Distinct Magma Research Articles

Post-collisional ultrapotassic volcanism in the Tangra Yumco-Xuruco graben, south Tibet: Constraints from geochemistry and Sr–Nd–Pb isotope

Major-trace elements and Sr–Nd–Pb isotopic data are presented for newly discovered felsic ultrapotassic lavas in the Tangra Yumco-Xuruco graben, south Tibet. The felsic ultrapotassic lavas are characterized by high SiO 2 and K 2O, low MgO and CaO contents. They have also high abundances of incompatible trace elements such as large ion lithophile elements (LILEs) and light rare earth elements (LREEs), especially Rb (562–893 ppm), Th (148–182 ppm) and La (101–131 ppm). One of the most striking features of these rocks is their high Th/La (1.15–1.53) and Rb/Sr (1.0–1.26) ratios with pronounced Eu and Ce anomalies (Eu/Eu⁎ = 0.56–0.60, Ce/Ce⁎ = 0.73–0.90), while the high field strength elements (HFSEs) and Sr are depleted relative to LILEs and HREEs. The felsic ultrapotassic lavas show highly radiogenic 87Sr/ 86Sr (0.7200–0.7282) and unradiogenic 143Nd/ 144Nd (0.51191–0.51197), radiogenic 206Pb/ 204Pb (18.794–19.063), 207Pb/ 204Pb (15.697–15.824) and 208Pb/ 204Pb (39.455–39.954). The similarity of trace element patterns and Sr–Nd–Pb isotopic components with the high-MgO ultrapotassic rocks in the province suggests that both the felsic ultrapotassic and mafic ultrapotassic lavas were derived from partial melting of the south Tibetan lithospheric mantle metasomatized by sediment-derived melts. However, the remarkable differences in some key trace element pairs between the felsic ultrapotassic lavas and mafic ultrapotassic rocks in the Tangra Yumco-Xuruco graben indicate that the felsic ultrapotassic lavas were not the highly-differentiated products from the mafic ultrapotassic magmas. Our new data suggest that these felsic ultrapotassic lavas have a distinct magma source beneath the south Tibetan crust, which is different from that of the mafic ultrapotassic lavas. Their high light-REE enrichment (La/Yb ( N) = 41.98–51.41) but low heavy-REE fractionation (Dy/Yb ( N) = 1.99–2.11) suggest an amphibole and phlogopite-rich mantle source in the genesis of the potassic lavas. In contrast, the mafic ultrapotassic lavas in the graben have variable elevated Dy/Yb ( N) (2.1–2.8) and La/Yb ( N) (39–128) ratios, reflecting their sources with residual garnet. By implication, the discrepancy in their source assemblages suggests that the depths of the felsic ultrapotassic sources could be shallower than that of the mafic ultrapotassic sources in the south Tibetan lithosphere. In addition, proposed mantle metasomatism by subducted sediment-derived melts for the genesis of the ultrapotassic lavas might also be applicable for along-strike variation in the post-collisional volcanism in south Tibet. Isotopic mixing calculation illustrates that the subducted sediments represented by the Himalayan basement are an effective reservoir for bulk assimilation of the south Tibetan lithospheric mantle, and hence sediment input of various proportions into the mantle sources could be responsible for the isotopic variation seen in the post-collisional calc-alkaline and potassic volcanics in the eastern part of the southern Tibet. Along-strike variation in the post-collisional volcanism was most probably caused by a westward increase in sediment input in the southern Lhasa block during the subduction of the Tethyan oceanic lithosphere.

Evolution of a small intraplate basaltic lava field: Jerrabattgulla Creek, upper Shoalhaven River catchment, southeast New South Wales

The Jerrabattgulla Creek basalts are in the upper catchment of the Shoalhaven River of southeastern New South Wales. The basalts erupted into a narrow, north-draining valley and modified the local drainage system, re-routing the paleo-Jerrabattgulla Creek, preserving a series of sub-basaltic quartzose gravels with silcretes in the paleovalley. The paleovalley indicates that a north-flowing drainage existed in this place in the Miocene. The high-relief, narrow valley has preserved a volcanic stratigraphy allowing the magmatic evolution of this small lava field to be determined. The lavas have a large compositional range from olivine nephelinite through to quartz tholeiite, which is unusual in such a small lava field. They represent three distinct magma batches, most likely from an amphibole–apatite metasomatised sub-continental lithospheric mantle, and underwent fractional crystallisation of olivine, clinopyroxene and plagioclase and assimilated upper crust. The lava field underwent temporal change from dominantly alkaline, to mixed alkaline and subalkaline, to dominantly alkaline magmatism over the course of its evolution.

Age constraints on the Late Cretaceous alkaline magmatism on the West Iberian Margin

The onshore sector of the West Iberian Margin (WIM) was the locus of several cycles of magmatic activity during the Mesozoic, the most voluminous of which was of alkaline nature and occurred between 70 and 100 Ma. This cycle took place in a post-rift environment, during the 35° counter-clockwise rotation of Iberia and initiation of the alpine compression. It includes the subvolcanic complexes of Sintra, Sines, and Monchique, the volcanic complex of Lisbon and several other minor intrusions, covering an area of approximately 325 km 2. Previous cycles were tholeiitic and transitional in nature, occuring around 200 Ma and 130–135 Ma, respectively. New LA-ICP-MS U-Pb, 40Ar/ 39Ar, K-Ar and Rb-Sr ages on several intrusions distributed along the onshore WIM are presented, which combined with previously published data allows us to constrain the duration of the Late Cretaceous alkaline cycle to circa 22 Ma (94–72 Ma) and define two pulses of magmatic activity. The first one (94–88 Ma) occurred during the opening of the Bay of Biscay and consequent rotation of Iberia and clusters above N38°20′. The second pulse (75–72 Ma) has a wider geographical distribution, from N37° to N39°. This final pulse occurred during the initial stages of the Alpine orogeny in Iberia that led to the formation of the Pyrenees and Betics and to tectonic inversion of the Mesozoic basins. Isotope and trace element geochemistry point to a sublithospheric source for the alkaline magmatism that clearly distinguishes it from the previous cycles which had an important lithospheric mantle component. Also, it allows the discrimination between the two different alkaline pulses in terms of trace element abundance and residual mantle minerology. It is speculated that these differences might be the result of distinct magma ascent rates due to either more or less favourable tectonic settings that avoided or allowed the interaction with metasomatized lithosphere and equilibration with K rich minerals like amphibole and/or phlogopite.

Geochemistry of the lava and its implications in Musicians Seamounts

Chemical and isotopic data of the lava samples dredged in the southern Bach Ridge and the northern Italian Ridge of the Musicians Seamounts province, northeast of Hawaii. Although most of the samples analyzed are generally altered, a few are fresh. The latter exhibits similar geochemical and isotopic characteristics to normal MORB (Mid-Ocean Ridge Basalts). There are systematic geochemical trends from hotspot to mid-ocean ridge in the province. Incompatible element and isotopic variations suggest that the flow field had at least two distinct parental magmas, one with higher and one with lower MgO concentrations. The two parental magmas could be related by a magma mixing model. The major and trace element modeling shows that the two parental magmas could not have been produced by different degrees of melting of a homogeneous mantle source, but they are consistent with melting of a generally depleted mantle containing variable volumes of embedded enriched heterogeneity enriched interbeds.

The contemporaneous emission of low-K and high-K trachybasalts and the role of the NE Rift during the 2002 eruptive event, Mt. Etna, Italy

Mount Etna volcano erupted almost simultaneously on its northeastern and southern flanks between October 27 and November 3, 2002. The eruption on the northeastern flank lasted for 8 days, while on the southern flank it continued for 3 months. The northeastern flank eruption was characterized by the opening of a long eruptive fracture system between 2,900 and 1,900 m.a.s.l. A detailed survey indicates that the fractures’ direction shifted during the opening from N10W (at the NE Crater, 2,900 m) to N45E (at its lowest portion, 1,900 m) and that distinct magma groups were erupted at distinct fracture segments. Based on their petrological features, three distinct groups of rocks have been identified. The first group, high-potassium porphyritic (HKP), is made up of porphyritic lavas with a Porphyritic Index (P.I.) of 20–32 and K2O content higher than 2 wt%. The second group is represented by lavas and tephra with low modal phenocryst abundance (P.I. < 20) named here oligo-phyric (low-phyric), and K2O content higher than 2 wt% (HKO, high-potassium oligophyric). The third group, low-potassium oligophyric (LKO), consists of tephra with oligophyric texture (P.I. < 20) but K2O content < 2 wt%. K-rich magmas (HKP and HKO) are similar to the magma erupted on the southern flank, and geochemical variations within these groups can be accounted for by a variable degree of fractionation from a single parent magma. The K-poor magma (LKO), erupted only in the upper segment of the fracture, cannot be placed on the same liquid line of descent of the HK groups, and it is similar to the magmas that fed the activity of Etna volcano prior to the eruption of 1971. This is the first time since then that a magma of this composition has been documented at Mt. Etna, thus providing a strong indication for the existence of distinct batches of magma whose rise and differentiation are independent from the main conduit system. The evolution of this eruption provides evidence that the NE Rift plays a very active role in the activity of Mt. Etna volcano, and that its extensional tectonics allows the intrusion and residence of magma bodies at various depths, which can therefore differentiate independently from the main open conduit system.

Mesoarchean sanukitoid rocks of the Rio Maria Granite-Greenstone Terrane, Amazonian craton, Brazil

The Archean sanukitoid Rio Maria Granodiorite yielded zircon ages of ∼2.87 Ga and is exposed in large domains of the Rio Maria Granite-Greenstone Terrane, southeastern Amazonian craton. It is intrusive in the greenstone belts of the Andorinhas Supergroup, in the Arco Verde Tonalite and Caracol Tonalitic Complex (older TTGs). Archean potassic leucogranites, younger TTGs and the Paleoproterozoic granites of Jamon Suite are intrusive in the Rio Maria Granodiorite. The more abundant rocks of the Rio Maria Granodiorite have granodioritic composition and display medium to coarse even-grained textures. These rocks show generally a gray color with greenish shades due to strongly saussuritized plagioclase, and weak WNW-ESE striking foliation. The significant geochemical contrasts between the occurrences of Rio Maria Granodiorite in different areas suggest that this unit corresponds in fact to a granodioritic suite of rocks derived from similar but distinct magmas. Mingling processes involving the Rio Maria Granodiorite and similar mafic to intermediate magmas are able to explain the constant occurrence of mafic enclaves in the granodiorite. The associated intermediate rocks occur mainly near Bannach, where mostly quartz diorite and quartz monzodiorite are exposed. The dominant rocks are mesocratic, dark-green rocks, with fine to coarse even-grained texture. The Rio Maria Granodiorite and associated intermediate rocks show similar textural and mineralogical aspects. They follow the calc-alkaline series trend in some diagrams. However, they have high-Mg#, Cr, and Ni conjugate with high contents of large ion lithophile elements (LILEs), typical of sanukitoids series. The patterns of rare earth elements of different rocks are similar, with pronounced enrichment in light rare earth elements (LREEs) and strong to moderate fractionation of heavy rare earth elements (HREEs). Field aspects and petrographic and geochemical characteristics denote that the granodiorites and intermediate rocks have sanukitoid affinity. However, geochemical data suggest that the intermediate rocks and the granodiorites are not related by a fractional crystallization process. It is concluded that the intermediate rocks derived from similar sources to the granodiorites, but probably result from a higher degree of melting, being both cogenetic, but not comagmatic rocks. Mineralogical aspects associated with experimental evidence suggest that the Rio Maria Granodiorite magma was relatively water-enriched (>4 wt.%), explaining the presence of hornblende at the liquidus and the absence of clinopyroxene and orthopyroxene in the studied rocks. The occurrence of well-preserved magmatic epidote crystals, admitting that the Rio Maria Granodiorite was emplaced at shallow crustal levels, points to a rapid ascent of the Rio Maria Granodiorite magma.

Newly discovered submarine flank eruption at Stromboli volcano (Aeolian Islands, Italy)

The NW submarine portion of Stromboli volcano has been investigated by deep‐towed sidescan sonar, bathymetric surveys, video camera runs and dredging during two research cruises in 2002 and 2004. The surveys resulted in the identification of an extensive pillow lava field (106‐107m3) at about 2300 m of water depth and 9 km from the shoreline of Stromboli Island. The pillow lavas have a unique composition that does not match any known subaerial product, although a limited affinity exists with those erupted during the Neostromboli eruptive cycle of the island (13–6 ka). This is the first finding of a submarine eruption on the northern side of Stromboli and improves the knowledge of its flank activity and volcanic hazards. This eruption is interpreted as marking the onset of a new volcanic cycle from the edifice periphery fed by a new, distinct magma mixed with traces of the previous magma that survived the emptying of the Neostromboli magma chamber.

Geochemical and geochronological study of the Sanshui basin bimodal volcanic rock suite, China: Implications for basin dynamics in southeastern China

Sanshui basin is one of the typical Mesozoic–Cenozoic intra-continental rift basins with voluminous Cenozoic volcanic rocks in southeastern China. Thirteen cycles of volcanic eruptions and two dominant types of volcanic rocks, basalt and trachyte–rhyolite, have been identified within the basin. Both basalt and trachyte–rhyolite members of this bimodal suit have high values of εNd (+2.3 to +6.2) and different Sr isotopic compositions (initial 87Sr/ 86Sr ratios are 0.70461–0.70625 and 0.70688–0.71266 for basalts and trachyte–rhyolite, respectively), reflecting distinct magma evolution processes or different magma sources. The results presented in this study indicate that both of the trachyte–rhyolite and basaltic magmas were derived from similar independent primitive mantle, but experienced different evolution processes. The trachyte-rhyolitic magma experienced significant clinopyroxene and plagioclase fractionational crystallization from deeper magma chamber with significant crustal contamination, while the basaltic magmas experienced significant olivine and clinopyroxene fractionational crystallization in shallower magma chamber with minor crustal contamination. New zircon U–Pb dating confirms an initial volcanic eruption at 60 Ma and the last activity at 43 Ma. Geologic, geochemical, and geochronological data suggest that the inception of the Sanshui basin was resulted from upwelling of a mantle plume. The Sanshui basin widened due to subsequent east–west extension and the subsequent volcanism constantly occurred in the center of the basin. Evidence also supports a temporal and spatial association with other rift basins in southeastern China. The upwelling mantle plume became more active during late Cenozoic time and most likely triggered opening of other basins, including the young South China Sea basin.

Millennial timescale resolution of rhyolite magma recharge at Tarawera volcano: insights from quartz chemistry and melt inclusions

Most rhyolite eruption episodes of Tarawera volcano have emitted several physiochemically distinct magma batches (∼1–10 km3). These episodes were separated on a millennial timescale. The magma batches were relatively homogeneous in temperature and composition at pumice scale (>4 cm), but experienced isolated crystallisation histories. At the sub-cm scale, matrix glasses have trace element compositions (Sr, Ba, Rb) that vary by factors up to 2.5, indicating incomplete mixing of separate melts. Some quartz-hosted melt inclusions are depleted in compatible trace elements (Sr, Ti, Ba) compared to enclosing matrix glasses. This could reflect re-melting of felsic crystals deeper in the crystal pile. Individual quartz crystals display a variety of cathodoluminescence brightness and Ti zoning patterns including rapid changes in melt chemistry and/or temperature (∼50–100°C), and point to multi-cycle crystallisation histories. The Tarawera magma system consisted of a crystal-rich mass containing waxing and waning melt pockets that were periodically recharged by silicic melts driven by basaltic intrusion.

Contrasting complexities in the evolution of calc-alkaline and alkaline melts of the Nigde volcanic rocks, Turkey: textural, mineral chemical and geochemical evidence

The Nigde volcanic province in central Turkey comprises four separate stratovolcanoes (Tepekoy, Cinarli, Melendiz and Keciboyduran) of Mio-Pliocene age (MPv) and several monogenetic cones of Pleistocene age (Pv). The MPv rocks are composed of large volume of andesitic lavas and pyroclastics with medium to high-K calc-alkaline character. They are strongly porphyritic with phenocryst assemblages of plag + opx + cpx + Fe-Ti oxides + bio ± amph ± quartz, and commonly exhibit disequilibrium textures such as complexly zoned plagioclases, reversely zoned pyroxenes, and reacted amphiboles and biotites. Geothermobaro-metric studies based on mineral chemistry data suggest that the MPv calc-alkaline magmas underwent a complex evolutionary history of cooling, crystallisation and mixing within two distinct magma chambers sited in the lower and in the upper crust, respectively. The rocks of Pv include small volumes of alkaline basalts showing poorly porphyritic textures and equilibrated phenocryst assemblages of ol +cpx +lesser plag and Fe-Ti oxides. Textural features, and strong compositional variations in single phenocrysts from andesites clearly reflect magma mixing-mingling and open-system modifications. Major and trace element evidence also argues against dominant fractional crystallisation processes and supports magma mixing for andesitic magmas. In contrast, textural, mineral chemical and geochemical evidence for Pv alkaline rocks indicate that magmas had a relatively simple evolutionary history dominated by small degrees of fractional crystallisation of mafic minerals along volcanic conduits. MPv show typical island-arc trace element signatures, such as negative anomalies in high-field strength element (HFSE) and high ratios between HFSE and large ion lithophile elements (LILE). Slight HFSE negative anomalies are also observed in Pliocene alkaline basalt, suggesting contamination by arc component, either in the source or during alkaline magma ascent.

Discrimination of diamond resource and non-resource domains in the Victor North pyroclastic kimberlite, Canada

The Jurassic Victor kimberlite complex comprises several steep-sided pipes, including the Victor North pyroclastic kimberlite (VNPK) pipe which encompasses one of the main diamond resources in the mine plan. The VNPK pipe also contains some low-grade domains that appear to be non-economic. Distinguishing between the high- and low-grade units within this pipe is complicated by the gradational nature of the boundary. In this paper, detailed petrography and mineral chemistry analyses are used to establish reliable discriminators for the high- and low-grade kimberlite units. The high- and low-grade units are both spinel-bearing carbonate group-I kimberlites; dolomite is more abundant than calcite in all units. Based on textural relationships and composition, it is inferred that the carbonate is primary. Oxygen fugacity estimates on groundmass intergrowths of rutile and ilmenite indicate similar oxidation states for the magmas that produced the high- and low-grade kimberlite deposits. However, chromite core compositions within zoned groundmass spinels are markedly different between the high- and low-grade kimberlite units. In addition, compositions of groundmass ilmenite and dolomite show the same pattern as core compositions of chromite: the groundmass minerals in the low-grade unit have lower magnesium-numbers than corresponding phases in the high-grade unit. The chemical compositions of groundmass chromite and ilmenite suggest crystallisation with olivine, and thus record the oxygen fugacity associated with the early crystallisation history of the kimberlite magma. These observations suggest that the high- and low-grade units were formed from two distinct magma pulses that had a different origin in the asthenosphere and/or experienced different crystallisation histories. The similar oxygen fugacity estimates for the high- and low-grade kimberlite magmas indicate that differential resorption of diamond was not responsible for the grade difference. Therefore, it is most likely that the individual magma pulses entrained contrasting amounts of diamond during their ascent towards the surface, causing the observed grade differences in the VNPK.

Petrology of hypabyssal kimberlites: Relevance to primary magma compositions

Hypabyssal kimberlites are similar in their mineralogy and geochemistry across the world, and thus form a distinct magma type that is produced repeatedly in time and space. This small volume magma is derived from the asthenosphere (200–?650 km), and as it nears the surface consists of olivine macrocrysts, i.e. xenocrysts (~ 25 vol.%), olivine phenocrysts (~ 25 vol.%), entrained in a Ba–Ti–Mg-rich, Al-poor carbonate-silicate liquid containing high, but unknown amounts, of dissolved CO 2, CH 4 and H 2O. Hypabyssal kimberlites are shown to be hybrid rocks formed by mixing with, and assimilation of, mantle-derived lherzolite and harzburgite. Orthopyroxene is not stable in these magmas and its resorption leads the precipitation of primary liquidus olivine. The magma crystallizes rapidly at crustal levels to uniform holocrystalline rocks that do not contain glass. In addition to spinel, perovskite, monticellite, apatite, and phlogopite–kinoshitalite, primary carbonates are typically present as low pressure liquidus phases. Serpentines (primary polygonal serpentine and chrysotile) and carbonate commonly crystallize together from volatile-rich (CO 2 + H 2O) deuteric residua as a polycrystalline mesostasis or discrete segregations; the latter are not in-filled vesicles. Kimberlite magmas have a very long crystallization interval ranging from > 1200 to ~ 300 °C, and there is a continuum from the magmatic crystallization of silicate and oxide minerals to carbo-hydrothermal residua forming the calcite-serpentine mesostasis. It is concluded that there are no hypabyssal kimberlites which are representatives of the initial primary magma; all kimberlites are hybrid and contaminated magmas which have undergone crystallization in the mantle prior to crystallization of the groundmass-mesostasis assemblages.

Oxygen Isotopic Variations in the Clinopyroxene from the Filicudi Volcanic Rocks (Aeolian Islands, Italy): Implications for Open-System Magma Evolution

Oxygen isotope data are reported for clinopyroxene phenocrysts from volcanic rock samples from Filicudi is- land, Italy, with the aim of investigating mechanisms of magma evolution and mantle source characteristics in a continen- tal arc volcano. Filicudi rocks range from calc-alkaline basalt to high-K andesite and dacite. Mafic rocks have MgO � 6-7 wt%, Mg# � 60, Ni < 50 ppm and Cr < 200 ppm, suggesting they represent evolved melts from more primitive man- tle-derived parents. Variations of Sr-Nd isotope ratios against silica and MgO suggest magma evolution under open- system. Oxygen isotope ratios on clinopyroxene phenocrysts from representative rocks are in the range  18 Ocpx = + 5.37 to + 6.20 ‰ SMOW, corresponding to  18 Omelt = + 5.6 to + 6.4 ‰ and display an overall, but poorly defined, positive corre- lation with 87 Sr/ 86 Sr and MgO and a rough negative correlation with SiO2. Basalts exhibit the highest  18 Ocpx values for the volcanic series studied. Together, major, trace element and isotopic data indicate complex, multistage polybaric evolu- tionary processes for the Filicudi magmas. A process of fractional crystallisation, accompanied by variable degrees of crustal assimilation, best explains the isotopic and petrological data of the Filicudi volcanics. Evolution processes are in- ferred to have occurred in distinct magma reservoirs sited at different depths. Magma contamination affected basalts more extensively than andesitic and dacitic magmas. This was a consequence of the higher temperature and lower viscosity of mafic melts that were able to assimilate higher amounts of crustal wall rocks. Therefore, andesites and dacites preserved stable and radiogenic isotopic compositions more closely than basalts. Combined trace element and isotopic data suggest that primitive magmas at Filicudi were generated in a heterogeneous and metasomatised mantle source, which underwent contamination by slab-derived fluids. When compared with the nearby islands, isotopic variations of Filicudi rocks re- semble more closely those found at Alicudi than at Salina and Vulcano. This suggests that the same kind of evolutionary processes occurred in the western island of Filicudi and Alicudi, whereas at Salina and Vulcano, in the central arc, mag- mas typically evolved by AFC processes, generating derivative melts that display higher Sr-O isotope compositions com- pared to parent basalts. These contrasting styles of magma evolution may be related to different structures of the plumbing systems of volcanic islands that were constructed in different sectors of the arc.

40Ar/ 39Ar age and geochemistry of the post-collisional Miocene Yamadağ volcanics in the Arapkir area (Malatya Province), eastern Anatolia, Turkey

The Neogene Yamadağ volcanics occupy a vast area between Sivas and Malatya in eastern Anatolia, Turkey. These volcanic rocks are characterized by pyroclastics comprising agglomerates, tuffs and some small outcrops of basaltic–andesitic–dacitic rocks, overlain upward by basaltic and dacitic rocks, and finally by basaltic lava flows in the Arapkir area, northern Malatya Province. The basaltic lava flows in the Arapkir area yield a 40Ar/ 39Ar age of 15.8 ± 0.2 Ma, whereas the dacitic lava flows give 40Ar/ 39Ar ages ranging from 17.6 through 14.7 ± 0.1 to 12.2 ± 0.2 Ma, corresponding to the Middle Miocene. These volcanic rocks have subalkaline basaltic, basaltic andesitic; alkaline basaltic trachyandesitic and dacitic chemical compositions. Some special textures, such as spongy-cellular, sieve and embayed textures; oscillatory zoning and glass inclusions in plagioclase phenocrysts; ghost amphiboles and fresh biotite flakes are attributable to disequilibrium crystallization related to magma mixing between coeval magmas. The main solidification processes consist of fractional crystallization and magma mixing which were operative during the soldification of these volcanic rocks. The dacitic rocks are enriched in LILE, LREE and Th, U type HFSE relative to the basaltic rocks. The basaltic rocks also show some marked differences in terms of trace-element and REE geochemistry; namely, the alkaline basaltic trachyandesites have pronounced higher HFSE, MREE and HREE contents relative to the subalkaline basalts. Trace and REE geochemical data reveal the existence of three distinct magma sources – one subalkaline basaltic trachyandesitic, one alkaline basaltic and one dacitic – in the genesis of the Yamadağ volcanics in the Arapkir region. The subalkaline basaltic and alkaline basaltic trachyandesitic magmas were derived from an E-MORB type enriched mantle source with a relatively high- and low-degree partial melting, respectively. The magmatic melt of dacitic rocks seem to be derived from an OIB-type enriched lithospheric mantle with a low proportion of partial melting. The enriched lithospheric mantle source reflect the metasomatism induced by earlier subduction-derived fluids. All these coeval magmas were generated in a post-collisional extensional geodynamic setting in Eastern Anatolia, Turkey.

Magma chamber evolution prior to the Campanian Ignimbrite and Neapolitan Yellow Tuff eruptions (Campi Flegrei, Italy)

The Campi Flegrei (Campanian Region, Italy) experienced two cataclysmic caldera-forming eruptions which produced the Campanian Ignimbrite (39 ka, CI) and the Neapolitan Yellow Tuff (15 ka, NYT). We studied the minor eruptions before both these large events to understand magma chamber evolution leading towards such catastrophic eruptions. Major, trace element, and Sr and Nd isotope compositions of pre-Campanian Ignimbrite and pre-Neapolitan Yellow Tuff products define distinct geochemical groups, which are here interpreted as distinct magma batches. These batches do not show any transitional trend towards the CI and NYT eruptions. The CI and NYT systems are decoupled geochemically and isotopically. At least one of the pre-CI and one of the pre-NYT erupted magma batches qualifies as mixing endmembers for the large CI and NYT eruptions, and thus, must have been stored in reservoirs for some time to remain available for the CI and NYT eruptions. The least evolved, isotopically distinct magma compositions that are typical of the last phases of the NYT and CI eruptions did not occur before caldera-forming events. Based on the new data, we propose the following scenario: Multiple magma chambers with distinct compositions existed below the Campi Flegrei before the CI and NYT eruptions and remained generally separated for some time unless new magma was recharged. In each case, one of the residing magma reservoirs was recharged by a new large-volume magma input of intermediate composition from a deeper differentiating magma reservoir. This may have triggered the coalescence of the previously separated reservoirs into one large chamber which fed the cataclysmic caldera-forming eruption. Large magma chambers in the Campi Flegrei may therefore be ephemeral features, interrupted by periods of evolution in individual, separated magma reservoirs.

Compositionally diverse magmas erupted close together in space and time within a Karoo flood basalt crater complex

Geochemical data and mapping from a Karoo flood basalt crater complex reveals new information about the ascent and eruption of magma batches during the earliest phases of flood basalt volcanism. Flood basalt eruptions at Sterkspruit, South Africa began with emplacement of thin lava flows before abruptly switching to explosive phreatomagmatic and magmatic activity that formed a nest of craters, spatter and tuff rings and cones that collectively comprise a crater complex >40 km2 filled by 9–18 km3 of volcaniclastic debris. Rising magma flux rates combined with reduced access of magma to external water led to effusion of thick Karoo flood basalts, burying the crater-complex beneath the >1.5 km-thick Lesotho lava pile. Geochemical data is consistent with flood basalt effusion from local dikes, and some lava flows likely shared or re-occupied vent sites active during explosive eruptions at Sterkspruit. Flood basalt magmas involved in Sterkspruit eruptions were chemically heterogenous. This study documents the rapid (perhaps simultaneous) eruption of three chemically distinct basaltic magmas which cannot be simply related to one another from one vent site within the Sterkspruit crater complex. Stratigraphic and map relationships indicate that eruption of the same three magma types took place from closely spaced vents over a short time during formation of the bulk of the crater-complex. Two magma types recognized there have not been recognized in the Karoo province before. The variable composition of flood basalts at Sterkspruit argues that magma batches in flood basalt fields may be small (0.5–1 km3) and not simply related to one another. This implies in turn that heterogeneities in the magma source region may be close to each other in time and space, and that eruptions of chemically distinct magmas may take place over short intervals of space and time without significant hybridisation in flood basalt fields.

Volatile-induced magma differentiation in the plumbing system of Mt. Etna volcano (Italy): evidence from glass in tephra of the 2001 eruption

Mount Etna volcano was shaken during the summer 2001 by one of the most singular eruptive episodes of the last centuries. For about 3 weeks, several eruptive fractures developed, emitting lava flows and tephra that significantly modified the landscape of the southern flank of the volcano. This event stimulated the attention of the scientific community especially for the simultaneous emission of petrologically distinct magmas, recognized as coming from different segments of the plumbing system. A stratigraphically controlled sampling of tephra layers was performed at the most active vents of the eruption, in particular at the 2,100 m (CAL) and at the 2,550 m (LAG) scoria cones. Detailed scanning electron microscope and energy dispersive x-ray spectrometer (SEM-EDS) analyses performed on glasses found in tephra and comparison with lava whole rock compositions indicate an anomalous increase in Ti, Fe, P, and particularly of K and Cl in the upper layers of the LAG sequence. Mass balance and thermodynamic calculations have shown that this enrichment cannot be accounted for by “classical” differentiation processes, such as crystal fractionation and magma mixing. The analysis of petrological features of the magmas involved in the event, integrated with the volcanological evolution, has evidenced the role played by volatiles in controlling the magmatic evolution within the crustal portion of the plumbing system. Volatiles, constituted of H2O, CO2, and Cl-complexes, originated from a deeply seated magma body (DBM). Their upward migration occurred through a fracture network possibly developed by the seismic swarms during the period preceding the event. In the upper portion of the plumbing system, a shallower residing magma body (ABT) had chemical and physical conditions to receive migrating volatiles, which hence dissolved the mobilized elements producing the observed selective enrichment. This volatile-induced differentiation involved exclusively the lowest erupted portion of the ABT magma due to the low velocity of volatiles diffusion within a crystallizing magma body and/or to the short time between volatiles migration and the onset of the eruption. Furthermore, the increased amount of volatiles in this level of the chamber strongly affected the eruptive behavior. In fact, the emission of these products at the LAG vent, towards the end of the eruption, modified the eruptive style from classical strombolian to strongly explosive.

Early to Middle Miocene intra-continental basaltic volcanism in the northern part of the Arabian plate, SE Anatolia, Turkey: geochemistry and petrogenesis

Continental basalts ranging in age from 16.5 to 19.08 Ma crop out throughout the northern part of the Arabian plate. The basalts have distinctive petrographic characteristics such as rounded and skeletal olivine phenocrysts with abundant melt inclusions, implying the mixing of two distinct magmas. All of the analysed basalts are tholeiitic in composition. The presence of quartz xenocrysts with clinopyroxene rims in some samples indicates that crustal assimilation was probably an important process during magma ascent to the surface, and low Mg number and high SiO2 contents of the basalts clearly show that they have experienced fractional crystallization as well as crustal contamination. Variations of the major and trace elements versus MgO show that olivine+clinopyroxene+plagioclase were the main fractionating minerals. In terms of incompatible trace elements, the basalts have OIB-like signatures with a slight depletion at Nb–Ta on primitive-mantle-normalized diagrams. The basalts have slightly LREE enriched patterns with La/YbN = 5.5 to 6.7. La/Nb ratios are close to unity, suggesting the melts may have originated in the asthenospheric mantle. Partial melting modelling based on REE data imply that the melts were not produced from a single mantle source depth, which is either purely a spinel- or garnet-peridotite end member. The samples lie on a binary mixing line between low-degree melts (&lt;5%) from garnet-peridotite and higher-degree melts (&gt;10%) from spinel-peridotite sources on a plot of La/Yb v. Dy/Yb, requiring interaction of melts derived from both garnet- and spinel-peridotite fields. Melts originating from both sources were initially tapped by distinct magma chambers, which subsequently hybridized into a single flow. Hybridized magma ascended to the surface along Neogene strike-slip faults, which are linked to the Dead Sea Fault Zone.

Trace element constraints on mantle sources during mid-Proterozoic magmatism: evidence for a link between the Gardar (South Greenland) and Abitibi (Canadian Shield) mafic rocks

Trace and major element compositions of mid-Proterozoic (1.20–1.16 Ga) basaltic lava flows and dikes from the Gardar Province (South Greenland) provide evidence for two geochemically distinct magma sources. Based on distinct features of incompatible trace element ratios, such as Th/Ta, Th/Tb, or Th/Hf, they differ by the composition of their mantle source and by their partial melting trends. One mantle source is compositionally transitional between mid-ocean ridge basalt (MORB)-type and ocean-island basalt (OIB)-type sources with relatively low Ta/Hf ratios (~0.2), moderate enrichment in light rare-earth elements (LREE), and slightly positive initial εNdvalues (+2). It can be attributed to either a lithospheric mantle source or a depleted astenospheric mantle plume component that has been enriched shortly prior to eruption. The other mantle source is characterized by high Ta/Hf ratios (~0.6), a more pronounced LREE enrichment, and initial εNdvalues around 0. Elevated CeN/YbN(7.0–9.8) and TbN/YbNratios (1.6–1.8) of the rocks derived from this source indicate the presence of garnet during melting, suggesting melt generation at depths &gt; 70 km. This mantle source has the geochemical characteristics of an OIB-type source and is interpreted as originating from a mantle plume. Samples from the slightly younger (1.14 Ga) Abitibi dike swarm (Superior Province, Canada), spatially connected to the Gardar Province, show very similar trace element characteristics and the same two distinct magma sources can be identified. The geochemical similarities between the magma sources in South Greenland and Canada support the idea of a genetic link between the two magmatic provinces. This link strengthens the idea that the system was a long-lived major intracontinental rift zone.

Magmatic History of Somma–Vesuvius on the Basis of New Geochemical and Isotopic Data from a Deep Borehole (Camaldoli della Torre)

A continuous-coring borehole recently drilled at Camaldoli dellaTorre on the southern slopes of Somma^Vesuvius provides constraints on the volcanic and magmatic history of the Vesuvian volcanic area since c. 126 ka BP.The cored sequence includes volcanic units, defined on stratigraphical, sedimentological, petrological and geochemical grounds, emitted from both local and distal vents. Some of these units are of known age, such as one Phlegraean pre-Campanian Ignimbrite, Campanian Ignimbrite (39 ka), Neapolitan Yellow Tuff (14 9 ka) and Vesuvian Plinian deposits, which helps to constrain the relative age of the other units.The main rock types encountered are shoshonite, phonotephrite, latite, trachyte and phonolite. The sequence includes, from the base upwards: a thick succession of pyroclastic units emplaced between 126 and 39 ka, most of them attributed to eruptions that occurred in the Phlegraean area; the Campanian Ignimbrite; the products of a local tuff cone formed between 39 ka and the deposition of the products of the earliest activity of the Mt. Somma volcano; the products of the Somma^Vesuvius volcano, which include from the base upwards a thick sequence of lavas, pyroclastic rocks and the products of a local spatter cone dated between 3 7 ka and AD 79.The data obtained from the study of the borehole show that, before the Campanian Ignimbrite eruption, low-energy explosive volcanism took place in the Vesuvian area, whereas mostly high-energy explosive eruptions characterized the Campi Flegrei activity. In the Vesuvian area, Campanian Ignimbrite deposition was followed by the eruption of a local tuff cone and a long repose time, which predated the formation of the Mt. Somma edifice. Since 18 3 ka (Pomici di Base eruption) the activity of Somma^Vesuvius became mostly explosive with rare lava effusions.The shallowest cored deposits belong to the Camaldoli dellaTorre cone, formed between the Pomici di Avellino and Pomici di Pompei eruptions (3 7 ka^AD 79).Newgeochemical and Sr^Nd^Pb^ B-isotopic data on samples from the drilled core, together with those available from the literature, allow us to further distinguish the volcanic rocks as a function of both their provenance (i.e. Phlegraean or Vesuvian areas) and age, and to identify different magmatic processes acting through time in the Vesuvian mantle source(s) and during magma ascent towards the surface. Isotopically distinct magmas, rising from a mantle source variably contaminated by slabderived components, stagnated at mid-crustal depths (8^10 km below sea level) where magmas differentiated and were probably contaminated. Contamination occurred either with Hercynian continental crust, mostly during the oldest stages of Vesuvian activity (from 39 to 16 ka), or with Mesozoic limestone, mostly during recent Vesuvian activity. Energy constrained assimilation and fractional crystallization (EC-AFC) modelling results show that contamination with Hercynian crust probably occurred during differentiation from shoshonite to latite. Contamination with limestone, which is not well constrained with the available data, might have occurred only during the transition from shoshonite to tephrite. From the ‘deep’ reservoir, magmas rose towards a series of shallow reservoirs, in which they differentiated further, mixed, and fed volcanic activity.