The geochemistry of Dinantian volcanism in south Kintyre and the evidence for provincialism in the southern Scottish mantle

Scottish Dinantian transitional to mildly alkaline volcanism is represented by abundant outcrops in the Midland Valley, Southern Uplands and Highlands provinces. Dinantian volcanic rocks from Kintyre in the Scottish Highlands range in composition from basalt through basaltic hawaiite, hawaiite, mugearite and benmoreite to trachyte, the compositions of the evolved types being largely due to differentiation from the basaltic parents.Recent geochemical investigations of Scottish Caledonian granitoids, Siluro-Devonian Old Red Sandstone (ORS) lavas and xenolith suites from numerous vents and dykes of Permo-Carboniferous to Tertiary age have revealed that the Scottish crust and upper mantle both increase in age and are increasingly enriched in incompatible elements towards the north and northwest. The upper mantle and lower crust below the Highlands province are therefore largely considered to be more enriched and in parts older than those below the Midland Valley and Southern Uplands. Dinantian alkali basalts from these latter two provinces have Nd values predominantly in the range +3 to +6, initial ⁸⁷Sr/⁸⁶Sr values of 0.7029–0.7041 and ²⁰⁷Pb/ ²⁰⁴Pb values of 15.48–15.60. However, similar basalts from Kintyre and Arran in the Highlands have lower Nd (+0.1 to +3.4) and ²⁰⁷Pb/²⁰⁴Pb (for given ²⁰⁶Pb/²⁰⁴Pb ratios; 15.49–15.51) and slightly higher ⁸⁷Sr/⁸⁶Sr (0.7033–0.7046). This regional variation correlates well with the differences seen between Midland Valley and Highland magmas in the ORS calc-alkaline suite (Thirlwall 1986) and it is suggested that both the ORS and Dinantian basic rocks are derived from similar types of mantle, although no lithospheric slab component is present in the later Dinantian suites. Isotopically-distinct portions of mantle therefore appear to have been present below the Highland and Midland Valley-Southern Upland provinces from at least Caledonian to Carboniferous times. The combined incompatible element and Sr-Nd-Pd isotopic evidence from Kintyre and Arran basaltic rocks does not allow unequivocal distinction between a lithospheric mantle and a sublithospheric mantle source. The observed correlation between isotopic composition of Dinantian basalts and the chemical composition of the lithosphere, together with the apparent involvement of long-term separated source reservoirs suggests that Kintyre and Arran lavas were derived largely from a lithospheric mantle source. On the other hand, the isotopic enrichment of Kintyre basaltic rocks is not extreme; trace element and isotopic compositions are still comparable to modem OIB. Sublithospheric mantle could therefore also be a viable source for Kintyre and Arran Dinantian volcanism.     

Columbia River volcanism: the question of mantle heterogeneity or crustal contamination

Basalts from the Columbia River flood basalt province of the northwestern U.S.A. show a large diversity in chemical and Nd and Sr isotopic compositions. ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ranges from 0.51303 to 0.51208 and is strongly correlated with variations in ${}^{87}\text{Sr}{}^{86}\text{Sr}$. This correlation suggests mixing between two end member compositions, one characterized by ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{> 0.51303}$ and ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{< 0.7035}$, and the other with ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{< 0.5120}$ and ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{> 0.715}$. The more radiogenic component could be mantle enriched in incompatible elements during the Precambrian, or Precambrian materials of the continental crust. A quartz-rich xenolith found in the Columbia lavas has Rb-Sr and Sm Nd model ages of ≈ 1.4Æ, implying the existence of old, isotopically evolved crustal basement which could serve as contaminant. Nevertheless, crustal contamination alone cannot explain the chemical variation of the samples studied, and other fractionation processes must have occurred simultaneously. A model involving combined assimilation and crystal fractionation reproduces the chemical and isotopic characteristics of the volumetrically dominant Grande Ronde unit for an assumed crystallizing component of plagioclase, low calcium pyroxene and minor olivine. The data are not consistent with the suggestion that a ‘primordial’ mantle is the source for this continental flood basalt province. Rather they suggest that the main volume of these lavas was originally derived from a mantle similar in isotopic composition to island arc and ocean island basalts of the north Pacific. The primary magma was modified chemically and isotopically by crystal fractionation and assimilation of sialic crustal materials during its transport through, or storage in the continental crust.     

Geochemical evidence for arc-type volcanism in the Aegean Sea: the blueschist unit of Siphnos, Cyclades (Greece)

Blueschists, eclogites, chlorite–actinolite rocks and jadeite-gneisses of the blueschist unit of Siphnos have been investigated for their geochemical composition. Their protolith nature is characterised and a geodynamic model for the pre-metamorphic evolution of these metavolcanic rocks is proposed on the basis of immobile elements, especially trace elements and rare earth elements (REE).

The protoliths of the eclogites are characterised as calc-alkaline basalts, andesites and Fe-rich tholeiites evolving in an island-arc setting. Trace element data indicate that subducted marine sediments were assimilated in the magma chamber, enriching the protoliths in LILE and Pb. Produced in the early stage of back-arc basin opening, a protolith with affinities to both island-arc and MORB formed the precursor of the chlorite–actinolite rocks. They were created by low degrees of partial melting of very primitive magmas, akin to spinel-peridotites and have affinities to boninites, probably through melting of the peridotitic mantle wedge. Tholeiitic basalts and andesites with N-MORB affinity, especially in their REE-patterns, were then produced by partial melting, possibly in an embryonic back-arc basin. These rocks were the protoliths of the blueschists of Siphnos. Their enrichment in some LILE and Pb indicates a N-MORB source contaminated by marine sediments, probably shales or other Pb-rich sediments. Because the jadeite-gneisses show affinities to MOR-granites and volcanic arc granites, intrusion of their protoliths in a back-arc environment is likely. The protoliths of the quartz-jadeite gneisses are rhyodacites/dacites and rhyolites, those of the glaucophane-jadeite gneisses were andesites.

The proposed geodynamic model, solely based on geochemical data, is consistent with geochemical data from neighbouring islands, though those rock units show much higher chemical variability. Consistent with geotectonic models, which are based on structural and geophysical data, the volcanic protoliths of the Siphnos blueschist unit reflect the transition from subduction to spreading environment and record in detail: subduction, formation of an island-arc, and the evolution of a back-arc basin.     

Development of a heterogeneity in the lithosphere: Geochemical evidence

Systematization of information on multivalent trace elements in peridotite xenoliths made it possible to reveal differences in the distribution of these elements in the subcontinental and suboceanic segments of the lithosphere, which reflects the development of a geochemical heterogeneity in the lithosphere during the early (Hadean) stage of its evolution. The vast extent of trace-element differentiation in Hadean peridotite xenoliths is most probably explained by the appearance of appreciable masses of condensed water and, consequently, active mantle metasomatism in the hydrated lithosphere. The latter formed the upper depleted (oceanic) zone underlain by an “undifferentiated” zone enriched in trace elements. The removal of trace elements from both zones, a process that does not rule out the participation of earlier accretion in it, gave rise to a crust strongly enriched in these elements. The existence of long-lived extensive lithosphere heterogeneity calls for revision of the concept of multistage crustal growth with a general tendency toward an increase in its bulk volume.     

Lithospheric extension and the opening of the Red Sea: sediment-basalt relationships in Yemen*

This paper analyses new field and radiometric age data on the timing of extension, sedimentation and magmatism related to the opening of the Red Sea. The first evidence of widespread extension is found in synsedimentary fissures in the fluviatile to shallow marine Tawilah Formation of Cretaceous to early Tertiary age. The first indication of magmatism is the eruption of alkaline basalts (43.5–21.2 Ma) on to marine and non-marine sandstones of lower to middle Tertiary age. The lack of an angular unconformity at the sediment-basalt contact, over a period of 25 Myr, corroborates a diachronous relationship between the Tawilah sandstones and the Yemen Volcanics. These data also indicate that the onset of volcanism in Yemen was in the Eocene, as is the case in Ethiopia and Saudi Arabia. A passive rifting model is supported by early extension and sedimentation followed by syn- and post-tectonic magmatism.     

Mesozoic thermal evolution of the southern African mantle lithosphere

The thermal structure of Archean and Proterozoic lithospheric terranes in southern Africa during the Mesozoic was evaluated by thermobarometry of mantle peridotite xenoliths erupted in alkaline magmas between 180 and 60 Ma. For cratonic xenoliths, the presence of a 150–200 °C isobaric temperature range at 5–6 GPa confirms original interpretations of a conductive geotherm, which is perturbed at depth, and therefore does not record steady state lithospheric mantle structure.

Xenoliths from both Archean and Proterozoic terranes record conductive limb temperatures characteristic of a “cratonic” geotherm (40 mW m⁻²), indicating cooling of Proterozoic mantle following the last major tectonothermal event in the region at 1 Ga and the probability of thick off-craton lithosphere capable of hosting diamond. This inference is supported by U–Pb thermochronology of lower crustal xenoliths [Schmitz and Bowring, 2003. Contrib. Mineral. Petrol. 144, 592–618].

The entire region then suffered a protracted regional heating event in the Mesozoic, affecting both mantle and lower crust. In the mantle, the event is recorded at 150 Ma to the southeast of the craton, propagating to the west by 108–74 Ma, the craton interior by 85–90 Ma and the far southwest and northwest by 65–70 Ma. The heating penetrated to shallower levels in the off-craton areas than on the craton, and is more apparent on the southern margin of the craton than in its western interior. The focus and spatial progression mimic inferred patterns of plume activity and supercontinent breakup 30–100 Ma earlier and are probably connected.

Contrasting thermal profiles from Archean and Proterozoic mantle result from penetration to shallower levels of the Proterozoic lithosphere by heat transporting magmas. Extent of penetration is related not to original lithospheric thickness, but to its more fertile character and the presence of structurally weak zones of old tectonism. The present day distribution of surface heat flow in southern Africa is related to this dynamic event and is not a direct reflection of the pre-existing lithospheric architecture.     

Geochemical and O-isotope constraints on the evolution of lithospheric mantle in the Ross Sea rift area (Antarctica)

Peridotite xenoliths found in Cenozoic alkali basalts of northern Victoria Land, Antarctica, vary from fertile spinel-lherzolite to harzburgite. They often contain glass-bearing pockets formed after primary pyroxenes and spinel. Few samples are composite and consist of depleted spinel lherzolite crosscut by amphibole veins and/or lherzolite in contact with poikilitic wehrlite. Peridotite xenoliths are characterized by negative Al₂O₃–Mg# and TiO₂–Mg# covariations of clino- and orthopyroxenes, low to intermediate HREE concentrations in clinopyroxene, negative Cr–Al trend in spinel, suggesting variable degrees of partial melting. Metasomatic overprint is evidenced by trace element enrichment in clinopyroxene and sporadic increase of Ti–Fe_tot. Preferential Nb, Zr, Sr enrichments in clinopyroxene associated with high Ti–Fe_tot contents constrain the metasomatic agent to be an alkaline basic melt. In composite xenoliths, clinopyroxene REE contents increase next to the veins suggesting metasomatic diffusion of incompatible element. Oxygen isotope data indicate disequilibrium conditions among clinopyroxene, olivine and orthopyroxene. The highest δ¹⁸O values are observed in minerals of the amphibole-bearing xenolith. The δ¹⁸O_cpx correlations with clinopyroxene modal abundance and geochemical parameters (e.g. Mg# and Cr#) suggest a possible influence of partial melting on oxygen isotope composition. Thermobarometric estimates define a geotherm of 80°C/GPa for the refractory lithosphere of NVL, in a pressure range between 1 and 2.5 GPa. Clinopyroxene microlites of melt pockets provide P–T data close to the anhydrous peridotite solidus and confirm that they originated from heating and decompression during transport in the host magma. All these geothermometric data constrain the mantle potential temperature to values of 1250–1350°C, consistent with the occurrence of mantle decompressional melting in a transtensive tectonic regime for the Ross Sea region.     

The Paleozoic evolution of Central Tianshan: Geochemical and geochronological evidence

A suite of Paleozoic granitoids in Central Tianshan was studied for both geochemistry and geochronology in an effort to constrain their origin and tectonic setting. We combined LA-ICP-MS dating of zircon, standard geochemical analyses and Hf-isotopic studies of zircon to develop our tectonic model. Based on our analysis, the granitoids formed in three distinctive stages: ~ 450–400 Ma, ~ 370–350 Ma and ca. 340 Ma. The first stage (450–400 Ma) granitoids exhibit metaluminous, magnesian, high-K to shoshonitic characteristics of I-type granitoids (arc-setting), that are enriched in LREE relative to HREE with high (La/Yb)_CN values, show negative Eu anomaly and are depleted in Nb, Ta and Ti. This phase of granitoid emplacement was most likely related to the southward subduction of the Paleo-Tianshan Ocean beneath the Tarim block and the subsequent Central Tianshan arc. In contrast, the second stage granitoids (370–350 Ma) are distinctly different and are classified as calc-alkaline or shoshonitic plutons with a weak positive Eu anomaly. Within the second stage granitoids, it appears that the earlier (~ 365 Ma) granitoids fit within the A-type field whereas the younger (~ 352 Ma) granitoids plot within the post-collisional potassic field. These granitoids formed during collisions between Central Tianshan and the Tuha terrane that occurred along the northern margin of Central Tianshan. Lastly, the ca. 340 Ma granitoids are typical of volcanic arc granitoids again that probably formed during the northward subduction of the South Tianshan Ocean beneath the Central Tianshan landmass or the subsequent southward subduction of the residual Paleo-Tianshan Ocean.The Hf isotopic data of zircons from all the studied granitoids were pooled and yielded three prominent Hf T_DM^C model age populations: ca. 2400 Ma, ca. 1400 Ma and ca. 1100 Ma. The Hf-data shows a significant input of juvenile crust in addition to crustal recycling. We interpret these three phases of juvenile crustal addition to phases of global growth of continental crust (~ 2400 Ma), the addition of juvenile crust during the breakup of the Columbia supercontinent (~ 1400 Ma) and the assembly of Rodinia (~ 1100 Ma).     

Velocity structure of the continental upper mantle: evidence from southern Africa

The velocity model for southern Africa of Qiu et al. [Qiu, X., Priestley, K., McKenzie, D., 1996. Average lithospheric structure of southern Africa. Geophys. J. Int. 127, 563–587] is revised so as to satisfy both the regional seismic waveform data and the fundamental mode Rayleigh wave phase velocity data for the region. The revised S-wave model is similar to the original model of Qiu et al. except that the high velocity, upper mantle lid extends to 160 km depth in the revised model rather than to 120 km in the original model. Sensitivity tests of the regional seismic data show that the minimum velocity in the S-wave low velocity zone can be as high as 4.45 km s⁻¹ compared to 4.32 km s⁻¹ in the Qiu et al. model. The vertical S-wave travel time for the revised south African model is compared with the vertical S-wave travel times for the global tomographic models S12WM13 and S16B30, and they are found to be similar.     

Helium isotopes provide no evidence for deep mantle involvement in widespread Cenozoic volcanism across Central Asia

Small-volume alkali basaltic volcanism has occurred intermittently for the past + 30 My across a vast area of thick continental crust from southern Siberia, through Mongolia to northeast China. With a lack of evidence for Basin-and-Range-type crustal extension or rifting, models to explain the widely dispersed, yet long-lived, volcanism tend to favour involvement of one or more mantle plume(s). We examine the range of ³He/⁴He isotope values in olivine phenocrysts from basalts, and their entrained mantle xenoliths, from Hamar Daban in southern Siberia, and Hangai in central Mongolia, in order to examine whether upwelling lower mantle appears to be present beneath central Asia and thus test the validity of the plume model for this region. Our results show that the maximum ³He/⁴He value for the Siberian basalts is 8.12 ± 0.2R_a, and the maximum value for Mongolian basalts is 9.5 ± 0.5R_a. These values suggest that there is no significant contribution from a high ³He/⁴He primordial component that would strongly argue a lower mantle source. Overlap with commonly reported values for MORB leads us to propose that the source of the magmatism derives from the shallow asthenosphere. Alternative models to a deeply sourced mantle plume that may be able to explain the magmatism include: a shallow thermal anomaly confined to the upper mantle but either fed laterally or caused by thermal blanketing of the large Asian landmass; replacement or delamination of the lowermost lithosphere in response to tectonic stresses; or large-scale mantle disturbance or overturn caused by a protracted history of subduction beneath central Asia that ended regionally with the Jurassic closure of the Mongol-Okhotsk Ocean, but continues further afield with the present Indo-Asia collision.     

Dynamic Earth: crustal and mantle heterogeneity

The dynamic processes within the Earth leave their record in geophysical and geochemical variation about the general stratification with depth. A snapshot of current structure is provided by geophysical evidence, whereas geochemical information provides a perspective over the age of the Earth. The combination of information on the distribution of heterogeneity from geophysical and geochemical sources provides enhanced insight into likely geodynamic processes. A variety of techniques can be used to examine crustal structure, but the major source of information on seismic heterogeneity within the Earth comes from tomographic studies, exploiting surface waves for the lithosphere and body waves for the bulk Earth. A powerful tool for examining the character of mantle heterogeneity is the comparison of images of bulk-sound and shear-wave speed extracted in a single inversion, since this isolates the dependencies on the elastic moduli. Such studies are particularly effective when a common path coverage is achieved for P and S as, for example, when common source and receiver pairs are extracted for arrival times of the phases. The relative behaviour of the bulk-sound and shear-wave speeds allows the definition of heterogeneity regimes. For subduction zones, a large part of the imaged structure comes from S-wave speed variations. The narrow segments of fast wave speeds in the lower mantle, in the depth range 900 – 1500 km, are dominated by S variations, with very little bulk-sound contribution, so images of P-wave speed are controlled by shear-wave variability. Deep in the mantle, there are many features with high seismic-wave speed without an obvious association with subduction in the last 100 million years, which suggests long-lived preservation of components of the geodynamic cycle. The base of the Earth's mantle is a complex zone with widespread indications of heterogeneity on many scales, discontinuities of variable character, and shear-wave anisotropy. Discordance between P- and S-wave speed anomalies suggests the presence of chemical heterogeneity rather than just the effect of temperature.     

The Nsuze Group,Pongola Sequence,South Africa: Geochemical evidence for Archaean volcanism in a continental setting

The Archaean Nsuze Group in southeast South Africa represents an important volcano-sedimentary succession that is markedly different compositionally and lithologically from older and contemporaneous sequences elsewhere in southern Africa.The Nsuze volcanic rocks cropping out in the vicinity of the Pongola River in northern Natal display a complete spectrum of chemical compositions from basalt to rhyolite, with lavas of intermediate compositions predominant but ultramafic lavas absent. Flows of different compositions are complexly interdigitated. The uppermost rocks of the Nsuze Group reflect a gradual decrease in volcanic activity accompanied by an increase in sedimentation.The Nsuze lavas are tholeiitic with total Fe (as Fe₂ O₃) contents approaching 17% in the basaltic andesites. All lithologies are characterized by moderate to strong light rare earth element (REE) enrichment. Heavy REE slopes range from moderate in the basalts and basaltic andesites to flat in the rhyolites, with the exception of two basaltic andesites and a dacite which have flat heavy REE slopes. Basalts have small or no negative Eu anomalies but increasingly larger negative anomalies are a feature of the intermediate and acid lavas. Variations in Cr content appear to be related to stratigraphic position. Basalts and basaltic andesites in the lower part of the volcanic sequence have higher Cr contents (by a factor of six to eight times) than lavas with similar MgO abundances in the upper part of the sequence.Provisional modelling of the available chemical data favours low pressure, crystal fractionation from and evolved basaltic parent, that could be derived by initial non-modal melting of a garnet-lherzolite source. Variable degrees of crustal contamination of the evolved magmas are considered probable.     

Structural evolution history of the Red Sea Rift

The Red Sea Rift has been an object of comprehensive studies by several generations of geologists and geophysicists. Many publications and open-file reports provide insights into the geological history of this rift. Paleogene and Cretaceous rocks, which are considered to be prerift, are locally exposed at the margins of the Red Sea Rift. At the same time, some evidence indicates that at least some of these rocks are related to the early stage of the evolution of the Red Sea Rift. The available geological data suggest that the Red Sea region started its active evolution in the Cretaceous. As follows from lithostratigraphic data, the Cretaceous-Paleogene trough that predated the Oligocene-Quaternary rift covered this region completely or partially. The pre-Oligocene magmatism and geological evidence show that the Cretaceous-Paleogene trough was of the rift type. The Cretaceous-Eocene and Oligocene-Quaternary phases of rifting were separated by an epoch of uplifting and denudation documented by the erosion surface and unconformity.     

Geochemical evolution of lithospheric mantle beneath S.E. South Australia

The record of mafic magmatism from the Proterozoic to the Holocene in southern Australia reflects episodic incompatible element enrichment of the sub-continental lithospheric mantle (SCLM) recording periodic interaction of asthenosphere and lithosphere. The composition of Jurassic and Cainozoic mantle derived magmas is strongly influenced by the geochemical impact on the SCLM of events which took place during the Neoproterozoic and Cambrian. These events include rifting, passive margin development and orogenesis.Neoproterozoic to Cambrian basalts are widespread in western New South Wales, South Australia and Tasmania and reflect mantle decompression during extension and rifting of the Australian–East Antarctic Craton during the development of the proto-Pacific passive margin. These basalts fall into two regionally extensive and very different suites: (i) a voluminous suite of tholeiites and (ii) a highly undersaturated alkaline (nephelinite–basanite) series.Both Jurassic kimberlite magmas from the Adelaide Fold Belt and highly undersaturated Quaternary analcimites and basanites from the Mt. Gambier district of S.E. South Australia, have geochemical characteristics like those of the Precambrian–Cambrian alkaline suites. They have high concentrations of large ion lithophile (LIL), rare earth (RE) and high field strength (HFS) elements, and high HFSE/LILE and LREE/HREE ratios with T_DM^Nd values of 0.5–0.8 Ga. The Jurassic kimberlites appear to sample lithospheric mantle enrichment zones of Late Neoproterozoic to Early Cambrian age. The Quaternary suites result from mixing of contemporary mantle plume components with this old lithospheric enrichment, which is also identified with the occurrence of metasomatic phlogopite, amphibole and apatite in lherzolite mantle xenoliths from a number of Cainozoic volcanoes in Western Victoria.A very different type of lithospheric mantle enrichment took place during the late stages of the Ross–Delamerian Orogeny. This yielded a crustally contaminated mantle zone that mirrors the Cambro-Ordovician position of that orogen. This zone of contaminated lithospheric mantle interacted with a large plume in the Jurassic to yield the highly anomalous Ferrar–Tasmanian–Kangaroo Island basalts and dolerites.     

Post-collisional Plio-Pleistocene shoshonitic volcanism in the western Kunlun Mountains, NW China: Geochemical constraints on mantle source characteristics and petrogenesis

Major and trace element, Sr–Nd–Pb isotope and mineral chemical data are presented for post-collisional late Cenozoic shoshonitic volcanic rocks from the western Kunlun Mountains, NW China. They are distributed in two approximately E–W striking sub-belts, with the lavas in the southern sub-belt having been generated earlier than those in the northern sub-belt. The mineralogy of the rocks reflects crystallization from moderate temperature magmas (700–1000 °C) with high oxygen and water fugacities. They are geochemically characterized by relatively low TiO₂, Al₂O₃ and FeO and high alkalies coupled with very high contents of incompatible element concentrations. Remarkably negative Nb, Ta and Ti anomalies are displayed on primitive mantle-normalized incompatible element patterns. In addition, they show a relatively broad range of low ε_Nd (−1.8 to −8.7) at more restricted ⁸⁷Sr/⁸⁶Sr ratios (0.7081–0.7090). Pb isotopes are characterized by a range of ²⁰⁷Pb/²⁰⁴Pb (15.48–15.74) and ²⁰⁸Pb/²⁰⁴Pb (38.30–39.12) ratios at relatively invariant ²⁰⁶Pb/²⁰⁴Pb (18.60–18.83) values, except one sample with a ratio of 18.262, leading to near-vertical arrays. The lavas from the northern sub-belt have relatively high ⁸⁷Sr/⁸⁶Sr ratios. All lavas have extremely high La/Yb ratios, probably reflecting that the magmas were derived from a metasomatized lithospheric mantle source containing phlogopite–hornblende garnet peridotite affected by subducted sediments and hydrous fluids, rather than from a depleted asthenopheric mantle source or mantle plume source. However, the lavas from the southern sub-belt were derived from a lower degree of melting of more highly metasomatized sub-lithospheric mantle in comparison with those from the northern sub-belt. Processes responsible for partial melting of metasomatized lithospheric mantle and post-collision magmatism in the western Kunlun could be a consequence of continuously conductive heating of upwelling, hot asthenospheric mantle following the delamination subsequent to thickening, which is consistent with the spatial and temporal geochemical variations in shoshonitic rocks in Tibet.     

Pollution and paradigms: lessons from Icelandic volcanism for continental flood basalt studies

This paper is based on the premise that research into the environmental impact of continental flood basalt (CFB) volcanism has paid insufficient attention to the potential ecosystem damage that would result from the direct deposition of hundreds of megatons (Tg) of sulphur and other volatiles. The environmental impacts of the 1783 Laki Fissure eruption are reviewed in outline. It is shown that in a relatively brief period of volcanic activity, volatiles emitted by the eruption damaged and destroyed vegetation from the Arctic Ocean to the Mediterranean. Air pollution was so intense that human health was affected and the national death rate increased dramatically in both England and France.

It is proposed that the events of 1783 may be used as a paradigm for the environmental impacts of a CFB lava flow, and the emissions of 1783 are scaled up to illustrate this point. Thus, if a Laki style event were to erupt for a year it would approach the physical scale of a single episode of the Roza flow in the Columbia River CFB and potentially yield 576 Tg of sulphur gases which could have been oxidised into approximately 945 Tg of aerosol. This could generate a tropospheric aerosol mass of approximately 708 Tg H₂SO₄.

The ecosystem impact of the deposition of acids on this scale would be profound and, as with the actual Laki event, be continental in scale. All parts of the plant life cycle would be disrupted, including photosynthesis and fruiting. Inevitably, with the disruption of food webs animals would also be affected. Poorly buffered inland waters would be acidified, as would Boreal soils, reducing their biodiversity.

In our already polluted and interdependent world, any future event on this scale would have serious consequences for human health and trade.     

Geochemical evidence for paired arcs in the Permian volcanics of southern New Zealand

Although remnants of a Permian volcanic arc in the South Island of New Zealand have hitherto been considered to comprise a single volcanostratigraphic unit, recent work in the southern part of the arc has revealed the presence of two lithologically distinct terranes, only one of which is definitely Permian in age. Major and trace element analyses of representative specimens show that these terranes are geochemically distinct. Comparison with modern volcanic arcs indicates that this ancient arc in the South Island was a paired arc system made up of primitive island arc tholeiites to the east and more evolved andesitic rocks to the west.     

Biotic effects of late Maastrichtian mantle plume volcanism: implications for impacts and mass extinctions

During the late Maastrichtian, DSDP Site 216 on Ninetyeast Ridge, Indian Ocean, passed over a mantle plume leading to volcanic eruptions, islands built to sea level, and catastrophic environmental conditions for planktic and benthic foraminifera. The biotic effects were severe, including dwarfing of all benthic and planktic species, a 90% reduction in species diversity, exclusion of all ecological specialists, near-absence of ecological generalists, and dominance of the disaster opportunist Guembelitria alternating with low O₂-tolerant species. These faunal characteristics are identical to those of the K–T boundary mass extinction, except that the fauna recovered after Site 216 passed beyond the influence of mantle plume volcanism about 500 kyr before the K–T boundary. Similar biotic effects have been observed in Madagascar, Israel, and Egypt. The direct correlation between mantle plume volcanism and biotic effects on Ninetyeast Ridge and the similarity to the K–T mass extinction, which is generally attributed to a large impact, reveal that impacts and volcanism can cause similar environmental catastrophes. This raises the inevitable question: Are mass extinctions caused by impacts or mantle plume volcanism? The unequivocal correlation between intense volcanism and high-stress assemblages necessitates a review of current impact and mass extinction theories.