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USGS1969

Coexisting amphiboles from blueschist facies metamorphic rocks

Four pairs of associated calcic and sodic amphiboles from blueschist facies metamorphic rocks were analyzed with the electron microprobe and studied by single-crystal X-ray diffraction techniques. Except for ranges in the ratios Mg/(Mg+Fe) and Fe 3+ /(Fe 3+ +Al+Ti), the sodic amphiboles are similar in chemical composition. The amount of calcium in the M(4)-site ranges only from 0·18 to 0·21 ion per formula unit. The calcic amphiboles, in addition to a range in Mg/(Mg+Fe), vary in Na/(Na+Ca) ratio (0·29–0·48). Three of the calcic amphiboles contain less than 1·5 calcium ions per formula unit, indicating a significant solid solution of sodic amphibole components in the calcic amphibole phase. The a and b unit-cell parameters of the calcic amphiboles decrease with increased content of the sodic component.

Journal of Petrology↗
USGS1969

Biotites from granitic rocks of the central Sierra Nevada batholith, California

Biotites from plutonic rocks of the central Sierra Nevada and Inyo Mountains, California, have been examined and characterized by powder X-ray diffraction and optical and chemical methods. Compositions of the biotites define a trend in the compositional triangle Fe+3 Fe+2Mg. When related to the experimentally studied ternary system KFe 3 +3AlSisO 12 H- 1 -KFe 3 +2 AlSi 3 O 10 (OH) 2 -KMg 3 AlSi 3 O 10 (OH) 2 and coupled with the estimated positions of biotite solid solutions for different oxygen buffers, the trend suggests that oxygen fugacities in magmas during biotite crystallization were slightly higher than those defined by the Ni-NiO buffer. The compositional data also suggest that magmas were ‘buffered’ with respect to oxygen by oxides existing within the magmas themselves. Correlation between the Fe/(Fe+Mg) ratio, an inferred temperature indicator, and other elements is generally poor, which suggests that factors other than temperature at the time of crystallization exerted an important influence on compositions.

California↗
USGS1969

Minor-element and revised major-element contents of some Mediterranean pantellerites and comendites

Optical emission spectrographic analysis of three pantellerites from Pantelleria and two comendites from Sardinia show high concentrations of B, Be, Ga, La, Mo, Nb, Sn, Y, Yb, and Zt and low contents of Ba, Co, Cr, Ni, Sr, and V. Minor-element trends of these specimens are very similar to those of pantellerites from southern Nevada and New Zealand and other peralkaline silicic rocks from various localities. Fluorine contents of the Pantellerian specimens range from 0.19 to 0.32 per cent by weight resulting in abnormally high Cl to F ratios. New analyses for MgO, Na 2 O, and K 2 O are almost identical with the values obtained by Zies. Revised major-element compositions and calculated glass-phase compositions for the three Pantellerian specimens are given.

Pantelleria, Sardinia↗
USGS1969

Rheology of basalt in the melting range

Experimental data have been obtained for viscosities of tholeiite melts at temperatures from 1300 to 1120 °Cat 1 atm, using a concentric cylinder viscometer. The apparent viscosity increases more than two orders of magnitude between 1200 and 1120 °C (0–25 per cent crystallization) for shear rates of about 10 sec -1 and even more for lower shear rates. Non-Newtonian behaviour of ‘pseudo-plastic type’ becomes extremely pronounced at temperatures below about 1130 °C. At these temperatures, differences of less than 5 °C can produce changes in apparent viscosity amounting to orders of magnitude. These observations have led to the conclusion that the heat of deformation must itself influence rheological behaviour in the melting range. An equation for thermal energy balances and their rates of change is constructed and placed in a non-dimensional form that has been given published solutions by I. J. Gruntfest (1963) relating the shear stress, rate of strain, and temperature through the temperature dependence of viscosity. The results show that in an adiabatic system the heating rate increases with time so that the temperature eventually runs out of bounds, a process termed ‘thermal feedback’ by Gruntfest. A hypothesis of shear melting is derived on the basis of a simplified viscosity function extrapolated to the solidus temperature. The hypothesis is applied to magma generation in the earth on the basis of dimensional arguments. It is also suggested that thermal instabilities give rise to a sort of viscous failure responsible for deep-focus earthquakes, and that the two phenomena have the same cause relating ultimately to a gravitational energy source.

Journal of Petrology↗
USGSDate not supplied

Xenoliths in the honolulu volcanic series, Hawaii

The Pleistocene to Holocene Honolulu Volcanic Series was erupted from about 37 vents scattered over the older Koolau tholeiite shield. The rocks of this series are compositionally zoned with respect to the shield; near the Koolau caldera the predominant rocks are melilitenepheline basalts, but these give way outward to nepheline basalts, and ultimately, at the apron of the shield, to alkalic olivine basalts. The xenoliths in these are likewise zoned: most of those in the caldera area consist of dunite, most of those at intermediate distances of lherzolite, and some of those in the apron of the shield consist of garnet pyroxenite and peridotite. The zoning of the xenoliths, however, does not coincide with that of the enclosing rocks. We believe that copious eruption of Koolau tholeiite produced a lateral and vertical heterogeneity in the mantle beneath Oahu, and that the zoning in both Honolulu lavas and their xenoliths is caused by that heterogeneity. The textures of the xenoliths indicate that the basalts were mainly produced by fractional melting rather than fractional crystallization. There is some evidence that the dunite xenoliths are mantle residua produced during the generation of the tholeiite, and that the Honolulu magmas were generated at greater depths than the Koolau magmas, probably as a result of elastic unloading. © 1970 Oxford University Press.

Hawaii↗
USGS1971

Origin of the differentiated and hybrid lavas of Kilauea Volcano, Hawaii

Kilauea Volcano has erupted lava from its summit caldera and from two rift zones that extend from the summit towards the east and south-west. Lavas erupted from the summit of the volcano differ from each other principally in their content of olivine and define lines of ‘olivine control’ on magnesia variation diagrams. Lavas erupted on the rift zones may be similar in composition to the summit lavas or may be differentiated by processes that involve minerals other than olivine. All of the differentiated lavas have less than 6·8 per cent MgO and plot off the extension of olivine control lines for the summit lavas. Prehistoric vents (before A.D. 1750) from which differentiated lavas have been erupted are found on the east rift zone and in the western Koae fault zone adjacent to the south-west rift zone; historic vents for differentiated lavas are confined to the east rift zone. Twenty-one new analyses are presented for several of the east rift differentiates and for the newly discovered differentiates adjacent to the south-west rift zone. The differentiates have MgO as low as 3·9 per cent and SiO 2 as high as 56 per cent; both extremes are found in the prehistoric lavas adjacent to the south-west rift. Detailed petrochemical studies suggest the following conclusions: The chemical composition of magma erupted at Kilauea summit varies with the date of eruption. Lavas erupted before 1750, during the eighteenth and nineteenth centuries, and in the twentieth century form groups that can be distinguished chemically. On a lesser scale, each Kilauea summit eruption in the twentieth century has a chemistry that is distinctive with respect to the chemistry of every other summit eruption. During late prehistoric time pockets of differentiated magma were formed within the rift zones by separation of the liquid remaining after partial crystallization of bodies of summit magma. This process presumably is still going on within the east rift zone, but the more recently separated liquids have not yet been erupted to the surface. The relative time at which these differentiated magmas were produced can be estimated from calculations based on their chemical compositions, which show that the differentiates could lie on the liquid line of descent for Kilauea summit magma of prehistoric composition but not on any liquid line of descent for younger summit magmas. Lava from some eruptions, notably the early part of the 1955 eruption on the lower east rift, has the composition of the liquid fraction as it is generated within the rift. Lava compositions of other eruptions, including those of the later lavas of 1955, are best explained by mixing of magma supplied from a central reservoir beneath Kilauea summit with the differentiated liquid in the rift. Lava from each summit eruption is unique chemically, so it is possible to recognize its presence or absence as components of mixing in such mixed lavas. It appears that summit magma of composition characteristic of the 1952 and 1961 Halemaumau eruptions contributed to the composition of the mixed lavas produced in the latter part of the 1955 eruption. Summit magma of 1961 composition is alone sufficient to explain the composition of mixed lavas erupted in 1960 and 1961. In rift lavas erupted from 1962 to 1965, the composition of lava erupted in Halemaumau in 1967, in addition to the 1961 composition, is a component of mixing, and it is the dominant summit component in the composition of the two 1965 eruptions. The proportion of summit magma to differentiated magma needed to explain the composition of lavas erupted on the upper east rift increases from 1961 to 1965; this increase indicates that the differentiated magma was being diluted and used up by repeated flooding of this part of the rift zone by magma supplied from the central reservoir. The fact that components of ‘summit composition’ appear in rift eruptions before they appear undiluted in Halemaumau suggests that the central reservoir is vertically zoned. Rift eruptions are fed from lower levels where younger magma is available, and summit eruptions are fed from the relatively older magma above. The chemical distinction between lava of successive summit eruptions implies that significant convective mixing of magma does not take place throughout the central reservoir. The unique and uniform composition of lava of each successive summit eruption also suggests that summit eruptions end when all of the magma of one composition has been erupted. The magma erupted from the upper levels of the reservoir during one cycle is continually replaced from below by younger magma of different composition. In order for eruption to be renewed in Halemaumau, new magma from the mantle must be held in storage at intermediate levels before it attains an ‘eruptive state’. The hypothesis presented in 2–4 above permits qualitative predictions concerning future lava compositions. The composition of the next lava to be erupted in Halemaumau is expected to be distinct from that of the 1967 eruption, and this composition will presumably be identified in rift eruptions occurring between 1967 and the time of its appearance in Halemaumau. Differentiates of prehistoric age also were apparently formed in the same way as those of historic age, but the mixing cannot be described quantitatively because of poor control on the stratigraphy and the compositions of erupted lavas. One lava in the Koae group, that from Yellow Cone, appears to be a mixture of a picritic magma (12 per cent MgO) with a differentiated liquid with less than 2·5 per cent MgO and nearly 60 per cent SiO 2 .

Hawaii↗
USGS1971

A chemical study of serpentinization — Burro Mountain, California

Serpentinized dunites and harzburgites from the Burro Mountain peridotite show no change in the ratio of iron and magnesia to silica when compared with the same ratio for the unserpentinized equivalents. The mineral assemblage resulting from serpentinization consists of lizardite-chrysotile, brucite, and magnetite and is determined by the original bulk composition of the peridotite. The chemical and mineralogical data indicate that serpentinization proceeded under isochemical conditions except for the introduction of water into the peridotite. Expansion accompanies serpentinization because the serpentine products occupy a greater volume than the peridotite protolith. Tectonic emplacement of the Burro Mountain peridotite was facilitated by serpentinization and the attendant expansion.

California↗
USGS1971

Origin and emplacement of the ultramafic rocks of the Emigrant Gap area, California

The ultramafic bodies of the Emigrant Gap area are part of a mafic complex within a large composite pluton of the northern Sierra Nevada. The pluton was magmatically emplaced and is surrounded by an aureole of hornblende-hornfels facies rocks. Inclusions of country rock in ultramafic rock are of pyroxene-hornfels facies and appear to have been partly melted. Gravity studies indicate that the ultramafic bodies have near-vertical contacts extending to depths of at least 1½ to 2½ km. The mafic complex shows rough concentric zoning of rock types: ultramafic bodies occur at the core; gabbro forms a discontinuous intermediate unit; and diorite, tonalite, and granodiorite occur at the margins. Within the ultramafic bodies, unserpentinized wehrlitic peridotite is dominant; dunite and olivine clinopyroxenite are present but greatly subordinate. The ultramafic rocks consist almost entirely of olivine (FO 80 ) and diopside (Ca 46 Mg 46 Fe 8 ); orthopyroxene, hornbolende, and plagioclase occur locally. The gabbro, diorite, tonalite, and granodiorite contain both ortho- and clinopyroxene. Both ultramafic and two-pyroxene-bearing rocks were emplaced nearly simultaneously, as partly crystallized magmas and magmatic crystal mushes that had similar temperature. In all the rocks the structures are dominantly magmatic and were produced by sorting and orientation of crystals by magmatic flow. Structures produced by post-consolidation deformation and replacement are minor and local. The structural and chemical relations within the mafic complex suggest that all the rocks are derived from a single gabbroic magma by crystal fractionation, with the ultramafic rocks formed by mechanical accumulation of early crystallized mafic minerals, and the two pyroxene-bearing granodiorite crystallized from a felsic differentiate. It is likely that flowage differentiation was the dominant process of crystal segregation. The Emigrant Gap mafic complex is similar in structure, rock texture, and mineralogy to zoned ultramafic complexes, such as those of south-eastern Alaska, and is very different from either stratiform or alpine-type bodies. Though unlike the Alaskan bodies in detail, it appears that this complex should be classed with the zoned complexes in any broad grouping of ultramafic occurrences.

California↗
USGS1973

Phase relations of basalts in their melting range at PH2O = 5 kb as a function of oxygen fugacity: Part I. Mafic phases

The phase relations of three basalts, the Picture Gorge tholeiite, the 1921 Kilauea olivine tholeiite, and the 1801 Hualalai alkali basalt, were studied at 5 kb water pressure, 680–1000°C, at the oxygen fugacities of the quartz-fayalite-magnetite (QFM) and hematite-magnetite (HM) buffers. In the range 680–850 °C, the crystalline assemblage on the QFM buffer is dominantly hornblende+ plagioclase, ± ilmenite, magnetite, sphene, fayalitic olivine, and phlogopitic mica. From 875 to 1000 °C the crystalline assemblage is hornblende+ olivine± augite+ ilmenite± magnetite. A melt phase is present from 700 to 1000 °C; a vapor phase was present in all charges. The hornblendes formed on the QFM buffer range in composition from common green hornblendes at low temperatures to kaersutitic hornblendes at 1000 °C. A1(IV) and Ti increase temperature. AI(VI) passes through a maximum near 825 °C, decreasing both above and below this temperature. AI(IV) is proportional to the sum A1(VI)+2Ti. There is a positive linear correlation of approximately 3 : 1 between AI(IV) and the number of cations in the A-site. The most likely explanation for this correlation at present is that the substitution of AI(VI) or Ti +4 for a divalent cation creates local charge imbalances in the amphibole structure which can be compensated only by further A-site substitution. There also appears to be a correlation between the a-cell dimension of hornblende and the A-site occupancy. Above a thresh hold value of approxmately 0.5 cations in A, a increases as A-site occupancy increases. Phase relations on the hematite-magnetite buffer are considerably simpler. The hornblendes show relatively little change in composition as temperature increases, and in the tholelitic compositions break down at or below 970 °C 35–60 °C above the first appearance of augite±olivine. The melting of hornblende is incongruent in all cases. The Fe-Ti oxides are pseudo-brookite and titanohematite; at 1000 °C these oxides make up 10 per cent by weight of the assemblage and contain most of the Tio 2 and FeO in the charge. The patterns of hornblende variation observed in this study compare closely with those reported in a wide range of experimental and field data. The appearance of high-TiO 2 kaersutitic hornblendes in the tholeities at 1000° C, P H2O = 5 kb on the QFM buffer implies that the restricted occurence of kaersutite in nature (where it is associated only with mafic to intermediate alkalic rocks) is controlled by volatile content (H 2 O , F 2 )rather than by differences in condensed bulk composition.

Journal of Petrology↗
USGS1973

Oxidation during magmatic differentiation, Finnmarka Complex, Oslo area, Norway: Part 2, the mafic silicates

Electron-microprobe analyses are presented for pyroxene, amphibole, and biotite from monzonite, granodiorite, and granite at Finnmarka, Norway. Compositional trends measured in biotite, present in all three rock types, and in amphibole, present in the monzonite and granodiorite, are markedly atypical and are interpreted as reflecting crystallization under progressively more oxidizing conditions. The average Fe/Fe + Mg for biotites from successively more silicic rock types changes from 0.64 → 0.35 → 0.28, and for amphiboles changes from 0.58 in the monzonite to 0.29 in the granodiorite. Analyses of selected areas within amphibole grains in the granodiorite show marked chemical variations, although single-crystal X-ray photographs are sharp and do not reveal multiple phases. On the basis of 33 such analyses, four coupled substitutions are identified as operative; the most unusual finding is the relation of 1 Ti cation to 4 Al IV cations in the unit cell. Variations within individual amphibole grains of the granodiorite resemble changes noted in evolution of amphibole composition from monzonite to granodiorite and are interpreted as reflecting progressive oxidation. Consideration of these data for the mafic silicates, data for the opaque oxides, and the extensive formation of sphene in the granodiorite, has allowed development of schematic reactions and an overall picture of magmatic environment and evolution at Finnmarka. Crystallization apparently took place at PH 2 o of 1000 bars or less and a temperature of about 700 °C. The trend of oxidation during differentiation is more extreme than any heretofore reported. Amphiboles, as well as biotites, may participate in oxidation reactions and may reflect the oxidation-reduction processes that occurred during magmatic evolution.

Oslo↗
USGS1974

Prehnite- and pumpellyite-bearing mineral assemblages, west side of the Appalachian metamorphic belt, Pennsylvania to Newfoundland

Prehnite- and/or pumpellyite-bearing meta-igneous rocks are found on the west side of the Appalachian metamorphic belt (1) near Jonestown, south-eastern Pennsylvania; (2) on Rensselaer Plateau, eastern New York; (3) near Quebec City, Quebec; and (4) at Little Port, Humber Arm, western Newfoundland. The assemblages critical to determining the conditions of metamorphism are (1) chlorite-epidote-hematite-pumpellyite-prehnite; actinolite-chlorite-hematite-pumpellyite-stilpnomelane; (2) actinolite-chlorite-epidote-stilpnomelane; chlorite-epidote-pumpellyite-stilpnomelane; chlorite-epidote-hematite-pumpellyite; (3) chlorite-epidote-hematite-pumpellyite-stilpnomelane; chlorite-epidote-pumpellyite-prehnite; and (4) chlorite-epidote-prehnite; chlorite-prehnite-stilpnomelane; chlorite-epidote-pumpellyite-prehnite. One pumpellyite-bearing rock from western Newfoundland shows a later vein of analcime-calcite. All the assemblages also include quartz, sphene, calcite, K-mica, and albite. Analysis of the mineral assemblages by the Schreinemakers method for the phases actinolite-chlorite-epidote-hematite-prehnite-pumpellyite-stilpnomelane shows that the different localities can be assigned different metamorphic grades. Though the detailed results of the Schreinemakers analysis depend on the assumed source of ferric iron in epidote, the major conclusions are not affected. The thermodynamic role of calcite is more problematic, but it appears that CO 2 did not behave as a boundary-value component during metamorphism. If calcite is treated as an excess phase, the Schreinemakers bundle decomposes to a net in a multisystem. Plotted on such a net, the various localities again occupy different parts signifying different metamorphic grades. The occurrence of pumpellyite-bearing assemblages on the west flank of the northern Appalachian metamorphic belt might suggest that these assemblages, contrary to the ideas of Miyashiro and of others, do not indicate high-pressure and low-temperature type of metamorphism. These assemblages, however, are compatible with an alternative interpretation as remnants of a high-pressure, low-temperature Taconic metamorphic regime, whose imprint within most of the Appalachian metamorphic belt has been obliterated by later events. Such a reconstruction is compatible with the suggestion that this zone, lying near the margin of the early Paleozoic craton, was an active subduction zone during the Taconic orogeny.

Appalachian metamorphic belt↗
USGS1961

Jadeite deposits of the clear creek area, New Idria district, San Benito county, California

Tectonic inclusions within the New Idria serpentine body contain jadeite in two distinct assemblages: (1) Lenslike inclusions containing a monomineralic green jadeite core surrounded by a calc-silicate rim. (2) Jadeite veins cross-cutting albite-crossite schist inclusions. In these veins jadeite coexists with low albite; green jadeite (Jd 75 Ac+Di+He 25) coexists with low albite within the host schist. Albite schists are related to the pre-metamorphic keratophyres and it can be shown that desilication of such rocks produces a bulk composition similar to that of jadeite. Metamor-phism of keratophyric tectonic inclusions within serpentine produced jadeite-albite schists along margins of the inclusions. Jadeite-albite veins formed from mobilized liquids rich in the jadeite molecule and deficient in water. Geological evidence such as would indicate extreme pressures or temperatures during jadeite formation is lacking; rather, a low silica and dry system allowed its formation at reduced temperatures and pressures.

California↗
USGS1961

Magmatic differentiation in the Uwekahuna Laccolith, Kilauea Caldera, Hawaii

Petrographic and chemicoal studies of a suite of rocks from the Uwekahuna laccolith of Kilauea Volcano show that the original mafic tholeiitic magma differentiated into tholeiitic picrite, tholeiitic olivine gabbro, and an aphanitic rock approaching quartz-basalt in composition. Mechanisms involved were an initial gravity settling of olivine and a final filter pressing of the residual liquid. The range of composition represented by the rocks of the laccolith is as great as that found among all hitherto analysed lavas of the volcano.

Hawaii↗
USGS1984

Metamorphic history of a high-grade blueschist exotic block from the Franciscan complex, California

A high-grade blueschist tectonic block from the Franciscan Complex of the northeast Diablo Range shows evidence of three episodes of retrograde blueschist facies metamorphism ± deformation developed under progressively declining P-T conditions. The first retrograde metamorphism involved formation of an outer rind of actinolite + chlorite + rutile ± phengite, and the growth of coarse-grained chlorite + pumpellyite within the block. During the second event the rind and outer edge of the blueschist were folded, sheared and fractured, and primary glaucophane within the blueschist was replaced by albite, medium-grained chlorite, and glaucophane-crossite. The third retrograde metamorphism was marked by the pseudomorphic replacement of rind actinolite by aragonite and quartz. Aragonite also crystallized extensively within th block, accompanied by lawsonite, chlorite, jadeitic pyroxene, and crossite; this last mineral assemblage is identical to that of the surrounding Franciscan metasedimentary rocks. Features characteristic of the first and second retrograde metamorphic events are readily observed in other high-grade tectonic blocks of the Franciscan Complex and the correlative Otter Point Formation of Oregon. In contrast, evidence of a third retrograde metamorphism, matching that of presently associated Franciscan and Otter Point rocks, has been found in some but not all blocks examined so far. The high P/T conditions of prograde metamorphism and the available metamorphic age determinations suggest that the tectonic blocks originally formed in a pre-Franciscan subduction zone setting. Fragments of blueschist and eclogite from this metamorphic terrain were tectonically incorporated in a serpentinite diapir, and they developed alteration rinds through interaction with the enclosing ultramafic rock. The available data suggest that the examined exotic block and at least some others were transferred to the Franciscan as detritus from a body of serpentinite that reached the earth's surface. Such blocks were then resubducted and metamorphosed along with their presently associated sedimentary sequences.

Journal of Petrology↗
USGS1986

Aluminum enrichment in silicate melts by fractional crystallization: some mineralogic and petrographic constraints.

The degree of aluminum saturation of an igneous rock may be described by its Aluminum Saturation Index (ASI) defined as the molar ratio Al 2 O 3 (CaO + K 2 O + Na 2 O). One suggested origin for mildly peraluminous granites (ASI between 1 and about 1.1) is by fractional crystallization of subaluminous (ASI < 1) magmas; hornblende, having ASI < 0.5, could be a major driving force in such a fractionation process. The efficacy of the process depends not only on precipitation of hornblende and its effective removal from the reacting system, but on the composition and nature of other coprecipitating phases, weighted by their modal abundances in the reactive system. Precipitation of feldspar (ASI = 1), for instance, would retard or even prevent aluminum enrichment in the melt if the ASI of melt is < 1, but would enhance such evolution if the ASI of the melt is > 1. Discussion of the efficacy of any mineral must be made in the context of the total reacting system. For hornblende to effectively cause a melt to evolve into a peraluminous composition, it must be able to coexist with peraluminous magmas. Experimental phase equilibrium data show that at pressure > 5 kb hornblende can coexist with strongly peraluminous melts (ASI ˜ 1.5). Scantily phyric volcanic rocks show that hornblende can coexist with granitic magma having ASI ˜ 1.1 –1.2. The aggregate ASI of last-stage minerals of a typical granite is less than this value; therefore, even after hornblende has reacted out, the residual magma may be expected to continue to evolve toward more aluminous compositions.

Journal of Petrology↗
USGS1986

Lower crustal xenoliths, Chinese Peak lava flow, central Sierra Nevada

An assemblage of pyroxenite, peridotite, and mafic granulite xenoliths contained in the toe of a 10 Ma trachybasalt flow remnant overlying Late Cretaceous granitoids indicates the presence of a mafic-ultramafic complex beneath the Sierra Nevada batholith. Olivine-free pyroxenites that include orthopyroxenites, websterites, and clinopyroxenites are dominant. Primary igneous textures are displayed by some pyroxenites, but commonly are masked by recrystallization. Fe-rich harzburgites and lherzolites are rare. A few of the ultramafic xenoliths contain ovoid opaque patches that are apparently pseudomorphs after garnet and have pyralspite garnet compositions. A pressure corresponding to a lower crustal depth of approximately 40 km has been determined from two of these xenoliths using a garnet-orthopyroxene geobarometer. Abundant mafic granulites can be subdivided into those containing 12 per cent or less A 12 O 3 and chemically gradational with pyroxenites and others containing more than 15 per cent A 12 O 3 and showing considerable scatter on oxide variation diagrams. The high-alumina granulite xenoliths have relatively low 87 Rb/ 86 Sr but high 87 Sr/ 86 Sr, whereas low-alumina and ultramafic xenoliths have a wide range of 87 Rb/ 86 Sr, but lower 87 Sr/ 86 Sr; the isotopic data indicate an age for the complex roughly the same as that of overlying granitoid plutons. However, the granitoids have initial 87 Sr/ 86 Sr ratios intermediate between the high-alumina and ultramafic xenoliths, suggesting that they may have resulted from mixing of basaltic magma, represented by the ultramafics, and crustal materials, with subsequent crystal fractionation. The trachybasalt may represent a partial melt of the ultramafic rocks. Rocks analogous to the Chinese Peak xenoliths are exposed in the Giles complex of central Australia, a series of several deformed layered mafic and ultramafic intrusions, emplaced in a granulite facies terrain. Contemporaneous development of mafic-ultramafic complexes and the Sierra Nevada batholith may explain the present day thick (˜ 50 km) crust in this region

Journal of Petrology↗
USGS1987

Geochemistry of high-silica peralkaline rhyolites, Naivasha, Kenya rift valley

The Recent (<15000 y) volcanic complex of southwest Naivasha, Kenya, consists of mildly peralkaline (comenditic) rhyolite domes, lava flows, air fall pumices, and lake sediments, with minor, peripheral, basalts and hawaiites. The comendites are either aphyric or sparsely porphyritic, few samples containing >5 per cent phenocrysts. Phenocryst minerals are quartz-sanidine-ferrohedenbergite-fayalite-titanomagnetite-ilmenite-riebeckite-arfvedsonite-aenigmatite-biotite-zircon. Ferrohedenbergite and zircon are restricted to less peralkaline, and amphibole, aenigmatite, and biotite to more peralkaline, rocks.The comendites show unusually strong enrichment in Cs, F, Hf, Nb, Rb, REE, Ta, Th, U, Y, Zn, and Zr, and extreme depletion in Mg, Ca, Ba, Co, and Sr. REE patterns are moderately LREE-enriched, with large, negative Eu anomalies. Values of LIL/HFS element ratios, such as Th/Ta and Rb/Zr, are unusually high for peralkaline rhyolites, and are consistent with a substantial crustal component in the comendites. Parameters such as LREE/HREE and Zr/Nb ratios indicate that the Naivasha rhyolites represent several pulses of closely related, but subtly different, magmas. Sanidine/glass partition coefficients for Ba, Pb, Rb, Sr, U, and the REE are presented for one specimen.Major and trace element modelling, and feldspar-rock relationships, show that closed system crystal fractionation cannot alone account for the overall compositional variations in the comendites. A model involving partial melting of variable crustal source rocks and migration of dissolved volatile-metal complexes may be appropriate at Naivasha. © 1987 Oxford University Press.

Journal of Petrology↗
USGS1962

Stability relations of the ferruginous biotite, annite

Annite , KFe 3 AISi 3 O 10 (OH) 2 a member of the iron biotites and the ferrous analogue of phlogopite, has been synthesized and its phase relations have been determined as functions of temperature, fugacity of oxygen (fo 2 ), and total pressure (P total ≈PH 2 O+PH 2 ). A method for controlling fo 2 at high total pressures is described, and data for the 'oxygen buffers' used are given. Buffers range from quartz+iron+fayalite assemblages (low fo 2 ) to magnetite-hematite assemblages (high fo 2 ). Optical properties and unit-cell dimensions of synthetic annites depend on the conditions of synthesis.By recalculating published analyses of natural iron-rich biotites it can be shown that one cannot assume a constant hydrogen content for such biotites. Oxidation may have occurred by drying at 115°C. Octahedral occupancy therefore cannot be calculated from such data.Phase relations of annite are presented in 2,070 and 1,035 bar sections. Depending on fo 2 -T values annite was found to decompose to one of the following assemblages: hematite+ sanidine, magnetite+sanidine, fayalite+leucite+kalsilite, iron+sanidine. All decompositions are dehydration and redox reactions and are sensitive to changes in fH 2 0 and fo 2 (or fH 2 0 and fH 2 ). At 2, 070 bars total pressure annite +magnetite+sanidine can coexist between 425°C and 825° C, depending upon the magnitude of fo 2 .In the presence of quartz the stability field of annite is more restricted. Phase equilibria in the system KAlSiO 4 -SiO 2 -Fe-O 2 -H 2 have been summarized schematically.Wherever possible, thermodynamic extrapolations are made to test the internal consistency of the data. Enthalpies of formation are calculated for both annite and phlogopite. Ranges of fo 2 values in nature as well as mechanisms for changes in fo 2 are investigated. It is useful to distinguish between assemblages which are internally buffered with respect to fo 2 changes and those which are not buffered. The applications of individual reactions involving annite to specific geologic problems are discussed with respect to igneous, metamorphic, and sedimentary rocks.

Journal of Petrology↗