low-K Tholeiitic Basalts Research Articles

Tholeiitic to calc-alkaline and alkaline volcanisms in an extensional arc setting of a Tethyan ophiolite: insights from small-scale compositional and temporal transitions from the Dali sector (Armenia)

• Dali's sector shows a transition from island-arc magmatism to more alkaline volcanism. • Alkaline lavas comprise E-MORB, transitional calc-alkaline basalts and atypical OIB. • The transition to alkaline lavas occurred in the Tithonian–Valanginian time interval. • The diversity of magmatic affinities suggests an extensional oceanic arc setting. New field and petrographic observations, combined with whole rock and mineral geochemical analyses are applied on volcanic rocks present in the Dali sector, east of Lake Sevan (Armenia). A small-scale sampling of the volcanic sequence allows us to identify, for the first time in Armenian ophiolites, two groups of lavas (groups B and C1) stratigraphically and geochemically intermediate between previously recognized arc tholeiite (group A) and OIB-like (group C2) basalts. Their age is constrained by two distinct intercalated radiolarite sequences, which were dated as Tithonian – Valanginian. Group B lavas are low-K tholeiitic basalts and basaltic andesites derived from an enriched mantle source (La N /Yb N 1.8–6.1; Sm N /Yb N 1.3–2.4; Nb/Yb 2.0–6.7). The composition of their clinopyroxenes ranges from Ti-poor augite to Ti-rich diopside augite in correlation with higher La/Yb and Nb/Yb ratios in bulk rocks. Group C1 lavas are basaltic andesites containing Ti and Na-rich amphiboles (kaersutites, hastingsites) as major mineral phases. Three analyzed diopside-amphibole-porphyritic samples are alkaline trachybasalts (La N /Yb N 20–21; Nb/Yb 16–18). Both the group B and C1 lavas exhibit variable, although moderate, enrichments in Th/Yb for given Nb/Yb ratios and Nb negative anomalies in normalized multi-elements patterns (Nb/La 0.53–1.02), which are not correlated with the degree of enrichment of their mantle sources. Our results suggest that transitional and OIB-like volcanic lavas were generated by low- (group B) and very low- (groups C1–C2) melting of a heterogeneous mantle source in an oceanic arc–back-arc system in extension, possibly in relation with slab break-off.

Petrogenesis of Havre Volcano in the Kermadec Arc: 2012 Eruption of a Chemically Homogeneous Rhyolite

The 2012 Havre submarine eruption produced a 1.5 km3 bulk rock volume or 0.52 km3 dense rock equivalent volume of rhyolite emplaced as minor lava flows, a field of sunken seafloor pumice, and a volumetrically dominant pumice raft. This moderately large volume of medium-K (1.4–1.6 wt% K2O) rhyolite pumice is relatively chemically homogeneous (71.5–73.0 wt% SiO2), and no trace element variation or cryptic zoning has been detected despite the textural diversity of pumice material. Radiogenic isotope ratios (87Sr/86Sr 0.703693–0.703744; 206Pb/204Pb 18.7648–18.7781; 208Pb/204Pb 38.587–38.605; 143Nd/144Nd 0.513001–0.513020) demonstrate the Havre rhyolite is sourced from mantle similar to regional eruptive products of the Kermadec arc volcanic front. Providing some further insight into the Havre magmatic system is an abundance of diverse volcanic rock fragments primarily embedded in the banded raft pumice. Embedded rock fragments represent a variety of fresh to hydrothermally altered lavas ranging in composition from basaltic to rhyolitic (50.6–72.3 wt% SiO2) and are likely sourced from varying depths within the volcanic conduit during explosive fragmentation. The diverse embedded volcanic rock fragments, therefore, represent earlier erupted lavas that constructed Havre volcano and are snapshots of the petrogenetic history of Havre. Magnesian augite in basaltic to basaltic andesite embedded rock fragments has a similar compositional range (En55Fs12Wo33 to En39Fs26Wo35) to the previously documented antecrystic clinopyroxene observed in the 2012 rhyolite pumice raft. Herein, we explain how this large volume of chemically homogeneous crystal-poor rhyolite can be generated in an oceanic arc setting based on major and trace element petrogenetic models. Rhyolite-MELTS crystal fractionation models indicate the antecrystic mineral compositions within the Havre pumice of plagioclase (An55–78), and magnesian augites (En53Fs10Wo37 to En40Fs26Wo34) are the primary phases that would crystallize in basaltic to andesitic melt compositions. Modeling indicates that the forerunner basaltic magma must be a relatively dry (∼1 wt% H20) low-K tholeiitic basalt in composition and would require ∼78% crystallization at different pressures to ultimately generate the Havre 2012 rhyolite.

From passive margin to continental collision: Geochemical and isotopic constraints for E-MORB and OIB-like magmatism during the neoproterozoic evolution of the southeast Brasília Belt

Metamorphic rocks derived from mafic rocks can be found in metamorphic belts, and their geochemical and isotopic characteristics may be valuable sources of information for interpretation of ancient geologic magmatic scenarios. The southern Brasília Belt, between the Amazonian and São Francisco cratons in Central Brazil exhibits different amphibolite rocks that were emplaced into Veríssimo Sequence and Araxá Group metasedimentary sequences, during the Neoproterozoic. The obtained results indicate that the studied amphibolite rocks from Pires do Rio-Catalão area represent three magmatic events associated with three magmatic environments: 1) a passive margin development at 979 Ma, a forearc/back-arc basin at ∼819 Ma, and a continental collision at ∼651 Ma.The first magmatic event, related to continental extension and passive margin development is represented by the emplacement at 979 ± 17 Ma of E-MORB low-K tholeiitic basalts with positive εNd(t) values between +4.4 and −2 and TDM between 1.26 and 1.82 Ga, related with the Veríssimo metavolcanosedimentary Sequence. The zircon populations include a high amount of metamorphic, inherited grains.The second magmatic event, related with an forearc/back-arc extension is represented by E-MORB basalts with negative Nb-Ta anomalies and slightly positive εNd(t) values of +0.78 to +2.71, cogenetic with the Araxá Group. An OIB-like basalt sample is also related to this stage and has a U-Pb age of 819.7 ± 6.3 Ma. εNd(t) values of +1.82 to +2.57, εHf(t) in zircon around-zero and TDM ages from 1.03 to 1.18 Ga.The third event is represented by syn- to late-tectonic OIB-like basaltic/gabbroic magmatism at 651 ± 6 Ma, with positive εNd(t) value of +2.57 and younger TDM age at 1.18 Ga, that can be associated to regional continental collision and closure of the Neoproterozoic ocean.Also, provenance studies into metasedimentary rocks support that passive and active margins could be successively superposed during the main thrusting tectonic event in the Brasiliano orogeny, but slices of different stratigraphic units preserve their depositional isotopic signature.These findings suggest that the occurrence of main mafic magmatic events over a timespan as long as the entire evolution of the Brasília Belt (1.0–0.6 Ma) might be a key to individualize superposed tectonic basins in Central Brazil.

Geochemistry and origin of metabasites from the granulite-gneiss complex of the Angara-Kan block, southwestern Siberian craton

In this paper, we present data on major and trace elements in highly metamorphosed mafic rocks from the granulite-gneiss complex of the Angara-Kan block (southwestern Siberian craton), identify igneous protoliths of the metabasites, and assess the mobility of elements during metamorphism. Two types of rocks with different geologic relations and compositions were recognized. Garnet-bearing two-pyroxene granulites (Cpx + Pl + Grt + Opx) occur as sheet- and boudin-like bodies, which were folded and deformed with their host paragneisses. Dikes, which in most cases underwent only brittle deformation, are composed of metabasites characterized by the assemblage Cpx + Hbl + Pl + Grt. The major element compositions of igneous protoliths for the mafic granulites and metabasite dykes correspond to variously differentiated basaltic magmas. The protoliths of the metabasites are depleted in K2O, LILE, Zr, Nb, and LREE and were derived from a depleted mantle source. The major and trace element compositions of the dike metabasites are similar to those of low-K tholeiitic basalts of oceanic island arcs. Continental intraplate basalts derived from an enriched mantle source are possible igneous protoliths for the mafic granulites enriched in Ba, LREE, Nb, Ta, Zr, and Hf. It is assumed that lower Rb, Th, and U contents in the mafic granulites compared with continental flood basalts, high K/Rb and La/Th, and moderate Th/U ratios reflect the loss of Rb, Th and U during granulite-facies metamorphism.

Water content of primitive low-K tholeiitic basalt magma from Iwate Volcano, NE Japan arc: implications for differentiation mechanism of frontal-arc basalt magmas

The water content of low-K tholeiitic basalt mag- ma from Iwate volcano, which is located on the volcanic front of the NE Japan arc, was estimated using multi- component thermodynamic models. The Iwate lavas are moderately porphyritic, consisting of ~8 vol.% olivine and ~20 vol.% plagioclase phenocrysts. The olivine and plagioclase phenocrysts show significant compositional variations, and the Mg# of olivine phenocrysts (Mg#78-85) correlates positively with the An content of coexisting plagioclase phenocrysts (An85-92). The olivine phenocrysts with Mg# > ~82 do not form crystal aggregates with plagioclase phenocrysts. It is inferred from these observations that the phenocrysts with vari- able compositions were primarily derived from mushy boundary layers along the walls of a magma chamber. By using thermodynamic calculations with the observed petrological features of the lavas, the water content of the Iwate magma was estimated to be 4-5 wt.%. The high water content of the magma supports the recent consensus that frontal-arc magmas are remarkably hydrous. Using the estimated water content of the Iwate magma, the water content and temperature of the source mantle were estimated. Given that the Iwate magma was derived froma primarymagmasolelybyolivinefractionation, the water content and temperature were estimated to be ~0.7 wt.% and ~1,310 °C, respectively. Differentiation mech- anisms of low-K frontal-arc basalt magmas were also exam- ined by application of a thermodynamics-based mass balance model to the Iwate magma. It is suggested that magmatic differentiation proceeds primarily through fractionation of crystals from the main molten part of a magma chamber when it is located at <~200 MPa, whereas magma evolves through a convective melt exchange between the main magma and mushy boundary layers when the magma body is located at

Geochemistry of gabbros and granitoids (M- and I-types) from the Nubian Shield of Egypt: Roots of Neoproterozoic intra-oceanic island arc

The Neoproterozoic intrusive rocks of the Wadi Ranga area, Nubian Shield of Egypt, comprise gabbros and granitoids emplaced during oceanic island arc and post-collision stages. The plutonic rocks of the island arc stage include hornblende gabbros (Dabbah pluton), trondhjemite (Abu Ghalaga pluton) and tonalites with subordinate quartz gabbro and quartz diorite (Reidi and Abu Ghusun plutons), whereas the post-collision intrusives include granodiorite and monzogranite (Helifi-Hamata pluton). The gabbros and granitoids of the island arc stage are largely calcic, low-K rocks which have either tholeiitic (gabbro and trondhjemite) or transitional tholeiitic to calcalkaline nature (tonalites). On the other hand, the granitoids of the post-collision stage are medium to high-K calcalkaline rocks. All the investigated granitoids are metaluminous. The spider diagrams, with enrichment in LILE and strong Nb depletion, and the almost flat to slightly LREE-depleted REE patterns of the gabbro and trondhjemite are similar to those of the Wadi Ranga low-K tholeiitic basalts and silicic volcanics, respectively, suggesting that the gabbro and trondhjemite are the plutonic equivalents of the Wadi Ranga immature island arc extrusives, and they were derived from mantle source at the early immature island arc stage. Similar to the trondhjemite, the tonalites show LILE enrichment and strong Nb depletions on the MORB-normalized spider diagrams. However, the tonalites have REE patterns which are enriched in LREE (La/Yb=1.71–5.54). The derivation of the tonalites through fractionation of the same magma produced the trondhjemite seems unlikely. Therefore, high degree partial melting of juvenile basaltic arc crust is favoured for the origin of tonalites during a late immature island arc stage. The post-collision granitoids show considerable enrichment in LILE and to a lesser extent in HFSE, slight negative Nb anomaly and strong negative P and Ti anomalies relative to N-MORB. Their REE patterns are LREE-enriched (La/Yb=5–19), with negative Eu anomaly. These characteristics are consistent with origin through lower degrees of partial melting of old basaltic arc crust and subsequent fractional crystallization. The geochemical characteristics of the trondhjemite and tonalites, and the granodiorite–monzogranite classify them as M-type and I-type granitoids, respectively. The partly tholeiitic intrusives of the Wadi Ranga area (South Eastern Desert) have lower K2O, Rb and LREE compared to the M-type calcalkaline intrusives of the North Eastern Desert, implying northwardly dipping subduction zone.The geochemical similarities between the intrusives of Neoproterozoic and Phanerozoic oceanic island arcs imply that they share similar style of subduction, which differs from that of the Archaean. The generation of high SiO2 (up to 74.5wt%), low K2O (0.56–1.78wt%) and slightly LREE-depleted trondhjemite in early immature oceanic island arc setting supports the arc origin of the primitive continental crust. Silicic magma production through partial melting of the early arc volcanic rocks during the evolution of the arc and the post-collision stage, drives the middle and upper oceanic arc crust towards a composition closer to that of the continental crust. The present study indicates that the intra-oceanic island arcs continued to play a role in the generation of the continental crust after the Archaean.

Primary Magmas at the Volcanic Front of the NE Japan Arc: Coeval Eruption of Crustal Low-K Tholeiitic and Mantle-derived Medium-K Calc-Alkaline Basalts at Azuma Volcano

It has long been thought that the low-K tholeiitic (TH) and medium-K calc-alkaline (CA) lavas in the NE Japan arc were produced by fractional crystallization from mantle-derived basalt magmas, and that the latter formed from mixing of mafic and felsic magmas, both derived from a common primary TH basalt through fractionation. An alternative view was recently proposed on the basis of Sr isotope microanalysis of plagioclase phenocrysts from the Zao volcano, suggesting that (1) the low-K TH basaltic andesites formed by melting of lower crustal amphibolite and that (2) the medium-K CA basalts to andesites formed by mixing of mantle-derived basalt and crustal TH melts. To investigate further the origin of the ‘primary’ low-K TH and medium-K CA basalts, we investigated basalts and andesites from Azuma volcano. Azuma is a Quaternary eruption center at the volcanic front in the NE Japan arc that has erupted two types of basalt: (1) radiogenic-Sr (€87Sr/€86Sr = 0·7058–0·7062) low-K TH basalt lavas without evidence of magma mixing and assimilation; (2) unradiogenic-Sr (€87Sr/€86Sr = 0·7039–0·7041) medium-K CA basalt lavas with subtle evidence for magma mixing. Associated intermediate lavas are voluminous and are all (3) mildly radiogenic-Sr (€87Sr/€86Sr = 0·7044–0·7055) medium-K andesites, all of which have CA affinities with evidence for rigorous magma mixing but no crustal assimilation. The low-K TH basalt has an isotopic composition similar to that of crustal granitoids beneath Azuma and has a composition indicating that it potentially formed from a high-degree lower crustal amphibolite melt. The medium-K CA basalt has a basaltic groundmass with Mg-rich olivine (Fo89) and calcic plagioclase phenocrysts (An90) and the most unradiogenic Sr (€87Sr/€86Sr = 0·7037–0·7038), suggesting that it originated from a primary mantle melt. Major and trace element microanalysis of the basaltic groundmass indicates that the primary magma composition is close to high-K. We conclude that the mantle-derived basalt at Azuma is the result of a high- to medium-K magma that was later mixed with a low-K TH basalt melt from the amphibolitic lower crust to form medium-K CA basalts and andesites. This supports the view of a lower crustal origin of the low-K TH basalts and simultaneously requires a reappraisal of the origin of the across-arc variation in K contents of the mantle-derived primary arc basalts, as the high- to medium-K CA basalt is geochemically fairly similar to the high-K rear-arc basalt in the NE Japan arc.

Mafic magmas from Mount Baker in the northern Cascade arc, Washington: probes into mantle and crustal processes

Five mafic lava flows located on the southern flank of Mount Baker are among the most primitive in the volcanic field. A comprehensive dataset of whole rock and mineral chemistry reveals the diversity of these mafic lavas that come from distinct sources and have been variably affected by ascent through the crust. Disequilibrium textures present in all of the lavas indicate that crustal processes have affected the magmas. Despite this evidence, mantle source characteristics have been retained and three primitive endmember lava types are represented. These include (1) modified low-K tholeiitic basalt (LKOT-like), (2) typical calc-alkaline (CA) lavas, and (3) high-Mg basaltic andesite and andesite (HMBA and HMA). The Type 1 endmember, the basalt of Park Butte (49.3–50.3 wt% SiO2, Mg# 64–65), has major element chemistry similar to LKOT found elsewhere in the Cascades. Park Butte also has the lowest overall abundances of trace elements (with the exception of the HREE), indicating it is either derived from the most depleted mantle source or has undergone the largest degree of partial melting. The Type 2 endmember is represented by the basalts of Lake Shannon (50.7–52.6 wt% SiO2, Mg# 58–62) and Sulphur Creek (51.2–54.6 wt% SiO2, Mg# 56–57). These two lavas are comparable to calc-alkaline rocks found in arcs worldwide and have similar trace element patterns; however, they differ from each other in abundances of REE, indicating variation in degree of partial melting or fractionation. The Type 3 endmember is represented by the HMBA of Tarn Plateau (51.8–54.0 wt% SiO2, Mg# 68–70) and the HMA of Glacier Creek (58.3–58.7 wt% SiO2, Mg# 63–64). The strongly depleted HREE nature of these Type 3 units and their decreasing Mg# with increasing SiO2 suggests fractionation from a high-Mg basaltic parent derived from a source with residual garnet. Another basaltic andesite unit, Cathedral Crag (52.2–52.6 wt% SiO2, Mg# 55–58), is an Mg-poor differentiate of the Type 3 endmember. The calc-alkaline lavas are least enriched in a subduction component (lowest H2O, Sr/PN, and Ba/Nb), the LKOT-like lavas are intermediate (moderate Sr/PN and Ba/Nb), and the HMBA are most enriched (highest H2O, Sr/PN and Ba/Nb). The generation of the LKOT-like and calc-alkaline lavas can be successfully modeled by partial melting of a spinel lherzolite with variability in composition of slab flux and/or mantle source depletion. The HMBA lavas can be successfully modeled by partial melting of a garnet lherzolite with slab flux compositionally similar to the other lava types, or less likely by partial melting of a spinel lherzolite with a distinctly different, HREE-depleted slab flux.

Polybaric degassing of island arc low-K tholeiitic basalt magma recorded by OH concentrations in Ca-rich plagioclase

Hydrogen in nominally anhydrous minerals (NAMs) in volcanic rocks can be used as a proxy for dissolved H 2O in melt prior to eruption. Plagioclase is a NAM that accommodates hydrogen in concentrations of up to hundreds of wt. ppm H 2O. The species of hydrogen in volcanic plagioclase is structural OH. We report the analytical results of OH concentrations in Ca-rich plagioclase from the 1986–1987 summit eruption of Izu-Oshima volcano, a frontal-arc volcano in Izu arc. We demonstrate that the island arc low-K tholeiitic basalt magmas erupting from the frontal-arc volcanoes are H 2O-saturated and undergo polybaric degassing during the magma ascent. The analyzed OH concentrations in plagioclase range from 20 to 300 wt. ppm H 2O, and three levels of OH (20–80 wt. ppm H 2O, 100–180 wt. ppm H 2O, and 220–300 wt. ppm H 2O) are found. These variations in OH indicate that crystallized plagioclase is equilibrated with H 2O-saturated melt at three depths beneath the Izu-Oshima volcano prior to eruption: near the surface level (≈ 1 wt.% H 2O in melt), at a 4-km-deep magma chamber (≈ 3 wt.% H 2O in melt), and at a 8–10-km-deep magma chamber (≈ 5 wt.% H 2O in melt). It is proposed that deep-seated island arc low-K tholeiitic basalt magmas erupting from frontal-arc volcanoes are richer in H 2O than previously thought, containing approximately 1 wt.% H 2O based on analyses of “leaked” melt inclusions and phase equilibrium studies at “low-pressure conditions”.

Synchronous Japan Sea opening Miocene fore-arc volcanism in the Abukuma Mountains, NE Japan: An advancing hot asthenosphere flow versus Pacific slab melting

During Japan Sea back-arc opening between 21 and 15 Ma, low-K tholeiitic basalts erupted in the Abukuma Mountains on the trench side of NE Japan. The basaltic rocks in this region are divided into two units by a hiatus. The lower unit consists of the 20 Ma Tenmyosan volcanic rocks, including many monogenetic basaltic volcanoes whose vents occur throughout the district. The upper unit is the Ryozen Formation that occurred at about 16 Ma. This Formation is composed of a subaerial basaltic polygenetic volcanic edifice, with a diameter of ca. 30 km. Minor amounts of andesitic and dacitic gravels occur in the distal portion of the Ryozen edifice. Low-K tholeiitic basalts of the Tenmyosan volcanic rocks and the Ryozen Formation have similar in major and trace element compositions. Their MgO content decrease from 13.2 to 3.4 wt.% with increasing incompatible element abundances and more pronounced negative Nb and Ta anomalies. Nd, Sr and Pb isotopic compositions of the Tenmyosan basalt display a range of values for SrI (0.7040–0.7050), NdI (0.5127–0.5129) and 206Pb/ 204Pb (18.2–18.5). These features suggest that the Tenmyosan monogenetic volcanism at the early stage of the Japan Sea opening was generated by intermittent ascent of minor mafic melts from heterogeneous sources. Isotopic data for the Ryozen basalt define a binary mixing trend between a depleted and enriched crust-like source, varying with SiO 2 content. These chemical changes show that, as the opening progressed, the asthenospheric source of the Ryozen basalts became more homogeneous and depleted (SrI; 0.7043, Nd; 0.5129, 206Pb/ 204Pb; 18.2), and continuous ascent of the basaltic melt was able to make a large magma chamber that was associated with significant crustal assimilation. On the other hand, hornblende dacite in the Ryozen Formation has SiO 2 contents between 66 and 74 wt.%, high Mg# from 41.7 to 54.3. The dacite also shows high Sr (417–650 ppm) and Sr/Y (35–112), and high La/Yb (12–20). Compared with the basaltic rocks, the Ryozen dacite has relatively low SrI (0.70436) and high NdI (0.5128–0.5129). These features show that the dacite has no genetic relationship with the basaltic rocks and can be considered an adakite, traditionally viewed as product of eclogitic slab melting. The 14 Ma Nodegamiyama andesite has SiO 2 in 54 to 55 wt.%, high Mg# (59.0–62.3), high Cr and high Ni contents. The isotopic ratios of this andesite are enriched (SrI; 0.7054–0.7055, NdI; ~ 0.5127, 206Pb/ 204Pb; ~ 18.5). This high-Mg andesite may be generated by melting of subducting sediments and interaction of such melts with overlying mantle peridotites. Although the downgoing Pacific plate was Cretaceous in age and thus too cold to melt under normal conditions, incursion of the hot asthenosphere above the slab during the Japan Sea opening may have caused melting to occur.

High-K alkali basalts of the Western Snake River Plain (Idaho): Abrupt transition from tholeiitic to mildly alkaline plume-derived basalts

Basaltic volcanism in the western Snake River Plain underwent an abrupt change circa ~ 700 ka to 900 ka, from low-K tholeiitic basalt and ferrobasalt to high-K transitional alkali basalt. The low-K tholeiitic basalts share major element, trace element, and isotopic characteristics with olivine tholeiites of the eastern Snake River Plain, and must have been derived by similar processes from similar sources. In contrast, the younger high-K alkali basalts share major element, trace element, and isotopic characteristics with plume-derived alkali basalts of ocean islands suites like Hawaii. We conclude that this abrupt transition reflects either or both the erosion of pre-existing mantle lithosphere in the wake of the Yellowstone–Snake River plume, or the depletion of this lithosphere in fusible components so that it no longer contributed to the overall mass flux of magma. The abruptness of the transition implies that it may have a catastrophic origin, such as lithospheric delamination caused by a Rayleigh–Taylor instability beneath the Idaho batholith.

Geochemical structure of the Early Carboniferous volcanic complexes of the Southern Urals

In this paper, the concept of a geochemical structure (Yaroshevskii, 2004) was applied to describe chemical variations in the Early Carboniferous volcanic complexes and their distribution over the tectonic zones of the Southern Urals and Transuralian region in order to clarify the geodynamic settings of their formation. The cluster analysis of a geochemical dataset including 325 analyses of volcanic rocks from the Magnitogorsk, Southern Ural, Transuralian, and Valer’yanovskii tectonic zones allowed us to reduce the geochemical diversity of rocks to eight large geochemical groups. Based on average compositions, these geochemical groups (clusters) can be classed with the following rocks: (1) low-K tholeiitic basalts, (2) high-Ti subalkaline basalts, (3) high-Al subalkaline basalts, (4) subalkaline andesites, (5) subalkaline rhyolites, (6) Na subalkaline rhyolites, (7) potassic subalkaline rhyolites, and (8) high-Al potassic trachyandesibasalts. The distribution of these clusters in tectonic zones of the Southern Urals and Transuralian makes it possible to organize these complexes into four groups. The first group includes a differentiated series from high-Ti subalkaline basalts to sodic subalkaline rhyolites with the predominance of aluminous subalkaline basalts and subalkaline andesites. This group is most widespread in the Magnitogorsk and Valer’yanovskii zones. The second group corresponds to a differentiated series from low-K basalts to Na subalkaline rhyolites with a strong prevalence of high-Ti subalkaline basalts and less abundant aluminous subalkaline basalts. This group is widespread in the Eastern Ural zone. The third group includes subalkaline andesites and rhyolites with subordinate ultrapotassic rhyolites and trachyandesibasalts, which compose the Uya-Novoorenburg suture. The fourth group comprises high-Ti subalkaline basalts occurring in the Transuralian zone. Such a distinct distribution of the geochemical types of volcanic rocks is well consistent with concepts on the formation of the Southern Ural volcanic belts at the East European paleocontinent margin in a Californian-type setting. The Valer’yanovskii belt was formed at the active margin of the Kazakhstan paleocontinent.

Alkaline volcanism in the history of the Norwegian-Greenland basin

The opening of the North Atlantic Ocean began in the Late Paleocene and was accompanied by the eruption of submarine and subaerial basalts, which built up submarine plateau and ridges, islands, and volcanoes. The volcanic rocks are dominated by low-K tholeiitic basalts, which associate with almost coeval alkaline rocks (subalkali and alkali basalts and their derivatives, basanites, nephelinites, and others). The oldest alkaline volcanics (58–56 Ma) were formed during the opening of the oceanic rift at its shoulders, in northeastern Greenland and the western Norwegian shelf. It was recently found that 55–53 Ma-old alkali-ultramafic rocks are much more widespread at the eastern coast of Greenland than it was previously thought. The younger occurrences of alkali volcanism with pulses at 30, 10, 5 Ma, and up to the present day were formed on the young oceanic plate and newly formed islands and seamounts. To compare the oceanic and continental volcanism of this region, oceanic volcanics dredged during Cruise 10 of the R/V Akademik Kurchatov were reanalyzed using modern analytical methods (XRF and ICP-MS). This study showed that the oceanic and continental alkaline rocks are significantly different in petrochemical and geochemical characteristics, which is caused by differences in magma generation depths and compositions of the mantle source material. The primary continental alkaline magmas were initially more enriched in incompatible trace elements than oceanic ones. During the shallow-level differentiation of oceanic magmas, trace elements and alkalis could be accumulated in residual melts, but these processes occurred on a minor scale and depended on tectonic conditions.

Petrology and geochemistry of metamorphosed basaltic pillow lava and basaltic komatiite in the Mauranipur area: subduction related volcanism in the Archean Bundelkhand craton, Central India

The occurrence of metamorphosed basaltic pillow lava in close association with serpentinized ultramafic rock, metamorphosed basaltic komatiite, volcaniclastic metasediment, and banded iron formation (BIF) in the Mauranipur area is the first explicit evidence for subduction related submarine volcanism in the Archean Bundelkhand craton, Central India. The Mauranipur pillow lava underwent greenschist to amphibolite facies metamorphism while retaining a geochemical signature of its igneous protolith. The pillow lava and associated massive volcanic rock is subalkalic, low-K tholeiitic basalt to basaltic andesite with SiO2 = 51.9-55.9 wt% and Mg/(Mg + Fetotal) × 100 = 63.4-67.2. The rock shows depletion in HFS elements with Nb/Nb* (0.07-0.17), and LIL elements are relatively enriched compared with present-day N-MORB compositions. Chondrite normalized REE patterns for the Mauranipur pillow lava are nearly flat with (La/Sm)N = 0.9-1.5, (Gd/Yb)N = 1.0-1.1, and Eu/Eu* = 0.81-1.10. The geochemical characteristics of the pillow lava are similar to those of low-K tholeiitic basalt reported from modern intra-oceanic arcs. The associated high-Mg andesite is compositionally similar to basaltic komatiite, enriched in LREE, with (La/Sm)N = 2.95-6.44, and depicts a nearly flat chondrite normalized HREE pattern with a low MREE/HREE ratio (Gd/Yb)N = 1.24-1.58. The basaltic komatiite displays remarkably similar geochemical characteristics to modern boninite. The present study, combined with available geological data, suggests that the supracrustal rocks of the Mauranipur area represent an Archean ophiolite sequence formed in a plate convergent setting.

The geochemical nature of the igneous rocks of the Sharbot Lake domain, Central Metasedimentary Belt, Ontario

Two bimodal mafic-silicic suites of igneous rocks, the Sharbot Lake volcanic rocks and the Lavant Igneous Complex, are identified geochemically in the Sharbot Lake domain of the Central Metasedimentary Belt in Ontario, and their genesis and thermotectonic environment are evaluated. The Sharbot Lake volcanic rocks comprise a series of basalts characterized by light rare-earth element (LREE) depletion and relatively high concentrations of Σ Fe2O3, TiO2, MnO, V, and Y, together with rhyolites and silicic pyroclastic rocks. They are intruded by rocks of the Lavant Igneous Complex, which comprises tholeiitic gabbros characterized by LREE enrichment and low concentrations of Σ Fe2O3, TiO2, MnO, V, and Y, and granitoid rocks. The trace element signatures of the mafic rocks of the Sharbot Lake volcanic sequences are most like those of back-arc tholeiitic basalts, and those of the Lavant Igneous Complex are comparable to those of low-K tholeiitic basalt suites. The trace element signatures of the silicic rocks associated with both suites are typical of those formed by crustal melting. Volcanic sequences with trace-element signatures very similar to those of the Sharbot Lake suites have been previously described in the Belmont and Grimsthorpe domains of the Central Metasedimentary Belt, suggesting that the three domains all belong to the Bancroft – Elzevir – Mazinaw – Sharbot Lake superterrane. The lithological, structural, and igneous characteristics of this superterrane suggest that it represents part of a complex back-arc basin underlain by areas of rifted and attenuated continental crust and oceanic crust.

Chemical zonation of volcanoes at the northern end of NE Japan arc: K-Ar ages and geochemistry of some Pliocene and Pleistocene basalts from the western region of Sapporo, southwestern Hokkaido.

Low-K tholeiitic basalts are found from Sapporo-dake and Asari-dake volcanoes in the western region of Sapporo considered as the back-arc side of SW Hokkaido. K-Ar ages of these basalts are 2.6±1.0 Ma (Asari-dake volcano) and 1.6±0.6 Ma (Sapporo-dake volcano) comparable with those of andesitic rocks distributed in the same area. These basalts are similar to the basalts from the trench-side of NE Honshu in the major and trace elements chemistry. Chemical zonation of volcanoes trending N-S direction in the central NE Japan arc can extend to its northern end based on the present results. Because the trench axis bends from N-S to NE-SW direction off the coast of SW Hokkaido, this zonation cannot be explained by the present arctrench system.

Early Geochemical Evolution of an Oceanic Island Arc and Backarc: Fiji and the South Fiji Basin

The initial 25 to 35 Ma of oceanic arc volcanism in Fiji (the arc stage) was dominated by eruption of LREE-depleted, low-K tholeiitic basalts and andesites. The oldest volcanics include the highest-HFSE group, MORB-like in composition, plus high-Mg basalts and basic andesites with flat REE patterns as low as 4x chondritic concentrations. These erupted either within the volcanic arc during its first 5 Ma or in an adjacent backarc during its first Ma of formation. Subsequent volcanics of the early arc stage apparently erupted onto early backarc basin crust. Most are intermediate between earlier extremes in HFSE contents, but at least one volcano produced strongly LREE-depleted island arc tholeiites. In contrast, medium-K tholeiitic and calc-alkaline andesites erupted during the mature arc stage. Parental basalts were enriched in LREE but depleted in HREE relative to earlier basalts due to a change in source composition. Counterflowing MORB-source asthenosphere within the mantle wedge dominated vol...

山形市東部，滝山火山の地質の岩石

Mt. Ryuzan (elevation 1, 362m) represents the oldest edifice in the Zao volcanic group of NE Japan. It consists of thick piles of lavas and pyroclastic rocks, and has been subjected to volcanic eruption or land slide activity which produced mud-flow (debris flow) deposits. Both K-Ar ages and paleomagnetic data of these lavas indicate that the volcano formed at about 1 million years ago. The analyzed rocks from Mt. Ryuzan fall within the SiO2 range of 47.4-60.3% (water free basis) and basalt is the predominant rock type. Alkali contents, especially K2O, are very low in those basalts, compared with rocks forming the main (Central) Zao volcano. Chemically, rocks of Mt. Ryuzan belong to the tholeiitic series (TH) of Miyashiro (1974). It is noteworthy that low-K tholeiitic basalts occur at the early stage of the formation of Zao volcano.

Broken Hill Pb-Zn-Ag deposit ? a product of mantle metasomatism

Broken Hill ore deposition occurred during the highest geothermal gradient coeval with an event of bimodal basic-rhydacitic volcanism. The depositional environment is interpreted as an ensialic rift on the basis of the sedimentary facies, ferro- and low-K tholeiitic basalts, and bimodal basic-calc alkaline (rhyolite, rhyodacite) volcanism. The orebody is of unusual composition characterised by abundant carbonate, fluorite and fluorapatite, abundant LIL-, K- and Rb-rich premetamorphic alteration assemblages, primordial S and Pb isotopic values, Sr isotopes possibly indicating a heterogeneous source, S : Se in the magmatic range, and zoning suggestive of cooling of the ore fluid.

Archaean greenstone belt from the Central African Republic (Equatorial Africa)

Abstract The Bandas belt, one of two prominent Archaean greenstone belts in the Central African Republic (Equatorial Africa), is ca. 250 km long. At the southernmost part of the belt, a metasedimentary—metavolcanic rock suite is preserved only in brachysynclines. The suite can be divided into two lithostratigraphic units. The lower unit is composed predominantly of volcanic rocks, while the upper one contains mainly metasedimentary rocks. The volcanic rocks, which are part of a sequence ca. 3600 m thick, can be sub-divided according to stratigraphic position, lithology and geochemistry into three groups. The lowermost group includes low-K tholeiitic basalts depleted in light REE. The second group consists of tholeiitic basalts with light REE-enriched patterns and the third, uppermost, group includes andesites, which are similar in several respects to Recent calc-alkaline andesites. The tholeiitic basalts of the first two groups are probably related to different upper mantle sources. The andesites of the third group were produced either by fractional crystallization from a basaltic magma enriched in light REE or equilibrium melting of eclogite or garnet amphibolite.