Phenocryst Assemblages Research Articles

A geochemical and Sr–Nd isotopic study of the Vendian greenstones from Gorny Altai, southern Siberia: Implications for the tectonic setting of the formation of greenstones and the role of oceanic plateaus in accretionary orogen

The Central Asian Orogenic Belt is considered to be the most important site of juvenile crustal formation since the Neoproterozoic. Gorny Altai of southern Siberia represents the first stage of tectonic evolution in the Central Asian Orogenic Belt. Greenstones of Gorny Altai occur either as blocks within an accretionary complex surrounded by a sedimentary/serpentinite matrix, or as a coherent ophiolite complex. The greenstones are overlain or intercalated by micritic limestones indicating that they formed in shallow water in an intra-oceanic environment. The phenocryst assemblage of the greenstones (clinopyroxene+plagioclase, plagioclase and clinopyroxene) is, however, different from that of mid-oceanic ridge basalt (MORB).Based on the geochemical characteristics, these greenstones are divided into four groups. Group 1 greenstones show light rare earth element (LREE) depletion but no negative Nb–Ta anomaly in primitive mantle normalized multi-element plots; they resemble normal (N-) MORB or back-arc-basin basalt (BABB). Group 2 rocks have slight LREE depletion or show flat REE patterns; they have a negative Nb–Ta anomaly and are similar to island arc basalt (IAB) or BABB. Group 3 rocks have LREE-enriched patterns, no negative Nb–Ta anomaly and are similar to enriched (E-) MORB or oceanic island basalt (OIB). Group 4 greenstones are represented by LREE-enriched patterns, with negative Nb–Ta anomaly; they resemble IAB. Although the co-existence of such heterogeneous geochemical signatures may suggest that the rocks were formed in different tectonic settings followed by tectonic mixing and aggregation in a subduction zone, the field relationships, petrological and mineralogical composition, and geochemical characteristics suggest that the greenstones were formed as part of an oceanic plateau generated by a mantle plume source which contained a recycled crustal component. An important implication is that oceanic plateaus may represent an important constituent in the development of accretionary orogens.

Are arc-type rocks the products of magma crystallisation? Observations from a simple oceanic arc volcano: Raoul Island, Kermadec Arc, SW Pacific

Raoul Island is the emergent summit of a large intra-oceanic strato-volcano in what is globally one of the simplest of subduction settings. In this simple setting erupted magmas span the compositional range from basalt to dacite but none have the high Mg-numbers and high Ni and Cr expected of primitive mantle-derived melts. The lavas range from aphyric to highly porphyritic and are characterised by phenocryst assemblages dominated by plagioclase accompanied by clinopyroxene, olivine, orthopyroxene and spinel. Phenocryst core compositions and zoning profiles are remarkably uniform irrespective of total phenocryst content or geochemical composition, indicating a decoupling of melt and crystal components in the system. A consistent model for the Raoul magmatic system is that primitive high-Mg magma generated in a melt column within the underlying mantle wedge is transformed into a series of derivative low-Mg magmas by fractional crystallisation within the lower crust. Low-Mg magma accumulates variable quantities of crystal cargo as it ascends toward the surface through a crystal mush zone. These processes are essentially those that characterise continental subduction-related magmatic systems but differ only in the absence of an evolved crustal component.

Differentiation and crystallization conditions of basalts from the Kerguelen large igneous province: an experimental study

Phase relations of basalts from the Kerguelen large igneous province have been investigated experimentally to understand the effect of temperature, fO2, and fugacity of volatiles (e.g., H2O and CO2) on the differentiation path of LIP basalts. The starting rock samples were a tholeiitic basalt from the Northern Kerguelen Plateau (ODP Leg 183 Site 1140) and mildly alkalic basalt evolved from the Kerguelen Archipelago (Mt. Crozier on the Courbet Peninsula), representing different differentiation stages of basalts related to the Kerguelen mantle plume. The influence of temperature, water and oxygen fugacity on phase stability and composition was investigated at 500 MPa and all experiments were fluid-saturated. Crystallization experiments were performed at temperatures between 900 and 1,160°C under oxidizing (log fO2 ~ ΔQFM + 4) and reducing conditions (log fO2 ~ QFM) in an internally heated gas-pressure vessel equipped with a rapid quench device and a Pt-Membrane for monitoring the fH2. In all experiments, a significant influence of the fO2 on the composition and stability of the Mg/Fe-bearing mineral phases could be observed. Under reducing conditions, the residual melts follow a tholeiitic differentiation trend. In contrast, melts have high Mg# [Mg2+/(Mg2+ + Fe2+)] and follow a calk-alkalic differentiation trend at oxidizing conditions. The comparison of the natural phenocryst assemblages with the experimental products allows us to constrain the differentiation and pre-eruptive conditions of these magmas. The pre-eruptive temperature of the alkalic basalt was about 950–1,050°C. The water content of the melt was below 2.5 wt% H2O and strongly oxidizing conditions (log fO2 ~ ΔQFM + 2) were prevailing in the magma chamber prior to eruption. The temperature of the tholeiitic melt was above 1,060°C, with a water content below 2 wt% H2O and a log fO2 ~ ΔQFM + 1. Early fractionation of clinopyroxene is a crucial step resulting in the generation of silica-poor and alkali-rich residual melts (e.g., alkali basalt). The enrichment of alkalis in residual melts is enhanced at high fO2 and low aH2O.

Progressive Melt Extraction from Upwelling Mantle Constrained by the Kita-Matsuura Basalts in NW Kyushu, SW Japan

Systematic petrological, geochemical, and chronological studies of Cenozoic intra-plate volcanism in southwesternJapan, in the KitaMatsuura area in northwestern Kyushu, reveal temporal and spatial changes in mantle melting processes on a time scale of 2 5 Myr and a horizontal scale of 35 km.The most extensive basaltic activity in the area occurred between 8 5 and 6 0 Ma. Four vertical sections through the volcanic sequence from the underlying basement to the top flows were studied to encompass the distribution of the KitaMatsuura basalt. In the central and western sections (Hirado, Senryu, and Ishimori) there is a temporal variation in the chemical composition of the basalts: volcanism initiated eruption of mildly alkaline basalts (lowto medium-SiO2 group) followed by major flows of sub-alkaline basalt (mediumto high-SiO2 group). The eastern section (Kunimi section) is characterized by mildly alkaline basalts (low-SiO2 group). Each SiO2 group has a distinctive phenocryst assemblage and major element composition. The Zr/Y, Nb/Th, and Nb/Y ratios of the Kita-Matsuura basalts are positively correlated, which cannot be explained by assimilation of crustal materials but instead is linked to melting processes in the mantle. The average segregation depth of the inferred parental magmas of the three groups decreases in the order of SiO2 enrichment, indicating a temporal decrease in melt segregation depth in the central and western sections.The correlation between Zr/Yand Nb/Y can be reproduced by a series of instantaneous melts resulting from decompressional critical melting of a primitive mantle source with 2% trapped melt. In each section, both Zr/Yand Nb/Y decrease upwards, which implies an increase in the degree of melting with time. The temporal and spatial variations in basalt chemistry are most plausibly accounted for by progressive melt extraction during mantle upwelling, which started in the garnet stability field ( 3 0 GPa) and ended in the spinel field ( 1 5 GPa). Upwelling was centred beneath the Hirado section, in the westernmost area in KitaMatsuura, with a diameter inferred to be greater than 70 km.

C2/c pyroxene phenocrysts from three potassic series in the Neogene alkaline volcanics, NE Turkey: their crystal chemistry with petrogenetic significance as an indicator of P–T conditions

Chemical and structural data are reported for C2/c pyroxene phenocrysts collected from three potassic series (Group A: basanite-tephrite, Group B: tephrite-pho- nolitic tephrite, Group C: alkaline basalt-trachybasalt) of the Neogene alkaline volcanics (NAVs) in northeastern Turkey, in order to investigate the evolution of the mag- matic plumbing system and the location of magma chamber(s) with crystallization conditions. The rock series hosting the clinopyroxene phenocrysts show generally porphyritic texture and have a variable phenocryst-rich nature (20-58%), with phenocryst assemblages character- ized by cpx ± ol ± plag ± foid ± amp ± bio. The clinopyroxene phenocrysts can be chemically classified as Ti- and Fe 3? -rich Al-diopsides for Groups A and B (AB- cpxs) and Ti- and Fe 3? -poor Al-diopsides for Group C (C- cpxs). They have poorly variable composition, clustering in the diopside field. Structurally, the diopside groups have nearly similar a (ranging from 9.73 to 9.75 A u ), Vcell (437.2-440.9 A u 3 ), and hbetai angle values (106.01- 106.23), but some differences in polyhedral parameters and geometries of the AB-cpxs and C-cpxs have been observed. For example, the AB-cpxs are characterized by

Groundmass crystallization in dacite dykes taken in Unzen Scientific Drilling Project (USDP-4)

Groundmass textural and compositional analyses of the drilled dacite dykes of the Unzen Scientific Drilling Project (USDP-4) identify the feeder dyke of the 1990–1995 eruption and elucidate the crystallization process of dykes at depth. In the drilling depth range of 1582–1996 m (“conduit zone”), four dacite dykes were recognized. The groundmasses of all but one of these dykes have textures ranging from cryptocrystalline to microcrystalline aggregate of crystals < 10 µm across forming an equigranular mosaic of plagioclase, alkali feldspar, quartz, and pyrite. The samples include a small number of coarser-grained plagioclase microlites (20 µm to 0.3 mm long). The compositions of groundmass consisting only of grains < 10 µm plotted at the lower pressure (< 50 MPa) ternary minimum in the Qz'–Ab'–Or' system suggests that the crystallization of plagioclase, alkali feldspar, and quartz took place nearly simultaneously. The compositions of coarser plagioclase microlites and groundmass, the plagioclase microlite textures, and the phenocryst assemblages show significant differences from historical lavas exposed in the summit area. This implies the possibility that most of the dacite dykes are not feeder dykes for the lavas at the summit and remained beneath the surface, perhaps because of high viscosity associated with high SiO 2. One sample C14-1-1 collected 1977 m, has a texture, composition, and phenocryst assemblage nearly identical to that of the dome lava of the 1990–1995 eruption, differing only in the presence of hydrothermal alternation. At this time we cannot definitely conclude that C14-1-1 was the feeder dyke for the 1990–1995 eruption until we can elucidate the time scale and the conditions governing hydrothermal alternation.

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

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

Evidence for a short-lived stratified magma chamber: petrology of the Z-To5 tephra layer ( c . 5.8 ka) at Zao volcano, NE Japan

Abstract Volcanic rocks from the Z-To5 tephra layer of Zao volcano, NE Japan, preserve petrological information that reflects the magmatic processes under the volcano. The Z-To5 rocks were formed by the mixing of three magmas that differed in composition and phenocryst assemblage: basalt (1150–1200 °C), with high Mg (Fo c . 81 ) olivine; basaltic andesite (1020–1100 °C), with Mg-rich orthopyroxene (Mg#= c . 78) and clinopyroxene (Mg#= c . 78), lower Mg olivine (Mg#= c . 78), and calcic (An c . 85 ) plagioclase; and andesite (900–1000 °C) with Mg-poor orthopyroxene (Mg#=61–66) and clinopyroxene (Mg#=64–68), and An-poor plagioclase. The basaltic magma was formed through fractionation of Fo c . 85 olivine from a less differentiated basaltic magma during its fast ascent from the depths. The andesitic magma, which occupied a shallow magma chamber, was heated by underplating of the basaltic magma, resulting in dissolution of some minerals. Subsequently, the basaltic andesite magma was formed by mixing of the basaltic and andesitic magmas in the chamber. Petrological evidence for the rapid growth of phenocrysts in the basaltic andesite magma suggests that the magma residence time was short. The basaltic andesite magma, mixing with a small portion of the andesitic magma, was withdrawn upon eruption. The rates of these processes are inferred to be rapid based on petrological considerations.

Adakitic Dacites Formed by Intracrustal Crystal Fractionation of Water-rich Parent Magmas at Nevado de Longaví Volcano (36·2°S; Andean Southern Volcanic Zone, Central Chile)

SUMMARY AND IMPLICATIONS The two Holocene dacitic episodes at NLV produced themost evolved magma compositions that are preservedat this volcano, although occurrences of amphibole-richcumulate xenoliths in earlier main-cone lavas indicatethat magmas lying along similar liquid lines of descentwere being generated at depth by the NLV magmaticsystem prior to the Holocene (Selle¤s, 2006). The erupteddacites have adakitic signatures, high B and fluid-mobile/immobile trace element ratios, and amphibole-rich pheno-cryst assemblages, and they lack clinopyroxene. Quenchedmafic enclaves in dacitic rocks represent inputs of unu-sually water-rich magmas, which also have relatively lowconcentrations of most incompatible elements, high B,high mobile/immobile trace element ratios, and a highmodal proportion of amphibole phenocrysts.These simila-rities and their mingled association in the Lomas Limpiasdacite and Castillo Andesite strongly support a closegenetic link in the form of a parental role for the enclavemagmas.A polybaric, two-stage crystal extraction modelconstrained by experimentally determined phase relationsand natural mineral compositions reproduces whole-rockmajor and trace element trends and is a logical conse-quence of differentiation of these extremely water-richmagmas. Anomalously low incompatible trace elementconcentrations in dacites (except forboron) are not consis-tent with significant upper crustal assimilation, but theyare easily reconciled with volumetrically important frac-tionation of amphibole, which in turn is supported by thepresence of amphibole-rich cumulate xenoliths. The traceelement abundances in the early, high-pressure stage ofevolution have been adjusted by the fractionation of 1 5%garnet. Although modal garnet is not observed, experi-mental studies (Mu«ntener et al., 2001; Ulmer & Mu«ntener,2005; Alonso-Pe¤rez, 2006) have shown that it could bestable inwet mantle-derived melts at pressures correspond-ing to inferred lower crustal pressuresbelow NLV.The lackof erupted primitive melts precludes rigorous modeling ofthe early, deep fractionation, which is inferred to be thekey to the adakitic signature of the Holocene NLVmagmas. The deep hot zone model proposed by Annenet al. (2006) provides an adequate thermo-mechanicalframework for generation of magmas derived by fractiona-tion of high-pressure assemblages from hydrous, mantle-derived magmas.A crystal fractionation origin for adakite-like rocks hasbeen proposed in earlier studies for other arc settings,notably in the geodynamically complex Philippine arc(Castillo et al., 1999; Prouteau & Scaillet, 2003; Solidumet al., 2003; Castillo & Newhall, 2004; Macpherson et al.,2006), but also in the Ecuadorian Andes (Chiaradia et al.,2004) and the Archean of South Africa (Kleinhanns et al.,2003). All of these studies concur that amphibole plays apreponderant role in the generation of the adakitic signa-ture (with or without garnet), as well as limiting theamount of plagioclase being fractionated. Previous studiescall upon elevated magmatic water contents to explain theparticipation of amphibole-rich assemblages, althoughthe reasons why elevated water fluxes occurred at a givenplace and time remain largely unexplored. We proposethat this unique occurrence of adakitic magmas in theQuaternaryAndean SVZ is ultimately related to the posi-tion of Nevado de Longav|¤ above the down-dip projectionof the oceanic Mocha Fracture Zone, which could trans-port large amounts of water hosted in serpentinite bodies(e.g. Bonatti & Crane, 1984; Ulmer & Trommdorff, 1995;Kerrick, 2002; Macleod et al., 2002; Ranero et al., 2003;Ranero & Sallare's, 2004).

Extreme geochemical heterogeneity in Afro-Arabian Oligocene tephras: Preserving fractional crystallization and mafic recharge processes in silicic magma chambers

Four Indian Ocean deep-sea tephras can be correlated to individual Oligocene on-land silicic pyroclastic units found in the Afro-Arabian flood volcanic province (Yemen and Ethiopia), providing valuable stratigraphic marker horizons. They also preserve among the largest geochemical heterogeneities observed in individual eruption events (SiO 2: 43 to 75 wt.% and 56 to 79 wt.%). The major, trace element and isotopic variations preserved in individual shards in these ash layers provide a unique series of snapshots of magma chamber processes that are used to elucidate the formation and petrogenetic evolution of large volume, chemically zoned silicic magmatic systems. Banded shards within individual ash layers represent up to 85% of the SiO 2 variation observed in the entire tephra on the scale of < 1 mm 3, clearly demonstrating that the observed compositional variations found within individual layers represent individual eruptive events. The silicic magmas are compositionally related to the underlying Main Flood Basalt phase of volcanism in Yemen, whereas the basaltic end-member found in the tephras is compositionally distinct and related to the Upper Series mafic lavas, found intercalated with the Main Silicic Series on-land ignimbrites and tuffs. The intermediate to silicic component of the tephras was generated by extreme fractional crystallization ( F = ca. 60%) of plagioclase, anothoclase, augite, magnetite and ilmenite, as observed in on-land phenocryst assemblages.

Genesis of post-hotspot, A-type rhyolite of the Eastern Snake River Plain volcanic field by extreme fractional crystallization of olivine tholeiite

Rhyolites occur as a subordinate component of the basalt-dominated Eastern Snake River Plain volcanic field. The basalt-dominated volcanic field spatially overlaps and post-dates voluminous late Miocene to Pliocene rhyolites of the Yellowstone–Snake River Plain hotspot track. In some areas the basalt lavas are intruded, interlayered or overlain by ~15 km3 of cryptodomes, domes and flows of high-silica rhyolite. These post-hotspot rhyolites have distinctive A-type geochemical signatures including high whole-rock FeOtot/(FeOtot+MgO), high Rb/Sr, low Sr (0.5–10 ppm) and are either aphyric, or contain an anhydrous phenocryst assemblage of sodic sanidine ± plagioclase + quartz > fayalite + ferroaugite > magnetite > ilmenite + accessory zircon + apatite + chevkinite. Nd- and Sr-isotopic compositions overlap with coeval olivine tholeiites (ɛNd = −4 to −6; 87Sr/86Sri = 0.7080–0.7102) and contrast markedly with isotopically evolved Archean country rocks. In at least two cases, the rhyolite lavas occur as cogenetic parts of compositionally zoned (~55–75% SiO2) shield volcanoes. Both consist dominantly of intermediate composition lavas and have cumulative volumes of several 10’s of km3 each. They exhibit two distinct, systematic and continuous types of compositional trends: (1) At Cedar Butte (0.4 Ma) the volcanic rocks are characterized by prominent curvilinear patterns of whole-rock chemical covariation. Whole-rock compositions correlate systematically with changes in phenocryst compositions and assemblages. (2) At Unnamed Butte (1.4 Ma) the lavas are dominated by linear patterns of whole-rock chemical covariation, disequilibrium phenocryst assemblages, and magmatic enclaves. Intermediate compositions in this group resulted from variable amounts of mixing and hybridization of olivine tholeiite and rhyolite parent magmas. Interestingly, models of rhyolite genesis that involve large degrees of melting of Archean crust or previously consolidated mafic or silicic Tertiary intrusions do not produce observed ranges of Nd- and Sr-isotopes, extreme depletions in Sr-concentration, and cogenetic spectra of intermediate rock compositions for both groups. Instead, least-squares mass-balance, energy-constrained assimilation and fractional crystallization modeling, and mineral thermobarometry can explain rhyolite production by 77% low-pressure fractional crystallization of a basaltic trachyandesite parent magma (~55% SiO2), accompanied by minor (0.03–7%) assimilation of Archean upper crust. We present a physical model that links the rhyolites and parental intermediate magmas to primitive olivine tholeiite by fractional crystallization. Assimilation, recharge, mixing and fractional melting occur to limited degrees, but are not essential parts of the rhyolite formation process.

Petrogenesis and volcanology of anorogenic rhyolites: a special issue dedicated to Bill Bonnichsen

The occurrence of large volume, high-temperature silicic volcanic rocks is apparently confined to continental regions and is a manifestation of significant energy input into the continental crust over extended time periods. In the western United States, such magmatism occurs predominantly in extensional settings in the Cenozoic Great Basin and TransPecos (west Texas) volcanic provinces. It is particularly prolific in the late Cenozoic Snake River Plain-Yellowstone province that forms a time-transgressive ‘hot spot’ track across southern Idaho, where rhyolites were extruded as large lava flows and even larger ignimbrites. Volumes of individual eruptions range up to at least 100 km 3 for the lava flows and to as much as 3,000 km 3 for the ignimbrites. Cumulative volumes of silicic magma produced likely exceed 10 5 km 3 , making this province one of the most prolific rhyolite outpourings on Earth. These rhyolites are characterized by high magmatic temperatures (sometimes exceeding 1,000°C), anhydrous phenocryst assemblages, low oxygen fugacities, and A-type trace element chemistry. In all these respects the “anorogenic” rhyolites contrast sharply with silicic magmas from “orogenenic” settings (i.e., convergent margins and mid-Cenozoic calcalkaline silicic magmas of the Great Basin and Colorado Plateau including

Crystallization of primitive basaltic magmas at crustal pressures and genesis of the calc-alkaline igneous suite: experimental evidence from St Vincent, Lesser Antilles arc

Near-liquidus crystallization experiments have been carried out on two basalts (12.5 and 7.8 wt% MgO) from Soufriere, St Vincent (Lesser Antilles arc) to document the early stages of differentiation in calc-alkaline magmas. The water-undersaturated experiments were performed mostly at 4 kbar, with 1.6 to 7.7 wt% H2O in the melt, and under oxidizing conditions (ΔNNO = −0.8 to +2.4). A few 10 kbar experiments were also performed. Early differentiation of primitive, hydrous, high-magnesia basalts (HMB) is controlled by ol + cpx + sp fractionation. Residual melts of typical high-alumina basalt (HAB) composition are obtained after 30–40% crystallization. The role of H2O in depressing plagioclase crystallization leads to a direct relation between the Al2O3 content of the residual melt and its H2O concentration, calibrated as a geohygrometer. The most primitive phenocryst assemblage in the Soufriere suite (Fo89.6 olivine, Mg-, Al- and Ti-rich clinopyroxene, Cr–Al spinel) crystallized from near-primary (Mg# = 73.5), hydrous (∼5 wt% H2O) and very oxidized (ΔNNO = +1.5–2.0) HMB liquids at middle crustal pressures and temperatures from ∼1,160 to ∼1,060°C. Hornblende played no role in the early petrogenetic evolution. Derivative HAB melts may contain up to 7–8 wt% dissolved H2O. Primitive basaltic liquids at Soufriere, St Vincent, have a wide range of H2O concentrations (2–5 wt%).

Experimental study of the joins forsterite–diopside–leucite– and forsterite–leucite–åkermanite up to 2.3 GPa [ P(H 2O) = P(Total)] and variable temperatures: Its petrological significance

We conducted a series of melting experiments in the join forsterite–diopside–leucite under 0.1 and 2.3 GPa and in the join forsterite–leucite–åkermanite under 2.3 GPa to understand paragenetic relationships amongst different types of lamproitic and lamprophyric magmas with K-rich mafic and ultramafic volcanic (kamafugitic) rocks. Both the joins were studied in the presence of excess water. The experimental results of the join forsterite–diopside–leucite at 0.1 GPa show that the five-phase point of forsterite (Fo) ss + diopside (Di) ss + leucite (Lc) ss + liquid (Liq) + vapour (V) (equivalent to ugandite lava) occurs at Fo 2Di 50Lc 48 at 880 ± 5 °C. Phlogopite appears as the last phase at 830 ± 15 °C. The final crystalline assemblage of forsterite ss + diopside ss + leucite ss + phlogopite is similar to the phenocryst assemblage of missourite lava. Present study suggests that an olivine leucitite (ugandite) can be derived from an olivine italite, a slightly potassic peridotite and a leucitite magma. A study of the join Fo–Di–Lc [ P(H 2O) = P(Total)] at 2.3 GPa shows that liquid compositions penetrate the primary phase volumes of forsterite ss, phlogopite ss, kalsilitess, K-feldspar ss and diopside ss. It has the following three five-phase points: 1) one occurring at Fo 9Di 49Lc 42 and 1005 ± 5 °C, where liquid and vapour coexists with forsterite ss, phlogopite ss and diopside ss (phlogopite-bearing madupite), 2) the second one at Fo 4Di 50Lc 46 and 990 ± 10 °C, where diopside ss, K-feldspar ss and phlogopite ss coexist with liquid and vapour (pyroxene-bearing minette), and 3) the third one at Fo 3Di 21Lc 76 and 775 ± 5 °C, where phlogopite ss, kalsilite ss and K-feldspar ss are in equilibrium with liquid plus vapour (kalsilite-bearing minette). The experimental results of the join Fo–Lc–åkermanite (Ak) show that the join 40 penetrates the primary phase volumes of forsterite ss, phlogopite ss, kalsilite, K-feldspar ss, diopside ss and merwinite ss. The data indicate the presence of four five-phase points: 1) one occurring at Fo 7Lc 42Ak 51 and 1165 ± 5 °C, where phlogopite ss, forsterite ss, diopside ss coexists with liquid and vapour (olivine-bearing madupite), 2) the second one at Fo 3Lc 49Ak 48 and 1140 ± 10 °C, where a liquid is in equilibrium with phlogopite ss, K-feldspar ss, diopside ss and vapour (pyroxene-bearing minette), 3) the third one at Fo 18Lc 21Ak 61 and 1255 ± 10 °C, where merwinite ss, forsterite ss and diopside ss are in equilibrium with liquid and vapour (merwinite-bearing wherlite), and 4) the fourth one at Fo 5Lc 73.5Ak 21.5 and 770 ± 5 °C, where kalsilite ss, phlogopite ss and K-feldspar coexist with liquid and vapour (kalsilite-bearing minette). The present data suggest that high pressure heteromorphic equivalent of a katungite magma is represented by a kalsilite-bearing minette, a pyroxene-bearing minette, or an olivine-bearing madupite.

A North-South Transect across the Central Mexican Volcanic Belt at 100 W: Spatial Distribution, Petrological, Geochemical, and Isotopic Characteristics of Quaternary Volcanism

Within the Zita¤ cuaro^Valle de Bravo (ZVB) region of the central Mexican Volcanic Belt (MVB), three lava series have erupted during the Quaternary: (1) high-K2O basaltic andesites and andesites; (2) medium-K2O basaltic andesites, andesites and dacites; (3) high-TiO2 basalts and basaltic andesites.The dominant feature of the first two groups is the lack of plagioclase accompanying the various ferromagnesian phenocrysts (olivine, orthopyroxene, augite, and hornblende) in all but the dacites. This absence of plagioclase in the phenocryst assemblages of the high-K2O and medium-K2O intermediate lavas is significant because it indicates high water contents during the stage of phenocryst equilibration. In contrast, the high-TiO2 group is characterized by phenocrysts of plagioclase and olivine.The spatial distribution of these three lava series is systematic.The southern section of the ZVB transect, 280^330 km from the Middle AmericaTrench (MAT), is characterized by high-K2Omelts that are relatively enriched in fluidmobile elements and have the highest Sr/Sr ratios. Medium-K2O basaltic andesite and andesite lavas are present throughout the transect, but those closest to theMATareMgO-rich (3 5^9 4 wt%) and have phenocryst assemblages indicative of high magmatic water contents (3 5^6 5 wt % water) and relatively low temperatures (950^10008C). In marked contrast, the northern section of the ZVB transect (380^480 km from the MAT) has high-TiO2, high field strength element (HFSE)-enriched magmas that have comparatively dry (51 5 wt % magmatic water) and hot (1100^12008C) phenocryst equilibration conditions. The central section of the ZVB transect (330^380 km from the MAT) is a transition zone and produces moderately light rare earth element (LREE) and large ion lithophile element (LILE)-enriched, medium-K2O lavas with phenocryst assemblages indicative of intermediate (1 5^3 5 wt %) water contents and temperatures. The high-K2O series compositions are the most enriched in LILE and LREE, with a narrow range of radiogenic Sr/Sr from 0 704245 to 0 704507, Nd/Nd values ranging from 0 512857 to 0 512927 (eNd1⁄4 4 27^5 63), and Pb/Pb values from 38 248 to 38 442, Pb/Pb values from 15 563 to 15 585, and Pb/Pb values from 18 598 to 18 688. The medium-K2O series compositions are only moderately enriched in the LILE and LREE, with a broader range of Sr/Sr, but similar Nd/Nd and Pb/Pb values to those of the high-K2O series. In contrast, the high-TiO2 series compositions have little enrichment in LILE or LREE and instead are enriched in the HFSE and heavy rare earth elements (HREE). The high-TiO2 lavas are isotopically distinct in their lower and narrower range of Nd/Nd. The isotopic variations are believed to reflect the upper mantle magma source regions as the low content of phenocrysts in most lavas precludes significant upper crustal assimilation or magma mixing, other than that represented by the presence of quartz xenocrysts (52 vol. %) with rhyolitic glass inclusions, which are found in many of these lavas.The systematic spatial variation in composition of the three lava series is a reflection of the underlying subduction-modified mantle and its evolution.

Nature and significance of a Cambro-Ordovician high-K, calc-alkaline sub-volcanic suite: the late- to post-orogenic Motru Dyke Swarm (Southern Carpathians, Romania)

The Motru Dyke Swarm intrudes the Precambrian Danubian basement of the Southern Carpathians (Romania). It is a marker of a sub-volcanic event that occurred during the early Palaeozoic (Cambrian to Ordovician). The geographical distribution of dykes on a ∼2,000 km2 area is heterogeneous; several areas of high dyke density have been the subject of a detailed petrological and geochemical study. Taken altogether, the 150 samples define a single complete magmatic series, from basaltic andesite to rhyolite. Whole-rock major element variations show a medium- to high-K, calc-alkaline magmatic suite. The compositional variations and the general decrease of trace element contents (both compatible and incompatible, including REEs) from basaltic andesite to rhyolite are consistent with 1) the fractionation of the observed phenocryst assemblages, Ca-amphibole (Ti-pargasite to magnesiohornblende) followed by intermediate plagioclase, clinopyroxene and accessory biotite and quartz and 2) the absence of lower and/or upper crustal contamination. Trace elements diagrams display typical arc patterns (LILE, Pb and LREE enrichment and relative depletion in Nb-Ta, Zr-Hf and Ti). The Th/U, Nb/Ta and Zr/Hf ratios are constant and close to the mantle values throughout the whole series, which argues that the parental magma was generated from a single and homogeneous enriched lithospheric mantle source. The field regional evidence implies that melting occurred during a late- to post-orogenic period of lithospheric extension, and thus took place quite lately after the cessation of Pan-African subduction.

Cosmogenic 10Be, 21Ne and 36Cl in sanidine and quartz from Chilean ignimbrites

Our initial results indicate that three cosmogenic nuclides: 10Be, 21Ne and 36Cl can be analyzed in sanidine. To uncover complex exposure histories or marked changes in denudation rates over time several nuclides with different half-lives (or stable) must be measured. Because of its shorter half-life, the combination of 36Cl and a long-lived nuclide 10Be or stable nuclide 21Ne will provide more information than the pairs 10Be and 26Al or 10Be and 21Ne (in quartz). Sanidine (alkali feldspar) is a common high temperature mineral and often dominates the phenocryst assemblage in silicic to intermediate volcanic rocks. Bedrock surfaces studied come from the Oxaya (erupted 19–23Ma) and Lauca (erupted 2.7Ma) ignimbrites of northern Chile. Quartz and sanidine phenocrysts coexist; therefore, we can check the viability of sanidine through direct comparison with nuclide concentrations in quartz. In addition, as quartz has no target for 36Cl in significant abundance we show that the unique power of sanidine is that 36Cl can be measured. We have obtained very good agreement between 10Be and 21Ne concentrations measured in sanidine and coexisting quartz. No meteoric 10Be was apparent in these sanidines. Concentrations of all three nuclides in mineral separates from rock sample CN309 from the Lauca ignimbrite in the Western Cordillera agree well and correspond to minimum exposure ages of 30–50ka. 10Be and 21Ne measured in both sanidine and quartz from three rock samples from the Oxaya ignimbrite (CN19, CN23, CN104a) in the Western Escarpment record low average landscape modification rates (<0.70m/Ma) over the last several million years. In contrast, 36Cl data from sanidine in CN23 seem to indicate shorter minimum exposures and more rapid maximum erosion rates.

Pre-eruptive and syn-eruptive conditions in the Black Butte, California dacite: Insight into crystallization kinetics in a silicic magma system

A series of experiments and petrographic analyses have been run to determine the pre-eruption phase equilibria and ascent dynamics of dacitic lavas composing Black Butte, a dome complex on the flank of Mount Shasta, California. Major and trace element analyses indicate that the Black Butte magma shared a common parent with contemporaneously erupted magmas at the Shasta summit. The Black Butte lava phenocryst phase assemblage (20 v.%) consists of amphibole, plagioclase (core An 77.5), and Fe–Ti oxides in a fine-grained (< 0.5 mm) groundmass of plagioclase, pyroxene, Fe–Ti oxides, amphibole, and cristobalite. The phenocryst assemblage and crystal compositions are reproduced experimentally between 890 °C and 910 °C, ≥ 300 MPa, X H2O = 1, and oxygen fugacity = NNO + 1. This study has quantified the extent of three crystallization processes occurring in the Black Butte dacite that can be used to discern ascent processes. Magma ascent rate was quantified using the widths of amphibole breakdown rims in natural samples, using an experimental calibration of rim development in a similar magma at relevant conditions. The majority of rims are 34 ± 10 μm thick, suggesting a time-integrated magma ascent rate of 0.004–0.006 m/s among all four dome lobes. This is comparable to values for effusive samples from the 1980 Mount St. Helens eruption and slightly faster than those estimated at Montserrat. A gap between the compositions of plagioclase phenocryst cores and microlites suggests that while phenocryst growth was continuous throughout ascent, microlite formation did not occur until significantly into ascent. The duration of crystallization is estimated using the magma reservoir depth and ascent rate, as determined from phase equilibria and amphibole rim widths, respectively. Textural analysis of the natural plagioclase crystals yields maximum growth rates of plagioclase phenocryst rims and groundmass microlites of 8.7 × 10 − 8 and 2.5 × 10 − 8 mm/s, respectively. These rates are comparable to values determined from time-sequenced samples of dacite erupted effusively from Mount St. Helens during 1980 and indicate that syneruptive crystallization processes were important during the Black Butte eruptive cycle.

Mafic xenoliths from Egyptian Tertiary basalts and their petrogenetic implications

Mineralogical data, coupled with whole-rock major and trace element data of mafic xenoliths from two occurrences of the Egyptian Tertiary basalts, namely Abu Zaabal (AZ) near Cairo and Gabal Mandisha (GM) in the Bahariya Oases, are presented for the first time. Chemically, AZ basalts are sodic transitional, while those of GM are alkaline. In spite of the different petrographic and geochemical features of the host rocks, mafic xenoliths from the two occurrences are broadly similar and composed essentially of clinopyroxene, plagioclase, alkali feldspar, and Fe–Ti oxides. The analytical results of host rocks, xenoliths and their minerals suggest that the xenoliths are cognate to their host magmas rather than basement material. The mafic xenoliths are olivine-free and contain alkali feldspar contrary to the phenocryst assemblage of the host rocks, confirming that they are not cumulates from the host magma. The geochemical and mineralogical characteristics show that the precursor magmas of these xenoliths are more fractionated and possibly contaminated compared to those of the host rocks. Estimated crystallization conditions are ∼ 1–3 kbar for xenoliths from both areas, and temperature of ∼ 950–1100 °C vs. 920–1050 °C for AZ and GM, respectively. These cognate xenoliths probably crystallized from early-formed, highly-fractionated anhydrous magma batches solidified in shallow crustal levels, possibly underwent some AFC during their ascent, and later ripped-up during fresh magma pulses. The xenoliths, although rare, provide an evidence for the importance of crystal fractionation at early evolution of the Egyptian Tertiary basalts.

Application of the QUILF thermobarometer to the peralkaline trachytes and pantellerites of the Eburru volcanic complex, East African Rift, Kenya

The Quaternary Eburru volcanic complex in the south-central Kenya Rift consists of pantelleritic trachytes and pantellerites. The phenocryst assemblage in the trachytes is sanidine + fayalite + ferrohedenbergite + aenigmatite ± quartz ± ilmenite ± magnetite ± pyrrhotite ± pyrite. In the pantellerites, the assemblage is sanidine + quartz + ferrohedenbergite + fayalite + aenigmatite + ferrorichterite + pyrrhotite ± apatite, although fayalite, ferrohedenbergite and ilmenite are absent from more evolved rocks (e.g. with SiO 2 > 71%). QUILF temperature calculations for the trachytes range from 709 to 793 °C and for the pantellerites 668–708 °C, the latter temperatures being among the lowest recorded for peralkaline silicic magmas. The QUILF thermobarometer demonstrates that the Eburru magmas crystallized at relatively low oxidation states (ΔFMQ + 0.5 to − 1.6) for both trachytes and pantellerites. The trachytes and pantellerites evolved along separate liquid lines of descent, the trachytes possibly deriving from a more mafic parent by fractional crystallization and the pantellerites from extreme fractionation of comenditic magmas.