Minerals in thin section at university of oklahoma studyblue. Plagioclase zoning includes oscillatory and patchy zonation and sieve textures. Structure and dynamics of the laacher see magma chamber. Mineral chemistry and thermobarometry of a southern. The oscillatory zoning is believed to have formed in response to recurrent supersaturation of the melt in anorthite adjacent to. Some of these samples contain sodic plagioclase so be certain to determine the sign of extinction when measuring extinction angles in anormal determinations. Can be confused with quartz if there is no twinning, however the optic axis figure will resolve this. Calcium depletion halos and femnmg zoning around faceted. Often a grey color in comparison to other feldspars. Albite and carlsbad twinning are visible in the xpl view, and also concentric growth zoning is seen in crystals near extinction. Maximum interference colors generally upper first to lower second order. Hornblendes are commonly unzoned, but some show simple, multiple or patchy zoning. Plagioclase phenocrysts in site 648 basalts range in size from patchy, and sector zoning are all apparent, even in individual thin sections. The plagioclase in some igneous bodies characteristically shows two or more distinct stages of corrosion and patchy zoning see figs.
Concentric zoning may be more subtle as with garnet and feldspar, for example. A plagioclase phenocryst can generally be divided into three parts. This is interpreted to be a result of assimilation of rocks of the aphanitic unit, given the nature of the contact between the two units see aphanitic unit section below. Patchy zoning of plagioclase was found in gro 95533 n with interior areas of grains more sodic blue in fig 1 and inclusionfree, while some rims are more calcic green in fig 1 and inclusion rich. In situ uthpbtotal dating of polychronous monazite in. Mar 23, 2011 the crystal structure remains the same but the composition changes from ca rich plagioclase, something like anorthite or labradorite to na rich plagioclase like albite. Citeseerx magma evolution and opensystem processes at. Twentytwo crystals from 1974 and thirtyseven crystals from 1999 ash were weighed, oriented, sectioned and optically characterized to delineate zoning differences. Interpretation and petrogenesis of zoned plagioclase. Because of incipient deformation, plagioclase often contains tapering twin lamellae and has undulatory extinction. False color rgb xray map of a plagioclase grain in gro 95533 n with patchy zoning.
In this paper, we discuss the age as well as textural and mineralogical evidences of magma hybridization in the western tatra granitoids. Geochemistry, petrogenesis, and tectonic setting of. Mineral zoning, p t x m phase relations, and metamorphic. Origin of patchy zoning in plagioc lase from gabbroic rocks of southwestern oregon. Mineral zoning, ptxm phase relations, and metamorphic. Many igneous plagioclases exhibit irregular zoning consisting of corroded cores embayed and surrounded in crystallographic continuity by more sodic plagioclase. Microcline and orthoclase are potassium feldspars kalsi 3 o 8, usually designated or in discussions involving their endmember composition. Strong coloration and pleochroism, 2 intersecting cleavages, and diamond shaped basal sections are diagnostic.
Plagioclase from bmg proximal to bag commonly displays disequilibrium features indicative of reaction with a more mafic magma, such as resorption surfaces, patchy zoning and especially reverse zoning with compositions approaching those of typical bag. Electron microprobe analyses of zoned columbitetantalite crystals from the granitic pegmatites of the erajarvi area in orivesi, southern finland indicate wide compositional variation within the series fenb 2 o 6feta 2 o 6mnnb 2 o 6mnta 2 o 6, especially in specimens from thin pegmatite dikes. Magma interaction recorded in plagioclase zoning in. The importance of accurate knowledge concerning the causes of composition zoning in plagioclases lies in the possibility that such zoning may throw light on the magmatic history of the rock of which the zoned felspar is a component. Pdf plagioclase zoning as an indicator of magma processes at.
Deep roots for midoceanridge volcanoes revealed by. Plagioclase is a series of tectosilicate framework silicate minerals within the feldspar group. This grain appears quite homogeneous in plane light, without concentric zones of inclusions that are commonly seen elsewhere. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or recordgroove effect. Plagioclase zoning includes oscillatory and patchy. The mafic amphibolites are concluded to have been derived from intrinsically basaltic protoliths, as determined by the presence of patchy zoning in relict igneous plagioclase, normative mineralogy, niggli trends, and a variety of diagnostic geochemical criteria.
Type 1 a, b and type 2 cg patchy zoning in plagioclase. Plagioclase textures and zoning patterns in the miocene. Dual view of a cleavage fragment of labradorite showing the multicolored labradorescence schiller effect and the same area viewed without the color. Calcium depletion halos and femnmg zoning around faceted plagioclase inclusions in garnet from a highgrade pelitic gneiss donna l. Origin of patchy zoning in plagioclase from gabbroic rocks. Pdf the processes that control compositional zoning in. The significance of plagioclase textures in midocean ridge basalt. This sometimes occurs in plagioclase crystals where irregularly shaped patches of the crystal show different compositions as evidenced by going extinct at angles different from other zones in the crystal. The processes that control compositional zoning in plagioclase phenocrysts. Four main types are present in plagioclase phyric oceanic basalts. In the studied rocks, plagioclase crystals display a variety of textures such as finescale oscillatory zoning, resorption surfaces, sieved and patchy zones, and a wide variability of anorthite an and trace element te content. Plagioclase phenocrysts in site 648 basalts range in size from zoning patterns even within individual thin sections. Probable relations between plagioclase zoning and magma. These textures provide us with information concerning conditions of magmatic evolution and histories of magmatic systems.
Mineral textures and zoning as evidence for open system. Mineral textures and zoning as evidence for open system processes. Also, albiteanorthite series plagioclase is characterized by a distinctive zoned structure. Isolated, ir regular inclusions of plagioclase in optical. Plagioclase exhibits both internal and external resorption ir and er, respectively in b and c. Poikilitic inclusions commonly present within the patches of sodic plagioclase are believed to have crystallized from the melt trapped in the cores after corrosion. Zoning in plagioclase is equivocal in some samples and systematic in others. Plagioclase is a major constituent mineral in the earths crust, and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of igneous rocks.
The plagioclase in this rock is more calcic than normal in a gabbro, and is classified as bytownite. Such plagioclase can be derived from both igneous and metamorphic sources. Phenocryst zonation in porphyryrelated rocks of the. In sar1, most plagioclase is an 46 and zoning is systematic to an 40 in the reaction zones and back to an 43 at the contact with pyroxene. Plagioclase zoning as an indicator of magma processes at. Alkali feldspar orthoclase and microcline are derived from both igneous and metamorphic sources. The optical parameters vary nearly linearly with composition, but because of the structural complexities, xray diffraction work is usually advised in identification of plagioclase feldspar. The interpretation of plagioclase zoning has been the subject of much speculation among petrologists, phemister having recently contributed 1934 a valuable discussion of earlier work, together with certain new suggestions as to the probable causes of the reverse and oscillatory types. This is a scanning electron microscope image where backscattered electrons are collected to yield an image that tells us about composition. The composition of plagioclase growing from a melt depends on physical parameters such as changes in temperature t. Plagioclase zone formation, especially those in the volcanic rocks is reflected in. Oscillatory zoning this sometimes occurs in plagioclase grains wherein concentric zones around the grain show thin zones of. The textures and kinetics of reaction between plagioclase and melts have been investigated experimentally, and origin of dusty plagioclase in andesites has been discussed.
Practical aspects of mineral thermobarometry plagioclase structure, sites and end members. Reverse and oscillatory zoning in plagioclase felspars. Plagioclase phenocrysts from mafic magmatic enclaves and plagioclase crystals from host granitoids of some plutons of the central part of the sierra nevada batholith are complexly zoned and commonly divided into three neatly distinct parts. Normal concentric, reverse, oscillatory, patchy, and sector zoning are all apparent, even in individual thin sections. Wnrrnny department of geological sciences, aj20, university of washington, seattle, washington 98195, u. In situ uthpbtotal dating of polychronous monazite in the. Monazites in the polygonized plagioclase matrix groupiii are variable in size 50500. Complex phenocryst textures and zoning patterns in andesites and. Growth of crystals from magma is one of the fundamental processes of geology. Plagioclase of the olivine gabbros shows very wide compositional ranges from an 66 to an 40 fig. Zoning in columbitetantalite crystals from the granitic.
This type of zoning may also be produced by physical inclusions of rock or other minerals. Please redirect your searches to the new ads modern form or the classic form. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more properly known as the plagioclase feldspar series from the ancient greek for oblique fracture, in reference to its two cleavage angles. The highly porphyritic andesite contains plagioclase phenocrysts with a wide array of disequilibrium textures and zoning patterns, providing an opportunity to reconstruct andesiteforming processes in a tectonic environment that evolved from subduction to transform motion. Plagioclase composition is very sensitive to temperature. Albite, carlsbad, and pericline twins cut across the composition zones. The core oligoclase displays a sieve texture, whereas the rim illustrates the growth of euhedral and zoned koligoclase. Patchy zoning of corrosion, is commonly rather uniform in the plagioclase of a given magmatic body. Ansrnlcr faceted, reversely zoned plagioclase inclusions surrounded by ca depletion halos and. As a result of discontinuous growth of a particular grain, plagioclase can have discontinuous zoning or patchy zoning. The color is due to optical interference on boggild exsolution lamellae. Anovrrz department of geosciences, university of arizona, tucson, arizona 85721, u. In crosspolarized light fine oscillatory zoning is visible, in which the composition varies back and.
Plagioclase is also required for balancing, and the result is the production of quartz. Recent work on oscillatory zoning in plagioclase springerlink. Petrological significance of zoned plagioclase in eldjurti. The boundary between the core and rim may be euhedral or somewhat irregular and embayed. Interference colors masked by minerals strong color. Plagioclase is rarely present in the peridotite unit, except as an intercumulus phase near the contact with the aphanitic unit. Backscattered electron bsederived zoning patterns of plagioclase phenocrysts are used to identify magma processes at bezymianny volcano, kamchatka, based on the 20002007 sequence of eruptive products. These mineralogical assemblages also provide information about the nature and source of metasomatizing fluids. Formula, naalsi3o8caal2si2o8, solid solution series between narich endmember albite and carich. Complex zoning in plagioclase feldspars from odp site 648.
A sequence of oscillatory cores and thin, normally zoned, more sodic rims is characteristic of many igneous plagioclases, especially those of quartz diorites and granodiorites. Zoning proves to be more gradational and mg and ca are relatively enriched toward the symplectite while fe is depleted. Plagioclase crystals in magmatic rocks possess various textural types and zoning such as spongy, boxy and sieve texture, and oscillatory, patchy and normal zoning. Ansrnlcr little information on prograde pz paths in granulites is generally preserved because. An example of this would be clay, hematite, or chlorite inclusions coating the surface of an inner layer of crystal growth. Millimetersized plagioclase phenocrysts were explosively erupted in highalumina basaltic ash from fuego volcano, guatemala on october 14, 1974.
If the plagioclase also shows zoning, then it is likely from a volcanic source. Make note of zoning patchy, oscillatory, normal, reversed, twinning, inclusions, alteration, synneusis, and determine the composition. Magma evolution and opensystem processes at shiveluch. The patchyzoned regions are relatively sodic but overlap the compositions of the brightly banded regions. It is important to note that this change in composition and resultant zoning is accomplished if the magma body is not involved in any mixing. Plagioclase exhibits complex zoning such as oscillatory oz and patchy zoning pz in b and c.
All the rockforming feldspars are alumino silicate minerals with the general formula at 4 o 8 in which a potassium, sodium, or calcium ca. The chemical variation in the composition of a solid solution mineral series from core to rim, as the result of separation of the mineral phases during its growth by loss of equilibrium in a continuous reaction series is called zoning. The western tatra granitoids common tatra type were formerly interpreted as purely stype rocks, formed during a prolonged 360350 ma magmatic episode kohut and janak 1994. Plagioclase phenocrysts from mafic enclaves and plagioclase from its host granite possess a pattern of complex zonation. In both cases, the composition of the final melt inclusion depends on element. Most euhedral plagioclase phenocrysts show welldeveloped zoning, or complex zonation with significant patchy zones in the core fig. Why are plagioclase feldspars typically found zoned in thin. Plagioclase xenocrysts and mafic magmatic enclaves in some. Phenocryst zoning patterns are used to identify opensystem magmatic processes in the products of the 2001 eruption of shiveluch volcano, kamchatka. Plagioclase thin section tectosilicates microckscopic. The patchy zoning of pyroxenes has been interpreted as reflecting a twostage crystallization history. Processes of crystal growth arent perfect for example, crystals that grow rapidly may develop skeletal or dendritic forms.
Feldspar value, price, and jewelry information home. The crystals have unzoned, patchy zoned and oscillatoryzoned parts. We compare plagioclase zoning relationships in the 1974 vulcanian and 1999 strombolian. Unzoned, inclusionpoor cores, up to 2 mm thick have round edges and compositions between an 97 and an 88. This chemical zoning is common, particularly in volcanic and hypabyssal intrusive rocks, and is expressed as a variation in the extinction angle from one zone to another.
Mineral zoning, ptxm phase relations, and metamorphic evolution of some adirondack granulites, new york frank s. Multistage growth of the trachytic lava dome of the puy. Shibata, t 1990, complex zoning in plagioclase feldspars from odp site 648, proceedings of ocean drilling program, scientific results, vol. Minerals belonging to a continuous solid solution such as. The presense of parallel striae on cleavage surfaces distinguishes plagioclase from kfeldspar. More information on this type of zoning is given by smith 1974. The birefringence is sligltly higher than normal resulting in a yellow tint to come crystals. Chemical zoning and disequilibrium textures in plagioclase phenocrysts sem x images. In sections cut parallel to 010 the included plagioclase typically forms irregular. Plagioclase microstructures and compositions as tracers of. Minerals free fulltext hfseree transfer mechanisms.
Changes in composition during growth produce a zonation resembling tree rings which is interpreted in a somewhat similar manner although the periods of growth resulting in zones are not necessarily annual. Patchy zoning is interpreted here in terms of decrease in confining pressure on waterdeficient magma during its rise in the crust. The interior of this crystal has patchy zoning rather than concentric, indicating skeletal early growth. Finally, mixing with a further mafic magma appears to have locally occurred also in the bag. Xray diffraction work is usually advised in identification of plagioclase feldspar. Pdf backscattered electron bsederived zoning patterns of plagioclase phenocrysts are used to identify magma processes at. Origin of patchy zoning in plagioclase from gabbroic rocks of.
Most plagioclase crystals are twinned according to various laws related to the crystal structures and also the. The lavas and pumices studied are hornblende plagioclase andesites with average preeruptive temperatures of 840c and fo2 of 1. Zoning the zoning is a compositional variations found in the crystals. Plagioclase grains are commonly homogeneous and in undeformed gabbros rarely show signs of compositional zoning fig. Chemical zoning is common the calcic core has a larger extinction angle than the sodic rim. Patchy zoning in plagioclase has been observed in gabbroic rocks in southwestern oregon. Zoning patterns in plagioclase can also reflect the character of the matrix overgrown. Interpretation of plagioclase zonation in calcic pelitic.
The sequence of development is visualized as follows. Markussen department of earth and environmental science, rensselaer polytechnic institute, troy, ny 12180, usa detailed petrologic analysis of ten metaanorthosites and related introduction. Learn vocabulary, terms, and more with flashcards, games, and other study tools. In et10, most plagioclase is an 50 and there are patches of an. Elemental maps of example 2 yield different results. Patchy zoning this sometimes occurs in plagioclase crystals where irregularly shaped patches of the crystal show different compositions as evidenced by going extinct at angles different from other zones in the crystal. Most plagioclase crystals are twinned according to various laws related to the crystal structures and also the distribution of atoms in the structures. Pdf the significance of plagioclase textures in mid. Groups of dully and brightly banded oscillatory zones correlate between grains and are respectively relatively calcic and sodic. Plagioclase phenocrysts in the plagioclase phyric basalt from hole u1433b of iodp expedition 349 in the south china sea show complex zoning patterns e. The purpose of this exercise is to give students the experience of utilizing the two component solid solution phase diagram of albiteanorthite and applying this diagram to the interpretation and petrogenesis of chemically zoned plagioclase. Petrography and mineral chemistry of the boroujerd pluton. This finding is consistent with the empirical formula of pyroxferroite ca 0.
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