Parental Mafic Magma Research Articles

Geochronology and Geochemistry of Mafic Rocks in the Xuhe, Shaanxi, China: Implications for Petrogenesis and Mantle Dynamics

AbstractThe Xuhe mafic rocks, located in Ziyang county of Shaanxi Province, are dominated by diabase‐porphyrite, gabbro–diabase, diabase, and pyroxene diorite. Primitive mantle‐normalized multi‐element patterns show that, the Xuhe mafic rocks are enriched in large ion lithophile elements (LITE), such as Ba and Pb, depleted in K and Sr for basic rocks, and are depleted in Sr, P and Ti for pyroxene diorite. Chondrite‐normalized REE patterns display LREE enrichment (LaN/YbN = 9.34–13.99) and have normalized patterns for trace element and REE similar to that of typical OIB. Detailed SEMS zircon U–Pb dating yields emplacement ages of 438.4 ± 3.1 Ma for Xuhe mafic rocks. The relatively low MgO (basic rock: 3.11–7.21 wt%; pyroxene diorite: 0.89–1.21 wt%) and Mg# (0.20–0.49) for Xuhe mafic rocks suggest that they were possibly originated from an extremely evolved magma. The rising parental mafic magmas underwent pyroxene and plagioclase fractionation. Crustal contamination of pyroxene diorite before emplacement occurred at a higher crustal level compared to other lithology in Xuhe mafic rocks. The degree of partial melt was low (5%‐10%) and in garnet‐spinel transition facies. Sr‐Nd isotope of pyroxene diorite and enrichment mantle characteristics for Xuhe mafic rocks suggest that mafic rocks in the North Daba Mountains were derived from a mixture of HIMU, EMU and small amount of EMI components. Furthermore, this study discusses mantle geodynamic significance of Xuhe mafic rocks in the Silurian, which indicates subduction and uplift of magma caused back‐arc extension.

Bimodal magmatism produced by progressively inhibited crustal assimilation.

The origin of bimodal (mafic-felsic) rock suites is a fundamental question in volcanology. Here we use major and trace elements, high-resolution Sr, Nd and Pb isotope analyses, experimental petrology and thermodynamic modelling to investigate bimodal magmatism at the iconic Carlingford Igneous Centre, Ireland. We show that early microgranites are the result of extensive assimilation of trace element-enriched partial melts of local metasiltstones into mafic parent magmas. Melting experiments reveal the crust is very fusible, but thermodynamic modelling indicates repeated heating events rapidly lower its melt-production capacity. Granite generation ceased once enriched partial melts could no longer form and subsequent magmatism incorporated less fertile restite compositions only, producing mafic intrusions and a pronounced compositional gap. Considering the frequency of bimodal magma suites in the North Atlantic Igneous Province, and the ubiquity of suitable crustal compositions, we propose 'progressively inhibited crustal assimilation' (PICA) as a major cause of bimodality in continental volcanism.

Geochemistry and petrogenesis of the 595 Ma shoshonitic Qunai monzogabbro, Jordan

The last stage in the formation of the Arabian Nubian Shield in Jordan was dominated by post-orogenic igneous activity of the ∼610–542Ma Araba Suite, including a monzogabbroic stock intruding the Saramuj Conglomerate, near the southeastern corner of the Dead Sea. The geological setting, petrography, geochemistry and geothermometry of the monzogabbro and other cogenetic varieties are used to shed light on the petrogenesis of this stock and reveal its magma source. The monzogabbro, megaporphyry dikes, and scattered syenite pockets are co-magmatic and alkaline, potassic and shoshonitic in nature. REE and trace elements patterns indicate that these magmas were produced from a mantle that had been modified by subduction-related metasomatism. The parental mafic magma could have been derived by 10% partial melting of LILE-enriched phlogopite-bearing spinel lherzolite, probably lithospheric mantle, in association with post-collisional extension. Fractional crystallization of this parental magma by olivine and pyroxene gave rise to the monzogabbroic magma.The megaporphyry dikes with their giant labradorite plagioclase megacrysts represent feeders of a voluminous volcanic activity that could have lasted for about 105 years.Thermodynamic modeling applying the MELTS software indicates crystallization of this suite in the temperature range of 1184–760°C at a pressure of 2kbars, agreeing with olivine-pyroxene, pyroxene, and two-feldspar thermometry. The modeled mineralogy and sequence of crystallization of constituent minerals using MELTS is in remarkable agreement with the observed modal mineralogy of the monzogabbro. Furthermore, a great degree of congruity exists between the modeled and observed chemistry of the major minerals with only minor discrepancies between modeled composition of biotite and olivine.

Geochemical, Sr–Nd isotopic, and zircon U–Pb geochronological constraints on the petrogenesis of Late Paleoproterozoic mafic dykes within the northern North China Craton, Shanxi Province, China

The ages and source of Precambrian mafic dolerite dykes of the North China Craton (NCC) were determined using geochronological, geochemical, and whole-rock Sr–Nd isotopic data. Laser ablation–inductively coupled plasma-mass spectrometry (LA–ICP-MS) U–Pb analysis of zircons yielded consistent ages of 1621±6 and 1621±7Ma for two mafic dykes (SYG01 and SYG02) within the NCC. The dykes are classified as the sub-alkaline series according to their K2O+Na2O concentrations (2.5–2.9wt%), and as the calc-alkaline series according to their K2O concentrations (0.05–0.72wt%). These dykes also contain low concentrations of the light rare earth elements [(La/Yb)N=1.5–2.4], with negligible Eu anomalies (δEu=0.9–1.1), positive Ba, Pb, and K anomalies, and depletions in Th, U, and high field strength elements (P and Ti). In addition, the dykes contain relatively low amounts of radiogenic Sr[(87Sr/86Sr)i=0.7025–0.7035] and have high ɛNd(t) values (5.6–5.8). These data suggest that the dykes were derived from a depleted mantle source that was hybridized during interaction with foundered lower crustal material. The parental mafic magmas underwent the fractionation of olivine, pyroxene, hornblende, and Fe–Ti oxides (e.g. rutile, ilmenite, titanite) during ascent, with negligible evidence for crustal contamination prior to emplacement at a high crustal level. The NCC extension was related to foundering of lower crust, and there were three episodes of lithosphere extension during Proterozoic time (i.e. 1.8–1.6Ga, 1.3–1.2Ga, and 0.8–0.7Ga).

Permian A-type granites in Tarim and western part of Central Asian Orogenic Belt (CAOB): Genetically related to a common Permian mantle plume?

Zircon U–Pb ages, whole-rock geochemical and zircon Hf isotope data are reported for the Permian A-type quartz syenites–granites in western Tarim Block, Northwest China. Zircon U–Pb dating indicates these alkali plutons were emplaced at 270–277Ma. Elemental geochemistry shows their typical A-type signatures, i.e., high total alkali, HFSE, REE contents and Ga/Al, FeOtot/(FeOtot+MgO) and Nb/Y ratios. Isotopically, their epsilon Nd values vary from −2.6 to 2.0. Based on zircon Hf isotope compositions, they could be divided into two sub-groups: Group I exhibits high positive εHf(t) ranging from 3 to 11 (mostly within the range of 5 to 9), which is highly comparable with the syenites in Bachu area located just south of these plutons. Group II has εHf(t) ranging from −3 to 2. The second group exhibits comparable Nd–Hf isotope compositions with the spatially and temporally related Piqiang ultramafic–mafic complex. Integrating geological, geochemical and Nd–Hf isotope data, we suggest that the A-type syenite–granites were formed via intensive crystal fractionation from a common plume-derived parental mafic magma, coupled with variable extent of crustal contamination. Moreover, the extent of the crustal contamination increase from inner Tarim towards South Tianshan Collisional Belt.A comprehensive synthesis of the coeval A-type granites in the western part of the Central Asian Orogenic Belt (CAOB) shows that they exhibit distinct geochemical features, e.g., large range of εNd(t) and (87Sr/86Sr)i values, high Y/Nb, Yb/Ta and low Nb/La ratios. All these features argue that they were derived from juvenile mafic crust with variable addition of the mafic materials from mantle sources and some old crustal materials or mixing of mantle-derived mafic magma with Precambrian crustal components. We suggest that both the Permian A-type granites from Tarim and western section of CAOB were genetically linked to the Permian Tarim mantle plume, which is concurrent with the two mantle domains for the Permian Tarim Large Igneous Province (LIP).

Petrogenesis of Early Cretaceous bimodal volcanic rocks in the Fanchang Basin, SE China: an energy-constrained assimilation–fractional crystallization model

Volcanic rocks in the Middle–Lower Yangtze River Valley (MLYRV) constitute a bimodal magmatic suite, with a significant compositional gap (between 50% and 63% SiO2) between the mafic and felsic members. The suite is characterized by a relatively wide spectrum of rock types, including basalts, trachytes, and rhyolites. The basaltic rocks have low-to-moderate SiO2 contents of 46.00–50.01%, whereas the trachytes and rhyolites possess SiO2 contents in the range of 63.08–77.61%. Rocks of the bimodal suite show moderate enrichment of LILEs, negative Nb, Ta, and Ti anomalies, and are significantly enriched in LREEs. The basalts were most likely generated by parental mafic magmas derived from enriched lithospheric mantle with minor assimilation of crustal materials involving coeval crystal fractionation during magma evolution. The results of energy-constrained assimilation and fractional crystallization simulations demonstrate that the felsic magma was produced by the mixing of 5–20% lower crustal anatectic melts with an evolved mafic magma (∼48% SiO2) and accompanied by extensive clinopyroxene, plagioclase, biotite, and Fe–Ti oxide fractionation. Our model for the genesis of felsic rocks in bimodal suites is different from the traditional models of crustal melting and fractional crystallization or assimilation–fractional crystallization of basaltic liquids.

Geochemistry and U–Pb zircon ages of metamorphic volcanic rocks of the Paleoproterozoic Lüliang Complex and constraints on the evolution of the Trans-North China Orogen, North China Craton

The Paleoproterozoic Lüliang Complex is situated in the central part of the western margin of the Trans-North China Orogen and consists of volcanic rocks, sedimentary rocks and Paleoproterozoic granitoid intrusions. The volcanic rocks and earlier granitoid rocks were strongly deformed and metamorphosed into the greenschist- to amphibolite-facies. These metamorphosed volcanic rocks are dominated by basalts to basaltic andesites. The parental mafic magmas of these metamorphosed volcanic rocks were mainly derived from the 5% to 30% partial melting of spinel lherzolites to spinel-garnet lherzolites which had been enriched by the subduction melts. Mafic magma experienced fractional crystallization and crustal assimilation.U–Pb zircon dating on two metamorphosed volcanic rocks from the Yejishan and Lüliang groups reveals that they formed at 2210±13Ma and 2213±47Ma, respectively, and were metamorphosed at ∼1832Ma. This suggests that the metamorphosed volcanic rocks in the Yejishan and Lüliang groups formed synchronously in the Paleoproterozoic. These new ages, integrated with recently reported U–Pb zircon ages for the Jiehekou Group and Paleoproterozoic granitoids, suggest that all of the lithological assemblages of the Lüliang Complex formed and were metamorphosed in the Paleoproterozoic, not in the Neoarchean.Petrological, geochronological and geochemical data suggest that the geodynamic evolution of the Paleoproterozoic Lüliang Complex was involved in the development of a magmatic arc system at an active continental margin, generating widespread arc-related magmatism at ∼2.2Ga. The Lüliang Complex then underwent intense deformation and metamorphism, and was incorporated into the Trans-North China Orogen during the 1.88–1.83Ga collisional event which was followed by post-collision extension at ∼1.80Ga.

Comment on 'A Metamodel for Crustal Magmatism: Phase Equilibria of Giant Ignimbrites' by S. J. Fowler and F. J. Spera

The generation and evolution of giant, supereruptionforming magma bodies is a contentious topic that has prompted much recent work, much of which has appeared in this journal. In a recent paper, Fowler & Spera (2010) summarize the state of the discussion (and disagreement) in very useful terms. On one side, Bindeman & Valley (2001) and Bindeman et al., (2008), using evidence from geochronology and oxygen-isotope geochemistry of Yellowstone zircons, argue in favor of a selfcannibalization, crustal melting origin of large volumes of low-dO rhyolites. On the other side, Bachmann & Bergantz (2004, 2008) emphasize the challenges posed by heat-flow constraints to an upper crustal melting scenario, and advocate step-wise crystal fractionation of mafic parental magmas. Interestingly, Fowler & Spera (2010) use phase equilibria information derived from MELTS simulations to place themselves in a more extreme position, and suggest that single-step fractional crystallization from mafic parental magmas is sufficient to explain the origin and evolution of giant bodies of rhyolitic magma. There are many critical assumptions in the treatment of Fowler & Spera (2010) that need careful scrutiny, but we concentrate on those that we feel better equipped to address. We focus on the evolution of the Bishop magma body, and we comment on (1) the compatibility of the proposed scenario with existing evidence from minerals, melt inclusions and pumice from the Bishop Tuff, and (2) the estimates for the longevity of the Bishop magma body derived from heat-balance considerations. It seems significant to point out that the model proposed by Fowler & Spera (2010) is essentially a transposition of the model they developed for the evolution of the Campanian Ignimbrite using similar methods (Fowler et al., 2007). However, there are significant differences between the trachyte^phonolite system characteristic of the Campanian Ignimbrite and the high-silica rhyolite systems discussed by Fowler & Spera (2010). Probably cognizant of some of the difficulties in transposing their model from one type of system to another, Fowler & Spera (2010) present what they call a ‘metamodel’, an abstract construct that is useful to understand the generation and evolution of large volumes of high-silica rhyolite. By doing so, they purposely distance themselves from the details of the various magmatic systems and focus on the first-order features of the geochemical evolution of the systems of interest. Importantly, they postulate evolution from parental mafic^intermediate magmas erupted in the same regions and with similar ages to the high-silica rhyolites, and carry forward the exercise of seeking inconsistencies between modeled evolutionary paths and data for the natural systems.

Diverse Permian magmatism in the Tarim Block, NW China: Genetically linked to the Permian Tarim mantle plume?

Zircon U–Pb ages and geochemical data are reported for the Piqiang oxide-bearing ultramfic–mafic complex, the Bachu mafic dyke swarm, the Yingan and Kaipaizileike basalts and the Halajun A-type granites in the Tarim Block, Northwest China. The Piqiang complex and the Halajun A-type granites were emplaced at ca. 276 Ma and ca. 278 Ma, respectively. Together with previously reported geochronological data, the diverse intrusive and extrusive rocks in Tarim show a peak age at ca. 275 Ma. Elemental and Nd isotope geochemistry suggests that the spatially and temporally related Piqiang complex (including some dolerite dykes or stocks) and the Halajun A-type granites were formed via crystal fractionation/accumulation of a common plume-derived parental mafic magma (melting degree > 10%), coupled with variable extents of crustal contamination. Crystal fractionation/accumulation in one or several magma chambers resulted in the diversity of rocks types. The Bachu mafic dyke swarm shares a similar mantle source with the intrusive rocks in the Piqiang–Halajun area but with a relatively lower degree of partial melting (~ 5%). In contrast, the basalts were derived from a time-integrated, enriched lithospheric mantle source as suggested by their high-Ti, LREE- and LILE-enriched trace element signature and negative ε Nd( t) values (− 2.0 ~ −2.6). The synchronous yet diverse range of Permian igneous rocks in Tarim can best be accounted for by a Permian mantle plume, which is about 15 Ma earlier than the Emeishan plume in southwestern China.

Small-scale mantle heterogeneity on the source of the Gran Canaria (Canary Islands) Pliocene–Quaternary magmas

New chemical and Sr–Nd–Pb isotopic data of the Plio-Quaternary mafic lavas of Gran Canaria are used to investigate their mantle source composition. The most prominent aspects of the new dataset are the slight isotopic differences between the Plio-Quaternary (Post-Roque Nublo Group) and the older Pliocene (Roque Nublo Group) mafic parental magmas, which reflect small-scale mantle heterogeneities. Melting of two mantle materials, one isotopically more depleted and similar to the Depleted Mantle (DM) and the other with more radiogenic Pb-isotope ratios comparable to a mantle with high U/Pb ratio (HIMU), accounts for the isotopic and trace element composition of the Pliocene–Quaternary magmas of Gran Canaria. Geochemical variations show that the Pliocene–Quaternary mantle source is compositionally and lithologically heterogeneous and supports the presence of a silica-deficient pyroxenite mantle component. The contribution of the pyroxenite component in the generation of the Roque Nublo and Post-Roque Nublo magmas is estimated to be in the range from 50 to 70%. Trace element ratios support mixing between the two mantle components (pyroxenite veins in a peridotite matrix) which obscure the original chemical and isotopic composition of these two end-members.

Role of cryptic amphibole crystallization in magma differentiation at Hudson volcano, Southern Volcanic Zone, Chile

Hudson volcano (Chile) is the southern most stratovolcano of the Andean Southern Volcanic Zone and has produced some of the largest Holocene eruptions in South America. There have been at least 12 recorded Holocene explosive events at Hudson, with the 6700 years BP, 3600 years BP, and 1991 eruptions the largest of these. Hudson volcano has consistently discharged magmas of similar trachyandesitic and trachydacitic composition, with comparable anhydrous phenocryst assemblages, and pre-eruptive temperatures and oxygen fugacities. Pre-eruptive storage conditions for the three largest Holocene events have been estimated using mineral geothermometry, melt inclusion volatile contents, and comparisons to analogous high pressure experiments. Throughout the Holocene, storage of the trachyandesitic magmas occurred at depths between 0.2 and 2.7 km at approximately ~972°C (±25) and log fO2 −10.33–10.24 (±0.2) (one log unit above the NNO buffer), with between 1 and 3 wt% H2O in the melt. Pre-eruptive storage of the trachydacitic magma occurred between 1.1 and 2.0 km, at ~942°C (±26) and log fO2 −10.68 (±0.2), with ~2.5 wt% H2O in the melt. The evolved trachyandesitic and trachydacitic magmas can be derived from a basaltic parent primarily via fractional crystallization. Entrapment pressures estimated from plagioclase-hosted melt inclusions suggest relatively shallow levels of crystallization. However, trace element data (e.g., Dy/Yb ratio trends) suggests amphibole played an important role in the differentiation of the Hudson magmas, and this fractionation is likely to have occurred at depths >6 km. The absence of a garnet signal in the Hudson trace element data, the potential staging point for differentiation of parental mafic magmas [i.e., ~20 km (e.g., Annen et al. in J Petrol 47(3):505–539, 2006)], and the inferred amphibolite facies [~24 km (e.g., Rudnick and Fountain in Rev Geophys 33:267–309, 1995)] combine to place some constraint on the lower limit of depth of differentiation (i.e., ~20–24 km). These constraints suggest that differentiation of mantle-derived magmas occurred at upper-mid to lower crustal levels and involved a hydrous mineral assemblage that included amphibole, and generated a basaltic to basaltic andesitic composition similar to the magma discharged during the first phase of the 1991 eruption. Continued fractionation at this depth resulted in the formation of the trachyandesitic and trachydacitic compositions. These more evolved magmas ascended and stalled in the shallow crust, as suggested by the pressures of entrapment obtained from the melt inclusions. The decrease in pressure that accompanied ascent, combined with the potential heating of the magma body through decompression-induced crystallization would cause the magma to cross out of the amphibole stability field. Further shallow crystallization involved an anhydrous mineral assemblage and may explain the lack of phenocrystic amphibole in the Hudson suite.

Transition from Ultrapotassic Kamafugitic to Sub-alkaline Magmas: Sr, Nd, and Pb Isotope, Trace Element and 40Ar–39Ar Age Data from the Middle Latin Valley Volcanic Field, Roman Magmatic Province, Central Italy

The Middle Latin Valley volcanic field forms part of the Roman Magmatic Province and includes Pleistocene monogenetic volcanism characterized by the emplacement of small lava flows and minor pyroclastic ejecta and flows.The absence of a main volcanic edifice and of a large, shallow-level magma reservoir allows the eruption of primitive magmas. Geochemical and petrological data suggest that at least four types of mafic parental magmas are present within the volcanic field: (1) melilite-bearing ultrapotassic (kamafugitic); (2) plagioclase-bearing and -free leucititic (HKS); (3) shoshonitic; (4) sub-alkaline. Ar^Ar dating reveals diachronous emplacement of mafic magmas with different levels of K enrichment; the kamafugitic lavas are the oldest and the sub-alkaline lavas the youngest. Incompatible trace element contents strictly follow K2O, but overall the groups of rocks show similar trace element fractionation, with high field strength elements less enriched than large ion lithophile elements. Despite a restricted range in MgO and SiO2 contents, the Middle LatinValley volcanic rocks have highly variable Sr, Nd and Pb isotopic compositions. The sub-alkaline rocks have the lowest Sr/Sr and the highest Nd/Nd, whereas the kamafugitic rocks have the highest Sr/Sr and the lowest Nd/Nd. Intermediate isotopic compositions between these two end-members are shown by leucitites^plagio-leucitites and shoshonites. A clear, time-dependent trend of isotopic variation is observed. This also holds true for Pb isotope compositions, with shoshonitic and subalkaline rocks showing the most radiogenic signatures and the kamafugitic rocks the least radiogenic signatures.The overall geochemical characteristics of the magmas can be reconciled in terms of a model involving recycling of marly shales within the upper mantle; this overprinted earlier pervasive metasomatism related to melts (supercritical fluids) derived from altered oceanic basalts. The crustal derived (marl) end-member is considered to have been concentrated within a metasomatic vein network within the lithosphere, whereas the supercritical fluid-metasomatized end-member occurs within the surrounding mantle. Early partial melting of veins produced strongly undersaturated melilite-bearing ultrapotassic magma (kamafugitic).The progressive exhaustion of the veined mantle increased the

The oxygen isotopic composition as an indicator of the genesis of “Basaltic” corundum

This paper reports an oxygen isotopic study of corundums and associated minerals from the Yogo lamprophyres (Montana, US), plagioclase-corundum inclusions in alkali basalt (Tunkin depression, Russia), and from modern alluvium of Podgelbanochnyi alkaline basaltic volcano (Primorye, Russia). It is shown that all sapphires genetically related to mafic magmatic rocks have a similar oxygen isotopic composition (the variations of δ18O are within 2.5‰) with most values plotting between +4.5 and +7.0‰ SMOW. The oxygen isotopic ratios in the associated minerals (olivines, pyroxenes, mica, and others) and host rocks are plotted in the same interval. This indicates that the sapphire crystallized during evolution of the parental mafic magma. However, there are xenogenic corundums, which were only transported by basaltic magma to the Earth’s surface. They have a sharply distinct oxygen isotopic composition, which suggests their disequilibrium with the host lavas, and, correspondingly, a different genetic nature.

Dioritic intrusions of the Slavkovský les (Kaiserwald), Western Bohemia: their origin and significance in late Variscan granitoid magmatism

Mafic and intermediate intrusions occur in the Slavkovský les as dykes, sills and minor tabular bodies emplaced in metamorphic rocks or enclosed in late Variscan granites near the SW contact of the Western Krusne hory/Erzgebirge granite pluton. They are similar in composition and textures to the redwitzites defined in NE Bavaria. Single zircon Pb-evaporation analyses constrain the age of a quartz monzodiorite at 323.4 ± 4.4 Ma and of a granodiorite at 326.1 ± 5.6 Ma. The P–T range of magma crystallization is estimated at ~1.4–2.2 kbar and ~730–870°C and it accords with a shallow intrusion level of late Variscan granites but provides lower crystallization temperatures compared to the Bavarian redwitzites. We explain the heterogeneous composition of dioritic intrusions in the Slavkovský les by mixing between mafic and felsic magmas with a minor effect of fractional crystallization. Increased K, Ba, Rb, Sr and REE contents compared to tholeiitic basalts suggest that the parental mafic magma was probably produced by melting of a metasomatised mantle, the melts being close to lamprophyre or alkali basalt composition. Diorites and granodiorites originated from mixed magmas derived by addition of about 25–35 and 50 vol.%, respectively, of the acid end-member (granite) to lamprophyre or alkali-basalt magma. Our data stress an important role of mafic magmas in the origin of late Variscan granitoids in NW Bohemian Massif and emphasize the effect of mantle metasomatism on the origin of K-rich mafic igneous rocks.

The origin of hydrous, high-δ18O voluminous volcanism: diverse oxygen isotope values and high magmatic water contents within the volcanic record of Klyuchevskoy volcano, Kamchatka, Russia

Klyuchevskoy volcano, in Kamchatka’s subduction zone, is one of the most active arc volcanoes in the world and contains some of the highest δ18O values for olivines and basalts. We present an oxygen isotope and melt inclusion study of olivine phenocrysts in conjunction with major and trace element analyses of 14C- and tephrochronologically-dated tephra layers and lavas spanning the eruptive history of Klyuchevskoy. Whole-rock and groundmass analyses of tephra layers and lava samples demonstrate that both high-Mg (7–12.5 wt% MgO) and high-Al (17–19 wt% Al2O3, 3–6.5 wt% MgO) basalt and basaltic andesite erupted coevally from the central vent and flank cones. Individual and bulk olivine δ18O range from normal MORB values of 5.1‰ to values as high as 7.6‰. Likewise, tephra and lava matrix glass have high-δ18O values of 5.8–8.1‰. High-Al basalts dominate volumetrically in Klyuchevskoy’s volcanic record and are mostly high in δ18O. High-δ18O olivines and more normal-δ18O olivines occur in both high-Mg and high-Al samples. Most olivines in either high-Al or high-Mg basalts are not in oxygen isotopic equilibrium with their host glasses, and Δ18Oolivine–glass values are out of equilibrium by up to 1.5‰. Olivines are also out of Fe–Mg equilibrium with the host glasses, but to a lesser extent. Water concentrations in olivine-hosted melt inclusions from five tephra samples range from 0.4 to 7.1 wt%. Melt inclusion CO2 concentrations vary from below detection (<50 ppm) to 1,900 ppm. These values indicate depths of crystallization up to ~17 km (5 kbar). The variable H2O and CO2 concentrations likely reflect crystallization of olivine and entrapment of inclusions in ascending and degassing magma. Oxygen isotope and Fe–Mg disequilibria together with melt inclusion data indicate that olivine was mixed and recycled between high-Al and high-Mg basaltic melts and cumulates, and Fe–Mg and δ18O re-equilibration processes were incomplete. Major and trace elements in the variably high-δ18O olivines suggest a peridotite source for the parental magmas. Voluminous, highest in the world with respect to δ18O, and hydrous basic volcanism in Klyuchevskoy and other Central Kamchatka depression volcanoes is explained by a model in which the ascending primitive melts that resulted from the hydrous melt fluxing of mantle wedge peridotite, interacted with the shallow high-δ18O lithospheric mantle that had been extensively hydrated during earlier times when it was part of the Kamchatka forearc. Following accretion of the Eastern Peninsula terrains several million years ago, a trench jump eastward caused the old forearc mantle to be beneath the presently active arc. Variable interaction of ascending flux-melting-derived melts with this older, high-δ18O lithospheric mantle has produced mafic parental magmas with a spectrum of δ18O values. Differentiation of the higher δ18O parental magmas has created the volumetrically dominant high-Al basalt series. Both basalt types incessantly rise and mix between themselves and with variable in δ18O cumulates within dynamic Klyuchevskoy magma plumbing system, causing biannual eruptions and heterogeneous magma products.

Petrology of Karoo volcanic rocks in the southern Lebombo monocline, Mozambique

The Karoo volcanic sequence in the southern Lebombo monocline in Mozambique contains different silicic units in the form of pyroclastic rocks, and two different basalt types. The silicic units in the lower part of the Lebombo sequence are formed by a lower unit of dacites and rhyolites (67–80 wt.% SiO 2) with high Ba (990–2500 ppm), Zr (800–1100 ppm) and Y (130–240 ppm), which are part of the Jozini–Mbuluzi Formation, followed by a second unit, interlayered with the Movene basalts, of high-SiO 2 rhyolites (76–78 wt.%; the Sica Beds Formation), with low Sr (19–54 ppm), Zr (340–480 ppm) and Ba (330–850 ppm) plus rare quartz-trachytes (64–66 wt.% SiO 2), with high Nb and Rb contents (240–250 and 370–381 ppm, respectively), and relatively low Zr (450–460 ppm). The mafic rocks found at the top of the sequence are basalts and ferrobasalts belonging to the Movene Formation. The basalts have roughly flat mantle-normalized incompatible element patterns, with abundances of the most incompatible elements not higher than 25 times primitive mantle. The ferrobasalt has TiO 2 ∼ 4.7 wt.%, Fe 2O 3 t = 16 wt.%, and high Y (100 ppm), Zr (420 ppm) and Ba (1000 ppm). The Movene basalts have initial (at 180 Ma) 87Sr/ 86Sr = 0.7052–0.7054 and 143Nd/ 144Nd = 0.51232, and the Movene ferrobasalt has even lower 87Sr/ 86Sr (0.70377) and higher 143Nd/ 144Nd (0.51259). The silicic rocks show a modest range of initial Sr-( 87Sr/ 86Sr = 0.70470–0.70648) and Nd-( 143Nd/ 144Nd = 0.51223–0.51243) isotope ratios. The less evolved dacites could have been formed after crystal fractionation of oxide-rich gabbroic cumulates from mafic parental magmas, whereas the most silica-rich rhyolites could have been formed after fractional crystallization of feldspars, pyroxenes, oxides, zircon and apatite from a parental dacite magma. The composition of the Movene basalts imply different feeding systems from those of the underlying Sabie River basalts.

Geochemistry, Petrogenesis and Geodynamic Relationships of Miocene Calc-alkaline Volcanic Rocks in the Western Carpathian Arc, Eastern Central Europe

We report major and trace element abundances and Sr, Nd and Pb isotopic data for Miocene (16� 5^11 Ma) calc-alkaline volcanic rocks from the western segment of the Carpathian arc. This volcanic suite consists mostly of andesites and dacites; basalts and basaltic andesites as well as rhyolites are rare and occur only at a late stage. Amphibole fractionation both at high and low pressure played a significant role in magmatic differentiation, accompanied by highpressure garnet fractionation during the early stages. Sr^Nd^Pb isotopic data indicate a major role for crustal materials in the petrogenesis of the magmas.The parental mafic magmas could have been generated from an enriched mid-ocean ridge basalt (E-MORB)type mantle source, previously metasomatized by fluids derived from subducted sediment. Initially, the mafic magmas ponded beneath the thick continental crust and initiated melting in the lower crust. Mixing of mafic magmas with silicic melts from metasedimentary lower crust resulted in relatively Al-rich hybrid dacitic magmas, from which almandine could crystallize at high pressure. The amount of crustal involvement in the petrogenesis of the magmas decreased with time as the continental crust thinned. A striking change of mantle source occurred at about 13 Ma. The basaltic magmas generated during the later stages of the calc-alkaline magmatism were derived from a more enriched mantle source, akin to FOZO. An upwelling mantle plume is unlikely to be present in this area; therefore this mantle component probably resides in the heterogeneous upper mantle. Following the calc-alkaline magmatism, alkaline mafic magmas erupted that were also generated from an enriched asthenospheric source. We propose that both types of magmatism were related in some way to lithospheric extension of the Pannonian Basin and that subduction played only an indirect role in generation of the calc-alkaline magmatism.The calc-alkaline magmas were formed during the peak phase of extension by melting of metasomatized, enriched lithospheric mantle and were contaminated by various crustal materials, whereas the alkaline mafic magmas were generated during the post-extensional stage by low-degree melting of the shallow asthenosphere. The western Carpathian volcanic areas provide an example of long-lasting magmatism in which magma compositions changed continuously in response to changing geodynamic setting.

Mantle source volumes and the origin of the mid-Tertiary ignimbrite flare-up in the southern Rocky Mountains, western U.S.

Voluminous intermediate to silicic composition volcanic rocks were generated throughout the southern Rocky Mountains, western U.S., during the mid-Tertiary “ignimbrite flare-up”, principally at the San Juan and Mogollon-Datil volcanic fields. At both volcanic centers, radiogenic isotope data have been interpreted as evidence that 50% or more of the volcanic rocks (by mass) were derived from mantle-derived, mafic parental magmas, but no consensus exists as to whether melting was largely of lithospheric or sub-lithospheric mantle. Recent xenolith studies, however, have revealed that thick (> 100 km), fertile, and hydrated continental lithosphere was present beneath at least portions of the southern Rocky Mountains during the mid-Tertiary. The presence of such thick mantle lithosphere, combined with an apparent lack of syn-magmatic extension, leaves conductive heating of lithospheric mantle as a plausible method of generating the mafic magmas that fueled the ignimbrite flare-up in this inland region. To further assess this possibility, we estimated the minimum volume of mantle needed to generate the mafic magmas parental to the preserved mid-Tertiary igneous rocks. Conservative estimates of the mantle source volumes that supplied the Mogollon-Datil and San Juan volcanic fields are ∼ 2 M km 3 and ∼ 7 M km 3, respectively. These volumes could have comprised only lithospheric mantle if at least the lower ∼ 20 km of the mantle lithosphere beneath the entire southern Rocky Mountains region underwent partial melting during the mid-Tertiary and if the resulting mafic magmas were drawn laterally for distances of up to ∼ 300 km into each center. Such widespread melting of lithospheric mantle requires that the lithospheric mantle have been uniformly fertile and primed for melting in the mid-Tertiary, a possibility if the lithospheric mantle had experienced widespread hydration and refrigeration during early Tertiary low angle subduction. Exposure of the mantle lithosphere to hot, upwelling sub-lithospheric mantle during mid-Tertiary slab roll back could have then triggered the mantle melting. While a plausible source for mid-Tertiary basaltic magmas in the southern Rocky Mountains, lithospheric mantle could not have been the sole source for mafic magmas generated to the south in that portion of the ignimbrite flare-up now preserved in the Sierra Madre Occidental of northern Mexico. The large mantle source volumes (> 45 M km 3) required to fuel the voluminous silicic ignimbrites deposited in this region (> 400 K km 3) are too large to have been accommodated within the lithospheric mantle alone, implying that melting in sub-lithospheric mantle must have played a significant role in generating this mid-Tertiary magmatic event.

Contrasting origins of Cenozoic silicic volcanic rocks from the western Cordillera of the United States

Two fundamentally different types of silicic volcanic rocks formed during the Cenozoic of the western Cordillera of the United States. Large volumes of dacite and rhyolite, mostly ignimbrites, erupted in the Oligocene in what is now the Great Basin and contrast with rhyolites erupted along the Snake River Plain during the Late Cenozoic. The Great Basin dacites and rhyolites are generally calc-alkaline, magnesian, oxidized, wet, cool (<850°C), Sr-and Al-rich, and Fe-poor. These silicic rocks are interpreted to have been derived from mafic parent magmas generated by dehydration of oceanic lithosphere and melting in the mantle wedge above a subduction zone. Plagioclase fractionation was minimized by the high water fugacity and oxide precipitation was enhanced by high oxygen fugacity. This resulted in the formation of Si-, Al-, and Sr-rich differentiates with low Fe/Mg ratios, relatively low temperatures, and declining densities. Magma mixing, large proportions of crustal assimilation, and polybaric crystal fractionation were all important processes in gener- ating this Oligocene suite. In contrast, most of the rhyolites of the Snake River Plain are alkaline to calc-alkaline, ferroan, reduced, dry, hot (830-1,050°C), Sr-and Al-poor, and Nb-and Fe-rich. They are part of a distinctly bimodal sequence with tholeiitic basalt. These characteristics were largely imposed by their derivation from parental basalt (with low f H2O and low fO2) which formed by partial melting in or above a mantle plume. The differences in intensive parameters caused early precipitation of plagioclase and retarded crystallization of Fe-Ti oxides. Fractionation led to higher density magmas and mid-crustal entrapment. Renewed intrusion of mafic magma caused partial melting of the intrusive complex. Varying degrees of partial melting, fractionation, and minor assimilation of older crust led to the array of rhyolite compositions. Only very small volumes of distinctive rhyolite were derived by fractional crystallization of Fe-rich intermediate magmas like those of the Craters of the Moon-Cedar Butte trend.

A high-pressure experimental study on the evolution of the silicic magmatism of the Main Ethiopian Rift

In this contribution we report the results of an experimental study that investigated equilibrium and fractional crystallization of hydrous, transitional alkaline basalt at low oxygen fugacity, under lower to middle crustal conditions to constrain the generation of subaluminous and peralkaline differentiation products that typically occur in rift systems. The experiments reveal that liquids produced by equilibrium crystallization in the range 0.7–1 GPa cannot cross the subaluminous/peralkaline compositional divide. In contrast, fractional crystallization experiments under isobaric and polybaric conditions approach closer the naturally observed trend from subaluminous to evolved peralkaline products suggesting that polybaric differentiation starting at elevated pressures can indeed lead to the transition from subaluminous to peralkaline derivative liquids. The presence of water in the parental magmas of silicic derivative products is of prime importance for the fractionation equilibria as well as for the mobility of such magmas toward shallow crustal levels. We suggest that peralkaline magmas in rift environments are indicative for differentiation under relatively low oxygen fugacity conditions in an extensional environment characterized by a high degree of crustal fracturing that allows rapid upward migration of mafic parental magmas and formation of shallow magma reservoirs. Crystallization–differentiation of parental, hydrous transitional alkaline basalt in such reservoirs is controlled by low pressure phase equilibria that typically evolve through early saturation of anorthite-rich plagioclase and suppressed amphibole crystallization resulting in ‘low-alumina’, peralkaline derivative liquids.