Journal articles: 'Plagioclase zoning'

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Hoshide, Takashi, and Masaaki Obata. "Zoning and resorption of plagioclase in a layered gabbro, as a petrographic indicator of magmatic differentiation." Earth and Environmental Science Transactions of the Royal Society of Edinburgh 100, no. 1-2 (March 2009): 235–49. http://dx.doi.org/10.1017/s1755691009016090.

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ABSTRACTThe Murotomisaki Gabbroic Intrusion is a sill-like layered gabbro emplaced in sedimentary strata of Tertiary age in southwest Japan. The zoning (including resorption structures) and the compositional variations of plagioclase from throughout the intrusion were studied, and it was found that the zoning pattern may be classified into four types, which may well correlated with the hosting rock types, the mode of occurrences and their stratigraphic positions in the intrusion. The plagioclase zoning was successfully decoded, and the sequence of events that took place during the magmatic differentiation was deduced and further interpreted in the context of a stratified basal boundary layer slowly ascending in a solidifying magma body. It was shown that various layered structures – modal layering, podiform gabbroic pegmatites and anorthositic layers – observed in the Murotomisaki Gabbro were formed within the moving basal boundary layer by flushing of H2O-rich fluid and fractionated silicate melts from below. By the fluxing of hydrous fluids, plagioclase crystals preferentially dissolved and then melt fraction increased in the basal boundary layer. Under these circumstances, plagioclase-rich fractionated melts diapirically segregated from the crystal pile. Calcic plagioclases, which are out of equilibrium in the central part of the intrusion, may have originated from the basal boundary layer in this manner.

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L'Heureux, Ivan. "Oscillatory zoning in plagioclase: thermal effects." Physica A: Statistical Mechanics and its Applications 239, no. 1-3 (May 1997): 137–46. http://dx.doi.org/10.1016/s0378-4371(96)00474-8.

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3

Firouzkouhi, Zahra, Ali Ahmadi, David Richard Lentz, and Ali-Asghar Moridi-Farimani. "Mixing of basaltic and andesitic magmas in the Bazman volcanic field of southeastern Iran as inferred from plagioclase zoning." Mineralogical Magazine 81, no. 4 (August 2017): 975–85. http://dx.doi.org/10.1180/minmag.2017.081.001.

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AbstractLate Cenozoic basalts of the Bazman volcanic field, Makran volcanic arc of southern Iran, contain two types of plagioclase feldspar phenocrysts with significant textural and compositional differences. The most common type is rather homogeneous with only weak zoning and maximum An content of 83 mol.%. The less common type of phenocryst exhibits complex zoning and, other than rims, is close in composition and similar in texture to those of associated andesites. This type of plagioclase phenocryst is characterized by an engulfed core with oscillatory zoning, which is overgrown by sieve-textured, moderately zoned mantle, and a relatively narrow rim. In both rock types, the An content of the core is between 40 and 63 mol.% with abrupt fluctuations. No significant correlation between An content and MgO, FeO, SrO and BaO is apparent in the core of phenocrysts in basalts. Anorthite content of the core of phenocrysts in andesites inversely correlates with SrO and BaO. The mantle of plagioclase phenocrysts in both rock types is characterized by sharp increases of An (up to 41 mol.%), MgO, and FeO, in the contact with the core. Anorthite correlates positively with MgO and FeO in the mantle, but correlation between An and SrO and BaO is not evident. It is assumed that plagioclase phenocrysts originally crystallizing from the host andesitic magma were interrupted by mixing with a hotter, juvenile basaltic magma. The resulting changes in temperature, composition, and H2O content of the surrounding melt caused compositional zonation, and the development of resorption in the cores and sieve texture in the mantles. As the An contents of the rims of the phenocrysts resemble the average An content of the groundmass plagioclases in both rock types, it is thought that the two involved magmas gained their independent physical identity before the formation of compositionally-distinct rims of plagioclase phenocrysts.

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PEARCE, T., and A. KOLISNIK. "Observations of plagioclase zoning using interference imaging." Earth-Science Reviews 29, no. 1-4 (October 1990): 9–26. http://dx.doi.org/10.1016/0012-8252(0)90024-p.

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5

Pearce, T. "Observations of plagioclase zoning using interference imaging." Earth-Science Reviews 29, no. 1-4 (October 1990): 9–26. http://dx.doi.org/10.1016/0012-8252(90)90024-p.

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6

NIU, Zhijian, Yue LIU, and Yongjun DI. "Plagioclase Zoning as a Record of Magma Evolution." Acta Geologica Sinica - English Edition 88, s2 (December 2014): 1477–78. http://dx.doi.org/10.1111/1755-6724.12382_18.

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7

Perugini, Diego, Giampiero Poli, and Luca Valentini. "Strange attractors in plagioclase oscillatory zoning: petrological implications." Contributions to Mineralogy and Petrology 149, no. 4 (April 16, 2005): 482–97. http://dx.doi.org/10.1007/s00410-005-0667-6.

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8

Durant, D. G., and A. D. Fowler. "Origin of reverse zoning in branching orthopyroxene and acicular plagioclase in orbicular diorite, Fisher Lake, California." Mineralogical Magazine 66, no. 6 (December 2002): 1003–19. http://dx.doi.org/10.1180/0026461026660073.

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Abstract An orbicular diorite from Fisher Lake, California, USA, contains multi-shelled, magmatic orbicules with branching and budding orthopyroxene crystals as well as feather and acicular plagioclase crystals that are oriented perpendicular to the growth horizon. Plagioclase and orthopyroxene show gradual, reverse compositional zoning along the long axes and normal zoning along the short axes. The reverse zoning varies from An87 to An93 and Mg68 to Mg74 over distances of 4 mm and 8 mm respectively. The close proximity of these two minerals makes it likely that only one mechanism is responsible for the reverse zoning. This zoning can be explained by using relevant temperature-composition diagrams and Gibbs free energy-composition plots. Under sudden and moderate undercoolings, which produce high growth but low nucleation rates, the difference in Gibbs free energy (ΔG) between the crystals and liquid is not initially maximized, i.e. initial compositions are not near-to-equilibrium. This results in crystal compositions that are closer to that of the bulk liquid than expected for crystallization under near-to-equilibrium conditions (i.e. very small ΔT). Over time, and under isothermal crystallization conditions, ΔG gradually increases to a maximum producing crystal compositions that also gradually attain near-to-equilibrium compositions. Subsequent to attaining these conditions, normal zoning occurs perpendicular to the crystal growth axes.

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9

Yang, Fan, Xiao-Long Huang, Yi-Gang Xu, and Peng-Li He. "Magmatic Processes Associated with Oceanic Crustal Accretion at Slow-spreading Ridges: Evidence from Plagioclase in Mid-ocean Ridge Basalts from the South China Sea." Journal of Petrology 60, no. 6 (May 10, 2019): 1135–62. http://dx.doi.org/10.1093/petrology/egz027.

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Abstract Magmatic processes associated with oceanic crustal accretion at slow-spreading mid-oceanic ridges are less well understood compared with those at fast-spreading ridges. Zoned plagioclase in the basalts might record these magmatic processes as a result of the very slow intra-crystal diffusion of CaAl–NaSi. Plagioclase phenocrysts in plagioclase-phyric basalt from Hole U1433B of International Ocean Discovery Program (IODP) Expedition 349 in the South China Sea show complex zoning patterns (e.g. normal, reverse, oscillatory and patchy). These samples provide a rare opportunity to determine the magma dynamics associated with oceanic crustal accretion at slow-spreading ridges through time. Igneous lithological units in Hole U1433B consist of a series of massive lava flows at the bottom and a thick succession of small pillow lava flows at the top. Most of the plagioclase phenocrysts in the massive lava show core–rim zonation with high-An cores (An ∼85%; in mole fraction; Pl-A) in equilibrium with melts that are more primitive than their host. Some high-An cores of Pl-A phenocrysts contain melt inclusions and are depleted in La, Ce, Y and Ti, but enriched in Sr and Eu; this is interpreted as resulting from dissolution–crystallization processes during reaction of hot melt with pre-existing plagioclase cumulates. In the pillow lavas, most of the plagioclase phenocrysts show normal core–mantle–rim zonation (Pl-B) with An contents decreasing gradually from the core to the mantle to the rim, suggesting extensive magma mixing and differentiation. Reversely zoned plagioclases (Pl-C) are sparsely present throughout the basalts, but mostly occur in the lower part of the drill hole. The cores of euhedral Pl-C phenocrysts are compositionally comparable with the mantles of Pl-B phenocrysts, suggesting that the evolved magma was recharged by a relatively primitive magma. Melt inclusion-bearing Pl-A phenocrysts occur mainly in the massive lava, but rarely in the pillow lava, whereas Pl-B phenocrysts are present dominantly in the pillow lava, which reflects reducing melt–rock interaction and enhanced magma mixing, recharging and differentiation from the bottom to the top of the hole. In addition, the extensive magma mixing and differentiation recorded by Pl-B phenocrysts in the pillow lava require the existence of a melt lens beneath the mid-ocean ridge. Consistently, the plagioclase phenocrysts in the pillow lava mostly lack melt inclusions, corresponding to very weak melt–rock reactions, which indicates that the magma was transported through plagioclase cumulates by channel flow and requires a higher magma supply to the magma conduit. Therefore, the textural and compositional variations of plagioclase phenocrysts in the samples reflect the changes in magma dynamics of the mid-ocean ridge basalt through time with respect to oceanic crustal accretion at slow-spreading ridges. Overall, the oceanic crustal accretion process is sensitive to the magma supply. In the period between two episodes of extension, owing to a low melt supply the primitive melt percolates through and interacts with the mush zone by porous flow, which produces melt inclusion-bearing high-An plagioclase through dissolution–crystallization processes. At the initial stage of a new episode of extension, the melt infiltrates the mush zone and entrains crystal cargoes including melt inclusion-bearing high-An plagioclase. During the major stage of extension, owing to a relatively high melt supply the melt penetrates the mush zone by channel flow and can pool as melt lenses somewhere beneath the dikes; this forms intermediate plagioclases and the reverse zoning of plagioclases by magma mixing, recharging and differentiation in the melt lens. Such magmatic processes might occur repeatedly during the episodic extension that accompanies oceanic crustal accretion at slow-spreading ridges, which enhances the lateral structural heterogeneity of the oceanic crust.

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10

Russell, J. K., G. T. Nixon, and T. H. Pearce. "Petrographic constraints on modelling the crystallization of basalt magma, Cow Lakes, southeast Oregon." Canadian Journal of Earth Sciences 25, no. 4 (April 1, 1988): 486–94. http://dx.doi.org/10.1139/e88-049.

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Thermodynamic calculations and models of olivine zoning profiles are used to estimate the crystallization history of a basaltic magma from Cow Lakes, southeast Oregon. The lava is an alkali olivine basalt containing olivine and plagioclase phenocrysts and microphenocrysts. The geometry and range of chemical zoning in the olivine phenocrysts have been delineated by laser interference microscopy and electron microprobe analysis. The olivine phenocrysts are characterized by homogeneous cores and rims that exhibit strong, continuous, normal zoning (ΔFo = 7–19 mol%).Thermodynamic modelling has been used to estimate the magmatic crystallization path of the Cow Lakes basalt on the basis of the phenocryst assemblage and mineral compositions. The calculated crystallization path begins at 1290 °C and 0.5 GPa ([Formula: see text]) with equilibrium crystallization of the olivine to 1265 °C. Plagioclase appears at 1225 °C, followed by clinopyroxene at 1205 °C. Intratelluric crystallization was terminated prior to crystallization of the clinopyroxene, which is seen in the groundmass but not as phenocrysts.The thermodynamic modelling provides a means to numerically simulate the zoning patterns in olivine defined by the laser interference microscopy. Simulated and observed zoning patterns both have compositionally flat cores and strongly zoned rims. The extent of zoning observed in the olivine phenocrysts is, however, approximately twice the predicted extent, and it appears that a significant proportion of olivine phenocrysts crystallized during ascent or upon eruption.

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11

Mørk, Mai Britt E. "Coronite and eclogite formation in olivine gabbro (Western Norway): reaction paths and garnet zoning." Mineralogical Magazine 50, no. 357 (September 1986): 417–26. http://dx.doi.org/10.1180/minmag.1986.050.357.06.

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AbstractGabbros and dolerites in the Western Gneiss Region of Norway have been subjected to a high P-T regime and preserve evidence of the existence of several stages in the consequent reaction sequence. Incomplete reaction stages are characterized by corona structures between relict igneous phases and by frequent pseudo-morphs after igneous olivine, plagioclase and augite. The increasing degree of reaction is recorded by successive increases of modal garnet, omphacite and phlogopite (±orthopyroxene) to produce eclogite, often with excellent pseudomorphic preservation of the igneous fabric.Pseudomorphic replacement of olivine by orthopyroxene (in coronas and aggregates) and of plagioclase by local assemblages of garnet, spinel, and sodic plagioclase, is interpreted as a transient reaction stage with restricted and selective diffusion between the original mafic and felsic domains. Complete eclogitization is compatible with more extensive diffusion, especially of Na and Al (on a mm scale) leading to omphacite production by replacement of olivine, augite, and orthopyroxene. Concomitant reactions in the felsic domains lead to total replacement of the transient phases by garnet, with or without inclusions of minute omphacite grains.Strong Ca zoning of garnet in the coronites is inter-preted as a relict growth zoning, attributed to local controls by diffusion and subreactions in the plagioclase host and to local garnet + plagioclase + spinel equilibria. In contrast, complete eclogitization is associated with diffusional homogenization of garnet by (Mg, Fe)Ca−1 and MgFe−1 exchange with omphacite ± phlogopite.

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12

Ustunisik, Gokce, Attila Kilinc, and Roger L. Nielsen. "New insights into the processes controlling compositional zoning in plagioclase." Lithos 200-201 (July 2014): 80–93. http://dx.doi.org/10.1016/j.lithos.2014.03.021.

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13

Popov, D. V., N. A. Nekrylov, and P. Yu Plechov. "Petrology of Late Albian tuffites from Bakhchysarai district (Southwestern Crimea)." Moscow University Bulletin. Series 4. Geology, no. 1 (February 28, 2016): 82–91. http://dx.doi.org/10.33623/0579-9406-2016-1-82-91.

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We present first data on the composition of melt inclusions in plagioclase and quartz crustal clasts and detailed study of plagioclase chemical zoning and mineral compositions from Bakhchysarai Late Albian tuffites. All magmatic melt compositions fall into field of high-K rhyolites. Measured water content in melt inclusions is up to 4 wt.%. Phenocrysts crystallized at the temperature within range 820-860 °C and pressure not less than 1 kbar. Bakhchysarai tuffites were formed in active continental margin geotectonical settings.

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14

L'Heureux, Ivan, and Anthony D. Fowler. "Dynamical model of oscillatory zoning in plagioclase with nonlinear partition relation." Geophysical Research Letters 23, no. 1 (January 1, 1996): 17–20. http://dx.doi.org/10.1029/95gl03327.

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15

Hattori, Keiko, and Hiroaki Sato. "Magma evolution recorded in plagioclase zoning in 1991 Pinatubo eruption products." American Mineralogist 81, no. 7-8 (August 1, 1996): 982–94. http://dx.doi.org/10.2138/am-1996-7-820.

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16

Carcangiu, G., M. Palomba, and M. Tamanini. "REE-bearing minerals in the albitites of central Sardinia, Italy." Mineralogical Magazine 61, no. 405 (April 1997): 271–83. http://dx.doi.org/10.1180/minmag.1997.061.405.10.

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AbstractRecent studies on albitite rocks located in the granodiorite complex of Central Sardinia have revealed that epidote has a widespread occurrence as a light rare-earth element (LREE)-bearing accessory common phase. Titanite has been recorded as a heavy rare earth element (HREE)-bearing mineral. The Hercynian granodiorite complex of Central Sardinia is composed chiefly of quartz, Ca-plagioclase, K-feldspar and biotite and of a wide variety of secondary assemblages, mainly allanite, titanite and zircon. Albitic plagioclase and quartz are the main mineral components of the albitites. Additional minerals include, besides allanite and epidote, a more calcic-plagioclase (oligoclase), K-feldspar, chlorite, titanite and more rarely muscovite. The mineral assemblages and REE-bearing minerals of albitites were analysed by wavelength dispersive spectrometry (WDS). Chemical data suggest that there is a near complete solid-solution between epidote and allanite whereas little variations in HREE of titanites were detected. In epidote-group minerals a pronounced zoning in REE was observed while titanite was recorded unzoned. Textural relations were studied by SEM to distinguish primary from secondary epidotes. Chemical criteria to recognize magmatic from alteration epidotes were also applied. The alteration epidotes mainly occur and generally originate from plagioclase alteration and from leaching of magmatic allanite. Comparison of textures using both the SEM technique and EPMA data showed that the characteristic ‘patchy zoning’, observed in epidotes, corresponds with different amounts of REE in these minerals.The schematic model proposed for the epidote-forming reactions during the metasomatic processes that affected the granodiorites involves: (i) the instability of the anorthitic component of plagioclase; (ii) the simultaneous formation of albite; (iii) the leaching of the magmatic allanite with a redistribution of REE in the epidotes of the albitites.

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17

Oziegbe, E. J., O. O. Ocan, and A. O. Buraimoh. "Petrography of Allanite-bearing Tonalite from Iwo Region, Osun State, Nigeria." Materials and Geoenvironment 67, no. 2 (July 27, 2020): 79–89. http://dx.doi.org/10.2478/rmzmag-2020-0006.

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AbstractPrimary, secondary and accessory minerals in tonalitic rocks from Iwo region of the Precambrian Basement Complex of Southwestern Nigeria were identified and analysed with the aim of determining the various processes involved during the crystallisation of magma. Thin sections of tonalite were prepared and studied with the aid of a petrographic microscope. The mineral assemblages observed are biotite, plagioclase, alkali-feldspar, amphiboles, pyroxene, quartz, muscovite and chlorite. Allanite, titanite, apatite and zircon occur as accessory minerals. Muscovite and chlorite are found to be secondary minerals. The mineral allanite has a characteristic form of zoning and shows evidence of metamictisation, and is surrounded by dark-coloured biotite having radioactive haloes. Titanite is anhedral to subhedral crystals and forms reaction rim round opaque minerals. Plagioclase shows evidence of compositional zoning as well as plastic deformation of the twin lamellae. The allanite observed is primary in nature and has undergone radioactive disintegration; chlorite and muscovite are formed by secondary processes of chloritization and sericitisation, respectively. The tonalite is formed as a result of rapid cooling of magma close to the Earth's surface.

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Shcherbakov, Vasily D., Pavel Yu Plechov, Pavel E. Izbekov, and Jill S. Shipman. "Plagioclase zoning as an indicator of magma processes at Bezymianny Volcano, Kamchatka." Contributions to Mineralogy and Petrology 162, no. 1 (November 3, 2010): 83–99. http://dx.doi.org/10.1007/s00410-010-0584-1.

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19

Riehle, J. R., J. R. Budahn, M. A. Lanphere, and D. A. Brew. "Rare earth element contents and multiple mantle sources of the transform-related Mount Edgecumbe basalts, southeastern Alaska." Canadian Journal of Earth Sciences 31, no. 5 (May 1, 1994): 852–64. http://dx.doi.org/10.1139/e94-078.

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Pleistocene basalt of the Mount Edgecumbe volcanic field (MEF) is subdivided into a plagioclase type and an olivine type. Olivine basalt crops out farther inboard from the nearby Fairweather transform than plagioclase basalt. Th/La ratios of plagioclase basalt are similar to those of mid-ocean-ridge basalt (MORB), whereas those of olivine basalt are of continental affinity. The olivine basalt has higher 87Sr/86Sr ratios than the plagioclase basalt.We model rare earth element (REE) contents of the olivine basalt, which resemble those of transitional MORB, by 10–15% partial melting of fertile spinel–plagioclase lherzolite followed by removal of 8–13% olivine. Normative mineralogy indicates melting in the spinel stability field. REE contents of an undersaturated basalt (sample 5L005) resemble those of Mauna Loa tholeiite and are modelled by 5–10% partial melting of fertile garnet lherzolite followed by 10% olivine removal. Plagioclase basalt resembles sample 5L005 in REE contents but is lower in other incompatible-element contents and 87Sr/86Sr ratios. Plagioclase basalt either originated in depleted garnet lherzolite or is a mixture of sample 5L005 and normal MORB; complex zoning of plagioclase and colinear Sc and Th contents are consistent with magma mixing.We conclude that olivine basalt originated in subcontinental spinel lherzolite and that plagioclase basalt may have originated in suboceanic lithosphere of the Pacific plate. Lithospheric melting seemingly requires vertical flow of mantle material, although there is no direct evidence at the MEF for crustal extension that might provide a mechanism for mantle advection. In any case, most MEF magmas are subalkaline because of moderately high degrees of partial melting at shallow depth.

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20

Grogan, S. E., and R. J. Reavy. "Disequilibrium textures in the Leinster Granite Complex, SE Ireland: evidence for acid-acid magma mixing." Mineralogical Magazine 66, no. 6 (December 2002): 929–39. http://dx.doi.org/10.1180/0026461026660068.

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Abstract The LGC (Leinster Granite Complex) in SE Ireland consists of five plutons or Units. Units 1 and 3 display complex magmatic disequilibrium textures described here for the first time. The textures indicative of disequilibrium include boxy cellular plagioclase, oscillatory zoning, calcic spike zones and combinations of these within individual plagioclase feldspar crystals. The textures are interpreted as having formed in a dynamic magmatic environment which facilitated acid-acid mixing on a variety of scales and at a variety of stages between source to emplacement. It is proposed that the restricted distribution of disequilibrium textures was a result of the complex strain regime operating within the crust during assembly of Units 1 and 3.

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21

Hébert, R., R. Hekinian, and D. Bideau. "Primitive intratransform volcanism at Garrett Transform Fault (East Pacific Rise)." Canadian Journal of Earth Sciences 34, no. 8 (August 1, 1997): 1101–17. http://dx.doi.org/10.1139/e17-089.

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The paper presents the results of a petrological study of samples collected by submersible from volcanic features that floor the intratransform domain of the Garrett Transform Fault. Most intratransform volcanics are typically highly porphyritic and primitive mid-ocean ridge basalt (MORB) (glasses have Mg# higher than 0.65), whereas most volcanics close to the East Pacific Rise – transform intersection zone are nearly aphyric evolved MORB (glasses have Mg# lower than 0.54). In the intratransform volcanics, phenocrysts and megacrysts are plagioclase and olivine and accessory spinel microphenocrysts in the magnesian lavas, and clinopyroxene and plagioclase in ferrobasalts. Variable mineral chemistry of plagioclase and spinel, chemical disequilibria between these phases and surrounding glass, and resorption features and oscillatory zoning in plagioclase suggest that limited magma mixing occurred during genesis of intratransform MORB. Aluminous- to chromian-spinel compositions are in agreement with melt interactions in small magma pockets. These processes are superimposed on complex partial melting events of a heterogeneous source region underlying the intratransform domain. Generation of most ferrobasalts can be explained by crystal fractionation of primitive MORB. Some ferrobasalts appear to derive from discrete magma bodies, since they are not simply connected to the magnesian tholeiites through fractional crystallization processes.

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Soda, Yusuke, Taku Matsuda, Sachio Kobayashi, Motoo Ito, Yumiko Harigane, and Takamoto Okudaira. "Reversely zoned plagioclase in lower crustal meta-anorthosites: An indicator of multistage fracturing and metamorphism in the lower crust." American Mineralogist 105, no. 7 (July 1, 2020): 1002–13. http://dx.doi.org/10.2138/am-2020-7284.

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Abstract This paper describes the formation mechanism of reversely zoned plagioclase, which has been observed frequently in lower crustal shear zones and is indicative of multistage fracturing and meta-morphism in the lower crust, by studying the microstructural and chemical characteristics of plagioclase in sparsely fractured anorthosites and anorthositic mylonites from the Eidsfjord shear zone, Langøya, northern Norway. Based on the field relationship between sparsely fractured anorthosite and anorthositic mylonite, the fracturing of anorthosite occurred before the formation of mylonite. In sparsely fractured anorthosites, transgranular fractures are observed; hydration-reaction products, including Na-rich plagioclase, occur within cracks and fractures, suggesting that hydration reactions occurred during or after fracturing. The hydration reactions in sparsely fractured anorthosites are estimated to have occurred at higher-pressure (P) amphibolite-facies conditions (~0.9–1.0 GPa and ~550–700 °C). In anorthositic mylonites, which are considered to have initiated by fracturing and subsequent hydration metamorphism at lower-P amphibolite-facies conditions (~0.7 GPa and ~600 °C), recrystallized plagioclase grains often show compositional zoning with an Na-rich core and a Ca-rich rim. Because the compositions of metamorphic plagioclase grains in the sparsely fractured anorthosites and those of the Na-rich cores of the reversely zoned plagioclase in anorthositic mylonites are similar to each other, the Na-rich cores of the matrix plagioclase in the anorthositic mylonites have recrystallized under higher-P amphibolite-facies conditions and then been overgrown or replaced by the Ca-rich rims under lower-P conditions. Consequently, the reversely zoned plagioclase observed frequently in lower crustal shear zones is an indicator of multistage brittle fracturing and subsequent hydration metamorphism during exhumation, providing information relevant to understanding the deep rupture process caused by repeated seismicity alternating with aseismic creep below the seismogenic zone.

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23

de Hoÿm de Marien, Luc, Pavel Pitra, Florence Cagnard, and Benjamin Le Bayon. "Prograde and retrograde P–T evolution of a Variscan high-temperature eclogite, French Massif Central, Haut-Allier." BSGF - Earth Sciences Bulletin 191 (2020): 14. http://dx.doi.org/10.1051/bsgf/2020016.

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The P–T evolution of a mafic eclogite sample from the Haut-Allier was studied in order to constrain the dynamic of the Variscan subduction in the eastern French Massif Central. Three successive metamorphic stages M1, M2 and M3, are characterized by assemblages comprising garnet1-omphacite-kyanite, garnet2-plagioclase, and amphibole-plagioclase, respectively, and define a clockwise P–T path. These events occurred at the conditions of eclogite (M1; ∼ 20 kbar, 650 °C to ∼ 22.5 kbar, 850 °C), high-pressure granulite (M2; 19.5 kbar and 875 °C) and high-temperature amphibolite facies (M3; < 9 kbar, 750–850 °C), respectively. Pseudosection modelling of garnet growth zoning and mineralogy of the inclusions reveal a prograde M1 stage, first dominated by burial and then by near isobaric heating. Subsequent garnet1 resorption, prior to a renewed growth of garnet2 is interpreted in terms of a decompression during M2. High-pressure partial melting is predicted for both the M1 temperature peak and M2. M3 testifies to further strong decompression associated with limited cooling. The preservation of garnet growth zoning indicates the short-lived character of the temperature increase, decompression and cooling cycle. We argue that such P–T evolution is compatible with the juxtaposition of the asthenosphere against the subducted crust prior to exhumation driven by slab rollback.

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Cherniak, D. J., and E. B. Watson. "Ti diffusion in feldspar." American Mineralogist 105, no. 7 (July 1, 2020): 1040–51. http://dx.doi.org/10.2138/am-2020-7272.

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Abstract Chemical diffusion of Ti has been measured in natural K-feldspar and plagioclase. The sources of diffusant used were TiO2 powders or pre-annealed mixtures of TiO2 and Al2O3. Experiments were run in crimped Pt capsules in air or in sealed silica glass capsules with solid buffers (to buffer at NNO). Rutherford backscattering spectrometry (RBS) was used to measure Ti diffusion profiles. From these measurements, the following Arrhenius relations are obtained for diffusion normal to (001):For oligoclase, over the temperature range 750–1050 °C:DOlig=6.67×10-12exp(-207±31kJ/mol/RT)m2s-1For labradorite, over the temperature range 900–1150 °C:DLab=of4.37×10-14exp(-181±57kJ/mol/RT)m2s-1For K-feldspar, over the temperature range 800–1000 °C:DKsp=3.01×10-6exp(-342±47kJ/mol/RT)m2s-1. Diffusivities for experiments buffered at NNO are similar to those run in air, and the presence of hydrous species appears to have little effect on Ti diffusion. Ti diffusion also shows little evidence of anisotropy. In plagioclase, there appears to be a dependence of Ti diffusion on An content of the feldspar, with Ti diffusing more slowly in more calcic plagioclase. This trend is similar to that observed for other cations in plagioclase, including Sr, Pb, Ba, REE, Si, and Mg. In the case of Ti, an increase of 30% in An content would result in an approximate decrease in diffusivity of an order of magnitude. These data indicate that feldspar should be moderately retentive of Ti chemical signatures, depending on feldspar composition. Ti will be more resistant to diffusional alteration than Sr. For example, Ti zoning on a 50 μm scale in oligoclase would be preserved at 600 °C for durations of ~1 million years, with Sr zoning preserved only for ~70 000 yr at this temperature. These new data for a trace impurity that is relatively slow-diffusing and ubiquitous in feldspars (Hoff and Watson 2018) have the potential to extend the scope and applicability of t-T models for crustal rocks based on measurements of trace elements in feldspars.

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Dunham, A. C., and F. C. F. Wilkinson. "Sulphide droplets-and the Unit 11/12 chromite band, Rhum: a mineralogical study." Geological Magazine 122, no. 5 (September 1985): 539–48. http://dx.doi.org/10.1017/s0016756800035457.

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AbstractA microscopic and electron microprobe investigation has been made of six samples of the allivalite–chromitite–peridotite band at the junction of Units 11 and 12, Eastern Layered Series, Rhum. Analyses are presented of olivine, plagioclase and pyroxene, the plagioclase showing reverse zoning in the chromitite. Sulphide droplets occur within and above the chromitite, and contain pyrrhotite, pentlandite, cubanite, bornite, digenite, chalcocite, native copper and electrum, as well as chromite zoned to magnetite, spinel and ilmenite. The variation from aluminous chromite to chromite in the chromitite is confirmed by analyses of five new traverses. The rocks formed during and just after the time when new magma entered the magma chamber. The variation in the chromite compositions is ascribed mainly to magmatic variation in composition due to the mixing process.

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SAKI, A. "Formation of spinel-cordierite-plagioclase symplectites replacing andalusite in metapelitic migmatites of the Alvand aureole, Iran." Geological Magazine 148, no. 3 (October 22, 2010): 423–34. http://dx.doi.org/10.1017/s0016756810000841.

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AbstractSpinel–cordierite–plagioclase symplectites partially replacing andalusite occur in the metapelitic migmatite rocks of the Alvand aureole within the Sanandaj–Sirjan metamorphic belt, Hamadan, Iran. The presence of melt shows that corona development occurred under partial melting conditions. Spinel is predicted to grow with cordierite at around 700°C. Exhaustion of the available SiO2 and/or separation of sillimanite/andalusite from SiO2-rich matrix domains by cordierite resulted in the formation of localized low-silica activity domains and thus triggered the growth of spinel in the rim of andalusite, the reaction Sil/And + Bt = Crd + Spl + Kfs + melt, as the most common reaction for the development of coronas in the metapelitic of Alvand aureole. The breakdown of garnet to plagioclase + sillimanite, dehydration melting and the formation of spinel–plagioclase symplectite could occur during heating or decompression; these textures are limited to the contact aureole in the studied area, so heating is perhaps the more likely explanation for formation of the symplectites in the metapelitic rocks of the Alvand aureole. The P–T diagram, inferred paths and zoning profiles of garnet do not account for the decompression history of the terrane.

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Barnes, C. G., K. Werts, V. Memeti, S. R. Paterson, and R. Bremer. "A tale of five enclaves: Mineral perspectives on origins of mafic enclaves in the Tuolumne Intrusive Complex." Geosphere 17, no. 2 (February 5, 2021): 352–74. http://dx.doi.org/10.1130/ges02233.1.

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Abstract The widespread occurrence of mafic magmatic enclaves (mme) in arc volcanic rocks attests to hybridization of mafic-intermediate magmas with felsic ones. Typically, mme and their hosts differ in mineral assemblage and the compositions of phenocrysts and matrix glass. In contrast, in many arc plutons, the mineral assemblages in mme are the same as in their host granitic rocks, and major-element mineral compositions are similar or identical. These similarities lead to difficulties in identifying mixing end members except through the use of bulk-rock compositions, which themselves may reflect various degrees of hybridization and potentially melt loss. This work describes the variety of enclave types and occurrences in the equigranular Half Dome unit (eHD) of the Tuolumne Intrusive Complex and then focuses on textural and mineral composition data on five porphyritic mme from the eHD. Specifically, major- and trace-element compositions and zoning patterns of plagioclase and hornblende were measured in the mme and their adjacent host granitic rocks. In each case, the majority of plagioclase phenocrysts in the mme (i.e., large crystals) were derived from a rhyolitic end member. The trace-element compositions and zoning patterns in these plagioclase phenocrysts indicate that each mme formed by hybridization with a distinct rhyolitic magma. In some cases, hybridization involved a single mixing event, whereas in others, evidence for at least two mixing events is preserved. In contrast, some hornblende phenocrysts grew from the enclave magma, and others were derived from the rhyolitic end member. Moreover, the composition of hornblende in the immediately adjacent host rock is distinct from hornblende typically observed in the eHD. Although primary basaltic magmas are thought to be parental to the mme, little or no evidence of such parents is preserved in the enclaves. Instead, the data indicate that hybridization of already hybrid andesitic enclave magmas with rhyolitic magmas in the eHD involved multiple andesitic and rhyolitic end members, which in turn is consistent with the eHD representing an amalgamation of numerous, compositionally distinct magma reservoirs. This conclusion applies to enclaves sampled <30 m from one another. Moreover, during amalgamation of various rhyolitic reservoirs, some mme were evidently disrupted from a surrounding mush and thus carried remnants of that mush as their immediately adjacent host. We suggest that detailed study of mineral compositions and zoning in plutonic mme provides a means to identify magmatic processes that cannot be deciphered from bulk-rock analysis.

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Eales, H. V., W. D. Maier, and B. Teigler. "Corroded plagioclase feldspar inclusions in orthopyroxene and olivine of the Lower and Critical Zones, Western Bushveld Complex." Mineralogical Magazine 55, no. 380 (September 1991): 479–86. http://dx.doi.org/10.1180/minmag.1991.055.380.17.

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AbstractThe occurrence of spheroidal, ovoid or deeply embayed plagioclase inclusions within cumulus bronzite and olivine grains of both pyroxenites and norites here implies the existence of plagioclase feldspar in melts prior to the separation of ultramafic cumulates from these melts. Variations of Sr-isotope initial ratio are shown to exist within the feldspar populations of such rocks, even on a grain-to-grain scale. Five separate stratigraphic intervals within the Lower and Critical Zones exhibit this texture, and in each case these are intervals wherein normal fractionation trends (decline in both whole-rock and orthopyroxene Mg/(Mg+Fe2+); decline in Cr contents of orthopyroxene) became reversed. The data support a model for evolution of the texture during major episodes of influx of hot, primitive magmatic liquid into the chamber. Progressive mixing of the new liquid with resident liquids (bearing feldspar on the liquidus) effected progressive changes in the composition of the hybrid liquids produced, and resorption and imposition of reversed zoning upon the inclusions.

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29

Sato, Yuto, and Kazuhito Ozawa. "Reconstruction of the lithosphere-asthenosphere boundary zone beneath Ichinomegata maar, Northeast Japan, by geobarometry of spinel peridotite xenoliths." American Mineralogist 104, no. 9 (September 1, 2019): 1285–306. http://dx.doi.org/10.2138/am-2019-6858.

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Abstract Accurate estimation of the depths of spinel peridotite xenoliths for which reliable geobarometers are not available is imperative to be able to reconstruct the precise structures of the lithosphereasthenosphere boundary (LAB). The LAB can be defined based on thermal, chemical, rheological, and petrological contrasts, and knowing its depth is crucial to understanding mantle dynamics. We attack this problem by examining spinel peridotite xenoliths from Ichinomegata maar in the back-arc side of Northeast Japan Arc. Extensive mineral compositions of nine xenolith samples revealed various patterns of chemical zoning in pyroxenes, suggesting diverse thermal histories. We examined the timescales of development of each zoning pattern and identified minerals, grain portions, and components closely approached equilibrium just before xenolith extraction as orthopyroxene and clinopyroxene, the outermost rims, and Ca-Mg-Fe components, respectively. Applying the best pair of geothermobarometers to the chosen analyses, plausible derivation depths of eight samples were obtained. They range from 0.72–1.6 GPa in pressure and from 830–1080 °C in temperature, which defines a high thermal gradient of 10 K/km or 290 K/GPa. There is an intimate correlation between the zoning patterns of pyroxenes and the depth estimates: pyroxenes in the deeper samples have zoning indicating cooling followed by heating just before xenolith extraction, and those of the shallower samples have zoning indicating monotonic cooling. Depth variations of rock microstructures, grain size of olivine, chemical compositions of minerals, and phase assemblage, including the presence or absence of glass or fluid phase, show that the mantle beneath Ichinomegata consists of two distinct layers. The shallower (28–32 km) layer is granular, less oxidized, amphibole- and plagioclase-bearing, and subsolidus, whereas the deeper (41–55 km) layer is porphyroclastic, amphibole- and plagioclase-free, oxidized, and partially molten. The contrasts between the two layers suggest that the upper layer represents a lithospheric mantle and the lower layer a LAB zone. These layers are similar to those reported from the bottom of subcontinental lithospheric mantle in various aspects, but the LAB beneath Ichinomegata is much shallower (40–60 km) and cooler (~1100 °C). The coincidence of (1) the depth of a rheological transition, marked granular to porphyroclastic textures, and (2) the depth of a phase transition, from subsolidus hydrous peridotite to a hydrous mantle with melt in localized pockets, is the remarkable feature of the LAB beneath Ichinomegata. This suggests that a rheological boundary zone in arc settings is governed by melting of the hydrous mantle and that the underlying asthenosphere is partially molten. The depth-dependent thermal history shown by chemical zoning in pyroxenes and the presence of melt as pockets suggest that the LAB beneath Ichinomegata was in a transient state that was affected by thermal and material transport.

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Tsune, A., and A. Toramaru. "A simple model of oscillatory zoning in magmatic plagioclase: Development of an isothermal undercooling model." American Mineralogist 92, no. 7 (July 1, 2007): 1071–79. http://dx.doi.org/10.2138/am.2007.2236.

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31

Zhong, Yun, Weiliang Liu, Zhilei Sun, Chris Yakymchuk, Kefa Ren, Jinnan Liu, Wei Li, Yaoliang Ma, and Bin Xia. "Geochemistry and Mineralogy of Basalts from the South Mid-Atlantic Ridge (18.0°–20.6°S): Evidence of a Heterogeneous Mantle Source." Minerals 9, no. 11 (October 27, 2019): 659. http://dx.doi.org/10.3390/min9110659.

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The South Mid-Atlantic Ridge is a typical slow-spreading ridge that represents a modern example to understand mantle composition and the evolution of mid-ocean ridge magmatism. In this paper, we investigate basalt samples dredged from four locations along the South Mid-Atlantic Ridge ranging from 18.0° to 20.6°S. The basalts belong to the tholeiitic series and exhibit normal mid-ocean ridge basalt (N-MORB) geochemical features with variable enrichments of Rb, Th, U, and Pb and depletions of Ba and Sr relative to primitive mantle. Some samples have negative Nb–Ta anomalies whereas others have positive Na–Ta anomalies to average N-MORBs. Plagioclase phenocrysts, microphenocrysts, and microlites occur in the in the matrix; phenocrysts and microphenocrysts are bytownite and labradorite in composition. Olivine phenocrysts are forsterite (Fo87 to Fo96). Chemical zoning in phenocrysts are interpreted to record crystal fractionation and magma mixing. Cores of plagioclase phenocrysts have higher anorthite values (An72–83) and estimated crystallization temperatures (~1180–1240 °C) that may suggest a xenocrystic origin. The lower anorthite proportions of rims of plagioclase phenocrysts (An65–71) and microphenocrysts (An54–72) yield lower estimated crystallization temperatures of ~1090–1120 °C and ~980–1060 °C, respectively. Rims of plagioclase phenocrysts and microphenocrysts may be generated in different environments such as magma chambers or magma channels, respectively. The basalt samples probably originated from partial melting of a depleted mantle spinel lherzolite source with a minor contribution of enriched materials possibly derived from the Saint Helena plume and subcontinental lithospheric mantle in the asthenosphere. Variable compositions of the basalt samples suggest heterogeneous mantle that includes depleted and enriched components at the South Mid-Atlantic Ridge between 18.0°–20.6°S.

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Petrakakis, Konstantin, Nathalie Schuster-Bourgin, Gerlinde Habler, and Rainer Abart. "Ca-rich garnets and associated symplectites in mafic peraluminous granulites from the Gföhl Nappe System, Austria." Solid Earth 9, no. 3 (June 19, 2018): 797–819. http://dx.doi.org/10.5194/se-9-797-2018.

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Abstract. Mafic peraluminous granulites associated with the mantle-derived peridotites of the Dunkelsteiner Wald provide evidence of the tectono-metamorphic evolution of the Gföhl Nappe System, Austria. They contain the primary assemblage garnet + Al-rich clinopyroxene + kyanite. Large Ca- and Mg-rich garnets are embedded in a granoblastic matrix of Al-rich clinopyroxene, Ca-rich plagioclase and minor hornblende. They were partially replaced by different generations of symplectites: (a) corundum + sapphirine + spinel + plagioclase formed around kyanite inclusions, (b) orthopyroxene + spinel + plagioclase ± hornblende formed at their rims and (c) clinopyroxene + orthopyroxene + spinel + plagioclase ± hornblende formed within cracks. Large garnets show complex compositional structure comprising several repeatedly occurring garnet types, which are characterized by specific compositions. The areal extent and the cross-cutting relations observed in element distribution maps allowed for the derivation of the relative timing of the formation of the different garnet types. The compositional features of the garnets indicate post-formational modification by intra-crystalline diffusion and metasomatic agents. The garnet composition isopleths in equilibrium assemblage diagrams are in line with compositions modification as indicated by the element distribution maps. They confirm the deviation of composition from equilibrium for all garnet types. Furthermore, at least the youngest garnet types show evidence of metasomatic (Fe + Mg) loss affecting their Ca content. Pressure–temperature (P–T) estimates are based on equilibrium assemblage diagrams that reproduce satisfactorily the observed mineral assemblages and measured mineral compositions. Criteria for checking the existence of preserved equilibrium compositions are suggested. The results call into question the invariability of the assumption that the Ca content and/or zoning in garnet preserves primary P–T information from garnet growth in every case. Recrystallization and compositional readjustment of the reactive garnet volume during symplectite formation led to the development of pronounced, secondary diffusion-induced zoning profiles overprinting the different garnet types and post-dating the complex garnet compositional structure. The primary assemblage is stable between 760 and 880 °C and pressures > 11 kbar. The bulk composition of the crack symplectites is almost isochemical to the oldest, broken-down garnet type. These symplectites were formed above 730 °C and pressures between 5 and 7.5 kbar. The rocks studied underwent more or less isothermal decompression from pressures above 11 to ∼ 6 kbar at temperatures of about 800 °C. Crack and rim symplectites were formed after decompression during the early stage of approximately isobaric cooling under conditions of low differential stress. Due to limited availability of fluids promoting symplectite formation, the timescale of symplectite formation calculated from secondary diffusion profiles associated with crack symplectites is shown to be geologically very short (< 0.5 ka).

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Stewart, M. L., and A. D. Fowler. "The nature and occurrence of discrete zoning in plagioclase from recently erupted andesitic volcanic rocks, Montserrat." Journal of Volcanology and Geothermal Research 106, no. 3-4 (May 2001): 243–53. http://dx.doi.org/10.1016/s0377-0273(00)00240-7.

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34

Ginibre, C., and J. P. Davidson. "Sr Isotope Zoning in Plagioclase from Parinacota Volcano (Northern Chile): Quantifying Magma Mixing and Crustal Contamination." Journal of Petrology 55, no. 6 (May 29, 2014): 1203–38. http://dx.doi.org/10.1093/petrology/egu023.

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35

Churikova, T. G., B. V. Ivanov, J. Eichelberger, G. Wörner, B. Browne, and P. Izbekov. "Major and trace element zoning in plagioclase from Kizimen Volcano (Kamchatka): Insights into magma-chamber processes." Journal of Volcanology and Seismology 7, no. 2 (March 2013): 112–30. http://dx.doi.org/10.1134/s0742046313020024.

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36

Doan, Vu Thi Anh. "Gemological characteristics and the quality of sapphire from Krong H’Nang - Dak Lak." Science and Technology Development Journal - Natural Sciences 1, T5 (November 29, 2018): 263–69. http://dx.doi.org/10.32508/stdjns.v1it5.559.

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Sapphires in Krong H'Nang area have been found mainly in deluvialor alluvial placers which related to Pliocene-lower Pleistocene basaltoid formations (βN2-Q1). Their main colors are dark green, yellow-green, yellow more rarely dark blue, yellow-blue-green. Sapphires in this area are transparency, translucency and opaqueness, most of them are opaque. The crystals are from 2–7 mm and sometimes their size can be up to 30–4 0mm. Besides the color zoning reduces color quality in dark blue sapphire, there are also many plagioclase or zircon, fingerprint and liquid-filled inclusions which reduce the clarity of the sapphires. Transparent sapphires with green, yellow-green, yellow are evaluated to be good quality and can be used in jewelry. For translucent and opaque sapphires with dark green, dark blue and transparent sapphires with color zoning, they can be used in jewelry but are not estimated well. Therefore, in order to improve the quality of the latter group, suitable methods such as heat treatment, beryllium diffusion and glass filling are needed.

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37

Liu, Wenheng, Xiaodong Liu, Jiayong Pan, Kaixing Wang, Gang Wang, Yuben Niu, and Wanjing Gong. "Magma Mixing Genesis of the Mafic Enclaves in the Qingshanbao Complex of Longshou Mountain, China: Evidence from Petrology, Geochemistry, and Zircon Chronology." Minerals 9, no. 3 (March 25, 2019): 195. http://dx.doi.org/10.3390/min9030195.

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The Qingshanbao complex, part of the uranium metallogenic belt of the Longshou-Qilian mountains, is located in the center of the Longshou Mountain next to the Jiling complex that hosts a number of U deposits. However, little research has been conducted in this area. In order to investigate the origin and formation of mafic enclaves observed in the Qingshanbao body and the implications for magmatic-tectonic dynamics, we systematically studied the mineralogy, petrography, and geochemistry of these enclaves. Our results showed that the enclaves contain plagioclase enwrapped by early dark minerals. These enclaves also showed round quartz crystals and acicular apatite in association with the plagioclase. Electron probe analyses showed that the plagioclase in the host rocks (such as K-feldspar granite, adamellite, granodiorite, etc.) show normal zoning, while the plagioclase in the mafic enclaves has a discontinuous rim composition and shows instances of reverse zoning. Major elemental geochemistry revealed that the mafic enclaves belong to the calc-alkaline rocks that are rich in titanium, iron, aluminum, and depleted in silica, while the host rocks are calc-alkaline to alkaline rocks with enrichment in silica. On Harker diagrams, SiO2 contents are negatively correlated with all major oxides but K2O. Both the mafic enclaves and host rock are rich in large ion lithophile elements such as Rb and K, as well as elements such as La, Nd, and Sm, and relatively poor in high field strength elements such as Nb, Ta, P, Ti, and U. Element ratios of Nb/La, Rb/Sr, and Nb/Ta indicate that the mafic enclaves were formed by the mixing of mafic and felsic magma. In terms of rare earth elements, both the mafic enclaves and the host rock show right-inclined trends with similar weak to medium degrees of negative Eu anomaly and with no obvious Ce anomaly. Zircon LA-ICP-MS (Laser ablation inductively coupled plasma mass spectrometry) U-Pb concordant ages of the mafic enclaves and host rock were determined to be 431.8 5.2 Ma (MSWD (mean standard weighted deviation)= 1.5, n = 14) and 432.8 4.2 Ma (MSWD = 1.7, n = 16), respectively, consistent with that for the zircon U-Pb ages of the granite and medium-coarse grained K-feldspar granites of the Qingshanbao complex. The estimated ages coincide with the timing of the late Caledonian collision of the Alashan Block. This comprehensive analysis allowed us to conclude that the mafic enclaves in the Qingshanbao complex were formed by the mixing of crust-mantle magma with mantle-derived magma due to underplating, which caused partial melting of the ancient basement crust during the collisional orogenesis between the Alashan Block and Qilian rock mass in the early Silurian Period.

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38

Ruprecht, Philipp, and Gerhard Wörner. "Variable regimes in magma systems documented in plagioclase zoning patterns: El Misti stratovolcano and Andahua monogenetic cones." Journal of Volcanology and Geothermal Research 165, no. 3-4 (September 2007): 142–62. http://dx.doi.org/10.1016/j.jvolgeores.2007.06.002.

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39

Ginibre, Catherine, Gerhard Wörner, and Andreas Kronz. "Minor- and trace-element zoning in plagioclase: implications for magma chamber processes at Parinacota volcano, northern Chile." Contributions to Mineralogy and Petrology 143, no. 3 (June 2002): 300–315. http://dx.doi.org/10.1007/s00410-002-0351-z.

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40

Ginibre, Catherine, Andreas Kronz, and Gerhard Wörner. "High-resolution quantitative imaging of plagioclase composition using accumulated backscattered electron images: new constraints on oscillatory zoning." Contributions to Mineralogy and Petrology 142, no. 4 (January 2002): 436–48. http://dx.doi.org/10.1007/s004100100298.

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41

Ahmed-Said, Y., and P. W. G. Tanner. "P-T conditions during emplacement, and D2 regional metamorphism, of the Ben Vuirich Granite, Perthshire, Scotland." Mineralogical Magazine 64, no. 4 (August 2000): 737–53. http://dx.doi.org/10.1180/002646100549599.

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AbstractThe 590±2 Ma Ben Vuirich Granite was intruded into late Proterozoic Dalradian rocks prior to the Grampian orogeny, during which it was affected by upper amphibolite facies regional metamorphism. Spotted cordierite and andalusite (chiastolite) hornfelses at the granite margin were altered to kyanite-and garnet-bearing assemblages during regional metamorphism. From the inferred mineralogy of the hornfels, together with the normative Qz–Ab–Or values of the granite and the application of a simple cooling model, we conclude that the country rocks immediately adjacent to the granite were hornfelsed at T = ∼600°C and P ≤ 2 kbar.The hornfelsed rocks were subsequently metamorphosed during the regional D2 event to form an equilibrium mineral assemblage of muscovite + biotite + garnet + plagioclase + quartz + kyanite (after chiastolite). Garnet which grew during this event shows unusual reversed chemical zoning, with Ca increasing systematically from core to rim as Fe, Mg and Mn decrease. A study of element partitioning between coexisting phases in equilibrium (including zoned garnets), and use of an internally consistent thermodynamic dataset, suggest that isothermal (T = 577±42°C) compression (from P = 6.2±1.6 kbar to 9.0±1.9 kbar) occurred during crustal thickening. K-feldspar-plagioclase-quartz veinlets found in the hornfels close the granite contact are demonstrated to be of igneous origin.

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42

Nikkola, Paavo, Enik˝o Bali, Maren Kahl, Quinten H. A. van der Meer, O. Tapani Rämö, Guðmundur H. Guðfinnsson, and Thorvaldur Thordarson. "Mid-crustal storage and crystallization of Eyjafjallajökull ankaramites, South Iceland." JOKULL 69 (February 2020): 83–102. http://dx.doi.org/10.33799/jokull2019.69.083.

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Our understanding of the long-term intrusive and eruptive behaviour of volcanic systems is hampered by a relatively short period of direct observation. To probe the conditions of crustal magma storage below South Iceland, we have analysed compositions of minerals, mineral zoning patterns, and melt inclusions from two Eyjafjallajökull ankaramites located at Brattaskjól and Hvammsmúli. These two units are rich in compositionally diverse macrocrysts, including the most magnesian olivine (Fo88-90) and clinopyroxene (Mg#cpx 89.8) known from Eyjafjallajökull. Olivine-hosted spinel inclusions have high Cr#spl (52–80) and TiO2 (1–3 wt%) and low Al2O3 (8–22 wt%) compared to typical Icelandic chromian spinel. The spinel-olivine oxybarometer implies a moderate oxygen fugacity of logFMQ 0–0.5 at the time of crystallization, and clinopyroxene-liquid thermobarometry crystallization at mid-crustal pressures (1.7–4.2 kbar, 3.0±1.4 kbar on average) at 1120–1195°C. Liquid-only thermometry for melt inclusions with Mg#melt 56.1–68.5 and olivine-liquid thermometry for olivine macrocrysts with Fo80.7-88.9 yield crystallization temperatures of 1155–1222°C and 1136–1213°C, respectively. Diffusion modelling of compositional zonations in the Brattaskjól olivine grains imply that the Brattaskjól macrocrysts were mobilized and transported to the surface from their mid-crustal storage within a few weeks (at most in 9–37 days). Trends in clinopyroxene macrocryst compositions and the scarcity of plagioclase indicate that the mid-crustal cotectic assemblage was olivine and clinopyroxene, with plagioclase joining the fractionating mineral assemblage later. In all, the crystal cargoes in the Brattaskjól and Hvammsmúli ankaramites are composed of agitated wehrlitic or plagioclase wehrlitic crystal mushes that crystallized over a large temperature interval at mid-crustal depths.

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Mavrogonatos, K., S. Flemetakis, A. Papoutsa, S. Klemme, J. Berndt, G. Economou, A. Pantazidis, I. Baziotis, and P. D. Asimow. "PHOSPHORUS ZONING FROM SECONDARY OLIVINE IN MANTLE XENOLITH FROM MIDDLE ATLAS MOUNTAINS (MOROCCO, AFRICA): IMPLICATIONS FOR CRYSTAL GROWTH KINETICS." Bulletin of the Geological Society of Greece 50, no. 4 (July 28, 2017): 1923. http://dx.doi.org/10.12681/bgsg.11933.

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Mantle xenolith samples in contact with basalt flows were collected from the Tafraoute maar in Morocco. Discrete melt veins are present in one xenolith sample, crosscutting primary layering and foliation. We used both optical microscopy and electron microprobe analysis to characterize the glasses and minerals in the melt veins. The melt veins consist of glass and crystals of olivine, clinopyroxene, plagioclase, spinel and apatite. The olivine in the melt veins is quite distinct from the same mineral within the matrix due to its characteristic P-enriched rims (up to 0.3 wt.%). Correlations between Al and P, as well as experimentally determined partition coefficient for P, point towards non-equilibrium partitioning during rapid crystal growth at the end of crystallization.

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Waight, Tod E., and Jakob B. Tørnqvist. "Sr isotope zoning in plagioclase from andesites at Cabo De Gata, Spain: Evidence for shallow and deep contamination." Lithos 308-309 (May 2018): 159–67. http://dx.doi.org/10.1016/j.lithos.2018.03.007.

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45

Sen, Gautam, Huai-Jen Yang, and Mihai Ducea. "Anomalous isotopes and trace element zoning in plagioclase peridotite xenoliths of Oahu (Hawaii): implications for the Hawaiian plume." Earth and Planetary Science Letters 207, no. 1-4 (February 28, 2003): 23–38. http://dx.doi.org/10.1016/s0012-821x(02)01146-9.

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46

Gibb, F. G. F., and C. M. B. Henderson. "The Shiant Isles Main Sill: structure and mineral fractionation trends." Mineralogical Magazine 60, no. 398 (February 1996): 67–97. http://dx.doi.org/10.1180/minmag.1996.060.398.06.

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AbstractThe Shiant Isles Main Sill, of Tertiary age, is a classic example of a composite, differentiated alkaline basic sill. The first unit to be intruded was a 2 m thick olivine teschenite which was emplaced with phenocrysts of olivine (mg> 83) [mg≡ Mg#] and, perhaps, plagioclase. This was intruded by a 24 m thick picrite sill consisting of a mush of melt and suspended olivine phenocrysts (mg> 83) with a D-shaped modal profile. The 140 m thick picrodolerite-crinanite unit was formed by a magma carrying ∼ 10% olivine (mg> 83) as the main phenocryst phase, together with some calcic plagioclase phenocrysts, being emplaced into the top of the picrite unit before the host rock was completely solidified. The olivine phenocrysts settled towards the bottom to form the picrodolerites.In-situdifferentiation processes occurred under conditions of almost perfect fractional crystallization, during which very strongly zoned ophitic crystals of olivine (fayalitic rims) and clinopyroxene (hedenbergitic rims), and zoned laths of plagioclase (anorthoclase rims), formed. The last unit consists of ∼ 2 m of granular olivine picrodolerite which was intruded into the upper crinanites, again before the host rock was fully solid.The mineral zoning patterns are interpreted using published cation diffusion coefficient data, and used to show that the picrite unit might have cooled to the blocking temperatures for Mg and Fe diffusion in < 5 years, and that even the relatively thick crinanite unit cooled very fast, so preserving the zoned Fe-Mg olivine and pyroxene compositions. The compositions of coexisting ilmenites and spinels define a redox trend which initially lies close to fayalite-magnetite-quartz buffer conditions, but later becomes more reducing and approaches magnetite-wustite buffer conditions. The final stages of development occurred during sub-solidus deuteric processes and involved formation of analcime and zeolites, as well as localized sulphide mineralization.

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47

Andersen, M. C., and T. C. R. Pulvertaft. "Occurrences of anorthositic rocks in the reworked Archaean basement in the Umanak area, central West Greenland." Rapport Grønlands Geologiske Undersøgelse 129 (December 31, 1986): 1–18. http://dx.doi.org/10.34194/rapggu.v129.7933.

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The main occurrence of anorthositic rocks in the Umanak area is the Tunulik sheet, a body up to 300 m thick with an areal extent of about 80 km2. The sheet is a composite body, consisting of blocks of crudely foliated anorthosite and leucogabbro 1-3 m in size in a granitic gneiss matrix. The blocks are jumbled together with no systematic distribution of blocks with different colour indices and no preferred orientation of the internal foliation within the blocks. The blocks are composed mainly of plagioclase, with small amounts of hornblende and biotite. The plagioclase shows patchy zoning; the composition is in the range An82-68. The dark minerals tend to be clustered in parallel schlieren. Epidotisation has given many blocks a yellowish colour. Textures are metamorphic. Both quartz- and nepheline-normative compositions are represented in the anorthositic blocks; it is suspected that compositions were modified with respect to Si, and possibly also Na and K, during metamorphism and incorporation in the granitic host. In addition to the Tunulik sheet, there are several small occurrences of anorthositic rocks in the Umanak area. These differ from the Tunulik sheet only in the fact that the anorthositic blocks in the minor occurrences are hosted in granodioritic-tonalitic gneiss no different from the country gneisses in the area, whereas in the Tunulik sheet the matrix gneiss is granitic.

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Gagnevin, D., T. E. Waight, J. S. Daly, G. Poli, and S. Conticelli. "Insights into magmatic evolution and recharge history in Capraia Volcano (Italy) from chemical and isotopic zoning in plagioclase phenocrysts." Journal of Volcanology and Geothermal Research 168, no. 1-4 (November 2007): 28–54. http://dx.doi.org/10.1016/j.jvolgeores.2007.07.018.

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49

White, S. "Composition and zoning of garnet and plagioclase in Haast Schist, northwest Otago, New Zealand: Implications for progressive regional metamorphism." New Zealand Journal of Geology and Geophysics 39, no. 4 (December 1996): 515–31. http://dx.doi.org/10.1080/00288306.1996.9514730.

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Borges, Melroy R., Gautam Sen, Garret L. Hart, John A. Wolff, and D. Chandrasekharam. "Plagioclase as recorder of magma chamber processes in the Deccan Traps: Sr-isotope zoning and implications for Deccan eruptive event." Journal of Asian Earth Sciences 84 (April 2014): 95–101. http://dx.doi.org/10.1016/j.jseaes.2013.10.034.

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Journal articles on the topic 'Plagioclase zoning'

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