Calc-alkaline Samples Research Articles

Partitioning and isotopic fractionation of Li between mineral phases and alkaline to calc-alkaline melts of explosive and effusive eruptions

Evaluating the lithium (Li) budget of alkaline to calc-alkaline melts and their co-existing mineral phases is crucial to understanding which magmatic or post-eruptive processes can influence the Li contents and isotopic compositions of volcanic deposits. This is also important to be able to better constrain how economically relevant Li deposits form. The lithium contents of many different eruptions from different tectonic settings (intraplate, subduction zone, extensional environment) and cooling environments (quenched, quickly cooled and slowly cooled) were studied by combining new and literature data that were obtained using LA-ICP-MS. We found that the Li concentrations in melt inclusions and re-entrants are independent of tectonic setting (hotspot, subduction zone and rifting) and rarely exceed 100 ppm. When melt inclusions are re-homogenised the Li concentration increases which can lead to the overestimation of the Li budget of a magma.The highest Li concentrations in the alkaline suite of samples were found in groundmass glass (reaching up to 318 ppm). The Li contents in the co-existing mineral phases decrease in the following order: biotite (0.8–99.3 ppm), plagioclase (0.9–25.1 ppm), clinopyroxene (0.6–31.0 ppm) and sanidine (0.5–38.7 ppm). The highest Li concentrations in the calc-alkaline suite of samples were found in the biotite crystals (10.2–2314 ppm) followed by the groundmass glass (2.1–176 ppm). The Li contents are lower in the other mineral phases, from plagioclase (1.4–35.0 ppm), quartz (3.2–21.1 ppm), olivine (1.0–16.3 ppm), sanidine (0.3–8.1 ppm) to clinopyroxene (0.7–21.8 ppm). The Li concentrations in groundmass glass and co-existing crystal phases are independent of the tectonic setting but appear to be more closely related to their cooling environment. To evaluate the influence of the different cooling environments the Li isotopic compositions (per mil deviation of 7Li/6Li ratio relative to L-SVEC reference material) of different eruptions were also determined. The Li isotopic compositions of groundmass glass and mineral phases reveal a range in δ7Li composition spanning up to 6 ‰ for alkaline samples and >10‰ between bulk sample and mineral phases for calc-alkaline samples. The cooling environment of the samples has a large impact on the Li isotopic compositions of the mineral phases with only quartz seemingly unaffected. Rapidly quenched samples may retain their magmatic δ7Li compositions or will only experience small amounts of diffusional overprinting (e.g., by degassing) whereas slowly cooled samples will experience a prolonged diffusional exchange between minerals and matrix overprinting their magmatic δ7Li compositions.The Li inventory in quartz crystals may be influenced by the tectonic setting and water concentration in the crystals. To further assess the influence of tectonic settings on Li behaviour we used a large data set which is based on the 13 alkaline to calc-alkaline eruptions from different tectonic settings and calculated the apparent partition coefficients (Kds) between groundmass glass and co-existing mineral phases. Biotite crystals (alkaline systems) return the highest Kds (subduction-extensional setting: 0.01–0.57, oceanic hotspot: 0.17–2.65) whereas sanidine crystals yield the lowest Kds. Sanidines from the subduction-extensional setting (alkaline trend) have the lowest Kds (0.01–0.07; one outlier ranging from 0.02 to 0.24), followed by the oceanic hotspot setting (alkaline trend, 0.02–0.09) and the extensional setting (calc-alkaline trend; 0.01–0.15). Plagioclase crystals from the subduction-extensional setting (alkaline trend) yield Kds between 0.01 and 0.29, crystals from subduction zone settings (0.06–0.81) and extensional settings (0.09–0.50) have larger ranges in their Kds. The clinopyroxene crystals from the subduction zone settings (calc-alkaline trend) have the largest range in Kds (0.03–0.99), followed by the oceanic hotspot setting (alkaline trend, 0.06–0.48) and subduction-extensional setting (alkaline trend, 0.01–0.22). Quartz crystals (only from the calc-alkaline trend) from subduction zone settings have lower Kds (0.09–0.13) compared to the extensional settings (Bishop Tuff: 0.14–0.28; Caetano Tuff: 0.73–0.94). The wide variety of processes that can affect final Li abundances in melt and crystals (degassing, scavenging into fluids, intra-crystal diffusion, hydration of glasses) makes estimating the partition coefficients particularly difficult and results in the wide ranges observed here. Targeted experimental work would thus help to better constrain the behaviour of Li in alkaline compositions.

Permian post-collisional basic magmatism from Corsica to the Southeastern Alps

Post-Variscan Early Permian magmatism is widespread from Corsica to the Eastern Alps. After the formation of the Variscan orogenic belt between Laurussia and Gondwana, felsic and mafic bodies were emplaced during an extensional tectonic phase. This study focuses on a mafic dyke swarm that intruded in the region of Ajaccio (Corsica, France) and on a gabbroic intrusive complex outcropping in the Eastern Alps (Bressanone/Brixen, South Tyrol, Italy) both of which were emplaced during this extensional event. New U-Pb data from zircon show that both of these intrusions were emplaced at ca. 282 Ma. Most Ajaccio dykes display a calc-alkaline affinity and are characterized by enrichment of LILE (Large Ion Lithophile Elements) and LREE over HREE (Light and Heavy Rare Earth Elements) and HFSE (Nb, Ta, Hf, Zr; High Field Strength Elements). Two dykes show tholeiitic affinity and are characterized by marked LREE and Th-U depletion and by minor negative Nb-Ta anomalies on multi-element diagrams, which is typical for within-plate basalts. Bressanone/Brixen gabbros display similar geochemical features to the Ajaccio calc-alkaline samples with LREE enrichment over MREE (Medium Rare Earth Elements) and HREE, and HFSE depletion. Such characteristics are also confirmed by trace element data obtained by LA-ICP-MS on clinopyroxene and plagioclase crystals.The calc-alkaline to tholeiitic dykes are characterized by enriched to depleted Sr-Nd-Pb isotopic compositions, respectively. Some Ajaccio dykes and the Bressanone/Brixen gabbros are clearly affected by crustal contamination. However, the combination of trace element and isotopic variations suggests that mantle source heterogeneities were also involved. Isotopic compositions of the Ajaccio dykes reveal the presence of two distinct mantle source signatures: an enriched mantle (EM) component, which dominates the composition of the calc-alkaline-like suite, while a DMM (Depleted MORB Mantle) component dominates the source of the tholeiitic suite. The EM mantle source likely formed during Variscan subduction, however the composition of the tholeiitic suite requires a source composed dominantly of depleted mantle, with a minor contribution from both the subducting slab and supra-subduction mantle wedge. The mantle source of the tholeiitic magmas possibly originated from greater depths (compared to the calc-alkaline-like suite) during the post-orogenic extensional phase potentially due to break-off and/or roll-back of the Variscan slab or mantle flow from the front or margins of the subducted slab.Notably, coeval Permian mafic intrusive bodies from throughout Corsica (Bocca di Tenda, Porto, Pila Canale) and from the Australpine and Southalpine Alps intruded at a similar time and have geochemical features that are very similar to the Ajaccio dyke swarm and Brixen gabbro. This indicates that a widespread Permian magmatic province developed in a post-orogenic extensional tectonic setting at the margin of the former Variscan belt. During the Middle-Late Triassic magmatism with similar enriched geochemical signatures occurred in the Southern Alps, Dinarides, Australpoine domains and at the end of the Triassic over central Pangea (Central Atlantic Magmatic Province). This would highlight the importance that the Variscan orogeny had in the development of a more fertile mantle.

Age and geological context of the Barby Formation, a key volcanic unit in the Mesoproterozoic Sinclair Supergroup of southern Namibia

Abstract Volcanic and sedimentary rocks of the Sinclair Supergroup occur in the Konkiep Terrane of Southern Namibia. Three volcanic and sedimentary cycles are recognised. In this work we describe and date volcanic rocks of the Barby Formation, a key unit in the Sinclair area. The coeval Spes Bona Syenite and the Tiras Granite Gneiss are also described and dated. The rock types in the Barby Formation are rhyolites, basaltic trachyandesites, trachybasalts and trachydacites as well as volcanoclastic rocks. The rocks are largely undeformed and partly altered by deuteric and contact metamorphic processes but not regionally metamorphosed. Our samples represent both the calc-alkaline and alkaline trends documented in previous work. U-Pb ion probe and laser ablation inductively coupled plasma (LA-ICP) multicollector mass spectrometer Lu-Hf microbeam analyses were made of zircon and baddeleyite grains from four samples. A felsic tuff sample from the base of the Barby Formation has a 207Pb/206Pb zircon age of 1214 ± 5 Ma (2σ). A rhomb porphyry sample from the top of an 8.5 km-thick stratigraphic section gives a 207Pb/206Pb baddeleyite age of 1217 ± 2 Ma. The Spes Bona Syenite which intrudes the top of the Barby Formation has a 207Pb/206Pb baddeleyite age of 1217 ± 3 Ma and an indistinguishable LA-ICP collision cell mass spectrometer Rb-Sr biotite isochron age of 1238 ± 20 Ma, showing that there was no >350°C regional metamorphic event. Multi-element diagrams for the calc-alkaline samples show a dominant signature of reworked crust which is superimposed on a possible subduction signature. However the alkaline samples contain clear subduction signatures which are not seen in the underlying 1.37 Ga Kumbis rhyolite. The Barby Formation samples and coeval Spes Bona Syenite have Lu-Hf crustal residence ages between 1682 and 1873 Ma, suggesting that both of these units formed from a mixture of juvenile mantle-derived and older crustal material. The Barby Formation is considered to have originated due to a subduction event which took place during the assembly of the Rodinia supercontinent. The duration of the Barby magmatic episode is constrained to a maximum 9 m.y. period between 1219 and 1210 Ma, and during this period the Konkiep Terrane was an active continental margin. The 1204 ± 9 Ma Tiras Granite Gneiss is slightly younger than the Barby Formation and intruded across the Lord Hills Shear Zone, which is the suture between the hardly metamorphosed Konkiep Terrane and the highly metamorphosed Grunau Terrane of the Namaqua-Natal Province. Its intrusion reflects the end of subduction-related volcanism, due to the collision of Namaqua terranes with the Konkiep Terrane.

Petrologic evolution of Miocene-Pliocene mafic volcanism in the Kangal and Gürün basins (Sivas-Malatya), central east Anatolia: Evidence for Miocene anorogenic magmas contaminated by continental crust

This study discusses the geochemical features of the Early-Middle Miocene and Pliocene basaltic (SiO2 = ~46–52; MgO = ~6–10 wt%) to andesitic (SiO2 = ~59; MgO = ~4 wt%) rocks exposed in the Gürün and Kangal basins (Sivas, eastern part of central Anatolia), respectively. The basaltic rocks are characterized by alkaline to tholeiitic affinities, while the more evolved andesitic samples show calc-alkaline affinity. Trace element variations reveal that they can be evaluated in three sub-groups, each represented by different contents of trace elements for given Nb contents. Primary magmas of each groups were likely produced by different degrees of partial melting (~1–2, 2–3, 7–10% respectively) from a common mantle source, subsequently underwent different degrees of fractionation and crustal contamination. Derivation from a common mantle source of the primitive magmas of each group is supported by similar Sr, Nd and Pb isotopic ratios. Increasing degrees of partial melting seem to be responsible for the alkaline to tholeiitic variation among the basaltic samples, while higher degrees of crustal contamination (AFC) resulted in calc-alkaline affinity of the more evolved samples.Most primitive Pliocene samples show intra-plate (anorogenic) geochemical features, while the more evolved Miocene calc-alkaline samples resemble geochemically subduction-related (orogenic) magmatic rocks. However, on the basis of detailed geochemical models, we propose that the calc-alkaline affinity among the Miocene samples can also be gained by crustal contamination of their primary magmas which were also anorogenic in character. If this is true, overall, the Miocene and Pliocene basaltic to andesitic rocks in the Gürün and Kangal basins appear to may have formed by variable degrees of partial melting of a common anorogenic mantle that had not been subject to subduction-related metasomatism. This is an alternative approach to the general view assuming the Early-Middle Miocene magmatic activity in the region was derived from subduction-modified mantle sources in response to subduction of the Arabian Plate under the Anatolian Plate. This hypothesis further implies that either delamination of the sub-continental lithosphere or slab break-off processes beneath the central to eastern Anatolia might took place well before the Miocene, thus allowing upwelling unaltered mantle to provide the source of the Miocene to Pliocene volcanic rocks.

Subduction-related Volatile Recycling and Magma Generation beneath Central Mexico: Insights from Melt Inclusions, Oxygen Isotopes and Geodynamic Models

The subduction-related Michoaca¤n^Guanajuato Volcanic Field (MGVF) in central Mexico contains 900 cinder cones and numerous larger shield volcanoes of Late Pliocene to Holocene age.We present data for major, trace and volatile (H2O, CO2, S, Cl) elements in olivine-hosted melt inclusions from eight calc-alkaline cinder cones with primitive magma characteristics and one more evolved alkali basalt tuff ring.The samples span a region extending from the volcanic front to 175 km behind the front. Relationships between H2O and incompatible trace elements are used to estimate magmatic H2O contents for 269 additional volcanic centers across the MGVF and central Mexico. The results show that magmatic H2O remains high (3^575 wt %) for large distances (150 km) behind the front. Chlorine and S concentrations are strongly correlated with melt H2O and are also high across most of the arc (700^1350 ppm Cl, 1500^2000 ppm S).The alkali basalt, located far behind the front (175 km), has much lower volatile contents (515wt%H2O, 200 ppm Cl, 500 ppm S), and is compositionally similar to other melts erupted in this region. Oxygen isotope ratios of olivine phenocrysts (56^6o) from the calc-alkaline samples are higher than for typical mantle-derived magmas but do not vary systematically across the arc. Calc-alkaline samples have high large ion lithophile element concentrations relative to Nb andTa, as is typical of subduction-related magmas, but alkali basalt samples far behind the front have high Nb and Ta and lack enrichments in fluidmobile elements. Modeling based on volatiles and trace elements suggests that the calc-alkaline magmas were generated by 6^15% partial melting of a variably depleted mantle wedge that was fluxed with H2O-rich components from the subducted slab. In contrast, the alkali basalts formed by small degrees of decompression melting of an ocean island basalt source that had not been fluxed by slabderived components. Based on high d18Oolivine values and trace element characteristics, the H2O-rich subduction components added to the mantle wedge beneath the MGVF are likely to be mixtures of oceanic crust derived fluids and sediment melts. Integrating these results with new 2-D thermo-mechanical models of the subduction zone beneath the MGVF, we demonstrate that the present-day plate configuration beneath theMGVFcauses fluids to be released beneath the forearc and volcanic front, and that sediment melts can be produced beneath the volcanic front by the waning stages of fluid released from the oceanic crust percolating through already dehydrated sediments. Down-dragging of serpentine- and chloritebearing peridotite in the lowermost mantle wedge probably plays a role in fluid transport from the forearc to beneath the arc. H2O-rich magmas located more than 50 km behind the volcanic front can be explained by mantle hydration related to a shallower slab geometry that existed at 3 Ma. Rollback of the slab over the last 2 Myr has resulted in strong mantle advection that forms low-H2O, high- Nb alkali basaltic magmas by decompression melting far behind the present-day volcanic front.

Shoshonite and sub-alkaline magmas from an ultrapotassic volcano: Sr–Nd–Pb isotope data on the Roccamonfina volcanic rocks, Roman Magmatic Province, Southern Italy

The Roccamonfina volcano is characterised by two stages of volcanic activity that are separated by volcano-tectonic caldera collapses. Ultrapotassic leucite-bearing rocks are confined to the pre-caldera stage and display geochemical characteristics similar to those of other volcanoes in the Roman Province. After the major sector collapse of the volcano, occurred at ca. 400 ka, shoshonitic rocks erupted from cinder cones and domes both within the caldera and on the external flanks of the pre-caldera Roccamonfina volcano. On the basis of new trace element and Sr–Nd–Pb isotope data, we show that the Roccamonfina shoshonitic rocks are distinct from shoshonites of the Northern Roman Province, but are very similar to those of the Neapolitan volcanoes. The last phases of volcanic activity erupted sub-alkaline magmas as enclaves in trachytic domes, and as lavas within the Monte Santa Croce dome. Ultrapotassic rocks of the pre-caldera composite volcano are plagioclase-bearing leucitites characterised by high levels of incompatible trace elements with an orogenic signature having troughs at Ba, Ta, Nb, and Ti, and peaks at Cs, K, Th, U, and Pb. Initial values of 87Sr/86Sr range from 0.70926 to 0.70999, 143Nd/144Nd ranges from 0.51213 to 0.51217, while the lead isotope rations vary between 18.788–18.851 for 206Pb/204Pb, 15.685–15.701 for 207Pb/204Pb, and 39.048–39.076 for 208Pb/204Pb. Shoshonites show a similar pattern of trace element depletions and enrichments to the earlier ultrapotassic leucite-bearing rocks but have a larger degree of differentiation and lower concentrations of incompatible trace elements. On the other hand, shoshonitic rocks have Sr, Nd, and Pb isotopes consistently different than pre-caldera ultrapotassic leucite-bearing rocks. 87Sr/86Sr ranges from 0.70665 to 0.70745, 143Nd/144Nd ranges from 0.51234 to 0.51238, 206Pb/204Pb ranges from 18.924 to 19.153, 207Pb/204Pb ranges from 15.661 to 15.694, and 208Pb/204Pb ranges from 39.084 to 39.212. High-K calc-alkaline samples have intermediate isotopic values between ultrapotassic plagioclase leucitites and shoshonites, but the lowest levels of incompatible trace element contents. It is argued that ultrapotassic magmas were generated in a modified lithospheric mantle after crustal-derived metasomatism. Interaction between the metasomatic agent and lithospheric upper mantle produced a low-melting point metasomatised veined network. The partial melting of the veins alone produced pre-caldera leucite-bearing ultrapotassic magmas. It was possibly triggered by either post-collisional isotherms relaxation or increasing T°C due increasing heat flow through slab tears. Shoshonitic magmas were generated by further melting, at higher temperature, of the same metasomatic assemblage with addition 10–20% of OIB-like astenospheric mantle material. We suggest that addition of astenospheric upper mantle material from foreland mantle, flowing through slab tearing after collision was achieved.

Viscosity measurements of hydrous rhyolitic melts using the fiber elongation method

We have devised a new, simple and easy technique to measure the viscosity of hydrous silicate melts by combining an autoclave for melt hydration and the fiber elongation method for viscosity measurement. Using this, we measured the viscosity of hydrous rhyolitic melts whose water content ranges from 0.02 to 0.58 wt%. We observed a drastic decrease in viscosity against water content: 0.1 wt% water decreases the viscosity about an order of magnitude. Even when the water content is only 0.02 wt%, the viscosity decreased about half an order of magnitude. These results clearly demonstrate that the effect of water on viscosity should not be ignored even when it occurs as a trace constituent. We compared our experimental data with those derived from a non-Arrhenian viscosity model, which is considered to be applicable to calc-alkaline samples. This model succeeded in expressing the viscosity variation against water content but was unable to accurately predict the measured viscosity of liquids.

Geochemical approach to magmatic evolution of Mt. Erciyes stratovolcano Central Anatolia, Turkey

Erciyes stratovolcano, culminating at 3917 m, is located in the Cappadocian region of central Anatolia. During its evolution, this Quaternary volcano produced pyroclastic deposits and lava flows. The great majority of these products are calc-alkaline in character and they constitute Kocdag and Erciyes sequences by repeated activities. Alkaline activity is mainly observed in the first stages of Kocdag and approximately first-middle stages of Erciyes sequences. Generally, Kocdag and Erciyes stages terminate by pyroclastic activities. The composition of lavas ranges from basalt to rhyolite (48.4–70.5 wt.% SiO 2). Calc-alkaline rocks are represented mostly by andesites and dacites. Some compositional differences between alkaline basaltic, basaltic and andesitic rocks were found; while the composition of dacites remain unchanged. All these volcanics are generally enriched in LIL and HFS elements relative to the orogenic values except Rb, Ba, Nb depleted alkaline basalt. 87 Sr/ 86 Sr and 143 Nd/ 144 Nd isotopic composition of the volcanics range between 0.703344–0.703964, 0.512920–0.512780 for alkaline basalts and change between 0.704322–0.705088, 0.512731–0.512630 for alkaline basaltic rocks whereas calc-alkaline rocks have relatively high Sr and Nd isotopic ratios (0.703434–0.705468, 0.512942–0.512600). Low Rb, Ba, Nb content with high Zr/Nb, low Ba/Nb, La/Yb ratio and low Sr isotopic composition suggest an depleted source component, while high Ba, Rb, Nb content with high La/Yb, Ba/Nb, low Zr/Nb and low 87 Sr/ 86 Sr ratios indicate an OIB-like mantle source for the generation of Erciyes alkaline magma. These elemental and ratio variations also indicate that the different mantle sources have undergone different degree of partial melting episodes. The depletion in Ba, Rb, Nb content may be explained by the removal of these elements from the source by slab-derived fluids which were released from pre-collisional subduction, modified the asthenospheric mantle. The chemically different mantle sources interacted with crustal materials to produce calc-alkaline magma. The Ba/Nb increase of calc-alkaline samples indicates the increasing input of crustal components to Erciyes volcanics. Sr and Nd isotopic compositions and elevated LIL and HFS element content imply that calc-alkaline magma may be derived from mixing of an OIB-like mantle melts with a subduction-modified asthenospheric mantle and involvement of crustal materials in intraplate environments.

Distinctive REE patterns for tholiitic and calc-alkaline magma series co-occurring at Adatara volcano, Northeast Japan.

Rare earth element (REE) concentrations were precisely determined by inductively coupled argon plasma/atomic emission spectrometry (ICP) for 9 tholeiitic and 13 associated calc-alkaline rocks from Adatara volcano, located at the volcanic front of Northeast Japan arc. The tholeiitic samples commonly display nearly flat REE patterns, whereas the calc-alkaline samples characteristically show concave light REE-enriched patterns. In both suites, the REEs, with the exception of Eu, are positively correlated to varying extents with K2O and other incompatible elements. Eu concentrations in the calc-alkaline suite do not vary significantly, resulting in significant negative Eu anomalies. In the tholeiitic suite, chondrite-normalized (Ce/Yb)CN ratios are low and show little variation (1.54–1.80) over a significant range in SiO2 content (52–62 wt%). In contrast, (Ce/Yb)CN in the calc-alkaline suite show much greater variation (2.17 to 2.94) over a smaller range in SiO2 content (56 to 62 wt%) and increase systematically with SiO2. REE data for the tholeiitic suite are consistent with a crystal fractionation model that successfully reproduces the observed major and trace-element variations. A similar fractionation model is compatible with major-element and some trace-element variations for the calc-alkaline suite, but results in excessively high heavy REE contents, and fails to reproduce the elevated (Ce/Yb)CN ratios for the most acidic samples. The preferential enrichment of light REEs may reflect additional processes such as: (1) assimilation, (2) incorporation of a differentiated liquid (or evolved magma) that has undergone fractionation of heavy REE-rich minerals, and (3) mixing of primary magma derived by a small degree of partial melting from a common (amphibolitic or hornblende gabbroic) source. Although the tholeiitic and calc-alkaline suites possess distinct 87Sr/86Sr, Rb/Ba and Zr/Nb ratios, different magma sources are not required to produce the distinct REE patterns of the coexisting suites. An equilibrium batch partial melting model can reproduce both the flat (tholeiitic) and concave (calc-alkaline) REE patterns from a common peridotitic source by larger (10–15%) and smaller (5%) degrees of partial melting, respectively. Therefore, both tholeiitic and calc-alkaline suites could have been derived from a common lherzolitic magma source, provided that the calc-alkaline suite underwent incorporation (assimilation) of isotopically distinct (lower crust-derived) materials prior to evolution.

Tholeiitic and calc-alkaline magma series at Adatara volcano, Northeast Japan: 2. Mineralogy and phase relations

Low alkali [low-K by Gill (1978) or island arc by Jakes and Gill (1970)] tholeiitic magma suites are closely associated with [medium-K by Gill (1978)] calc-alkaline suites in the outer volcanic zone (called Nasu volcanic zone; NVZ) of Northeast Japan. Chemistry and phase relations of phenocrysts are studied for representative 5 low-alkali tholeiitic and 8 calc-alkaline rock samples of Adatara volcano in NVZ, in order to examine the mineralogical evolutionary mechanisms of the two series of magmas. Also clarified are differences in the magmatic temperature and oxygen fugacity between the two magma series. The correlations between whole rock chemical compositions and mineralogy in the tholeiitic suite at this volcano are well-documented, supporting the argument that the members of this suite are related through fractional crystallization of observed phenycrystic phases (Part 1; Fujinawa, 1988). Mineralogical observations in the calc-alkaline suite are compatible with the views based on their whole rock chemical compositions; although fractional crystallization process is dominant in the evolution of the calc-alkaline magma, intermittent mixing of (small amounts of) magnesian high-temperature magmas operates concurrently. Temperature estimates with the two-pyroxenes geothermometer range from 1080°C to 970°C in the tholeiitic suite, whereas from 990° to 850°C in the calc-alkaline suite. Temperature-oxygen fugacity trends indicate that the low-alkali tholeiitic magma evolves under conditions of higher temperature and lower oxygen fugacity than the calc-alkaline magma. In view of the fact that the hyperstene and augite phenocryst cores in the calc-alkaline samples are compositionally distinguishable from the relevant phases in the tholeiitic samples, it is unlikely that magma mixing played an essential role in the generation of the calc-alkaline suite at Adatara volcano.

Tholeiitic and calc-alkaline magma series at Adatara volcano, northeast Japan: 1. Geochemical constraints on their origin

Low-alkali [“island arc” by Jakeš and Gill (1970) or “low-K” by Gill (1978)] tholeiitic rocks are closely associated with calc-alkaline rocks in the outer volcanic zone (called the Nasu volcanic zone) of northeast Japan. Major-element, trace-element (Ni, Cr, Co, Sc, V, Zr, Nb, Zn, Rb, Cs, Ba, Sr, Ce and Y) and Sr isotope compositions are presented for 9 low-alkali tholeiitic and 16 calc-alkaline samples of Adatara volcano belonging to the Nasu volcanic zone. In the trace elements vs. SiO 2 diagram the tholeiitic series and calc-alkaline series show different trends to each other for Co, Sc, V, Zr, Rb, Sc, Ba and Sr. As expected from K 2O contents, the calc-alkaline samples are enriched in incompatible elements such as Rb, Cs, Ba, Ce and Zr compared with the tholeiitic samples. On the other hand, contents of Nb in the calc-alkaline samples are somewhat lower than those in the tholeiitic samples. Plots of Ni and Cr contents vs. SiO 2 for the tholeiitic samples come near smooth trend lines whereas those for the calc-alkaline samples are scattered. Sr isotopic ratios in the tholeiitic series lie within the range between 0.7056 and 0.7058 and between 0.7048 and 0.7052 in the calc-alkaline series. Smooth major- and trace-element compositional trends in the tholeiitic series are successfully modelled by simple fractional crystallization of plagioclase, hypersthene, augite, titanomagnetite and minor olivine. Similarly, crystal fractionation models can closely reproduce major- and most of the trace-element variations in the calc-alkaline series, but failed to account for irregular variations of Ni and Cr. The scattering of Ni and Cr probably reflects simultaneous crystal fractionation and (subordinate) magma mixing in the subvolcanic chamber. The inferred fractionated phases for the calc-alkaline series are richer in augite and titanomagnetite and slightly poorer in plagioclase than those for the tholeiitic series. The tholeiitic magma and calc-alkaline magma are not derived from a common parental (tholeiitic) magma and the calc-alkaline parental magma is richer in SiO 2 and incompatible elements (in particular Rb and Zr), poorer in radiogenic Sr and possibly more oxidized than the tholeiitic parental magma.

Petrochemical constraints on the models of Cretaceous-Eocene tectonic evolution of the Eastern Pontic chain (Turkey)

Sixteen selected samples from the Upper Cretaceous volcanic belt of the Eastern Pontids have been analysed for major elements, Rb, Sr and Zr. On the basis of the K 2O versus SiO 2 distribution, two groups of rocks have been distinguished, one with calc-alkaline affinity and a second group with shoshonitic character. The calc-alkaline rocks have porphyritic texture with clinopyroxene, plagioclase and orthopyroxene as phenocryst and in the groundmass. The orthopyroxene is lacking in the shoshonites where plagioclase, clinopyroxene and, in the more evolved terms, amphibole and biotite are the main phenocryst minerals. The shoshonitic rocks have higher K 2 O Na 2 O ratio, K 2O, P 2O 5 and Rb, contents with respect to the calc-alkaline samples. The TiO 2 content is invariably low, never exceeding approximately 1%. The occurrence of volcanic rocks ranging in composition from calc-alkaline to shoshonitic in the Upper Cretaceous volcanic belt of the Eastern Pontids suggests that the Upper Cretaceous volcanic cycle reached its mature stage before the onset of the Eocene calc-alkaline volcanism which is believed to be neither genetically nor tectonically related with the Upper Cretaceous volcanism.