Clinopyroxene compositions in the Deccan and Rajmahal Traps and their bearing on magma types and evolution

Electron probe analyses of clinopyroxenes from several areas of the Deccan and Rajmahal Traps consisting mostly of subalkalic and alkalic basalts, picritic basalts and a few dolerite dykes have been obtained. Evaluation of the data indicate the absence of pigeonite from subalkalic basalts that occur in close spatial association with mild or strongly alkalic basalts in areas such as Rajpipla, Navagam and central Kachchh. Co-existence of augite and pigeonite, however, has been noticed in subalkalic basalts/dykes and picritic basalts from a number of Deccan localities such as Sagar, Igatpuri, Kalsubai, Triambak, Pavagarh and Girnar besides the one sample from Rajmahal. Diopside, salite, and wollastonite-rich compositions dominate the basanites and foidites of Kachchh whereas chrome-diopside and salite are the main types in the picrite basalt samples from Anila, Botad and Paliyad in Saurashtra akin to those found in contiguous areas in the east from borehole flows at Dhandhuka and Wadhwan studied in detail previously. Compositional variations in zoned clinopyroxenes indicate differentiation of the parental magma and also mixing of different magma types (subalkalic and alkalic) from areas such as Igatpuri, Rajpipla and Kachchh. Based on host-rock chemistry, total alkalis-silica plot, CIPW norms, estimated temperatures of eruption and augite – pigeonite thermometry, it has been inferred that clinopyroxene compositions, especially the incidence of pigeonite, appear to be very sensitive to bulk chemistry of host rocks, especially their Na₂O, K₂O, SiO₂, total iron and TiO₂ contents. Non-quadrilateral cationic components in the clinopyroxenes, such as Al in tetrahedral and octahedral positions together with Si, Na, Ti and Cr abundances have been found to be useful to discriminate clinopyroxenes from alkalic and subalkalic basalt types besides inferences on the ferric iron component in them. Evaluation of host-rock compositions in the ternary olivine–clinopyroxene-quartz plot indicate polybaric conditions of crystallization and evolution especially in samples that are picritic (e.g. Pavagarh, Anila and Kachchh) and which could also breach the olivine–clinopyroxene-plagioclase thermal divide that exists in part between alkalic and subalkalic basalts under atmospheric conditions.     

Some chemical controls on igneous amphibole compositions

The composition of igneous amphiboles is extremely sensitive to magma composition and the nature of the co-existing phases. The extent of solid solution in amphiboles may be depicted in the basalt tetrahedron. They are more silica-undersaturated than the liquid from which they have crystallized for liquids containing about 11% normative nepheline. For liquids containing more than 11% normative nepheline the reverse is true. The existence of this compositional divide may be demonstrated for both intrusive and extrusive suites. Amphiboles are generally nepheline-normative, except when crystallizing from granodioritic to granitic magmas.Fractionation factors between amphibole and magma composition for MgO and FeO appear to increase with decreasing temperature; those for TiO₂ and Na₂O decrease with increasing pressure. The CaO content of amphiboles appears to be insensitive to the CaO content of the magma.     

Contributions to the mineral chemistry of Hawaiian rocks

Feldspar phenocrysts, microphenocrysts, groundmass feldspar, interstitial material of feldspar composition, and residual SiO₂-K₂O-rich glass in 24 rocks of the tholeiitic, alkalic, and nephelinic suites from Haleakala and West Maui volcanoes, Maui, Hawaii, were analyzed quantitatively with the electron microprobe. Rocks studied include tholeiite, olivine tholeiite, oceanite, alkalic olivine basalt, alkalic basalt, hawaiite, mugearite, trachyte, basanite, and basanitoid. Results and conclusions: i) In all rocks studied, An decreases and Or increases from phenocrysts to microphenocrysts to groundmass feldspar to interstitial material of feldspar composition. ii) Phenocrysts occur in rocks of the tholeiitic and alkalic suites and, in spite of differences in bulk rock compositions, overlap in composition. iii) Groundmass feldspar in rocks of the tholeiitic suite are nearly identical in composition; the same is true for rocks of the nephelinic suite. However, in the highly differentiated alkalic suite, groundmass feldspar composition ranges from labradorite to sanidine; i.e. the higher the bulk rock CaO, the higher is the An content, and the higher the bulk K₂O, the higher is the Or content. iv) In general, rocks with phenocrysts have groundmass feldspar less An-rich than those without phenocrysts. v) In rocks of the tholeiitic suite, normative feldspar approaches modal feldspar. However, in rocks of the alkalic and nephelinic suites, normative feldspar, because of the presence of highly alkalic interstitial material and the absence of nepheline in the mode but its presence in the norm, is drastically different from modal feldspar. vi) Hawaiites contain labradorite and not andesine, as per definition, and mugearite contains andesine and not oligoclase, as groundmass feldspar. In fact, when considering phenocrysts and interstitial material of feldspar composition, hawaiites range from bytownite to sanidine and mugearite from andesine to sodic sanidine, but normative feldspar plots in the andesine field for hawaiites and the oligoclase field for mugearite. vii) Rocks of the three suites can be distinguished on the basis of Or and An in groundmass feldspar, the presence of thin rims of groundmass composition of phenocrysts of rocks of the alkalic suite, and the presence of interstitial material of anorthoclase to sanidine composition in rocks of the alkalic and nephelinic suites. iix) Rocks transitional between the tholeiitic and alkalic suites are observed and are characterized by transitional mineral compositions.This paper was first presented as a talk before the 68. Annual Meeting of the Cordilleran Section of the Geological Society of America, Honolulu, Hawaii, March 29–April 1, 1972.     

Experimental constraints on the low-pressure evolution of transitional and mildly alkalic basalts: multisaturated liquids and coexisting augites

Basaltic liquids, saturated at one-atmosphere in augite, olivine, and plagioclase, show a strong linear trend in pseudo-quaternary projections from nepheline to slightly quartz normative compositions. Coexisting augites show a corresponding variation from slightly nepheline to hypersthene normative compositions. Consequently, alkalic and mildly alkalic liquids coexist with nepheline normative augites; tholeiitic liquids coexist with hypersthene normative augites; and transitional liquids coexist with augites compositionally intermediate between those crystallizing in alkalic and tholeciitic melts. The variation in augite composition is due to a decrease in wollastonite and non-quadrilateral components, going from mildly alkalic, to transitional, and tholeiitic augites. The observed liquid and augite trends lack correlations between normative composition and crstallization temperature. This is an artifact of plotting unrelated liquid lines of descent. Individual liquid lines of descent, in contrast, display good correlation between normative composition and crystllization temperature, which, for both liquids and coexisting augites, are highly oblique to nearly perpendicular to the general trend. Because the composition of the first augite to appear on the liquidus varies as a function of the bulk system, the location of the thermal divide shows corresponding variations. As a consequence, unifying cotectics and pseudo-quaternary normative projections should not, without specific experimental knowledge, be used to address liquid lines of descent and common magmatic parentages.     

Alkalic Igneous Rocks of the Balcones Province, Texas

In late Cretaceous time, subsilicic, alkalic magmas formed sills,laccoliths, plug-like bodies, small volcanoes, and a few dikesin Cretaceous sedimentary rocks of the Texas coastal plain alongan arcuate trend approximately coinciding with the buried Ouachitastructural belt. Transverse structural features apparently localizedthe igneous activity in two centers. In the larger center nearUvalde, Texas about half of the bodies are melilite-olivinenephelinite, a third olivine nephelinite and the remainder analcitephonolite, olivine basalt, and nepheline basanite, in orderof decreasing abundance. Basaltic and nephelinitic magmas were the primary magmas fromwhich all igneous rocks of the province were derived. Removalof olivine gave rise to a small range in chemical compositionof the basaltic rocks, but not gradation toward silica undersaturation.Nephelinitic magmas with compositions near that at which normativecalcium orthosilicate appears differentiated along two trendsto form melilite-olivine nephelinite as one end product andthrough nepheline basanite to analcite phonolite as the other.These two trends arose independently, their courses being determinedby the composition of the primary nephelinitic magma and theplane Fo-Di-Ne which has no piercing point at low pressure.     

汉诺坝玄武岩化学及其演化趋势

汉诺坝玄武岩于中新世喷出。除了碱性玄武岩以外,夏威夷岩广泛分布,还有苦橄玄武岩和马盖尔岩。有三个岩浆演化趋势:地壳岩浆房内橄榄石和单斜辉石斑晶组合的分离作用产生了从夏威夷岩到马盖尔岩的肯尼迪分异趋势;莫霍面附近的岩浆房内石榴石、普通辉石、歪长石和钛铁矿巨晶组合的分离作用导致了由原生夏威夷岩和碱性玄武岩经过进化的夏威夷岩和碱性玄武岩到拉斑玄武岩的跨式B型分异趋势;原生岩浆演化趋势。     

江西广丰早白垩世中晚期盆地火山-侵入杂岩活动序列确认及地质意义

江西广丰早白垩世盆地座落在124.51±11.9Ma(Rb-Sr等时线)石溪组火山岩系之上,是一个由湖进到湖退、具完整2级层序的湖相盆地;盆地湖进期沉积岩中穿插有斑岩岩脉群(正长斑岩、花岗斑岩),Rb-Sr等时线年龄为110.83±3.16Ma,U-Pb不一致曲线下交点105.4±1.3Ma,盆地最大湖面时期伴有碱性玄武岩的溢出,K-Ar年龄为100～104Ma,盆地湖退时期伴有亚碱性拉斑玄武岩溢出,K-Ar年龄为98～103Ma。同位素年代学无法区分其早晚,但野外迭置关系及沉积岩序可判别出早晚,这就构成了伴随盆地形成过程的火山-侵入杂岩活动序列,即中酸性岩脉→碱性玄武岩→拉斑岩武岩,该序列对探索该区早白垩中晚期拉张盆地的成因机制具有启迪意义。     

Age and petrology of alkalic postshield and rejuvenated-stage lava from Kauai,Hawaii

At the top of the Waimea Canyon Basalt on the island of Kauai, rare flows of alkalic postshield-stage hawaiite and mugearite overlie tholeiitic flows of the shield stage. These postshield-stage flows are 3.92 Ma and provide a younger limit for the age of the tholeiitic shield stage. The younger Koloa Volcanics consist of widespread alkalic rejuvenated-stage flows and vents of alkalic basalt, basanite, nephelinite, and nepheline melilitite that erupted between 3.65 and 0.52 Ma. All the flows older than 1.7 Ma occur in the west-northwestern half of the island and all the flows younger than 1.5 Ma occur in the east-southeastern half. The lithologies have no spatial or chronological pattern. The flows of the Koloa Volcanics are near-primary magmas generated by variable small degrees of partial melting of a compositionally heterogeneous garnet-bearing source that has about two-thirds the concentration of P₂O₅, rare-earth elements, and Sr of the source of the Honolulu Volcanics on the island of Oahu. The same lithology in the Koloa and Honolulu Volcanics is generated by similar degrees of partial melting of distinct source compositions. The lavas of the Koloa Volcanics can be generated by as little as 3 percent to as much as 17 percent partial melting for nepheline melilitite through alkalic basalt, respectively. Phases that remain in the residue of the Honolulu Volcanics, such as rutile and phlogopite, are exhausted during formation of the Koloa Volcanics at all but the smallest degrees of partial melting. The mantle source for Kauai lava becomes systematically more depleted in ⁸⁷Sr/⁸⁶Sr as the volcano evolves from the tholeiitic shield stage to the alkalic postshield stage to the alkalic rejuvenated stage: at the same time, the lavas become systematically more enriched in incompatible trace elements. On a shorter timescale, the lavas of the Koloa Volcanics display the same compositional trends, but at a lower rate of change. The source characteristics of the Koloa Volcanics, considered along with those of the Honolulu Volcanics, support a mixing model in which the source of rejuvenated-stage lava represents large-percent melts of a plume source mixed with small amounts of small-percent melts of a heterogeneous mid-ocean-ridge source.     

Experimental and major element constraints on the evolution of lavas from Lihir Island,Papua New Guinea

The major element chemistry of SiO₂-undersaturated arc lavas from Lihir Island, Papua New Guinea, and 1 atmosphere experiments on an alkali basalt from this island show complex polybaric fractionation affected this suite of lavas. Low Ni and MgO are typical of these arc lavas and result from olivine fractionation, probably at high pressure. Fractionation at low pressure (<5 kb) produces two evolutionary trends. Separation of clinopyroxene, plagioclase and minor olivine from the primitive lavas results in increasing normative nepheline contents and major element trends similar to those of the experiments. In contrast, addition of magnetite and amphibole to the fractionating assemblage in the evolved lavas results in decreasing normative nepheline and major element trends which are markedly different from those of the experiments. The composition of experimental glasses and 1 atmosphere liquid lines of descent, derived from anhydrous melting experiments run at the fayalite-magnetite-quartz (FMQ) buffer and at higher oxygen fugacities, are displaced from the lavas on oxide-oxide plots. HighfO₂ produces high Fe³⁺/Fe²⁺ and the early crystallization of abundant magnetite, and high H₂O contents are responsible for crystallization of amphibole. Crystal fractionation of these phases and the high Fe³⁺/Fe²⁺ are responsible for the displacement of the lavas and experimental glasses in mineral projection schemes from the 1 atmosphere olivine-clinopyroxene-plagioclase saturation boundary of Sack et al. (1987).     

Mineralogy,petrology and chemistry of lithic fragments from Luna 20 fines: origin of the cumulate ANT suite and its relationship to high-alumina and mare basalts

Bulk analyses of 157 lithic fragments of igneous origin and analyses of their constituent minerals (plagioclase, pyroxene, olivine, Mg-Al spinel, chromite, ilmenite, armalcolite, baddeleyite, zirkelite, K-feldspar, interstitial glass high in SiO₂ and K₂O) have been used to characterize the lunar highland rock suites at the Luna 20 site. The predominant suite is composed of ANT (anorthositic-noritic-troctolitic) rocks, as found at previous Apollo and Luna sites. This suite consists of an early cumulate member, spinel troctolite, and later cumulate rocks which are gradational from anorthosite to noritic and troctolitic anorthosite to anorthositic norite and troctolite; anorthositic norite is the most abundant rock type and its composition is close to the average composition for the highland rocks at this site. Spinel troctolite is a distinctive member of this suite and is characterized by the presence of Mg-Al spinel, magnesian olivine (average, Fo₈₃), and plagioclase. High-alumina basalt with low alkali content is another important rock type and melt of this composition may be parental to the cumulate ANT suite. Alkalic high-alumina basalt (KREEP) was not found in our sample, but may be genetically related to the ANT suite in that it may have formed by partial melting of rocks similar to those of the ANT suite. Fractional crystallization of low alkali, high-alumina basalt probably cannot produce alkalic high-alumina basalt because the enrichment in KREEP component is many times greater than the simultaneous change in major element components. Formation of alkalic high-alumina basalt by mechanical mixing of ANT rocks with very KREEP-rich components is not likely because the high-alumina basalt suite falls on a cotectic in the anorthiteolivine-silica system. Mare basalts may also be genetically related in that they may have been derived by remelting of rocks formed from residual liquids of fractional crystallization of parental low-alkali, high-alumina basalt, plus mafic cumulate crystals; the resultant melt would have a negative Eu anomaly and high $\text{Fe}\text{Mg}$ and $\text{pyroxene}\text{plagioclase}$ ratios.     

Experimental constraints on the low-pressure evolution of transitional and mildly alkalic basalts: the effect of Fe-Ti oxide minerals and the origin of basaltic andesites

One-atmosphere, melting experiments, controlled at the fayalite-magnetite-quartz oxygen buffer, on mildly alkalic and transitional basalts from Iceland show that these begin to crystallize Fe-Ti oxide minerals (magnetite and/or ilmenite) at 1105±5°C, apparently independently of bulk composition and the order of silicate and oxide mineral crystallization. Most samples crystalline plagioclase and olivine as the first two crystalline phases, augite as the third phase, and an Fe-Ti oxide mineral as the fourth phase. The main effects of Fe-Ti oxide crystallization are a marked decrease in FeO and TiO₂ in the liquid, and a notable increase in SiO₂ and Al₂O₃, and the minor oxides K₂O and P₂O₅, with decreasing temperature. The most silicic glasses are compositionally mugearitic and shoshonitic basaltic andesites. Because the smallest amount of glass that could be analyzed with the microprobe represents 20–55 percent liquid remaining, it can be expected that more silicic liquids will occur at lower temperatures. On normative, pseudoternary projections, the general effect of Fe-Ti oxide crystallization for mildly alkalic and transitional basalts is a marked increase in normative quartz. This is caused by a strong systematic convergence, with the appearance of Fe-Ti oxides, of the bulk solid precipitates toward the liquid compositions, as projected on the triangle plagioclase-diopside-olivine. For alkalic basalts, the bulk solid precipitate shows an increase in normative diopside with falling temperature and Fe-Ti oxide crystallization. This causes the liquids to move toward decreasing normative diopside and relatively little variation in nepheline. The experimental observations imply that mildly alkalic and transitional magmas, without stabilizing a Fe-Ti oxide mineral, will not evolve toward early silica saturation.     

Melting and phase relations in the plane tholeiite-lherzolite-nepheline basanite to 40 kilobars with geological implications

Six crystalline mixtures, picrite, olivine-rich tholeiite, nepheline basanite, alkali picrite, olivine-rich basanite, and olivine-rich alkali basalt were recrystallized at pressures to 40 kb, and the phase equilibria and sequences of phases in natural basaltic and peridotitic rocks were investigated.The picrite was recrystallized along the solidus to the assemblages (1) olivine+orthopyroxene+ clinopyroxene +plagioclase+spinel below 13 kb, (2) olivine+orthopyroxene+clinopyroxene+spinel between 13 kb and 18 kb, (3) olivine+orthopyroxene+clinopyroxene+ garnet+spinel between 18 kb and 26 kb, and (4) olivine+clinopyroxene+garnet above 26 kb. The solidus temperature at 1 atm is slightly below 1,100° and rises to 1,320° at 20 kb and 1,570° at 40 kb. Olivine is the primary phase crystallizing from the melt at all pressures to 40 kb.The olivine-rich tholeiite was recrystallized along the solidus into the assemblages (1) olivine+ clinopyroxene+plagioclase+spinel below 13 kb, (2) clinopyroxene+orthopyroxene+ spinel between 13 kb and 18 kb, (3) clinopyroxene+garnet+spinel above 18 kb. The solidus temperature is slightly below 1,100° at 1 atm, 1,370° at 20 kb, and 1,590° at 40 kb. The primary phase is olivine below 20 kb but is orthopyroxene at 40 kb.In the nepheline basanite, olivine is the primary phase below 14 kb, but clinopyroxene is the first phase to appear above 14 kb. In the alkali-picrite the primary phase is olivine to 40 kb. In the olivine-rich basanite, olivine is the primary phase below 35 kb and garnet is the primary phase above 35 kb. In the olivine-rich alkali basalt the primary phase is olivine below 20 kb and is garnet at 40 kb.Mineral assemblages in a granite-basalt-peridotite join are summarized according to reported experimental data on natural rocks. The solidus of mafic rock is approximately given by T=12.5 P _Kb+1,050°. With increasing pressure along the solidus, olivine disappears by reaction with plagioclase at 9 kb in mafic rocks and plagioclase disappears by reaction with olivine at 13 kb in ultramafic rocks. Plagioclase disappears at around 22 kb in mafic rocks, but it persists to higher pressure in acidic rocks. Garnet appears at somewhat above 18 kb in acidic rocks, at 17 kb in mafic rocks, and at 22 kb in ultramafic rocks.The subsolidus equilibrium curves of the reactions are extrapolated according to equilibrium curves of related reactions in simple systems. The pyroxene-hornfels and sanidinite facies is the lowest pressure mineral facies. The pyroxene-granulite facies is an intermediate low pressure mineral facies in which olivine and plagioclase are incompatible and garnet is absent in mafic rocks. The low pressure boundary is at 7.5 kb at 750° C and at 9.5 kb at 1,150° C. The high pressure boundary is 8.0 kb at 750° C and 15.0 kb at 1,150° C. The garnet-granulite facies is an intermediate high pressure facies and is characterized by coexisting garnet and plagioclase in mafic rocks. The upper boundary is at 10.3 kb at 750° C and 18.0 kb at 1,150° C. The eclogite facies is the highest pressure mineral facies, in which jadeite-rich clinopyroxene is stable.Compositions of minerals in natural rocks of the granulite facies and the eclogite facies are considered. Clinopyroxenes in the granulite-facies rocks have smaller jadeite-Tschermak's molecule ratios and higher amounts of Tschermak's molecule than clinopyroxenes in the eclogite-facies rocks. The distribution coefficients of Mg between orthopyroxene and clinopyroxene are normally in the range of 0.5–0.6 in metamorphic rocks in the granulite facies. The distribution coefficients of Mg between garnet and clinopyroxene suggest increasing crystallization temperature of the rocks in the following order: eclogite in glaucophane schist, eclogite and granulite in gneissic terrain, garnet peridotite, and peridotite nodules in kimberlite.Temperatures near the bottom of the crust in orogenic zones characterized by kyanitesillimanite metamorpbism are estimated from the mineral assemblages of metamorphic rocks in Precambrian shields to be about 700° C at 7 kb and 800° C at 9 kb, although heat-flow data suggest that the bottom of Precambrian shield areas is about 400° C and the eclogite facies is stable.The composition of liquid which is in equilibrium with peridotite is estimated to be close to tholeiite basalt at the surface pressure and to be picrite at around 30 kb. The liquid composition becomes poorer in normative olivine with decreasing pressure and temperature.During crystallization at high pressure, olivine and orthopyroxene react with liquid to form clinopyroxene, and a discontinuous reaction series, olivine orthopyroxene clinopyroxene is suggested. By fractional crystallization of pyroxenes the liquid will become poorer in SiO₂. Therefore, if liquid formed by partial melting of peridotite in the mantle slowly rises maintaining equilibrium with the surrounding peridotite, the liquid will become poorer in MgO by crystallization of olivine, and tholeiite basalt magma will arrive at the surface. On the other hand, if the liquid undergoes fractional crystallization in the mantle, the liquid may change in composition to alkali-basalt magma and alkali-basalt volcanism may be seen at a late stage of volcanic activity.Publication No. 681, Institute of Geophysics and Planetary Physics, University of California, Los Angeles.     

Mineralogy and petrology of Grenada,Lesser Antilles island arc

The mineralogy and petrology of volcanic and plutonic rocks from the island of Grenada are described. The volcanic rocks include basanitoids, alkalic and subalkalic basalts, andesites and dacites. Phenocryst phases in the basanitoids and basalts are olivine (Fo_90–71), zoned calcic augite, spinel ranging from ferrian pleonaste through chromite to titaniferous magnetite, and plagioclase. Some of the basalts contain pargasitic amphibole. Andesites and dacites generally contain hypersthene and augite, and one pigeonite-hypersthene-augite-bearing andesite was found. Apatite commonly occurs as a phenocryst in the andesites and dacites and quartz is present in some dacites as well as being a possible xenocryst in both alkalic and subalkalic basalts. Plutonic cumulates found as ejected fragments in tuffs and ashes are composed of variable proportions of olivine, magnetite, calcic augite, amphibole and plagioclase. One peridotitic (ol-cpx-opx) fragment was found but spinel or garnet peridotitis are absent. Despite the alkalic nature of the association, calcalkalic characteristics such as calcic plagioclase, restricted Feenrichment in coexisting pyroxenes and generally low TiO₂ content relative to oceanic suites are present in Grenada. Estimates of conditions of equilibration of the basanitoids with potential upper mantle materials using the results of high-pressure experiments are compared with estimates from thermodynamic data. Equating and basanitoid with hypothetical garnet peridotite assemblages gives a pressure and temperature of equilibration in the region of 35–38 kbar and 1550–1625 ° K. Experimental results are not supportive of these estimates.     

Pyroxene-Olivine-Quartz Assemblages in Rocks Associated with the Nain Anorthosite Massif, Labrador

Unusually iron-rich pyroxene and olivine occur in rocks associatedwith the Nain anorthosite massif, Labrador. Adamellite and granodioritecontain orthopyroxene (inverted from pigeonite) as iron-richas Ca₆Fe₈₂Mg₁₂; comparison with experimental data suggests aminimum pressure of crystallization of 5 kb. Some of these iron-richpyroxene crystals have broken down, apparently upon decreasingpressure, to yield intergrowths of less iron-rich orthopyroxene(near Ca₇Fe₇₂Mg₂₁), ferroaugite, fayalite (near Fo₉), and quartz.Other rocks, monzonites, contain pyroxenes with calcium-poorcores and ferroaugite rims, as well as crystals composed ofbroad lamellae of ferroaugite and orthopyroxene in sub-equalproportions. Analysis of one such crystal with unusually thinand closely spaced lamellae yielded a bulk composition of Ca₂₄Fe₅₈Mg₁₈.Such pyroxenes probably crystallized near or above the crestof the augite-pigeonite two-phase region, probably above 925°C. This high temperature suggests that the monzonites crystallizedfrom relatively dry magmas. If they represent a residual fractionderived from the same magma as the anorthosite, then that magmamust have been nearly anhydrous. Pigeonite rather than orthopyroxene was the primary magmaticCa-poor pyroxene in most of the Nain rocks studied here. Nucleationrates apparently were low under subsolidus conditions, and low-Capigeonite (Ca₂Fe₇₈Mg₂₀) is present in grains where orthopyroxenedid not nucleate as pigeonite cooled and exsolved ferroaugite.Iron-rich orthopyroxene (Ca₂Fe₇₉M₁₉) crystallized instead ofpigeonite in a Greenland quartz syenite that contains more abundanthydrous phases.     

Experimental investigations of low-Ca pyroxene stability and olivine-pyroxene-liquid equilibria at 1-atm in natural basaltic and andesitic liquids

This paper reports experiments carried out at 1-atm under conditions of controlled oxygen fugacity, using natural andesites and andesite mixed with augite+synthetic pigeonite or augite+orthopyroxene. The experimental results are used (1) to investigate the controls of Mg^# (Mg/[Mg+Fe²⁺]) and temperature on low-Ca pyroxene stability (pigeonite vs orthopyroxene), (2) to quantify the effects of variations in bulk composition on the position of multiple saturation boundaries in mineral component projection schemes and (3) to develop a thermodynamic model for silica activity for melts saturated with olivine and pyroxene. Over the Mg^# range of 0.80–0.30 the minimum temperature of pigeonite stability in natural compositions is equivalent to the Lindsley (1983) boundary determined for pure Ca-Mg-Fe pigeonites. For the low variance, 5-phase assemblage oliv-aug-low-Ca pyroxene-plag-liquid, expressions involving liquid (Na₂O+K₂O)/(Na₂O+K₂O+CaO),Mg^# and TiO₂ content predict temperature and the movement of multiple saturation boundaries in pseudoternary projections in response to changing melt composition. The equilibrium for the low pressure melting of low-Ca pyroxene to olivine+liquid is formulated as a geothermometer and monitor of silica activity. Equilibrium constants estimated from thermochemical data and activities calculated for experimentally produced olivine and pyroxenes are used to develop a model for silica activity in liquid.     

Lineages and Fractionation Trends in Undersaturated Volcanic Rocks from the East Otago Volcanic Province (New Zealand) and Related Rocks

The petrology of the East Otago Volcanic Province (late Miocene),including the Dunedin Complex, is dominated by undersaturatedsodic lavas and shallow intmsives whose compositions range fromalkali basalt, basanite, and nephelinite to trachyte and phonolite.A range of relatively potassic volcanic rocks is also developedin the Province. The degree of undersaturation and ratios suchas (FeO+Fe₂O₃):MgO and K₂O:Na₂O vary widely in rocks with comparabledifferentiation indices. Chemical and mineralogical data onwhole rock-glass pairs and variants of shallow intrusives inthe Province, assessed in conjunction with similar data fromother volcanic provinces, demonstrate the production of trachyticliquids from alkali basaltic parents and of phonolitic liquidsfrom basanitic parents. Fractionation trends in both sodic andpotassic series, defined by variation in differentiation indexand normative nepheline, indicate that the degree of undersaturationof the derivative salic liquids is dependent on that of theparent; the slope of the fractionation trend lines is influencedby additional factors, including Po₂. The unequivocal productionof phonolite from trachyte in the East Otago Province has notbeen demonstrated. The mafic variants of theralites from Waihola and a basanite-pegmatoidassociation from Omimi are similar in composition, but the mostsalic differentiates from these differentiation sequences displaypronounced differences in their KO:Na₂O ratios, tending towardsleuco-theralitic (cf. lugarite) and malignitic compositionsrespectively. A multicomponent spectrum of fractionation lineages is suggestedfor individual differentiated bodies and for lava series. Sodiclineages include: (I) alkali basalt–hawaiite–mugearite–benmoreite–trachyte;(2) a more undersaturated series from basanite through nephelinehawaiite, nepheline mugearite, and nepheline benmoreite, tophonolite; (3) a nephelinite series, more restricted in compositionalrange. The relatively high Fe: Mg ratios characterizing themugearitic variants are sometimes exhibited by members of themore potassic series in which, however, Fe:Mg ratios tend tobe characteristically lower. This feature is correlated withthe frequent occurrence of kaersutitic amphibole or its resorptionproducts, indicative of intratelluric fractionation under relativelyhydrous conditions and probably relatively high Po₂. Certainnepheline trachyandesites from East Otago are described in detail.Trachy-basalt-trachyandesite-tristanite-trachyte and sanidinebasanite-nepheline trachyandesite-nepheline tristanite-phonolitelineages are also proposed, and the nomenclature of these moreseries discussed.     

Mineralogy as a function of depth in the prehistoric Makaopuhi tholeiitic lava lake,Hawaii

The electron probe X-ray microanalyzer has been used to determine the compositional variability of the groundmass minerals and glass in 10 specimens from a complete 225-foot section of the prehistoric tholeiitic lava lake of Makaopuhi Crater, Hawaii. The order of beginning of crystallization was: (1) chromite, (2) olivine, (3) augite, (4) plagioclase, (5) pigeonite, (6) iron-titanium oxides and orthopyroxene, (7) alkali feldspar and apatite, and (8) glass.Although the lake is chemically tholeiitic throughout, the occurrence of ferromagnesian minerals is as though there were a gradation from alkali olivine basalt in the upper chill downwards to olivine tholeiite. Groundmass olivine decreases downwards and disappears at about 20 feet. Pigeonite is absent in the uppermost 5±2 feet, then increases in amount down to 20 feet, below which augite and pigeonite coexist in constant 21 proportions. Strong zoning and metastable compositions characterize the pyroxenes of the chilled zones, but these features gradually disappear towards the interior of the lake to give way to equilibrium pyroxenes. Relatively homogeneous poikilitic orthopyroxene ( Ca₄Mg₇₀Fe₂₆) occurs in the olivine cumulate zone, having formed partly at the expense of pre-existing olivine, augite, and pigeonite ( Ca₈Mg₆₆Fe₂₆). The growth of orthopyroxene is believed to have been facilitated by the slower cooling rate and higher volatile pressure at depth, and by the rise in Mg/Fe ratio of the liquid due to the partial dissolution of settled olivine.Unlike olivine and pyroxene, feldspar is least zoned in the upper and lower chilled regions. The greatest range of compositional zoning in feldspar occurs at 160 to 190 feet, where it extends continuously from Or_1.0Ab₂₂An₇₇ to Or₆₄Ab₃₃An₃. The feldspar fractionation trend in the An-Ab-Or triangle gradually shifts with depth toward more equilibrium trends, even though the zoning becomes more extreme. The variation with depth in the initial (core) composition of the plagioclase suggests the influence of either slow nucleation and growth (undercooling) or slow diffusion in the liquid, relative to the rate of cooling.Idiomorphic opaque inclusions in olivine phenocrysts are chrome-spinels showing continuous variation from 60 percent chromite to 85 percent ulvospinel and to magnetite-rich spinel. A pre-eruption trend of increasing Al with decreasing Cr can be recognized in chromites from the upper chill. Most of the inclusions show a trend of falling Cr and Al, toward an ulvospinelmagnetite solid solution which is progressively poorer in Usp with depth. This trend was produced by solid state alteration of the chromite inclusions during cooling in the lava lake. Ilmenite (average Ilm₉₁Hm₉) coexists with variably oxidized titaniferous magnetite in the basalt groundmass. Estimated oxygen fugacities agree well with other independent determinations in tholeiitic basalt. No sulfide phase has been detected.Fractional crystallization produced a groundmass glass of granitic composition. Average, in percent, is: SiO₂, 75.5; Al₂O₃, 12.5; K₂O, 5.7; Na₂O, 3.1; CaO, 0.3; MgO, 0.05; total FeO, 1.2; and TiO₂, 0.8. Normative Or> Ab. Minor changes in glass composition with depth are consistent with a greater approach towards the granite minimum. Incipient devitrification precluded reliable analysis of glass from the lower half of the section. The SiO₂-phase associated with devitrification contains alkalis and Al and is believed to be cristobalite. Needle-like apatite crystals in the groundmass glass are Siand Fe-bearing fluorapatites containing appreciable rare earths (predominantly Ce) and variable Cl.The grain-size and maximum An content of the cores of plagioclase grains were controlled by cooling rate and are at a maximum at the center of the section. The most homogeneous pyroxene (and olivine, Moore and Evans, 1967), most equilibrium pyroxene trends, most abundant alkali feldspar, and most equilibrium feldspar trends are found at 160 to 190 feet, which is appreciably below that part of the lake which was slowest to crystallize. Volatile pressure, increasing with depth, possibly controlled the degree of attainment of equilibrium more than cooling rate.Since they are dependent on cooling history, some of the modal criteria commonly used for recognizing basalt types, such as the absence of Ca-poor pyroxene, presence of groundmass olivine, and the presence of alkali feldspar, should be applied with caution. Petrographic comparison of basalts from one flow, volcano, or province, with another, should recognize the possible variations due to cooling history alone.Publication authorized by the Director, U.S. Geological Survey     

Mafic and ultramafic extrusives of the Barberton Mountain Land in terms of the CMAS system

Evolutionary trends among the mafic and ultramafic extrusives of the Onverwacht Group are revealed by subprojections within the CMAS tetrahedron. The parental magma of the Onverwacht extrusives had a composition close to that of the ultramafic rocks occurring at the base of the pile. The development of this parental magma is satisfactorily accounted for by partial melting of a four-phase lherzolite mantle at depths of 90–100 km or more. Polybaric olivine and orthopyroxene fractionation were important in controlling the development of mafic and ultramafic komatiite-type extrusives and associated tholeiitic basalts occurring at stratigraphically higher levels. Attention is drawn to the presence of nepheline normative extrusives in the upper formations of the Onverwacht Group. The accumulation and preservation of 15,000 m of mantle-derived material required the presence of a pre-Onverwacht floor.     

Skeletal crystallization and residual glass compositions in a cellular alkalic pyroxenite nodule from Oldoinyo Lengai

The alkalic pyroxenite nodule consists of megacrysts of diopside, apatite, perovskite and titanomagnetite in a groundmass consisting of diopside, apatite, titanomagnetite, nepheline, melilite, garnet and vishnevite crystals of various shapes, including previously undescribed skeletal and dendritic shapes, together with vesicles and residual glass. The residual glass is poor in SiO₂ (38–40 wt%), and extraordinarily rich in Na₂O (12.8–15 wt%), SO₃ (1–1.5 wt%), and Cl (0.25–0.7 wt%), as a result of rapid, non-equilibrium crystallization of groundmass phases from a CO₂-rich nephelinite melt.The Oldoinyo Lengai alkalic carbonatite lavas do not represent extreme products of the fractional crystallization of pyroxene, wollastonite, nepheline and alkali feldspar from the carbonated nephelinite melt. The most likely connection between the carbonatite and silicate magma types is one of liquid immiscibility, probably involving phonolite melt.