Mineralogy and geochemistry of the « Low potassium » series of the roccamonfina volcanic suite (Campania,South Italy)

In the Campania area volcanic rocks belonging both to the trachytic (low K) and tephritic (high K) series occur, the high K series being well subordinate to the low K one. Volcanology of the trachytic series shows that: 1) the most widespread products are trachytes and alkali-trachytes, 2) in the stratigraphic sequences the most evolved (salic) volcanics are the oldest. The Roccamonfina trachytic series includes several Hy and Q normative lavas, whereas the majority of the Phlegraean Fields rock samples are Ne normative. In the A F M diagram, the lack of the Fe-rich phases gives a linear trend similar to the calc-alkaline series. An important feature of the Roccamonfina trachytic series is the coexistence of two clinopyroxenes of diopsidic and salitic composition observed in all the studied samples. The phenocrystic assemblage (clinopyroxenes, Mg-olivine, Mg-biotite) indicates that the crystallization of the melt probably occurred under relatively highpH₂O conditions. The crystallization of the diopsidic pyroxene was controlled mainly by a relatively high silica activity of the magma, whereas the salitic pyroxene, which crystallized at a lithostatic pressure lower than the diopside, might be related to the increase of the alumina activity in the final trachytic liquid. The occurrence of two clinoproxenes suggests two main episodes of crystallization of the parental latitic magma, one at a depth of about 15 km producing diopside phenocrysts and the second at a shallower depth where salite was in equilibrium with the residual melt. The volcanic rocks of the Roccamonfina trachytic series (latites, trachytes and alkalitrachytes) have likely been generated by fractionation of a latitic melt, whereas the strongly porphyritic trachybasalts formed by crystal settling of pyroxenes and olivine from a more evolved latitic melt.     

Volcanic rocks of methana,South Aegean arc,Greece

Cale-alkalic lavas and associated intrusives and cognate xenoliths, from the peninsula of Methana, are comparable geochemically with similar lavas from elsewhere in the South Aegean arc. At least four differentiation series have been recognized, largely of basaltic andesite to rhyodacite type, in which hydrous and dry conditions alternated. The compositional range of minerals other than plagioclase is limited. The chemical mineralogy of olivines and pyroxenes has been studied in detail. An aluminous bronzite that is found, contrary to previous interpretations, does not appear to be a high pressure phase. The chemical composition of the basic rocks and basic xenoliths suggests that the parent material of these lavas was of high alumina basalt composition. The abundance of accumulative xenoliths and the change of the mafic minerals to more iron-rich compositions towards the more acid end of each series, suggest that phenocryst fractionation has been the major process for the later stages of the evolution of all these rocks.     

Open-system processes in the genesis of silica-oversaturated alkaline rocks of the Rallier-du-Baty Peninsula, Kerguelen Archipelago (Indian Ocean)

The Rallier-du-Baty Peninsula forms the southwestern part of the Kerguelen Archipelago (Indian Ocean), whose magmatic activity is related to the long-lived 115-Ma Kerguelen plume. The peninsula is mostly made of alkaline rocks constituting two well-defined ring complexes. This paper focuses on the northern ring complex, which is not yet known. Recent field studies have revealed seven discrete syenitic ring dykes ranging in age from 6.2 to 4.9 Ma, and two later volcanic systems. ⁴⁰Ar/³⁹Ar dating of a trachytic ignimbrite linked to the Dôme Carva volcano complex yields an age of 26±3 Ka. This represents the last major eruptive event on the Kerguelen Archipelago. The volcanism is bimodal with trachybasalts and trachyandesites constituting the mafic lavas and trachytes and rhyolites constituting the felsic lavas. The volume of erupted felsic magma is by far the larger, and is represented by abundant pyroclastic deposits and lava flows. Boulders of plutonic rocks are found to the northwest of Dôme Carva, and represent intermediate rocks (i.e. monzogabbros and monzonites) that are not present at the surface. Basic rocks are mostly trachybasalts and trachyandesites, while true basalts are scarce. Their mineralogy consists chiefly of plagioclase, olivine, diopside and oxides. Sieve-textured plagioclase is common, as well as corroded olivine and diopside phenocrysts. Peralkaline commenditic trachytes are the most abundant type of acid volcanic rocks. They consist of abundant sanidine, augite and magnetite phenocrysts and interstitial quartz, aegerinic pyroxenes and Na-amphiboles. Ring dykes of quartz-poor alkali feldspar syenites display the same mineralogy, except hornblende is common and replaces diopside. Hornblende is particularly abundant in intermediate monzogabbros. Major and trace element variations of volcanic rocks emphasise the predominant role of fractional crystallisation with a general decrease of MgO, CaO, P₂O₅, TiO₂, FeO, Ba, Sr and Ni from basic to felsic rocks. However, the scattering of the data from the basic rocks indicates that other processes have operated. The overall evolution from trachyte to rhyolite is in agreement with the fractionation of sanidine as the major control. An increase of incompatible elements from trachyte to rhyolite is observed. The felsic lavas display an increase of ⁸⁷Sr/⁸⁶Sr_(i) without any significant variations in the Nd isotopic composition. The genesis of the basic rocks is complex and reflects concomitant processes of fractional crystallisation, mixing between different basic magmas and probable assimilation of Ba-rich oceanic crust. Major and trace element modelling confirms the possibility of producing the trachytes through continuous differentiation from a basaltic alkaline parent. Discrepancies observed for some trace elements can be explained by the crystallisation of amphibole at an intermediate stage of magma evolution. The overall evolution from trachyte to rhyolite is thought to be controlled by crystal fractionation. High ⁸⁷Sr/⁸⁶Sr_(i) of the trachytes is interpreted to reflect interaction with an ocean-derived component, probably during assimilation of hydrothermally altered oceanic crust. Boulders of amphibole-bearing monzonites and monzogabbros found to the northwest of Dôme Carva are thought to represent intermediate magma composition that formed at depths but did not erupt.     

Sub-volcanic crystallization at Stromboli (Aeolian Islands, southern Italy) preceding the Sciara del Fuoco sector collapse: evidence from monzonite lithic suite

 We describe texture, mineralogy and whole-rock composition of cognate monzonite sub-volcanic clasts within debris flow deposits related to the 5000 years catastrophic phreatomagmatic eruption probably linked to the Sciara del Fuoco sector collapse. The debris flows are at the top of accretionary lapilli-rich ash deposits overlying potassic (KS, shoshonites) lavas of the Neostromboli period. The monzonites are inferred to be crystallized in situ, at low P, at the side walls and/or roof margins of a shallow magma chamber and to be cogenetic with the KS Neostromboli extrusives. They can be considered "ideal orthocumulates" since they approximately retain a bulk liquid composition and possibly represent "slowly cooled equivalents" of their KS shoshonite host rock. The "closure temperature" of final solidification of the monzonite lithic suite was estimated through ternary-feldspar geothermobarometry, plagioclase–K-feldspar and K-feldspar–biotite equilibria and is in the range of 750–790  °C with a maximum –logfO₂ around 15.1–15.3. The estimated pressure of crystallization is <0.5 kbar. Potassic lavas and dikes, previously emplaced during the Neostromboli period, also resemble the monzonites in both major trace elements and mineral chemistry. The cogenetic relationship between KS Neostromboli extrusives and the monzonite host-rock magma from which the sub-volcanic clasts were derived is clear evidence that a shallow magma chamber existed between the caldera collapse of the Vancori period and the Sciara del Fuoco sector collapse (i.e. between 13 000 and 5000 years). The monzonite clasts were derived from crystallization at very shallow depth (ca. 1 km) and strongly support the hypothesis of violent decompression of the shallow magmatic plumbing system during the Sciara del Fuoco sector collapse. Climax of the regressive landslide event, with maximum disruption of the chamber walls, took place during emplacement of the debris flows, i.e. during the late stage of the Neostromboli phreatomagmatic eruption. Received: 15 September 1996 / Accepted: 5 May 1997     

Bimodal volcanism at the Katla subglacial caldera,Iceland: insight into the geochemistry and petrogenesis of rhyolitic magmas

The Katla subglacial caldera is one of the most active and hazardous volcanic centres in Iceland as revealed by its historical volcanic activity and recent seismic unrest and magma accumulation. A petrologic and geochemical study was carried out on a suite of mid-Pleistocene to Recent lavas and pyroclastic rocks originated from the caldera. The whole series is characterised by a bimodal composition, including Fe-Ti transitional alkali basalts and mildly alkalic rhyolites. Variations in trace-element composition amongst the basalts and rhyolites show that their chemical differentiation was mainly controlled by fractional crystallisation and possible assimilation. The petrology and chemistry of the few intermediate extrusive rocks show that they were derived from magma mingling or hybridisation. The absence of extrusive rocks of true intermediate magmatic composition and the occurrence of amphibole-bearing felsic xenoliths support the hypothesis of partial melting of the hydrated basalt crust as the main process leading to the generation of rhyolites. The ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr values of Katla volcanic rocks fit the general isotopic array defined by late Quaternary to Recent lavas from Iceland. A few rock specimens are distinguished by low ¹⁴³Nd/¹⁴⁴Nd values suggesting assimilation and mixing of much older crustal material. Despite their similar whole-rock chemical compositions, the postglacial rhyolitic extrusives differ from the felsic xenoliths by their glass composition and the absence of amphibole. This, together with the general chemical trend of volcanic glasses, indicates that the postglacial rhyolitic extrusives were probably derived by a process involving late reheating and partial melting of crustal material by intrusion of basaltic magmas.     

Mafic and ultramafic xenoliths in San Bartolo lava field: New insights on the ascent and storage of Stromboli magmas

Mafic and ultramafic xenoliths are well represented within a large basaltic lava field of Stromboli. These basalts, known as San Bartolo lavas, show a high-K calc-alkaline (HKCA) affinity and were erupted <5 ka BP. Xenoliths consist of olivin-gabbro, gabbronorite, anorthosite, dunite, wehrlite and clinopyroxenite. Thermobarometric estimates for the crystallization of gabbroic materials show minima equilibration pressures of 0.17–0.24 GPa, at temperatures ranging from 940 to 1,030°C. These materials interacted with hydrous ascending HKCA basaltic magmas (with temperatures of 1,050–1,100°C) at pressures of about 0.2–0.4 GPa. These pressure regimes are nearly identical to those found for the crystallization of phenocrystic phases within HKCA basaltic lavas. Gabbroic inclusions are regarded as cumulates and represent crystallized portions of earlier HKCA Strombolian basalts.Dunite and wehrlite show porphyroclastic-heterogranular textures, whereas the clinopyroxenite exhibit a mosaic-equigranular texture typical of mantle peridotites. These ultramafic materials are in equilibrium with more primitive basaltic magmas (under moderately hydrous and anhydrous conditions) at pressures of 0.8–1.2 GPa, which is below the crust-mantle transition, located at about 20 km depth under Stromboli.Major and trace element distributions indicate comagmatism between the host basaltic lava and the mafic and ultramafic inclusions. REE patterns for mafic nodules are relatively regular and overlap the field of basaltic lavas (HKCA). They show moderate to high LREE enrichments and moderate enrichments in HREE relative to chonrites. Spider diagrams also show significant similarities between the lavas and the mafic-ultramafic xenoliths as well.During their ascent, primitive Strombolian magmas may be stored in upper-mantle regions where they interact with peridotitic materials and partly differentiate (to give dunite and wehrlite) before migrating to upper crustal levels. In this region, hydrous basaltic magmas (with estimated water contents of 2–3.5 wt%) are stored in the subvolcanic environment, and are allowed to crystallize the gabbroic materials before reaching the surface under nearly anhydrous conditions.An erratum to this article can be found at     

Cooling history of pyroxene chondrules in the Yamato-74191 chondrite (L3)—an electron microscopic study

Fine textures of clinopyroxene in an excentroradial pyroxene chondrule (EPC) and a comb-like pyroxene chondrule (CPC) in the Yamato-74191 chondrite (L3) have been studied by analytical electron microscopy. Both pyroxenes consist of three regions different in composition and texture; core, mantle and marginal regions, though the pyroxenes of the CPC are more Fe-rich than those of the EPC. The core region is the most Mg-rich with no Ca component and commonly shows polysynthetic (100) twins. The mantle region is slightly calcic, and the marginal region shows a rapid increase of Ca outward.The polysynthetic twins, cracks and subgrain boundaries in the core in the EPC and CPC must have formed during the transition from proto-type to clino-type pyroxenes. The exsolution textures in the mantle and marginal regions indicate initial crystallization of pigeonite-C followed by decomposition into pigeonite-P and augite. The decomposition must have taken place by nucleation growth in the mantle region and by spinodal decomposition in the marginal region. The periodicity of 15–20 nm in the spinodal decomposition textures indicates that the cooling rate of the pyroxenes, when passing through about 1000°C, was of the order of a few tens to several degrees centigrade per hour. The cooling history of the chondrules has been explained by a monotonous cooling controlled by the cooling rate of the surrounding medium.     

RbSr systematics and crustal contamination models for calc-alkaline igneous rocks

New strontium isotopic data of calc-alkaline Pliocene-to-Quaternary lavas (southern Peru) confirm their anomalous isotopic composition compared to those of calc-alkaline rocks from active margins where continental crust is not involved. Gradual enrichment of radiogenic Sr occurs during fractional crystallization of calc-alkaline magma. The variation of the isotopic composition of these lavas as well as⁸⁷Sr/⁸⁶Sr versus 1/(⁸⁶Sr) diagrams form the basis for a model involving processes of fractional crystallization combined with mixing and addition of radiogenic Sr that originated in continental crust and was transported by a fluid phase.     

The quaternary caldera volcano emuruangogolak,kenya rift,and the petrology of a bimodal ferrobasalt-pantelleritic trachyte association

Emuruangogolak is a Quaternary basalitrachyte volcano situated in the Suguta graben of the northern Kenva rift, and probably erupted last early in this century. Following the construction of an early trachytic shield volcano, two episodes of caldera collapse occurred. each preceded by explosive pvroclastic activity. Post-calelera volcanism consisted of alternating phases of basalt and trachyte eruption. The basic lavas are high-Ti ferrobasalts of a mildly alkaline ‘transitional’ composition and the trachytes are peralkaline and oversaturated. A distinct compositional bimodality exists and no rocks in the range 49–59°. SiO, have been found. Major and trace element analyses suggest that the trachytes are genetically related to the basalts. Associations of almost identical lavas occur in Ethiopia. Pantelleria and the Azores but with the presence of intermediate terms Fractional crystallization is the mechanism currently preferred to account for the origin of the trachytes. The ‘Daly gap’ may be a consequence of a crystallization process which limits the volume of intermediate magma available at any time. In addition, the physical properties and spatial distribution of the different magmas probably discriminate against the cruption of lavas of intermediate composition.     

Petrology and geochemistry of volcanic rocks of Aegina,Greece

The Plio-Pleistocene cale-alkalic lavas of Aegina are comparable geochemically with similar lavas from elsewhere in the South Aegean arc. At least four differentiation series, largely of basaltic andesite to rhyodacite type, have been recognized in Aegina. Petrographical and geochemical evidence shows that lavas and xenoliths have a common origin. The mineralogy of the rocks suggests an alternation of hydrous and dry conditions during their formation. The chemical composition of the basic rocks, and of basic cognate xenoliths in the lavas, suggest that the parent material of all the lavas was of basic composition. Fractionation of the early formed phases is believed to have been the main process for the formation of the more acidic differentiates.     

Paths and sequence of intrusion of alkaline rocks on Turja Peninsula

The Turja Peninsula consists of porphyritic granite overlain by sandstone, cut by numerous dikes and veins of varying composition, form, and orientation. The relative ages of the dikes and veins has been established by their mutual intersections, and correlation of these with compositions of the rocks demonstrates the existence of three periods of igneous activity. The dikes and veins of the first period strike predominantly NNE and dip ESE. Most of those of the second period strike E-W and dip N, but those of a subordinate group strike NNE and dip ESE. Those of the third period strike predominantly N-S and dip E. The first and third periods are characterized by fine grained rocks resembling effusives, the second by coarse grained typically intrusive rocks and intense metasomatism that altered both dikes and veins and the wall rocks. The difference in texture of rocks formed at the same depth horizon is attributed to a difference in temperature of the wall rocks and the rate of rise of the magma through them.     

江西省广丰县李家膨润土矿床成因及找矿标志

广丰县李家膨润土矿床赋存于白垩系上统南雄组第二岩性段酸性火山岩的顶底部。膨润土的主要矿物成分为蒙脱石,矿石结构构造与成矿原岩有关。矿石化学成分与酸性熔岩相近,矿体形态及空间分布受酸性熔岩流相带的控制。因此认为,膨润土矿床成矿原岩为酸性玻质熔岩,矿床是由酸性玻璃质熔岩经水解脱玻化而形成,其成因类型为风化淋滤蚀变岩型矿床。膨润土可自然改型,由原来的钠基膨润土改型成钙基膨润土和偏镁的钙基膨润土。对时代较新的酸性火山岩,在玻质熔岩发育处都有可能找到有利用价值的膨润土矿。由于膨润土常由珍珠岩、沸石脱玻化形成,三者常共生出现,是很好的找矿标志。     

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 FeO_tot/(FeO_tot+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 (ɛ_Nd = −4 to −6; ⁸⁷Sr/⁸⁶Sr_i = 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. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites.     

Melt inclusions in quartz phenocrysts: The acid rocks of the Bannaya–Karymshina area,Kamchatka

The Bannaya–Karymshina area is situated in southern Kamchatka west of the East Kamchatka Volcanic Belt in the backarc part of the Kuril–Kamchatka island arc. The area is unique in that it contains abundant ejecta of calc-alkaline, acid, mostly ignimbrite, volcanism for a period of 4 Ma. Three rock complexes can be identified with rhyolitic and rhyodacitic compositions: Middle Pliocene ignimbrites, crystalloclastic tuffs of Eopleistocene age that fill in the Karymshina caldera, and Early Pleistocene intrusions. All of these are composed of rocks with normal total alkalinity, while the concentration of potassium places them at the boundary between moderate and high-potassium rocks. We sought to determine the composition of primary acid melts by studying the composition of the silicate phase in homogeneous melt inclusions that were conserved in quartz phenocrysts hosted by volcanic rocks of varying ages. Practically all the melt inclusions we analyzed show increased total alkalies and are in the class of trachyrhyodacites and trachyrhyolites, with the varieties of the highest alkali content being alkaline rhyolites and comendites; the concentration of K₂O classifies them as subalkaline rocks; one also notes the increased alumina of the acid melts. The compositions and spatial locations of the melt inclusions in quartz phenocrysts provide evidence of a three-phase crystallization in magma chambers at different depths. According to the experimental data, the quartz phenocrysts crystallized in a water-saturated melt at pressures of 0.1 to 3.5 kbars.     

Origine des grenats des roches calco-alcalines du Sud-Est de l’Espagne

Tertiary dacites and rhyodacites in SE Spain contain garnets, both as phenocrysts in the lavas and within their enclosed metamorphic venoliths. Microprobe analyses show that calc-alkaline and metamorphic rocks garnets are quite similar in composition (almandine-rich). However, the zoning patterns displayed in the two types of garnets differ. Garnets included in lavas show normal zoning (or a slight enrichment of Mn at the borders), while garnets in xenoliths have Mn-enriched rims or reverse zoning. The differences in zonation, as well as in petrographic features, indicate that the garnets in volcanic rocks crystallized directly from the calc-alkaline magma at an early stage of its crystallization. On the other hand, the uniform composition of dacitic and rhyodacitic garnets, as well as its low pyrope and grossular content, suggest that the conditions in which the garnets crystallized were rapidly exceeded.     

Magma-reservoir crystallization processes: small-scale dikes in cumulate gabbros, Mauna Kea Volcano, Hawaii

 Gabbroic xenoliths that represent cumulate environments within Mauna Kea Volcano are, in rare examples, penetrated by small-scale (<7 cm) dikes. We examined four dike/host composite xenoliths to establish how this evidence for magma seemingly injected into cumulate gabbro fits into the evolution of igneous processes in shield volcano magma reservoirs. Olivine, clinopyroxene, and plagioclase compositions in both host gabbros and dikes are characteristically tholeiitic and evolved (Fo_71–66, cpx-Mg # 79–77, An_72–51) with respect to Hawaiian magmatism. Dikes, however, when compared with their host gabbros, have slightly greater abundances of some incompatible elements and slightly more evolved olivine compositions (e.g., Fo₆₈ vs Fo₇₁₎. Compared with Mauna Kea lava compositions, both host gabbros and dikes have lower incompatible-element abundances, positive Eu anomalies, and, notable for dikes, major-element compositions unlike those of lavas (e.g., SiO₂<46 wt.%). The small-scale dikes, therefore, also have cumulate characteristics. We interpret them as representing late-stage liquids (e.g., <5 wt.% MgO, based on <Fo₇₀) "squeezed" from solidifying cumulus piles of evolved (e.g., ∼Fo₇₀) gabbroic assemblages. The compositions of the dikes, however, do not match those of the most evolved liquids expected in reservoirs because they appear to have lost interstitial liquids (e.g., positive Eu anomalies, low abundances of some trace elements). Because minerals in the dikes were in equilibrium with highly evolved liquids, conditions for small-scale dike formation in cumulate environments apparently occur only at the last stages of reservoir magma differentiation and solidification. Received: 25 February 1997 / Accepted: 14 June 1997     

Geological and geochemical studies of the sintra alkaline igneous complex,portugal

The Sintra igneous complex, Portugal was an important centre of activity in late Cretaceous times. The great proportion of thealkaline rocks are felsic and include five large quartz syenite intrusions and trachyandesite, trachyte and alkali rhyolite lavas and dykes, most of which are oversaturated. Mafic rocks are sparse, but vary widely from alkaline and highly undersaturated types containing high K₂O, TiO₂ and Ba, similar to the contemporaneous Lisbon lavas, to hypersthene normative trachybasalts and one hypersthene normative basalt. The various magma types are intimately associated and a well-developed netveined complex of alkali gabbro, monzonite and syenite is recognised at Cabo da Roca. A study of the dyke distributions, intersections and orientations suggest a close propinquity of both oversaturated and undersaturated and of both felsic and matic magmas. The basic magmas of Sintra and Lisbon show a continuous range in undersaturation (0 to 16% normative nepheline) and rare hypersthene normative basalts. Derivation of the hypersthene normative and mildly undersaturated basalts from the more undersaturated melts by low pressure fractionation or contamination by siliceous crust is shown to be unlikely. High pressure eclogite fractionation of a hypersthene normative basalt or variations in the percentage partial melting of a mantle under conditions where titanphlogopite is a low melting fraction are both processes compatible with the variations in undersaturation and proportions of TiO₂, K₂O and Ba. The quartz syenites and over satured felsic lavas of Sintra are thought to be derived from hypersthene nor mative parents.     

The petrogenesis of peridotitic komatiites: Evidence from high-pressure melting experiments

The compositions of liquidus olivines and orthopyroxenes of natural specimens of spinifex- and quench-textured peridotitic komatiites from the Belingwe Greenstone Belt, Rhodesia have been determined for pressures between 10 and 40 kbars. In conjunction with the chemical variation exhibited by these peridotitic komatiites it is concluded that the more magnesian lavas cannot be derived by concentration of olivine phenocrysts, fractional crystallisation or equilibrium partial melting. The peridotitic komatiites could evolve by polybaric assimilation and complete melting of garnet lherzolite into an initial liquid containing about 24% MgO.     

Endogenous growth in channelized komatiite lava flows: evidence from spinifex-textured sills at Pyke Hill and Serpentine Mountain,Western Abitibi Greenstone Belt,Northeastern Ontario,Canada

Spinifex-textured sills (i.e., veins) characterized by komatiitic magmas that have intruded their own volcanic-piles have long been recognized. For instance, in the early 1970s, Pyke and coworkers, in their classic work at Pyke Hill in Munro Township, noted that not all spinifex-bearing ultramafic rocks formed as lava flows, rather some were clearly emplaced as small dikes and sills. Several hypotheses have been proposed to explain spinifex-textured sills: intrusion into a cold host, filter pressing, or drainage of residual liquid. However, these do not satisfactorily explain the phenomenon. Field and petrographic observations at Pyke Hill and Serpentine Mountain demonstrate that spinifex-bearing komatiite sills and dikes were emplaced during channel inflation processes when new magma was intruded into a cooler, semi-consolidated but permeable cumulate material. Komatiitic liquids were intruded into the olivine cumulate rocks near the boundary between the spinifex and the cumulate zones of well-organized to organized komatiite flows. Spinifex-textured sills are generally tabular in morphology, stacked one above another, with curviplanar contacts sub-parallel to stratigraphy. Some sills exhibit complex digitated apophyses. Thinner sills typically have a random olivine spinifex texture similar, though generally composed of coarser crystals, to that of komatiite lava flows. Thicker sills exhibit more complex organization of their constituent crystals characterized by zones of random olivine spinifex, overlying zones of organized coarse spinifex crystals similar to those found in lava flows. They have striking coarse dendritic spinifex zones composed of very large olivine crystals, up to several centimetres long and up to 1 cm wide that are not observed in lava flows. Typically, at the sill margins, the cumulate material of the host flow is composed of euhedral to subhedral olivine crystals that are larger than those distal to the contact. Many of these margin-crystals have either concentric overgrowth shells or dendritic olivine overgrowths that grew from the cumulate-sill contact toward the sill interior. The dendrites grew on pre-existing olivine cumulate at the contact in response to a sharp temperature gradient imposed by the intrusion of hot material, whereas the concentric overgrowths formed as new melt percolated into the unconsolidated groundmass of the host-flow cumulate material. Spinifex-textured sills and dikes occur in well-organized to organized flows that are interpreted to have formed by “breakouts” above and peripheral to lava pathways (channels/conduits) as a result of inflation that accompanied voluminous komatiitic eruptions responsible for the construction and channelization of komatiitic flow fields. The spinifex-textured dikes and sills represent komatiitic lava that was originally emplaced into the channel roof during periods of episodic inflation that resulted in lava breakouts and was subsequently trapped in the “roof rocks” during periods of channel deflation. Accordingly, the occurrence of spinifex-textured sills and dikes may indicate proximity to, and aid in the identification and delineation of lava channel-ways that could potentially host Ni–Cu–(PGE) mineralization within komatiitic lava flow-fields.     

Eruptive history and petrology of Mount Drum volcano,Wrangell Mountains,Alaska

Mount Drum is one of the youngest volcanoes in the subduction-related Wrangell volcanic field (80×200 km) of southcentral Alaska. It lies at the northwest end of a series of large, andesite-dominated shield volcanoes that show a northwesterly progression of age from 26 Ma near the Alaska-Yukon border to about 0.2 Ma at Mount Drum. The volcano was constructed between 750 and 250 ka during at least two cycles of cone building and ring-dome emplacement and was partially destroyed by violent explosive activity probably after 250 ka. Cone lavas range from basaltic andesite to dacite in composition; ring-domes are dacite to rhyolite. The last constructional activity occurred in the vicinity of Snider Peak, on the south flank of the volcano, where extensive dacite flows and a dacite dome erupted at about 250 ka. The climactic explosive eruption, that destroyed the top and a part of the south flank of the volcano, produced more than 7 km³ of proximal hot and cold avalanche deposits and distal mudflows. The Mount Drum rocks have medium-K, calc-alkaline affinities and are generally plagioclase phyric. Silica contents range from 55.8 to 74.0 wt%, with a compositional gap between 66.8 and 72.8 wt%. All the rocks are enriched in alkali elements and depleted in Ta relative to the LREE, typical of volcanic arc rocks, but have higher MgO contents at a given SiO₂, than typical orogenic medium-K andesites. Strontium-isotope ratios vary from 0.70292 to 0.70353. The compositional range of Mount Drum lavas is best explained by a combination of diverse parental magmas, magma mixing, and fractionation. The small, but significant, range in ⁸⁷Sr/⁸⁶Sr ratios in the basaltic andesites and the wide range of incompatible-element ratios exhibited by the basaltic andesites and andesites suggests the presence of compositionally diverse parent magmas. The lavas show abundant petrographic evidence of magma mixing, such as bimodal phenocryst size, resorbed phenocrysts, reaction rims, and disequilibrium mineral assemblages. In addition, some dacites and andesites contain Mg and Ni-rich olivines and/or have high MgO, Cr, Ni, Co, and Sc contents that are not in equilibrium with the host rock and indicate mixing between basalt or cumulate material and more evolved magmas. Incompatible element variations suggest that fractionation is responsible for some of the compositional range between basaltic andesite and dacite, but the rhyolites have K, Ba, Th, and Rb contents that are too low for the magmas to be generated by fractionation of the intermediate rocks. Limited Sr-isotope data support the possibility that the rhyolites may be partial melts of underlying volcanic rocks. Received March 13, 1993/Accepted September 10, 1993