Origin of Fe-Ni-Cu (Co) sulfide and Fe-Ti oxide minerals in the c. 1.77 Ga dolerite dyke, Singhbhum Craton (eastern India)

Dolerite dyke swarms are widespread within the Singhbhum Craton (eastern India) that emplaced from the Neoarchean to Paleoproterozoic era just after the stabilization of crust before c. 3 Ga. These dyke swarms are oriented in NE - SW to NNE - SSW, NW - SE to WNW - ESE, E - W, and N - S directions. The WNW - ESE trending c. 1.77 Ga Pipilia dyke swarm is sampled from the Satkosia area of the Orissa state. The dyke shows a noticeable disparity in terms of the modal proportion and grain size of pyroxenes, plagioclase, Fe-Ti-oxide minerals and texture across the trend. At places the primary silicates are altered to secondary hydrated mineral assemblages of amphibole, chlorite and sericite. Primary silicates are clinopyroxene (augite: Mg# = 65.7 - 82.6; En37-48Fs11-17Wo36-41), orthopyroxene (clinoenstatite: Mg# = 68.5 − 78; En63-70Fs20-29Wo4-5), plagioclase (An11-39Ab44-82Or1-7) and Fe-Ti oxides are titanomagnetite (FeO = 34.38 − 39.50 wt%, Fe2O3 = 48.26 − 56.21 wt%, TiO2 = 5.05 − 9.60 wt%) and ilmenite (FeO = 40.75 − 43.79 wt%, Fe2O3 = 3.54 − 10.03 wt%, TiO2 = 47.82 − 50.87 wt%). Application of two-pyroxene thermometry yields an equilibration temperature range of 1065oC to 978oC, and coexisting titanomagnetite-ilmenite pairs reveal 731.39oC to 573.37oC at the oxygen fugacity (fO2) condition NNO+0.3 to FMQ-1.03. The dyke contains disseminated sulfides at the interstices of Fe-Ti-oxides, and silicates. Major sulfide minerals are pyrite, chalcopyrite, and vaesite; Pyrite-vaesite assemblages occur in association with secondary silicate minerals. Pyrite grains contain variable concentration of Co = 0.01 − 5.70 wt% and Ni = 0.02 − 1.95 wt%. Coexisting vaesite contains Co = 2.42 − 10.44 wt%, Ni = 26.40 − 47.88 wt%, and Fe = 7.32 − 26.55 wt%. Texture, sulfide-silicate assemblage, and presence of low metal/S sulfides such as the pyrite-vaesite assemblage indicate primary Fe-Ni- sulfides (pyrrhotite-pentlandite) that segregated from immiscible sulfide liquid at high temperature is modified by late magmatic/hydrothermal fluid activities. Numerous sulfide-bearing deposits hosted in ultramafic-mafic intrusions of Paleoproterozoic age have been recorded globally and the occurrence of Fe-Ni-sulfides in the c. 1.77 Ga Pipilia dyke swarm in the Singhbhum Craton enhances the exploration potential of this craton in eastern India.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5643989

Geochronological and Geochemical Constraints on the Petrogenesis of Permian Dolerite Dyke Swarms in the Beishan Orogenic Belt, NW China

Extensive Early Permian mafic-ultramafic intrusions, doleritic dykes, and basalts crop out within the Beishan area, southern Central Asian Orogenic Belt (CAOB). We present new geochronological and geochemical data for Gubaoquan dolerite dyke swarms in the Beishan orogenic belt. Zircon U-Pb Dating of the Gubaoquan dykes indicates that they were emplaced during the Early Permian (280.7 ± 4 Ma), that was coeval with Yinaoxia and Podong mafic dykes in Beishan area. The dykes are characterized by low Mg# (47–84) in the clinopyroxene crystals, and the content of whole-rock Fe2O3 (t), MgO, and alkali (Na2O + K2O) range from 12.5–17.4, 4.06–5.51, and 2.8–4.4 wt.%, respectively. The samples from the Gubaoquan dykes have high and variable Ba/La (5.93–14.2) and Ba/Nb (15.0–37.3) ratios but low Th/Yb (0.17–0.24) ratios. The rocks show slightly enrichments in LREE, HFSE, Th, and Hf, and depletion in Nb and Ta. The εNd (t = 280 Ma) values and initial 87Sr/86Sr ratios of the Gubaoquan dykes show variations ranging from 6.4 to 6.8 and 0.706240 to 0.707546, respectively. These data suggest that the parental magmas for the Gubaoquan dykes were probably derived from partially decompressed melting of upwelling depleted asthenosphere mantle that was metasomatized previously by subducted fluids.

Implication of Mesoproterozoic (∼1.4 Ga) magmatism within Sette-Daban (Southeast Siberia)

<p>Numerous Mesoproterozoic mafic dyke swarms are known in Siberia. The main intrusions are concentrated in the northern part of the platform and in Sette-Daban (southeastern part of Siberia), and single intrusions are known on all the outcrops of the crystalline basement in the southern part. The largest dyke swarms are located on the Anabar shield and Sette-Daban (with ~1500 Ma and 1000-950 Ma, respectively [1,2]). In the period 1400-1300 Ma, single intrusions are known: 1382 ± 2 Ma [3] on the Anabar shield, 1385 ± 30 Ma [4] on the Udzha uplift, Listvyanka and Goloustnaya dykes in the south of the platform –  1350 ± 6 Ma [5] and 1338 ± 3 Ma [6], respectively. Also, there is the north-trending dolerite dyke at Sette-Daban, which cuts the Lower Riphean sediments of the Uchur Group. The age of this dike was estimated as 1339 ± 59 Ma employing Sm-Nd isochrone [7]. We report here a new U-Pb age on apatite, Nd isotopy, and geochemistry for this dolerite dyke.</p><p>A typical apatite grain used for the U-Pb dating. On the  Tera-Wasserburg diagram, the regression line intercepts in the lower part the concordia line at 1419 ± 15 Ma. The chemical composition of this dyke corresponds to subalkaline basalts (SiO<sub>2</sub> = 45.6, Na<sub>2</sub>O+K<sub>2</sub>O = 3.9 wt%). The rocks correspond (Mg# = 61) to the calc-alkaline series (FeO*/MgO = 1.1) with a low content of TiО<sub>2</sub> (1.25 wt %). A clear negative Nb-Ta anomaly on the multielement diagram suggests an IAB affinity. Incompatible element ratios such as Th/Yb, Nb/Th, Nb/Yb, Zr/Nb also suggest that these dolerites are close to arc-related basalts in composition. Eps(Nd) calculated to the initial value at 1400 Ma shows a slightly negative value -0.2, which is considered as mantle source with contribution from the enriched source.</p><p>Geochemical and Nd isotopy characteristics show the affinity of the Sette-Daban dyke with low-Ti series of the Phanerozoic flood basalt provinces (e.g. Karoo, Siberian traps, etc. [8,9]) with the suggestion that these dolerites were generated from a metasomatized subcontinental lithospheric mantle source. Assuming geochemical characteristics and new U-Pb age of the dolerite we propose flood basalt province in the southeast Siberia in Mezoproterozoic (~1400 Ma).</p><p>The research was supported by the Russian Science Foundation grant (19-77-10048).</p><p> </p>

Deciphering the Vássačorru Igneous Complex within the Seve Nappe Complex, Scandinavian Caledonides

<p>The Seve Nappe Complex (SNC) of the Scandinavian Caledonides comprises Neoproterozoic sedimentary and igneous rocks that experienced high-pressure metamorphism and deformation during subduction and exhumation. Fieldwork was conducted in the Kebnekaise region in northern Sweden, focusing on the Aurek metagabbro and the Vistas metaigneous rocks within the Vássačorru Igneous Complex (VIC), hosted within SNC metasediments. Field observations show that the Aurek metagabbro is locally sheared with well-defined foliation and lineation. In contrast, the Vistas metaigneous rocks, consisting of both granite and gabbro bodies, are only locally foliated. Furthermore, the granite is intruded by ENE-WSW striking dolerite and rhyolite dykes that parallel the local foliation, and are weakly deformed, whereas a NNE-SSW striking syenite dyke is observed in a portion of undeformed gabbro.</p><p>The Aurek metagabbro mineral assemblages consist of garnet, amphibole, plagioclase, biotite, chlorite, and pyroxene. The Vistas gabbro and dolerite dyke both consist of plagioclase, pyroxene, and amphibole. The Vistas granites and rhyolite dyke include quartz, feldspar, biotite, muscovite, ± garnet, and the syenite dyke contains feldspar, plagioclase, pyroxene, amphibole, quartz, and biotite. The Vistas metaigneous rocks generally show primary igneous assemblages.</p><p>Bulk rock chemistry shows that the Aurek and Vistas gabbros, and the Vistas dolerite dyke, are classified as tholeiites. For the Aurek gabbros, Th/Yb of 0.06-1.86 and Nb/Yb of 0.11-5.14 indicate that they have N-MORB to E-MORB compositions, with possible crustal input. The Vistas gabbro (Th/Yb of 0.09 and Nb/Yb of 1.15) and the dolerite dyke (Th/Yb of 0.12 and Nb/Yb of 0.66) also suggest such trend. The Vistas granites, rhyolite, and syenite dyke all have calc-alkaline composition. Trace elements confirm volcanic arc affinity for the granites and the syenite dyke (Nb: 3.1-5.9 ppm, Rb: 116.5-177.5 ppm, Y: 12.9-18.0 ppm, Ta: 0.3-0.4 ppm, Yb: 2.04-3.19 ppm), whereas the rhyolite dyke (Nb: 38.2 ppm, Rb: 247.8 ppm, Y: 72.6 ppm, Ta: 2.8 ppm and Yb: 12.62 ppm) reflects a within plate setting.</p><p>Combining the field relationship with geochemistry of the studied metaigneous rocks, we tentatively propose that the VIC is composed of three pulses of magmatism: (1) mafic MORB magmatism represented by the gabbros, emplaced in an extensional regime; (2) felsic calc-alkaline magmatism represented by granites and syenite, emplaced in an active continental margin environment; and (3) bimodal within-plate magmatism or crustal assimilation in a volcanic arc represented by dolerite and rhyolite dykes. However, the only existing age is from U-Pb zircon dating of the Vistas granite, which yielded 845±14 Ma (Paulsson & Andreasson, 2002). Further zircon U-Pb geochronology will be conducted to obtain ages of the various lithologies of the VIC to better understand temporal relationships and to link the VIC with tectonic events in the Scandinavian Caledonides.</p><p>This study was supported by the National Science Centre (Poland) grant no. 2019/33/B/ST10/01728 to J. Majka.</p><p>References</p><p>Paulsson, O., Andreasson, P.-G., 2002. Attempted break-up of Rodinia at 850 Ma: Geochronological evidence from the Seve-Kalak Superterrane, Scandinavian Caledonides. J. Geol. Soc. 159, 751–761. https://doi.org/10.1144/0016-764901-156</p>

Reconstructing the Ediacaran to Ordovician history of the Baltoscandian Margin of continent Baltica

<p>In the Scandes, the lower thrust sheets of the Caledonian allochthons provide unambiguous stratigraphic evidence of correlation with the successions of the Baltoscandian platform. Cambrian successions, including the Alum Shale Formation, providing the footwall for the main Caledonian decollement in Scandinavia, can be followed at least 200 km westwards from the thrust front into the hinterland of the orogen. The overlying early Palaeozoic strata provide evidence of facies changes into foreland basin deposits in the mid Ordovician and early Silurian; also of Ediacaran and Cryogenian successions, including Marinoan tillites. The amount of internal shortening in the Lower Allochthon is not uncontroversial, but certainly amounts to more than 100 km, implying that all the overlying alllochthons in the Scandes were derived from west of the Norwegian coast.</p><p>The metamorphic grade of the units in the Lower Allochthon increases from low to high greenschist facies, from the thrust front westwards into the deep hinterland. Overlying thrust sheets of the Middle Allochthon are of higher metamorphic grade and more ductilely deformed. The basal parts are usually dominated by basement-derived units and Neoproterozoic sedimentary rocks. They are overthrust by dolerite dyke-intruded thrust sheets, the Särv Nappes, with host-rocks dominated by Cryogenian and Ediacaran sandstones, the former including subordinate limestones and Marinoan tillites. The Baltoscandian margin dolerite dyke swarms amount to up to c. 35% of these thrust sheets.</p><p>The overlying, highest tectonic units in the Middle Allochthon (the Seve Nappe Complex, SNC) are of amphibolite and higher metamorphic grade. They include a greater variety of lithologies, including some that are very similar to those in the underlying Särv Nappes (e,g. quartzites and eclogitized dolerites). The metasedimentay host rocks include a wide range of paragneisses and marbles. Abundant mafic rocks include metamorphosed gabbros, basalts and peridotites and, together with the dyke swarms, can totally dominate the composition of some thrust sheets. The similar geochemistry and early Ediacaran age (c. 600 Ma) of the mafic rocks in the Särv and Seve nappes define the Baltoscandian outermost margin and continent-ocean transition zone (COT). Iapetus Ocean terranes comprise the overlying thrust sheets of the Upper Allochthon (e.g. the Köli Nappe Complex).</p><p>The metamorphism of the different thrust sheets in the SNC provide clear evidence that some parts were subducted; others not. A wide range of isotope age data constrain the timing of subduction, with the earliest ages in the mid Cambrian (c. 505 Ma) to early Ordovician (c. 483 Ma). It has been suggested that the deposition of the Alum Shale Formation on the Baltscandian platform, was related to this early Caledonian subduction. A more probable interpretation is that subduction along the outermost edge of this highly extended COT did not influence the edge of the platform till the early Tremadoc.</p><p>Some authors have introduced cryptic sutures into the Baltoscandian outer margin, described above. They should reassess their data and better define the evidence for their conviction.</p>

A mantle plume origin for the Scandinavian Dyke Complex: a “piercing point” for 615 Ma plate reconstruction of Baltica?

<p>The origin of Large Igneous Provinces (LIPs) associated with continental breakup and the reconstruction of continents older than c. 320 million years (pre-Pangea) are contentious research problems. Here we study the petrology of a 615 - 590 Myr dolerite dyke complex that intruded rift-basins of the magma-rich margin of Baltica and now is exposed in the Scandinavian Caledonides. These dykes are part of the Central Iapetus Magmatic Province (CIMP), a LIP emplaced in Baltica and Laurentia during opening of the Iapetus Ocean within the Caledonian Wilson Cycle. The >1000 km long dyke complex displays lateral geochemical zonation from enriched to depleted basaltic compositions from south to north. Geochemical modelling of major and trace elements shows these compositions are best explained by melting hot mantle 75-250°C above ambient mantle. Although the trace element modelling solutions are non-unique, the best explanation involves melting a laterally zoned mantle plume with enriched and depleted peridotite lithologies, similar to present-day Iceland and to the North Atlantic Igneous Province. The origin of CIMP appears to have involved several mantle plumes. This is best explained if rifting and breakup magmatism coincided with plume generation zones at the margins of a Large Low Shear-wave Velocity Province (LLSVP) at the core mantle boundary. If the LLSVPs are quasi-stationary back in time as suggested in recent geodynamic models, the CIMP provides a guide for reconstructing the paleogeography of Baltica and Laurentia 615 million years ago to the LLSVP now positioned under the Pacific Ocean. Our results provide a stimulus for using LIPs as piercing points for plate reconstructions.</p>

The setting and style of manganese mineralization in the Constantiaberg Massif, Cape Peninsula, South Africa

Manganese oxyhydroxide mineralization is widespread in the Constantiaberg Massif. It is largely hosted by west-northwest – east-southeast trending brittle structures in the competent Ordovician arenites of the Peninsula Formation of the Cape Supergroup. Manganese is also found impregnating more porous Peninsula Formation arenites and Quaternary scree. This study proposes that the more significant deposits at Hout Bay and Constantiaberg differ from most of the mineralization in that they are spatially associated with saprolithic dolerite dykes belonging to the 132 Ma False Bay dolerite dyke swarm. It is suggested that this deep weathering may be related to a Miocene palaeo-landsurface, yielding a maximum age for these more important Mn deposits. However, there is evidence for mineralization of different ages through the Quaternary Period. Apart from Mn-mineralization hosted by rare breccias containing hydrothermal quartz, most of the mineralization is supergene, having been leached and transported in reduced acidic groundwater and precipitated at or near a redox front in a near-surface environment.

Petrology and Geochemistry of Hama Koussou Dolerite Dyke Swarms (North Cameroon, Central Africa)

The Pan African granitoid basement of Hama Koussou Cretaceous half basin in North Cameroun (Central Africa) is transected by near N-S, NE-SW and ENE-WSW giant doleritic dykes trending along the same Pan African directions. Hama Koussou dolerites are compliant with the regional distension that occurred after the Pan African basement consolidation prior to the development of West and Central African Rift System at Late Jurassic-Early Cretaceous times. Studied lavas are composed of large clinopyroxene oïkocrysts, plagioclase and alkali feldspar laths and oxides phenocrysts exhibiting ophitic, sub-ophitic and intercertal textures. Microprobe chemical analyses carry out on the main mineral phases show that clinopyroxenes are diopside and augite, plagioclases are labradorite, andesine, oligoclase and albite and alkali feldspars are mainly sanidine with a few percent of orthoclase. ICP-MS and ICP-AES geochemical analyses of Hama Koussou lavas exhibit basalt, basaltic trachyandesite and trachyandesite compositions of continental tholeiite features. Tholeiite basalts of Hama Koussou are the results of high partial melting of E-MORB mantle source of spinel lherzolite composition, located at 65-55 km depth. More evolved tholeiite lavas of Hama Koussou basin are the products of tholeiite basalt differentiation trough assimilation and fractional crystallization coupled with fluids circulation.

Geochemical Characteristics and Petrogenesis of Basement Rocks in Idoani Area, Ondo State, Southwestern Nigeria

Idoani area of Ondo State is an extension of the Upper Proterozoic Igarra Schist Belt of the Southwestern Nigerian Basement Complex. The area comprises mainly of metasedimentary rocks with subordinate amount of intrusives. Quartz-mica schist, phyllite, quartzite and banded gneiss are the dominant rock types in the area. These metasediments are intruded by the Older granite and other minor intrusives (pegmatite, aplite, dolerite dyke, quartz vein). Idoani rocks, as revealed by petrographic studies, are rich in quartz, alkali and plagioclase feldspars and mica with a low amount of opaque minerals. Geochemical studies and petrochemical variation diagrams show that the rocks are sub-alkaline, medium-K calc-alkaline to shoshonitic, peraluminous, and characterized by intermediate-high silica content and low-moderate amount of mafic oxides. Petrological and geochemical studies revealed that the quartz-mica schist and phyllite are metapelites while the quartzite is a meta-arkose, derived from a granitic – quartz-monzonitic source. The banded gneisses were probably derived from a hybrid sedimentary–igneous protoliths while the granite is of magmatic origin.