Plume-lithosphere Interaction Research Articles

Mantle flow beneath La Réunion hotspot track from SKS splitting

If upper mantle anisotropy beneath fast-moving oceanic plates is expected to align the fast azimuths close to the plate motion directions, the upper mantle flow pattern beneath slow-moving oceanic plates will reflect the relative motion between the moving plate and the underlying large-scale convecting mantle. In addition to the non-correlation of the fast azimuths with the plate motion direction, the flow and anisotropy pattern may be locally perturbed by other factors such as the upwelling and the sublithospheric spreading of mantle plumes. Investigating such plume–lithosphere interaction is strongly dependent on the available seismological data, which are generally sparse in oceanic environment. In this study, we take the opportunity of recent temporary deployments of 15 seismic stations and 5 permanent stations on the Piton de la Fournaise volcano, the active locus of La Réunion hotspot and of 6 permanent stations installed along or close to its fossil track of about 3700km in length, to analyze azimuthal anisotropy detected by SKS wave splitting and to decipher the various possible origins of anisotropy beneath the Western Indian Ocean. From about 150 good and fair splitting measurements and more than 1000 null splitting measurements, we attempt to distinguish between the influence of a local plume signature and large-scale mantle flow. The large-scale anisotropy pattern obtained at the SW-Indian Ocean island stations is well explained by plate motion relative to the deep mantle circulation. By contrast, stations on La Réunion Island show a complex signature characterized by numerous “nulls” and by fast split shear wave polarizations trending normal to the plate motion direction and obtained within a small backazimuthal window, that cannot be explained by either a single or two anisotropic layers. Despite the sparse spatial coverage which precludes a unique answer, we show that such pattern may be compatible with a simple model of sublithospheric spreading of La Réunion plume characterized by a conduit located at 100–200km north of La Réunion Island. Anisotropy beneath the new GEOSCOPE station in Rodrigues Island does not appear to be influenced by La Réunion plume-spreading signature but is fully compatible with either a model of large-scale deep mantle convection pattern and/or with a channeled asthenospheric flow beneath the Rodrigues ridge.

The role of recycled oceanic crust in magmatism and metallogeny: Os–Sr–Nd isotopes, U–Pb geochronology and geochemistry of picritic dykes in the Panzhihua giant Fe–Ti oxide deposit, central Emeishan large igneous province, SW China

The picritic dykes occurring within fine-grained gabbro in the marginal zone and in the surrounding Proterozoic wall-rock marbles of the Panzhihua Fe–Ti oxide deposit closely correspond in bulk composition with the nearby Panzhihua intrusion. These dykes offer important constraints on the nature of the mantle source of the Panzhihua ore-bearing intrusion and its possible link to the Emeishan plume. U–Pb zircon dating of the picritic dyke yields a crystallization age of 261.4 ± 4.6 Ma, coeval with the timing of the main Panzhihua gabbroic intrusion and Late Permian Emeishan flood basalts. The Panzhihua picritic dykes contain 37.63–43.41 wt% SiO2, 1.15–1.56 wt% TiO2, 11.43–13.25 wt% TFe2O3, and 20.96–28.87 wt% MgO. Primitive-mantle-normalized patterns of the rocks are comparable to those of ocean island basalt. The rocks define a relatively small range of Os isotopic compositions and a low Os signature of −0.13 to +2.76 for γOs (261 Ma). In combination with their Sr–Nd–Os isotopic compositions, we interpret that these rocks were derived from the Emeishan plume sources as well as the interactions of plume melts with the overlying lithosphere which had been extensively affected by eclogite-derived melts from the deep-subducted oceanic slab. Partial melting induced by an upwelling mantle plume that involved an eclogite or pyroxenite component in the lithospheric mantle could have produced the parental Fe-rich magma. Our study suggests that plume-lithosphere interaction might have played a key role in generating many world-class Fe–Ti oxide deposits clustered in the Panxi area.

Geochronology, geochemistry and petrogenesis of Neoproterozoic basalts from Sugetbrak, northwest Tarim block, China: Implications for the onset of Rodinia supercontinent breakup

Two layers of basaltic flows intercalated with Late Neoproterozoic (Sinian) sandstones have been identified in the Sugetbrak region in the northwest Tarim block, Northwest China. The basaltic rocks are composed of augite and plagioclase phenocrysts set in a groundmass of plagioclase laths with interstitial subophitic clinopyroxene grains and minor anhedral opaque minerals (magnetite and ilmenite). LA-MC-ICP-MS U–Pb dating of zircons from the lower basaltic flows yields a crystallization age of 783.7±2.3Ma. Both layers of the basaltic rocks are characterized by low SiO2 and high total FeO (>12wt.%), TiO2 (>3wt.%) and P2O5 (>0.5wt.%) contents and extremely high Na2O/K2O ratios, and display a Fenner trend of differentiation that could be ascribed to significant fractional crystallization of clinopyroxene and plagioclase. The mineralogical and geochemical characteristics suggest the affinities of a transitional series between alkaline basalt and tholeiite. However, the lower basaltic flows have higher Nb/Y ratios than the upper ones, indicating that they are more alkaline. Positive age-corrected Nd isotope ratios [ɛNd(t)=+0.24–1.07] and positive ɛHf(t) values (+1.1–4.5) of the basaltic rocks suggest absence of any significant crustal contamination. High ɛNd(t) lavas are isotopically similar to those of several modern oceanic hotspots, and have ocean island-like patterns of incompatible elements. The estimated potential mantle temperature is ∼100–150°C higher than normal asthenospheric mantle, consistent with a plume-head origin. Moderate ratios of light rare earth elements (REE) to heavy REE indicate that the source magma was probably generated by partial melting of garnet–spinel transition facies of peridotite, but the upper basaltic rocks were derived from a relatively shallower mantle source, reflecting progressive lithosphere thinning possibly through plume–lithosphere interaction. We correlate the Sugetbrak basalts to the second Neoproterozoic mantle plume event (780–745Ma) related to the breakup of the Rodinia supercontinent.

Geochemical characteristics and petrogenesis of Permian basaltic rocks in Keping area, Western Tarim basin: A record of plume-lithosphere interaction

The two basalt flows in Kupukuziman (库普库兹满) Formation of Keping (柯坪) area are the typical products of Permian magmatism. Based on systematic field investigations, we carried out geochemical studies on representative Keping basalts. The results show that the SiO2 contents in basalts range from 44.69 wt.% to 51.68 wt.%, and the total alkalis range from 4.05 wt.% to 5.5 wt.%, belong- ing to alkaline basalts. The Ti/Y=468.27-565.35 and TiO2=(2.88-3.82) wt.% compared to those of the high-Ti basalts in Emeishan (峨眉山) large igneous province (LIP) (TiO2=(3.58-5.21) wt.%, Ti/Y>500, Xiao et al., 2004). The contents of MgO and compatible elements Cr (12.9-18.6 ppm), Ni (13.7-22.7 ppm), and Co (39.6-50.2 ppm) are low, which suggest that the samples have undergone varying grades of fractional crystallization. All the samples have uniform REE and other trace element characteristics, with enriched LREE and LILE relative to HREE and HFS, similar REE patterns without Eu anomaly. The basalts are also show slightly depleted of Ta and Nb. They have higher initial 87 Sr/ 86 Sr ratios (0.707 211-0.708 300), lower eNd(t) values (-2.47 to -4.14), and a narrow range of ( 208 Pb/ 204 Pb)t=37.535 2-

Mechanisms of continental crust formation in the Central Asian Foldbelt

Geological and isotopic study of rocks occurring in the Early and Late Baikalian, Caledonian, Hercynian, and Indosinian fold regions of Central Asia is carried out. The juvenile crust formation occurred in these fold regions have determined the systematic differences in isotopic compositions of the crust. In the course of the subsequent (postaccretion) evolution, the crust of these domains underwent multiple reworking. These processes were accompanied by variations in the Nd isotopic compositions of the crust, which, in turn, are recorded in the isotopic compositions of granites and felsic volcanics as products of crust melting. Three types of crust differing in Nd isotopic composition and structure and, as a consequence, in formation mechanisms, are distinguished. The isotopically homogeneous crust is a source of igneous rocks with constant model Nd isotopic age (TNd(DM2st)) irrespective of the age of the crustal igneous rocks. These are the isotopic provinces, the crust of which remained isolated from addition of alien materials during postaccretion evolution. The axial zone of the Hercynides in the Central Asian Foldbelt is an example. The isotopically heterogeneous layered crust consists of fragments differing in isotopic composition. The products of its melting are characterized by widely scattered ɛNd(T) and (TNd(DM2st). The appearance of alien sources of melt is considered in terms of underplating. This mechanism develops either due to subduction of the juvenile oceanic lithosphere beneath the mature continental lithosphere at convergent boundaries or as a result of plume-lithosphere interaction. The first mechanism operated during the formation of granitoids pertaining to the Tuva-Mongolia microcontinent. The second mechanism was responsible for the formation of batholiths in the zonal Hangay, Barguzin, and Mongolia-Transbaikalia magmatic fields. The isotopically heterogeneous mixed crust is composed of fragments differing in isotopic composition, which are tectonically mixed, resulting in the formation of an isotopically uniform reservoir in the domain of magma generation. As a result, the products of melting acquire isotopic parameters substantially distinct from the juvenile rocks of the corresponding structural zone. The formation of such a crust is related to the tectonic delamination, which provides for juxtaposition and a high degree of tectonic mingling of heterogeneous fragments at deep levels. The Caledonides of the Central Asian Foldbelt are characterized by such a mechanism of crust formation.

Geochronology–geochemistry of the Cida bimodal intrusive complex, central Emeishan large igneous province, southwest China: petrogenesis and plume–lithosphere interaction

The Cida complex is situated in the Panxi region and is predominantly composed of mafic-ultramafic and syenitic rock units; minor amounts of intermediate rocks occupy the contact zone between the two major rock types. The intermediate unit is mineralogically heterogeneous and typically exhibits a mottled structure. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb zircon dating shows that the mafic-ultramafic rocks and syenitic rocks formed almost coevally (243 ± 0.77 Ma and 240.5 ± 0.76 Ma, respectively). These ages may represent the end phase of the Emeishan large igneous province (ELIP) magmatism. Most of these three rock types possess alkaline and metaluminous affinities. The mafic-ultramafic, syenitic, and intermediate units have K2O + Na2O contents of 1.85–5.16, 6.55–10.46, and 9.55–11.54 wt.%, and SiO2 contents of 40.06–46.70, 61.74–68.54, and 51.57–54.13 wt.%, respectively. The mafic-ultramafic unit displays ocean-island basalt (OIB)-like primitive-mantle-normalized incompatible element patterns, coupled with low initial 87Sr/86Sr ratios (0.7048–0.7064), positive ϵNd(t) (0.32–2.23), and zircon ϵHf(t) (4.53–14.17) values, consistent with a mafic plume-head origin, whereas one exceptional sample with negative ϵNd(t) (–0.22) can be interpreted as due to the involvement of considerable amounts of enriched subcontinental lithospheric mantle. The relatively low (La/Yb) N ratios (3.40–7.69) reflect a spinel-facies lherzolite source. The syenitic unit is characterized by enrichment in large ion lithophile elements (e.g. Rb, K, Pb) and light rare earth elements (LREEs), relative to high field strength elements (e.g. Nb, Ta, P, Ti) and heavy rare earth elements (HREEs), respectively. These features, together with their metaluminous affinities, low SiO2 contents, lower initial 87Sr/86Sr ratios (0.7043), positive ϵNd(t) (0.18), and zircon ϵHf(t) (2.63–10.09) values as well as modelling of REEs, can be plausibly explained by crustal partial melting of juvenile basic materials beneath the Yangtze Block. In contrast, the field, petrographic observations, and geochemical signatures (e.g. the linear correlations between FeO* and MgO, K/Ba and Rb/Ba ratios) suggest that the intermediate unit may have resulted from magma mixing between the syenitic and basaltic magmas that in turn had evolved from a parental mafic-ultramafic liquid. Thus, the formation of the Cida complex can be attributed to the plume–lithosphere interaction plus partial melting of juvenile basic lower crust in response to heating of underplated plume-derived basaltic magma.

Occurrence of inherited supra-subduction zone mantle in the oceanic lithosphere as inferred from mantle xenoliths from Dragon Seamount (southern Tore–Madeira Rise)

Spinel-bearing peridotite and pyroxenite xenoliths dredged from the Dragon Seamount (southern Tore–Madeira Rise, West Iberia and Morocco margin) give an insight into the composition of the underlying lithosphere. These xenoliths are devoid of evidence of strong host lava–peridotite interaction and re-equilibration or late impregnation in the plagioclase facies. The spinels and pyroxenes from the Dragon peridotites have compositions distinct from those of both lherzolites and harzburgites from the Iberia margin and the Mid-Atlantic Ridge. They display a highly depleted composition, in particular, high Cr-number, up to 0.63 in the spinels, consistent with a melting degree between 12 and 19%. Because of the strong chemical similarities between the Tore–Madeira Rise, Newfoundland peridotites, and peridotites from supra-subduction zones, we propose that the Dragon peridotites formed in a similar context. The pyroxenites display a cumulate texture and are probably a high-temperature–high-pressure cumulate formed by fractional crystallization from a melt. The Tore–Madeira Rise peridotites may represent a former mantle wedge in an oceanic arc, later included into the continental lithosphere and finally tectonically disseminated within the lithosphere during the rifting of the Newfoundland–Iberia continental lithosphere. As a consequence, rifting processes may produce heterogeneities in the oceanic lithosphere and influence isotopic compositions of ocean island basalt-type lavas during plume–lithosphere interactions, as inferred for the southern Tore–Madeira Rise. Supplementary material: Further information on the analytical results is available at http://www.geolsoc.org.uk/SUP18512 .

A new metallogenic model of the Panzhihua giant V–Ti–iron oxide deposit (Emeishan Large Igneous Province) based on high-Mg olivine-bearing wehrlite and new field evidence

The Panzhihua layered intrusion hosts a giant V–Ti–iron oxide deposit with ore reserves estimated at 1333 Mt. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb zircon dating of comagmatic anorthosite yields a crystallization age of 259.77 ± 0.79 million years, coeval with the Emeishan flood basalts. Recently, we identified a small wehrlite dike in microgabbroic rocks and marbles. The wehrlite consists of high-Mg olivine phenocrysts with up to 90.44 wt.% Fo. Incompatible element-normalized patterns between bulk wehrlite and clinopyroxenes in gabbro suggest that they are cogenetic. The Panzhihua parental magma is estimated to have been picritic (∼10 wt.% FeO and ∼16 wt.% MgO), produced by partial fusion of garnet peridotite. Much of the melting occurred in garnet-facies mantle at an initial melting temperature of about 1530°C and pressure of ∼3.4 GPa, suggesting involvement of a mantle plume. The degree of partial melting was rather modest and could have been generated by plume–lithosphere interaction or ascending plume-derived melting contaminated by lithospheric mantle. Field relationships show sharp contacts between the massive ores and gabbro, between wehrlite and fine-grained gabbro, and between disseminated ores and gabbro. Considering the entire intrusion, which is locally cut by dikes or veins of anorthosite, together with the occurrence of a breccia made up of gabbro clasts cemented by disseminated ores, we suggest that different types of magmas were generated by liquid differentiation in a deeper-level chamber. This differentiation could have resulted from double-diffusive convection cells, with melt later intruding into a higher-level chamber, rather than by crystal settling or in situ growth on the floor of the intrusion. However, rhythmic layering produced by in situ crystallization only occurs in the middle of the Panzhihua intrusion and was caused by periodic fluctuation in water pressure.

Petrogenesis of lamprophyres from Chhota Udepur area, Narmada rift zone, and its relation to Deccan magmatism

Geological setting, petrology and geochemistry of a new find of lamprophyre dykes, intruding the Precambrian basement gneisses from the Chhota Udepur area, Narmada rift zone, Western India, are presented. Of the three lamprophyre dykes, two of them display E–W trend paralleling that of the Narmada rift, extend up to ∼2.5 km in length and constitute the longest yet recorded lamprophyre dykes from the Indian shield. The Chhota Udepur lamprophyres (CUL) show several overlapping mineralogical and geochemical characteristic features displayed by alkaline- and calc-alkaline lamprophyres and hence it is not straight forward to pigeon-hole them in existing lamprophyre classification schemes. They are porphyritic–panidiomorphic with euhedral to sub-hedral phenocrysts and microphenocrysts of olivine (Fo 78.33–78.42 Fa 21.24–21.37), diopsidic augite to titaniferous augite (Wo 54.83–59.03 En 36.75–41.36 Fs 0.1–5.66), biotite and amphibole (kaersutite). Feldspar (orthoclase perthite and plagioclase) is essentially confined to the groundmass along with accessory phases such as apatite and spinel (ti-magnetite). Rare carbonate ocelli are also present. In terms of major element geochemistry the CUL are nepeheline-normative and predominantly sodic (Na 2O > K 2O); yet their K 2O and SiO 2 contents are sufficiently high to signal traits of calc-alkaline lamprophyres. On the other hand, the CUL exclude negative spikes at Nb–Ta which are considered to be a characteristic of subduction-related (calc-alkaline) magmas, are highly evolved (Mg#: 27.82–55.45), enriched in incompatible trace elements (high La/Nb, Zr/Nb) and have fractionated REE distribution patterns (La/Yb N ratios: 35–44). Post-emplacement alteration and crustal contamination have had little influence on their geochemistry. Th/Yb, Ta/Yb, Nb/U and Ce/Pb ratios of the CUL imply an OIB type-enriched mantle source, similar to that displayed by other Deccan-related lamprophyres. A metasomatised (enriched) garnet lherzolite mantle source with very low degrees of partial melting that has subsequently undergone a large degrees of fractional crystallisation is capable of accounting observed REE geochemistry of the CUL. We infer the lamprophyres of Chhota Udepur area to be a part of the alkaline rock spectrum of the Deccan Large Igneous Province and their emplacement, together with that of other alkaline complexes in the Chhota Udepur area, is envisaged to be strongly controlled by pre-existing crustal weakness accentuated by the mantle plume–lithosphere interaction during the end-Cretaceous in the Narmada rift zone.

The Song Da magmatic suite revisited: A petrologic, geochemical and Sr–Nd isotopic study on picrites, flood basalts and silicic volcanic rocks

Permian volcanic rocks, including picrites, flood basalts and silicic volcanic rocks, crop out in the Song Da district, northern Vietnam. They occur as a ∼300-km-long belt emanating from the southernmost part of the Emeishan large igneous province, SW China. The belt represents a detached part from the province and was displaced ∼600 km southeastward along the Ailao Shan-Red River shear zone during the mid-Tertiary in response to the India–Eurasia collision. We report here geochemical and Sr–Nd isotopic data on the Song Da volcanic rocks to decipher the petrogenetic processes responsible for their formation. The picrites show evidence of olivine fractionation and formed from high-Mg magmas (MgO = 20–25 wt.%) originated from depleted mantle peridotites from the Emeishan plume head. The plume head is characterized by εNd( t) value of +8. Some picrites and low-Ti basalts have low Nb/La, Nb/U and εNd( t), which are indicative of plume–lithosphere interaction in various degrees. Contamination of continental crustal rocks (<20%) is able to produce the spectra of the observed trace element patterns and isotopic ratios. The silicic volcanic rocks are metaluminous or peraluminous (aluminium saturation index = 0.92–1.24) and are sufficiently different from those present elsewhere in the Emeishan large igneous province that are dominantly peralkaline. The Song Da silicic volcanic rocks have εNd( t) values of −0.1 to +0.6 and show strong depletion in compatible trace elements (e.g. Ni = 0.8–3 ppm, V = 4–55 ppm). They most likely formed by extensive fractional crystallization from the associated high-Ti basalts, possibly with some crustal contamination.

Geochemical variations at intraplate hot spots caused by variable melting of a veined mantle plume

Three-dimensional geodynamic models of plume-lithosphere interaction were used to explore the causes of spatial patterns of magmatic compositions at intraplate hot spots. The models couple mantle flow, heat transfer, and the melting of multiple components present in the mantle as small blobs or veins with different solidi and composition. Predicted magma compositions evolve from having a strong signature from the deepest-melting component in the early stages of volcanism to a strong signature from the shallowest-melting component in the later stages. This compositional trend arises by progressive melt extraction of the different components and a horizontal displacement of their melting zones due to shear flow associated with plate motion. When three or more components are present, the composition of a volcano evolves along arrays in isotope space that trend toward mixed compositions of the components rather than the components themselves. Models explain the average Pb isotope trends in the Hawaiian Scientific Drilling Program core at Mauna Kea volcano. Observed scatter about the average trends and the distinction between the Kea and Loa subchains are explained by spatial variability in the relative proportions of the components in the mantle. Monte Carlo simulations show that linear Pb isotope arrays are unlikely to originate from nonsystematic, binary mixing if the scale of the magma capture zone is much larger than the scale of isotopic heterogeneity. However, systematic sampling by progressive melt extraction naturally generates such linear arrays if the capture zone is large compared to the scale of heterogeneity.

Upper mantle seismic anisotropy in the intra-continental Kachchh rift zone, Gujarat, India

Shear wave splitting study of 411 SKS/SKKS phases covering backazimuth range of 13 ° to 305 ° recorded by 12 broadband stations in the Kachchh rift has led to estimates of fast axis orientations and splitting times for 118 good measurements. The average vector mean of fast axis orientation (86 ± 14 °) corresponds to the E–W axis of the Kachchh rift and the delay time (~ 1.6 s) is attributed to the ~ 184 km-thick upper mantle layer with 4% anisotropy. The anisotropic character observed for the Kachchh rift (KR) is comparable to other continental rifts and these are related to the high-temperature, lattice-preferred orientation fabric of olivine, inherited from the mantle flows. The source of the rift-axis parallel anisotropy is traced to the rift-parallel flows within the 76 ± 6 km-thick lithosphere. Additionally, the rift-parallel pockets of partial melts also induce anisotropy within the asthenosphere. Both these are inherited from the plume–lithosphere interaction during the Deccan/Reunion plume episode (~ 65 Ma).

Crustal structure beneath the Galápagos Archipelago from ambient noise tomography and its implications for plume-lithosphere interactions

[1] To constrain the seismic velocity structure of the crust beneath the Galapagos Archipelago, we conducted a tomographic study using high-frequency Rayleigh waves obtained from cross correlations of ambient noise. We analyzed waves with periods between 5 and 8.5 s, sensitive to shear wave velocity (VS) structure between about 3 and 10 km depth, after accounting for the effect of water depth. Crustal velocities are up to 25% lower than those of very young crust at the East Pacific Rise and are comparable to those of Hawaii. We attribute the lower than normal velocities to the combined effect of heating and the presence of melt in the crust above the Galapagos plume as well as the construction of a highly porous volcanic platform emplaced atop preexisting oceanic crust. On average, VS between 3 and 10 km depth beneath the western archipelago is up to 15% lower than beneath the eastern archipelago. We attribute the west-to-east velocity increase to a decrease in porosity of the volcanic platform and to cooling of the crust after its passage above the Galapagos plume. The results of this study, in combination with previous work, indicate that many of the unusual aspects of the Galapagos Archipelago are the result of variations in the thickness and internal structure of the chemical and thermal lithospheres. Our findings indicate that observed variations in the flexural response to loading observed in the Galapagos cannot be explained by the current thermal state of the lithosphere. Instead, the flexural response likely represents varying elastic strength at the time of loading. We also propose that the northwest and northeast trending alignments of volcanic centers found throughout the archipelago (the Darwinian lineations) may be associated with preexisting zones of weakness in the lithosphere formed during earlier episodes of ridge jumping and ridge propagation that were later reactivated by stresses generated by plume-lithosphere interactions.

Geochemistry and geochronology of the mafic lavas from the southeastern Ethiopian rift (the East African Rift System): assessment of models on magma sources, plume–lithosphere interaction and plume evolution

Major and trace element and isotopic ratios (Sr, Nd and Pb) are presented for mafic lavas (MgO > 4 wt%) from the southwestern Yabello region (southern Ethiopia) in the vicinity of the East African Rift System (EARS). New K/Ar dating results confirm three magmatic periods of activity in the region: (1) Miocene (12.3–10.5 Ma) alkali basalts and hawaiites, (2) Pliocene (4.7–3.6 Ma) tholeiitic basalts, and (3) Recent (1.9–0.3 Ma) basanite-dominant alkaline lavas. Trace element and isotopic characteristics of the Miocene and Quaternary lavas bear a close similarity to ocean island basalts that derived from HIMU-type sublithospheric source. The Pliocene basalts have higher Ba/Nb, La/Nb, Zr/Nb and 87Sr/86Sr (0.70395–0.70417) and less radiogenic Pb isotopic ratios (206Pb/204Pb = 18.12–18.27) relative to the Miocene and Quaternary lavas, indicative of significant contribution from enriched subcontinental lithospheric mantle in their sources. Intermittent upwelling of hot mantle plume in at least two cycles can explain the magmatic evolution in the southern Ethiopian region. Although plumes have been originated from a common and deeper superplume extending from the core–mantle boundary, the diversity of plume components during the Miocene and Quaternary reflects heterogeneity of secondary plumes at shallower levels connected to the African superplume, which have evolved to more homogeneous source.

A thermochemical model of Eurasian Permo-Triassic mantle plumes as a basis for prediction and exploration for Cu-Ni-PGE and rare-metal ore deposits

Four stages of the thermochemical plume—lithosphere interaction generating a broad mushroom-like head of plume and especially fourth regressive cooling phase have the important role for metallogeny. The analysis of a thermochemical plume model together with recent geological and geochronological data on magmatic ore systems in the Siberian, Tarim, Emeishan, Central European, and some other large igneous provinces (LIPs) enabled the following characteristics of the metallogeny in large igneous provinces to be revealed: (1) the specific combination of mineralization types, which include magmatic Cu-Ni-Pt and Fe-Pt; hydrothermal Ni-Co-As (±Ag, U, Au), Au-As, Ag-Sb, Au-Hg, Sb-Hg and stratiform Cu (copper-bearing sandstones and shales enriched in Co, Ni, Ag, Pt); (2) the areal or spot-like pattern of the location of mineralization types (opposed to the linear-belt localization in subduction and rift settings); (3) the zoned distribution of mineralization types relative to LIPs' centers, with Cu-Ni-Pt, Fe-Pt and stratiform Cu mineralization localized in a LIP center, and hydrothermal mineralization bound to a LIP periphery; (4) the essential contemporaneity of the formation of each mineralization type in LIPs, and the existence of coeval but spatially separated Cu-Ni-Pt, Ni-Co-As and Au-As deposits; (5) the close linkage between different mineralization types and particular pulses of mafic, alkaline mafic, and felsic magmatism; (6) the unified succession of ore-forming events; (7) the close relationship between the extent of mineralization and LIP igneous volumes, which in turn depend on the plume heat power. The major characteristics of localization of different mineralization types within LIPs, the timing and genetic relationships between mineralization and types of magmatism, as well as specific geological controls on ore formation provide a basis for establishing new geological, magmatic, litological, and geochemical criteria crucial for predicting and targeting new mineralization within LIPs.

Petrogenesis of the end-Cretaceous diamondiferous Behradih orangeite pipe: implication for mantle plume–lithosphere interaction in the Bastar craton, Central India

We present mineral chemistry, geochemistry and Sr and Nd isotope data of drillcore samples from the Late Cretaceous (65 Ma), diamondiferous Behradih ultramafic pipe, Bastar craton, Central India, which is emplaced synchronous with the Deccan flood basalt eruption. The rock is affected by pervasive serpentine–talc–carbonate alteration and consists of pelletal lapilli and variously sized olivine and phlogopite macrocrysts, set in a groundmass of abundant clinopyroxene, chrome spinel, apatite, Fe-rich perovskite (<50 μm), zircon, titanite, rutile and calcite. Mineralogical studies identify the Behradih pipe as orangeite (formerly termed as Group II kimberlite) and establish the occurrence of such rocks outside the Kaapvaal craton, southern Africa. As the age of the Behradih orangeite overlaps with that of the main phase of the Deccan flood basalt magmatism, we infer a common tectonomagmatic control vis-a-vis the Deccan-related mantle plume. Trace element ratios and the Nd isotope signatures of the Behradih pipe imply that the Deccan plume has only contributed heat, but not substantial melt, to the Behradih magma with a cause-and-consequence relationship between them. Our study highlights (a) a striking similarity in the genesis of Late Cretaceous orangeites associated with the continental flood basalts in the Kaapvaal and Bastar cratons but related to different mantle plumes and (b) the role of plume–lithosphere interaction in the generation of orangeites.

Variety and complexity of the Late-Permian Emeishan basalts: Reappraisal of plume–lithosphere interaction processes

New petrographic, major element, trace element and isotopic data (Sr and Nd) for 17 Emeishan basalts from the Lijiang-Shangri-La area of SW China are integrated with published data comprising ~ 300 mafic rocks from the Emeishan large igneous province (LIP) to create a comprehensive model for this LIP event. We find large compositional variations in the Emeishan basalts that were generated by melting of heterogeneous mantle sources and interaction between the Emeishan plume and lithosphere. The Emeishan basalts can be generally grouped into high-Ti and low-Ti classes, but our samples argue that a more detailed classification scheme is required, and the two classes are better separated by the line TiO 2 = −0.08MgO + 2.91. We suggest that high-Ti basalts are products of deep melting plume head material similar to oceanic island basalts (OIBs), with little lithospheric overprint. The low-Ti basalts, alternatively, were generated from shallower melting of the plume head and have significant lithospheric inputs via either direct crustal contamination or by inherited subduction components in the lithosphere. The Vietnam picrites and low-Ti basalts are distinct from other ELIP samples, and probably derived from a much more depleted mantle component, either intrinsic to the mantle plume itself or entrained in the plume head during its ascent. We infer that the thermal structure, spatially distributed compositional heterogeneities within the plume head, and varying degrees of interaction with overlying lithosphere and crust control the composition of lavas from a given locality.

Plume-Lithosphere Interaction during Migration of Cretaceous Alkaline Magmatism in SW Portugal: Evidence from U-Pb Ages and Pb-Sr-Hf Isotopes

Large massifs of alkaline rocks are exposed along ∼250 km of the Atlantic coast of Portugal. Their origin is still poorly understood, including the precise timing of their emplacement and their rel ...

Permian basaltic rocks in the Tarim basin, NW China: Implications for plume–lithosphere interaction

There are large areas of Permian basaltic rocks in the Tarim basin (PBRT) in northwestern China. Precise Ar–Ar dating of these rocks revealed an eruption age span of 262 to 285 Ma. Most of the PBRT is composed of alkaline basaltic rocks with high TiO 2 (2.43%–4.59%, weight percent), high Fe 2O 3 + FeO (12.63%–17.83%) and P 2O 5 (0.32%–1.38%) contents. Trace elements of these rocks have affinities with oceanic island basalts (OIB), as shown in chondrite normalized rare earth elements (REE) diagrams and primitive mantle normalized incompatible elements diagrams. The rocks show complex Sr–Nd isotopic character based on which they can be subdivided into two distinct groups: group 1 has relatively small initial ( t = 280 Ma) 87Sr/ 86Sr ratio (∼ 0.7048) and positive εNd( t) (3.42–4.66) values. Group 2 has relatively large initial 87Sr/ 86Sr ratio (0.7060–0.7083) and negative εNd( t) (from − 2.79 to − 2.16) values. Lead isotopes are even more complex with variations of ( 206Pb/ 204Pb) t, ( 207Pb/ 204Pb) t and ( 208Pb/ 204Pb) t ranging from 17.9265 to 18.5778, 15.4789 to 15.6067 and 37.2922 to 38.1437, respectively. Moreover, these two groups have different trace elements ratios such as Nb/La, Ba/Nb, Zr/Nb, Nb/Ta and Zr/Hf, implying different magmatic processes. Based on the geochemistry of basaltic rocks and an evaluation of the tectonics, deformation, and the compositions of crust and lithospheric mantle in Tarim, we conclude that these basaltic rocks resulted from plume–lithosphere interaction. Permian mantle plume caused an upwelling of the Tarim lithosphere leading to melting of the asthenospheric mantle by decompression. The magma ascended rapidly to the base of lower crust, where different degrees of assimilation of OIB-like materials and fractionation occurred. Group 1 rocks formed where the upwelling is most pronounced and the assimilation was negligible. In other places, different degrees of assimilation and fractionation account for the geochemical traits of group 2.

Neoproterozoic riftogenic subalkali basites of the Central Urals: Geochemical specifics of clinopyroxene

Trace-element distribution in clinopyroxenes of different generations was used to decipher the intricate melt fractionation history of the trachybasalts, trachyandesites, and gabbroids from different rift-related magmatic complexes, which were formed during evolution of the Neoproterozoic passive margin of the East European craton (western slope of the Central Urals). It was established that chromian and high-magnesian cores of the early Cpx phenocrysts in the trachybasalts are not xenogenic, but represent relict minerals that were formed at the early stage of fractionation of a high-Mg melt. The fact that the trachybasalts and trachybasaltic andesites contain high-Mg Cpx with trace-element patterns similar in shape and element abundance indicates their formation via differentiation of a common melt. However, their subsequent evolution was different: trachybasaltic andesites were subjected to significant crustal contamination, which was recorded in composition of late Cpx. All subalkali basites were presumably formed from comparatively deepseated melts, because, according to thermodynamic calculations, early cores in the gabbroids crystallized at temperature more than 1200°C and pressure between 10 and 14–15 kbar, i.e., at depths of approximately 35–50 km. Some differences observed between trace-element composition of Cpx from the studied subalkali basatoids and gabbroids are inconsistent with their derivation from a common source, but similarity of their Cpx in many characteristics undoubtedly indicates close depths and compositions of their sources. Geochemical peculiarities of Cpx in the Neoproterozoic rift rocks from the western slope of the Central Urals testify that they were derived from melts formed by relatively low degree partial melting with garnet in residue. The geochemical specifics of clinopyroxenes from the Neoproterozoic riftogenic subalkali basites of the Central Urals and basaltoids from the Paleozoic Tagil structure of the same region showed that this island-arc system reflects the composition of melt source and its reworking by mantle fluids, which were different in the course of plume-lithosphere interaction and suprasubduction processes.