites have been derived from broadly similar source rocks, i.e., from a continental crust that initially formed in Palaeoproterozoic time (ca. 2.10–1.86 Ga), and subsequently underwent intracrustal partial melting, differentiation and rejuvenation via mafic underplating in Mesoproterozoic time.

Deep continental roots and cratons.

The formation and preservation of cratons-the oldest parts of the continents, comprising over 60 per cent of the continental landmass-remains an enduring problem. Key to craton development is how and when the thick strong mantle roots that underlie these regions formed and evolved. Peridotite melting residues forming cratonic lithospheric roots mostly originated via relatively low-pressure melting and were subsequently transported to greater depth by thickening produced by lateral accretion and compression. The longest-lived cratons wereassembled during Mesoarchean and Palaeoproterozoic times, creating the stable mantle roots 150 to 250 kilometres thick that are critical to preserving Earth's early continents and central to defining the cratons, although we extend the definition of cratons to include extensive regions of long-stable Mesoproterozoic crust also underpinned by thick lithospheric roots. The production of widespread thick and strong lithosphere via the process of orogenic thickening, possibly in several cycles, was fundamental to the eventual emergence of extensive continental landmasses-the cratons.

Geochronology and geochemistry ofca. 2.48 Ga granitoid gneisses from theYudongzi Complexin the north‐westernYangtze Block,China

The Early Precambrian magmatism in the Archean–Palaeoproterozoic basement Yudongzi Complex is critical for understanding the early crustal formation and evolutionary history of the Yangtze Block in the South China Craton. In this paper, combined zircon UPb geochronological, zircon Hf isotopic, and whole‐rock geochemical studies are carried out on the granitoid gneisses from the Yudongzi Complex, north‐western Yangtze Block. The magmatic zircons from the granitoid gneiss yield a UPb upper intercept age of 2,477±18 Ma, interpreted as the crystallization age of the granitoid gneiss. The granitoid gneiss samples are characterized by high Al2O3contents, moderate to high Sr/Y, (La/Yb)Nand Na2O/K2O ratios, no obvious Eu and Sr anomalies, and depleted Nb, Ta, and Ti values that are similar to those of Archean trondhjemite–tonalite–granodiorite (TTG) suites, indicating that the investigated rocks belonging to TTG‐series rocks. They display pronounced enriched‐LREE patterns with high La/Yb values and relatively high Zr/Sm and Eu/Eu* but low Nb/Ta and Dy/Yb ratios, implying that these rocks were formed by partial melting within amphibolite‐facies conditions with mainly residual amphibole, garnet, and accessory rutile in the magma source. These rocks also display apparently low MgO, Mg#, Ni, and Cr contents, suggesting derivation from partial melting of thickened ancient lower crust. Zircon εHf(t) values from the granitoid gneiss sample vary from −10.1 to −6.9 with TDM2ofca. 3.59–3.40 Ga, what demonstrates their crustal source nature as well. Thus, taking all these factors together, we propose that theca. 2.48 Ga granitoid gneisses in the Yudongzi Complex might have been produced by partial melting of ancient crust material under amphibolite‐facies conditions, with primarily residual assemblages of amphibole, garnet, and attached rutile. Taking together our new data and such from previous studies, the Yangtze Block has been probably involved into a significant reworking of ancient crust event during the Early Palaeoproterozoic time.

When Proterozoic Crusts Became Thick: New Insights from Magma Petrology

The Earth’s continental crust represents the outermost envelope of the solid Earth, controlling exchanges within the geosphere and reflecting geodynamics processes. One of the fundamental issues of Earth Science aims to determine crustal thickness in past geodynamic environments in order to discuss the evolution of certain geodynamic processes through time. Despite presenting a continuing challenge, the evolution of crustal thickness during the last 3 billion years can be investigated using indirect clues yielded by the chemical signature of mafic magmas and associated ferromagnesian minerals (pyroxene, amphibole). Here, we present a new statistical assessment of a global database of magmatic and mineral chemical information. Analysis reveals the increasing occurrence of high-temperature pyroxenes and amphiboles growing in Ca-rich, Fe-poor magma since ~1 Ga, which contrasts with lower temperature conditions of minerals crystallization throughout the Meso- and Palaeoproterozoic times. This is interpreted to reflect temporal changes in the control of Earth’s crust on mantle-derived magma composition, related to changes in lithospheric thickness and mantle secular cooling. We propose that thick existing crust is associated with deeper, hotter magmatic reservoirs, potentially elucidating the mineral chemistry and the contrasting iron content between primary and derivative mafic magmas. Based on both the chemical and mineral information of mafic magma, an integrated approach provides qualitative estimates of past crustal thickness and associated magmatic systems. Our findings indicate that the Proterozoic was characterized by thicker crustal sections (>40–50 km) relative to the Phanerozoic and Archean (<35 km). This period of crustal thickening appears at the confluence of major changes on Earth, marked by the onset of mantle cooling and Plate Tectonics and the assembly of Columbia, the first supercontinent.

Primary Sedimentary Structures and MISS in Gulcheru Quartzite along SW Part of Cuddapah Basin

ABSTRACT Sedimentary structures are very useful in paleocurrent analysis and interpretation of siliciclastic shallow marine environment. These interesting sedimentary structures such as parallel bedding, cross bedding, ripples, and mud cracks as well as synaeresis cracks, are best studied in the field. They are formed by a variety of sedimentary processes, including fluid flow, sediment-gravity flow, soft-sediment deformation and biogenic activity. Gulcheru Formation has evidence of interaction of microbial communities with clastic sedimentation during Palaeoproterozoic time. Because of high porosity-permeability of siliciclastics of Gulcheru Formation, palaeontologists do not expect many fossils preserved in such rocks and thus they have been overlooked. Microbially induced sedimentary structures (MISS) are highlighted in the Gulcheru Formation. They are indicative of shallow marine environment. Gulcheru Formation shows alluvial fan to shallow marine shelf environments within a limited thickness of about ∼ 110m, which indicates an alluvial plain coast where alluvial deposits are modified by wave forces.

The Palaeoproterozoic global carbon cycle: insights from the Loch Maree Group, NW Scotland

Two Palaeoproterozoic events have particularly interested Earth scientists. These are the global Lomagundi–Jatuli Event, the greatest magnitude positive carbonate carbon isotope excursion in Earth history, and the Shunga Event, the world’s largest organic carbon burial event. Analysis of newly acquired high-resolution C–O isotope data and U–Pb zircon geochronology refine understanding of carbon isotope characteristics and timing of deposition of the Palaeoproterozoic Loch Maree Group of NW Scotland. Petrographic examination reveals a basal unconformity between the Loch Maree Group and Archaean basement, permitting a stratigraphy and younging direction to be assigned. Detrital zircon ages from immediately above the unconformity are dated at c . 2.3 Ga. δ 13 C carbonate data on two temporally discrete carbonate packages range from c . +15 to 2‰ in the older unit and c . 2 to −5‰ in the younger carbonate unit. Current age constraints indicate that the Loch Maree Group is too young to be fully coeval with the Lomagundi–Jatuli Event but is within the age range of the Shunga Event. This revives consideration of a straightforward mass-balance process involving burial of organic carbon as an explanation for at least some of the C-cycle perturbations of Palaeoproterozoic time. Supplementary Material: Sample descriptions from the Loch Maree Group, geochemical data, sample preparation and geochronology data are available at http://www.geolsoc.org.uk/SUP18865 .

U–Pb zircon ages and (isotope) geochemical signatures of the Kamanjab Inlier (NW Namibia): constraints on Palaeoproterozoic crustal evolution along the southern Congo craton

Abstract The formation of the Kamanjab Inlier (KI) in NW Namibia is poorly known and constrained to Palaeoproterozoic times. With the Epupa complex (EC) and Grootfontein Inlier (GI), the KI marks the southwestern Congo craton margin. Our new geochemical data for granitoids and orthogneisses indicate formation along an active continental margin. Single zircon ages frame granitoid emplacement to 1.86–1.83 Ga, roughly 75 myr older than ages from the northern EC and approximately 100 myr younger than from the GI. The southern EC is the only known Archaean Namibian basement with εNd 1.85 Ga of −10.2 to −6.3, in contrast to northern EC (−1.8 to 4.4) and KI (−6.2 to 2.6). Thus, earlier speculation that the southern EC is an exotic terrane, among the Namibian basement complexes, is supported by our data. In contrast, the KI is geochemically comparable to the northern EC and GI. The c. 2.0 Ga Lufubu metamorphic complex roughly 1000 km further east shows similar geochemistry, and a common evolution in the Kamanjab–Bangeweulu magmatic arc has already been proposed. Therefore, our new data point to a major Palaeoproterozoic crustal growth event at the southwestern margin of the Congo Craton starting in the present east and gradually moving towards the present NW.

Highly depleted cratonic mantle in West Greenland extending into diamond stability field in the Proterozoic

This study presents electron microprobe data for dunite xenoliths from a lamprophyre dyke located on the island of Qeqertaa, West Greenland. The minimum age of this dyke is Palaeoproterozoic and it experienced amphibolite facies metamorphism and deformation during that era. The samples consist of nearly 200 xenoliths with a size range of 0.5-8cm. These dunite xenoliths have olivine Mg#, that range from 80.3 to 94.6 (n=579) with a mean of 92.6. Orthopyroxene is found in three xenoliths and garnet in five others. The latter suggests the depth of the Qeqertaa xenolith suite to be near the diamond stability-field, which is substantiated by the finding of diamonds in bulk samples of the Qeqertaa dyke. This further indicates the presence of a lithospheric mantle domain dominated by high-Mg# dunite to this depth in Palaeoproterozoic time. Cr-rich spinel, in the 0.1–0.2mm size range, is found within and between olivine grains in individual xenoliths. These Cr-spinels yield Fe–Mg exchange temperatures of 400–600°C. However, the presence of intermediate spinel compositions spanning the lower temperature solvus suggests that equilibration temperatures were >550°C. Fe3+#, expressed as 100×Fe3+/(Fe3++Al+Cr), is shown to be a useful parameter in order to screen for altered spinel (Fe3+#>10) with disturbed Mg# and Cr#. The screened spinel data (Fe3+#<10) show a distinctly different trend in terms of spinel Cr# versus Mg#, compared to unmetamorphosed xenoliths in Tertiary lavas and dikes from Ubekendt Ejland and Wiedemann Fjord, respectively, also located within the North Atlantic craton. This difference likely reflects amphibolite facies metamorphic resetting of the Qeqertaa xenolith suite by Fe–Mg exchange. Given the similarity of the Qeqertaa xenolith suite with the Ubekendt and Wiedemann suites, in terms of their olivine Mg# and spinel Cr# distribution, high-Mg# dunite is likely to be an important component of the subcontinental lithospheric mantle beneath the North Atlantic craton and appears to have spanned a vertical distance of at least 150km in this region, even during the Palaeoproterozoic.

High-resolution magnetotelluric studies of the Archaean-Proterozoic border zone in the Fennoscandian Shield, Finland

SUMMARY The Archaean–Proterozoic collisional zone is a complex mixture of the Archaean complexes [e.g. Iisalmi Complex (IC)], Proterozoic supracrustal belts [e.g. Kainuu Belt (KB) and Savo Belt (SB)] and oceanic arc lithologies in the central Fennoscandian Shield. The zone was formed in the Savo orogeny when the Keitele microcontinent collided with the Archaean Karelian craton in the Palaeoproterozoic time. The crustal architecture of this palaeosuture is studied using new broad-band magnetotelluric data from 104 sites. 2-D conductivity models across the border zone between the Palaeoproterozoic Svecofennian Domain and the Archaean Karelian province are constrained using the recent, partly collocated reflection seismic data from the Finnish Reflection Experiment (FIRE). Dimensionality analyses, in particular the Q-function analysis, show that magnetotelluric data represent reasonably well regional 2-D structure at periods <100 s, which is the longest period used in this study. Strike determinations gave a stable strike of N15W. For the inversions, the data are projected into three parallel profiles with an azimuth of N75E. The determinant inversion is selected as the most suitable method for the data set. Especially the phase data are useable only from the determinant since one of the polarizations have the out-of-quadrant phase at several sites. The interpreted final, geological more appropriate models, where smoother thick conductive areas are replaced by thinner layers, are constructed from the results of the unconstrained smooth inversions with the help of forward modelling, synthetic and prior model inversions and reflection seismic models. The two major sets of crustal conductors are identified. They have an opposite dip and together they form a bowl-shaped conductor. In the west, the eastward dipping SB conductors are located at the bottom of the formation underlain by the Keitele microcontinent. The SB conductors extend to the east possibly cutting the westward dipping conductors of the KB. The conductive KB is flanked above and below by the resistive Archaean IC/Rautavaara and Eastern Finland Complexes and represents the remnants of a basin that was squeezed between two Archaean blocks in the collision. The crustal conductors revealed by the inversion extend to the surface, where they can be associated with the near-surface conductors mapped by airborne electromagnetic surveys. Geological mapping has shown that the near-surface conductors are caused by graphite and/or sulphide-bearing metasedimentary rocks. Thus the deep conductors related with the Proterozoic KB and SB were formed on the surface and transported into the upper and middle crustal levels in the Savo orogeny. The crust in the Karelian Craton is highly resistive having only minor resistivity variations in our research area. Also the Archaean lower crust is resistive on the contrary to the conductive lower crust of the Proterozoic SB and the Keitele microcontinent.

Palaeomagnetism of Precambrian dyke swarms in the North China Shield: The ∼1.8 Ga LIP event and crustal consolidation in late Palaeoproterozoic times

The North China Shield (NCS) is cut by a laterally-extensive dyke swarm emplaced at 1.78–1.76 Ga when an extensional regime succeeded regional metamorphism and completion of cratonisation by ∼1.85 Ga. Palaeomagnetic study of these dykes and adjoining metamorphic country rocks identifies a dominant shallow axis comprising a contiguous population with NE to N declinations and rare opposite polarity. Dykes with NE shallow magnetic declination (A1, D/ I = 36/−1°) recognised from previous study and emplaced in granulite terranes in the north are displaced by more northerly declinations (A2, D/ I = 8/2°) in lower grade metamorphic terranes to the south. Contact tests indicate a primary cooling-related origin to these magnetisations although tests are in part ambiguous because magnetisations in the granulite basement are comparable. Petrologic and rock magnetic considerations imply that magnetisation of the dykes occurred during uplift from depths as deep as 20 km following the peak of metamorphism at ∼1.85 Ga. A temporal migration A2 → A1 is implied by the higher crustal level and earlier acquisition of the former, and the deeper source and later acquisition of the latter. A third population of dyke magnetisations (A3, D/ I = 18/43°) is distributed towards steeper inclinations and close to the Mesozoic-Recent palaeofield. These are either partial or complete overprints of A1–A2 magnetisations with greater degrees of alteration indicated by demagnetisation and thermomagenetic spectra, or are much younger dykes of Mesozoic-Tertiary age. A minority fourth (later Precambrian but presently undated) dual polarity population has a magnetisation (11 dykes, D/ I = 108/7°) with contact tests indicating a primary cooling-related origin. The ∼1.78–1.76 Ga time of emplacement of the dominant dyke swarms in this study is widely represented by contemporaneous igneous rocks in other major shields linked to major Large Igneous Province (LIP)-related events. The new definition of a ∼1.83–1.76 Ga APW swathe from the North China Shield permits a comparison with other shields and yields a constraint to continental configurations during the late Palaeoproterozoic. A quasi-integral reconstruction of Palaeopangaea is tested here and supported by conformity of predominantly of uplift-related palaeopoles from the ∼1.90–1.70 Ga tectono-thermal belts and from SW → NE trending APW implied by the distribution of poles from the ∼1.80 Ga igneous suites including the LIP events. This trend incorporates the A2 → A1 migration and the granulite terrane cooling polar swathe from North China. The reconstruction indicates that continental crust consolidated in Palaeoproterozoic times by accretion of ∼2.3–1.7 Ga orogenic belts around a hemispheric and crescent-shape core already established by Late Archaean times. The North China Shield is interpreted to have bordered the western cratonic margin of the Indian Shield in a proximity supported by correlation of geological features and suggested by a number of previous workers. The Central Orogenic Zone of the North China shield characterised by tectono-thermal activity prior to ∼1.85 Ga was then contiguous with a comparable zone running through the centre of the Indian Shield and continuing into the Capricorn Belt of Western Australia. The ∼1.78–1.76 Ga dykes in North China continue into dyke swarms in the South India Shield and may have been sourced in a plume-related LIP focussed near the continental margin in the Xiong’er Aulacogen.

LA-(MC)-ICPMS U-Pb zircon geochronology and Lu-Hf isotope compositions of the Taihua complex on the southern margin of the North China Craton

Widely distributed on the southern margin of the North China Craton, the Taihua complex extends roughly in an east-west direction and the relatively complete successions are found in Lushan County, Henan Province. Like many other Archean terranes, the Taihua Complex can be divided into two major lithological units along the Dangze River, namely gneisses series and supracrustal rocks. The former is located on the north side of the river and chiefly composed of TTG gneisses and amphibolites; the latter on the south side of the river is mainly supracrustal rock. Coupled with the previous studies, the results obtained by this study show that the Taihua complex was formed in a large time span from Neoarchean to Palaeoproterozoic. The TTG gneisses and amphibolites are dated at Neoarchean (2794–2752 Ma). The 2.9 Ga and 3.1 Ga zircons in amphibolites could be xenocrysts. In the North China Craton, the 2.8–2.7 Ga old rocks crop out in several areas e.g. western Shandong Province, Jiaodong Peninsula and Lushan area of Henan Province. In addition, 2.8–2.7 Ga detrital zircons or xenocryst zircons have been recognized in Huai’an, Fuping, Wutai areas of North China Craton and also reported in the Early Paleozoic diamondiferous kimberlites in Mengyin and Fuxian. All these age results indicate that the 2.8–2.7 Ga rocks may have been developed much broader region than today’s outcrops. Zircon Hf and whole rock Nd isotopes show that the 2.8–2.7 Ga tectono-thermal event represents an important period of crustal growth with minor ancient crust reworked in the North China Craton. However the formation of supracrustal rocks is limited to 2.2–2.0 Ga in the Palaeoproterozoic time, not the Archaean, as previously believed. Combined with the chronological data of aluminium-rich metamorphic rocks (Khondalite series) on the southern margin of the North China Craton and adjacent areas, it is suggested that the above areas have widely developed Paleoproterozoic passive continental margin environment.

Interaction of microbial communities with clastic sedimentation during Palaeoproterozoic time — An example from basal Gulcheru Formation, Cuddapah basin, India

The siliciclastic basal Gulcheru Formation (∼ 1.8 Ga) of the Proterozoic Cuddapah basin preserves abundance of mat-induced sedimentary structures like old elephant skin, wrinkle structure, kinneyia ripples, palimpsest ripples etc. in the vicinity of Pullivendla town (Kottalu village), Andhra Pradesh, India in a low gradient tidal-flat deposional setting. This is the first report of interaction of microbial communities with clastic sedimentation during Palaeoproterozoic time in Indian Purana stratigraphy and probably from the viewpoint of Global Proterozoic biosedimentation. Various types of cracks on bed-top, hitherto considered as of trace-fossil in origin, may be considered to be formed on exposed surface due to dessication or under water due to synaeresis in presence of microbial communities.

Protopangaea: Palaeomagnetic definition of Earth's oldest (mid-Archaean-Palaeoproterozoic) supercontinent

The geological record of consolidation and uplift of crustal protolith identifies three major groupings of Archaean cratonic nuclei (Rogers, 1996). These comprise ‘Ur’ (∼3.0 Ga cratons of Southern Africa, Western Australia), ‘Arctica’ (∼2.5 Ga cratons of Greenland, Fennoscandia, Laurentia and Siberia) and ‘Atlantica’ (∼2.0 Ga cratons of Western Africa and South America). In this paper the ∼2.9–2.0 Ga record from these cratons is used to develop a palaeogeographic model based on (i) the proximities recognised from geological evidence and (ii) the correlation of ∼2.7–2.2 Ga palaeomagnetic poles between Ur and Arctica (Piper, 1982, 2003). The solution shows that Archaean continental crust had aggregated into a crescent-shaped supercontinent, ‘Protopangaea’, by mid-Archaean times. The outer wings comprised core elements of Ur and Arctica which appear to have retained internal quasi-integrity with respect to each other until the end of the Proterozoic eon. Crust in between was still consolidating in early Palaeoproterozoic times and would ultimately include the Atlantica cratonic grouping. Geological tracers, including straight belts and mineral provinces, form lineaments parallel to the arcuate shape of the protocontinent and, as presently defined, the reconstruction indicates that a minimum of 35–56% of the present continental crust had been extracted from the mantle by these times. Derivation of the reconstruction is facilitated because the bulk of the ∼2.7–2.2 Ga poles imply very low rates of apparent polar wander (APW, <10 mm/year) during this ∼5 Ga interval. These low APW rates correlate with the later granite–greenstone tectonics and are presumed to reflect dominant small scale convection. In contrast rapid APW after ∼2.2 Ga records a transition to typical Proterozoic tectonics with large scale mobility reflected in the accretion of mobile belts to the core Ur-Atlantica-Arctica assemblage. The reason why continental crust accretes into supercontinents of symmetrical crescent-shaped form is also addressed. By analogy with Phanerozoic Pangaea and the present-day geoid it is interpreted to have resulted from large-scale, presumably whole mantle, convection systems driving the continental crust towards regions of minimum gravitational potential.

Palaeoproterozoic high‐pressure granulite overprint of the Archean continental crust: evidence for homogeneous crustal thickening (Man Rise, Ivory Coast)

AbstractThe character of mountain building processes in Palaeoproterozoic times is subject to much debate. Based on the discovery of high‐pressure granulites in the Man Rise (Côte d’Ivoire), several authors have argued that Eburnean (Palaeoproterozoic) reworking of the Archean basement was achieved by modern‐style thrust‐dominated tectonics.A mafic granulite of the Kouibli area (Archean part of the Man Rise, western Ivory Coast) displays a primary assemblage (M1) containing garnet, diopsidic clinopyroxene, red‐brown pargasitic amphibole, plagioclase (andesine), rutile, ilmenite and quartz. This assemblage is associated with a subvertical regional foliation. Symplectites that developed at the expense of the M1 assemblage contain orthopyroxene, clinopyroxene, plagioclase (bytownite), green pargasitic amphibole, ilmenite and magnetite (M2). Multiequilibrium thermobarometric calculations and P–T pseudosections calculated with thermocalc suggest granulite facies conditions of ∼ 13 kbar, 850 °C and &lt;7 kbar, 700–800 °C for M1 and M2, respectively. In agreement with the qualitative information obtained from reaction textures and chemical zoning of minerals, this suggests an evolution dominated by decompression accompanied by moderate cooling. A Sm–Nd garnet – whole‐rock age of 2.03 Ga determined on this sample indicates that this evolution occurred during the Palaeoproterozoic. It is argued that from the geodynamic point of view the observed features are best explained by homogeneous thickening of the margin of the Archean craton, re‐heated and softened due to the accretion of hot, juvenile Palaeoproterozoic crust, as well as coeval intrusion of juvenile magmas. Crustal shortening was mainly accommodated by transpressive shear zones and by lateral crustal spreading rather than large‐scale thrust systems.

Apparent partial loss 40Ar/39Ar age spectra of hornblende from the Palaeoproterozoic Lapland-Kola orogen (arctic European Russia): insights from numerical modelling and multi-method in-situ micro-sampling geochronology

40Ar/39Ar age spectra with progressively increasing step ages are well known for metamorphic hornblende and have been classically interpreted by partial loss of radiogenic argon by diffusion processes during younger thermo-tectonic reworking. Application of a number of numerical modelling tools based on diffusion theory and that assume thermally activated loss of radiogenic 40Ar by solid-state volume diffusion suggests that staircase-shaped age spectra of Neoarchaean tschermakitic hornblende from the Lapland-Kola Orogen are due to argon losses of 40–50% during reheating to 450 ± 25 °C in Palaeoproterozoic time. However, in hornblende samples that yielded staircase-type age spectra, biotite occurs in the matrix, as well as intimately and abundantly intergrown with the amphibole along grain boundaries, cleavages, fractures and other defects. Drilling of 1.5 mm diameter discs from carefully selected hornblende grains in petrographic thin sections permitted to minimise the effects of contaminant biotite inclusions and/or compositional zoning of the amphibole. 40Ar/39Ar laser probe step-heating of drilled biotite-free hornblende discs yielded flat age spectra, suggesting absence of thermally activated radiogenic 40Ar loss. This would imply unrealistically contrasting temperature histories for neighbouring grains. Apparent-loss age spectra, thus, result from differential gas release of hornblende and an included, earlier degassing minor contamination of much younger biotite, which had apparently not been completely eliminated from the amphibole separate, despite careful handpicking. This is confirmed by the Ca/K ratio spectra — a proxy for 37ArCa/39ArK — of hornblende that are flat for drilled biotite-free hornblende grains, but initially low for hornblende separates. A drilled disc and a separate of hornblende from a biotite-free amphibolite did not yield apparent loss spectra, but flat age and Ca/K ratio spectra, confirming the interpretation of the role of biotite.

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.

The palaeomagnetically viable, long-lived and all-inclusive Rodinia supercontinent reconstruction

Abstract Palaeomagnetic apparent polar wander (APW) paths from the world's cratons at 1300–700 Ma can constrain the palaeogeographic possibilities for a long-lived and all-inclusive Rodinia supercontinent. Laurentia's APW path is the most complete and forms the basis for superposition by other cratons' APW paths to identify possible durations of those cratons' inclusion in Rodinia, and also to generate reconstructions that are constrained both in latitude and longitude relative to Laurentia. Baltica reconstructs adjacent to the SE margin of Greenland, in a standard and geographically ‘upright’ position, between c . 1050 and 600 Ma. Australia reconstructs adjacent to the pre-Caspian margin of Baltica, geographically ‘inverted’ such that cratonic portions of Queensland are juxtaposed with that margin via collision at c . 1100 Ma. Arctic North America reconstructs opposite to the CONgo+São Francisco craton at its DAmaride–Lufilian margin (the ‘ANACONDA’ fit) throughout the interval 1235–755 Ma according to palaeomagnetic poles of those ages from both cratons, and the reconstruction was probably established during the c . 1600–1500 Ma collision. Kalahari lies adjacent to Mawsonland following collision at c . 1200 Ma; the Albany–Fraser orogen continues along-strike to the Sinclair‐Kwando‐Choma‐Kaloma belt of south-central Africa. India, South China and Tarim are in proximity to Western Australia as previously proposed; some of these connections are as old as Palaeoproterozoic whereas others were established at c . 1000 Ma. Siberia contains a succession of mainly sedimentary-derived palaeomagnetic poles with poor age constraints; superposition with the Keweenawan track of the Laurentian APW path produces a position adjacent to western India that could have persisted from Palaeoproterozoic time, along with North China according to its even more poorly dated palaeomagnetic poles. The Amazonia, West Africa and Rio de la Plata cratons are not well constrained by palaeomagnetic data, but they are placed in proximity to western Laurentia. Rift successions of c . 700 Ma in the North American COrdillera and BRAsiliano-Pharuside orogens indicate breakup of these ‘COBRA’ connections that existed for more than one billion years, following Palaeoproterozoic accretionary assembly. The late Neoproterozoic transition from Rodinia to Gondwanaland involved rifting events that are recorded on many cratons through the interval c . 800–700 Ma and collisions from c . 650–500 Ma. The pattern of supercontinental transition involved large-scale dextral motion by West Africa and Amazonia, and sinistral motion plus rotation by Kalahari, Australia, India and South China, in a combination of introverted and extroverted styles of motion. The Rodinia model presented here is a marked departure from standard models, which have accommodated recent discordant palaeomagnetic data either by excluding cratons from Rodinia altogether, or by decreasing duration of the supercontinental assembly. I propose that the revised model herein is the only possible long-lived solution to an all-encompassing Rodinia that viably accords with existing palaeomagnetic data.