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The Rosario-6 is a non-diamondiferous hypabyssal kimberlite located above the Rio de la Plata craton and near the south-eastern edge of the Parana Basin, in southern Brazil. It is petrographically an inequigranular texture, macrocrystal kimberlite, fresh and the groundmass exhibits a microporphyritic texture and round megacrysts of olivine, which are derived from disaggregated mantle xenoliths. Olivine is also present as macrocrysts, microphenocrysts and in the groundmass together with phlogopite and apatite. These microphenocrysts are immersed in a groundmass of olivine, monticellite, phlogopite, CaTiO3-perovskite, apatite, Mg-chromite and Mg-ulvospinel and melilite. A mesostasis assemblage of phlogopite, melilite, soda melilite, akermanite and calcium carbonate is segregated from the groundmass. Its geochemical signature is similar to those of transitional kimberlites of Kaapvaal Craton, South Africa, and the U Pb ages of ~ 128 Ma on perovskite reveal that Rosario-6 kimberlite post-dates the main pulse of volcanism in the Parana-Etendeka Large Igneous Province (LIP). The high Ti content of some minerals, such as Mg-chromite, Mg-ulvospinel, phlogopite and melilite, and the presence of perovskite suggest a Ti-rich source. The petrographic, geochemical and isotopic data indicate that the Rosario-6 kimberlite source is a depleted mantle metasomatized by H2O-rich fluids, CO2-rich and silicate melts derived from the recycling of an ancient subducted oceanic plate (eclogite) before the South Atlantic opening. Although several authors indicate the influence of Tristan da Cunha plume for the generation of alkaline magmatism associated to the Parana-Etendeka flood basalts, our data demonstrates that Tristan da Cunha plume has no chemical contribution to the generation of Rosario-6 kimberlite, except by its thermal influence.

Highlights • At least parts of the Beata Ridge formed during the main CLIP stage at 95–83 Ma. • Sampling of numerous volcanic rocks indicates a broad extrusive magmatic event. • Depleted and enriched geochemical signatures point to a heterogeneous mantle source. • Geochemical heterogeneities can occur on a small scale of tens of kilometers. Abstract The Caribbean Large Igneous Province (CLIP), a Cretaceous oceanic flood basalt province, presumably formed at the initiation of the Galapagos hotspot. During the M81 cruise of the German R/V METEOR, we sampled the Beata Ridge, a prominent submarine structure in the Caribbean Sea belonging to the CLIP. The ridge offers the opportunity to directly sample basement sequences of the central, submarine part of the CLIP, complementing numerous studies of accreted CLIP sequences exposed on land around the margins of this LIP. The majority of the recovered Beata Ridge samples are volcanic, implying that at least parts of the Beata Ridge were formed during a large extrusive event in contrast to previous assumptions that the structure is primarily composed of intrusive rocks. Several stratigraphically controlled profiles were sampled along the western slope of the Beata Ridge using the remotely operated vehicle (ROV) Kiel 6000 and revealed variously alternating sequences of magmatic rocks (lavas, pillow breccias, tuffs and gabbros) and sediment plains. We report new 40Ar/39Ar age and geochemical (major and trace element, Sr-Nd-Hf-Pb isotope) data for the recovered magmatic samples. Although the 40Ar/39Ar analyses display disturbed age spectra, they suggest an age range of 92.4–76.9 Ma. Thus our age data show for the first time that the Beata Ridge also formed during the main magmatic stage of the CLIP (~95–83 Ma). Previous studies suggested that the Beata Ridge was formed during a second, lower-volume magmatic phase of the CLIP (~81–71 Ma), possibly related to decompression melting during an extensional phase in the Caribbean. Most samples display relatively flat chondrite-normalized rare earth element (REE) patterns commonly observed throughout the CLIP, but light REE enriched and depleted compositions are also present. The occurrence of enriched and depleted incompatible element and radiogenic isotope signatures implies a heterogeneous mantle source region, as is observed for other LIPs worldwide. Since a high degree of geochemical variability is observed over short stratigraphic intervals within the ROV profiles, melt homogenization did not operate as effectively as commonly assumed for LIPs. Instead the plume head probably preserved some domains of enriched and depleted components, whereas most of the melts during the main stage have intermediate compositions (with flat REE patterns), representing mixtures of the enriched and depleted components.

Magmatism of the Povungnituk Group of the Cape Smith Belt, northern Superior craton, was formed in three stages: (i)early alkaline magmatism and associated carbonatites (undated), (ii) a main flood basalt sequence (Beauparlant Formation) (constrained between 2040 and 1991 Ma), and (iii) a late stage alkaline pulse (Cecilia Formation) (ca. 1959 Ma). We suggest that the main stage of magmatic activity (middle pulse) was of short duration. A new U Pb baddeleyite age of 1998 ± 6 Ma is obtained from a dolerite sill intruding the uppermost section of the Beauparlant Formation. This age has regional significance because it matches the previously obtained 1998 ± 2 Ma age for the Watts Group (Purtuniq) ophiolite of the northern Cape Smith Belt and the 1998 ± 2 Ma U-Pb age of the Minto dykes intruding the craton to the south. These coeval units, along with additional units correlated on paleomagnetic grounds (Eskimo Formation), are interpreted to define a large igneous province (LIP), extending over an area of >400,000 km2, which we herein define as the Minto-Povungnituk LIP. Geochemical comparison between the Watts Group ophiolite, Minto dykes and the mafic Povungnituk Group shows significant differences allowing these data to be divided into two groups and domains within the LIP. A northern domain, comprising the Povungnituk and Watts groups, shows mixing between a depleted mantle source and a more enriched mantle plume-sourced melt. A southern domain comprising the Minto dykes and the paleomagnetically linked Eskimo Formation shows signs of an even more enriched source, while these magmas also show the effect of crustal contamination. Two distinct source mechanisms can be responsible for the observed geochemical differences between the two domains. First, a difference in lithospheric sources, where melting of different portions of Superior craton lithosphere caused the different melt signatures in the interior of the craton. In this case magmatism in the two domains is only related by having the same heat source (e.g.,a mantle plume) interpreted to be located on the northwestern side of the northern Superior craton. Second, two distinct deep mantle sources that remained separated within the ascending plume. This is analogous to some current hotspots interpreted to sample both large low shear velocity provinces (LLSVP) and adjacent ambient deep mantle. This latter interpretation would allow for the use of bilateral chemistry in LIPs as a potential tool for the recognition and mapping of the LLSVP boundaries throughout Earth's history.

© 2017 Elsevier B.V. Phase equilibrium experiments have been performed on an extremely high-Ti (5.4 wt.% TiO2) picrite from the base of the Paleogene (~55 Ma) East Greenland Flood Basalt Province. This sample has a high CaO/Al2O3 ratio (1.14), a steep rare-earth elements (REE) profile, is enriched in incompatible trace elements, and is in chemical equilibrium with highly primitive olivine. This all suggests that the picrite is a near-primary melt that did not suffer major chemical evolution during ascent from the mantle source and through the crust. Near-liquidus phase relations were determined over the pressure range of 1 atm, 1 to 1.5 GPa and at temperatures from 1094 to 1400°C. They provide an important constraint on the petrogenesis of these lavas. The high-Ti picritic melt is multi-saturated with olivine (Ol) + orthopyroxene (Opx) at 1 GPa but has only Ol or Opx on the liquidus at lower and higher pressures, respectively. This indicates the primitive melt was last equilibrated with its mantle source at relatively shallow pressure (~ 1 GPa). Melting probably started at 2-3 GPa and the picritic melt was produced by 15-30% melting of the mantle source. Such a degree of partial melting requires a mantle with a high potential temperature (1480-1530˚C). The relatively low CaO content and high FeO/MnO ratios of the most primitive East Greenland picrites, the high Ni content of olivine phenocrysts and the presence of low-Ca pyroxene (i.e., pigeonite) at high pressure in our experiments all suggest that the mantle source contained a major component of garnet pyroxenite. Residual garnet in the source could adequately explain the low Al2O3 content (7.92 wt.%) and steep REE patterns of the picrite sample. However, simple melting of a lherzolitic source, even with a major pyroxenite component, cannot explain the formation of magmas with the very high Ti contents observed in some East Greenland basalts. We therefore propose that magmas highly-enriched in Ti were produced by melting of a metasomatized mantle source containing Ti-enriched amphibole and/or phlogopite. ispartof: LITHOS vol:300 pages:20-32 status: published

The Late-Permian Emeishan mantle plume event caused large-scale flood basalts and related mafic-ultramafic layered mafic intrusions in the inner zone of the Emeishan large igneous province (ELIP). However, the related crustal magma response of this mantle plume event in the inner zone of the ELIP remains poorly understood. In this contribution, we report contemporaneous (~259 Ma) ferrosyenite and A-type granite in the inner zone of the ELIP, and their chronological and geochemical data suggest simultaneous crustal melting at different depths. The ferrosyenites display moderate SiO2 (60.14 to 63.58 wt%), high Fe2O3T (4.28 to 8.52 wt%) and alkaline (Na2O + K2O = 10.91 to 12.75 wt%) contents. In combination with their positive zircon eHf(t) values (+1.5 to +12.9), we propose that the ferrosyenites were formed by high temperature melting of Fe-rich refractory juvenile lower crust at H2O-poor condition, which was induced by underplating of the high-temperature mantle plume. Rhyolite–MELTS modeling results show that ~7% melting of ferrodiorite-like source at ~1120 °C and 8 kbar can produce similar compositions with least evolved ferrosyenite under a relatively dry condition. The Baima granites have high SiO2 (70.58 to 74.26 wt%) contents and 10000*Ga/Al ratios (3.00 to 3.28), indicating the affinity to A-type granites. Their evolved zircon eHf(t) values (−8.1 to −0.6) indicate an evolved crustal source region. Rcrust modeling indicates that ~9% partial melting of ancient granodiorite-like crustal material under low pressure condition (4 kbar) can produce the observed granitic compositions. Considering the close spatial and temporal relationship between ferrosyenites and A-type granites, we propose that the emplacement of ferrosyenitic magma resulted in low-degree partial melting at a relatively shallow crustal level, responsible for the formation of the A-type granites. In summary, this study highlights the significant impact on crustal magma response to the high-temperature mantle plume event in the inner zone of ELIP.

Two typical lamproitic dykes were found in Noril'sk region of the north-western Siberian Craton, which according to mineralogical, geochemical and isotopic criteria belong to anorogenic, non-diamondiferous type of lamproites. According to the geologic relationships, they cut through the Noril'sk-1 intrusion of the Siberian flood basalt province and thus are younger than ~ 251 Ma. 40Ar/39Ar dating of the two dykes yielded ages of 235.24 ± 0.19 Ma and 233.96 ± 0.19 Ma, showing that they were emplaced in Carnian of the Late Triassic, about 16 Ma after the flood basalt event. There are some indications that there were multiple lamproitic dyke emplacements, including probably emplacement of diamondiferous lamproites, which produced Carnian-age diamond-rich placer deposits in other parts of the Siberian Craton and in adjacent regions. Lead isotope modelling shows that the source of the studied lamproites was formed with participation of recycled crust, which underwent modification of its U/Pb ratio as early as 2.5 Ga. However, the exact mechanism of the recycling cannot be deciphered now. It could be either through delamination of the cratonic crust or subduction of a mix of ancient terrigenous sediments into the mantle transition zone.

Thermal events and metasomatic processes have influenced the Kaapvaal craton in South Africa. High-density fluids (HDFs) trapped as microinclusions in diamond are main metasomatic agents which provide an insight to these processes in the Earth's mantle. Here we present data for 15 fibrous, HDF-bearing diamonds from the Voorspoed mine, South Africa, reflecting multiple diamond-forming events in a cooling lithosphere. Analyzed by FTIR and EPMA, the Voorspoed fibrous diamonds reveal three populations that differ in their nitrogen aggregation and HDF composition. A silicic‑carbonatitic group containing 11–30% B-centers, a saline group containing 5–16% B-centers, and a single high-Mg carbonatitic diamond with 0% B-centers. The distinct nitrogen aggregation of the fibrous diamond groups in Voorspoed and the lack of clear major element evolutionary trends for each HDF type or intermediate compositions between the different types suggest different time-temperature formation histories. Thermobarometry of mineral inclusions in non-fibrous monocrystalline Voorspoed diamonds (Viljoen et al., 2018) indicates that the Voorspoed lithosphere cooled by 100–200 °C since their host diamonds crystallized at high initial temperatures. High temperatures in Voorspoed lithosphere can be correlated with the eruption of the Ventersdorp flood basalts at the central Kaapvaal (ca. 2.7 Ga) or the Bushveld complex (ca. 2.06 Ga), and cooling rates of the lithosphere provide a time frame for a cooling process that originated ~2–3 Ga. Combining these data with the nitrogen aggregation systematics of fibrous and monocrystalline Voorspoed diamonds, we suggest that most Voorspoed diamonds formed during 4 metasomatic events: the oldest one recorded took place between 2 and 3 Gyr as a result of a major thermal perturbation, whereas the following three occurred between 200 and 600 Myr, 30–90 and

Silicic rocks occur in the uppermost units of the longest volcanic succession (~ 5000 m thick) in the Binchuan area of the Permian Emeishan flood basalt province of SW China. They are predominantly rhyolites and to a lesser extent trachytes, both containing potassium feldspar megacrysts as the dominant phenocryst phase up to approximately 20 mm in size. These megacrysts contain domains of albite arranged in vein-like networks, likely formed by post-magmatic alteration. Crystal size distributions (CSD) suggest that these megacrysts grew in a stable magmatic system, consistent with relatively uniform core-to-rim compositional (K2O: ~ 14–16 wt%) and isotopic profiles 208Pb/206Pb ratios (~ 2.06–2.08 ± 0.005). Both whole-rock trace elements and Pb isotope ratios of these silicic rocks are similar to the Emeishan basalts, suggesting a common source for both mafic and felsic units and a limited role of crustal melting in genesis of the felsic units. Major and trace element models further indicate that these rocks could not have formed exclusively by re-melting of old crust or solidified basaltic rock, but must have formed through crystal fractionation from the flood basalts or possibly partial melting of basaltic rock followed by fractional crystallization. K-feldspar-bearing rhyolites are also observed in the last stages of other large igneous provinces. We suggest that they represent final melt fractions and their appearance in the magmatic system coincides with waning of voluminous magmatic activities.

SHRIMP zircon Pb/U dating of Cenozoic volcanic rocks in the Tengchong area, western Yunnan Province, China, shows that the dacite and andesitic breccia lavas from Qushi village were intruded at 480 ± 10 ka and 800 ± 40 ka, respectively. Moreover, Pb/U dating of trachyandesite from Tuantian village and olivine basalt from Wuhe village give weighted mean 206 Pb/ 238 U ages of 2.82 ± 0.08 Ma and 12.28 ± 0.30 Ma. Corrections for initial 230 Th disequilibrium of zircon were used for the former two younger ages. The Tengchong volcanic rocks have a large range of SiO 2 (48.6–66.9 wt.%) and mostly belong to a high-K calc-alkaline series. The lavas originated from heterogeneous sources and were modified by subsequent fractional crystallization. The REE and other trace element patterns of the Tengchong volcanic rocks resemble magmas having a large component of continental crust. All have similar degrees of LREE and HREE fractionation and are enriched in LILE, La, Ce and Pb, with depletions in Nb, Ta, Ti, Sr and P relative to primitive mantle. Zircon δ 18 O values of 6.96 ± 0.17 and 7.01 ± 0.24‰ and highly varied negative e Hf (t) values of − 1.5 to − 11.0 and − 10.3 to − 13.7, as well as the presence of inherited zircon grains in the studied samples, indicate that the magmas contain crustal material on a large scale. The Tengchong volcanic rocks have HFSE ratios (e.g., Nb/Ta, La/Nb, Zr/Y) similar to continental flood basalts, indicative of an intra-plate extensional tectonic setting. Widespread distributed faults might have facilitated upwelling of mantle-derived melts and eruptions from shallow crustal magma chambers to form the large volcanic field.

Cenozoic magmatism in East Africa results from the interplay between lithospheric extension and material upwelling from the African Large Low Shear Velocity Province (LLSVP). The modern focusing of East African magmatism into oceanic spreading centers and continental rifts highlights the modern control of lithospheric thinning in magma generation processes, however the widespread, and volumetrically significant flood basalt events of the Eocene to Early Miocene suggest a significant role for material upwelling from the African LLSVP. The slow relative motion of the African plate during the Cenozoic has resulted in significant spatial overlap in lavas derived from different magmatic events. This complexity is being resolved with enhanced geochronological precision and a focus on the geochemical characteristics of the volcanic products. It is now apparent that there are three distinct pulses of basaltic volcanism, followed by either bimodal lavas or silicic volcanic products during this period: (A) Eocene Initial Phase from 45 to 34 Ma. This is a period of dominantly basaltic volcanism focused in Southern Ethiopia and Northern Kenya (Turkana). (B) Oligocene Traps phase from ~ 33.9 to 27 Ma. This period coincides with a significant increase in the aerial extent of volcanism with broadly age equivalent 1 to 2 km thick sequences of dominantly basalt centered on the NW Ethiopian Plateau and Yemen, (C) Early Miocene resurgence phase from ~ 26.9 to 22 Ma. This resurgence in basaltic volcanism is seen throughout the region at ca. 24–23 Ma, but is less volumetrically significant than the prior two basaltic pulses. With our developing understanding of the persistence of LLSVP anomalies within the mantle, I propose that the three basaltic pulses are ostensibly manifestations of the same plume–lithosphere interaction, requiring revision to the duration, magmatic extent, and magma volume of the African–Arabian Large Igneous Province.

The Faroe Islands Basalt Group (FIBG) comprises a gross stratigraphic thickness of over 6.5 km of dominantly extrusive basaltic facies erupted during the Late Palaeocene to Early Eocene. In this study we present 140 major and trace element analyses from flow by flow field and borehole sample profiles, through the Enni Formation, which comprises the final phase of volcanism preserved on the Faroe Islands. The sample profiles target geographically spaced and overlapping stratigraphic sequences tied relative to a 3D ArcGIS surface for the regionally extensive volcaniclastic Argir Beds marker unit. From these profiles five geochemical groups including one low TiO 2 (Low-Ti 2 (High-Ti > 1.5 wt%) groups differentiated by Nb, Zr, Y and V variations are identified in conjunction with previous studies. The spatial and stratigraphic distribution of these groups is mapped across the islands and demonstrates a complex inter-digitated flow field evolution. Within the finer scale variations, broad spatial and temporal development trends are identified demonstrating the potential for correlation within the volcanic succession at the local, tens of kilometers scale. Low-Ti lavas formed in association with lithospheric thinning and developed extensive flow fields between the Faroe Islands and East Greenland contemporaneous to the eruption of High-Ti smaller melt fraction lava flows in both locations. The progression of High-Ti lava groups preserved on either side of the developing rift zone is very similar, but is not, however, chronostratigraphic due to multiple inter-digitations of the chemical types. We tentatively suggest that a previously proposed rift-oblique transfer zone between the Faroe Islands and East Greenland enabled non-uniform lithospheric thinning and the preservation of a near-continuous High-Ti melting region between these areas beyond the onset of Low-Ti eruptions which were initially fed from the west. This study highlights the complex nature of late stage flood basalt plumbing systems and eruption dynamics in a rift proximal setting.

Isotopic compositions of continental flood basalts are essential to understand their genesis and to constrain the character of their mantle sources. Because of potential crustal contamination, it needs to be evaluated if and to which degree these basalts record original isotopic signals of their mantle sources and/or crustal signatures. This study examines the Sr, Nd, Hf, and Pb isotopic compositions of the late Cenozoic Xinchang–Shengzhou (XS) flood basalts, a small-scale continental flood basalt field in eastern China. The basalts show positive correlations between 87 Sr/ 86 Sr and 143 Nd/ 144 Nd, and negative correlations between 143 Nd/ 144 Nd and 176 Hf/ 177 Hf, which deviate from compositional arrays of crustal contamination and instead highlight variations in magmatic recharge intensity and mantle source compositions. The lava samples formed by high-volume magmatic recharge recorded signals of recycled sediments in the mantle source, which are characterized by moderate Ba/Th (91.9–106.5), excess 208 Pb/ 204 Pb relative to 206 Pb/ 204 Pb, and excess 176 Hf/ 177 Hf relative to 143 Nd/ 144 Nd. Thus, we propose that magmatic recharge buffers the original isotopic compositions of magmas against crustal contamination. Identifying and utilizing the isotope systematics of continental flood basalts generated by high volumes of magmatic recharge are thus crucial to trace their mantle sources.

The Sarnu-Dandali alkaline complex in Rajasthan, northwestern India, is considered to represent early, pre-flood basalt magmatism in the Deccan Traps province, based on a single 40 Ar/ 39 Ar age of 68.57 Ma. Rhyolites found in the complex are considered to be 750 Ma Malani basement. Our new 40 Ar/ 39 Ar ages of 88.9–86.8 Ma (for syenites, nephelinite, phonolite and rhyolite) and 66.3 ± 0.4 Ma (2σ, melanephelinite) provide clear evidence that whereas the complex has Deccan-age (66 Ma) components, it is dominantly an older (by ~ 20 million years) alkaline complex, with rhyolites included. Basalt is also known to underlie the Early Cretaceous Sarnu Sandstone. Sarnu-Dandali is thus a periodically rejuvenated alkaline igneous centre, active twice in the Late Cretaceous and also earlier. Many such centres with recurrent continental alkaline magmatism (sometimes over hundreds of millions of years) are known worldwide. The 88.9–86.8 Ma 40 Ar/ 39 Ar ages for Sarnu-Dandali rocks fully overlap with those for the Indo-Madagascar flood basalt province formed during continental breakup between India (plus Seychelles) and Madagascar. Recent 40 Ar/ 39 Ar work on the Mundwara alkaline complex in Rajasthan, 120 km southeast of Sarnu-Dandali, has also shown polychronous emplacement (over ≥ 45 million years), and 84–80 Ma ages obtained from Mundwara also arguably represent post-breakup stages of the Indo-Madagascar flood basalt volcanism. Remnants of the Indo-Madagascar province are known from several localities in southern India but hitherto unknown from northwestern India 2000 km away. Additional equivalents buried under the vast Deccan Traps are highly likely.

We present new trace element compositions of amphiboles, Sr–Nd–Hf isotope compositions of clinopyroxenes and mineral modes for spinel peridotite xenoliths that were entrained in a Miocene alkali basalt (Gundeweyn, northwestern Ethiopian plateau), in order to understand the geochemical evolution and variation occurring within the continental lithospheric mantle (CLM) in close proximity to the East African Rift system, and its dynamic implications. With the exception of a single amphibole-bearing sample that is depleted in LREE (La/Yb N = 0.45 × Cl), amphiboles in lherzolites and in one harzburgite show variable degrees of LREE enrichment (La/Yb N = 2.5–12.1 × Cl) with flat HREE (Dy/Yb N = 1.5–2.1 × Cl). Lherzolitic clinoyroxenes have 87 Sr/ 86 Sr (0.70227 to 0.70357), 143 Nd/ 144 Nd (0.51285 to 0.51346), and 176 Hf/ 177 Hf (0.28297 to 0.28360) ranging between depleted lithosphere and enriched mantle. LREE-enriched clinopyroxenes generally have more enriched isotope compositions than depleted ones. While lherzolites with isotope compositions similar to those of the Afar plume result from the most recent metasomatic overprint, isotope compositions more depleted than present-day MORB can be explained by an older melt extraction and/or isotopic rehomogenisation event, possibly related to the Pan-African orogeny. Several generations of amphibole are recognized in accord with this multi-stage evolution. Texturally unequilibrated amphibole occurring within the peridotite matrix and in melt pockets attest to continued hydration and refertilization of the lithospheric mantle subsequent to Oligocene flood basalt magmatism, during which an earlier-emplaced inventory of amphibole was likely largely consumed. However, a single harzburgite contains amphibole with the highest Mg# and lowest TiO 2 content, which is interpreted as sampling a volumetrically subordinate mantle region beneath the Ethiopian plateau that was not tapped during flood basalt magmatism. Strikingly, both trace-element enriched and depleted lherzolites have high clinopyroxene and orthopyroxene and low olivine contents (median 15, 24 and 56 vol.%), combined with primitive olivine Mg# (median 89.5), indicating the presence of refertilized mantle beneath Gundeweyn. Despite its fertility and FeO-rich character (hence high inferred density), and impingement by the Afar plume, the CLM beneath the Ethiopian plateau, though apparently thinned through thermochemical erosion, has so far resisted whole-sale delamination or dripping. This is tentatively ascribed to insufficient stress and density contrasts at the periphery of the Afar plume, which reached its greatest thermochemical buoyancy in the Afar region, northeast of Gundeweyn.

LIP printing is a term adapted from forensic science to describe the use of geochemical proxies for tectonic and petrogenetic fingerprinting of Large Igneous Provinces (LIPs). Here, we investigate in detail the LIP printing of basic lavas, sills and dykes using two immobile element proxies: Th/Nb, a crustal input proxy, to monitor subduction-metasomatism and crustal assimilation and Ti/Yb, a residual garnet proxy, to monitor depth and degree of melting. The LIP printing diagram, a plot of Th/Nb against Ti/Yb for intraplate, plume-derived magmas, is characterised by two distinct arrays: a subduction-modified lithospheric mantle (SZLM) array and a MORB-OIB-OPB (plume) array (where OPB = oceanic plateau basalt). LIP basalt suites divide into three categories on this diagram: Type I plots entirely within the MORB-OIB-OPB array indicative of a significant plume source; Type II plots entirely within the SZLM array indicative of a significant sub-continental lithospheric mantle source, and Type III plots on a variety of trends between the two arrays indicative of significant plume-lithosphere interactions. Modelling demonstrates how the three LIP types, and the observed trends within and between individual LIPs, can be explained by differences in the compositions and relative contributions of lithospheric and asthenospheric (plume) mantle, in temperature and depth of melting and in the extent and nature of magma-crust interactions. This large genetic, and hence compositional, variability within and between LIPs relates to differences in geological and geodynamic setting and supports the forensic concept that ‘no two LIP prints are alike’. The potential applications of the LIP printing diagram are demonstrated here using four types of examples that highlight temporal and spatial LIP print diversity: flood basalt terranes related to Atlantic breakup (NAIP, Parana-Etendeka and CAMP); giant dyke swarms (Superior Craton Late Archean to Early Proterozoic dyke swarms and the Mackenzie dyke swarm); mineralization-related LIP terranes (Bushveld and Noril’sk); and early (c. 3.5 Ga) Earth and extraterrestrial lavas (terrrestrial Paleoarchean basalts and komatiites, lunar mare basalts and Martian shergottites).

The link of the widespread Late Permian–Triassic magmatism in the southwestern corner of the South China block to a subduction zone or the Emeishan mantle plume is debated. We have carried out an integrated geochronological-petrological-geochemical study of four mafic dikes in the Gulinjing–Nanxi area of this region to shed new light on the issue. Zircon grains from the Gulinjing–Nanxi mafic dikes yield U Pb ages from 254 ± 2 to 250 ± 2 Ma, indicating that they are coeval with abundant felsic intrusive-extrusive rocks in the region, but are 6–7 myr younger than the nearby Emeishan flood basalts and associated mafic-ultramafic intrusions. The Gulinjing–Nanxi mafic dikes are characterized by light rare-earth element enrichments and pronounced negative Nb Ta anomalies, similar to typical arc basalts worldwide, plus enriched Sr-Nd-Hf isotope compositions (initial 87Sr/86Sr from 0.7088 to 0.7103, eNd(t) from −6.6 to −4.6, and zircon eHf(t) from −3.7 to −0.3). The isotopes and incompatible trace element ratios indicate that the parental magmas for the Gulinjing–Nanxi mafic dikes are consistent with contaminated arc mafic magmas, but not contaminated Emeishan flood basalts in the region. This, together with a significant age difference between the mafic dikes and the Emeishan flood basalts and associated mafic-ultramafic intrusions in the nearby areas, support the interpretation that the Gulinjing–Nanxi mafic dikes are unlikely to be parts of the Emeishan mantle plume. Based on regional igneous association and the distribution of known paleo subduction zones in the surrounding regions, we conclude that the Gulinjing–Nanxi mafic dikes are linked to Paleo-Pacific subduction to the south, and not Paleo-Tethys subduction to the west.

Continental basalts generally display enrichment of fluid-mobile elements and depletion of high-field-strength elements, similar to those that evolved in the subduction environment, but different from oceanic basalts. Based on the continental flood basalt database for six large igneous provinces, together with rift-related basalt data from the Basin and Range Province, this study aimed to test the validity of geochemical tectonic discrimination diagrams in distinguishing arc-like intra-continental basalts from arc basalts and to further investigate the role of deep-Earth water cycling in producing arc-like signatures in large-scale intra-continental basalts. Our evaluation shows that arc-like intra-continental basalts can be distinguished from arc basalts by integrating the following factors: (1) the FeO, MgO, and Al 2 O 3 concentrations of the primary melt; (2) Ti V, Zr Zr/Y, Zr Ti, and Ti/V Zr/Sm Sr/Nd discrimination diagrams; (3) the coexistence of arc-like and OIB-like subtype basalts within the same province; (4) primitive mantle-normalized trace element distribution patterns. The similarity of enrichment in fluid-mobile elements (Ba, Rb, Sr, U, and K) between arc-like and true arc basalts suggests the importance of water flux melting in producing arc-like signatures in continental basalts. Experimentally determined liquid lines of descent (LLD) imply high magma water concentrations for continental flood basalts (CFBs) and the Basin and Range basalts. Furthermore, estimates based on the Al 2 O 3 –LLD method indicates 4.0–5.0 wt% pre-eruptive magma H 2 O concentration for CFBs and the Basin and Range basalts. The tight relationships between H 2 O/Ce and Ba/La, Ba/Nb and Rb/Nb based on global arc basalt data were further used to estimate the primary H 2 O concentrations. With the exception of the Emeishan CFBs (mainly containing 4.0–5.6 wt% H 2 O), all other CFBs investigated have similar estimated primary H 2 O contents, with values ranging from 1.0 to 2.0 wt%. The estimated primary H 2 O content of the Basin and Range basalts is extremely high and up to 10.0 wt%. Thus, this study demonstrates that water flux melting played an important role in the generation of many intra-continental igneous provinces. This new finding was further employed to investigate the tectonic setting of 320–270 Ma basalts in Inner Mongolia, North China. Most basalts from three key rock units (i.e. Amushan, Benbatu, and Dashizhai formations) from the Central Asian Orogenic belt are classified as non-arc types. The estimated magma H 2 O concentrations suggest a strong link between H 2 O content and arc-like geochemical signatures. Together with established geological evidence, we proposed that these 320–270 Ma basaltic rocks were most likely produced in a post-orogenic extensional environment facilitated by subducted slab-driven deep-Earth fluid cycling. We propose a mantle transition zone water-filtering model that links deep-Earth fluid cycling, large-scale intra-continental basaltic magmatism, and supercontinent cycles into a self-organized system.

The Southern Oklahoma Aulocogen (SOA) stretches from southern Oklahoma through the Texas panhandle and into Colorado and New Mexico, and contains mafic through silicic magmatism related to the opening of the Iapetus Ocean during the early Cambrian. Cambrian magmatic products are best exposed in the Wichita Mountains (Oklahoma), where they have been extensively studied. However, their ultimate derivation is still somewhat contentious and centers on two very different models: SOA magmatism has been suggested to occur via [1] continental rifting (with or without mantle plume emplacement) or [2] transform-fault related magmatism (e.g., leaky strike–slip faults). Within the SOA, the subsurface in and adjacent to the Arbuckle Mountains in southern Oklahoma contains thick sequences of mafic to intermediate lavas, intrusive bodies, and phreatomagmatic deposits interlayered with thick, extensive rhyolite lavas, thin localized tuffs, and lesser silicic intrusive bodies. These materials were first described in the Arbuckle Mountains region by a 1982 drill test (Hamilton Brothers Turner Falls well) and the best available age constraints from SOA Arbuckle Mountains eruptive products are ~ 535 to 540 Ma. Well cuttings of the mafic through intermediate units were collected from that well and six others and samples from all but the Turner Falls and Morton wells are the focus of this study. Samples analyzed from the wells are dominantly subalkaline, tholeiitic, and range from basalt to andesite. Their overall bulk major and trace element chemistry, normative mineralogy, and Sr Nd isotope ratios are similar to magmas erupted/emplaced in flood basalt provinces. When compared with intrusive mafic rocks that crop out in the Wichita Mountains, the SOA well cuttings are geochemically most similar to the Roosevelt Gabbros. New geochemical and isotope data presented in this study, when coupled with recent geophysical work in the SOA and the coeval relationship with rhyolites, indicates that the ~ 250,000 km3 of early Cambrian mafic to silicic igneous rocks in the SOA were emplaced in a rifting event. This event is suggested to result from the break-up of Pannotia and the formation of the failed arm of a three-armed radial rift system.

New analyses of siderophile–lithophile elements and Sr–Nd isotopes in the Permian basalts and picrites from northern Vietnam, the southernmost occurrence of the Emeishan flood basalt province, together with previously published data, are used to address the question of whether any meaningful correlation between these elements and isotopes exists at a province scale. The available data show that negative correlations between e Nd, ( 87 Sr/ 86 Sr) i and mantle-normalized (Nb/Th) n are present in the basalts but not in the associated picrites. This indicates that crustal contamination is negligible in the picrites but significant in some of the basalts. The picrites and basalts from the entire province show negative correlations between (Rh/Ru) n , (Pt/Ru) n , (Pd/Ru) n and Mg-number. This indicates that Ru behaves compatibly whereas Rh, Pt and Pd behave incompatibly during magma differentiation. The incompatible behavior of Rh in natural basaltic systems is also supported by the fact that (Pt/Rh) n remains constant with decreasing Mg-number in the lavas. Depletions of Pd and Pt, and to a lesser degree Cu, in some basaltic samples characterized by relatively low e Nd and (Nb/Th) n support the notion that sulfide saturation in the magmas was triggered by a combination of siliceous crustal contamination and addition of external sulfur. Within the entire flood basalt province only the picrites from Song Da, northern Vietnam show clear depletion in Ir relative to Ru. These picrites are also characterized higher Al 2 O 3 /TiO 2 and lower mantle-normalized La/Yb (0.2–2.4) than those from elsewhere in the province, possibly due to the involvement of an Ir-depleted, fertile mantle component in magma generation at this location.

Extensive volcanic fields on the western Arabian Plate have erupted intermittently over the last 30 Ma following emplacement of the Afar flood basalts in Ethiopia. In an effort to better understand the origin of this volcanism in western Saudi Arabia, we analyzed 3 He/ 4 He, and He, CO 2 and trace element concentrations in minerals separated from xenoliths and lava flows from Harrat Hutaymah, supplemented with reconnaissance He isotope data from several other volcanic fields (Harrat Al Birk, Harrat Al Kishb and Harrat Ithnayn). Harrat Hutaymah is young ( 3 He/ 4 He trapped within most xenoliths, with a weighted mean of 7.54 ± 0.03 R A (2σ, n = 20). This homogeneity occurs over at least eight different xenolith types (including spinel lherzolite, amphibole clinopyroxenite, olivine websterite, clinopyroxenite and garnet websterite), and encompasses ten different volcanic centers within an area of ~ 2500 km 2 . The homogeneity is caused by volatile equilibration between the xenoliths and fluids derived from their host magma, as fluid inclusions are annealed during the infiltration of vapor-saturated magmas along crystalline grain boundaries. The notable exceptions are the anhydrous spinel lherzolites, which have a lower weighted mean 3 He/ 4 He of 6.8 ± 0.3 R A (2σ, n = 2), contain lower concentrations of trapped He, and have a distinctly depleted light rare earth element signature. 3 He/ 4 He values of ~ 6.8 R A are also commonly found in spinel lherzolites from harrats Ithnayn, Al Birk, and from Zabargad Island in the Red Sea. Olivine from non-xenolith-bearing lava flows at Hutaymah spans the He isotope range of the xenoliths. The lower 3 He/ 4 He in the anhydrous spinel lherzolites appears to be tied to remnant Proterozoic lithosphere prior to metasomatic fluid overprinting. Elevated 3 He/ 4 He in the western harrats has been observed only at Rahat (up to 11.8 R A ; Murcia et al., 2013), a volcanic field situated above thinned lithosphere beneath the Makkah-Medinah-Nafud volcanic lineament. Previous work established that spinel lherzolites at Hutaymah are sourced near the lithosphere-asthenosphere boundary (LAB), while other xenolith types there are derived from shallower depths within the lithosphere itself (Thornber, 1992). Helium isotopes are consistent with melts originating near the LAB beneath many of the Arabian harrats, and any magma derived from the Afar mantle plume currently appears to be of minor importance.

The Deccan basalt erupted at the K-T boundary and comprises multiple flow units with spatial variations in physiography and geochemistry. The Koyna borehole-7 (KBH-7 of MoES-BGRL) of the Koyna Seismic Zone is situated in the western part of the Deccan continental flood basalt province, where the lava pile is 1251 m thick. Thirty-one samples of four flow units (named FA, FB, FC, FD; 1248.20–1110.78 m) from the lower part and another four flow units (named FV, FW, FX, FY; 202.79–42.85 m) from the upper part of the KBH-7 borehole show significant petrographic, mineral chemical and bulk-rock geochemical variations. Individual flows are characterized by higher abundances of plagioclase and clinopyroxene phenocrysts in their lower than upper parts; conversely, their upper parts are relatively enriched in magnetite and ilmenite. In addition, the flows become amygduloidal near their tops. Seven samples from the lower part of the FB flow unit have relatively higher MgO (≈ 8.94–12.9 wt%) and lower TiO2 (≈ 1.43–1.67 wt%) contents compared to samples from other flow units and are identified as low-Ti high-Mg basalt. Clinopyroxene phenocrysts from low-Ti high-Mg basalts are more magnesian (Mg-number ≈ 62.4 to 82.3) and chromium-rich, and not in equilibrium with plagioclase phenocrysts in the rocks. The magnesian-rich clinopyroxene phenocrysts were probably concentrated in the flow by flow differentiation. The trace element geochemistry indicates that the flows in the KBH-7 borehole are crustally contaminated. Extremely low Nb/Ta ( 1) and Ni/MgO (≈ 1.9–163) of the KBH-7 basalts also indicate the absence of an earlier sulfide liquation of the magma. This may be related to the absence of crustal sulfur from the tonalitic basement rocks or the Gondwana sediments (greywacke) in conjunction with high iron content (FeO(T) ≈ 11.9–15.9 wt%) of the basaltic magma.

This geochemical study is based on 69 volcanic rocks representing the entire volcano-stratigraphy of the 12 lava formations of the Deccan Traps (DT) of the Indian peninsula. The DT constitutes one of the largest continental flood basalt province (CFB) in the world. More than 90% of the lavas of this province are considered to have erupted in less than one million years by plume volcanism, coinciding with the Cretaceous-Paleogene (K-Pg) boundary, leading to the speculation that this volcanism had a major role in the end-Cretaceous mass extinction. The current study was undertaken to fully geochemically characterize the DT plume with respect to major-trace element composition as well as their correlated Sr-Nd-Pb and Hf isotope geochemistry of each lava formation in step-by-step sampling the entire Deccan vertical stratigraphy of over 60 samples mostly on the Western Ghat Escarpment as collected by Beane (1988) . The upper lava formations from Poladpur to Panhala-Desur, the Wai subgroup, are sharply distinguishable geochemically by all the geochemical parameters from the lower lava formations of the Lonavala and Kalsubai subgroups. This distinction is remarkable as they are on samples in a continuous supposedly vertical sequence in space and time. The Wai subgroup shows restricted range in FeO/MgO ratios at a given MgO wt% but indicating a more Fe-rich mantle source than many other CFB (average Mg# 46.7 as opposed to the 53.3 Mg# for the lower subgroups) province; the lower subgroups show a widely variable FeO/MgO ratios, interpreted as results of partial melting of subducted eclogitic slab. This interpretation is supported by the trace elements including the rare earths, and the isotopic ratios of Nd, Sr, Pb and Hf that were all measured in the same lava samples. Our high precision Pb isotopic measurements reveal the gist of the Deccan plume characteristics as Pb content in the mantle is low in contrast with their higher abundance in the continental crust. The Wai subgroup demonstrates a relatively consistent initial 206Pb/204Pb ratios as opposed to their heterogeneous compositions in the lower formations of Lonavala and Kalsubai. In addition, the Pb isotopes in a 206Pb/204Pb vs 207Pb/204Pb plot define an age of 2604 ± 140 Ma that we interpret as the “provenance” age of the subducted slab that sourced the lower formation lavas. Interestingly, this “errorchron” is also consistent with the “Dupal anomaly” concept developed on the sub Indian Oceanic mantle derived basalts. The combined multiple isotopic volcano-stratigraphy shows the geochemical contrast more dramatically than any other previous study on the Deccan. This high precision multiple isotopic study of the Deccan is consistent with recent U-Pb and Ar-Ar geochronologic studies on the rapid eruption of the Deccan, mostly the plume head, interpreted here constituting the upper lava formations of the Wai subgroup as without much interaction with the continental crust.

International audience; Mount Girnar is one of the most conspicuous alkaline complexes (gabbro-diorite-syenite-lamprophyre), which intrude some of the earliest erupted basalts (ca. 69 Myr) of the Deccan Large Igneous Province (Deccan LIP) in the Kathiawar plateau of NW India. Petrography, bulk-rock geochemistry, Sr and Nd isotopes and 40Ar/39Ar mineral ages of fractionated (Mg#: 36.3–43.6) lamprophyre dykes (younger intrusives) are reported from two widely separated domains from Mt. Girnar. Petrography and mineral chemistry reveal that major mineral assemblages (pargasite and kaersutite varieties of amphibole, diopside, biotite and feldspar) in the lamprophyres are pristine and devoid of alteration. The lamprophyres belong to the alkaline variety in general and the camptonite - monchiquite series in particular. The bulk-rock major and trace elements of the Girnar lamprophyres display very good correlation with each other and also with those of associated rocks (syenites, diorites and gabbros) which support their genetic relationship. Trace-element ratios do not evidence crustal contamination and reveal derivation of the lamprophyres from partial melting of a lithospheric mantle source significantly modified by interaction with asthenospheric-derived melts, and resembles other alkaline rocks from the Deccan LIP in this regard. Initial 87Sr/86Sr (0.7052–0.7053) and 143Nd/144Nd (0.5125–0.5127) of the Girnar lamprophyres and associated rocks such as syenite, diorite and gabbro are tightly clustered and further attest to their derivation from a cogenetic parental melt. Their positive εNdi values (+0.8 to +3.4) require a mantle source that has experienced moderate long-term depletion of light rare-earth elements. Neoproterozoic/Early Cambrian depleted-mantle (TDM) Nd model ages of ~414–588 Myr are closer to the timing of break-up of the Rodinia supercontinent as well as coincide with the Ediacaran-Cambrian Malagasy orogeny. 40Ar/39Ar dating of three mineral separates (amphibole and biotite) from the lamprophyres gave precise plateau ages of 65.9 ± 0.3 Myr to 66.1 ± 0.4 Myr demonstrating that the emplacement of the Mt. Girnar igneous complex was close to the Cretaceous-Paleogene boundary. The emplacement of a range (110 Myr to 68.5 Myr) of other spatially related alkaline as well as silicic plutonic complexes, such as the Mundwara, Sarnu-Dandali, and Barda complexes prior to the main flood basalt event at ca. 66.0–65.1 Myr, highlights the role of extensional events in pre-existing rift/fault zones preceding the plume-lithosphere interaction in the Deccan LIP.

Made available in DSpace on 2020-12-10T17:33:58Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-07-01 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) The origin of the Parana Continental Flood Basalts (PCFB), which constitute one of largest magmatic provinces in the world, remains a very controversial subject, particularly whether melts are sourced in the mesosphere (deep mantle plume), asthenosphere and/or lithosphere. In an effort to scrutinize those competing hypotheses, new measurements of Os, Nd, Sr and Pb isotopes, along with major and trace elements are presented for low-TiO2 tholeiites from the province. This suite occurs discreetly in the central-north of the province (termed Ribeira magmatype), whose lithospheric structure is characterized by higher P- and S- wave velocities, and high electrical resistivity. The initial Os-187/Os-188 isotopic compositions in Ribeira lavas, ranging from 0.10660 to 0.12575, are more unradiogenic than the other magma-types of the PCFB, as well as the estimates of the contemporary Depleted Mantle and are lower than any osmium isotopic ratio yet reported for continental flood basalts. These remarkably unradiogenic Os-187/Os-188 ratios preclude significant continental crust contamination and require the involvement of an ancient subcontinental lithospheric mantle source that evolved in a very low Re/Os environment. The fact that the Ribeira rocks occur on a peculiar lithospheric geophysical structure and have a unique unradiogenic osmium isotope signature reveals for the first time the existence of Archean lithosphere concealed by the Parana Basin, inserted in the Paranapanema fragmented lithosphere. On the other hand, the other magmatypes of the province, whose osmium isotopic signature is similar to the modern fertile mantle, occur close to more electrically conductive lithospheric structures. Thus, osmium isotopic information integrated with recent crustal and upper mantle geophysical soundings provides crucial information about the PCFB mantle sources. To account for the combined Os, Nd, Sr and Pb isotopic compositions of the Ribeira rocks, we propose that the primary melts formed from the heterogeneous lithospheric mantle (including Archean lithosphere fragments) that was variably hybridized by melts derived from recycling of eclogites related to multiple Neoproterozoic-Cambrian suture zones that surround the Parana Basin and subduction processes. (C) 2020 Elsevier B.V. All rights reserved. Univ Fed Bahia, Inst Fis, Dept Fis Terra & Meio Ambiente, BR-40170115 Salvador, BA, Brazil Univ Sao Paulo, Inst Astron Geofis & Ciencias Atmosfer, Rua Matao 1226, BR-05508090 Sao Paulo, SP, Brazil Univ Sao Paulo, Inst Geociencias, BR-05422970 Sao Paulo, SP, Brazil Univ Fed Parana, Dept Geol, BR-81270460 Curitiba, PR, Brazil Univ Estadual Paulista, Inst Geociencias & Ciencias Exatas, BR-13506900 Rio Claro, SP, Brazil Univ Estadual Paulista, Inst Geociencias & Ciencias Exatas, BR-13506900 Rio Claro, SP, Brazil FAPESP: 2013/02748-2 FAPESP: 2012/060826

A critical review of the available multidisciplinary data on the Parana-Etendeka Province allows for the reconciliation of the controversial aspects of its origin in a coherent tectonomagmatic scenario, in which continental flood basalt (CFB) magmatism evolved from the Parana plateau s.s. (Stage 1) to progressive continental rifting in Etendeka (Stage 2), and then opened to the South Atlantic at the same latitude; the CFB magmatism is triggered by the prolonged impingement of the proto Tristan plume on the western Gondwana lithosphere. The provinciality of the CFB is evidenced by the incompatible element distribution and Sr-Nd-Pb isotopic data, where the Parana and Etendeka magmas are more akin to lithospheric and asthenospheric (plume-related) components, respectively. Stage 1 consisted of the rapid outpouring of the Parana Plateau s.s. CFB (135–134 Ma, ~ 800,000 km3, and eruption rate ~0.8 km3/a) that was zonally arranged with prevailing high-TiO2 (HT) basalts in the central-northern part and low-TiO2 (LT) suites at the southern periphery. The differentiated nature (MgO = 8–4%) of these plateau magmas suggests the variable extent of fractional crystallisation during rise through a relatively thick lithosphere. Petrological modelling, isotopic signatures (87Sr/86Sr 0.70483–0.70620, and eNd(t) from −1.27 to −5.78), and incompatible element distributions approaching the EM1 mantle component suggest that the HT and LT Parana basalts may be derived from mantle sources located in the lower lithosphere at P 3–4 GPa and a potential temperature (Tp) of 1500–1550 °C. The high Tp recorded (and the relative Texcess 250–300 °C thermal anomaly) can be attained after several million years of lithospheric heating, beginning from the first plume impact, which is represented by precursor alkaline events (145–138 Ma) at the westernmost border of the Parana plateau. The subsequent rifting (Stage 2, mostly 134–128 Ma) developed SE of the Parana plateau in relation to the NW drifting of the Gondwana plate over the rising plume and was accompanied by progressive lithospheric arching, thinning, and rifting, which culminated in continental breakup close to the Etendeka border. The concomitant and exclusive appearance, in both the HT and LT suites of this region, of the hottest and deepest (Tp up to ~1590 °C and P up to ~5 GPa) high-MgO sub-lithospheric magmas (87Sr/86Sr 0.70319–0.70533 and eNd(t) from 9.08 to 0.53) is consistent with their generation from the axial zone of the upwelling plume. The basic–acidic bimodal character of CFB magmatism, since the beginning of this stage, must be related to the intensive block faulting of the rifted margins that favoured magma trapping and crystal fractionation to trachydacite and dacite-rhyolite differentiates from the respective HT and LT basalts. Owing to the higher SiO2 and viscosity, the prevailing dacite-rhyolite magmas were more prone to pond in crustal magma chambers, where they experienced assimilation fractional crystallisation (87Sr/86Sr 0.71466–0.72558 and eNd(t) from −5.50 to −9.31) before erupting as extensive rheo-ignimbrites. During the final rifting stage, CFB activity continued until 128 Ma (with sporadic episodes until 122 Ma), and the plume dynamic support gradually vanished beneath the two conjugate South American/South African continental margins, up to the opening of the South Atlantic and hot-spot volcanism of the Rio Grande and Walvis Ridges.

The onset of the Middle Permian Emeishan Large Igneous Province (ELIP), is commonly implicated in the Guadalupian mass extinction, one of the largest Phanerozoic extinction events, yet the contribution of the ELIP to this biotic event remains unresolved because of a lack of appropriate age constraints. In this study, we examined claystone beds overlying the cherty limestone of the Maokou Formation and underlying the flood basalts of the ELIP that were emplaced around the Guadalupian-Lopingian Boundary. Integrated petrographic, mineralogical, elemental, and isotopic features indicate that the studied claystones contain significant components from mafic volcanic ash that was likely associated with the emplacement of the ELIP. Zircons from the claystones yield U Pb ages of ~262.5 Ma, which predates the subaerial eruptive phase of the ELIP (259.1–260.4 Ma), but is comparable to the J. altudaensis conodont biostratigraphic zone (~263 Ma, Middle Capitanian Stage) which is contemporaneous with the emplacement of early-stage Emeishan subaqueous pillow basalts and mafic volcaniclastics. Our data provide a crucial new constraint on the lower age limit for the onset of the ELIP, which was likely associated with violent submarine eruptions and thus could have contributed to the loss of marine fauna during the Guadalupian mass extinction.

A comprehensive study on zircon U-Pb age dating, whole-rock geochemistry and Sr-Nd isotope data has been conducted on the mafic rocks of the Xiazhuang uranium ore district and adjacent regions in South China. Based on field work and petrographic features, three rock types (the Kuzhukeng gabbro, the WNW-trending dolerite dykes and the NNE-trending lamprophyre dykes) are distinguished. Early Jurassic SHRIMP and LA-ICPMS ages of zircon for the Kuzhukeng gabbro (198 ± 1 Ma) and WNW-trending dolerite dykes (193 ± 4 Ma) have been obtained, which are 50 Ma older than previously thought (being Cretaceous). These geochronologic data provide new evidence for the rarely identified Early Jurassic magmatisms in South China. Whole-rock geochemical data for the Kuzhukeng gabbro and WNW-trending dolerite dykes are similar, both of which being higher in FeO and TiO 2 but lower in SiO 2 and K 2 O than the NNE-trending lamprophyre dykes. Trace element characteristics and Sr-Nd isotope data indicate arc-like signatures similar to the Cretaceous southeast coast basalts of China for the lamprophyre dykes, but an OIB-like geochemical affinity for the high-TiO 2 mafic rocks similar to the Permo/Triassic Emeishan flood basalts and the Middle Jurassic Ningyuan alkaline basalts. We propose that the lamprophyre dykes formed in an arc volcanic system driven by the subduction of the paleo-Pacific plate. In contrast, the Kuzhukeng gabbro and associated dolerite dykes record the post-orogenic (Indosinian) extension event in the Tethyan tectonic regime. This further implies that the Indosinian extension may have lasted until the Early Jurassic, and therefore, the subduction of the paleo-Pacific plate in south China was probably later than this period. Most U deposits of the Xiazhuang area are located at the intersection between the WNW-trending dolerite dykes and the NNE-trending faults within the Triassic granites of eastern Guidong complex, South China. Previous metallogenesis studies assumed that intrusion of the WNW-trending mafic dykes induced upward migration of mantle-derived CO 2 -rich fluids, which leached U from the Triassic granite and subsequently precipitated at the intersection between mafic dykes and NEE-trending faults. Our new age data for the WNW-trending dolerite dykes reveal an age gap with respect to the associated U deposits (85–135 Ma) of at least 50 Ma, arguing against this model. Here, we propose that the role played by the dolerite dykes for U mineralizations is more likely to provide a favorable physicochemical environment promoting the precipitation of U from oxidized fluids. The fluids themselves, however, are probably related to the NNE-trending fault system and associated lamprophyre dykes rather than to the dolerites.

We tested the accuracy of trace element discrimination diagrams for basalts using new datasets from two petrological databases, PetDB and GEOROC. Both binary and ternary diagrams using Zr, Ti, V, Y, Th, Hf, Nb, Ta, Sm, and Sc do a poor job of discriminating between basalts generated in various tectonic environments (continental flood basalt, mid-ocean ridge basalt, ocean island basalt, oceanic plateau basalt, back-arc basin basalt, and various types of arc basalt). The overlaps between the different types of basalt are too large for the confident application of such diagrams when used in the absence of geological and petrological constraints. None of the diagrams we tested can clearly discriminate between back-arc basin basalt and mid-ocean ridge basalt, between continental flood basalt and oceanic plateau basalt, and between different types of arc basalt (intra-oceanic, island and continental arcs). Only ocean island basalt and some mid-ocean ridge basalt are generally distinguishable in the diagrams, and even in this case, mantle-normalized trace element patterns offer a better solution for discriminating between the two types of basalt.

The Palisades Sill at Upper Nyack, NY contains evolved rocks that crystallized as ferrodiabase and ferrogranophyre and occupy 50% to 60% of the local thickness. 143 Nd/ 144 Nd isotope values for rocks representing Palisades diversity range between 0.512320 and 0.512331, and indicate a homogeneous source for the Palisades and little or no contamination from shallow crustal sediments. Petrographic analysis of ferrodiabase suggests that strong iron enrichment was the result of prolonged quiescence in cycles of magmatic input. Ferrogranophyres in the updip northern Palisades at Upper Nyack are members of a suite of cogenetic rocks with similar composition to ‘sandwich horizon’ rocks of the southern Palisades at Fort Lee, NJ, but display distinct mineralogical and textural features. Differences in textural and mineralogical features are attributed to a) updip (lateral) migration of residual liquid as the sill propagated closer to the surface; b) deformation caused by tectonic shifts; and c) crystallization in the presence of deuteric hydrothermal fluids resulting in varying degrees of alteration. A model connecting multiple magmatic pulses, compaction and mobilization of residual liquid by compositional convection, closed-system differentiation, synchronous with tapping of the sill for extrusion of coeval basaltic subaerial flows is presented. The persistence of a low-temperature mushy layer, represented by ferrogranophyres, supports the possibility of a long-lived conduit subject to reopening after periods of quiescence in magmatic input, leading to the extrusion of the multiple flows of the Orange Mountain Basalt and perhaps even subsequent Preakness Basalt flows, depending on solidification conditions. A sub-Newark Basin network of sills subjected to similar protracted input of pulses as hypothesized for the Palisades was likely responsible for 600 ka of magmatic activity required to emplace a third set of Watchung flood basalts, the Hook Mountain Basalt.

We present new age determinations, derived from 40 Ar– 39 Ar incremental heating experiments, for basaltic lava flows from the Mandla lobe, located on the eastern margin of the main Deccan volcanic province, some ~ 1000 km from the Western Ghats escarpment. The most reliable estimates of crystallization ages come from 5 plateau ages from plagioclase separates, from a stratigraphically controlled succession of 37 lava flows. We detect no statistically significant age difference from bottom to top (range 63–65 Ma) and calculate a weighted mean age for the section at 64.21 ± 0.33 Ma. These lava flows are significantly younger than the majority of the main Deccan volcanic activity documented from the Western Ghats (67–65 Ma). The new ages are consistent, however, with geochemical correlation of the Mandla lobe lavas with the uppermost succession (Poladpur–Ambenali–Mahabaleshwar Formations) of the SW Deccan, and indicate that this post K/PB youngest phase of flood basalt activity erupted over much of the province.

Major flood basalt emplacement events can dramatically alter the composition of the sub-continental lithospheric mantle (SCLM). The Siberian craton experienced one of the largest flood basalt events preserved in the geologic record — eruption of the Permo-Triassic Siberian flood basalts (SFB) at ~ 250 Myr in response to upwelling of a deep-rooted mantle plume beneath the Siberian SCLM. Here, we present helium isotope ( 3 He/ 4 He) and concentration data for petrologically-distinct suites of peridotitic xenoliths recovered from two temporally-separated kimberlites: the 360 Ma Udachnaya and 160 Ma Obnazhennaya pipes, which erupted through the Siberian SCLM and bracket the eruption of the SFB. Measured 3 He/ 4 He ratios span a range from 0.1 to 9.8 R A (where R A = air 3 He/ 4 He) and fall into two distinct groups: 1) predominantly radiogenic pre-plume Udachnaya samples (mean clinopyroxene 3 He/ 4 He = 0.41 ± 0.30 R A (1σ); n = 7 excluding 1 outlier), and 2) ‘mantle-like’ post plume Obnazhennaya samples (mean clinopyroxene 3 He/ 4 He = 4.20 ± 0.90 R A (1σ); n = 5 excluding 1 outlier). Olivine separates from both kimberlite pipes tend to have higher 3 He/ 4 He than clinopyroxenes (or garnet). Helium contents in Udachnaya samples ([He] = 0.13–1.35 μcm 3 STP/g; n = 6) overlap with those of Obnazhennaya ([He] = 0.05–1.58 μcm 3 STP/g; n = 10), but extend to significantly higher values in some instances ([He] = 49–349 μcm 3 STP/g; n = 4). Uranium and thorium contents are also reported for the crushed material from which He was extracted in order to evaluate the potential for He migration from the mineral matrix to fluid inclusions. The wide range in He content, together with consistently radiogenic He-isotope values in Udachnaya peridotites suggests that crustal-derived fluids have incongruently metasomatized segments of the Siberian SCLM, whereas high 3 He/ 4 He values in Obnazhennaya peridotites show that this section of the SCLM has been overprinted by Permo-Triassic (plume-derived) basaltic fluids. Indeed, the stark contrast between pre- and post-plume 3 He/ 4 He ratios in peridotite xenoliths highlights the potentially powerful utility of He-isotopes for differentiating between various types of metasomatism (i.e., crustal versus basaltic fluids).

Mantle-derived spinel peridotite and spinel plagioclase pyroxenite xenoliths hosted in Cenozoic alkali basalts (20 Ma) are found in the Xikeer area, western Tarim Block, NW China. Based on petrographic and geochemical characteristics, the peridotite xenoliths are divided into three groups. Group 1 peridotite xenoliths have experienced high degree melt extraction (~ 17% fractional melting) and weak, incipient metasomatism. Group 2 and 3 peridotite xenoliths have undergone extensive silicate melt metasomatism, resulting in clinopyroxenes with spoon-shaped and highly fractionated REE patterns respectively. Although their present texture is metamorphic, the pyroxenite xenoliths were initially of metasomatic origin, with high pressure protolith mineral assemblage of orthopyroxene + clinopyroxene + spinel ± olivine. Numerical modeling of the Mg-number shows that the Xikeer pyroxenites may have resulted from reaction between Group 1 peridotite and a primitive Permian picritic melt at a high melt/rock ratio (> 15) and that the host basanite is incapable of being the metasomatic melt. The Re–Os isotopic systematics of the Xikeer peridotites and pyroxenites yield an apparent isochron of ~ 290 Ma, virtually identical to the age of Tarim flood basalts. Their eNd(t = 290 Ma) ranges from + 9.5 to + 14.3, typical of convecting mantle. We propose that the Xikeer xenolith suite may have been initially formed by melt extraction from the convecting mantle and, shortly after, was refertilized by Tarim mantle plume melts during the Early Permian, a process which is referred to as mantle ‘auto-refertilization’.

The Early Permian magmatism in Tarim, NW China comprises diamondiferous kimberlites, lamprophyres, flood basalts, Fe–Ti oxide ore-bearing layered mafic–ultramafic intrusions, bi-modal dyke swarms, alkaline igneous complexes (including syenites and A-type granites), rhyolites and pyroclastic rocks. The extent of this intraplate magmatism exceeds 250,000 km2, making it comparable to Large Igneous Provinces (LIPs). Screening of available radiometric ages reveals three main magmatic episodes in the Tarim LIP, with the first being marked by ~ 300 Ma small-volume kimberlites, followed by two phases of bimodal magmatism at ~ 290 Ma and at ~ 280 Ma, respectively. This relatively long time interval of the Early Permian magmatism is consistent with a low eruption rate of the Tarim LIP and is supported by the intercalation of volcanic rocks with sediments in outcrops and drill holes. Although the spatial distribution of each magmatic episode in the Tarim LIP is far from assessed, it seems that the ~ 290 Ma flood basalts are widespread across the province, whereas ~ 300 Ma kimberlites and ~ 280 Ma ultramafic–mafic–felsic intrusions and dyke swarms only occur in the Bachu Uplift and around the margins of the Tarim craton. We propose that the ~ 300 Ma kimberlites were derived from deep part of the metasomatized sub-continental lithospheric mantle (SCLM), while the ~ 290 Ma flood basalts were likely formed as a result of mixing of plume-derived melts with SCLM-derived melts (e.g., lamproitic melt) as they rose through the SCLM. In contrast, the ~ 280 Ma magmas were most likely derived from the convecting mantle. A plume incubation model is proposed to account for the temporo-spatial distribution of the Tarim LIP, in which different styles of plume–lithosphere interaction are recognized. In the first two episodes, the mantle plume incubating the base of the craton provides the heat that triggered melting of the enriched components in the SCLM. In contrast, adiabatic decompression melting within the plume produced the ~ 280 Ma magmatic phase. Thermal modeling suggests that lithospheric thinning by thermal erosion might have been associated with the upwelling mantle plume, with the greatest thinning occurring in the Bachu area. Thinned spots and weak zones at the margins of cratons and mobile belts caused preferential channeling of plume flow and subsequent decompression melting. This explains the localized distribution of ~ 280 Ma magmas in the Tarim LIP.

The Permian mafic dyke swarms are widely distributed in the Tarim Block, East Tianshan and the western Junggar Block in northwestern China. These mafic dyke swarms, together with coeval basalts, mafic–ultramafic intrusions and A-type granitic plutons, constitute the Permian Tarim large igneous province (LIP), which is considered to be mantle plume-derived. In this study, multi-source high-resolution remote sensing images are used to identify the geometry and emplacement mode of the mafic dykes, sills and flood basalts in Keping and its adjacent areas including Bachu (northwestern Tarim Block), Beishan and Kuluketag (northeastern Tarim Block) and Kelamyi (western Junggar Block). Structural analysis and paleomagnetic results are used to restore the primary geometry of the mafic dykes. The results indicate that the lengths and thicknesses among a total of 117 extracted mafic dykes in Keping follow negative exponent size distributions. The mafic dykes have a mean thickness of 3.8 m with a maximum of 21.4 m. The length of unconnected mafic dyke segments ranges from 0.127 km to 17.1 km, with an arithmetic length of 1.8 km. The restoration of geometry of the mafic dykes indicates that the Permian mafic dyke swarms in Keping may have extended as far as 61–69 km along a primary orientation of about N320°W, and the flood basalts of the Tarim LIP may have extended to Keping so that the areal extent of the Tarim LIP could be enlarged for about 12,000 km 2 compared with that originally reported (about 250,000 km 2 ). The geometric features of the mafic dyke swarms in Keping and its adjacent areas are different from those of the giant radiating dyke swarms due to radial fractures associated with domal uplift. The mafic dyke swarms and sills in the Tarim LIP make up the plumbing system of the mantle plume. Accompanied by the upwelling of mantle plume, many eruptive centers and regional dykes are generated in Beishan, East Tianshan and Tarim basin. The flood basalts in Keping are fissure-type, which may have been fed by magma conduits from the plume center; whereas in Bachu, the central volcanic eruption is dominant.

The Early Permian Tarim Large Igneous Province (LIP) in northwestern China contains a large area of silicic volcanics (~ 48,000 km 2 ) which are spatially and temporally associated with mafic–ultramafic rocks. In order to understand the behavior of crust above a mantle plume, selected rhyolitic samples are investigated in terms of U–Pb zircon dating, geochemical and isotopic analyses. The Tarim rhyolites have high A/CNK ratios (= molar Al 2 O 3 /CaO + Na 2 O + K 2 O), Fe # , Ga/Al ratios, concentrations of high field strength elements (HFSEs) such as Zr and Nb, and rare earth elements (REEs), along with high zircon saturation temperatures (872–940 °C), typical of aluminous A-type granitoids. Two contrasting rock types have been recognized. The low Nb–Ta type rhyolites are mainly associated with the first phase of the Tarim flood basalt magmatism at ~ 290 Ma. They are characterized by negative Nb–Ta anomalies, low e Nd (t) and e Hf (t) values, and high 87 Sr/ 86 Sr(t) and δ 18 O zircon values, consistent with a derivation from continental crustal source. The high Nb–Ta type rhyolites and their plutonic equivalents are associated with the second episode of Tarim magmatism (283–272 Ma). They are characterized by small negative to positive Nb–Ta anomalies, oceanic island basalt (OIB)-like trace element ratios, low 87 Sr/ 86 Sr(t) and high e Nd (t) and e Hf (t) values. These high Nb–Ta rhyolites are best interpreted as hybrid products of crystal fractionation of mafic magmas, coupled with crustal assimilation. The temporal and compositional evolution of the Tarim rhyolites reflects various extents of thermal and mass exchange between mantle-derived basaltic magma and crustal material above a mantle plume. When the plume head rises to the base of the Tarim craton, it first melts enriched components in the lithospheric mantle (~ 290 Ma), part of which may have ponded near the crust–mantle boundary and induced crustal anatexis leading to the formation of the low Nb–Ta type rhyolites. At ~ 280 Ma, large magma chambers and plumbing systems were formed due to increasing magma supply rate during decompression melting of the mantle plume. This led to the formation of a mafic–ultramafic and felsic association of which the high Nb–Ta type rhyolites are a part.

The Tarim continental flood basalts (CFBs) provide important clues about the genesis and magmatic evolution of the Early Permian Tarim Large Igneous Province (Tarim LIP) in northwestern China. Here we present results of LA–MC–ICPMS Lu–Hf isotope analysis on Early Permian (ca. 290 Ma) zircons extracted from the Tarim CFBs in the Keping area, northwest of the Tarim Basin. Zircons from two sub-groups of Keping basalts (Groups 1a and 1b) have similar Lu–Hf isotopic compositions and exhibit a relatively large range of 176Hf/177Hf ratios between 0.282422 and 0.282568. Their negative eHf(t) values (− 6.8–− 1.4) are generally lower than the whole-rock eHf(t) values of their host basalts (− 2.8–2.1), and are distinct from other known intrusive rocks (− 0.3–7.1) in the Tarim LIP and their hosted zircons (4.9–8.8). Systematic studies of Hf isotopic data from Tarim and its adjacent regions reveal that these zircons are probably xenocrysts, sourced from coeval igneous rocks in the South Tianshan Orogen (e.g., the Lower Permian Xiaotikanlike Formation volcanic and pyroclastic rock suite). This, together with the presence of Precambrian zircons in Keping basalts, clearly indicates crustal contamination during their eruptions and provides hints about the potential contaminant sources. Geochemical modeling further suggests that the earlier erupted Group 1b basalts experienced more contamination, predominantly by some high Th–U–Pb rock components, most likely from the South Tianshan Orogen. The later erupted Group 1a basalts in the Keping area have been less contaminated with mainly the Tarim Precambrian rocks. Another group of the Tarim CFBs in the Northern Tarim Uplift (Group 2) appears to have undergone negligible crustal contamination but possesses evidence for variable source compositions. The modeling also indicates that the uncontaminated parental magmas of various Tarim LIP rocks (from the picrites and basalts to ultramafic–mafic and syenitic intrusive rocks) exhibit a wide range of eNd(t) values (ca. − 5–5), reflecting source isotopic heterogeneity, which may be a consequence of plume–lithosphere interaction during the generation of the Tarim LIP.

The age of the Emeishan lavas in SW China remains poorly constrained because the extrusive rocks are (1) thermally overprinted and so represent an open system unsuitable for 40 Ar/ 39 Ar geochronology and (2) in most cases devoid of zircon so that it is impossible for the application of U–Pb geochronology. Existing radiometric age constraints of Emeishan large igneous province are mainly from the application of SIMS and LA-ICP-MS U–Pb techniques to zircons from mafic and felsic intrusions, which represent indirect constraints for the lavas. In an attempt to directly determine the age of the Emeishan lava succession, high-resolution chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) zircon U–Pb techniques have been used on the felsic ignimbrite at the uppermost part of the Emeishan lava succession. These techniques have yielded a weighted mean 206 Pb/ 238 U age of 259.1 ± 0.5 Ma (n = 6; MSWD = 0.7). We interpret this age as the termination age of the Emeishan flood basalts. The age of the Guadalupian–Lopingian boundary is still unconstrained by high-resolution geochronology but is likely to be close to our new age for this felsic ignimbrite.

article i nfo The Early Permian Panjal Traps of northern India represent a significant eruption of volcanic rocks which occurred during the opening of the Neotethys Ocean. Basaltic, basaltic-andesites, dacitic and rhyolitic rocks collected from Guryal Ravine and Pahalgam show evidence for subaerial and subaqueous eruptions indicating that they are contemporaneous with the formation of a shallow marine basin. The major and trace element geochemistry of the basalts is consistent with a within-plate setting and there are basalts which have high-Ti (TiO2 N 2.0 wt.%) and low-Ti (TiO2 b 1.8 wt.%) compositions. The 'high-Ti' basalts are similar to OIB whereas the 'low-Ti' basalts are similar to continental tholeiites. The identification of 'high- and low-Ti' basalts within the Panjal Traps is analogous to other large igneous provinces (e.g. Karoo, Deccan, Parana, Emeishan). The Sr-Nd isotopic values (eNd(T) = −5.3 to +1.3; ISr = 0.70432 to 0.71168) of both types of basalts overlap indicating that the rocks may have originated from the same ancient subcontinental lithospheric (i.e. EMII-like) mantle source (TDM = ~2000 Ma). The two groups of basalts can be modeled by using a primitive mantle source and different degrees of partial melting where the high-Tirocksare producedby ~1% partial melting of a spinel peridotite source where- as the low-Ti rocks are produced by ~8% partial melting. Trace elemental and isotope modeling indicatesthat some of the basalts assimilated ≤ 10% crustal material. In contrast, the basaltic-andesites are likely formed by mixing be- tween basaltic magmas and crustal melts which produced rocks with higher SiO2 (~55 wt.%) content and enriched isotopic signatures (eNd(T) = −6.1; ISr = 0.70992). The Panjal Trap volcanism was likely due to partial melting of the SCLM within a passive extensional setting related to the rifting of Cimmeria from Gondwana. Contemporaneous volcanic and plutonic granitic rocks throughout the Himalaya are probably not petrogenetically related but are likely part of the same regional tectonic regime.

The lithosphere–asthenosphere boundary (LAB) depth represents a fundamental parameter in any quantitative lithospheric model, controlling to a large extent the temperature distribution within the crust and the uppermost mantle. The tectonic history of Ireland includes early Paleozoic closure of the Iapetus Ocean across the Iapetus Suture Zone (ISZ), and in northeastern Ireland late Paleozoic to early Mesozoic crustal extension, during which thick Permo-Triassic sedimentary successions were deposited, followed by early Cenozoic extrusion of large scale flood basalts. Although the crustal structure in Ireland and neighboring offshore areas is fairly well constrained, with the notable exception of the crust beneath Northern Ireland, the Irish uppermost mantle remains to date relatively unknown. In particular, the nature and extent of a hypothetical interaction between a putative proto Icelandic mantle plume and the Irish and Scottish lithosphere during the Tertiary opening of the North Atlantic has long been discussed in the literature with diverging conclusions. In this work, the present-day thermal and compositional structure of the lithosphere in Ireland is modeled based on a geophysical–petrological approach (LitMod3D) that combines comprehensively a large variety of data (namely elevation, surface heat flow, potential fields, xenoliths and seismic tomography models), reducing the inherent uncertainties and trade-offs associated with classical modeling of those individual data sets. The preferred 3D lithospheric models show moderate lateral density variations in Ireland characterized by a slightly thickened lithosphere along the SW-NE trending ISZ, and a progressive lithospheric thinning from southern Ireland towards the north. The mantle composition in the southern half of Ireland (East Avalonia) is relatively and uniformly fertile (i.e., typical Phanerozoic mantle), whereas the lithospheric composition in the northern half of Ireland (Laurentia) seems to vary from moderately depleted to fertile, in agreement with mantle xenoliths erupted in northwestern Ireland.

We present petrology, geochemistry and radiogenic isotope (Sr and Nd) data of thirteen post-Deccan lamprophyre dykes in the Narmada rift zone from the Chhotaudepur alkaline province of the Deccan Large Igneous Province (DLIP). Mineralogically, these dykes show affinity towards alkaline (sannaite and camptonite) as well as ultramafic (damtjernite) varieties of lamprophyres. Their major oxides and certain trace element ratios increase with increasing silica content highlighting the strong influence of fractionation processes. Their Nb/U and Ce/Pb ratios are similar to the mantle array defined by MORBs and OIBs and suggests an uncontaminated nature. Major oxide (K2O, Na2O, SiO2 and TiO2) contents show geochemical similarity towards shoshonitic volcanic series, whereas elevated Zr/Hf and Nb/La coupled with suppressed Rb/Nb and Zr/b display their affinity towards HIMU-type intraplate basalts. Their radiogenic initial 87Sr/86Sr (0.706034–0.710582) and sub-chondritic initial ɛNd (−8.6 to 2.1) are akin to those of the (i) ca. 65 Ma Ambadongar carbonatite, NW India, and (ii) ca. 65 Ma orangeites from Bastar Craton, central India, highlighting an enriched lithospheric mantle source. REE inversion modeling suggests ~3% enrichment of an undepleted mantle followed by small degrees of melting of this enriched mantle source are sufficient- as in the case of ocean island basalts (OIB)- to reproduce their observed REE concentrations. Their TDM Nd model ages (564–961 Ma) are consistent with widespread convergent margin-related magmatism during the amalgamation of the Rodinia supercontinent. We propose that enriched lithospheric mantle developed during the Neoproterozoic was metasomatized by small-volume CO2-rich melts imparting a HIMU-type geochemical character during Late Cretaceous, when the mantle plume (viz., Reunion) responsible for the flood basalt eruption, impinged at the base of the NW Indian lithosphere. From the presence of F-rich apatite and high K/Rb in mica, we infer the (i) presence of F-phlogopite in their source regions, and (ii) that the depth of post-Deccan lithosphere-asthenosphere boundary (LAB) beneath NW India was at least ~100 km at ca. 65 Ma.

The 1.27 Ga Coppermine continental flood basalt (CFB) province in northern Canada represents the extrusive manifestation of the 2.7 Mkm2 Mackenzie large igneous province (LIP) that includes the Mackenzie dyke swarm and the Muskox layered intrusion. New Re–Os isotope and highly siderophile element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundance data are reported together with whole-rock major- and trace-element abundances and Nd isotopes to examine the behaviour of the HSE during magmatic differentiation and to place constraints on the extent of crustal interaction with mantle-derived melts. Mineral chemistry and petrography are also reported for an unusual andesite glass flow (CM19; 4.9 wt.% MgO) found in close proximity to newly recognised picrites (> 20 wt.% MgO) in the lowermost stratigraphy of the Coppermine CFB. Compositions of mineral phases in CM19 are similar to the same phases found in Muskox Intrusion chromitites and the melt composition is equivalent to inclusions trapped within Muskox chromites. The apparently conflicting elevated HSE contents (e.g., 3.8 ppb Os) and mantle-like initial 187Os/188Os (γOs = + 2.2), versus stable isotope (δ18O = + 12‰) and lithophile element evidence (eNdi = − 12.8) for extensive crustal contamination, implicate an origin for CM19 as a magma mingling product formed within the Muskox Intrusion during chromitite genesis. Combined with Nd isotope data that places the feeder for lower Coppermine CFB picrites and basalts within the Muskox Intrusion, this result provides compelling evidence for direct processing of some CFB within upper-crustal magma chambers. The Coppermine CFB defines a 187Re–187Os isochron with an age of 1263 + 16/− 20 Ma and initial γOs = + 2.2 ± 0.8. The initial Os isotope composition for the Coppermine CFB is slightly higher than the near-primitive-mantle initial 187Os/188Os for the Muskox Intrusion (γOs = + 1.2 ± 0.3). This result is interpreted to reflect greater crustal contamination in extrusive CFB and the sensitivity of Os isotopes, compared with absolute HSE concentrations, for tracking crustal contributions. Modelling of absolute and relative HSE abundances in global CFB reveals that HSE concentrations decrease with increasing fractionation for melts with 13.5 wt.% MgO) from CFB (n = 98; 1.97 ± 1.77 ppb) having higher Os abundances than ocean island basalt (OIB) equivalents (n = 75; 0.95 ± 0.86 ppb). The differences between CFB and OIB picrite absolute Os abundances may result from higher degrees of partial melting to form CFB but may also reflect incorporation of trace sulphide in CFB picrites from magmas that reached S-saturation in upper-crustal magma chambers. Significant inter-element fractionation of (Re + Pt + Pd)/(Os + Ir + Ru) are generated during magmatic differentiation in response to strongly contrasting partitioning of these two groups of elements into sulphides and/or HSE-rich alloys. Furthermore, fractional crystallization has a greater role on absolute and relative HSE abundances than crustal contamination under conditions of CFB petrogenesis due to the dilution effect of continental crust, which has low total abundances of the HSE. Combined data for the basaltic and intrusive portions of the Mackenzie LIP indicate a mantle source broadly within the range of the primitive upper mantle. The majority of Archaean komatiites and Phanerozoic CFB also require mantle sources with primitive upper mantle to chondritic Re/Os evolution, with exceptions typically being from analyses of highly-fractionated MgO-poor basalts.

Despite considerable efforts, the Paleozoic tectonic setting (subduction vs. post-collision vs. mantle plume) in the Central Asian Orogenic Belt (CAOB) remains controversial. With aims of solving this issue, we examined zircon saturation temperatures of granitoids emplaced in different settings, and demonstrated that zircon saturation thermometry of felsic igneous rocks, especially when integrated with their lithology and geochemistry, can be a simple and powerful tool for tectonic discrimination. This method is then applied in northern Xinjiang where the Early Permian granitoids have higher saturation temperatures (> 800 °C) than the Silurian–Carboniferous granitoids (mainly

In this paper, we present geochronological, geochemical and palaeomagnetic results from the Hlagothi Complex and a NW-trending dolerite dyke swarm on the southeastern region of the Kaapvaal Craton in northern KwaZulu-Natal, South Africa. The Hlagothi Complex consists of layered sills of meta-peridotite, pyroxenite and gabbro intruding into the Pongola Supergroup. U-Pb baddeleyite ages on the Hlagothi Complex and a NW-trending dyke of 2866 +/- 2 Ma and 2874 +/- 2 Ma, respectively, reveal a ca. 2.87 Ga magmatic event on the southeastern Kaapvaal Craton. The geochemical signature of the Hlagothi Complex recognises two discrete groupings, with a magmatic source that is chemically distinct from those of the older rift-related Nsuze and Dominion groups. Additional units on the Kaapvaal Craton can be linked with this new 'Hlagothi' event based on spatial and temporal association, and geochemistry: 1) the Thole Complex, 2) parts of the Usushwana Complex, and 3) flood basalts within the Mozaan Group and Central Rand Group. The association between all these units suggests a previously unrecognised large igneous province in the southeastern Kaapvaal Craton. Our palaeomagnetic data identifies a possible primary magnetisation within the least-altered lithologies of the Hlagothi Complex (with a virtual geographic pole at 23.4 degrees N, 53.4 degrees E, dp=8.2 degrees and dm=11.8 degrees). The bulk of samples however, displayed two episodes of remagnetisation. These are likely to be associated with 2.85 to 2.75 Ga aged granitoids across the southeastern Kaapvaal Craton, and tectonic activity in the nearby Meso- to Neoproterozoic Namaqua-Natal mobile belt A short-lived (

The Sao Francisco Craton (SFC) is an Archaean craton that hosts a significant number of mafic intrusions. Previous attempts to date the igneous emplacement of these rocks are limited to a few dates of poor precision. Here, two dyke swarms from the Uaua Block in the northeastern portion of the craton were dated using U-Pb on baddeleyite (ID-TIMS) and zircon (SIMS). The older dyke swarm trends mainly N-S to NW-SE (but also NE) and comprises norite dykes, whereas the younger dykes are tholeiite and trend NE-SW. The ages of 2726.2 +/- 3.2 and 2623.8 +/- 7.0 Ma date dykes of noritic and tholeiitic compositions, respectively. The older age matches the ages of several Archaean-Large Igneous Provinces (tholeiitic-komatiite greenstone belts and continental flood basalts) worldwide and represents part of a feeder system, but the younger age is similar to Andean style continental margin magmatism in the Sao Francisco and Zimbabwe cratons. The two dyke swarms are tholeiitic in composition but the noritic dykes are more enriched in both compatible and incompatible elements and have higher (La/Yb)(N) ratios than the tholeiite dykes. The norite dykes are interpreted as low degrees of melting from enriched refractory mantle sources, whereas the tholeiite dykes represent high degrees of partial melting of more depleted mantle sources; the two mafic dyke swarms may be related to each other by progressive extension of the continental lithosphere. It is suggested that the Uaua Block with its dyke swarms is a small piece of Archaean crust dispersed after the break-up of a major Archaean supercraton. (C) 2012 Elsevier B.V. All rights reserved. (Less)