A Model of Magmatic Crystallization

A computer model simulating fractional crystallization at oneatmosphere pressure incorporates nine broadly-defined minerals—magnetite,olivine, hypersthene, augite, quartz, plagioclase, orthoclase,leucite, and nepheline. The crystallization temperature of eachmineral is considered to be a smooth function of the compositionof the magmatic liquid. These mineral temperature equationsare obtained by multiple linear regression analysis of informationfrom published silicate systems and rock melting experiments.The nine equations are solved for any primary liquid, withinthe broad range of common magma types, to select the crystallizingmineral or minerals. Partition ratios from published experimentsand analyses of lavas and phenocrysts permit calculation ofthe composition of the crystallizing mineral assemblage. Subtractionof a small amount of that composition from the primary liquidyields a new liquid, which may be recycled to yield a sequenceof liquids during fractional crystallization. The crystallizationmodel handles assemblages of co-precipitating minerals, andcan trace progressive saturation in new minerals, substitutionof a new mineral for an old mineral, and cessation of crystallizationof a mineral. The sequences of minerals and liquids derivedfrom a broad set of primary liquids are geologically realistic,so the model is useful in predicting phenocrysts in volcanicrocks and events during crystallization of shallow intrusions.     

Chemical composition of rock-forming minerals and crystallization physicochemical conditions of the Middle Eocene I-type Haji Abad pluton,SW Buin-Zahra,Iran

The Haji Abad intrusion is a well-exposed Middle Eocene I-type granodioritc pluton in the Urumieh–Dokhtar magmatic assemblage (UDMA). The major constituents of the investigated rocks are K-feldspar, quartz, plagioclase, pyroxene, and minor Fe–Ti oxide and hornblende. The plagioclase compositions fall in the labradorite, andesine, and oligoclase fields. The amphiboles range in composition from magnesio-hornblende to tremolite–hornblende of the calcic-amphibole group. Most pyroxenes principally plot in the field of diopside. The calculated average pressure of emplacement is 1.9 kbar for the granodioritic rocks, crystallizing at depths of about 6.7 km. The highest pressure estimated from clinopyroxene geobarometry (5 kbar) reflects initial pyroxene crystallization pressure, indicating initial crystallization depth (17.5 km) in the Haji Abad granodiorite. The estimated temperatures using two-feldspar thermometry give an average 724 °C. The calculated average temperature for clinopyroxene crystallization is 1090 °C. The pyroxene temperatures are higher than the estimated temperature by feldspar thermometry, indicating that the pyroxene and feldspar temperatures represent the first and late stages of magmatic crystallization of Haji Abad granodiorite, respectively. Most pyroxenes plot above the line of Fe³⁺?=?0, indicating they crystallized under relatively high oxygen fugacity or oxidized conditions. Furthermore, the results show that the Middle Eocene granitoids crystallized from magmas with H₂O content about 3.2 wt%. The relatively high water content is consistent with the generation environment of HAG rocks in an active continental margin and has allowed the magma to reach shallower crustal levels. The MMEs with ellipsoidal and spherical shapes show igneous microgranular textures and chilled margins, probably indicating the presence of magma mixing. Besides, core to rim compositional oscillations (An and FeO) for the plagioclase crystals serve as robust evidence to support magma mixing. The studied amphiboles and pyroxenes are grouped in the subalkaline fields that are consistent with crystallization from I-type calc-alkaine magma in the subduction environment related to active continental margin. Mineral chemistry data indicate that Haji Abad granodiorites were generated in an orogenic belt related to the volcanic arc setting consistent with the subduction of Neo-Tethyan oceanic crust beneath the central Iranian microcontinent.     

Petrology of Lamproites from Smoky Butte, Montana

Hyalo-armalcolite-phlogopite lamproites and sanidine-phlogopitelamproites occurring at Smoky Butte, Montana are rocks formedfrom rapidly quenched, high temperature, uncontaminated lamproiticmagma. Petrographic variations are attributable to differentcooling histories of several batches of compositionally identicalmagma. Compared with other occurrences of lamproite, the rocksare unusually rich in TiO₂ and are characterized by the presenceof abundant armalcolite and the most TiO₂-rich phlogopites yetfound in this paragenesis. Compositional data are given fortitanian phlogopite, olivine, diopside, titanian potassian richterite,armalcolite, sanidine, analcite, and glass. The mafic mineralsare Al-deficient and exhibit very little compositional variation.Original leucite has been pseudomorphed by sanidine or analcite.The latter mineral was probably formed at the same time thatthe glass lost K, and gained Na, during alteration by groundwater.All of the lamproites are strongly enriched in Ta, Hf, and thelight REE (La /Yb = 162–280), and have high MgO and Crcontents. Mineralogical, geochemical, and previously publishedisotopic data are combined in developing a petrogenetic modelwhich suggests that these lamproites were derived from an ancient(2.5 Ga) doubly metasomatized harzburgitic source, and thatthey represent relatively primitive lamproites which were intrudedat near-liquidus temperatures.     

A geochemical assessment of the middle and lower Mvoti river system,KwaZulu-Natal,South Africa

This paper presents one of the first investigations into the geochemical enrichment of the middle and lower Mvoti river system. Chemical elements are naturally present in aquatic sediments, but their concentrations tend to rise to potentially toxic levels via both natural and anthropogenic processes. This study evaluated the concentrations of aluminum, arsenic, boron, barium, cadmium, calcium, cobalt, chromium, copper, iron, lead, magnesium, manganese, molybdenum, nickel, phosphorous, selenium, silicon, strontium, titanium, vanadium and zinc, using inductively coupled plasma optical emission spectroscopy. The levels of elements present were used to assess their spatial distribution within the river and to determine the contamination factors and enrichment factors for each element. The pollution load index (PLi) is another contributing parameter that was calculated to determine the degree of pollution at each site. The results indicate that the sediments of the Mvoti are low to moderately polluted and deteriorating with time (average PLi value of 5.19), and that a major contributing factor to this contamination is natural sources.     

Experimental Melting of Cordierite Gneiss and the Petrogenesis of Syntranscurrent Peraluminous Granites in Southern Brazil

To understand the petrogenesis of peraluminous granites syntectonicto the Dorsal de Canguçu Transcurrent Shear Zone in theSul-rio-grandense Shield, Brazil, melting experiments were performedon one of the potential protoliths, a cordierite-bearing semi-peliticmetasedimentary gneiss (PE-1). Experiments were conducted atpressures of 5, 10 and 15 kbar, at temperatures of 700–900°C,and under fluid-absent and 5% H₂O-present conditions. The experimentsshow that fluid-absent melting begins at near-solidus conditions,around 700°C, promoted by participation of retrogressivephengitic muscovite in the reaction Mus + Kf ± Qz = melt± Fe–Ti oxide ± Als, producing a very smallamount of melt (<9%) with widely ranging composition. Allhypersolidus experiments (>800°C) produced S-type graniticmelts promoted by participation of biotite or cordierite inthe reactions Bio + Pl + Crd + Qz = Px + Fe–Ti oxide +melt at 5 kbar, and Bio + Pl + Crd ± Qz = Grt + Als ±Kf + melt at 10 and 15 kbar, both producing a high amount ofmelt (10–63% by volume). The melt compositions obtainedat 900°C and 15 kbar under fluid-absent conditions, promotedby biotite or cordierite breakdown, are similar to the syntectonicgranites. However, it is unlikely that the granites were formedat this pressure (corresponding to a depth of melting of     

Postsedimentation transformation of triassic terrigenous rocks in West Chukotka as an indicator of folding conditions

Postsedimentation alteration and structural assemblies of the Triassic sedimentary complexes of West Chukotka are discussed. Zoning of the alteration is based on examination of newly formed structural and mineral assemblages, the chemical composition, and the polytypes of clay minerals. Three zones of postsedimentation transformation of sandstones are distinguished: (1) the zone of chlorite, illite, and mixed-layer disordered chlorite-smectite; (2) the zone of illite and chlorite; and (3) the zone of phengite and ferroan chlorite. The grade of postsedimentation transformation and the composition of the newly formed micas are correlated with the cleavage type. The development of two-three types of cleavage leads to the highest degree of rock transformation. The assemblages of clay minerals and the crystal chemistry of the authigenic phengite show that the grade of postsedimentation transformation of the Triassic rocks attains the stage of greenschistfacies metamorphism in the zone of development of two cleavage types. Where the second cleavage is not documented or poorly developed, the rocks remain unmetamorphosed. Evidence is given that postsedimentation transformation of terrigenous rocks in the foldbelt is controlled largely by deformation.     

Raman spectroscopic study of K-lingunite at various pressures and temperatures

K-lingunite is a high-pressure modification of K-feldspar that possesses the tetragonal hollandite structure. Variations of the Raman spectra of K-lingunite were studied up to ~31.5 GPa at room temperature, and in the range 79–823 K at atmospheric pressure. The Raman frequencies of all bands were observed to increase with increasing pressure, and decrease with increasing temperature for K-lingunite. This behavior is in line with those observed for most of other materials. New sharp Raman bands appear at pressures greater than 13–15 GPa, suggesting a phase transition in K-lingunite with increasing pressure. The transition is reversible when pressure was released. The appearance of these new Raman bands may correspond to the phase transition revealed earlier at around 20 GPa by X-ray diffraction studies. Instead of transforming back to its stable minerals, such as orthoclase, microcline or sanidine, K-lingunite became amorphous in the temperature range 803–823 K at atmospheric pressure.     

Metamorphic evolution of garnet-bearing ultramafic rocks from the Gongen area, Sanbagawa belt, Japan

Garnet‐bearing ultramafic rocks including clinopyroxenite, wehrlite and websterite locally crop out in the Higashi‐akaishi peridotite of the Besshi region in the Cretaceous Sanbagawa metamorphic belt. These rock types occur within dunite as lenses, boudins or layers with a thickness ranging from a few centimetres to 1 metre. The wide and systematic variation of bulk‐rock composition and the overall layered structure imply that the ultramafic complex originated as a cumulate sequence. Garnet and other major silicates contain rare inclusions of edenitic amphibole, chlorite and magnetite, implying equilibrium at relatively low P–T conditions during prograde metamorphism. Orthopyroxene coexisting with garnet shows bell‐shaped Al zoning with a continuous decrease of Al from the core towards the rim, consistent with rims recording peak metamorphic conditions. Estimated P–T conditions using core and rim compositions of orthopyroxene are 1.5–2.4 GPa/700–800 °C and 2.9–3.8 GPa/700–810 °C, respectively, implying a high P/T gradient (> 3.1 GPa/100 °C) during prograde metamorphism. The presence of relatively low P–T conditions at an early stage of metamorphism and the steep P/T gradient together trace a concave upwards P–T path that shows increasing P/T with higher T, similar to P–T paths reported from other UHP metamorphic terranes. These results suggest either (1) down dragging of hydrated mantle cumulate parallel to the slab–wedge interface in the subduction zone by mechanical coupling with the subducting slab or (2) ocean floor metamorphism and/or serpentinization at early stage of subduction of oceanic lithosphere and ensuing HP–UHP prograde metamorphism.     

Glauconites from the Late Palaeocene — Early Eocene Naredi Formation,western Kutch and their genetic implications

Glauconitic minerals are considered as one of the valuable input parameters in sequence stratigraphic analysis of a basin. In the present study glauconitic minerals are reported from subtidal green shale facies in the lower part of the Late Paleocene-Early Eocene Naredi Formation of western Kutch. On the basis of the foraminiferal assemblage the glauconite bearing beds are interpreted to have formed in a mid shelf depositional settings of an unstable marine conditions. XRD studies confirm the glauconite mineralogy of the green pellets and provide an estimation of glauconite maturity. Textural attributes of the glauconites confirm their derivation by different degrees of alteration of precursor feldspar grains. Because of the authigenic origin and autochthonous nature, these glauconites hold promise for understanding sequence stratigraphy of the Palaeogene succession of the western Kutch.     

Shear wave velocities and primary influencing factors of Campi Flegrei–Neapolitan deposits

The Campi Flegrei caldera (Italy) has been the site of intense seismic activity over the past decades. This area is densely populated and includes important towns such as Napoli with historical sites and supporting many industries. For the proper use and management of the region, the evaluation of the dynamic properties of near-surface rocks is necessary.

The volcanological pattern has been reconstructed from the lithostratigraphies of several drillings. The most interesting and widespread pyroclastic products are the pozzolana deposit (soil) and the Neapolitan Yellow Tuff (rock). Both pozzolana and tuff products are covered by recent eluvial and coastal sandy deposits and younger volcanic products (<12 000 years). The characteristic ranges of the shear wave velocity (V_s) of the Campi Flegrei–Neapolitan soils and tuffs are defined and the primary influencing factors are evaluated. For the sandy deposits, the results show that eluvial and lacustral products have lower shear wave velocities than coastal products. For the volcanic products younger than 12 000 years b.p. the influence of vertical pressure is emphasized. As regards the pozzolana deposit (soil) and the Neapolitan Yellow Tuff (rock), a major influencing factor is shown by the textural characteristics and the different hardening degrees as a consequence of the diagenetic processes. The scattering of the V_s velocities for the same formation is so wide that only the variability ranges can be individuated. These results suggest a need to carry out detailed V_s measurements or, at least, to make a parametric study of the effect of the V_s ranges on seismic response analysis in order to give safe building codes.