Showing papers in "Journal of Petrology in 1992"

Petrologic Evolution of Anorthoclase Phonolite Lavas at Mount Erebus, Ross Island, Antarctica

Abstract: Mount Erebus, Ross Island, Antarctica, is an active, intraplate, alkaline volcano. The strongly undersaturated sodic lavas range from basanite to anorthoclase phonolite, and are termed the Erebus lineage (EL). The lavas are porphyritic with olivine (Fo88_51), clinopyroxene (Wo43_j3En36_41Fs8_30), opaque oxides (Usp31_76), feldspar (An72_uX and apatite. Rare earth element (REE) contents increase only slightly with increasing differentiation compared with other incompatible elements. The light REE are enriched (LaN/YbN = 14-20) and there are no significant Eu anomalies. Sr/Sr is uniform and low (~O7030) throughout the EL, suggesting derivation of the basanites from a depleted asthenospheric mantle source, and lack of significant crustal contamination during fractionation of the basanite. Regular geochemical trends indicate that the EL evolved from the basanites by fractional crystallization. Major element mass balance calculations and trace element models show that fractionation of 16% olivine, 52% clinopyroxene, 14% Fe-Ti oxides, 11% feldspar, 3% nepheline, and 3% apatite from a basanite parent leaves 23-5% anorthoclase phonolite. Minor volumes of less undersaturated, more iron-rich benmoreite, phonolite, and trachyte are termed the enriched iron series (EFS). The trachytes have Sr/Sr of O704, higher than other EFS and EL rocks, and they probably evolved by a combined assimilation-fractional crystallization process. The large volume of phonolite at Mt. Erebus requires significant basanite production. This occurs by low degrees of partial melting in a mantle plume (here termed the Erebus plume) rising at a rate of about 6 cm/yr.

Regional Metamorphism of the Waits River Formation, Eastern Vermont: Delineation of a New Type of Giant Metamorphic Hydrothermal System

Abstract: Metamorphic isograds and time-integrated fluid fluxes were mapped over the ~ 1500 km2 exposure of the Waits River Formation, eastern Vermont, south of latitude 44°30'N. Isograds based on the appearance of oligoclase, biotite, and amphibole in metacarbonate rocks define elongated metamorphic highs centered on the axes of two large antiforms. The highest-grade isograd based on the appearance of diopside is closely associated spatially with synmetamorphic granitic plutons. Pressure, calculated from mineral equilibria, was fairly uniform in the area, 7± 1-5 kb; calculated temperature increases from ~480°C at the lowest grades in the area to ~575°C in the diopside zone. Calculated XCC)2 of equilibrium metamorphic fluid increases from <0O3 at the lowest grades to ~0-2 in the amphibole zone and decreases to ~ 0-07 in the diopside zone. Time-integrated fluid fluxes increase with increasing metamorphic grade, with the following mean values for each metamorphic zone (in cm3/cm2): ankerite-oligoclase zone, 1 x 104; biotite zone, 7 x 10*; amphibole zone, 2 x 105; diopside zone, 7 x 10'. The mapped pattern of time-integrated fluxes delineates two large deep-seated (~25-km depth) regional metamorphic hydrothermal systems, each centered on one of the major antiforms. Fluid flowed subhorizontally perpendicular to the axis of the antiforms from their low-temperature flanks to their hot axial regions and drove prograde decarbonation reactions as they went. Along the axes of the antiforms fluid flow was further focused around synmetamorphic granitic intrusions. In the hot axial region fluid changed direction and flowed subvertically out of the metamorphic terrane, precipitating quartz veins. Estimates of the total recharge, based on progress of prograde decarbonation reactions, nearly match estimates of the total discharge, based on measured quartz vein abundance, (2-10) x 1012 cm3 fluid per cm system measured parallel to the axes of the antiforms. Within the axial regions fluids had lower XCOl and rocks record greater time-integrated fluxes close to the intrusions than at positions more than ~ 5 km from them. The differences in both fluid composition and time-integrated flux can be explained by mixing close to the intrusions of regional metamorphic fluids of XQQ^O-I with fluids from another source with XCOl~0 in the approximate volume ratio of 1:2.

Rare Earth Element Behaviour During Evolution and Alteration of the Dartmoor Granite, SW England

Abstract: The distribution of rare earth elements (REE) within the compositionally zoned Dartmoor pluton is used to constrain models of granite evolution and to assess the effects of pervasive hydrothermal alteration on REE mobility. The main process of magma evolution was crystal fractionation of early plagioclase, biotite, and accessory minerals (apatite, monazite, zircon, and xenotime). Concentrations of REE (particularly LREE and Eu) and other elements (Fe2O3t, MgO, CaO, TiO2, Zr, Ba, and Sr) decrease strongly with evolution of the pluton from 71 to 74% SiO2. These trends, and the inward zoning of the pluton, are compatible with differentiation by crystal fractionation at the level of emplacement, a process that gave rise to a marginal cumulate granite (CGM) modified by country rock assimilation, a body of inner granite (PM), and a late-stage evolved granite (FG) that intruded the earlier types. REE modelling of the Dartmoor granite types by fractional crystallization of REE-enriched accessory minerals from a parent PM-granite shows that the FG-granite cannot have formed from a residual liquid left by crystallizati on of the CGM-granite. Two discrete stages of crystallization occurred; side-wall cumulate CGM-granite crystallization dominated by LREE-enriched monazite fractionation followed by a late-stage mobile residual FG-granite in which fractionation was dominated by HREE-enriched apatite and zircon. Modelling supports the idea that largescale assimilation of country rock was not the dominant process during Dartmoor granite evolution. Pervasive hydrothermal alteration locally affected all Dartmoor granite types, altering primary plagioclase, biotite, apatite, monazite, and, to a lesser extent, zircon and xenotime. During pervasive sericitization, chloritization, and tourmalinization, REE were mobilized over distances of centimetres only and redistributed into the secondary alteration products seridte, chlorite, tourmaline, allanite, and sphene. Whole-rock REE abundances were not affected

Synmetamorphic High-Density Carbonic Fluids in the Lower Crust: Evidence from the Nilgiri Granulites, Southern India

Abstract: High-density CO2 inclusions occur abundantly in granulite facies rocks (age of metamorphism approximately 2.5 b.y.) of the Nilgiri massif, southern India. The chronology of carbonic inclusions in the widespread enderbitic granulites studied in relation to the development of micro-textures and mineral assemblages indicates that randomly oriented, negative-crystal-shaped CO2 inclusions (4-20-mu-m) in garnet and quartz grains (qtz I) armoured by garnet entrap syn-peak-metamorphic pore fluids. The more abundant trail-bound CO2 inclusions in the deformed, polygonized, and partially recrystallized matrix quartz grains (qtz II and III) and plagioclase grains were formed in connection with a stage of compressional deformation and subsequent annealing related to the development of the late-Proterozoic Bhavani shear zone. These inclusions resulted from local re-equilibration of the former peak-granulitic carbonic inclusions and re-entrapment of released fluids. The presence of pure CO2 in all the inclusions is confirmed by microthermometric data and laser-excited Raman microspectrometry. Temperatures of homogenization (liquid phase) are in the range of -50 to +20-degrees-C, and the corresponding CO2 densities are between 1.154 and 0.807 g/cm3. Mineralogical thermobarometry on the enderbitic granulites documents a continuous gradient of near-peak metamorphic conditions from 750-degrees-C, 9-10 kb in the northern part to 730-degrees-C, 7 kb in the southwestern part of the Nilgiri massif. Uniform P, T estimates (600-650-degrees-C, 6-7 kb) for late coronitic garnet + quartz assemblages in enderbites and metadolerites indicate that differential uplift of the massif to mid-crustal levels was accomplished before late compressional deformation. In conformity, carbonic inclusions in quartz II and III are characterized by uniformly high density (1.154-1.08 g/cm3). In contrast, early carbonic inclusions in garnet and quartz I preserve the density contrast reflecting the regional P, T gradient during near-peak metamorphic fluid entrapment. The fluid inclusion systematics indicate `near-isochoric; uplift of the northern high-P domain, but near-isobaric cooling of the southwestern low-P domain. The carbonic fluids are thought to have been derived either from internal sources during dehydration-melting processes or from freezing synmetamorphic intrusives into the lower crust.

The Koloa Volcanic Suite of Kauai, Hawaii

Abstract: The Koloa lavas of the post-erosional volcanic suite of Kauai vary in composition from melilitite to alkali olivine basalt. 87Sr/86Sr varies from ˜0.7030 to 0.7033, the range being similar to that of other Hawaiian post-erosional suites. The chemical composition within single lava flows is variable and forms major-element trends that are different from the main Koloa trend. The main trend can be related to increased degrees of partial melting during decreasing pressure. The combined evidence from Sr and Nd isotopes and rare earth elements (REEs) could suggest that an ocean island basalt (OIB) plume was infiltrated by a carbonate-rich melt with a high concentration of Sr and REEs derived from the mid-ocean ridge basalt (MORB) source. However, the Pb-isotope systematics of the Hawaiian islands do not suggest that a MORB source was involved during the generation of the posterosional lavas. Instead, we suggest that the Hawaiian tholeiitic and nephelinitic magma suites are derived from a zoned plume with a low 87Sr/86Sr core and a high 87Sr/86Sr concentric margin. The trends of the single flows and their variation in isotope ratios suggest that they formed from magmas accumulated mainly from the central core of the zoned plume.

Parameterization of the Melting Regime of the Shallow Upper Mantle and the Effects of Variable Lithospheric Stretching on Mantle Modal Stratification and Trace-Element Concentrations in Magmas

Abstract: Parameterization of melting phenomena in the upper mantle has primarily focused on two basic themes, namely the physical and chemical processes that govern partial melting. Parameterization of physical processes mainly refers to establishing relationships between parameters such as the temperature, pressure, matrix and melt flow geometry, lithospheric stretching, and volume of magma. By contrast, parameterization of chemical processes largely implies unravelling the relationships between type and degree of melting, and source and melt composition. Few attempts have been made, however, to interrelate the two processes. The present work is an effort to provide a link between physical and chemical parameters associated with mantle melting and to allow in-depth modelling of partial melting processes in upwelling asthenosphere in a rigorous yet simplified manner. Several correlations among the most important physical parameters (e.g., equilibration and extrusion temperature and pressure of magma, melt fraction and thickness, stretching factor, etc.) are explored. On this basis, a model for the compositional stratification of the lithosphere is proposed, and its bearing on the nature of intra-oceanic arc magmatism is emphasized. Trends of melting residues in terms of modal olivine and clinopyroxene are calculated for a wide range of possible potential temperatures that may be applied to xenolith or abyssal peridotite suites to constrain further their original depth of upwelling. Dry solidus equations for depleted peridotite compositions are also derived that may be used to infer the effects of volatiles on the melting of refractory supra-subduction zone mantle. The sensitivity of certain elements to temperature variations during melting in a column of ascending mantle is highlighted using Ni as an example, and the dangers of using single-value distribution coefficients to predict concentrations of transition metals in magmas are emphasized. MORB-normalized multi-element profiles calculated using a variety of sources, mantle potential temperatures, and stretching factors are presented, and the differences between instantaneous and pooled melts are discussed. A technique to calculate mineral proportions during transformation of garnet lherzolite to spinel lherzolite, together with estimates of the modal composition of fertile spinel and garnet lherzolite are included. Selected trace-element abundances in various sources [bulk silicate Earth, depleted MORB (mid-ocean ridge basalt) mantle, N-MORB] and distribution coefficients for common rock-forming minerals are also tabulated.

Pressure, Temperature, and Structural Evolution of West-Central New Hampshire: Hot Thrusts over Cold Basement

Abstract: Pressure-temperature (P-T) paths have been calculated from pelites and amphibolites of several major Acadian structures in west-central New Hampshire by using both inclusion thermobarometry and differential thermodynamics (the Gibbs method). P-T paths calculated for rocks exposed in the Orfordville and Bronson Hill anticlinoria are 'clockwise' and show 1-2-5 kb of exhumation with 30-100 °C of heating. Because this type of path is characteristic of the lower plate of overthrust terranes, these rocks are interpreted to be (para)autochthonous. P-T paths for rocks exposed in an intervening synclinorium (the Hardscrabble synclinorium) show isothermal loading of 1-3 kb followed by possible isobaric cooling. This behavior is characteristic of rocks occupying a middle-plate structural position within a multiple thrust package, and so these rocks are interpreted to be allochthonous. The interpretation that the Hardscrabble rocks are allochthonous differs from previous models, but better explains the petrologic data and is consistent with the stratigraphic and structural data on which other models have been based. Correlation of the P-T paths with deformational events through kinematic and textural analysis indicates that during nappe stage deformation, the synclinorial rocks were transported westward, and that the anticlinorial and synclinorial rocks were buried to depths of 25-30 and 20-25 km respectively. The exhumation with heating recorded by the anticlinorial samples occurred during the dome stage of deformation, and differentially uplifted the anticlinorial rocks relative to the synclinorial rocks; this differential uplift may have been accommodated through reactivation of early thrust faults with normal movement sense. P-T paths of the Hardscrabble synclinorium rocks are suggestive of a relatively elevated initial geothermal gradient for their pre-nappe source terrane, which is interpreted to have been between the Kearsarge-Central Maine basin and the Bronson Hill parautochthon.

Potassic Mafic Lavas of the Bearpaw Mountains, Montana: Mineralogy, Chemistry, and Origin.

Abstract: The volcanic rocks of the Bearpaw Mountains are part of the Montana high-potassium province, emplaced through Archaean rocks of the Wyoming Craton between 54 and 50 Ma ago. Extrusive rocks, dominated by minettes and latites, have a volume of U 825 km3. The minettes range in composition from 20 to 6% MgO. The more magnesian varieties contain the phenocryst assemblage forsterite + Cr-spinel + diopside ± phlogopite. More evolved rocks are olivine-free, with an assemblage of either salite + phlogopite + pseudoleucite or salite + phlogopite + analcime. The analcime is thought to be secondary after leucite, produced by loss of potassium from the minettes. Mineral chemistry and textures, especially of clinopyroxenes, indicate that mixing between minette magmas of varying degrees of evolution was commonplace. Compositional variation was further extended by accumulation of olivine + spinel + clinopyroxene phenocrysts, and by the preservation of mantle xenocrysts in the minettes. The primary minette magmas are inferred to have had 12–14% MgO and to have been generated at 30 kb from an olivine + diopside + phlogopite-bearing source. The primary magmas evolved dominantly by fractionation of olivine + diopside. The minettes have high contents of large ion lithophile elements (LILE) and light rare earth elements (LREE), with K2O up to 6.18%, Ba 5491 ppm, Sr 2291 ppm, and Ce 99 ppm. (87Sr/86Sr)0 ranges from 0.707 to 0.710 and £Nd varies from –10 to –16. These data, plus high LILE/HFSE (high field strength elements) values, are interpreted to show that the minettes contain at least three different mantle components. The lithosphere was initially depleted in Archaean times, but was metasomatically enriched in the Proterozoic and in the late Cretaceous and early Tertiary. The latites have many chemical features in common with the minettes, such as potassic character and high LILE/HFSE values. They formed by fractional crystallization of minette magma in combination with assimilation of crustal rocks; this process enriched the magmas in SiO2 and raised Na2O/K2O and 87Sr/86Sr values. Chemical data for phenocrysts and bulk rocks in minettes suggest mixing between minette and latite magmas.

Thermobarometry, Geochronology and the Interpretation of P–T–t Data in the Britt Domain, Ontario Grenville Orogen, Canada

Abstract: Textural evidence, thermobarometry, and geochronology were used to constrain the pressuretemperature-time (P-T-t) history of the southern portion of the Britt domain in the Central Gneiss Belt, Ontario Grenville Province. Typical metapelitic assemblages are quartz + plagioclase+ biotite + garnet + kyanite ± alkali feldspar ± sillimanite ± rutile ± ilmenite ± staurolite ± gahnite ± muscovite. Metatonalitic assemblages have quartz + plagioclase + garnet + biotite + hornblende + rutile + ilmenite. Metagabbroic rocks contain plagioclase + garnet + clinopyroxene + biotite + ilmenite ± hornblende ± rutile± quartz. Notable textural features include overgrowths of sillimanite on kyanite and of spinel on staurolite. The spinel overgrowths can be modeled by the breakdown of staurolite via the reaction Fe-staurolite = hercynite +kyanite + quartz+ H2O. The decomposition of staurolite to hercynite has a steep dP/dT slope and constrains the late prograde path of a staurolite metapelite. Garnet-Al2SiOj-plagioclase-quartz (GASP) barometry applied to metapelitic garnets that preserve calcium zoning reveals a pressure decrease from ~ 11 to 6 kb at an assumed temperature of 700 °C. Garnet-plagioclase-ilmenite-rutile-quartz and garnet-clinopyroxene-plagioclase-quartz barometry is in good agreement with pressures obtained with the GASP barometer. Geochronologic data from garnet, allanite, and monazite in metapelitic rocks give ages that fall into two groups, ~ 1-4 Ga and ~ 1 • 1 Ga, suggesting the presence of at least two metamorphic events in the area. It is most reasonable to assign the 1-4 Ga age to the high-pressure data and the 1-1 Ga age to the lower-pressure data. Collectively the P-T-t data indicate a complex and protracted history rather than a single cycle of burial and uplift for this part of the Grenville Province.

Petrogenesis of Ultramafic Xenoliths from the 1800 Kaupulehu Flow, Hualalai Volcano, Hawaii

Abstract: The 1800 Kaupulehu flow on Hualalai Volcano, Hawaii, contains abundant xenoliths of dunite, wehrlite, and olivine clinopyroxenite with minor gabbro, troctolite, anorthosite, and websterite. The petrography and mineral compositions of 41 dunite, wehrlite, and olivine clinopyTOxenite xenoliths have been studied, and clinopyroxene separates from eight of these have been analyzed for Ba, K, Rb, Sr, rare earth elements, 87Sr/*6Sr, and 143Nd/14*Nd. Temperatures of equilibration obtained by olivine-spinel and pyroxene geothermometry range from 1000 to 1200 °C. Mineralogical data combined with published fluid inclusion data indicate depths of origin in the range of 8-30 km. The rarity of orthopyroxene, the presence of Fe-rich olivine (Fog, _89) and clinopyroxene (Fs3 _,2X and the occurrence of high TiO2 in spinel (0-9-2-8 wt.%) and clinopyroxene (0-35—1-33 wt.%) all indicate that the xenoliths are cumulates, not residues from partial fusion. The separated clinopyroxenes have 87Sr/*6Sr (0-70348-0-70367) and '"Nd/'^Nd (O-51293-O-51299) values that are different from Sr and Nd isotope ratios of Pacific abyssal basalts ( 0-5130, respectively). Also, clinopyroxenes and spinels in the xenoliths have generally higher TiO2 contents (>O-35 and >0-91 wt.%, respectively) than their counterparts in abyssal cumulates (<0-40 and <0-70wt%, respectively). These differences indicate that the xenoliths are not a normal component of oceanic crust Because the xenoliths and alkalic to transitional Hualalai lavas have similar values for Cr/(Cr +Al)

Petrology of the Great Abitibi Dyke, Superior Province, Canada

Abstract: The Great Abitibi Dyke (GAD) which can be traced northeast, for > 700 km, across the Abitibi Belt in the southeastern Superior Province of the Canadian Shield, is composed of olivine gabbro to monzodiorite, weakly saturated to undersaturate d in silica. All rocks of the GAD can be derived by mainly plagioclase and olivine fractionation from a parental magma corresponding in composition to chilled margin samples. Two units can be distinguished , a marginal unit (Unit 1) representing 0-50% crystallization and a central unit (Unit 2\ found over about half of the dyke length, representing 50-70% crystallization. Modelling, using Pearce elemental ratio analysis, quantifies the fractionation history and allows mass-balance calculations over the present exposure level of the dyke. The approximate balance between the amounts of cumulate and fractionated rocks suggests that Unit 1 differentiated essentially in situ as a closed system. In contrast, Unit 2 rocks were formed by loss of substantial plagioclase and olivine from the parental magma. This fractionate must have either been lost to depth or left behind in an external chamber. Unit 2 rocks show depletion in plagioclase and enrichment in mafic minerals along strike towards the southwest (deeper exposure level?), a trend explained by density stratification. Regional variation in Unit 1 chemistry is interpreted in terms of lateral magma injection towards the northeast from the locus of Keweenawan rift volcanism. Feldspar, olivine, and augite compositions are linearly correlated with equilibrium temperature and extent of magma evolution. The parent magma had a trace element chemistry corresponding to a 'within-plate' setting and was probably derived from an incompatible-element enriched mantle similar to the source for ocean island basalts (OIBs).