Neoproterozoic-Early Cambrian Research Articles

Metallogeny and temporal-spatial distribution of sedimentary phosphorite mineralization in Iran, relation to tethyans oceans evaluation and implications for future exploration

There are several deposits and mineral occurrences of sedimentary phosphorite (SP) mineralization in Iran. The main tectonic-structural zones of Iran that host SP deposits are: (1) Alborz Magmatic Belt (AMB), (2) the Central Iran and (3) Zagros Folded Thrust Belt (ZFTB). These deposits were formed during separate time: (1) Neoproterozoic-Early Cambrian (Soltanieh Formation), (2) Ordovician-(Silurian) (Shirgesht, Mila and Seyahou Formations), (3) Upper Devonian (Jeirud Formation), (4) Permian to Jurassic (Abkhory, Taleqan and Hosseinabad area) and (5) Cretaceous to Paleogene (Gurpi and Pabdeh Formations). The stratigraphic sequences of these deposits are commonly formed from sedimentary rocks, which are similar to other SP districts in the Middle East and North Africa. The main period of SP mineralization in Iran was from the Neoproterozoic-Early Cambrian, Upper Devonian and Late Cretaceous to Paleogene. The Jeirud Formation of the Upper Devonian and the Pabdeh Formation of the Paleocene-Eocene contained the largest sedimentary phosphorites in Iran. The formation and distribution of sedimentary phosphorite deposits in the different structural zones can be explained by divergent and convergent events related to the Proto, Paleo, and Neo-Tethyan oceans. The SP deposits hosted by the Neoproterozoic-Early Cambrian Soltanieh Formation are related to the Proto-Tethys Ocean. Mineralization in the Ordovician Mila and Shirgesht formations and Upper Devonian Jeirud Formation are age related to the Paleo-Tethys Ocean, and finally, the SP deposits formed in the Cretaceous- Paleogene (Gurpi and Pabdeh formations) sequence were formed the evolution of the Neo-Tethys Ocean cycle. Passive margins are the principal plate tectonic settings for Iranian SP deposits. Iranian SP deposits are stratigraphically controlled in which phosphorite mineralization occurs as laminae, disseminated grains, and replacement. SP mineralization is commonly closely related to framboidal pyrite and biologic events. The AMB and ZFTB are the main tectonic zones for Iranian sedimentary phosphorite deposits. These zones are the most favorable metallogenic provinces for the exploration of sedimentary phosphorite deposits, representing in excess of 80% of the as of now known SP deposits and occurrences in Iran. High sea level, low oceanic anoxic events (OAE's), warm climates, latitude and longitude close to the equator and shallow slope environments are among the most important factors controlling Iranian SP deposits.

Relics of ophiolite-bearing accretionary wedges in NE Brazil and NW Africa: Connecting threads of western Gondwana´s ocean during Neoproterozoic times

Neoproterozoic breakup of Rodinia resulted in the formation of several oceanic realms between dispersing cratons, which were later consumed during the assembly of Gondwana. In its western portion, the interior orogenic belts of Gondwana formed during the Brasiliano-Pan African Orogeny in the late Neoproterozoic-early Cambrian. Available geophysical, structural and petrological data suggest that the complex network of shear zones that once connected the Borborema province (NE Brazil), Tuareg shield (Hoggar) and Central African domain (NW Africa) likely represent ancient sutures that mark collisional episodes between Archean-Paleoproterozoic paleocontinents such as Amazonian-West African and São Francisco-Congo. Mafic, ultramafic and sedimentary sequences associated with this set of structures respresent dismembered ophiolite slices interpreted as oceanic remnants (sensu lato) that were emplaced during the late stages of the Gondwana assembly. For instance, the composite Transbrasiliano-Khandi-In-Tedeini-Silet shear system crosscuts rock assemblages preserving a complex history of oceanic-crust-transition development (Novo Oriente complex) in association with primitive to evolved magmatic arcs and UHP rocks both in the Borborema province and NW Africa. In the central Borborema province, preserved ophiolitic slices are strongly overprinted by ductile and brittle deformation events, but partially preserved MORB-like amphibolites are akin to subduction-related-types that crystallized in early- and late Neoproterozoic times docked via terrane accretion and dispersed by strike-slip shear zones. In the southern Borborema province, an example of a Neoproterozoic ophiolitic assemblage is the Monte Orebe complex, that encompasses T-MORB mafic rocks, ultramafic lenses, and exhalative sedimentary rocks akin to early to late stages of oceanic basin spreading, emplaced during convergent plate motions between the Pernambuco-Alagoas superterrane and the São Francisco craton. Correlative units are found in Cameroon, including the strongly hydrotermalized ultramafic rocks of the Lomié and Boumnyebel complexes, that are structurally controlled by top-to-the-south verging nappes found in the N-NW margin of the Congo craton. In all scenarios, the ophiolitic complexes are related to intra-oceanic and continental magmatic arcs as well as to geophysical signatures comparable to Phanerozoic suture zones. Although strongly dismembered, scrapped off Neoproterozoic oceanic crust partially preserved within the major belts of western Gondwana demonstrate the role of accretion-collisional orogenesis during its assembly.

Carbon, oxygen and strontium isotopic and elemental characteristics of the Cambrian Longwangmiao Formation in South China: Paleoenvironmental significance and implications for carbon isotope excursions

The biosphere experienced episodic perturbations during the Neoproterozoic–early Cambrian, accompanied by major fluctuations of the carbon cycle. However, compared to the Ediacaran–early Cambrian, geochemical records of the Cambrian Toyonian Stage (Longwangmiao), which contains the significant Redlichiid–Olenellid Extinction Carbon Isotope Excursion (ROECE), are scarce. To address this data gap, we conducted measurements of carbon, oxygen and strontium isotopes and element contents of a continuous series of carbonate samples from the Longwangmiao Formation of Well HS2, located in the central Sichuan Basin of South China. Our specific aims were to reconstruct the paleoenvironment and to determine the origin of the ROECE. Strict sample screening was used to ensure that the isotopes and elements were not affected by diagenesis. The 87Sr/86Sr values of Well HS2, and for sites elsewhere, show that sea level gradually decreased during the Cambrian Toyonian Stage. Additionally, the pronounced negative δ13C excursion in the late Toyonian is globally associated with the ROECE, which is widely thought to have resulted from extinctions caused by a transgression. However, a transgression is not shown in the 87Sr/86Sr profile of Well HS2, which we attribute to local karst formation and erosion caused by frequent regional regressions. The interrelationships among several paleoenvironmental proxies show that continental weathering was climatically controlled, which affected the supply of terrigenous detritus, and that the carbonate platform was semi–restricted. Furthermore, profiles of paleoenvironmental proxies indicate that the environment was warm and humid, favoring high biological productivity and diversity during Interval I (early Toyonian), and that it gradually became colder and drier during Interval II (middle Toyonian). We suggest that by the time of Interval III (late Toyonian), the adverse effects of a cold and arid environment were superimposed on the transgression, triggering the ROECE. Our results provide a reference and possible interpretation for other carbon isotope excursion events.

A hidden crust beneath the central Urumieh‐Dokhtar Magmatic Arc revealed by inherited zircon ages, Tafresh, Iran

Laser ablation ICP‐MS U–Pb dating on inherited zircons from Neogene volcanic rocks of the Tafresh area in the central part of Urumieh‐Dokhtar Magmatic Arc (UDMA) yielded Paleoproterozoic, Late Neoproterozoic–Early Cambrian, Carboniferous, and Cretaceous ages. These ages improve our understanding of the geodynamical evolution of central UDMA. The 2,379 Ma age (i.e., Paleoproterozoic) is the oldest age reported in the central UDMA and is similar to the age of detrital zircons from Central Iran. The Paleoproterozoic zircons may come from the African Craton and fed from Central Iran sedimentary sequences. The Late Neoproterozoic–Early Cambrian ages (605 ± 11 Ma to 533 ± 9.2 Ma) indicate a basement for the UDMA, which is similar to that of the Sanandaj‐Sirjan Zone, Central Iran, and northwest Iran. The Carboniferous zircon age (328.4 Ma) is interpreted as being derived from the Eurasian arc produced by northward subduction of the Paleo‐Tethys Oceanic Plate. The Late Cretaceous zircon ages (106 to 71.2 Ma) might be regarded as indicating the Late Cretaceous magmatic episode, marking the subduction of oceanic crust that followed the Neo‐Tethys subduction.

Nd isotopic stratigraphy of Paleoproterozoic to late Paleozoic sedimentary strata of the southwestern Yangtze Block and implications for its tectonic evolution

ABSTRACT A systematic Sm-Nd isotopic study is reported for Paleoproterozoic to Late Paleozoic strata from the Dongchuan area in the southwestern Yangtze Block. The results, combined with the available detrital zircon U-Pb ages and Hf isotope data, constrain the provenances of these sedimentary rocks and further identify three important tectono-magmatic activities. Most of the Paleo-Mesoproterozoic samples (Dongchuan Group) display a wide T DM2 range of 1.92–2.52 Ga with corresponding ε Nd(t) values of +4.0 to −3.5, suggesting Paleoproterozoic-dominated provenances mixed with mantle-derived materials. This corresponds to the ~1.7–1.5 Ga mafic magmatic activities commonly occurred in the southwestern Yangtze Block, which are related to the early breakup of the Columbia supercontinent. The obvious vale of T DM2 and apex of ε Nd(t) occurred in the Neoproterozoic strata (~0.8 Ga) of the southwestern margin over the whole Yangtze Block. This is consistent with the widely recognized mantle-derived magmatism around the Yangtze Block related to the breakup of Rodinia. However, the decreases in Nd model ages are different among various regions, indicating that the Neoproterozoic mantle inputs are more profound in the southwestern and central Yangtze Block than the southeastern Yangtze and the Jiangnan orogenic belt. The late Ediacaran to early Cambrian strata from the southwestern Yangtze exhibit a decrease in T DM2 (from 2.00 to 1.67 Ga) and increase in ε Nd(t) (from −9.0 to −5.2). This is in accordance with the coeval juvenile crustal materials discovered in the northwestern Yangtze, which were probably derived from the assembly of the Gondwana continent. Thus, a Gondwanan affinity is suggested for the southwestern Yangtze Block. Overall, the Nd isotopic studies of the Paleoproterozoic to Late Paleozoic sedimentary strata from the southwestern Yangtze Block identified three major episodes of magmatic activities, late Paleoproterozoic (~1.7 Ga), Neoproterozoic (~0.8 Ga) and late Neoproterozoic-early Cambrian (~0.55 Ga) in the context of Columbia, Rodinia and the subsequent Gondwana supercontinents.

Hot dry rock geothermal resource potential in the Wudalianchi volcanic field, NE CHINA: Implications from geophysical exploration

Hot dry rock is regarded as the clean energy of the future and is an important part of geothermal resources, with great research value and development prospects. The Weishan volcano, located in the northern part of the Wudalianchi volcanic group, was formed in the Pleistocene, and its age is only (0.57–0.31) ± 0.05 Ma. This area has a geothermal setting similar to but a younger age than that of the Fenton Hill volcano (formed 1.40–1.10 Ma) where a hot dry rock power project has operated since 1973. In this study, we used ambient noise tomography and magnetotelluric sounding methods to investigate deep heat sources beneath the Weishan volcano. The results show good geothermal conditions for hot dry rock formation in this region. A cooling magma chamber that might be the hot dry rock heat source is indicated in the upper crust at 6.5–13 km depth, with a volume of ∼200 km3. Our study suggests that the monzonitic granites of the Indosinian Xiaohongshan unit, which lies beneath the Late Cretaceous Nenjiang Formation and the Neoproterozoic-Early Cambrian Beikuanhe Formation, should be the main target for further hot dry rock geothermal energy exploration and development.

U-Pb zircon dating, geochemistry and Sr-Nd-Pb isotopic ratios from Azna-Dorud Cadomian metagranites, Sanandaj-Sirjan Zone of western Iran

An integrated study of U–Pb zircon dating, whole-rock geochemistry and Sr-Nd-Pb isotopes was accomplished on metagranites from the Azna-Dorud region, Sanandaj-Sirjan Zone (SaSZ) of Iran. U–Pb zircon dating display that the crystallization of the protolith was occurred at age of 525.6 ± 4 Ma (Early Cambrian). The new dating reveal that Iranian basement rocks with ages Late Neoproterozoic-Early Cambrian from SaSZ are similar to the Central Iran region. Zircons from the rocks show oscillatory zoning and high Th/U ratios (0.36–1.23) that imply their magmatic source. The zircons are considered as late magmatic phase on the basis of the geochemistry of their trace elements. The petrographic investigations show the mineral assemblage mainly consisting of quartz, plagioclase, alkali feldspar (microcline) and biotite, as well as accessory minerals of titanite, allanite, epidote and zircon. Based on the chemical composition, the rocks have high SiO2 (71–75 wt%), total alkali contents (Na2O = 5.77–6.22%; K2O = 0.34–0.39 wt%) and low CaO and Al2O3 and are classified as metaluminous to slightly peraluminous (A/CNK = 0.95–1.01). The metagranites are depleted in Sr, Ba, P and Ti and enriched in Th and Ce relative to primitive mantle. These characteristics together with various geochemical discriminant diagrams offer a sodic A2-type nature for them. Initial 87Sr/86Sr and 143Nd/144Nd ratios vary from 0.7049 to 0.7121 and from 0.51189 to 0.51190, respectively. Also, the εNd(t) values of the metagranites vary from −1.2 to −1.5. The lead isotopic ratios are as (206Pb/204Pb)t = 18.135–18.596, (207Pb/204Pb)t = 15.628–15.655 and (208Pb/204Pb)t = 37.717–37.939. As a consequence, the Sr-Nd-Pb isotopic evidence reveals the Cadomian metagranites of the Azna-Dorud region generated from partial melting of a metabasite (i.e. amphibolite), which had its source in an enriched metasomatic mantle.

Gondwanan basement terranes of the Variscan–Appalachian orogen: Baltican, Saharan and West African hafnium isotopic fingerprints in Avalonia, Iberia and the Armorican Terranes

Iberia, Avalonia and the “Armorican” terranes form key constituents of the Variscan–Appalachian orogen, but their Neoproterozoic origins along the northern Gondwanan margin continue to be strongly debated. Here, we present a new detrital zircon U–Pb–Hf dataset from Neoproterozoic–Silurian sedimentary sequences in NW Iberia and Avalonia, in conjunction with the comprehensive existing datasets from potential source cratons, to demonstrate that the provenance of each terrane is relatively simple and can be traced back to three major cratons. The enigmatic Tonian–Stenian detrital zircons in autochthonous Iberian rocks were derived from the Saharan metacraton in the latest Neoproterozoic–early Cambrian. Avalonia is commonly considered to have been derived from the Amazonian margin of Gondwana, but the hafnium isotopic characteristics of the detrital zircon grains in early Neoproterozoic rocks bear much stronger similarities to Baltica. The hafnium isotopic array also suggests the early Avalonian oceanic arc was built on a sliver of “Grenvillian-type crust” (~2.0–1.0Ga) possibly of Baltican affinity at ~800Ma, prior to accretion with a continental margin at ~640Ma. The Upper Allochthon of Iberia is frequently linked to the West African Craton in the late Neoproterozoic–early Cambrian, however the hafnium isotopic array presented here does not support this connection; rather it is more similar to the hafnium array from Avalonia. The Armorican terranes have strong detrital zircon isotopic links to the West African Craton during the late Neoproterozoic–Cambrian.

Detrital zircon geochronology and Nd isotope geochemistry of the basal succession of the Taebaeksan Basin, South Korea: Implications for the Gondwana linkage of the Sino-Korean (North China) block during the Neoproterozoic–early Cambrian

The paleogeographic configuration of continental blocks around East Gondwana during the Neoproterozoic–early Cambrian is controversial. This study reports the U–Pb ages of detrital zircons and Nd isotopic composition of the Neoproterozoic–early Cambrian succession developed on the Precambrian basement in South Korea, which formed the southeastern portion of the Sino-Korean block (SKB) in its present configuration. Three stratigraphic units are addressed in this study: the Neoproterozoic Jangsan, early Cambrian Myeonsan, and early–middle Cambrian Myobong Formations. Both the Jangsan (white to pink quartz sandstone) and Myeonsan (dark gray ilmenite-rich sandstone/shale) formations are barren and are unconformably and conformably overlain, respectively, by the dark gray, fossiliferous fine-grained Myobong Formations. The Jangsan and Myeonsan Formations contain zircons with Archean–Paleoproterozoic ages, indicative of detritus derived from the local Precambrian basement. In contrast, the Myobong Formation is dominated by Mesoproterozoic to Neoproterozoic zircons, which are not represented in the local Precambrian basement. The Sm–Nd model ages of the Myobong Formation are younger than those of the underlying strata, indicative of significant changes in provenance during the deposition of this formation. Comparison with coeval sediment having Gondwana signatures in the southern margin of the SKB and the Tethyan Himalayan terrane strongly suggests that the Myobong Formation was derived from orogens in East Gondwana. The results of this study reveal that the timing of the Myobong Formation deposition marks the onset of a sedimentation episode on the southeastern margin of the SKB, which was related to the emergence of a vast source province in East Gondwana, possibly aided by the Cambrian transgression onto the SKB. In comparison with the published literature, we argue for the paleogeographic continuity of the SKB with the northern margin of East Gondwana, possibly between northwestern Australia and northeastern India during the Neoproterozoic–early Cambrian.

Neoproterozoic-Early Cambrian biota and ancient niche: A synthesis from molecular markers and palynomorphs from Bikaner-Nagaur Basin, western India

The time frame of present study is Neoproterozoic-Early Cambrian, an enigmatic and consequential time in the history of Earth. The present study elucidates the terminal Proterozoic biota and palaeoenvironment by characterizing the soluble and insoluble fractions of organic matter extracted from sediment core samples recovered from Marwar Supergroup, Bikaner-Nagaur Basin, western India. The normal alkanes range from C15 to C33 wherein C19 to C21n-alkanes predominate and carbon preference index ranges from 1.12 to 1.19. Pristane/phytane ratio varies between 0.9 and 1.2. The kerogen matter occurs as homogenous laminae and discrete organic remains which are mostly derived from microbial mat. Triterpenoids include cheilanthanes and hopanes which are confirming evidences of microbial input. Algal presence has also been confirmed from palynological data. Green algae evolved to be a major biota during the Neoproterozoic-Early Cambrian time and this is validated by the appreciable abundance of C29 steranes in the bitumen fraction. The amorphous pieces of kerogen residues lack any visibly distinct internal features. However, a very few simple ornamented acritarchs like Micrhystridium sp., Lophosphaeridium sp. and Leiosphaeridia sp. are observed in the organic residues. Micro-FTIR spectroscopy of the amorphous organic particles reveals the presence of long chain aliphatic components and unusually high CC aromatic ring stretching band. The organic remains were deposited in restricted marine water and elevated saline condition as indicated by several salinity biomarkers. The present study clearly demonstrates a microbial mat dominated niche with significant eukaryotic green algae that had been capable of surviving in the high salinity of the terminal Proterozoic oceans.

Petrology, geochemistry and zircon U–Pb dating of Band-e-Hezarchah metabasites (NE Iran): An evidence for back-arc magmatism along the northern active margin of Gondwana

The Band-e-Hezarchah granitoids (BHG) is located in the northern margin of the central Iran, where the very old continental crust of Iran is found. The BHG mainly include granodiorite, granite and leucogranite. Small meta-gabbroic stocks and dykes are associated with BHG. U–Pb zircon dating of the BHG granites and metabasites yield 238U/206Pb crystallization ages of ca. 553.6 and 533.5Ma respectively (Ediacaran–early Cambrian). The metabasites have calc-alkaline signature and their magmas seem to have originated from a mantle wedge above a subduction zone. These rocks are thought to be formed in a continental back-arc setting, related to the oblique subduction of Proto-Tethys oceanic lithosphere beneath the northern margin of Gondwanan supercontinent during Ediacaran–Cambrian time. The initial 87Sr/86Sr ratios and ɛNd (t) values for metabasites are change from 0.705 to 0.706 and −3.5 to −3.6 respectively. Sr–Nd isotope composition of metabasites indicates that these rocks were derived from a subcontinental lithospheric mantle source. The BHG and associated metabasites are coeval with other similar aged metagranites and gneisses from Iranian basements exposed in central Iran, Sanandaj-Sirjan and Alborz zones. These rocks were formed due to continental arc magmatism of Neoproterozoic–early Cambrian, bordering the northern active margin of Gondwana.

Research on carbon and oxygen isotopes in phosphorus-bearing rock series of the Late Neoproteroic-Early Cambrian Taozichong Formation in Qingzhen City, Guizhou Province, Southwest China

In the study of the phosphate-bearing stratum at the bottom of the Cambrian system, the authors found that there occurred carbon isotope negative anomalies in the Taozichong section phosphate-bearing stratum in Qingzhen, Guizhou Province; they can be correspondingly compared with other synsedimentary carbon isotope negative anomalies both at home and abroad. The results showed that there occurred three negative anomalies of carbon isotopes in the Neoproterozoic-Early Cambrian Taozichong Formation, indicating that the temporal pa- laeo-oceanographical environment changed significantly, in which there happened two times of intensive carbon isotope variation, corresponding to creature extinction. Meanwhile, it is believed that the carbon isotope negative anomalies in phosphorites were caused by the ascending water mass of ocean current with the negative carbon iso- topic composition of deep-ocean hydrothermal deposits.

Geochronology of igneous rocks in the Sierra Norte de Córdoba (Argentina): Implications for the Pampean evolution at the western Gondwana margin

U-Pb zircon data (secondary ion mass spectrometry [SIMS] and thermal ionization mass spectrometry [TIMS] analyses) from igneous rocks with differing structural fabrics in the Sierra Norte de Cordoba, western Argentina, suggest that the sedimentary, tectonic, and magmatic history in this part of the Eastern Sierras Pampeanas spans the late Neoproterozoic–Early Cambrian. A deformed metarhyolite layer in metaclastic sedimentary rocks gives a crystallization age of 535 ± 5 Ma, providing a limit on the timing of the onset of D 1 deformation and metamorphism. The new data coupled with published Neoproterozoic zircon dates from a rhyolite beneath the metaclastic section and detrital zircon ages from the section indicate a late Neoproterozoic to Early Cambrian depositional age, making this section time equivalent with the Puncoviscana Formation (sensu lato) of northwest Argentina. A synkinematic granite porphyry gives a crystallization age of 534 ± 5 Ma, providing a limit on the age of dextral mylonitization in the Sierra Norte area (D 2 event). The new age is consistent with ages of 533 ± 4 Ma from a mylonitic granite with dextral sense-of-shear fabrics and 531 ± 4 Ma from a late-synkinematic dacitic porphyry, which broadly indicates the final age of dextral deformation. A crystallization age of zircons from the postkinematic, high-level El Tio granite (530 ± 4 Ma) suggests that both stages of Pampean deformation and regional metamorphism, accompanied by synkinematic intrusions, were followed by uplift and took place during a very short time span in the Early Cambrian. This is supported by zircon dates of 523 ± 5 Ma from a rhyolite to dacite in the western part of the Rodeito area and dates from the undeformed El Escondido rhyolite and granite of 519 ± 4 Ma and 521 ± 4 Ma, respectively. These three crystallization ages also indicate that ductile dextral shearing and mylonitization associated with the Pampean D 2 event terminated in the Early Cambrian. Both stages of Pampean deformation in this segment of the western pre-Andean Gondwana margin seem to represent a continuous event that can be related to oblique dextral convergence between the overriding plate in the east and the subducting and finally colliding plate in the west. The postkinematic intrusions and extrusions are related to the late stage of the Pampean magmatic history, which terminated before Early Ordovician (Famatinian) time.

Infrastructure and provenance of Ganderia: Evidence from detrital zircon ages in the Brookville terrane, southern New Brunswick, Canada

The Brookville terrane in southern New Brunswick contains some of the oldest known sedimentary rocks of the Ganderian microcontinent of the northern Appalachian orogen. It includes the stromatolitic metacarbonate- and quartzite-dominated Ashburn Formation and metasiltstone-dominated Martinon Formation, in mylonitic contact with the Brookville Gneiss, an assemblage of low-pressure/high-temperature paragneiss and tonalitic orthogneiss. All of the metasedimentary units were intruded by the ca. 550–528Ma subduction-related Golden Grove Plutonic Suite. Analysis by LA-ICP-MS and TIMS of zircon grains from the Ashburn Formation quartzite yielded no <10% discordant ages younger than ca. 1200Ma, a predominance of Meso- and Paleoproterozoic ages back to ca. 2100Ma, and a few Neoarchaean ages. A sample from the metasiltstone matrix of a carbonate olistostrome in the Martinon Formation contains rounded grains with ages mainly between ca. 1000Ma and 2200Ma, indicating provenance similar to that of the Ashburn Formation. However, the sample also contains euhedral grains with ages averaging around 650Ma, indicating the maximum depositional age. A quartzite/calc-silicate sample from the Brookville paragneiss yielded detrital zircon grains with most ages between 1150Ma and 2000Ma, indicating provenance similar to that of the Ashburn and Martinon formations. An orthogneiss sample yielded euhedral (igneous) zircon grains with a concordia age of 615±4Ma, indicating an interval of continental margin subduction at least 60Ma older than that represented by the Golden Grove Plutonic Suite. The Ashburn and Martinon formations and Brookville paragneiss are evidence for development of a Late Cryogenian passive margin on the Proto-Andean-Caribbean edge of Amazonia after it had separated from Laurentia by ca. 650Ma to open the Puncoviscana Ocean. By 615Ma at least part of this margin had become a subduction zone which remained active during the Late Neoproterozoic-Early Cambrian, part of the Pampean arc system. Brookville and related terranes rifted away in the mid-Cambrian to form the foundation of Early Paleozoic Ganderia, which ultimately collided with composite Laurentia during the Salinic orogeny.

Diversity of Rhyacian granitoids in the basement of the Neoproterozoic-Early Cambrian Gurupi Belt, northern Brazil: Geochemistry, U–Pb zircon geochronology, and Nd isotope constraints on the Paleoproterozoic magmatic and crustal evolution

The Gurupi Belt, in northern Brazil, is a Neoproterozoic-Early Cambrian orogen developed at the south-southwestern margin of the São Luís-West African Craton. Several plutonic bodies are exposed as basement units of the Gurupi Belt and represent a variety of granitoid types emplaced at different ages and all show zircon inheritance and chemical and isotopic features that imply participation of reworked Archean to Paleoproterozoic crust in the magma genesis in clear contrast with the juvenile characteristics of the predominant magmatic unit of the neighboring São Luís cratonic fragment, the Tromaí Intrusive Suite of 2167–2147Ma. The weakly peraluminous, high-K, calc-alkaline biotite-bearing Cantão Granodiorite of 2163±4Ma has TDM model ages of 2.21–2.92Ga and ɛNd values of +2.7 to −7.1. The 2142±9Ma-old, weakly peraluminous, biotite- and muscovite-bearing Jonasa Granodiorite underwent metamorphism and deformation at 525Ma and shows TDM model ages of 2.14–2.40Ga, with ɛNd values of +3.9 to −0.2. Both are, probably, related to a continental arc setting. A relatively large event of strongly peraluminous granitic magmatism took place between 2116 and 2079Ma, and comprises the crust-derived, biotite- and muscovite-bearing granites and leucogranites of probable collisional setting belonging to the Japiim, Tamancuoca, Moça and Maria Suprema units. Despite broadly similar, these units show some chemical differences that are interpreted to result from mixtures of variable amounts of sedimentary and igneous sources of Archean to Paleoproterozoic ages. These granites show TDM model ages of 2.19–2.62Ga and the ɛNd values range from +3.9 to −0.2, and probably represent collision rocks. Associated in space and time with part of the strongly peraluminous granites, the 2100±21Ma-old, metaluminous to slightly peralkaline granites, quartz-monzonites and quartz-syenites of the Anelis Intrusive Suite have characteristics of potassic to shoshonitic post-collisional rocks. The suite shows TDM model ages of 2.27–2.48Ga and ɛNd values of +0.1 to −2.6. The last known event is represented by the post-orogenic, 2085±4Ma-old, weakly peraluminous, high-K, calc-alkaline, biotite-bearing Timbozal Monzogranite, with TDM model ages of 2.27–2.58Ga and ɛNd values of +1.1 to −2.8.The strongly peraluminous and different generations of potassic granitoids represent magmatic events with characteristics that may be related to both continental arc subduction and collision processes and indicate that the granitoid bodies represent disrupted fragments of a Rhyacian orogenic belt (continental arc and collision belt) that remains today as basement inliers within the Gurupi Belt in the margin of the São Luís-West African Craton. These sequences, together with the juvenile rocks of the São Luís cratonic fragment, are part of a protracted Rhyacian orogen that evolved from ∼2240Ma to ∼2050Ma, involving accretionary and collision processes. These sequences also correlate in time and several aspects of evolutionary processes with Eburnean-Birimian terranes of the West-African Craton and Trans-Amazonian belts of the Amazonian Craton.

Potential geodynamic relationships between the development of peripheral orogens along the northern margin of Gondwana and the amalgamation of West Gondwana

The Neoproterozoic-Early Cambrian evolution of peri-Gondwanan terranes (e.g. Avalonia, Carolinia, Cadomia) along the northern (Amazonia, West Africa) margin of Gondwana provides insights into the amalgamation of West Gondwana. The main phase of tectonothermal activity occurred between ca. 640–540 Ma and produced voluminous arc-related igneous and sedimentary successions related to subduction beneath the northern Gondwana margin. Subduction was not terminated by continental collision so that these terranes continued to face an open ocean into the Cambrian. Prior to the main phase of tectonothermal activity, Sm-Nd isotopic studies suggest that the basement of Avalonia, Carolinia and part of Cadomia was juvenile lithosphere generated between 0.8 and 1.1 Ga within the peri-Rodinian (Mirovoi) ocean. Vestiges of primitive 760–670 Ma arcs developed upon this lithosphere are preserved. Juvenile lithosphere generated between 0.8 and 1.1 Ga also underlies arcs formed in the Brazilide Ocean between the converging Congo/Sao Francisco and West Africa/Amazonia cratons (e.g. the Tocantins province of Brazil). Together, these juvenile arc assemblages with similar isotopic characteristics may reflect subduction in the Mirovoi and Brazilide oceans as a compensation for the ongoing breakup of Rodinia and the generation of the Paleopacific. Unlike the peri-Gondwanan terranes, however, arc magmatism in the Brazilide Ocean was terminated by continent-continent collisions and the resulting orogens became located within the interior of an amalgamated West Gondwana. Accretion of juvenile peri-Gondwanan terranes to the northern Gondwanan margin occurred in a piecemeal fashion between 650 and 600 Ma, after which subduction stepped outboard to produce the relatively mature and voluminous main arc phase along the periphery of West Gondwana. This accretionary event may be a far-field response to the breakup of Rodinia. The geodynamic relationship between the closure of the Brazilide Ocean, the collision between the Congo/Sao Francisco and Amazonia/West Africa cratons, and the tectonic evolution of the peri-Gondwanan terranes may be broadly analogous to the Mesozoic-Cenozoic closure of the Tethys Ocean, the collision between India and Asia beginning at ca. 50 Ma, and the tectonic evolution of the western Pacific Ocean.

The Chemistry of Manganese Ores through Time: A Signal of Increasing Diversity of Earth-Surface Environments

Almost all economic manganese ores are ultimately or directly derived from hydrothermal vents associated with intermediate volcanic rocks. This source is in contrast to deep-sea nodules, which likely have a larger component of sediment-derived manganese and whose volcanic sources are more mafic. Manganese deposits can be divided into sedimentary rock-hosted, volcanic rock-hosted and karst-hosted, in order of predominance. Two genetic types of sedimentary rock-hosted deposits can also be identified: those with Mn derived via upwelling from oxygen-minimum zones and those formed on the margins of euxinic basins. Most of the large tonnage deposits appear to form by the euxinic mechanism. Manganese ores, like those of Fe, show a strong concentration of deposits in the Paleoproterozoic and a lesser occurrence in the Neoproterozoic, but, unlike Fe, there is an additional strong peak in the Oligocene. Therefore, Mn is not controlled entirely by the level of oxygen in the Earth’s atmosphere. At each peak of Mn deposition, the associated ore deposits are concentrated in a few districts, suggesting a more local than global control on manganese metallogenesis. Age trends can, however, be discerned in some chemical properties of manganese deposits. Overall, there is a trend to progressive increases in chemical diversity from the Archean to the Recent, with a particularly steep increase in the Neoproterozoic-Early Cambrian, corresponding in time to the radiation of metazoans. Also beginning in the Cambrian is the development of upwelling-linked deposits. There is another sharp increase in chemical diversity at the Jurassic-Cretaceous boundary, which includes increased SiO 2 /Al 2 O 3 ratios and corresponds to the radiation of diatoms. There is a conspicuous gap in sedimentary rock-hosted Mn deposits between 1800 and 800 Ma that may correspond to a monotonous, low-oxygen ocean, but one without sulfidic deep water. Alternatively, Mn may have been precipitated entirely in the deep ocean, beneath a sulfidic oxygen minimum layer. The positive Eu anomalies, which in iron formations are equated to vent-sourced metals, are not seen in most Mn deposits, although they are found in Mn-rich iron formations. By contrast, Fe deposits interbedded with major Mn ores lack the usual Eu signal. Therefore, mechanisms of transport between hydrothermal vents and the sites of deposition differed for Fe and for Mn deposits in the Archean-Paleoproterozoic. The dominant pattern in the time trend of Mn deposition is increasing chemical diversity, which reflects an increasing compartmentalization of the Earth’s depositional environments. This compartmentalization was a response to, but also provided a spur to, the diversification of life forms.

Comparative evolution of the Iapetus and Rheic Oceans: A North America perspective

Differences in the origin and evolution of the Iapetan and Rheic oceans profoundly affected the development of the Ouachitan–Appalachian–Variscan orogen. The Iapetus Ocean was initiated in the Late Neoproterozoic–Early Cambrian by the separation of large continental landmasses (Laurentia, Baltica, Gondwana), whereas the Rheic Ocean originated in the Late Cambrian–Early Ordovician as the result of the separation of several ribbon continents, collectively termed peri-Gondwanan terranes, from the northern margin of Gondwana. Compared to the classic passive margin developed along the Laurentian margin of Iapetus, passive margin successions to the Rheic Ocean are more limited in extent. During convergence, both margins of the Iapetus Ocean developed western Pacific-type arc–back-arc systems, whereas subduction of the Rheic Ocean was primarily directed beneath Laurentia and was Andean in style. Closure of Iapetus involved a number of arc–back-arc accretionary events and had occurred by the Middle Silurian as a result of Laurentia–Baltica collision (to form Laurussia) and the accretion of the peri-Gondwanan terranes. Rheic Ocean closure occurred in the Carboniferous by way of continent–continent collision between Laurussia and Gondwana, with Gondwana on the lower plate. Within Iapetus, the formation of ophiolites occurred early in the orogenic cycle, and was followed soon after by obduction, HP–LT metamorphism and foreland-directed thrusting that is intimately related to accretionary events. These features dominate the northern Appalachian orogen where HT–LP metamorphism is attributed to post-accretionary slab break-off events. In contrast, HP–LT metamorphism within the Rheic Ocean reflects A-type subduction during Laurussia–Gondwana collision, and is rapidly succeeded by HT–LP metamorphism attributed to orogenic collapse. Ophiolites are rare and were formed either very early (Early Ordovician) or very late (Devonian–Carboniferous) in the evolution of the ocean. The Early Ordovician ophiolites were not obducted until the Carboniferous, at least 100 million years after their protoliths were formed. Rheic Ocean closure resulted in the formation of large-scale bivergent structures within the orogen, with symmetry changing on either side of the Rheic suture. Comparison of the evolution of the Iapetus and Rheic oceans suggests that differences in rift configuration, passive margin development, and subduction zone geometry within the two oceans profoundly influenced the subsequent style of orogenesis within the Ouachita–Appalachian–Variscan orogen.

The Basement of the Central Andes: The Arequipa and Related Terranes

The basement of the Central Andes provides insights for the dispersal of Rodinia, the reconstruction of Gondwana, and the dynamics of terrane accretion along the Pacific. The Paleoproterozoic Arequipa terrane was trapped during collision between Laurentia and Amazonia in the Mesoproterozoic. Ultrahigh-temperature metamorphism correlates with the collapse of the Sunsás-Grenville orogen after ∼1000 Ma and is related to slab break-off and dispersal of Rodinia. The Antofalla terrane separated in the Neoproterozoic, forming the Puncoviscana basin. Its closure was coeval with the collision of the eastern Sierras Pampeanas. The rift-drift transitions of the early Paleozoic clastic platform showed a gradual younging to the north, in agreement with counterclockwise rotation based on paleomagnetic data of Antofalla. North of Arequipa arc magmatism and high-grade metamorphism are linked to collision of the Paracas terrane in the Ordovician, during the Famatinian orogeny in the Sierras Pampeanas. The early Paleozoic history of the Arequipa massif is explained by a backarc, which further south changed to open oceanic conditions and subsequent collision. The Antofalla terrane reaccreted to the continental margin by the late Ordovician. These accretions and subsequent separations during the Mesoproterozoic, Neoproterozoic–early Cambrian, and late Cambrian–middle Ordovician are explained by changes in absolute motion of the Gondwana supercontinent during plate global reorganization.

Metamorphosed Ordovician Fe- and Mn-rich rocks in south-central Maine: From peri-Gondwanan deposition through Acadian metamorphism

abstra C t The Wilson Cove Member of the Cushing Formation is a thin (up to 120 m thick) metamorphosed Fe- and Mn-rich unit exposed discontinuously over a distance of >75 km in southern Maine. Cathodoluminescence imaging of zircon grains from the unit reveal texturally isolated detrital cores surrounded by distinct metamorphic overgrowths. U-Pb SHRIMP core ages range from 463 to 2058 Ma with a strong peak in the Neoproterozoic-Early Cambrian. This distribution of ages is consistent with a peri-Gondwanan source region and a Middle to Late Ordovician depositional age. Zircon rims have an age of 373 ± 4 Ma, consistent with growth during late Acadian metamorphism. Whole-rock geochemistry reveals considerable major- and trace-element variability with generally elevated abundances of Fe2O3(t) (15–43 wt%), MnO (0.1–12.1 wt%), Ba (4–2503 ppm), and As (7–1161 ppm). Geochemical discrimination diagrams suggest the protoliths were mixtures of hydrothermal exhalatives and terrigenous clastic sediment, with these materials most likely having been deposited in a marine back-arc basin proximal to a peri-Gondwanan continental source region. Late Devonian low-pressure, amphibolite-facies metamorphism of these bulk compositions produced assemblages dominated by grunerite + garnet + biotite + quartz. These and other mineral assemblages found in the Wilson Cove unit in south-central Maine are consistent with peak metamorphic conditions of 550 to 600 °C and 3–4 kbar previously determined from nearby metapelites. Mineral assemblages, mineral modes, and mineral compositions in these Fe- and Mn-rich rocks are strongly influenced by whole-rock bulk compositional variability. In particular, the compositions of co-existing garnet and grunerite vary systematically as a function of whole-rock MnO concentration.