Phase Equilibria in Three Assemblages of Kyanite-Zone Pelitic Schists, Lincoln Mountain Quadrangle, Central Vermont

Petrologic and geologic arguments suggested that a close approachto chemical equilibrium at a single temperature and pressuremight be found in the rocks near Mt. Grant in central Vermont.An area 2,800 feet by 4,000 feet was selected and studied indetail. The major assemblages are: kyanite-chloritoid-chlorite-quartz-muscovite-paragonite-rutile;garnet-chloritoid-chlorite-quartz-muscovite-paragonite-ilmenite;and garnet-chlorite-biotite-quartz-muscovite-albite-ilmenite.All minerals were separated from a primary sample for each ofthese assemblages and purified, complete gravimetric and spectrographicanalyses performed, and optical properties, X-ray properties,and densities measured. Chlorite, garnet, and chloritoid orbiotite were separated from an additional nine samples of theseassemblages and spectrographic and partial gravimetric analysesperformed. Distribution coefficients of each mineral pair for Mg/Fe andMn/Fe+ Mg and for the minor elements are similar in nearly everysample for a given assemblage. Distribution coefficients forgarnet-chlorite differ in the two assemblages which containthis pair; this difference is attributed to the difference inAl-content of the chlorite from the two assemblages. The smallrange of distribution coefficients, despite a wide range inthe relative proportions of the ferromagnesian phases, is convincingevidence that an equilibrium partition of the major and minorcations between the phases in each sample had been attainedand that the equilibration temperature was the same for eachsample. The coexistence of kyanite+muscovite+quartz and comparison ofthe composition of coexistent Ca-free muscovite and paragoniteand of O¹⁸/O¹⁰ ratios for coexistent quartz and magnetite withexperimental values indicate that these rocks formed at P_s 11 kb, T 550? C, and aH₂o low enough (Pe(H₂o) sufficiently lessthan P_s) to depress the pyrophyllite breakdown temperature byabout 40? C. The cations and the ao₂ are equilibrated locally(within each sample), but all the samples have equilibratedto a common O¹⁸/O¹⁶ ratio. A possible explanation is that afluid medium had sufficient oxygen in H₂O to control the O¹⁸/O¹⁶ratio of the rock and its phases, but that the rock system hadsufficient buffer capacity to control the ao₂ of the fluid.     

Geochemistry of the Mt Wright Volcanics from the Wonominta Block,northwestern New South Wales

The Mt Wright Volcanics are located in the Wonominta Block of northwestern New South Wales. Detailed regional mapping has shown that the block is a composite tectonic unit and that the metavolcanic rocks described as the Mt Wright Volcanics may have been emplaced at different time from Late Proterozoic (northern section: Packsaddle, Nundora) to Early Cambrian (southern section: Mt Wright). Geochemical investigations, including major and trace elements, as well as analyses of relic clinopyroxene, show that the rocks have affinities with alkali basalt with light‐rare‐earth‐element‐enriched compositions. An intra‐plate extensional environment (such as rift‐ and/or plume‐related) is considered most likely for the formation of the rocks. Though metamorphosed to various degrees, the rocks apparently retain much of their primary Sr isotopic character (initial ⁸⁷Sr/⁸⁶Sr about 0.7032) and, apart from their age, resemble the Tertiary intraplate volcanism in eastern Australia. The Nd isotope analyses yield remarkably similar results between the two sections of the Mt Wright Volcanics, with ¹⁴³Nd/¹⁴⁴Nd between 0.51260 to 0.51271 and _εNd(T) 4.7 ±0.4 (calculated to 525 Ma). A kaersutite‐bearing xenolith found in the northern section of the volcanic sequence has a Nd isotope composition more depleted than its hosts with _εNd(T) of 7.7. The isotope results suggest that the Mt Wright Volcanics were derived from a depleted mantle source without significant crustal contribution. It is proposed that the Mt Wright Volcanics possibly represent the products of a rifting event that led to the breakup of the Proterozoic supercontinent during Early Cambrian in eastern Australia.     

Magmatic history of the Eastern Creek Volcanics,Mt Isa,Australia: insights from trace-element and platinum-group-element geochemistry

The Paleoproterozoic basalts of the Eastern Creek Volcanics are a series of continental flood basalts that form a significant part of the Western Fold Belt of the Mt Isa Inlier, Queensland. New trace-element geochemical data, including the platinum-group elements (PGE), have allowed the delineation of the magmatic history of these volcanic rocks. The two members of the Eastern Creek Volcanics, the Cromwell and Pickwick Metabasalt Members, are formed from the same parental magma. The initial magma was contaminated by continental crust and erupted to form the lower Cromwell Metabasalt Member. The staging chamber was continuously replenished by parental material resulting in the gradual return of the magma composition to more primitive trends in the upper Cromwell Metabasalt Member, and finally the Pickwick Metabasalt Member formed from magma dominated by the parental melt. The Pickwick Metabasalt Member of the Eastern Creek Volcanics has elevated PGE concentrations (including up to 18 ppb Pd and 12 ppb Pt) with palladium behaving incompatibly during magmatic fractionation. This trend is the result of fractionation under sulfide-undersaturated conditions. Conversely, in the basal Cromwell Metabasalt Member the PGE display compatible behaviour during magmatic fractionation, which is interpreted to be the result of fractionation of a sulfide-saturated magma. However, Cu remains incompatible during fractionation, building up to high concentrations in the magma, which is found to be the result of the very small volume of magmatic sulfide formation (0.025%). Geochemical trends in the upper Cromwell Metabasalt Member represent mixing between the contaminated Cromwell Metabasalt magmas and the PGE-undepleted parental melt. Trace-element geochemical trends in both members of the Eastern Creek Volcanics can be explained by the partial melting of a subduction-modified mantle source. The generation of PGE- and copper-rich magmas is attributed to melting of a source in the subcontinental lithospheric mantle below the Mt Isa Inlier which had undergone previous melt extraction during an older subduction event. The previous melt extraction resulted in a sulfur-poor, metal-rich metasomatised mantle source which was subsequently remelted in the Eastern Creek Volcanic continental rift event. The proposed model accounts for the extreme copper enrichment in the Eastern Creek Volcanics, from which the copper has been mobilised by hydrothermal fluids to form the Mt Isa copper deposit. There is also the potential for a small volume of PGE-enriched magmatic sulfide in the plumbing system to the volcanic sequence.     

The age of the Mt Read Volcanics in the Que River area,western Tasmania

Sediments associated with the Mt Read Volcanics in the Que River area contain fossils that indicate that the Mt Read Volcanics (in part at least) are late Middle Cambrian (Ptychagnostus punctuosus or P. nathorsti Zone) in age, and that they are in part marine. The sediments are defined as the Que River Beds.     

Significance of MORB-derived Amphibolites from the Aracena Metamorphic Belt, Southwest Spain

The Aracena metamorphic belt, in the southwest Iberian Massif,is characterized by the presence of MORB-derived amphibolitesand continental rocks deformed and metamorphosed during theHercynian orogeny. Geochemical relationships of these amphibolitesindicate the existence of a multiple fractionation process froma set of parental magmas, implying the existence of a multi-chambersystem beneath the ridge where the basalt protolith was extruded.Neodymium isotopic ratios are typical of MORB, and oxygen isotopesindicate that these amphibolites have been derived from theuppermost part of the oceanic crust. Thermal evolution, revealedfrom the study of chemical variations in the amphibole chemistry,is interpreted as resulting from subduction in a low-pressureregime in which the thermal structure of the continental hanging-wallplayed an important role. This continental wall was previouslyheated by subduction of a slab window resulting from migrationof a triple junction along the continental edge during plateconvergence. Three petrologic arguments support this tectonicmodel. These are: (1) the low-pressure inverted metamorphicgradient of amphibolites of the oceanic domain; (2) the high-temperature-low-pressure metamorphism of the continental hanging wall; (3)the early intrusion of boninites into the continental domain. ^*Corresponding author.     

Betekhtinite from Mt. Lyell,tasmania

Betekhtinite occurs in specimens of recrystallised galena from the North Lyell orebody. Its chemical composition is Cu 59.0, Pb 17.3, Fe 2.7, S 20.6, total 99.6. The thermal stability of betekhtinite is briefly discussed.

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Zusammenfassung In Proben von rekristallisiertem Bleiglanz des North Lyell Erzkörpers kommt Betechtinit vor. Dessen chemische Zusammensetzung ist Cu 59.0, Pb 17.3, Fe 2.7 und S 20.6; total 99.6. Die thermische Stabilität des Betechtinites wird kurz diskutiert.

     

The petrogenesis of volcanics from Mt. Bulusan and Mt. Mayon in the Bicol arc,the Philippines

Pliocene to recent volcanic rocks from the Bulusan volcanic complex in the southern part of the Bicol arc (Philippines) exhibit a wide compositional range (medium- to high-K basaltic-andesites, andesites and a dacite/rhyolite suite), but are characterised by large ion lithophile element enrichments and HFS element depletions typical of subduction-related rocks. Field, petrographic and geochemical data indicate that the more silicic syn- and post-caldera magmas have been influenced by intracrustal processes such as magma mixing and fractional crystallisation. However, the available data indicate that the Bicol rocks as a group exhibit relatively lower and less variable ⁸⁷Sr/⁸⁶Sr ratios (0.7036–0.7039) compared with many of the other subduction-related volcanics from the Philippine archipelago. The Pb isotope ratios of the Bicol volcanics appear to be unlike those of other Philippine arc segments. They typically plot within and below the data field for the Philippine Sea Basin on ²⁰⁷Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb diagrams, implying a pre-subduction mantle wedge similar to that sampled by the Palau Kyushu Ridge, east of the Philippine Trench. ¹⁴³Nd/¹⁴⁴Nd ratios are moderately variable (0.51285–0.51300). Low silica (<55 wt%) samples that have lower ¹⁴³Nd/¹⁴⁴Nd tend to have high Th/Nd, high Th/Nb, and moderately low Ce/Ce* ratios. Unlike some other arc segments in the Philippines (e.g. the Babuyan-Taiwan segment), there is little evidence for the involvement of subducted terrigenous sediment. Instead, the moderately low ¹⁴³Nd/¹⁴⁴Nd ratios in some of the Bicol volcanics may result from subduction of pelagic sediment (low Ce/Ce*, high Th/Nd, and high Th/Nb) and its incorporation into the mantle wedge via a slab-derived partial melt.     

Petrogenesis of the gabbros from Mt. St. Hilaire,Quebec, Canada

Mt. St. Hilaire occurs as a small funnel-shaped intrusion in the Monteregian petrographic province of Quebec and consists of alkali gabbros and later nepheline syenites. Based on field relations, petrography, and geochemistry, five types of gabbro are recognized. In order of intrusion these are: leucogabbro, foliated gabbro, kaersutite-biotite gabbro, kaersutite gabbro, and a gabbro-melagabbro series. Based on analyses of the early-forming ilmenite-titanomagnetite, the gabbros crystallized under high fO₂ conditions which lead to subsequent crystallization of olivines with high MnO contents. Fractionation of ilmenite and titanomagnetite was a major control on the Ti and A[TV]concentrations in the clinopyroxenes. Plagioclase compositions in the gabbros became richer in Ab contents in the sequence gabbro-melagabbro to leucogabbro. Whole-rock analyses suggest that the parental magma of alkali basaltic composition was fairly evolved prior to emplacement. Lack of olivine in the cumulate gabbro-melagabbros and low Ni and Cr in all gabbros may reflect either extreme olivine fractionation and/or a very low olivine content in the source material for these basalts. Differentiation of the gabbros occurred both pre- and post-emplacement, probably by a process of crystal-liquid fractionation at depths between 3-5 and 8 km. This is in accordance with geophysical measurements for other Monteregian intrusions. A model is presented for the mechanism of emplacement.     

Phase Equilibria of Hornblende-Bearing Eclogite in the Western Dabie Mountain,Central China

The high-pressure (HP) eclogite in the western Dabie Mountain encloses numerous hornblendes,mostly barroisite.Opinions on the peak metamorphic P-T condition,PT path and mineral paragenesis of it are still in dispute.Generally,HP eclogite involves garnet,omphacite, hornblendes and quartz,with or without glaucophane,zoisite and phengite.The garnet has compositional zoning with X_(Mg) increase,X_(Ca) and X_(Mn) decrease from core to rim,which indicates a progressive metamorphism.The phase equilibria of the ...     

Phase Equilibria in the Metamorphic Rocks of St. Paul Island and Cape North, Nova Scotia

A suite of schists—one from the garnet zone, 19 from thestaurolite zone, 2 from the kyanite isograd, and one from thekyanite zone—were separated into their constituent minerals.Chemical analyses of one chlorite and of 23 sets of coexistingbiotites and garnets were carried out by photometric and titrametricprocedures. Plots of garnet-biotite tie-lines from divariantassemblages on appropriate phase diagrams result in intersectingtie-lines which cannot be ascribed to experimental error. Theoreticalconsiderations argue that at equilibrium, at the same pressureand temperature or at constant pressure and varying temperature,tie-lines of divariant assemblages should not intersect. Possibleexplanations require that diffusion equilibrium of Fe and Mgbe restricted to volumes smaller than that of a hand specimenor that P as well as T varies considerably. Emission spectrographicdeterminations of Fe and Mg in biotite indicate that the Fe/Mgratio varies among biotites little more than a centimeter apart.Such a variation would argue more in favor of a lack of diffusionequilibrium.     

Geology of the Lick Observatory Quadrangle,California

The Lick Observatory 7.5-minute quadrangle exposes evidence of geologic events that range from subduction of Mesozoic Franciscan Complex, through accumulation of marine Miocene porcellanite and clastics, to the development of the San Andreas fault system and deformation within it. The active Calaveras fault zone, with its linear valleys and subparallel strike-slip strands, transects the quadrangle and, northwest of San Filipe Valley, joins and incorporates the older Madrone Springs fault. The topography has formed in the past 1 to 2 million years and rises northeastward from the East Evergreen range-front thrust, across the Calaveras and several inferred mountain-building faults, to the 1280 m crest of Mt. Hamilton.

The stratigraphy includes coherent, variously schistose metagraywacke of the late Mesozoic Franciscan Complex; discordant zones of melange of sheared shale and blocks that include blueschist and eclogite; serpentine that may represent the Coast Range Ophiolite; relatively undeformed sandstone, shale, and conglomerate of the late Mesozoic Great Valley sequence; marine Miocene Claremont Porcellanite, mudstone, and Briones Sandstone; and deformed nonmarine gravels of the Pleistocene and Pliocene Santa Clara Formation.

The Franciscan sandstones are complexly deformed and discordantly transected by tectonically emplaced melange zones; a local chert mass marks the remnant of a discordantly overlying thrust sheet. Southwest of the Calaveras zone, folded Miocene rocks are faulted over the more strongly deformed Great Valley sequence. Those rocks, in turn, are thrust over small windows of Franciscan rock, and the entire mountain mass is thrust over Santa Clara gravels at the foot of the range. These latter structures postdate the 3.5 Ma imposition of compression across the plate margin suggested by plate tectonic reconstructions.     

Phase Equilibria of Margarite-Bearing Schists and Chloritoid + Hornblende Rocks from Western New Hampshire, USA

A variety of uncommon garnet-grade assemblages have been foundin rocks from three outcrops in the western part of centralNew Hampshire, and include the associations Grt+MrgCld, Grt+Bt+CldMrg,and Mrg+Cld+HblGrt (all rocks contain Ms, Chl, Ilm, and Qtz).These unusual rocks coexist with more typical Grt+Bt+Chl+Plmetapelites and amphibolites. Rim P–T conditions are {smalltilde}49035C and 5•751•25 kbar. Projection of the assemblages from Qtz, H₂O, and Ilm into theCa–Al'–Na–(Fe+Mg) tetrahedron, and from Qtz,Ilm, H₂O, and Chl into the Ca–Al'–Fe'–Mn tetrahedronindicates that Ca/(Ca+Na) and Mn differ among the assemblagesin a systematic fashion. Common Grt+Bt+Chl+Pl assemblages arerestricted to relatively high Mn and low Ca/(Ca+Na) values,whereas Cld+Bt+Mrg and Cld+Hbl+Mrg assemblages are stable atlow Mn and high Ca/(Ca+Na). These data suggest that at thisgrade Cld+Bt is more stable than Grt+Chl in the KFMASH system,whereas in the Ca—KFMASH system, Hbl+Cld assemblages arestable. Composition space analysis using the singular value decompositionmethod indicates that compositions of minerals from individualsamples are consistent with local equilibrium, but that differentoutcrops may not have all equilibrated at the same P–T–a_H2Oconditions. Thermodynamic analysis suggests that a garnet-zoneprograde sequence of ferromagnesian associations for these bulkcompositions would be Hbl+Cld+Grt+ChlBt+Cld+Grt+ChlBt+Grt+Chl. Staurolite-grade rocks from the same stratigraphic units areexposed across strike, and contain the assemblage Grt+StBtPl(all rocks contain Ms, Qtz, Chl, and Ilm). Margarite is commonlypresent as inclusions in the cores of garnets, but is absentas inclusions near garnet rims and from the matrix; conversely,staurolite inclusions are present towards the rims of the garnets,but are absent from the cores. These inclusion relations suggestthat margarite may react to form staurolite and garnet withincreasing grade via a reaction such as chlorite+margarite=staurolite+garnet+H₂O. Biotite is common in the matrix but is not typically abundant,and appears to have been the last phase to join the assemblage.Biotite is inferred to have joined the Grt+St+Chl assemblagesafter margarite breakdown through the reaction Grt+Chl+Ms=St+Bt+H₂O. Thus, uncommon margarite assemblages may evolve into commonGrt+Bt+St+Chl assemblages. ^* Present address: Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin 53706.     

Mineral reactions in a calc-mica schist from Gassetts,Vermont, U.S.A.

A petrographic and electron microprobe study of an interbedded calc-mica schist from Gassetts, Vermont reveals the complexities of Fe-Mg crystalline solution and gradients in X _H2O/X _CO2 during regional metamorphism. The common association of microcline+diopside ± zoisite formed from biotite +calcite+quartz may be produced by continuous (Fe-Mg) exchange reactions, despite the implied metastability in the CaO-K₂O-MgO-Al₂O₃-SiO₂-H₂O-CO₂ system. Different assemblages are produced in a reaction zone between carbonate and pelite beds of varying thickness. This illustrates the importance of exhaustion of fluid buffering assemblages due to mineral reactions proceeding on a local scale. Siliceous dolomites at the same metamorphic grade have produced significantly different assemblages to the interlayered carbonatepelite sequence.     

Metamorphic P-T paths from calcic pelitic schists from the Strafford Dome, Vermont, USA

Metamorphism of the Gile Mountain Formation and Waits River Formation in the Strafford Dome and Townshend-Brownington Syncline in east-central Vermont records two nappe-style events, D1 and D2, followed by doming. D1 formed a muscovite + biotite ± ilmenite schistosity subparallel to compositional layering, SO, and was followed by heating to garnet grade. The temperature and pressure at the end of D1 are estimated to be c . 450 C and 6-8 kbar. D2 variably crenulated and folded S1 during a nearly isothermal pressure increase of 1-2 kbar, calculated from compositions of garnet, which have inclusions trails with progressive crenulation and rotation of the S1 fabric. Similar P-T paths are computed for most of the area, suggesting that the later schistosity developed during emplacement of a regional nappe 3-6 km thick. There is a general lack of D3 (dome-stage) microstructures.
Near the Strafford-Willoughby Arch, staurolite and kyanite overgrew S2 in pelites, and plagioclase with increasing X _An overgrew S2 in calcic pelites, reflecting post-D2 heating to a maximum of 550-600 C. Metamorphic pressures at the end of D2 are fairly constant on the west side of the dome, indicating minor dome-stage uplift. In contrast, pressures at the thermal peak of metamorphism decrease by more than 4 kbar east of the dome. The observed pattern of isotherms and isobars is mainly the result of post-metamorphic, differential uplift and unroofing.
Finally, a minor, retrograde metamorphism produced the assemblage albite + epidote + K-feldspar + muscovite + chlorite, with grade increasing east toward the Connecticut River.     

Fractionation and Assimilation Processes in the Alkaline Augite Syenite Unit of the Ilimaussaq Intrusion, South Greenland, as Deduced from Phase Equilibria

The early augite syenite unit in the 1·13-Ga-old Ilímaussaqintrusive complex, South Greenland, consists of a magmatic assemblageof ternary alkali feldspar + fayalitic olivine + augite + titanomagnetite+ apatite + baddeleyite ± nepheline ± quartz ±ilmenite ± zircon. Feldspar, nepheline and QUILF thermometryyield T = 1000–700°C, at P = 1 kbar, which is derivedfrom fluid inclusion data from other parts of the complex. Ternaryfeldspar was the first major liquidus phase. It crystallizedat temperatures between 950 and 1000°C from a homogeneousmagma with a_SiO2 = 0·8 and f_O2 about 1·5–2log units below the fayalite–magnetite–quartz (FMQ)buffer. Later, closed system fractionation produced nepheline-bearingassemblages with a_SiO2 = 0·4 and log f_O2 = FMQ –3 to FMQ – 5. Assimilation of wall rocks produced localvariations of melt composition. Four traverses through the unitwere sampled parallel to the assumed direction of crystallization.They exhibit significant differences in their mineral assemblagesand compositions. The chemical zoning and calculated intensiveparameters of four sample suites reflect both closed systemfractional crystallization and local assimilation of wall rocks. KEY WORDS: alkaline magmatism; assimilation; fractionation; redox equilibria; QUILF     

Geology and Mineral Chemistry of the Deosai Volcanics, Baltistan, N. Pakistan

GEOLOGY AND MINERAL CHEMISTRY OF THE DEOSAI VOLCANICS, BALTISTAN, N. PAKISTAN     

Phase Equilibria of the Aqueous Systems Containing Lithium and Carbonate Ions

正1 Introduction Alkaline lakes are widely distributed in the area of the Qinghai-Tibet Plateau.Most of the salt lakes are famous for their high concentration of lithium,potassium,magnesium,boron(Ma,2000).In recent years,as a new energy material,lithium and its compounds are widely used in the new area,such as aerospace industry,nuclear     

攀枝花大田地区前震旦纪斜长角闪岩地球化学特征及构造意义

对攀枝花大田地区斜长角闪岩进行了系统的主微量地球化学特征、锆石LA-ICP-MS U-Pb年代学以及Lu-Hf同位素特征研究.结果表明:斜长角闪岩的SiO₂含量为47.88%~50.05%,原岩为亚碱性-碱性玄武岩.斜长角闪岩稀土总量(ΣREE)较高,稀土元素配分模式为轻稀土富集的右倾型,与洋岛玄武岩相似.微量元素原始地幔标准化蛛网图为"隆起"型,与板内玄武岩特征类似.Zr/Nb、Hf/Th等比值均表明其与板内玄武岩类似,而与岛弧玄武岩具有明显的差异.锆石U-Pb定年结果表明岩浆结晶年龄为816.0~833.6 Ma,同期岩浆结晶锆石的ε_Hf(t)值在-6.8~+3.8之间,其岩浆源区为与OIB类似的富集地幔源区且受到了地壳物质的混染.综合上述资料,认为其形成于Rodinia超级地幔柱活动导致的大陆裂谷环境.      

Partially Fused Granite Blocks from Mt. Elephant, Victoria, Australia

Petrological and chemical evidence is presented to show thatsmall patches of brown glass found in blocks of granite eruptedin basaltic scoria at Mt. Elephant are the result of the partialfusion of biotite and quartz. The glass has moderate SiO₂ (63–66per cent), high AI₂O₃ (17.5–19 per cent), high FeO (3.8–5.2per cent) and very high K₂O (7.0–7.8 per cent) relativeto Na₂O (3.3–3.7 per cent) and is similar in compositionto many high K alkali syenites and trachytes.