Mesoproterozoic Rocks Research Articles

Spatial and temporal variation in detrital zircon age provenance of the hydrocarbon-bearing upper Roper Group, Beetaloo Sub-basin, Northern Territory, Australia

The subsurface Beetaloo Sub-basin of the McArthur Basin, Northern Territory, Australia, comprises a succession of shallow-water, dominantly marine, clastic sedimentary rocks that formed in the main depocentre of the Mesoproterozoic Roper Group. This group contains the oldest commercial hydrocarbons known, whose presence has been linked to changing nutrient flux controlled by a changing provenance. LA-ICP-MS detrital zircon U–Pb age data presented here provide new age constraints on the upper Roper Group and reveal spatial and temporal provenance variations illustrating the evolution of the basin and its margins that are linked to a major provenance change caused by the coeval collision of the combined South Australian Craton/North Australian Craton with the West Australian Craton.The maximum depositional ages of the Bessie Creek Sandstone and the Velkerri Formation of the Roper Group are constrained to 1386 ± 13 Ma and 1308 ± 41 Ma, respectively, whereas the overlying Moroak Sandstone has no younger detrital zircons, so its maximum depositional age is also constrained as 1308 ± 41 Ma. The Kyalla Formation was deposited after 1313 ± 47 Ma, and two, as yet, informally defined and ungrouped latest Mesoproterozoic to Neoproterozoic sedimentary units, the lower and upper Jamison sandstone, have maximum depositional ages of 1092 ± 16 Ma and 959 ± 18 Ma, respectively. Large detrital zircon age datasets (of 1204 near-concordant analyses) indicate that zircons from the Maiwok Subgroup were originally sourced from Palaeoproterozoic and earliest Mesoproterozoic rocks. These are consistent with derivation from the surrounding exposed basement. Detrital zircon age variations up-section suggest a systematic temporal change in provenance. The oldest formation analysed (Bessie Creek Sandstone) has a major source dated at ca. 1823 Ma. Rocks of this age are common in northern basement exposures. Samples from the overlying Velkerri Formation, show derivation from a ca. 1590 Ma source, consistent with rocks exposed in Queensland, or the Musgrave Province. The Moroak Sandstone and the Kyalla Formation show progressively more ca. 1740 Ma detritus, which we suggest likely reflects new sources in the Arunta Region to the south.We suggest that the provenance variation initially records exposure and denudation of western Queensland rocks at ca. 1400 Ma due to rifting between Laurentia and the North Australian Craton. From then until at least ca. 1320 Ma, the increased ca. 1740 Ma detritus suggests uplift of the Arunta Region that we interpret as reflecting collision between the southern North Australian Craton and the West Australian Craton as ca. 1300–1400 Ma. This tectonically-controlled provenance change is interpreted to have included erosion of nutrient rich arc-rocks that may have caused a bacterial bloom in the Roper Seaway. The Jamison sandstone and overlying Hayfield mudstone represent a marked change in provenance and were deposited after the Musgrave Orogeny, representing a newly-recognised siliciclastic basin that may have formed a shallow, long wavelength foreland basin to areas uplifted during the Musgrave Orogeny.

The biostratigraphic conundrum of Siberia: Do true Tonian–Cryogenian microfossils occur in Mesoproterozoic rocks?

Abundant and diverse microfossils from shales of the ∼1500-Ma-old Ust’-Il’ya Formation in northern Siberia document early Mesoproterozoic life along the Siberian Platform. Similar to many Meso- and Neoproterozoic microbiota, the Ust’-Il’ya assemblage is dominated by cyanobacteria, both filamentous (oscillatorialeans and nostocaleans, represented primarily by cellular trichomes and empty sheaths) and coccoidal (chroococcaleans, including solitary and colonial specimens). Microbiotas from shallow-water settings that contain morphologically simple and small microfossils were originally taken to be representative of Mesoproterozoic life. However, the Ust’-Il’ya and other compressed organic-walled microfossil assemblages in open shelf facies include diverse microscopic eukaryotes: large (up to ∼1-mm diameter) megasphaeromorph acritarchs, branched filaments that are most likely of eukaryotic algae and other morphologically complex microorganisms.The Ust’-Il’ya assemblage contains numerous taxa previously reported from Tonian and Cryogenian successions (e.g., Elatera, Pterospermopsimorpha, Rugosoopsis and Chuaria), but these are long-ranging genera and no Neoproterozoic index fossils have been recorded. The microbiota is considered to be Mesoproterozoic in age because it contains a few taxa reported from the Calymmian microfossil assemblages (e.g., Lineaforma aff. L. elongata, Satka favosa and Elatera aff. E. minor). The Ust’-Il’ya mixed assemblage of prokaryotic and eukaryotic microorganisms records an evolutionary stage transitional between the prokaryote-dominated Paleoproterozoic and the eukaryote-dominated Neoproterozoic world.As reported in this study, 38 taxa were identified in the Ust’-Il’ya assemblage (of which six forms are described informly) that are assigned to 17 genera of microscopic eukaryotes (presumably 17–20 entities) and prokaryotes. A new species is proposed: Heliconema ustilense.

Constraints of molybdenite Re–Os and scheelite Sm–Nd ages on mineralization time of the Kukaazi Pb–Zn–Cu–W deposit, Western Kunlun, NW China

The Kukaazi Pb–Zn–Cu–W polymetallic deposit, located in the Western Kunlun orogenic belt, is a newly discovered skarn-type deposit. Ore bodies mainly occur in the forms of lenses and veins along beddings of the Mesoproterozoic metamorphic rocks. Three ore blocks, KI, KII, and KIII, have been outlined in different parts of the Kukaazi deposit in terms of mineral assemblages. The KI ore block is mainly composed of chalcopyrite, scheelite, pyrrhotite, sphalerite, galena and minor pyrite, arsenopyrite, and molybdenite, whereas the other two ore blocks are made up of galena, sphalerite, magnetite and minor arsenopyrite and pyrite. In this study, we obtained a molybdenite isochron Re–Os age of 450.5 ± 6.4 Ma (2σ, MSWD = 0.057) and a scheelite Sm–Nd isochron age of 426 ± 59 Ma (2σ, MSWD = 0.49) for the KI ore block. They are broadly comparable to the ages of granitoid in the region. Scheelite grains from the KI ore block contain high abundances of rare earth elements (REE, 42.0–95.7 ppm) and are enriched in light REE compared to heavy REE, with negative Eu anomalies (δEu = 0.13–0.55). They display similar REE patterns and Sm/Nd ratios to those of the coeval granitoids in the region. Moreover, they also have similar Sr and Nd isotopes [87Sr/86Sr = 0.7107–0.7118; eNd(t) = −4.1 to −4.0] to those of such granitoids, implying that the tungsten-bearing fluids in the Kukaazi deposit probably originate from the granitic magmas. Our results first defined that the Early Paleozoic granitoids could lead to economic Mo–W–(Cu) mineralization at some favorable districts in the Western Kunlun orogenic belt and could be prospecting exploration targets.

Current Literature Survey

The Imiter inlier (eastern part of the Moroccan Anti-Atlas) is located on the northwestern border of the West African Craton (WAC) and exhibits a range of Pan-African granitoids. Three massifs that crosscut the Imiter Saghro Group were targeted in this work: the Igoudrane granodiorite, Bou Teglimt granodiorite and Bou Fliou granite. We present here additional geochemical analyses (major and trace elements) and Sm-Nd isotopic data, which define two distinct groups: (i) the Igoudrane massif (677 Ma) and (ii) the Bou Teglimt granodiorite (576 Ma) and the Bou Fliou granite (550 Ma).Geochemical data confirm the calc-alkaline signature of the studied granitoids. Both groups of granitoids are slightly peraluminous and show strong negative anomalies in Nb, Ta and Ti in multi-element plots normalized to the primitive mantle. The granitoids have low 143Nd/144Nd initial ratios (0.5116–0.5117), with TDM model ages ranging from 1.73 to 1.52Ga. The εNd(t) values are negative, decreasing from the Igoudrane samples (−1.1 to −3.1) to the Bou Teglimt granodiorite (−3.0 to −3.3) and Bou Fliou (−4.2 to −4.8). All these data suggest a mixed magmatic origin involving a juvenile mantle source and an old, at least Paleoproterozoic crust. Given Mesoproterozoic rocks are lacking or very scarce in the Anti-Atlas, these results confirm the existence of an old cratonic basement beneath the eastern Anti-Atlas, and therefore suggest that the northern border of the West African Craton must be placed further to the north, as suggested by previous work in this region.

Platinum group element mineralization at Musongati (Burundi): concentration and Pd-Rh distribution in pentlandite

1. Introduction The platinum group elements (PGE: Ru, Rh, Pd, Os, Ir & Pt) are considered as critical metals (European Commission, 2014) and are highly valued for their high-tech applications. They are being recycled and intensely mined, but still deficits are experienced and expected in the coming years (European Commission, 2014). Since the large PGE deposits, such as the Bushveld Complex in South Africa and the Noril’sk-Talnakh deposits in Russia, will become depleted with time, new deposits need to be explored for their PGE potential, to sustain future demand. The mafic-ultramafic intrusions in Burundi, which are part of the Kabanga-Musongati alignment, are such potential deposits. They intruded the Mesoproterozoic rocks of the Karagwe-Ankole belt around 1375 Ma and form a SW-NE alignment of nine intrusions in Burundi, with further continuation towards Tanzania (Fig. 1; Fernandez-Alonso et al., 2012). Several drilling campaigns have been executed between 1970 and 1990 to explore the nickel and PGE potential of these intrusions (PNUD-UNDP, 1977; Exploration und Bergbau Gmbh, 1985;Deblond, 1994; Deblond & Tack, 1999). Although some limited data on the concentration of PGE in the boreholes of these campaigns is available (e.g. Klerkx, 1975, 1976), not much is known about the PGE distribution. In addition, the petrogenesis of the intrusions needs further elaboration, expanding on the work of e.g. Bandyayera (1997) and Duchesne et al. (2004). Figure 1. (A) Regional geology o

Nature of Late Mesoproterozoic to Early Neoproterozoic magmatism in the western Gyeonggi massif, Korean Peninsula and its tectonic significance

The western margin of the Gyeonggi massif, southern Korean Peninsula, has preserved N–S trending Neoproterozoic and sporadic Late Mesoproterozoic metaigneous rocks. Here we present the results from systematic field mapping, sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb dating, and whole-rock geochemical analyses of the Mesoproterozoic and Early Neoproterozoic metaplutonic rocks in the Hongseong area, together with previously published data from the western Gyeonggi massif. The SHRIMP ages of these rocks are categorized into three groups: (1) Late Mesoproterozoic (ca. 1.25–1.15Ga), (2) Early Neoproterozoic (ca. 900–770Ma), and (3) late Early Neoproterozoic (ca. 762–730Ma). The geochronological and geochemical features of the Late Mesoproterozoic rocks suggest that they were possibly formed in association with convergent plate motion. The Early and late Early Neoproterozoic rocks are interpreted to arc-related orogenic and rift-related post-orogenic environments, respectively. These age results and the tectonic signatures provide insight into the convergence process along the margins of the Rodinia supercontinent.

Sources of Mesoproterozoic igneous rocks and formation time of the continental crust of the Kokchetav Massif (Northern Kazakhstan)

Within the Kokchetav massif (Northern Kazakhstan), Mesoproterozoic granites and acid volcanics are widespread: these are the youngest Precambrian igneous rocks forming basement of the region. The Nd isotopic characteristics (eNd(t)–4.4 ÷–9.6, t Nd(DM) 2.1–2.6 Ga) obtained for these rocks indicate that the source of their melts was the Early Precambrian continental crust. Thus, the continental crust of the Kokchetav Massif had basically been formed by the beginning of the Mesoproterozoic and during the Late Precambrian: later it became a source for the granitoid melts.

Stratigraphy and Correlation of Mesoproterozoic Rocks Associated with Mohar Cauldron, Shivpuri District, Madhya Pradesh

Abstract: A circular structure, termed as cauldron of volcanic origin, was located near Mohar village in Shivpuri district (M.P.) in the year 2000. Subsequently, the same structure was called as Dhala structure of impact origin. There may be debate over the origin and evolution of this circular structure, but it is characterized by a unique lithological set-up within the Bundelkhand craton. The circular structure is defined by annular disposition of igneous and sedimentary rocks. This includes a set of felsic volcanic rocks and associated breccias named as Mohar Formation, exposed in the outer rim of the circular structure. The inner part of the circular structure has sedimentary sequence, termed as Dhala Formation. The field relations indicate that the Mohar and Dhala foarmations are younger than Bundelkhand granitoid complex but older than Kaimur Group. This period in Indian stratigraphy corresponds to Semri Group which consists of Porcellanite Formation, the rocks of which have formed due to deposition of volcanic ash. The geochronological data and field relations between different litho-units indicate that the Mohar volcanism which generated large volume of volcanic ash was a possible source for the formation of Porcellanite Formation. The deposition of sedimentary sequence in main Vindhyan basin was continued, whereas the volcanic activity in Mohar area continued till H ≈ 1.0 Ga. Since, acid volcanic activity has been reported in different parts of the world at H ≈ 1.0 Ga., it is possible that the Mohar acid volcanic activity is not an isolated event; instead it may be a part of global volcanic activities around H ≈ 1.0 Ga.

Geochemistry and Sm–Nd isotopic composition of the Imiter Pan-African granitoids (Saghro massif, eastern Anti-Atlas, Morocco): Geotectonic implications

The Imiter inlier (eastern part of the Moroccan Anti-Atlas) is located on the northwestern border of the West African Craton (WAC) and exhibits a range of Pan-African granitoids. Three massifs that crosscut the Imiter Saghro Group were targeted in this work: the Igoudrane granodiorite, Bou Teglimt granodiorite and Bou Fliou granite. We present here additional geochemical analyses (major and trace elements) and Sm-Nd isotopic data, which define two distinct groups: (i) the Igoudrane massif (677 Ma) and (ii) the Bou Teglimt granodiorite (576 Ma) and the Bou Fliou granite (550 Ma).Geochemical data confirm the calc-alkaline signature of the studied granitoids. Both groups of granitoids are slightly peraluminous and show strong negative anomalies in Nb, Ta and Ti in multi-element plots normalized to the primitive mantle. The granitoids have low 143Nd/144Nd initial ratios (0.5116–0.5117), with TDM model ages ranging from 1.73 to 1.52Ga. The εNd(t) values are negative, decreasing from the Igoudrane samples (−1.1 to −3.1) to the Bou Teglimt granodiorite (−3.0 to −3.3) and Bou Fliou (−4.2 to −4.8). All these data suggest a mixed magmatic origin involving a juvenile mantle source and an old, at least Paleoproterozoic crust. Given Mesoproterozoic rocks are lacking or very scarce in the Anti-Atlas, these results confirm the existence of an old cratonic basement beneath the eastern Anti-Atlas, and therefore suggest that the northern border of the West African Craton must be placed further to the north, as suggested by previous work in this region.

Geochronology and geochemical characteristics of the Dongping ore-bearing granite, North China: Sources and implications for its tectonic setting

The Dongping gold deposit, located in Chongli County (Hebei Province) about 200km northwest of Beijing, is one of the largest gold-producing areas along the northern margin of the North China Craton. It is located in the of Shuiquangou alkaline igneous complex of Middle Devonian age (394.3±3.2Ma), composed chiefly of highly alkaline syentite and quartz syenites. This study reveals the age of the Carboniferous in the deposit at 351.7±2.8Ma (MSWD=1.9). The Dongping deposit is locally hosted in Cretaceous (~143±1Ma) alkali granites that intruded the older and the gold mineralization is closely associated genetically with this event. Hydrothermal zircons in the alkali granites have Th/U ratios mostly ranging between 0.01 and 0.7 indicating oscillatory zoning. A few grains with high Th/U ratios (1.31–2.07) may be from metamorphic domains. Negative εHf(t) values of the zircon mainly range between −19.75 and −16.93, suggesting that they originated principally by the melting of recycled continental crust. Less abundant zircons with εHf(t) ranging from −25.76 to −23.46, with Hf model ages (TDM2) of 2.54 to 2.67Ga, (mainly 2.2 to 2.3Ga) suggest that recycled Neoarchean basement was also present in the source region. The Devonian syenites and quartz syenites have TDM1 ages ranging from 1.96 to 2.08Ga. Zircons from these rocks have εHf(t) values of −11.9 to −18.9. Certain zircons from the gold-bearing granite of Paleozoic age have an initial 176Hf/177Hf ratio of 0.281816 to 0.282058 and 0.282147 to 0.282348, reflecting a homogenous distribution of hafnium isotopes typical of magmatic sources. The TDM1 and TDM2 of the latest intrusion varying 1.33 to 1.59Ga and 1.72 to 2.11Ga respectively, indicating that the Neoproterozoic to Mesoproterozoic rocks of this area are an important source for the younger magma which are important to forming ore deposits. The TDM2 indicate that the magma may be derived from a very old crustal basement (~2.67Ga) in the northern margin of North China Craton by partial melting.

Detrital zircon in a supercontinental setting: locally derived and far-transported components in the Ordovician Natal Group, South Africa

U–Pb and Lu–Hf signatures of detrital zircon from conglomerates and sandstones of the Ordovician Natal Group, South Africa were determined using laser ablation inductively coupled plasma mass spectrometry. The basal conglomerates are dominated by Palaeo- to Mesoarchaean detrital zircon with ε Hf values from +3 to −4 with minor Mesoproterozoic input, indicating a proximal source in the Kaapvaal Craton and minor input from rocks of the Natal Sector of the Mesoproterozoic Namaqua–Natal Province. The sandstones are all dominated by a combination of juvenile Mesoproterozoic zircon and Neoproterozoic zircon derived from Mesoproterozoic rocks that were reworked during the Pan-African Orogeny. Several sedimentary sequences from former Gondwana with Neoproterozoic to Permian depositional ages show similar detrital zircon signatures. Sedimentary sequences of such vast temporal and geographical distribution are unlikely to have been fed by a single source, making it more likely that these sequences were fed by several different (Pan-Gondwana) source terranes with closely similar U–Pb and Lu–Hf zircon signatures. The results show that source terrane non-uniqueness can make ascertaining sedimentary provenance from detrital zircon impossible, and should be taken as a reminder when using detrital zircon as evidence for far-reaching conclusions in basin evolution studies and palaeogeography. Supplementary materials: U–Pb and Lu–Hf data are available at http://www.geolsoc.org.uk/SUP18880 .

Zircon U-Pb, Hf and O isotope constraints on growth versus reworking of continental crust in the subsurface Grenville orogen, Ohio, USA

Combined U-Pb, O and Hf isotope data in zircon allows discrimination between juvenile and reworked crust, and is therefore a useful tool for understanding formation and evolution of the continental crust. The crustal evolution of basement rocks in central North America (Laurentia) is poorly constrained, as it is almost entirely overlain by Palaeozoic cover. In order to improve our understanding of the evolution of this region we present U-Pb, O and Hf isotope data from zircon in drill-core samples from the subsurface basement of Ohio. The Hf isotope data suggests juvenile crust formation at ∼1650 Ma followed by continued reworking of a single reservoir. This 1650 Ma reservoir was tapped at ∼1450 Ma during the formation of the Granite-Rhyolite Province and subsequently reworked again during the Grenvillian orogeny. The ∼1650 Ma crust formation model age for the suite of samples along with the presence of ∼1650 Ma magmatic rocks suggests an eastward extension of the Mazatzal Province (or Mazatzal-like crust) and makes it a possible protolith to the subsurface basement of Ohio and surrounding Mesoproterozoic (i.e. Grenville-age) rocks. The eastward extension of this ∼1650 Ma crustal reservoir into Ohio requires a revision of the crustal boundary defined by Nd isotopic data to be located further east, now overlapping with the Grenville front magnetic lineament in Ohio. In fact, the easternmost sample in this study is derived from a more depleted reservoir. This limits the extent of >1.5 Ga basement in subsurface Ohio and constrains the location of the crustal boundary. Further, syn-orogenic magmatism at ∼1050 Ma suggests a potential extrapolation of the Interior Magmatic Belt into Ohio. Oxygen isotopic data in zircon suggests that during Grenvillian metamorphism, zircon recrystallisation occurred in the presence of heavy δ18O fluids resulting in zircon with elevated δ18O values.

Palaeomagnetism of Mesoproterozoic limestone and shale successions of some Purana basins in southern India

AbstractThe ‘Purana’ basins were long considered Neoproterozoic basins until geochronology and palaeomagnestism showed parts of the Chattisgarth and lower Vindhyan basins to be a billion years older. Historically, the successions in the Chattisgarth Basin are correlated with similar successions in the Pranhita–Godavari and Indravati basins. In India, differentiating between early–late Mesoproterozoic rocks and those spanning the Mesoproterozoic–Neoproterozoic boundary is possible by comparing magnetic declination and inclination; palaeomagnetism is therefore a very useful correlation tool. Here we report a new Stenian-aged palaeopole (50.1°N, 67.4°E, radius of cone of 95% confidence A95 = 12.4°, precision K = 30.1) from carbonate and shale successions of the Pranhita–Godavari and Chattisgarth basins (the C+/– magnetization). In addition, an early diagenetic remagnetization (component A) was identified. No primary or early diagenetic magnetizations were identified from the Indravati Basin. Here, as well as in stratigraphically higher parts of the other two successions, widespread younger magnetic overprints were identified (B+ and B– magnetic components). Our C+/– palaeopole is constrained by palaeomagnetic stability field tests, is different from known 1.4 Ga and 1.0 Ga Indian palaeopoles, but similar to a 1.19 Ga palaeopole. Penganga Group (Pranhita–Godavari Basin) deposition was probably initiated at around 1.2 Ga. A similar palaeomagnetic signature confirms its correlation with the Raipur Group (Chattisgarth Basin), of which the deposition spans most of the Stenian period (c. 1.2–1.0 Ga). Sedimentation in these groups began significantly later than c. 1.4 and c. 1.6 Ga, as suggested by ages reported from below the Raipur and Penganga groups, respectively.

Petrogenetic study of Mesoproterozoic volcanic rocks of North Delhi fold belt, NW Indian shield: implications for mantle conditions during Proterozoic

Mesoproterozoic North Delhi fold belt of NW Indian shield comprises three volcano-sedimentary basins viz. Bayana, Alwar and Khetri aligned parallel to each other from east to west. Each basin contains excellent exposures of mafic volcanic rocks. Major, trace and rare earth element abundances of volcanic rocks of the three basins are significantly diverse. Bayana and Alwar volcanics are tholeiites bearing close similarity with low Ti-continental flood basalts. However, Bayana volcanics are characteristically enriched in incompatible trace elements and REEs while Alwar volcanics display least enriched incompatible trace element abundances and flat REE patterns. The Khetri volcanics exhibit a transitional composition between tholeiite and calc-alkaline basalts. REE based source modeling suggests that Bayana suite was formed from the melts derived from 1 % to 10 % (avg. 4 %) of the partial melting of a spinel lherzolite source giving a residual mineralogy of 56 % Olv, 25 % Opx and 19 % Cpx. Whereas Alwar suite evolved through 12 %–20 % (avg. 15 %) partial melting of the same source with a residual mineralogy 61 % Olv, 25 % Opx and 14 % Cpx. Khetri volcanics are exposed at two localities Kolihan and Madhan–Kudhan. The Kolihan volcanics were derived from 1 % to 6 % (avg. 4 %) partial melting with residual mineralogy 56 % Olv, 25 % Opx and 19 % Cpx whereas the magma of Madhan Kudhan volcanic suite was generated by 15%–30 % partial melting of the same source leaving behind 64 % Olv, 25 % Opx and 11 % Cpx as residual mineralogy. This source modeling proves that melts of Bayana and Alwar tholeiites were generated by partial melting of a common source within the spinel stability field under the influence of mantle plume. During the course of ascent, Bayana melts were crustally contaminated but Alwar melts remained unaffected. There was two tier magma production in Khetri region, one from the partial melting of the mantle wedge overlying the subducted oceanic plate which formed Kolihan suite and two the melting of the subducted plate itself generating Madhan–Kudhan volcanics. It is interpreted that during Mesoproterozoic (1,800 Ma), the continental lithosphere of NW Indian shield suffered stretching, attenuation and fracturing in response to a rising plume. Consequently, differential crustal extension coupled with variable attenuation brought the asthenosphere to shallower setting which led to the production of tholeiitic melts. These melts enroute to the surface suffered variable lithospheric contamination depending upon the thickness of traversed crust. The Khetri basin attained maturity which resulted in the generation of true oceanic crust and its subsequent destruction through subduction. The spatial existence of three suites of mafic volcanics of diverse chemical signatures is best example of subduction–plume interaction. It is therefore, proposed that the Mesoproterozoic crust of NW Indian shield has evolved through the operation of a complete Wilson cycle at about 1,832 Ma, the age of mafic volcanics of Khetri basin.

Geochemical and Sr–Nd isotopic constraints on the mantle source of Neoproterozoic mafic dikes of the rifted eastern Laurentian margin, north-central Appalachians, USA

Abundant and widely distributed unmetamorphosed mafic dikes intrude Mesoproterozoic rocks of the New Jersey Highlands. The age of the dikes is imprecisely known but interpreted to fall between 615 and 576 Ma, which is consistent with the range of ages of mafic dikes from Labrador and Nova Scotia south to Pennsylvania that were emplaced along the rifted eastern Laurentian margin. New Jersey Highlands dikes are a few cm to 18 m wide and have lengths of as much as several km. They have sharp, largely discordant contacts against enclosing Mesoproterozoic rocks and aphanitic chilled margins that grade into coarser grained interiors. Columnar joints are present locally and suggest emplacement at a shallow crustal level. Geochemical compositions of the dikes range from alkalic to less common tholeiitic basalt having generally high TiO 2 , P 2 O 5 , Zr, Nb, Y, and La/Yb, and low MgO, Cr, and Ni. TiO 2 contents define high-Ti and low-Ti dikes that differ in high field strength elements (HFSE) and light rare earth elements (LREE) but overlap in abundances of most other elements. Dike magma evolved in an ascending mantle plume of OIB-like asthenosphere from enriched higher TiO 2 compositions to more depleted lower TiO 2 compositions. Subtle differences in the dike compositions are due to variations in the amount of partial melting within the plume and the depth of melt segregation. Sr–Nd isotope values of both dike compositions overlap and are characterized by ε Nd (T) of + 1.5 to + 3.8 and initial 87 Sr/ 86 Sr ranging from 0.7032 to 0.7077. Higher Sr isotope ratios are interpreted as resulting from local interaction of the dike magma with heterogeneous, high 87 Sr/ 86 Sr lithospheric mantle having EMI or EMII-like geochemical characteristics. Dikes form tabular structures that have long segments striking an average of N44°E and short segments striking about east–west. Their regional geometries form right-stepping, rhomb-shaped patterns due to emplacement into rift-related dilational fractures likely formed through a combination of southeast-directed extension and strike-slip shear stresses. Geochemical compositions of the dikes are the same regardless of their structural trend or location implying they formed during a single magmatic event. They, along with other mafic dikes in the north-central Appalachians, were emplaced in a within-plate tectonic setting along the rifted margin of eastern Laurentia, prior to opening of the Iapetus Ocean. • Mantle sources of Neoproterozoic mafic dikes were modeled from Sr–Nd isotopes and geochemistry. • Dikes formed from partial melting of a garnet-free, enriched OIB-like source. • Geochemical similarity to other regional Neoproterozoic dikes along the eastern Laurentian margin • Dikes emplaced from single magmatic event during rifting that formed the Iapetus Ocean

Early Neoproterozoic metagabbro-tonalite-trondhjemite of Sør Rondane (East Antarctica): Implications for supercontinent assembly

New data for intrusive meta-igneous rocks from the Southwest terrane of the Sør Rondane Mountains confirm the view that this is a juvenile oceanic arc terrane, with the main phase of subduction-related magmatic activity around 995–975Ma. Younger magmatism (960–925Ma) is more varied: a high Sr/Y (‘adakitic’) suite is present, as well as high-Ti mafic dykes, and one sample of A-type granite. This is interpreted as reflecting the end of subduction. The occasional presence of Archaean inherited zircons suggests proximity of Sør Rondane to an older continental nucleus from which detritus was shed. Although the ‘meta-igneous sector’ appears to be unique in representing a juvenile oceanic arc terrane, igneous ages and isotopic compositions around 1000–900Ma suggest a broad coherence between outcrops ranging from Schirmacher Oasis (11° E) to Yamato Mts (35° E). This area seems unrelated to the slightly older, and isotopically and geochemically more enriched Mesoproterozoic rocks of central and western Dronning Maud Land. A closer relation appears to exist with Sri Lanka-India-Madagascar during the earliest Neoproterozoic than with southern Africa.

Borehole logs from the Precambrian basement on Bornholm, eastern Denmark: geology and groundwater flow

Bornholm is situated in the Sorgenfrei–Tornquist Zone that separates the North-West European Craton from the Baltic Shield and the East European Platform (Fig.1). The Precambrian basement of northern and eastern Bornholm consists of different granitic and gneissic Mesoproterozoic rocks that are dated to c. 1455 Ma (Waight et al. 2012). It appears from the age data that granitic magmatism, deformation and metamorphism occurred over a relatively short time period. The rocks contain abundant pegmatite and aplite bodies. More than 250 mafic dykes occur. The dykes were intruded during three Precambrian events at c. 1326 Ma, 1220 Ma, 950 Ma and during a Permian event at c. 300 Ma (Holm et al. 2010).

Fluid Inclusions and their Geological Significance of Huajian Gold Deposit, Hebei Province, China

The Huajian gold deposit is located in the metallogenic belt of the northern part of the North China block. This deposit's ore bodies are mainly hosted in metamorphosed Neoarchean and Mesoproterozoic sedimentary rocks, of which Mesozoic volcano-intrusive complexes are closely associated with the Gold mineralization. The FIs of the Huajian deposit are primarily aqueous FIs with minor gas FIs. The pure gas or liquid FIs are very few. The ore-forming fluids were characterised by moderate–low temperature, low salinity and high oxygen fugacity and belonged to an H2O–NaCl ± CO2system. The FIs in quartz veins primarily developed in temperature intervals of 202–380°C, 191–407°C and 170–307°C., corresponding to salinities of 3.85wt.% to 11.23 wt.%, 3.69wt.% to 10.99 wt.% and 2.06wt.% to 17 wt.% NaCl eq.., respectively. The trapping pressures of the FIs from high temperature fluids in the quartz veins are 10-90 MPa, corresponding to depths of 1.0–10 km, assuming a density of the overlying rocks of 0.54 g/cm3–0.98 g/cm3. Multiple stages of phase separation or immiscibility of ore-forming fluid was critical for the formation of the Huajian deposit.

Organic-walled microfossils from the early Neoproterozoic Liulaobei Formation in the Huainan region of North China and their biostratigraphic significance

Biostratigraphic subdivision and correlation of early Neoproterozoic strata is hampered by the limited number of widely distributed and age diagnostic fossils. The acanthomorphic acritarch Trachyhystrichosphaera aimika, among a few other microfossil taxa, has emerged as a potential early Neoproterozoic index fossil, but its temporal and spatial ranges have not been thoroughly documented. To improve our knowledge about early Neoproterozoic biodiversity, we carried out a micropaleontological investigation of the early Neoproterozoic Liulaobei Formation in northern Anhui of North China. Our investigation using a low-manipulation maceration technique revealed a diverse assemblage of organic-walled microfossils dominated by sphaeromorphs and filaments, with relatively few acanthomorph taxa. A total of 23 taxa were recovered, including three new species (Eotylotopalla? grandis n. sp., Siphonophycus gigas n. sp., and Trachyhystrichosphaera botula n. sp.). Also present in the Liulaobei Formation are Trachyhystrichosphaera aimika (a species widely present in pre-Cryogenian Neoproterozoic and latest Mesoproterozoic rocks) and Pololeptus rugosus (a species characterized by an ellipsoidal vesicle with transverse annulations). The new data add to the growing diversity of early Neoproterozoic fossils and strengthen the basis for improved biostratigraphic correlation of early Neoproterozoic strata. Correlation with other geochronologically dated successions that contain Trachyhystrichosphaera confirms Trachyhystrichosphaera and T. aimika as promising index fossils to define and subdivide the pre-Cryogenian Neoproterozoic Era in terms of Global Boundary Stratotype Section and Point (GSSP), as opposed to current system using Global Standard Stratigraphic Age (GSSA). Available biostratigraphic data, including the occurrence of Trachyhystrichosphaera, Chuaria, Tawuia, and Sinosabellidites, suggest that the Liulaobei Formation is of pre-Cryogenian Neoproterozoic age, not Cryogenian–Ediacaran age as some have suggested in the past.

Precambrian geology of China

Precambrian geology of China