Palaeoproterozoic Times Research Articles

About this title - Palaeoproterozoic Supercontinents and Global Evolution

The Palaeoproterozoic era (2500–1600 Ma) was a critical period of Earth history, with dynamic evolution from the deep planetary interior to its surface environment. Several lines of geological evidence suggest the existence of at least one pre-Rodinia supercontinent, named Nuna or Columbia, which formed near the end of Palaeoproterozoic time. Prior to this assembly, there may have been an older supercontinent (Kenorland) or perhaps only independently drifting supercratons. The tectonic records of amalgamation and dispersal of these ancient landmasses provide a framework that links processes of the deep Earth with those of its fluid envelope. The sixteen papers in this volume present reviews and new analytical data that span the geological record of Palaeoproterozoic Earth and provide a current picture of Palaeoproterozoic research. The volume provides a useful reference book for students and professional geoscientists interested in this important period of global evolution.

Palaeomagnetic and rock magnetic study of charnockites from Tamil Nadu, India, and the ‘Ur’ protocontinent in Early Palaeoproterozoic times

Palaeomagnetic and magnetomineralogical results are reported from charnockites in basement terrane at the eastern sector of the WSW–ENE granulite belt of South India. Magnetite is the dominant ferromagnet identified by rock magnetic and optical study; it is present in several phases including large homogeneous titanomagnetites and disseminated magnetite in microfractures linked to growth stages ranging from primary charnockite formation to uplift decompression and exhumation within the interval ∼2500–2100 Ma. Several components of magnetization are resolved by thermal demagnetization and summarized by four pole positions; in the northern (Pallavaram) sector these are P1 (33°N, 99°E, d p/d m = 8/9°) and P2 (79°N, 170°E, d p/d m = 3/6°), and in the southern (Vandallur) sector they are V1 (23°N, 116°E, d p/d m = 8/9°) and V2 (26°S, 136°E, d p/d m = 5/10°). These magnetizations are linked to uplift cooling of the basement and unblocking temperature spectra suggest acquisition sequences P1 → P2 and V1 → V2 in each case implying movement of the shield from higher to lower palaeolatitudes sometime between 2500 and 2100 Ma. Palaeomagnetic poles from the cratonic nuclei of Africa, Australia and India all identify motion from higher to lower palaeolatitudes in Early Palaeoproterozoic times, and this is dated ∼2400 and ∼2200 Ma in the former two shields. The corresponding apparent polar wander (APW) segments match the magnetization record within the charnockite basement terranes of southern India to yield a preliminary reconstruction of the ‘Ur’ protocontinent, the oldest surviving continental protolith with origins prior to 3000 Ma. Although subject to later relative movements these nuclei seem to have remained in proximity until the Mesozoic break-up of Gondwana.

São Luís Craton and Gurupi Belt (Brazil): possible links with the West African Craton and surrounding Pan-African belts

Abstract The São Luís Craton and the Palaeoproterozoic basement rocks of the Neoproterozoic Gurupi Belt in northern Brazil are part of an orogen having an early accretionary phase at 2240–2150 Ma and a late collisional phase at 2080±20 Ma. Geological, geochronological and isotopic evidence, along with palaeogeographic reconstructions, strongly suggest that these Brazilian terrains were contiguous with the West African Craton in Palaeoproterozoic times, and that this landmass apparently survived subsequent continental break-up until its incorporation in Rodinia. The Gurupi Belt is an orogen developed in the southern margin of the West African–São Luís Craton at c . 750–550 Ma, after the break up of Rodinia. Factors such as present-day and possible past geographical positions, the timing of a few well-characterized events, the structural polarity and internal structure of the belt, in addition to other indirect evidence, all favour correlation between the Gurupi Belt and other Brasiliano/Pan-African belts, especially the Médio Coreaú domain of the Borborema Province and the Trans-Saharan Belt of Africa, despite the lack of proven physical links between them. These Neoproterozoic belts are part of the branched system of orogens associated with amalgamation of the Amazonian, West Africa–São Luís, São Francisco and other cratons and minor continental blocks into the West Gondwana supercontinent.

Geochemistry, zircon ages and whole-rock Nd isotopic systematics for Palaeoproterozoic A-type granitoids in the northern part of the Delhi belt, Rajasthan, NW India: implications for late Palaeoproterozoic crustal evolution of the Aravalli craton

Determination of zircon ages as well as geochemical and Sm–Nd isotope systematics of granitoids in the Khetri Copper Belt of the Aravalli mountains, NW India, constrain the late Palaeoproterozoic crustal evolution of the Aravalli craton. The plutons are typical A-type within-plate granites, derived from melts generated in an extensional tectonic environment. They display REE and multi-element patterns characterized by steep LREE-enriched and almost flat HREE profiles and distinct negative anomalies for Sr, P and Ti. Initial εNd values range from −1.3 to −6.2 and correspond to crustal sources with mean crustal residence ages of 2.5 to 2.1 Ga. A lower mafic crustal anatectic origin is envisaged for these granitoids, and the heterogeneous εNd(t) values are inferred to have been acquired from the magma source region. Zircon Pb–Pb evaporation and U–Pb ages indicate widespread rift-related A-type magmatism at 1711–1660 Ma in the northern Delhi belt and also suggest a discrete older magmatic event at around 1800 Ma. The emplacement ages of the compositionally distinct A-type granitoid plutons, and virtually coeval granulite metamorphism and exhumation in another segment of the Aravalli mountains, further signify that part of the Aravalli crust evolved during a widespread extensional event in late Palaeoproterozoic time.

Revised geochronology of magmatism in the western Capricorn Orogen at 1805–1785 Ma: Diachroneity of the Pilbara‐Yilgarn collision

The June Hill Volcanics occupy a central stratigraphic position in the tectonic evolution of the Capricorn Orogeny that occurred along the southwestern margin of the Pilbara Craton in Palaeoproterozoic time. The volcanic rocks have been interpreted alternatively as due to continental rifting, foreland‐basin development, or backarc extension. Previous multigrain zircon U–Pb results from these rocks, as well as from an unnamed rhyolitic porphyry that is interstratified in turbidites of the overlying Ashburton Formation, are influenced by the combined effects of Pb loss and xenocrystic inheritance. We report new SHRIMP zircon ages of 1799 ± 8 and 1786 ± 11 Ma, respectively from the June Hill Volcanics and the unnamed porphyry, both of which have been deformed during the Capricorn Orogeny. Close similarity between these ages and results from various late‐stage Capricorn granitoids (including our new SHRIMP result of 1795 ± 8 Ma for the Minnie Creek granite) imply rapid development of the western Ashburton Trough, accumulating and deforming ∼10 km of turbiditic sediments in about 10 million years. Comparison of our new June Hill age with previous SHRIMP results from the central Ashburton Trough implies that foreland‐basin development of the Capricorn Orogeny was significantly diachronous, younging westward during oblique Pilbara‐Yilgarn convergence.

Petrogenesis of the Post-kinematic Magmatism of the Central Finland Granitoid Complex I; Radiogenic Isotope Constraints and Implications for Crustal Evolution

Post-kinematic granitoids of the Central Finland Granitoid Complex INTRODUCTION (CFGC) were emplaced into the Palaeoproterozoic crust of central In many Precambrian shield areas, extensive PalaeoFinland shortly after the >1·89 Ga peak of the Svecofennian proterozoic granitoid magmatism marks the mature stage orogeny. They are found as discordant, generally non-foliated plutons of crustal accretion that led to rapid growth of juvenile with Aand C-type granite characteristics and a bimodal (mafic– terranes around pre-existing Archaean cratons (Patchett felsic) magmatic association. Conventional and ion microprobe et al., 1981; Van Schmus & Bickford, 1981; Patchett & U–Pb data show that the post-kinematic magmatism commenced Bridgwater, 1984; Huhma, 1986; Sadowski & Bettenin the northeastern CFGC at >1·885 Ga and gradually moved court, 1996). On a global scale, these processes led to the to the west, where the plutons are dated at >1·870 Ga. Radiogenic establishment of the Palaeoproterozoic supercontinent isotope (Nd, Sr, Pb) composition of the post-kinematic plutons NENA (Northern Europe–North America; Gower et al., indicates homogeneous initial ratios: Nd (at 1·875 Ga) >0, Sr/ 1990; Rogers, 1996) the scattered remnants of which Sr >0·703, and Stacey & Kramers-type Pb with low longcurrently constitute a substantial fraction of the conterm Th/U (>2). These help to constrain the protolith history of tinental crust of the northern hemisphere. the post-kinematic magmatism. Nd and Pb isotopic data for In the Finnish part of the Fennoscandian (or Baltic) Palaeoproterozoic granitoids, Pb isotopic data for associated sulphShield (Fig. 1), the evolution of the >1·9 Ga Svecoides, and Nd and Pb isotopic composition of the classic >1·6 Ga fennian orogen involved several magmatic events (e.g. Finnish rapakivi granites reveal several distinct crustal domains in Korsman et al., 1999). Emplacement of early orogenic the Finnish Svecofennian orogen, and indicate that the central part (1·93–1·91 Ga) tonalites was followed by predominant of the Svecofennian orogen consists of several terranes (microsynorogenic (1·90–1·87 Ga) magmatism characterized by continents) that were amalgamated in Palaeoproterozoic times. The I-type tonalite, granodiorite, and granite and arc-type CFGC region is the oldest (>2·0 Ga) among these terranes. volcanism. At 1·84–1·81 Ga, late-orogenic (mainly Stype) granites were emplaced as a broad belt across southernmost Finland. These and associated mafic rocks have been ascribed to crustal shortening (Vaisanen &

Petrogenesis of Mesoproterozoic (Subjotnian) rapakivi complexes of central Sweden: implications from U–Pb zircon ages, Nd, Sr and Pb isotopes

ABSTRACTU-Pb zircon geochronology of Mesoproterozoic (Subjotnian) rapakivi complexes in central Sweden yields: 1526 ± 3 Ma (Mullnäset), 1524 ± 3 Ma (Mårdsjö), 1520 ± 3 Ma (Nordsjö) and 1497 ± 6 Ma (Rödön). Together with complexes further S in Sweden, they constitute the westernmost, youngest (1·53−1·47 Ga) belt of rapakivi magmatism in the Fennoscandian shield.The low initial εNd values (−8·9 to −4·8) of all studied Subjotnian basic, intermediate and silicic rocks, require an input from an old (Archaean) low-radiogenic source component, as evidence for Palaeoproterozoic protoliths in the age range 2·5−2·1 Ga is lacking in this region. Crustal, early Svecofennian + Archaean (roughly 30−40%) sources are suggested for the Subjotnian A-type granites and syenites, where the granites derive from undepleted, granodioritic, and the syenites from monzodioritic (±depleted crustal) protoliths. The basic rocks originate from a depleted mantle acquiring the enriched Nd isotopic signatures during interaction with an Archaean lower crust (20−40%), largely depleted after rapakivi melt extraction. Pb isotope data from feldspars (207Pb/204Pb to 15·018−15·542) support the presence of Archaean components in the magmas.The results indicate that an Archaean basement is underlying relatively wide areas of Svecofennian formations in central Sweden. This old basement section was most likely rifted off the Archaean craton in the NE in Palaeoproterozoic times.

Palaeomagnetism, rock magnetism and magnetic fabrics in Precambrian metamorphic terranes of the Huabei Shield, China

This study has investigated magnetic remanence, rock magnetism and anisotropy of magnetic susceptibility (AMS) in granulite and amphibolite grade metamorphic terranes of the Huabei Shield between Inner Mongolia in the west and the Bohai Sea in the east. Rock magnetic studies identify annealed metamorphic magnetite grains with multidomain properties as the remanence carriers; a widely recorded stable remanence was probably fixed by grain shape effects. Granulite facies terranes are typically between one and two orders more strongly magnetised than amphibolite terranes and AMS fabrics correlate mostly with metamorphic mineral fabrics observed in the country rocks. Progressive thermal demagnetisation identifies a range of two and three component structures resident in magnetite. An important component recognised as a partial or complete remagnetisation by Late Mesozoic–Tertiary tectonic/magmatic activity is present in basement at the southern margin of the outcrop (Miyun terrane) and where extensive granite plutonism has occurred (Zhunhua terrane). These components have directions corresponding to remanence in the Yunmeng Shan Granite (119–114 Ma, D/I=33/58°, 39 samples, a 95=3.5°, palaeopole at 201°E, 64°N). Most remanence elsewhere was probably acquired during post-tectonic uplift and cooling of the basement between ∼2200 and 1850 Ma because palaeomagnetic directions are removed from the Phanerozoic palaeofield path and they are distinct from the palaeomagnetic record in the overlying Jixian Supergroup deposited at ∼1840–900 Ma. These latter magnetisations are considered reliable indicators of the palaeofield during Late Palaeoproterozoic times because deformation of overlying supracrustal rocks is mostly slight and no prominent deflection of magnetic remanence by magnetic fabrics is observed. Palaeofield directions and poles attributed to the time of uplift-related cooling are: Qian’an Terrane (D/I=215/71°, a 95=9°, 17 samples, pole at 99°E, 10°N) and North Qianxi Terrane (D/I=44/−45°, a 95=4°, 41 samples, pole at 79°E, 11°S). In addition, a more widely-preserved shallow northerly component correlates with a NW→E swathe of components recorded by uplift-related cooling within the Datong–Huan’an granulite terrane in the west of the shield. A preliminary Palaeo-Mesoproterozoic apparent polar wander path for the Huabei Shield is defined from the Palaeoproterozoic record in the metamorphic basement rocks and the Meso-Neoproterozoic record in the overlying Jixian Supergroup. It incorporates a loop between ∼2200 and 1850 Ma and exhibits a general east to west trend in subsequent times.

Archaean to Neoproterozoic magmatic events in the Kaoko belt of NW Namibia and their geodynamic significance

Age relationships in the N–S trending Neoproterozoic (Pan-African) Kaoko belt of northwestern Namibia are still poorly constrained. U–Pb and Pb–Pb zircon age determinations by single grain evaporation, conventional multigrain fraction analyses and ion microprobe (SHRIMP) from a profile along the E–W Hoanib River reveal various episodes of zircon growth, ranging from late Archaean to late Neoproterozoic. From the eastern part of the profile we report crystallization ages of gneiss protoliths from 2645 to 2585 Ma, the oldest ages so far found in Namibia. Sm–Nd isotope data suggest the involvement of still older crustal material in the generation of these gneisses. A thermal event at ca 2285 Ma ago caused new zircon growth in the Archaean rocks, and major igneous activity between 1985 and 1960 Ma is represented by granitoid intrusions. This demonstrates that these gneisses were part of the Congo craton since Palaeoproterozoic times. An emplacement age of ca 1500 Ma was determined for the protolith of a granitic orthogneiss within the central part of the Hoanib River profile. This is the first record of a magmatic event of this age in this region. The western part of the profile consists of granitoid rocks that were generated during the Neoproterozoic Damara orogeny. This terrain has previously been interpreted as a Neoproterozoic metasedimentary domain and subsequently as pre-Damaran basement, but our study proves these rocks to be igneous in origin with emplacement ages ranging from 552 to 656 Ma. Sm–Nd isotope data and zircon xenocrysts demonstrate the involvement of older crustal material in the generation of these granitoids. We interpret a coastal granite, dated at ca 564 Ma, as being emplaced during the last peak of high-grade metamorphism in the Kaoko belt, which is approximately synchronous with the last metamorphic event in the Damara belt of central Namibia.

Emplacement of a large igneous province as a possible cause of banded iron formation 2.45 billion years ago

LATEST Archaean and earliest Palaeoproterozoic times (from 2.6 to 2.2 billion years ago) have generally been viewed as a largely quiescent period of Earth history; the geological record indicates the very slow deposition of pelagic and chemical sediments1,2, and bears only a limited record of magmatic and tectonic activity3–5. Such quiescence is consistent with the contention that the Earth's main banded iron formations (BIFs)—finely laminated chemical sedimentary rocks, rich in iron oxide—formed slowly as oxygen abundances in the oceans gradually increased, thus reducing the capacity of sea water to retain dissolved iron6–10. Here we show that a large igneous province, comprising >30,000km3 of dolerite, basalt and rhyolite, accompanied deposition of a Hamersley Province BIF 2,449 ±3 million years ago. This observation indicates that Hamersley BIFs formed during a major tectono-magmatic event and were deposited very much faster than previously thought, at similar rates to (or faster than) modern pelagic sediments. Thus the largest Palaeoproterozoic BIFs, rather than simply reflecting a gradual increase in the oxygen content of the oceans during a period of tectonic quiescence, are more likely to have formed as a result of an increased supply of suboxic iron- and silica-rich sea water upwelling onto continental shelves during a pulse (or pulses) of increased submarine magmatic and hydrothermal activity.

Insight into the enigma of Neoproterozoic manganese and iron formations from the perspective of supercontinental break-up and glaciation

Abstract The genesis of manganese and iron formations (MnF and IF) is a closely related process during the Proterozoic, in which oceanic crustal venting provided metal-rich solutions to be deposited on adjacent continental shelves. Their deposition in condensed marine sequences occurred when continental break-up initiated a major transgression and provided favourable conditions for precipitation of chemical sediments from seawater. The Neoproterozoic Rodinian supercontinental break-up shows the relationship between this tectonic process, climate, and MnF/IF genesis. Metallogenesis followed particular rift-drift transitions during supercontinental break-up. MnFs/IFs are often associated with glaciomarine sedimentary rocks. Higher O 2 /CO 2 during continental rifting would, through inducing glacial conditions and withdrawing seawater, increase ocean salinity and thereby allow both deposition of glaciomarine sediments and precipitation of evaporites. Salinities would have been particularly enhanced in the restricted oceanic basins provided by early supercontinental rifting, where atmospheric evaporation would also have played a part. Accumulation of Mn and Fe in the Neoproterozoic may be directly related to a probably glacially derived increased ocean salinity that promoted accumulation and storage of Mn and Fe in ocean water through increased Mn, Fe, and Ba(?) solubility. Very similar processes may have been effective in Palaeoproterozoic times, although then a higher amount of Fe and Mn discharge and different ocean chemistry would have superseded the effect of ocean salinity. Chemical Mn and Fe precipitates related to supercontinental break-up in the Proterozoic as well as the Phanerozoic indicate a similar relationship between tectonics and ore accumulation. The resultant close time and space relationship between potent oceanic source and suitable continental-shelf depositional settings optimised accumulation and preservation of ancient Mn-Fe deposits. This scenario is at variance from present-day environments where source and suitable precipitation areas of long-term preservation potential are mostly widely separated in space.

Petrology of the Proterozoic mafic dyke swarms of Uruguay and constraints on their mantle source composition

Three dyke swarms in Uruguay range in age from Palaeoproterozoic (1.86 Ga, Florida region) to Neoproterozoic (∼0.7 Ga, Nico Perez and Treinta y Tres regions). The Florida and Nico Perez swarms are basalts, basaltic andesites and andesites with tholeiitic affinity, characterized by LILE and LREE enrichment with respect to HFSE, Nb depletion with respect to K and La, K Rb < 260 , Ba Rb < 19 , Zr Nb > 13 , (La Yb) n > 4.5 . Both swarms have positive ϵ Sr and negative ϵ Nd, but the Florida isotopic array is dominated by ϵ Nd variations (EM1 type) and that of Nico Perez by ϵ Sr variation (EM2 type). The Treinta y Tres swarm consists of slightly ne-normative transitional or alkaline basalts with geochemical characteristics resembling those of OIB (e.g., Gough Island), but suggestive of a less enriched OIB source, and have slightly positive ϵ Nd and ϵ Sr. Crustal contamination does not appear to be important in the petrogenesis of the swarms. It is proposed that the geochemical characteristics of the Florida swarm derive from melting of lithospheric mantle infiltrated by hydrous fluids released from the thermal breakdown of hydrous phases, either contained in underplated oceanic crust or formed by interaction of asthenospheric fluids with the lithospheric mantle. The EM1-type isotopic features are considered as unrelated with possible fluid addition, but as a pre-existing feature. The geochemical and isotope characteristics of the Nico Perez swarm are attributed to time-integrated enrichment processes, which affected the Nico Perez lithospheric mantle during the Palaeoproterozoic Florida episode. The geochemistry of the Treinta y Tres swarm may be explained by partial melting of a lithospheric mantle which did not suffer Palaeoproterozoic LILE and LREE enrichment but which was isotopically reset in Palaeoproterozoic times. Alternatively, a residual mantle related to the Florida event is required. Mantle melting and dyke intrusion probably occurred in an ensialic environment. The geochemical and isotopic characteristics of the Uruguay dyke swarms are similar to those of the Mesozoic basalts related to the Gondwana break-up, supporting the possibility that the latter derived from a heterogeneous lithospheric mantle source which recorded Proterozoic enrichment processes.

U‐Pb age of the Yxsjöberg Tungsten‐Skarn deposit, Sweden

Abstract The Yxsjoberg scheelite‐skam deposit of western Bergslagen (south‐central Sweden) formed in Palaeoproterozoic time when aqueous fluids migrating along lithologic discontinuities replaced limestones in acid volcanic rocks. The age of the mineralization and, thus, the source of the fluids are controversial. U‐Pb dating of titanite that formed during the skarnification of the limestones yields an age of 1789±2 Ma. This age is not compatible with an exhalati ve origin of the deposit or a genetic relation to Svecofennian arc magmatism, as has been suggested. Instead, the U‐Pb titanite age corresponds to the age of Y, Rb, F, and Nb‐rich post‐kinematic granitoids from the Bergslagen area and granitoids of the Transscandinavian Igneous Belt (TIB) farther to the west. As the TIB granitoids are not fertile with respect to W‐mineralizations, the U‐Pb titanite age indicates that the post‐kinematic granitoids were the source of heat, metals, and probably fluids for the formation of the Yxsjoberg scheelite‐ska...