Plume Activity Research Articles

The Middle Jurassic Cheshmeh-Ghasaban High-Mg gabbro (Hamedan, NW Iran): A plume-influenced intracontinental rifting event?

The Jurassic mafic to felsic magmatism affecting the older Ediacaran-to-Cambrian basement of the Sanandaj-Sirjan Zone of Iran has been traditionally interpreted as the product of arc and/or back-arc magmatism related to the early stages of Neo-Tethys subduction beneath Iran in the early Jurassic. Recent works and new compositional and geochronological data have started challenging this commonly accepted model in favor of scenarios involving continental rifting, mantle plume activity, and/or passive margin formation. In the Hamedan area of the central sector of the Sanandaj-Sirjan Zone, the Jurassic Cheshmeh-Ghasaban gabbro (ca. 165 Ma) is a key formation to better understand the tectono-magmatic framework of the whole area. Our new data, combined with the existing literature, suggest a transitional to alkaline OIB-like compositional character for this gabbro similar to the nearby but slightly younger (ca. 145 Ma) Panjeh and Ghalaylan basaltic complexes (in the Songhor-Ghorveh area). When integrated with the existing geochemical data of Jurassic mafic rocks from the central Sanandaj-Sirjan Zone, our results point to a scenario of intracontinental rifting, possibly involving the upwelling old metasomatized (by Proto-Tethys subduction?) mantle or mantle-plume activity.

HDO and SO2thermal mapping on Venus

Since January 2012, we have been monitoring the behavior of sulfur dioxide and water on Venus using the Texas Echelon Cross-Echelle Spectrograph (TEXES) imaging spectrometer at the NASA InfraRed Telescope Facility (IRTF, Mauna Kea Observatory). We present here new data recorded in 2021 and 2022, after an 18-month interruption due to the Covid pandemic. Most of the observations were recorded in two spectral ranges: the 7.4 µm (1345 cm−1) range, where SO2, CO2, and HDO (used as a proxy for H2O) transitions are observed at the cloud top, at an altitude of about 62 km, and the 19 µm (530 cm−1) range, where SO2and CO2are probed within the clouds at an altitude of about 57 km. We recently added the 8.6 µm (1162 cm−1) range, which probes a few kilometers above the cloud top (z= 67 km). As in our previous studies, the volume mixing ratio of SO2is estimated using the SO2/CO2line depth ratio of weak transitions; the H2O volume mixing ratio is derived from the HDO/CO2line depth ratio, assuming a D/H ratio of 200 times the Vienna Standard Mean Ocean Water (VSMOW). As reported in our previous analyses, the SO2mixing ratio shows strong variations with time and also over the disk, showing evidence of the formation of SO2plumes. These local maxima appear sporadically on the SO2maps and stay visible over a few hours. In contrast, the H2O abundance is remarkably uniform over the disk and shows moderate variations as a function of time. The present dataset shows significant differences with respect to the 2012–2019 dataset: (1) the SO2mixing ratio at the cloud top has decreased by a factor of about 3 with respect to the maximum value observed in July 2018; (2) the long-term anti-correlation between SO2and H2O previously observed between 2014 and 2019 is no longer visible; (3) a very high SO2plume activity was observed in Nov. 2021, in spite of the low SO2mixing ratio at the cloud top. In contrast, the distribution of the SO2plume appearance over the disk is confirmed, with a maximum along the equator and around the morning terminator. Information on the SO2vertical gradient is retrieved from the simultaneous analysis of SO2at 7.4 µm and 19 µm. The gradient is constant in most cases with a few exceptions, especially in November 2021 when the plume activity was high. Finally, the temperature distributions retrieved from the continuum maps in September and November 2021 show an unusual pattern possibly associated with gravity waves, as previously observed by the longwave infrared camera camera aboard the Akatsuki spacecraft.

The Paleoarchean Buffalo River komatiites: Progressive melting of a single large mantle plume beneath the growing Kaapvaal craton

Several Archean granitoid-greenstone terranes are exposed on the southeastern Kaapvaal craton in South Africa, but they received little scientific attention compared to the archetypal greenstone belt successions of the Barberton Mountain Land at the eastern craton margin. This study reports on a detailed field and geochemical survey of the Buffalo River Greenstone Belt at the southern Kaapvaal craton margin in KwaZulu-Natal, with focus on hitherto unstudied komatiites and basaltic rocks from this volcanic succession. Cross-cutting relationships and new U-Pb zircon age determinations for several granitoid units establish a minimum age of 3.26 Ga for komatiitic volcanism, possibly as old as ca. 3.5 Ga if a 3.47 Ga granodiorite sheet is interpreted as ‘intrusive’ into the greenstone succession.Geochemical data reveal three types of Paleoarchean komatiites at Buffalo River. Spinifex textured lava flows represent Al-depleted komatiites, with subchondritic Al2O3/TiO2 ratios and enrichment of LREE over HREE. The second type comprises Al-undepleted komatiites that have chondritic Al2O3/TiO2 and flat REE patterns. The third type identified comprises Al-enriched komatiites that display suprachondritic Al2O3/TiO2 ratios, with significant LREE depletion. The Al-depleted and Al-undepleted komatiites from Buffalo River are geochemically similar to komatiites from the 3.48 Ga Komati and 3.26 Ga Weltevreden formations of the Barberton Supergroup respectively, whereas the Al-enriched komatiites resemble the 3.33 Ga Commondale komatiites on the southeastern Kaapvaal craton.To explain the co-occurrence of three discrete komatiite types within a single volcanic succession at Buffalo River, we suggest that each major komatiite magmatic pulse originated from the same upwelling mantle source, from which melt was extracted at different pressure but similarly hot temperature conditions. 187Os/188Os data for the Al-depleted komatiites suggest an ultimate magma origin from a primitive mantle reservoir. The contrasting γOs values for Kaapvaal craton komatiites (zero to positive) and peridotitic mantle xenoliths (zero to negative) support a complementary nature of these lithologies as high-degree melts and depleted residues linked by vigorous mantle plume activity at around 3.5 Ga. Such a relationship can explain the contrasting Re/Os systematics of komatiites and lithospheric mantle peridotites, which creates the contrasting γOs over time.The highly siderophile element patterns of the Al-depleted komatiites from Buffalo River are similar to those of Barberton-type komatiites, for which an origin from the deepest upper mantle with high melt retention in an upwelling plume source was suggested. We confirm that this ca. 3.5 Ga mantle source had only 60–80 % of the platinum-group element budget of the modern ambient mantle, which points indirectly to a location at great depth in the aftermath of the meteoritic late accretion. Progressive melting of such an upwelling mantle source, to the point of majoritic garnet exhaustion, may explain the Al-undepleted and Al-enriched komatiites at Buffalo River. The presence of all three major komatiite types within a single volcanic succession may be linked to deep critical melting of a large mantle plume associated with growth of the Kaapvaal ‘continent’ at 3.5 Ga.

Brine–Melts and Fluids of the Fe-F-P-(Ba)-(Sr)-REE Central Asian Carbonatite Province (Southern Siberia and Mongolia): The Petrogenetic Aspects

The carbonatite complexes of the Central Asian carbonatite province comprise the Siberian carbonatites of the Western Transbaikalia and the Central Tuva regions, as well as those from the Mushugai-Khudag complex in Southern Mongolia. They are confined to Late Mesozoic rift structures and have endured considerable tectono-magmatic processes caused by intense plume activity, which also accompanied their formation. A systematic study of melt and fluid inclusions revealed that these carbonatites formed as a result of immiscibility processes in silicate–carbonate (salt) melts, as well as fractional crystallization. Alkaline–carbonatite rocks crystallized in the presence of brine–melts with different compositions, i.e., alkaline–fluorine, carbonate, sulfate, phosphate, and chloride. These melts are responsible for mineralization during the orthomagmatic stage and the primary phase of Fe-F-P-(Ba)-(Sr)-REE ore formation at temperature ranges of 850–830 °C, 650–610 °C, and 560–440 °C and pressures between 290 and 350 MPa. At a later stage, the brine–melts evolved into saline hydrothermal fluids, which are considered to be the source of the second stage of F-(Ba)-(Sr)-REE ore mineralization. The saline crystal–fluid inclusions consist mainly of fluorine–sulfate–carbonate–chloride and bicarbonate–chloride compositions, with temperatures of approximately 480–250 °C and pressures below 250 MPa. The shift from melt to fluid in carbonatite complexes could occur more frequently in nature than previously believed and could also apply to other F-REE carbonatite complexes that are linked to rifting and plume activity in mountain-building zones.

Crustal structure of the Tibetan Plateau and adjacent areas revealed from ambient noise tomography

How the Tibetan Plateau deformed in the Indian-Eurasian continental collision has been long debated, more specifically, over the relationship between the deep processes and surface structural complexity. Here, we use ambient noise tomography to obtain a high-resolution crustal S-wave velocity model beneath the Tibetan Plateau and adjacent areas involving a comprehensive dataset from over 500 stations. Our images reveal that the crustal flow should be in a limited scale according to the intermittent low-velocity zones (LVZs) observed in the middle crust at 20–40-km depth of the Tibetan Plateau. The distributions and strengths of LVZs further imply that different deep processes promote the surface deformation in various regions of the Tibetan Plateau. The LVZs in the northern plateau, collocated potassic magmatism and low velocity anomalies in the upper mantle, should be originated from the lithospheric delamination. However, in the southern plateau, the S-wave velocity showed an apparent lateral segmentation feature correlated with the north–south trending rifts. The feature indicates that the LVZs were likely controlled by the lateral tearing of the subducted Indian mantle lithosphere, which promotes the rifting deformation. Moreover, the LVZ in the central Tibet should have contributed to the formation of the conjugate strike-slip fault system. In the Tarim Basin, our model showed a high-velocity anomaly in the lower crust that may be related to ancient mantle plume activity.

Sm-Nd isotope systematics of Indian shales constrain the growth of continental crust: Implication for supercontinent cycle and mantle plume activity

Sm-Nd isotope systematics of Indian shales constrain the growth of continental crust: Implication for supercontinent cycle and mantle plume activity

Petrogenesis of the alkali basalt and trachy-andesite suite in the northern Tarim Basin, NW China: Implications for crust-mantle interactions controlled by the Permian mantle plume

The Permian Tarim large igneous province (LIP), which contains both large volumes of basalt and silicic volcanic sequences, has been intensively studied in the past decades, but some issues still remain controversial, including the petrogenesis of each component in the LIP, their interrelationships, and the geodynamic processes that produced them. In the Halahatang area of the northern Tarim Basin, a well-developed eruptive suite composed of basalt and trachy-andesite has been penetrated by numerous boreholes, which provide us an opportunity to better understand these issues. In this paper, we carried out an integrated study on the eruptive suite, including petrography, geochronology and geochemistry. The results indicate that the eruptive suite formed at around ∼290 Ma, generally synchronous with the main episode of magmatism within the Permian Tarim LIP. The basalts have similar compositions to typical oceanic island basalts (OIBs), which have alkaline, high-Ti and sodic compositions and show enrichments in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs). Whole-rock Sr–Nd isotopes are slightly enriched, with εNd(t) and (87Sr/86Sr)i values from −4.5 to −3.4 and 0.7065 to 0.7086, respectively. The trachy-andesites are alkaline, metaluminous and ferroan, and display highly enriched LREEs and LILEs. High Ga/Al ratios and Zr + Nb + Ce + Y values constrain their A-type affinity and A1-type nature. They have εNd(t) values from −8.2 to −4.5 and (87Sr/86Sr)i values from 0.7083 to 0.7189, more enriched than those of the basalts. Based on these results, we proposed that the different components in the igneous suite have close petrogenetic relationships. The alkali basalts may have been formed by mantle plume-derived melts coupled with somewhat assimilation of continental crust, while the trachy-andesites were products of variable fractionation of the basaltic magmas involving more continental crustal components in the magma evolution. In combination with the existing results in Tarim, we further suggested that the alkali basalt and trachy-andesite suite may have been generated via a crust–mantle interaction process related to the Permian mantle plume activity.

Depositional response to the Emeishan mantle plume: evidence from the eastern Sichuan Basin, South China

ABSTRACT The Emeishan large igneous province (ELIP) is among the prominent large igneous provinces in the world and has been associated with mantle plume activity. In this study we investigated the impact of the Emeishan mantle plume on generating the sedimentary rock suites of the eastern Sichuan Basin. We present petrology, detrital zircon U – Pb ages, trace elements, and Hf isotopic compositions of the Middle-Late Permian sedimentary sequences in the Huayingshan region to constrain the provenance and characterize the depositional response to the Emeishan mantle plume activity. The detrital zircon U – Pb ages of the Middle Permian Maokou Formation indicate that terrigenous detritus was mainly sourced from the Kangdian paleoland in the southwestern margin of South China. Detrital zircon grains from the bottom of the Late Permian Longtan Formation yield a lower intercept age of 257 ± 28 Ma, interpreted as the contemporaneous metamorphism related to the ELIP magma. The detrital zircon U – Pb ages of the middle part of the Longtan Formation are characterized by unimodal age spectra with a peak at 259 Ma. Provenance analysis indicates that the Longtan Formation was mainly sourced from the Emeishan volcanic rocks in the Huayingshan region. We propose that the Kangdian paleohigh may have been regionally uplifted as the result of the pre-eruption rise of the Emeishan mantle plume in the late Middle Permian. Moreover, this paleohigh was translated into a denuded zone and provided voluminous detritus into the eastern Sichuan Basin. In the early Late Permian, the ELIP magmas upwelled and outpoured from the Huayingshan fault, and flowed past the peripheral rocks to the low terrain in the Huayingshan region. The ELIP volcanic rocks in the Huayingshan region underwent intense post-eruption erosion, resulting in the deposition of voluminous volcanic detritus in the eastern Sichuan Basin.

The evolution of Kerguelen mantle plume and breakup of eastern Gondwana: New insights from multistage Cretaceous magmatism in the Tethyan Himalaya

The relationship between the breakup of eastern Gondwana and Kerguelen mantle plume remains extremely controversial. The crucial conundrum is that when and where the Kerguelen mantle plume initially formed and its evolutionary history are unclear. In this paper, we report the oldest Cretaceous bimodal magmatism (144–140 Ma) associated with the Kerguelen mantle plume from the Comei large igneous province in the Tethyan Himalaya. The mafic rocks are dominated by oceanic island basalt (OIB)-like dykes with occasional normal mid-ocean ridge basalt (N-MORB)-like intrusions, while the felsic rocks show A-type granites affinities. Our results suggest that the > 140 Ma bimodal magmatism was clearly a response to early Kerguelen plume activity, and the Kerguelen plume initially formed underneath the Tethyan Himalaya but not the triple junction between Australia, Antarctica, and Greater India. We argue that the Cretaceous magmatism associated with the Kerguelen plume in the Tethyan Himalaya can be divided into three pulses of ca.141 Ma, ca.132 Ma, and ca.117 Ma, which correspond to vertical underplating, continuous ascent, and a southward migration process, respectively. Moreover, we propose that the multistage OIB-like magmas in the Tethyan Himalaya are mainly the products of interactions between the lower crust and mantle plume, while the contribution of the lithospheric mantle is negligible. This proposal implies that the lithosphere was significantly thinned before ca.141 Ma, and the Kerguelen plume had been incubating underneath the Tethyan Himalaya prior to this. Our findings suggest that the Kerguelen plume may not have been the trigger for the initial breakup of eastern Gondwana, but was complementary to the later complete breakup process. We also innovatively consider that the northward dragging and slab-pull forces caused by the Cretaceous intra-oceanic subduction of the Neo-Tethys Ocean may have played an active role in the evolution of the Kerguelen plume and the breakup of eastern Gondwana.

The evolution of the Galápagos mantle plume.

The lavas associated with mantle plumes may sample domains throughout Earth's mantle and probe its dynamics. However, plume studies are often only able to take snapshots in time, usually of the most recent plume activity, leaving the chemical and geodynamic evolution of major convective upwellings in Earth's mantle poorly constrained. Here, we report the geodynamically key information of how the lithology and density of a plume change from plume head phase to tail. We use iron stable isotopes and thermodynamic modeling to show that the Galápagos plume has contained small, nearly constant, amounts of dense recycled crust over its 90-million-year history. Despite a temporal evolution in the amount of recycled crust-derived melt in Galápagos-related lavas, we show that this can be explained by plume cooling alone, without associated changes in the plume's mantle source; results are also consistent with a plume rooted in a lower mantle low-velocity zone also sampling primordial components.

Bifurcation of mantle plumes by interaction with stagnant slabs in the mantle transition zone: Evidence from late Cenozoic basalts within Southeast Asia

Mantle plumes can encounter stagnant slabs in the mantle transition zone (MTZ) during their ascent from the lower mantle to Earth’s surface. However, the interactions between stagnant slabs and mantle plumes remain poorly understood. This study presents new data on the spatial thermal−lithological−chemical heterogeneity in the mantle over a vast area of Southeast Asia that reflects the variable influence of a late Cenozoic mantle plume. Site U1433 of International Ocean Discovery Program (IODP) Expedition 349 in the southwest sub-basin of the South China Sea (SCS) is located near the center of this seismically imaged mantle upwelling. This area also contains mid-ocean ridge basalt (MORB) derived from a peridotite-dominated and normal temperature mantle source with a depleted Nd-Hf and enriched 207Pb/206Pb isotopic composition. In contrast, MORB from Site U1431 of IODP Expedition 349 within the eastern sub-basin of the SCS and other intraplate basalts from the Hainan-Leizhou area, the eastern Indochina block, and elsewhere in the SCS are dominantly sourced from higher temperature pyroxenite-enriched mantle material that records mantle plume activity. These observations are in contrast with previous models involving a single plume rising from the lower mantle beneath the SCS that then tilted toward Hainan Island, China. Instead, it is likely that the observations from this region are indicative of plume-stagnant slab interaction. The shallow upper mantle beneath Site U1433 has not been influenced by a mantle plume as the presence of stagnant slabs within the MTZ prevented further upwelling of the plume. In contrast, a mantle plume passed through the weaker MTZ within the eastern sub-basin and circum-SCS areas, allowing plume-related magmatism to occur. The bifurcation of a deep-rooted mantle plume by stagnant slabs in the MTZ produced a cluster of small mantle upwellings that ascended to shallow depths, consistent with new multiscale global tomographic data for Southeast Asia.

Mantle Anisotropy in NW Namibia From XKS Splitting: Effects of Asthenospheric Flow, Lithospheric Structures, and Magmatic Underplating

AbstractThe presence of the Etendeka flood basalts in northwestern Namibia is taken as evidence for the activity of the Tristan da Cunha mantle plume during the continental breakup between Africa and South America. We investigate seismic anisotropy beneath NW Namibia by splitting analysis of core‐refracted teleseismic shear waves (XKS phases) to probe mantle flow and lithospheric deformation related to the tectonic history of the region. We present the results of the joint splitting analysis of XKS data collected from 34 onshore stations and 12 ocean‐bottom seismometers. The fast polarization directions (FPDs) are consistent with a model that combines the effects of lithospheric deformation and large‐scale mantle flow due to the NE motion of the African plate. The dominantly NNW‐SSE‐oriented FPDs in the northern part are likely caused by shallow lithospheric structures. Our observations do not show any strong evidence of a pervasive effect of the Tristan da Cunha mantle plume.

A comparison of hydrothermal events and petroleum migration between Ediacaran and lower Cambrian carbonates, Central Sichuan Basin

Hydrothermal minerals and associated organic matter have been frequently found in the Ediacaran and lower Cambrian carbonate reservoirs, separated by internal shale source rocks, in the Central Sichuan Basin. This work provides a thorough comparison of petrography, geochemical signatures, fluid inclusions and ages between the hydrothermal minerals in the Ediacaran Dengying Formation (Fm) and the lower Cambrian Longwangmiao Formation. This comparison leads to a better understanding of the behaviour and migration history of hydrothermal fluids and petroleum. During the Ediacaran–Cambrian transition, the flux of silicon-rich hydrothermal fluid triggered by seafloor-related volcanic events, resulted in the exclusive occurrence of silicification in the Dengying Fm. During P3–T3, two different hydrothermal systems were found in the Dengying and Longwangmiao formations. The hydrothermal fluids in the Dengying reservoirs were dominated by a mixture of basement-derived Mississippi Valley Type [MVT] ore fluids and shale-derived oil-bearing brines, while those in the Longwangmiao Fm were mainly the oil-bearing brines. The upwards migration of the MVT ore fluids, characterized by very hot, saline, radioactive and Eu-rich conditions, was promoted by the activity of the Emeishan mantle plume 259.4–248 Ma and then blocked by the lower Cambrian shales and trapped only within the underlying Dengying reservoirs. The expulsion of the warm, less saline and moderately radioactive brines and oil was induced by thermal maturation of the organic-rich shales, which might have occurred over a longer duration (261–218.7 Ma). During 170.7–125.8 Ma (J2–K2), the combined effects of the overmature source rocks and Yanshanian tectonic compression facilitated the migration of much hotter shale-derived brines, as well as gas hydrocarbons, across the Ediacaran to lower Cambrian carbonate reservoirs. This comparative study provides some insights into the behaviour of hydrothermal fluids and petroleum in carbonates with internal organic-rich shales.

A Mantle Plume Connection for Alkaline Lamprophyres (Sannaites) from the Permian Tarim Large Igneous Province: Petrological, Geochemical and Isotopic Constraints

AbstractThe origin of lamprophyres associated with large igneous provinces (LIPs) remains controversial, particularly whether they are derived by direct melting of mantle plumes, or from previously metasomatized domains in thermally perturbed subcontinental lithosphere. Here, we report the petrological and geochemical characteristics of a recently identified suite of alkaline lamprophyres (sannaites) that represent the final pulse of magmatism in the Permian Tarim LIP in NW China. The sannaites display porphyritic texture with phenocrysts of olivine, clinopyroxene, hornblende, phlogopite, and titanomagnetite in a groundmass of plagioclase, clinopyroxene, nepheline, hornblende, biotite, and titanomagnetite with minor pyrite and apatite. Carbonate ocelli and almost pure albite in the groundmass are interpreted to have crystallized from immiscible carbonate and hydrous fluids, respectively, produced by late-stage magmatic segregation. The rocks show low to moderate SiO2 (37.7–49.3 wt.%) and MgO (2.74–9.91 wt.%), together with high Fe2O3T (up to 22.7 wt.%) and alkali contents (up to 9.02 wt.% Na2O + K2O). They are characterized by high incompatible element abundances, especially a marked enrichment in large-ion lithophile elements (Rb and Ba) and light rare-earth elements (e.g. La and Ce) relative to P and high-field-strength elements (e.g. Ti). They show a relatively restricted range of δ66Zn values between 0.22‰ and 0.46‰ with an average of 0.37 ± 0.04‰ (2SE, n = 10), which is marginally heavier than that of MORBs (0.27 ± 0.05‰). Their (87Sr/86Sr)t values range from 0.7035 to 0.7061, εNd(t) from −0.97 to +5.62, and δ26Mg from −0.36‰ to −0.17‰ (n = 8), the latter being consistent with those of global MORBs. Based on the correlation between Zn isotopes and TiO2–FeO concentrations, we infer that the heavy Zn isotopes in some of the sannaites resulted from fractional crystallization of Fe–Ti oxide minerals. The whole rock geochemical features of these rocks (negative K anomalies and enrichment in large-ion lithophile elements) and rhyolite–MELTS simulations suggest that the primary magmas of the sannaites were derived from an amphibole-bearing enriched lithospheric mantle source. Metasomatism and related formation of amphibole-bearing metasomatized mantle may be linked to sublithospheric melts/fluids derived from the Tarim plume in the earlier stages of plume activity, rather than slab-derived fluids or carbonate melts as suggested in previous studies for other alkaline mantle-derived magmas. Partial melting may have been triggered by the thermal input from the Tarim plume during a later stage. This study suggests that exotic, alkali-rich magmas can be produced during the multi-stage evolution of large mantle plumes, involving complex cycles of lithospheric mantle metasomatism and later melting of previously enriched domains.

Petrology and amphibole-plagioclase chemistry of amphibolites in the Ilesa area, Southwestern Nigeria

Amphibolites of Itagunmodi and Igun, within the Ilesa Area were studied in order to determine their petrochemical affinities, tectonic setting and conditions of formation; ultimately for an improved understanding of the evolution of the Ilesa Schist Belt. The amphibolites are massive in texture and occur as small to large lensoid boulders associated with quartz-biotite schist, quartzite and talc-chlorite schist. They comprise magnesio-hornblende, actinolite, labrodorite, albite and accessory titanite. Average elemental composition and ratios in the amphibolites were: Ba (872 ppm), Sr (609 ppm), Ni (89 ppm), Rb (72 ppm), Y (27 ppm), Zr (168 ppm), Nb (83 ppm), Cr (200 ppm), La (34 ppm), Cr/Ni (2.30), Y/Nb (0.63), Rb/Sr (0.10), Nb/La (2.55), and Zr/Nb (4.62). The major and trace element characteristics revealed that they are ortho-amphibolites with tholeiitic and calc-alkaline affinities, plotting in Within Plate Basalt field of the Zr vs Zr/Y diagram. The amphibolites exhibit enhanced mid-ocean ridge basalt (E-MORB) elemental characteristics with LREE > HREE enrichment and slight negative Eu anomaly. The calc-alkalic affinity of some of the amphibolites is a result of crustal contamination, and not an arc-related signature as earlier proposed. Elemental abundances and ratios revealed that the amphibolites were formed in an extensional tectonic setting where mantle plume activity and lithospheric contamination occurred. The mineralogical assemblage reflects amphibolite facies metamorphism, at temperature of formation of 450–687 °C and pressure of 0.5–4.3 kbar constrained by plagioclase-hornblende thermobarometry.

A greenstone belt in southeast Tibet: An accreted middle–late Permian oceanic plateau

Studies of accreted oceanic plateau sections provide crucial information on their structures, compositions, and origins. We investigate the petrogenesis of ultramafic–mafic rocks in the Tangjia–Sumdo greenstone belt of southeast Tibet using petrography, whole-rock geochemistry, and U-Pb zircon geochronology. These rocks are divided into four groups based on geochemical characteristics that include depleted and tholeiitic mafic rocks, transitional mafic rocks, enriched and alkaline mafic rocks, and picritic ultramafic rocks. Depleted and tholeiitic mafic rocks have the oldest crystallization ages (∼272 Ma), followed by picritic ultramafic rocks (∼270 Ma), transitional mafic rocks (267–254 Ma), and enriched and alkaline mafic rocks (252–250 Ma). Hafnium and neodymium isotope ratios of depleted and tholeiitic mafic rocks (εHf(t) = +13.1–+16.9; εNd(t) = +6.9–+7.1), transitional mafic rocks (εHf(t) = +1.8–+16.9; εNd(t) = +0.8–+5.5), enriched and alkaline mafic rocks (εHf(t) = +0.5–+5.4; εNd(t) = −1.5 to +1.9) and picritic ultramafic rocks (εHf(t) = +14.9–+17.2; εNd(t) = +7.8–+9.0) are similar to those of N-MORB, E-MORB, OIB and depleted-type picritic mafic rocks in other oceanic plateaus, respectively. The geochemical characteristics of the depleted and tholeiitic mafic rocks suggest that they formed by partial melting of depleted spinel lherzolite in a mid-ocean ridge setting, whereas the picritic ultramafic rocks suggest a high degree of partial melting of depleted lherzolite in a hot mantle plume head. The transitional mafic rocks formed by partial melting of moderately enriched garnet lherzolite. The youngest rocks (enriched and alkaline mafic rocks) formed by partial melting of a more enriched garnet lherzolite (compared to transitional mafic rocks) at relatively low temperatures. We propose that the depleted and tholeiitic mafic rocks represent normal oceanic crust of the Sumdo Paleo-Tethys Ocean and the transitional mafic rocks, enriched and alkaline mafic rocks and picritic ultramafic rocks are the fragments of the oceanic plateau, which were related to middle–late Permian mantle plume activity in the Sumdo Paleo-Tethys Ocean. We further suggest that the majority of the Tangjia–Sumdo greenstone belt represents a middle–late Permian oceanic plateau that reflects a previously unrecognized middle–late Permian mantle plume.

Turonian-coniacian definition of the Caribbean plate: tectonic and metamorphic record in the Median Belt, Dominican Republic

ABSTRACT Uplift and unroofing of Jurassic-Cretaceous, mantle and crust, arc- and plume-related rock units in the Median Belt of the Dominican Republic exposed basement rocks with a protracted record of tectono-thermal events delineating the evolution of the northern edge of the Caribbean plate. In this article we focus on crustal rock units in the north-eastern half of the Median Belt. First 40Ar/39Ar dating of metamorphic ferri-winchite (86.4 ± 2.5 Ma; crystallization date) and albite (82.3 ± 5.8 and 79.8 ± 1.6 Ma; cooling dates) in metabasites of boninitic photolith from the Aptian-Albian Maimón Formation in the Ozama shear zone points to a tectono-metamorphic event in the Upper Cretaceous. Beltwide, this event caused syn-metamorphic N- to NNE-directed, simple-shear dominated, mylonitic and phyllonitic deformation of the Maimón Formation at peak metamorphic conditions of 8.2 kbar and 380°C. Such conditions are consistent with subduction of a coherent portion of forearc (represented by the Maimón Formation) to depths of ~25-29 km. The tectono-metamorphic event dated here overlaps with the inception of Turonian-Coniacian SW-dipping subduction and metamorphic sole formation in a back-arc position recorded in the Moa-Baracoa ophiolite complex in neighboring Eastern Cuba. Contemporaneity between the subduction inception of forearc and back-arc portions of the Caribbean arc and the main pulse of plume activity recorded in the Caribbean Large Igneous Province (CLIP) suggests that plume activity promoted general plate instability leading to a regional-scale plate reorganization. This mantle-plume-induced plate margin reorganization was coeval with the inception of the NE-dipping subduction of the Farallón plate beneath Central America leading to the definition of the Caribbean Plate by double-verging subduction zones along its northern and southwestern margins.

MESOPROTEROZOIC BIMODAL MAGMATISM OF THE ULUTAU TERRANE OF CENTRAL KAZAKHSTAN

In the southern part of the Ulutau terrane (Central Kazakhstan), a complex of volcanogenic rocks of basalt-rhyolite composition of the Zhiide series has been studied. Based on the U–Pb (SIMS) geochronological study of accessory zircons, an estimate of the age of rhyolites was obtained for the first time – 1338 ± 5 million years, which corresponds to the first half of the Ectasia period of the Mesoproterozoic. The features of the composition of the basic and acidic effusions indicate their formation in the context of continental rifting caused by the activity of the mantle plume.

Geochemical Signatures of Platinum Group Elements in Ultramafic Rocks of Chotanagpur Gneissic Terrain, Eastern India and Their Genetic Control

Ultramafic rocks occur as intrusives in the form of lensoid bodies in the northwestern part of Chotanagpur Gneissic Terrain and consist mainly of olivine, orthopyroxene and clinopyroxene and are devoid of plagioclase. Based on distinct geochemical characteristics, the rocks have been identified as komatiites. The rocks are geochemically analogous to Al-undepleted Munro type (Al2O3/ TiO2=17.88-54.73) with distinctly high MgO (26.2-35.62 wt%), Ni (958-1902 ppm) and Cr (21.32-3320 ppm) contents. These rocks are characterised by low CaO/Al2O3, (Gd/Yb)n, (La/Yb)n with positive Zr, Hf, Ti anomalies suggesting high degree partial melting of mantle under anhydrous conditions at shallow depth with garnet as a residual phase in the mantle restite. These high MgO volcanic rocks having elevated concentrations of Ni and Cr are potential hosts for Platinum Group Elements (PGE) owing to their primitive mantle origin and eruption at high temperatures. These rocks have low ΣPGE (29-269.02ppb) content with Ir (0.1-0.8ppb) and Ru (1.05-5.78ppb) among Iridium group PGE (IPGE); and Pt (5.04-18.72ppb), Pd(3.5-18.0ppb), Rh (0.22-0.84ppb) among Platinum group PGE (PPGE). The PGE abundances in komatiites were controlled by olivine fractionation. The Major, trace, REE and PGE composition of the rock suggest melting under anhydrous condition at shallow depth above the garnet stability field under S-undersaturated condition. Anhydrous melting associated with mantle plume activity gave rise to the rock which subsequently contaminated by lower crustal materials during magma ascent and emplacement. Keywords: Chotanagpur Gneissic Terrain, Komatiite, Platinum Group Elements, Sulphur Undersaturation, Plume

Computational fluid dynamics calculations of binary representation of Ni-base superalloy solidification in frustum geometries

CFD calculations were carried out on a number of frustum geometries from the experimental literature on freckling of superalloys. The effects of two imposed thermal fields were investigated, flat and inclined. Experiments document that when thermal fields are inclined the cold side is more prone to formation of freckles; our results conclude that rising liquid plumes in advance of the solidification front, which can correlate to freckles, are more abundant and intense on the cold side. Also, modelled plume activity is greater in inward sloping frustum geometries as opposed to outward sloping ones, which also agrees with established literature. Complex flow patterns were observed in inclined geometries and there is a clear interplay and competition between both the thermal and solutal aspects of flow, with flow being significantly changed, sometimes reversed in direction in some capacity. Our results also offer insight into the incubation length for plume and resultant freckle formation after the transition between frustums, with some critical distance of advance of the solidification front necessary to initiate and establish other plumes that feed into the main one circulating liquid flow which is locally intense enough to potentially create a freckle.