Plume Activity Research Articles

Mesozoic dike swarms in Borborema Province (NE Brazil): A structural analysis based on airborne geophysical data and field work

Mapping and analysis of dike swarms support a detailed understanding of the orientation, geometry, spatial distribution, geochronology, emplacement mechanism, structural control, and environments at the time of the magmatic events. Therefore, dike swarms have become a key tool for studying mantle plume activity, large igneous provinces, and continental breakup. Before the Pangea breakup, the Neoproterozoic Borborema Province (NE Brazil) was intensively intruded by sets of dike swarms, whose precise mapping and classification are essential for understanding the tectonic plate movements and evolution of the South American and African continents. In this sense, we promoted a regional mapping of dike swarms and their relationships between orientation, emplacement mechanism and structural control to provide a better understanding of the role of Mesozoic and Cenozoic large magmatic events in their tectonic evolution. We apply anomaly enhancement techniques to high-resolution airborne magnetic data to obtain the geometry and estimate the depth of causative sources. A structural analysis was carried out integrating magnetic patterns, field data and a compilation of previous geological maps to describe the detailed distribution of dike swarms in Borborema Province. Our study demonstrated that dike swarms extend over 5.4 × 105 km2. The 1388 mapped mafic dikes are grouped into three distinct dike swarms (DS): 1135 – Rio Ceará-Mirim DS; 168 – Canindé DS; and 85 – Riacho do Cordeiro DS. The dike swarms show three preferential trends: NE-SW, E-W and NW-SE. The geometry and structural analysis of the dikes allowed us to establish the paleostress field active at the time of intrusion. For the Rio Ceará-Mirim DS, the paleostress trajectories represent an N–S extension, rotating to NW-SE in the western portion of the swarm. For the Canindé DS, the paleostress orientation shows a NW-SE extension rotating to NE-SW. For the Riacho do Cordeiro DS, the trajectories are similar to those traced for the Rio Ceará-Mirim DS, showing a predominant NW-SE trend.

Neoproterozoic protracted arc basaltic magmatism in the southern margin of the Yangtze Block, south China: New constraints from mafic-ultramafic intrusive rocks

Mantle plume activity and subduction are two popular geodynamic models that have been used to explain the widespread Neoproterozoic basaltic magmatism in the Jiangnan Orogenic Belt along the southern margin of the Yangtze Block, South China. We have evaluated the competing hypotheses using new geochronological, petrological and geochemical data of two mafic–ultramafic intrusions in the Yuanbaoshan (YBS) area of this orogenic belt, namely the YBS south and west intrusions. Zircons from the YBS south intrusion yielded a U-Pb age of 835 ± 3 Ma. This new age and the previously reported age for another mafic–ultramafic intrusion situated < 10 km to the northwest (the YBS west intrusion) reveal a protracted (12–20 Myr) episode of mafic magmatism in a very small area. Such temporal-spatial relationship is more consistent with the style of subduction-related magmatism than that of mantle plume-related magmatism. The YBS south and west intrusions are characterized by arc-like geochemical features such as light REE enrichments and pronounced negative Nb-Ta anomalies in whole rocks. Olivine Mn/Fe and Ca contents, clinopyroxene AlZ/TiO2 and Cr-spinel TiO2/Al2O3 of these intrusions are similar to those of some undisputed subduction-related mafic–ultramafic intrusions elsewhere in the world. Zircons from the YBS south intrusion have mantle-likeHf-O isotope compositions, with εHf(t)varying from + 3.3 to + 8.7 and δ18Oof5.32 ± 0.29‰. The whole rock samples yielded εNd(t) from + 5.2 to + 8.8 and (87Sr/86Sr)i from 0.7034 to 0.7051 for the YBS south intrusion, and εNd(t) from + 1.7 to + 5.2 and (87Sr/86Sr)i from 0.7036 to 0.7051 for the YBS west intrusion. The Sr-Nd isotope compositions of these intrusions are similar to those of global Cenozoic arc basalts, but are also consistent with mantle plume-derived magma with < 5% of crustal contamination. However, the lack of correlation between Th/Nb and εNd(t) or (87Sr/86Sr)i for the intrusive rocks is inconsistent with the contamination model. The results from this study support for a popular view that northward oceanic subduction beneath the southern margin of the Yangtze Block took place in the Neoproterozoic.

Magneto-thermometric modeling of Central India: Implications for the thermal lithosphere

We estimated the Curie depth using magnetic anomalies in the tectonically complex central Indian shield. The modified centroid method for the scaling distribution of sources is applied to aeromagnetic data and satellite magnetic data (EMAG2) to estimate the Curie depths. We selected 80% overlapping with a window length of 300 × 300 km2 for Curie depth estimation. The estimated Curie depths are 22–46 km being shallower in the Bastar craton, northern end of the Aravalli-Delhi fold belt, and eastern part of Central Indian tectonic zone in the Deccan Volcanic Province. Deeper Curie depths are found in the eastern parts of the Aravalli-Delhi fold belt, Deccan Volcanic Province to the south of Tapti basin, parts of the Ganga and Vindhyan basins. The heat flow values estimated from the Curie depths vary from 30 to 90 mW/m2 being higher are in the northern part covering the Aravalli-Delhi fold belt and the Ganga basin and low heat flow values in the remaining study region. The computed thermal lithospheric thickness ranges 62–120 km, being 62 km thin in the Vindhyan basin, 68 km in the Aravalli-Delhi fold belt, and 73 km in the Bastar craton. We noticed a relatively thick thermal lithospheric thickness of 99 km beneath the foreland Ganga basin and 100 km in the Bundelkhand craton. The thermal lithosphere beneath the Deccan Volcanic Province is found to have a thickness ~ 120 km , which is interpreted as due to the rapid cooling of the lithosphere after Reúnion plume activity. Probably, the plume activities in the past could have modified the thermal lithospheric structure beneath central India. The upper mantle in the Deccan Volcanic Province to the south of the Tapti basin and the Aravalli-Delhi fold belt show magnetic character.

Prolonged metamorphism of garnet-orthoamphibole gneisses from the Fuyun area: New insights into metamorphic evolution of the southern Chinese Altai orogen

Garnet–orthoamphibole gneisses (GOG) represent a metamorphic lithology originated from hydrothermally altered metasediments or metabasites, and usually comprise low–variance mineral assemblages and are suitable to recover the P–T trajectory. In this study, we firstly report the occurrence of the GOG in the southern Chinese Altai orogen, NW China. Revealing metamorphic P–T–t path for GOG is crucial for better understanding of the orogenic evolution. Petrographic observations, mineral chemistry and phase equilibrium modeling suggest a three–stage metamorphic evolution: 1) M1 assemblage mainly consists of garnet + hornblende + gedrite + biotite + plagioclase + quartz + sillimanite + rutile ± ilmenite, with P–T conditions of 7.7–9.0 kbar and 700–790 °C; 2) M2 stage is represented by symplectitic intergrowths of orthopyroxene + cordierite + ilmenite and local presence of spinel, with P–T conditions of 5.9–6.5 kbar and 800–880 °C; and 3) M3 stage is characterized by an assemblage of anthophyllite/biotite + plagioclase + quartz + ilmenite + cordierite ± staurolite, with P–T conditions of 5–6 kbar and < 650 °C. Therefore, GOG in this region have undergone a generalized clockwise P–T path. LA–ICP–MS dating results for zircon and rutile indicate that the M1 amphibole–facies metamorphism occurred at ∼320 Ma, and the M2 granulite–facies overprint and the post–peak cooling occurred at 300–250 Ma, with a concordant age of 275 ± 4 Ma. The GOG protoliths are considered as chemically altered igneous rocks that originally formed in the Devonian (386 ± 6 Ma). The protracted metamorphic evolution of GOG (>80 Ma) may have witnessed a long–lasting tectonic evolution from continent–arc collision (i.e. between the Junggar terrane and the Chinese Altai orogen) to post–orogenic extension in the late Paleozoic. The latter episode was likely companied by underplating and intruding of mantle–derived magmas as a result of the slab break–off or the nearby Tarim mantle plume activity.

Inter-cratonic geochronological and geochemical correlations of the Derim Derim–Galiwinku/Yanliao reconstructed Large Igneous Province across the North Australian and North China cratons

Large igneous provinces (LIPs) are often used as a powerful tool for constraining continental reconstructions, due to their potential to correlate radiometric emplacement ages across scattered fragments of once continuous igneous bodies, and establish the presence of coeval magmatic suites. Mesoproterozoic magmatism in northern Australia and north China has been suggested to be the dispersed fragments of a once contiguous LIP. Here we present new geochronology and geochemical data from the under-resolved northern Australian fragments of this province (the Derim Derim and Galiwinku suites), and review existing data on related Mesoproterozoic mafic sills in the North Australian Craton and the North China Craton. We propose an updated framework for shared magmatic genesis and cratonic proximity during magmatic emplacement. Apatite geochronology has been applied to date dolerite intrusions, using cogenetic minerals to constrain the initial common-Pb constraints of the dolerite samples which provides a significant advantage for dating mafic rocks. The ages of samples collected throughout the extent of the mafic magmatic province in northern Australia and north China supports coincident ca. 1330–1295 Ma magmatism across both regions, while geochemical evidence indicates a mantle plume derived melt as the probable source for both suites. Coeval intrusion of geochemically analogous sills within temporally and stratigraphically comparable sedimentary basins is supportive of plume activity proximal to a once continuous sedimentary system that covered much of the North Australian Craton and North China Craton, which supports a close connection between the cratons within the supercontinent Nuna in the Mesoproterozoic.

Intracontinental rift-related magmatism in the eastern Emeishan Large Igneous Province traced by zircon oxygen isotopes

Rift systems associated with mantle plume activity are potential conduits for the transfer of voluminous magmas and fluids, and also contribute to ore formation in magmatic and hydrothermal systems. The Panxi (Panzhihua-Xichang) rift within the ~260 Ma Emeishan Large Igneous Province (LIP) is a typical case where world-class Fe-Ti-V oxide and relatively small Cu-Ni sulfide deposits are hosted in mafic-ultramafic intrusions. However, whether similar rifts exist in other parts of the Emeishan LIP remains unclear. Here we carry out an integrated SIMS analysis of zircon U-Pb-O isotopes from volcanic tuffs from the eastern part of the Emeishan LIP. Our results indicate that magmatic zircons with U-Pb ages coeval with the timing of the Emeishan magmatism have δ18O values lower than that of mantle, and as low as +2.94‰. In conjunction with previously reported evidence from sedimentary units, we propose that the silicic magmas with relatively low δ18O values in the eastern Emeishan LIP were derived from crustal recycling of hydrothermally altered rocks within an extensional setting associated with mantle plume activity. Our results combined with previous data suggest that the rifts triggered by the mantle plume activity in the eastern Emeishan LIP may be the potential locales for the exploration of magmatic sulfide deposits.

Compared structure and evolution of the conjugate Demerara and Guinea transform marginal plateaus

The Demerara and Guinea plateaus are conjugate Transform Marginal Plateaus (TMPs) formed at the junction of the Jurassic Central Atlantic Ocean and the Cretaceous Equatorial Atlantic Ocean. We compare their structure and evolution through a combined industrial/academic seismic dataset tied by well data. We show that these TMPs record a complete evolution history from the Jurassic Central Atlantic to the Equatorial Atlantic breakup and seafloor spreading phases. Both plateaus first formed as volcanic margins displaying successive SDR wedges migrating towards the newly forming Central Atlantic domain. In this context, Demerara and Guinea, conjugates of the Bahamas, seem to have formed in relation to plume activity at the southern end of the Central Atlantic domain. Our dataset suggests that the Demerara and Guinea plateaus initially represented two distinct extrusive centres separated by a crustal basement high. Later, during the Cretaceous Equatorial opening phase, both plateaus separated in a transform mode following this discontinuity. Deformation is notably asymmetric during this phase: Aptian to Albian folding, strike-slip, transtensive deformation and complex vertical movements on the Demerara side and only minor deformation on the Guinean side, except on its divergent Sierra Leone termination. The deformation is sealed on both plateaus by a regional upper Albian erosion unconformity. Extensional reactivation occurs on the Guinea side (probably through a general collapse?). To conclude, this study provides new insights into the nature and origin of TMPs and the key tectono-sedimentary archives they may contain to understand the polyphase breakup conditions of Gondwana.

No plate tectonics necessary to explain Eoarchean rocks at Isua (Greenland)

Abstract Trace element and isotopic data for basalts from the Isua greenstone belt, West Greenland, indicate that they were derived from a range of mantle reservoirs that included depleted lower mantle, the mantle transition zone, and a primitive mantle reservoir probably located in the shallow upper mantle. Modeling of trace element compositions indicates that the Isua basalts were formed through the mixing between and refertilization of these diverse sources and their resultant melts and that this took place in the shallow upper mantle. It is proposed that the melting and mixing were driven by the heat transferred from hot deep mantle sources. This geochemical interpretation leads to a geodynamic model in which deep mantle domains rise to melt in the shallow mantle where there is mixing between a range of sources and melts. There is no evidence for material descending from the shallow to deeper mantle and no necessity for the involvement of crustal materials. These processes imply the activity of a mantle plume and/or heat pipe.

Variability of Natural Methane Bubble Release at Southern Hydrate Ridge

AbstractCurrent estimations of seabed methane release into the ocean (0.4–48 Tg yr−1) are based on short‐term observations and implicitly assume that fluxes are constant over time. However, the intensity of gas seepage varies significantly throughout a seep lifetime. We used instruments operated by the Ocean Observatories Initiative's Regional Cabled Array to monitor variations of gas emissions over the entire Southern Hydrate Ridge summit. We show that bubble plumes emanate from distinct and persistent vents. Multiple plumes can occur within each vent and the location of their outlets may shift progressively. Active bubble plumes vary temporally in number and intensity, even within single vents. Gas emission fluctuations are partly periodic and linked to the local tide. However, short‐term variability and high ebullition events unrelated to tidal cycles are also commonly observed. Our data indicate that small‐scale processes beneath or at the sediment surface are responsible for the short‐term variability of the venting activity that is otherwise modulated by tides. Furthermore, a decrease of venting at one vent may coincide with an increase in plume activity at other vents. Our results depict a spatially and temporally dynamic seep environment, the variability of which cannot be fully characterized without systematic and comprehensive monitoring of the entire area. These results indicate that flux estimations may be largely overestimated or underestimated depending on the time, duration, and place of observation. Although sudden ebullition bursts are hardly predictable, we argue that tidal cycles must be taken into consideration when estimating gas fluxes.

Magmatism in the Siang window of the Eastern Himalayan Syntaxis, NE India: a vestige of Kerguelen mantle plume activity

Abstract We report new U–Pb zircon ages for mafic plutonic (gabbro) and volcanic (andesite) rocks, along with the whole-rock chemistry of a mafic–felsic suite of volcanic rocks from the Siang window of the Eastern Himalayan Syntaxis, NE India. Field relationships, and mineralogical and geochemical characteristics, of the studied mafic–intermediate–felsic rocks suggest their co-magmatic linkage that was generated in an extensional tectonic environment. Incompatible trace elements and low concentrations of large ion lithophile elements (LILEs) and REE behaviour reflect both the enriched nature of the mafic rocks and the limited influence of crustal contamination in their genesis. Partial melting and fractional crystallization processes have played a major role during the genesis of these felsic volcanics from the parental mafic magma. The laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb ages suggest that the mafic plutonic rock was emplaced at c. 121.18 ± 1 Ma and intermediate volcanic rock was emplaced at c. 135.48 ± 0.50 Ma during the Early Cretaceous period. The new ages are consistent with earlier reported zircon U–Pb ages (133.0 ± 1.9–130.7 ± 1.8 Ma) of felsic volcanic rocks from the present study area. Our new field observations, and mineralogical and geochemical characteristics, in conjunction with the U–Pb isotopic database suggest that the major magmatic event in the core of the Siang window of the Eastern Himalaya is coeval with the Rajmahal–Sylhet–Mikir–Shillong flood basalts of eastern and northeastern India, and the Comei–Bunbury Large Igneous Province of southeastern Tibet and SW Australia. These events are related to the break-up of eastern Gondwana and outbreak of the Kerguelen plume.

The Tethyan Himalaya Igneous Province: Early Melting Products of the Kerguelen Mantle Plume

AbstractThe Kerguelen large igneous province (LIP) has been related to mantle plume activity since at least 120 Ma. There are some older (147–130 Ma) magmatic provinces on circum-eastern Gondwana, but the relationship between these provinces and the Kerguelen mantle plume remains controversial. Here we present petrological, geochronological, geochemical, and Sr–Nd–Hf–Pb–Os isotopic data for high-Ti mafic rocks from two localities (Cuona and Jiangzi) in the eastern Tethyan Himalaya igneous province (147–130 Ma). Zircon grains from these two localities yielded concordant weighted mean 206Pb/238U ages of 137.25 ± 0.98 Ma and 131.28 ± 0.78 Ma (2σ), respectively. The analyzed mafic rocks are enriched in high field strength elements and have positive Nb–Ta anomalies relative to Th and La, which have ocean island basalt-like characteristics. The Cuona basalts were generated by low degrees of melting (3–5 %) of garnet lherzolites (3–5 vol% garnet), and elsewhere the Jiangzi diabases were formed by relatively lower degrees of melting (1–3%) of garnet lherzolite (1–5 vol% garnet). The highly radiogenic Os and Pb isotopic compositions of the Jiangzi diabases were produced by crustal contamination, but the Cuona basalts experienced the least crustal contamination given their relatively low γOs(t), 206Pb/204Pbi, 207Pb/204Pbi, and 208Pb/204Pbi values. Major and trace element geochemical and Sr–Nd–Hf–Pb–Os isotope data for the Cuona basalts are similar to those for products of the Kerguelen mantle plume head. Together with high mantle potential temperatures (&amp;gt;1500 °C), this suggests that the eastern Tethyan Himalaya igneous province (147–130 Ma) was an early magmatic product of the Kerguelen plume. A mantle plume initiation model can explain the temporal and spatial evolution of the Kerguelen LIP, and pre-continental breakup played a role in the breakup of eastern Gondwana, given the &amp;gt;10 Myr between initial mantle plume activity (147–130 Ma) and continental breakup (132–130 Ma). Like studies of Re–Os isotopes in other LIPs, the increasing amount of crustal assimilation with distance from the plume stem can explain the variations in radiogenic Os.

Spatial distribution of the rheological heterogeneities at crustal depths underneath the Harrat Rahat, western Saudi Arabia and their correspondence to Bouguer anomalies

The Harrat Rahat (HR) is the largest harrat in the Large Igneous Province in Saudi Arabia. Evolution of HR is enigmatic; two main debated postulates are the Red Sea rift margin volcanism vis-a-vis the Afar plume activity. Our objective in the present study is to investigate the rheological parameters for the crustal columns beneath the HR; computed from Receiver Function analysis results based on 11 seismic stations in the Northern HR (NHR) and three broadband stations from the Southern HR (SHR). The computed rheological parameters (Vp, Vp/Vs, Lame's First Constant, Shear Modulus, Density, Poisson's Ratio, Young's Modulus, Bulk Modulus and Acoustic Impedance) are utilized to investigate the rheological character individually for the upper and lower crust (UC and LC) underneath the NHR and SHR by adopting the following: (i) mutual correlation between the computed elastic constants in order to make inferences on the probable rock composition prevalent at crustal depths, (ii) the rheological parameters are then compared to those derived for the host terrain, the Arabian Shield Transitional Crust (ASTC) for understanding on the intrinsic changes locally introduced in HR by magmatic intrusions, (iii) spatial mapping for selected rheological parameters unravel a pervasive ‘Zone of Anomalous Rheological Parameters’ (ZARP), the resultant maps provide the variation of rheological properties in UC and LC. The gross features appearing on the ZARP maps demonstrate the rheological heterogeneity within the bounds of NHR and SHR. The anomalous zones are further examined for their physical property correspondence to the Bouguer gravity anomaly map for NHR. The gravity axial low zone in NHR is regarded as probable locale for fractured dyke invaded crust which act as feeder zone; where a raised thermal regime heralded gross compositional changes in LC in consequence to crust-mantle interaction.

Triton: Topography and Geology of a Probable Ocean World with Comparison to Pluto and Charon

The topography of Neptune’s large icy moon Triton could reveal important clues to its internal evolution, but has been difficult to determine. New global digital color maps for Triton have been produced as well as topographic data for &lt;40% of the surface using stereogrammetry and photoclinometry. Triton is most likely a captured Kuiper Belt dwarf planet, similar though slightly larger in size and density to Pluto, and a likely ocean moon that exhibited plume activity during Voyager 2′s visit in 1989. No surface features or regional deviations of greater than ±1 km amplitude are found. Volatile ices in the southern terrains may take the form of extended lobate deposits 300–500 km across as well as dispersed bright materials that appear to embay local topography. Limb hazes may correlate with these deposits, indicating possible surface–atmosphere exchange. Triton’s topography contrasts with high relief up to 6 km observed by New Horizons on Pluto. Low relief of (cryo)volcanic features on Triton contrasts with high-standing massifs on Pluto, implying different viscosity materials. Solid-state convection occurs on both and at similar horizontal scales but in very different materials. Triton’s low relief is consistent with evolution of an ice shell subjected to high heat flow levels and may strengthen the case of an internal ocean on this active body.

Crustal SiO2 Content of the Emeishan Large Igneous Province and its Implications for Magma Volume and Plumbing System

AbstractThe Permian Emeishan large igneous province (LIP) in Southwestern China is recognized as a remnant of mantle plume activity. Its crust composition has a significant bearing on understanding the magmatic plumbing system in a LIP and the volume of magmas generated, which is critical to the viability of the mantle plume model and its potential environmental impact but is difficult to constrain. Here, a SiO2‐VP/VS‐VS relationship, compiled from existing laboratory measurements, is used to estimate the crustal SiO2 content of the Emeishan LIP based on the joint constraint of the crustal bulk VP/VS ratio and average VS. The crust underneath the northeastern inner zone of the Emeishan LIP has a SiO2 content of ∼55.7 wt%, in contrast with the bulk continental crust's SiO2 content of 62.1 ± 2.1 wt% but largely overlapping with the exposed mafic‐ultramafic intrusions. Such an unusually “mafic” crust is attributable to the intensive magmatic underplating and intraplating associated with the Emeishan mantle plume. It further allows a minimum estimate of the volume of ∼1.45 × 106 km3 for erupted magmas and ∼2.39 × 106 km3 for total emplaced magmas, which is comparable to the volume of typical plume‐generated LIPs.

Picrite-basalt complex in the Baoshan-Gongshan Block of northern Sibumasu: Onset of a mantle plume before breakup of Gondwana and opening of the Neo-Tethys Ocean

Abstract Mantle plumes are thought to play key roles in Earth's geodynamics, including mantle convection, continental formation, and plate tectonics. The connection between plume activity and continental dispersion, as exemplified by the breakup of Gondwana and the generation of the Neo-Tethys Ocean, is a key question for the geosciences. Here, we present detailed investigations for the picrite-basalt sequence in the Baoshan-Gongshan Block of the northern Sibumasu terrane, southwest China. Field relations and petrological and geochemical data reveal that these volcanic rocks are continental flood picrites and basalts, consistent with a mantle plume origin. The estimated mantle potential temperatures range from 1527 ± 86 °C to 1546 ± 98 °C, and melting depths vary from the spinel to garnet stability fields (1.1–5.3 GPa), similar to Cenozoic Hawaiian picrites. Zircon geochronological data show that the mantle plume activity started at ca. 335 Ma and lasted to 280 Ma; this range is earlier than the breakup of the Gondwana continent and opening of the Neo-Tethys Ocean (270–260 Ma). We conclude that the long-lived mantle plume impacted the continental lithosphere but it did not drive continental breakup and the opening of Neo-Tethys Ocean, which took place because of the subduction-induced stress generated by initial subduction of the Paleo-Tethys Ocean.

Petrology, geochemistry, and zircon U–Pb‐Lu–Hf isotopes of granitoids from the Ivindo Basement Complex of the Souanké Area, Republic of Congo: Insights into the evolution of Archean continental crust

The Ivindo Basement Complex (IBC) comprises the largest exposure of Meso‐ to Neoarchean rocks in the northwestern segment of the Congo Craton in the Republic of Congo. The age and petrogenetic setting of the IBC are not clear due to a lack of detailed studies. In the Souanké area, which is on the northeastern margin of the IBC, the most exposed rocks are charnockites, tonalite/granodiorite, and trondhjemite. Based on geology, geochemistry, and zircon U–Pb‐Lu–Hf isotopes, this paper constrains the petrogenesis and tectonic significance of these granitoids for the evolution of the IBC. Poor‐quality zircons did not allow for age determination and Lu–Hf isotopic characteristics for the charnockite. However, zircon U–Pb dating of the tonalite/granodiorite and trondhjemite yielded the weighted mean 207Pb/206Pb ages of 2,895 ± 9.4 Ma and 2,889 ± 9.2 to 2,897 ± 5.0 Ma, respectively. Zircon Hf isotopic (εHf[t]) values of +1.22 to +4.55 and Th/La &gt;0.12 for the tonalite/granodiorite and trondhjemite indicate that these rocks were derived from partial melting of an older mafic reservoir (TDM2 ages range from 3,028 to 3,230 Ma), possibly triggered by mantle upwelling or plume activity. All the studied IBC granitoids are peraluminous, calcic‐alkalic, relatively sodium‐rich (Na2O/K2O &gt; 1), and contain low Ni, Cr, and Yb contents, with strongly fractionated rare‐earth element (REE) patterns. On standard‐normalization diagrams, the IBC rocks exhibited similar distribution patterns in the trace element geochemistry as average Archean TTGs, except for high U, Nb, and Ta concentrations in the IBC trondhjemite. These high values may indicate amphibole dehydration melting (which sequesters Ta and Nb) at low‐pressure conditions at the source for the trondhjemite generation, while the other IBC rocks experienced high‐pressure melting. The 2,900–2,800 Ma tectono‐thermal event registered in the IBC from our study was also previously documented for the Ntem Complex TTGs of Cameroon (northwestern equivalent of the IBC) and elsewhere in other cratons such as in the Bação Complex (São Francisco Craton) of Brazil and, therefore, represents an important period for the Archean crustal evolution in geologic history.

Petrology and geochemistry of early Permian mafic–ultramafic rocks in the Wajilitag area of the southwestern Tarim Large Igneous Province: Insights into Fe-rich magma of mantle plume activity

The majority of global Fe–Ti-oxide deposits hosted in mafic–ultramafic layered intrusions are closely related to mantle plume activity. The metallogenesis of these Fe–Ti-oxide deposits is still debated, especially with regard to their mantle source and relationship to magmatism. Here we report detailed petrology and geochemistry for early Permian (ca. 280 Ma) mafic–ultramafic igneous rocks in the Wajilitag area of the western Tarim Large Igneous Province (TLIP) to establish the relationships among mantle source, magmatism, and metallogenesis in mantle plume activity. The mafic rocks (i.e., gabbro and diabase) show alkaline affinities and OIB-like trace-element patterns. Clinopyroxenites closely related to Fe–Ti-oxide deposits were derived from the same magmatic system as that of the mafic rocks. The primary magma of the Wajilitag mafic rocks was enriched in FeOT and TiO2 and depleted in SiO2, and has high FC3MS (FeOT/CaO − 3MgO/SiO2) and Fe/Mn, indicating the involvement of Si-poor pyroxenite/eclogite in the mantle source. Primary olivines from the Wajilitag gabbro have consistently high NiO and Fe/Mn. Wajilitag mafic rocks have variable Sr–Nd–Hf isotopes owing to their derivation from a hybrid mantle source with the involvement of subducted oceanic crust and sediments. Subducted slabs within the upper mantle are inferred to have been entrained by the upwelling mantle plume and preferentially melted to produce mainly Fe-rich magma. The occurrence of an adjacent oceanic subduction event just prior to the operation of a mantle plume played a crucial role in the formation of large-scale Fe–Ti-oxide deposits in LIPs.

PLATINUM METAL MINERALIZATION OF THE SOUTH URALS MAGMATIC COMPLEXES: GEOLOGICAL AND GEODYNAMIC CHARACTERISTICS OF FORMATIONS, PROBLEMS OF THEIR GENESIS, AND PROSPECTS

In the South Urals, we have identified and investigated two platinum-bearing formations – ophiolite chromitebearing complexes, and the Khudolaz differentiated mafic-ultramafic complex with sulfide Cu-Ni mineralization. The ophiolite chromite-bearing complexes include fragments of the upper mantle and lower crust of the Paleouralian Ocean, which were induced by collision onto the edge of the East European platform. The origin of the Khudolaz complex is related a mantle plume activity. Here, we review and compare the main features of platinum-metal mineralization (PMM) in these two formations.The article presents the results of mineralogical and geochemical studies of PMM associated with chromite and sulfide Cu-Ni ores. In association with chromitites, two types of PMM are distinguished: (1) predominating refractory platinoids in chromitites of the mantle unit of the section, and (2) predominating platinum and palladium in chromitites of the transitional wehrlite-clinopyroxenite complex. Compositions of platinum group minerals (PGM) and relations between their elements and host minerals suggest that the minerals of the ophiolite chromite-bearing complexes are of a restite origin, while the Khudolaz complex results from a combination of magmatic processes and solid-phase redistribution of material. Palladium (michenerite, froodite, merenskyite, borovskite, sudburyite) and platinum (sperrylite, moncheite) minerals are found in magmatic sulfide ores of the Khudolaz complex, which were subjected to hydrothermal metasomatization. Texture observations using electron microscope and optical (reflected light) images, as well as LA ICP MS analyses of sulfides suggest late- and post-magmatic crystallization of PMM in three phases: (1) immiscible metalloid or highly fractionated residual sulfide melts trapped in sulfides; (2) segregation of isomorphic impurities of platinum group elements (PGE) and chalcogenide elements from sulfide solid solutions; and (3) interaction of hydrothermal fluids with soluble sulfides.Prospective for PMM are extended bodies of disseminated chromitites in marginal dunites of the Kraka and Nurali massifs, and wehrlite-clinopyroxenite complexes of the same massifs containing PGE (above 500 ppb). In the Khudolaz complex, promising PMM bodies are low-metasomatized parts of sulfide ore bodies (1 ppm of ΣPGE and above) located in the largest massifs, Severny Buskun and Zapadny Karasaz. Exocontact zones of these intrusions are also promising for PMM.

Mantle sources and magma evolution in Europe's largest rare earth element belt (Gardar Province, SW Greenland): New insights from sulfur isotopes

Alkaline igneous complexes are often rich in rare earth elements (REE) and other metals essential for modern technologies. Although a variety of magmatic and hydrothermal processes explain the occurrence of individual deposits, one common feature identified in almost all studies, is a REE-enriched parental melt sourced from the lithospheric mantle. Fundamental questions remain about the origin and importance of the mantle source in the genesis of REE-rich magmas. In particular, it is often unclear whether localized enrichments within an alkaline province reflect heterogeneity in the mantle source lithology (caused by prior subduction or plume activity) or variations in the degree of partial melting and differentiation of a largely homogeneous source. Sulfur isotopes offer a means of testing these hypotheses because they are unaffected by high temperature partial melting processes and can fingerprint different mantle sources. Although one must be careful to rule out subsequent isotope fractionation during magma ascent, degassing and crustal interactions.Here, we present new S concentration and isotope (δ34S) measurements, as well as a compilation of major and trace element data, for a suite of alkaline magmatic units and crustal lithologies from the Mesoproterozoic Gardar Province. Samples span all phases of Gardar magmatism (1330–1140 Ma) and include regional dykes, rift lavas and the alkaline complexes Motzfeldt and Ilímaussaq, which represent two of Europe's largest REE deposits. We show that the vast majority of our 115 samples have S contents >100 ppm and δ34S of −1 to 5‰. Only 8 samples (with low S contents, <100 ppm) show evidence for crustal interactions, implying that the vast majority of Gardar melts preserve the S isotopic signature of their magma source. Importantly, samples from across the Gardar Province have δ34S above the canonical mantle range (≤−1.4‰) and therefore require recycled surface S in their mantle source. Elevated δ34S values are explained by a period of Andean-style subduction and mantle metasomatism which took place ∼500 Ma before rift onset and are also supported by trace elements signatures (e.g. Ba/La) which match modern subduction zones.Comparing the various generations of Gardar magmas, we find that δ34S values, large ion lithophile elements (K, Ba, P) and selective incompatible elements (Nb and light REE) are particularly enriched in the Late Gardar dykes, alkaline complexes and clusters of silica-undersaturated dykes spatially associated with the alkaline complexes. These data indicate that subduction-related metasomatism of the Gardar mantle was spatially heterogeneous, and that alkaline complexes are sourced from localized mantle domains highly enriched in 34S, REE, alkalis and volatiles (particularly, F). Since alkalis and volatiles play an essential role in driving extreme differentiation of alkaline melts and fluids, we suggest the co-location of these species plus incompatible metals at high concentrations in the lithospheric mantle is a critical first-step in the genesis of a world-class alkaline REE deposit. S isotopes are powerful tools for identifying enriched mantle domains and the sources of mineralized alkaline igneous bodies.

The cause for Nuna breakup in the Early to Middle Mesoproterozoic

The dynamic mechanism responsible for the breakup of Nuna supercontinent (1.6–1.3 Ga) is a key for understanding the early to middle Mesoproterozoic environment, life and mineralization on Earth. Although much research has been done to unravel the dispersion of young supercontinents (e.g., Pangea), efforts by sorting out critical geological records to disclose the driving force for Nuna breakup are still rare. Here we focus on this issue by integrating new whole-rock geochemical data, zircon U–Pb ages, Hf-in-zircon and Nd isotopes for Mesoproterozoic granitoids in the Chinese Central Tianshan (CTB) at the Central Asian Orogenic Belt (CAOB). Moreover, global geological data in the early to middle Mesoproterozoic are compiled to place further constraints. The studied granitoids are I-type granites emplaced at ca. 1480–1450 Ma. They were formed in an active continental margin of CTB that once belonged to the Fennoscandia in the margin of Nuna. These results, together with the available geological records in CTB, CAOB and Fennoscandia, indicate a subduction system existed along the periphery of these domains in the early to middle Mesoproterozoic. This subduction system was temporally and spatially linked to the 1.6–1.3 Ga accretionary belts in the peripheral blocks of Nuna supercontinent, suggesting an encircling subduction system surrounding Nuna supercontinent. The encircling subduction system was accompanied by intermittent Mesoproterozoic plume magmatism, some of which were geochemically overprinted with subduction-related signatures, suggesting a dominant continuous circum-supercontinent subduction operating on the breakup of Nuna supercontinent. Moreover, these episodic plume-related magmatism are temporally and geodynamically linked to the exterior subduction surrounding the Nuna supercontinent. Our study therefore demonstrates that the development of an exterior subduction system gave rise to the breakup of Nuna supercontinent, which was accompanied by subordinate plume activities.