Host Lavas Research Articles

Petrology of tholeiitic lavas from Tanna Island (New Hebrides): Importance of cumulative processes in Island arc magmatism

Tanna, one of the southernmost islands of the New Hebrides volcanic arc, is made of Late Pliocene to Recent island arc tholeiitic basalts and andesites, with SiO2 contents ranging from 45 to 57%. These lavas are highly porphyritic (30–50% in volume): phenocrysts of plagioclase are the most abundant, together with olivine and clinopyroxene. The groundmass contain plagioclase, augite, olivine, magnetite and glass; pigeonite, tridymite, sanidine and, rarely, biotite may also occur. The olivines and clinopyroxenes show an iron enrichment from the cores of phenocrysts to their rims and the groundmass crystals, but their compositional variations are not correlated with the Mg/Fe ratio of bulk host rocks, the most Fe-rich compositions being found in Mg-rich lavas. Plagioclase compositions range from An95 to An60 in the basalts and An60 to An50 in the andesites, but, within each group, they are not correlated with SiO2 or Na2O contents of host lavas. Consequently, the bulk major element compositions of Tanna volcanic rocks cannot be considered as primarily controlled by crystal separation from successive liquids. The oxyde-SiO2 variations diagrams, and the modal compositions and mineral chemistry show that crystal accumulation is the predominant mechanism accounting for bulk rock compositions. However, this does not exclude fractional crystallization: the variation of the calculated groundmass mineralogy strongly suggest the occurrence of crystal removal mainly clinopyroxene and magnetite.

Green clinopyroxenes and associated phases in a potassium-rich lava from the Leucite Hills, Wyoming

Green, salitic pyroxenes occur as megacrysts and as cores in diopsidic pyroxene phenocrysts and microphenocrysts in a wyomingite lava from Hatcher Mesa, Leucite Hills, Wyoming. Al-rich phlogopite (16–21% Al2O3), apatite, Fe-Ti-oxide, Mg-rich olivine (Fo93) and orthopyroxene (En61) also occur as megacrysts or as inclusions in diopside phenocrysts. All of these phases are found in ultramafic xenoliths in the host lava, and petrographic and chemical evidence is presented that the megacrysts originate by the disaggregation of the xenoliths. It is concluded that the latter are accidental fragments of the wall rocks traversed by the wyomingite magma and it is suggested that the clinopyroxene-rich xenoliths, from which the green pyroxenes are derived, formed in the upper mantle as a result of local metasomatism or by crystallization from magmas of unknown composition during an earlier igneous event. The precise role of the clinopyroxene-rich xenoliths (which also contain apatite, Fe-Ti-oxide and amphibole) in the genesis of the Leucite Hills magmas cannot be elucidated on the basis of the available data, but it is unlikely that they represent the source material from which these magmas are derived.

The nature of grain boundaries in dunite and lherzolite xenoliths: Implications for magma transport in refractory upper mantle material

Using X ray photoelectron spectroscopy (XPS), we have determined that the composition of intergranular regions on olivine crystals of unaltered dunite and lherzolite xenoliths from flows in Hawaii, Australia, and Arizona are markedly different from the compositions of both the olivine and the host transporting lavas. The boundaries are without low‐melting components, such as alkalis, but are rich in aluminum, silicon, magnesium, and iron. While XPS spectra offer only semiquantitative analyses, the compositions of the intergranular regions appear to be such that they would remain unmelted at temperatures existing in the xenolith source regions. We infer that within their source regions, refractory dunite bodies, of which the Hawaiian dunites are samples, have solid mechanical contacts along the olivine grain boundaries. Furthermore, we feel that the compositions of surface films are established so as to minimize the surface free energy along intergranular faces as melt is gradually removed. This is consistent with the observed inability of the host lavas to penetrate these xenoliths during their ascent to the surface. As such, the parent refractory material within the mantle appears to be effectively nonporous to the magma, and hence magma transport within the upper mantle would have to proceed by some mechanism other than the migration of melt along channels developed along wetted intergranular surfaces.

利尻火山・くつ形溶岩流の内部構造

Three types of vesicular segregated bodies are recognized in the Kutsugata lava flows, Rishiri Volcano; namely, “segregation layer”, “segregation cylinder”, and “segregation stock”. In transverse section. the segregation layer is a flat-lying parallel banding from several mm to more than 10cm thick. The segregation cylinder is a slender vertical column which is connected with the segregation layer at the upper end of the column. The segregation stock is defined as a pulge or pulme segregated from the host lava, and is associated with the segregation layer. These dark-colored segregated bodies must have formed during consolidation of the lava flaws which had ceased to move on the flat ground, in the definite sequence with segregation layers first, then segregation cylinders and finally segregation stocks. During cooling of basaltic magma, crystal particles and residual 1iquid with bubbles must have composed a dispersed or framewarked salid-fluid system. The field evidence suggests that the segregation layer was derived from the extension fracture developed perpendicular to the 1east principa1 stress direction in the frameworked so1id-fluidcomp1ex. The segregation cy1inde and stock, however, must have originated in the f1ow by the thixotropic break dawn and the release of stress-hardening of the solid-fluid system. It is considered that there were three mechanisms of material transport in the Kutsugata 1ava f1ows, viz., f1ow of a viscous 1iquid in the extensioh fractures, vo1ati1e channel1ing in the three-phase f1uidized system, and diapiric uprise of low density plastic masses. Eight chemica-analyses of the segregated bodies are compared with six from their hostparts in a typical thick lava flow together with all published bulk rock analyses from the Rishiri Volcano in order to trace fractionation trends of the magma after extrusion.

Multiple origins of clinopyroxenes in alkali basaltic rocks

Geochemical and textural data on clinopyroxenes in individual alkali basaltic flows from provinces in eastern Australia and the Massif Central can be used to differentiate four different modes of origin (three at high pressure) for these clinopyroxenes. Many single flows from the two provinces contain clinopyroxenes of three, or even four, of these origins. Rare flows contain core clinopyroxene with overgrowths of clinopyroxenes of two distinct generation. Each of the overgrowth is compositionally analogous to clinopyroxenes occurring in xenoliths or as discrete crystals in the same host lava. Such rimming relationships provide evidence that the host magma has undergone high pressure crystallisation and confirm that some xenoliths and megacrysts are cognate. With decreasing pressure the major changes in clinopyroxene chemistry are an increase in the ratio Al lv/Al vi, a linear increase of atomic proportions of Ti and Al with decreasing Si, and an increase in the Ti: (100 Mg/(Mg + Σ Fe)) ratio of the pyroxenes. Al 2O 2 wt, % is an unreliable potential geobarometric indicator; consideration of tetrahedral and octahedral site occupancies by Al (Al lv/Al vl ratio) is necessary. High-pressure fractionation dominated by clinopyroxene is postulated for some basaltic-composition, resulting in SiO 2 depletion, alkali enrichment and decrease in the Mg/(Mg + Fe 2+) value of the host magma.

Clinopyroxene composition of ophiolitic metabasalts in the Mediterranean area

Clinopyroxenes belonging to ophiolitic metabasalts from the Mediterranean area were analysed by microprobe. They exhibit a wide range of composition in agreement with the geochemical variations of the host lavas. The analysed clinopyroxenes show a high Ti-Al and Si-Ti correlation. Their Ti/Al ratio ranges from about 0.5 to very low values as in the Troodos and Pindos samples. The data on the clinopyroxenes give further support to the hypothesis that the Western Mediterranean ophiolitic basalts with “oceanic” characters are generated under various conditions and from magmas with variable chemical characters. In particular the data contradict the hypothesis that all the Northern Apennines and Corsica metabasalts were produced at “normal ridge segments”. Moreover, the clinopyroxenes occurring in some lavas from Troodos (and Pindos?) resemble the clinopyroxenes of the island arc basalts. Thus, although geochemical features and clinopyroxene composition do not necessarily impose a generation at destructive margins for these lavas, this hypothesis cannot be rejected.

Ultramafic and mafic nodule suites in shield-forming lavas from Mauritius

An ultramafic-mafic suite of nodules characterises the basic-intermediate shield-forming lavas on Mauritius. A close mineralogical coherence is seen between highly porphyritic host lavas and nodule type, with peridotite occurring only in picrite basalt, olivine gabbro in olivine basalt, bytownite anorthosite and feldspathic gabbro in feldspar-phyric basalt, and kaersutite gabbro, labradorite-andesine anorthosite and titanomagnetite-pyrrhotite assemblages in hawaiite. Prominent layering fabrics, crystallisation sequences, variations in chemical mineralogy and in mineral assemblages, together with phase equilibria constraints suggest that the nodules are derived from a compositionally and cryptically layered cumulate sequence which crystallised in a subjacent magma chamber at depths of 5–10 km (1.5–3.0 kb). The extensive development of kaersutite-rich assemblages and of marked compositional reversals in plagioclase is interpreted as evidence for the development of significantly hydrous conditions in the magma chamber by the hawaiitic stage of evolution. Combined olivine-clinopyroxene and plagioclase geother-mometry calculations suggest temperatures of c. 1000°C with P H 2 O /P Tot ratios possibly fluctuating between 0.5–1.0 at this stage. Fractionation of kaersutite-rich assemblages is thought to have maintained the transitional character of the magma until the final stages of evolution. The mineralogical coherence of the nodule/lava association indicates a simple model of magma chamber evolution. A process relating eruption of phenocryst-enriched magma from close to the magma/cumulate interface, with possible incorporation of unconsolidated crystal mushes, and elutriation of only the highest, most recently consolidated cumulates is proposed.

Ca-enrichment in olivines from volcanic rocks

Data are presented on the Ca-content of olivines in a range of volcanic rocks from a variety of suites. These include olivine basalts through to trachytes from Gough, St. Helena and Tristan da Cunha Islands; trachytes from central Victoria, Australia, and a leucitite and phonclitic-tephrite series from Bufumbira, Uganda. In olivine crystals from basaltic lavas the Ca- and Mn-contents are low and Fe shows the most significant zoning from core to rim. In Fe-rich olivines from trachytic differentiates Ca and Mn frequently show more significant variation than Fe. While the Mn-content is proportional to the Fe-content of these olivines, Ca-zoning, in many cases, is unrelated to Fe-content. The marked Ca-enrichment in olivines occurs with the absence of plagioclase in the host lavas. The Ca(Al  Na  K) ratio is shown to be related not only to the Ca-content of the olivines in the lava, but may be used to predict olivine stability in evolved compositions.

The Alkalic Rock Suite of Bogoslof Island, Eastern Aleutian Arc, Alaska

The historic eruptions of Bogoslof volcano were characterized by hornblende andesite in A.D. 1796, and nepheline-normative, hornblende basalt in A.D. 1883 and 1926-1927. Rare earth element (REE) abundances for the 1796 and 1926-1927 eruptions show enrichment of the light REE relative to the heavy REE, overall higher abundances in the andesite and a slight positive Eu anomaly in the basalt. The $$^{87}Sr/^{86}Sr$$ ratios are $$0.70343 \pm 7$$ for the andesite and $$0.70296 \pm 7$$ for the 1927 basalt. Lead isotopic abundances are similar for the two eruptions and the rocks are more radiogenic than mid-ocean ridge basalts. Microprobe data for phenocrysts of potassio ferroan pargasite, calcic augite and plagioclase show a considerable compositional range and degree of compositional overlap. Mafic inclusions in basalt and andesite contain the same phases as the host lavas and a similar compositional range, suggesting a syngenetic origin for inclusions and host. One inclusion contains quartz and biotite which ...

Clinopyroxene composition in mafic lavas from different tectonic settings

Many metamorphosed and weathered basalts contain fresh clinopyroxene crystals set in an altered groundmass. Microprobe analysis of these relict grains can be used to identify the magma type of the host lava. Statistical discrimination of clinopyroxenes from known magma types provides a test of the effectiveness of this method, showing that any attempt to classify an unknown clinopyroxene as either an ocean-floor basalt, a volcanic arc basalt, a within plate tholeiite or a within plate alkali basalt magma type should have a 70% chance of success. Identification of within plate alkali basalts is most likely to be successful because their pyroxenes characteristically have high Na and Ti and low Si contents. Within plate tholeiites can usually be distinguished from volcanic arc basalts because their pyroxenes contain more Ti, Fe and Mn. However, neither of these last two magma types can be easily distinguished from ocean floor basalts on the basis of pyroxene analyses. Diagrams of pyroxene composition based on discriminant functions and on Na2O vs MnO vs TiO2, SiO2 vs TiO2 and SiO2 vs Al2O3 provide the basis for visual discrimination. The discrimination achieved is mainly due to differences in the bulk chemistry of the host magmas and in the partitioning of cations into the pyroxene lattice; differences in temperature and crystallization histroy of the magmas are of lesser, but nevertheless finite, importance. Application of this technique to pyroxenes in metabasalts from Othris, Greece gave results consistent with, but more ambiguous than, results obtained from immobile trace element studies.

Petrogenetic significance of some new xenolithic alkaline rocks from East Africa

SummaryNew occurrences of xenolithic alkaline rocks from three East African volcanoes are described. Micro-ijolites and alkali pyroxenites occur in the carbonate-rich tufts and lavas of Mount Elgon; kaersutite-rich pyroxenites and a suite of fenitized basement rocks in the caldera flows of Mount Meru, and apparently unique melanocratic hortonolite nepheline syenites in the central foyaite plug of Mount Kenya. The 87Sr/86Sr ratios indicate mantle origin, and all are enriched in trace elements such as Sr, Nb, and Zr. The xenoliths from Mount Kenya are cumulate equivalents of the glassy kenyte lavas of the mountain; the pyroxenites of Mount Elgon are also cumulates whose predominant titanaugite is of identical composition to that of the host lavas. The contrast between the 'dry' Elgon pyroxenites and the 'wet' amphibole-rich types from Mount Meru is believed to reflect different CO2/H2O ratios in the volatile phase accompanying the alkaline vulcanicity. Strongly alkaline, carbonate-rich suites such as that of Mount Elgon derive from fairly dry alkali-basaltic magma under high CO2 activity by dominant pyroxene fractionation, whereas the mildly alkaline lineage seen in some areas of Mount Meru forms under more hydrous conditions in which amphibole fractionation is important. Mount Meru in addition shows a transition to rock-types typical of the strongly alkaline lineage, and this is reflected in the occurrence of relatively amphibole-poor pyroxenites with the amphibole-rich types.

High pressure cognate inclusions in the Newer Volcanics of Victoria

High pressure pyroxene- and amphibole-rich inclusions are found in a number of Victorian Newer Volcanics volcanoes. The host lavas range from nepheline basanite to nepheline hawaiite and nepheline mugearite. The wide variation in chemistry and mineralogy of the inclusions is explained by crystallization from basaltic magmas under varying P-T and PH2O conditions at depth. At moderate pressure wehrlite inclusions (ol+cpx) form, whereas at higher pressures pyroxenites (opx+cpx) and genetically related megacrysts form. Under relatively anhydrous conditions the clinopyroxene megacrysts show a trend of Ca enrichment whereas under hydrous conditions, when amphibole is also stable, the pyroxene shows a trend to greater iron enrichment. The trend nepheline basanite to nepheline mugearite has developed by extensive fractionation of amphibole at elevated pressures under hydrous conditions. Under less hydrous conditions where clinopyroxene assumes the dominant role during crystal fractionation, derivative liquids display a trend of increasing K2O/Na2O ratio, with little modification of their level of undersaturation. Olivine plays a decreasing role in crystal fractionation processes with increasing pressure. The available evidence indicates that the only magma which could have been parental to the observed basanites was a more picritic basanite.

Geological Relationships Between Massive Sulfide Bodies and Ophiolitic Volcanic Rocks near York Harbour, Newfoundland

Small lenses of massive copper- and zinc-rich sulfide occur in ophiolitic volcanic rocks capping the Bay of Islands Igneous Complex near York Harbour, Newfoundland. Ore was discovered in 1897 and about 50 000 tons were produced between 1898 and 1912. Recent exploratory work has outlined several ore bodies totaling 282 000 tons that contain 1.92% Cu and 4.67% Zn, assuming 20% dilution.The deposits are stratabound, occurring along an intensely altered zone that separates two members of the layered pillow lava succession. Two types of mineralization are present. Lenticular massive bodies ranging in size up to 37 000 tons are composed of brecciated pyrite, sphalerite and chalcopyrite. These are stratigraphically underlain by chalcopyrite-rich stringer mineralization in altered lava.Microscopic and petrochemical investigations indicate that the host lavas have undergone pervasive spilitic alteration and intensive propylitic alteration near ore. Geochemical studies of Co:Ni ratios in pyrite show that Ni content is highest in disseminated pyrite from the propylitized lavas and Co is highest in pyrite from stringer ore. The Co:Ni ratio for all analysed pyrite is greater than unity, suggesting a magmatic-volcanic derivation for the deposits.The geological evidence suggests that the sulfides are volcanogenic and were formed from hydrothermal fluids rich in Fe, Zn, Cu and S. These moved upward along zones of primary brecciation in the lavas, leached Fe, Ni, and Ti from the ferromagnesian minerals and deposited the massive lenses by chemical sedimentation or precipitation from the metal-rich exhalations on or near the sea floor.Comparison of the York Harbour deposits to the recent heavy metal bearing sediments of the Red Sea 'deeps', and to the Cretaceous lavas and orebodies of Cyprus, supports the above genetic interpretation, and indicates that these ores are clearly an integral part of ophiolite complexes. The comparison provides new information about sulfide mineral paragenesis during initial sedimentation, diagenesis, burial, and later metamorphism.

Melting of acid xenoliths into a basanite: An approach to the possible mechanisms of crustal contamination

Biotite gneiss xenoliths included in a basanitic flow in the Causses region (France) show several kinds of partial melting. Rhyolitic glasses appear at the expense of the quartz + feldspar assemblage; initially undersaturated latitic glasses at the expense of biotite + feldspar (+ quartz); lastly hyperaluminous products derivating from large phenoblasts of alkali feldspar. The microprobe study of these glasses, of their contacts with the host lava, as well as that of the neogenic mineral phases, show that contamination occurs through, at least, four possible processes: Qualitative magmatic contamination in alkalies (mainly K) and silica results from all these processes; quantitatively, the most important of them seems to be 4.

Oxides of low pressure origin from alkali basaltic rocks, southern highlands, N.S.W., and their bearing on the petrogenesis of alkali basaltic magmas

Abstract Alkali basaltic rocks from the Southern Highlands, N.S.W., contain oxide phases of both high and low pressure origin. The two phases are readily distinguished using chemical and textural criteria. Chemical data for low‐pressure Fe‐Ti oxides indicate that oxygen fugacities of the host lavas range from 10‐12.8 to 10‐8 atm at 950° to 1110°C. In most cases, the oxygen fugacities of the individual lava flows appear to be principally a function of temperature and intrinsic chemical equilibria existing at the time of formation of the basaltic liquid. However, some relatively differentiated flows shows a high degree of oxidation due to volatile enrichment with fractionation. Rare glassy flows show dendritic crystallization of Fe‐Ti oxides. Most flows in which abundant olivine was the first phase to be precipitated also contain Cr‐rich spinels associated, and apparently coeval, with the earliest‐crystallizing olivine.

Nature of source material for ultramafic minerals from Salt Lake Crater, Hawaii, from measurement of helium and argon diffusion

Helium and argon determinations on minerals contained in ultramafic xenoliths from Salt Lake crater, Oahu, Hawaii, show low values for the ratios of these radiogenic gases, incompatible with the radioactive parent (uranium-thorium and potassium) content. It is suggested that the gas content has been altered by diffusion during ascent of the xenoliths in the erupting lava. The diffusion of helium and argon contained in the augite of a pyroxenite (‘eclogite’) and in the olivine, enstatite, and chrome diopside of lherzolite nodules was measured at 1100°C. The 4He/40Ar ratio of the inert gases released from the minerals varied linearly with the square root of the heating time. Extrapolation of the ratio backward in time to the point where the minerals of the lherzolite nodules contained the same inert-gas ratio gave a value of 1.2 ± 0.2, which would be the gas ratio in the environment from which the gases were trapped. From this it was possible to estimate the time of exposure of the nodules to the high temperatures of the erupting lavas, which gave values between 6500 hours and 8000 hours. The possible use of such data to estimate the depth of the source of the nodules is pointed out. The 4He/40Ar ratio indicates gas accumulation for at least 3.5 b.y. from a material with the same U-Th/K ratio as the host lava of the ultramafics or as the garnet pyroxenite type of xenoliths.

Kinetic disequilibrium in trace element partitioning between phenocrysts and host lava

By means of general crystallization models it is shown that in general the observed partition coefficients for trace element distribution between phenocrysts and host lava are not equilibrium coefficients. This disequilibrium between the host lava and the phenocrysts, as far as trace element partitioning is concerned is due to an imbalance between the rate of crystal growth and trace element diffusion in the lava and in the crystal. It is suggested that the inverse correlation of grain size and certain trace-element concentrations for lunar basalts and oceanic tholeiites might be due to these kinetic effects.

Ore environment and ore genesis in Ramagiri gold field, Andhra Pradesh, India

Metamorphic origin, mobilization of gold during metamorphism of Precambrian host lava with aid of fluids derived from metamorphism of associated sediments, concentration in quartz bodies (formation due to release of large quantity of silica during metamorphism)

Partition coefficients of rare-earth elements between igneous matrix material and rock-forming mineral phenocrysts—II

Solid-liquid partition coefficients between phenocrysts and the host lavas have been measured for rare-earth elements by an isotope dilution technique. Phenocrysts analyzed include clinopyroxenes, orthopyroxenes, olivines, micas, amphiboles, garnet, plagioclases and K-feldspar. The consistency of much of the data suggests that most of the phenocrysts crystallized under equilibrium, or quasi-equilibrium, conditions. There is sufficient data on pyroxenes and feldspars to indicate that relative and absolute partition coefficients can be correlated with the major element composition of the phenocrysts. The size of the positive Eu anomaly in feldspars increases with increasing alkali content of the feldspar.

Partition of uranium and thorium between augite and host lavas

Contents of U and Th in mineral fractions separated from some volcanic lavas and a peridotite nodule have been measured. Th U ratios were larger in the groundmass than in the augite phenocryst of the lava. In the peridotite nodule, U and Th were most enriched in diopside. Ionic substitution of Ca in clinopyroxene by U and Th under equilibrium conditions is proposed for the possible partition mechanism.