Orbicular texture

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An orbicular texture is a concentric crystallization of common igneous rock minerals . It takes place around foreign inclusions, individual mineral grains or grain aggregates of the affected rock ( orbiculite ). Orbicular textures should not be confused with similar-looking circular sections of magma tubes.

etymology

The term orbicular texture is derived from the Latin orbiculus , a diminution of orbis circle, rounding.

history

The orbicular texture was first scientifically mentioned in 1802 by Leopold von Buch in the granite of the Giant Mountains . Its original find went back to 1796/1797. However, a first reading was made in 1785 by apprentices under de Barral in France. Further descriptions were made by Frederick Henry Hatch (1888), Karl Heinrich Rosenbusch (1907), Jakob Johannes Sederholm (1928), Albert Johannsen (1932 and 1937) and Pentti Eskola (1938 and 1963), as well as in a comprehensive revision by David J. . Leveson in 1966.

description

Orbicular texture polished slab of rock from Mount Magnet , Western Australia

Orbicular textures are built up from one, but mostly from several, very regularly shaped, concentric, spherical structures . These balls can be composed differently and have different grain sizes . Their spatial distribution is usually statistically disordered, but can also take place as densely packed groupings.

Usually an inner area (core) can be seen, which is girded by one or more shells. The individual orbicules , which can reach a diameter of 10 centimeters and more (with a range of 1 to 40 centimeters), are embedded in a uniform or porphyry matrix - the matrix . Overall, the core and matrix show the same minerals in most cases, but there are also differences in the composition. Very large orbicules are made up of several shells. In general, orbicular size and chemistry can be correlated with one another - ultramafic and carbonatitic orbicules are very small (centimeter range). With increasing silicon content, the diameter grows - very large orbicules between 30 and 40 centimeters occur, for example, in granodiorite and quartz monzonite .

The occurrences of orbicular textures are spatially very limited and take up well under a square kilometer (hectare area) on the surface. They can be found at the edges of intrusions , in passages and in igneous tube structures.

The orbicular textures of an occurrence are mostly similar in structure and size, but in some cases different types of orbiculars can be found next to each other.

core

The core area can be very different. As a rule, it consists of an agglomeration of fine-grain minerals. In their place, however, xenolites - foreign rock inclusions such as metamorphites (metapelites, dark slates, migmatites, amphibolites, peraluminous residues) - or autolithic magmatites (e.g. microdiorites, hornblendites and granitoids), large single crystals (pheno- or megacrystals) or whose fragments and crystal aggregates appear. Plagioclase or alkali feldspar and occasionally also mafic minerals such as hornblende function as crystallization nuclei. The core of ultramafites is serpentine (created by olivine decomposition), diopside or a mineral mixture of orthopyroxene , clinopyroxene and amphibole .

The transition from the core to the shell area is not always sharp, but is often blurred and can then only be determined under the microscope.

Due to previous magma mixing , nuclei can have a very complex structure. Complex cores can also be composed of one or two proto-orbiculites (see below) or a combination of xenolite and proto-orbiculite.

Peel)

Crystal growth in the shells very often takes place radially from the inside to the outside, but tangential growth parallel to the shell layers also occurs. The mineral grains are often only arranged in a ring without any preferred adjustment. All of these forms of growth vary from case to case and can alternate with one another. Outside, the mostly centimeter-thick shells often show concentric, rhythmically repeating drawings consisting of up to 40 different mineral layers. This shell pattern in the millimeter range is usually caused by the smallest dark mineral grains. Their rhythmicity is associated with periodic reactions with the magma under changing water pressure (p H 2 O) or under a changing rate of hypothermia (Δ T). Comb textures are also observed in the pods .

The core / shell ratio can vary widely. If the core is well developed, the shell area recedes - sometimes only one shell is left. Conversely, the shells gain in importance with relatively small cores, so that several layers follow one another. The orbicular / matrix ratio is also variable. Usually the matrix area predominates, common ratios are 35:65.

Peel detachment can also be observed occasionally.

matrix

The matrix tends to have a porphyry structure, but can also appear relatively uniformly grained. Their composition may or may not match the host rock. Their grain sizes can also differ from orbicules and host rocks. Their structure is very diverse and appears grain-like, slate-like, layered, etc. Contact with the host rock can occur abruptly or gradually. Some occurrences even have two or more matrices.

Host rocks

Orbicular textures can be found in many igneous rocks of intrusive (predominant) and extrusive nature (relatively rare) - for example in gabbros , norites , basalts , diorites and granites , but also in monzonites , syenites and rare carbonatites and lamprophyren . They can also occur in matamorphic rocks. Manifestations of the host are corridors , storage corridors and batholiths , rarely also lava flows, tuffs, gneiss, slate, horn rock, migmatite and even chromite.

Proto-Orbicularites

In the associated proto-orbicularites , the shell areas are often indistinct or only slightly developed. Usually only one or two thin and often interrupted shells are present in them besides the core. They can be viewed as transition stages to the host rock. Their contact with the actual orbicularites can be fluent, but they are usually sharp with the host. It can also happen that single or rarely two proto-orbicularites are again surrounded by orbicular texture. They are then to be seen as the core.

Mineral content

The light areas in the cores of orbicular textures are usually made up of feldspars (mainly plagioclase and some alkali feldspar ) and quartz , whereas in the shells, in addition to plagioclase, iron-magnesium minerals such as biotite , amphiboles (especially hornblende ), clinopyroxes , orthopyroxes and iron oxides how magnetite and ilmenite predominate. Minerals also found in orbicularites are olivine , nepheline , titanite , epidote , cordierite , sillimanite , chromite , tourmaline and calcite . The overall chemistry therefore essentially corresponds to diorites , granodiorites , monzonites and syenites .

corrosion

Occasionally whole shell parts are missing. The melting of the orbicular led to dissolution and corrosion at the edge - caused by the hot magma flowing by from the remaining melt. The corrosion can occur very irregularly and in places reach very deeply into several layers of the shell. A new, grainier generation of shells can later settle on this corrosion surface.

Deformations

Often deformations observed at individual Orbikulen - they are no longer circular, but elliptical, flattened and pressed together. As a result, they were plastically deformable structures, surrounded by a flowable melt. The deformation can go so far that the interstitial matrix is ​​drastically reduced or completely squeezed out. Crushed areas are also corroded in places.

Mineralized veins can also run through orbicules, sometimes with an offset. The orbicules were therefore stretched and displaced in the not yet completely solidified state. Completely broken, moving orbicules can also be observed.

Emergence

Orbicular granite from caldera in Chile

Orbicular textures are igneous structures , but their origin is still controversial. None of the proposed hypotheses can provide a general, all-encompassing development model. However, numerous mechanisms of formation have been proposed which reflect the versatility of the textures in both design and chemistry. Their versatility is explained by differences in the host rocks, in the matrix, in the core and shell structure.

It is obvious that orbicular textures have to be linked to very special physical / chemical conditions during crystallization, simply because of their very rare occurrence . Based on the mineralogical findings, it can be assumed that orbicular textures form around initial crystallizates in a magma chamber or its edge areas. Relatively cold condensation nuclei are either single grains (mostly feldspar phenocrystals), grain aggregates or xenolites.

Older hypotheses attempted to interpret the phenomenon, among other things, the immiscibility of magmas, fluctuations in pressure and temperature in the vicinity of the eutectic, reactions between magma and inclusions, rhythmic crystallization comparable to the formation of Liesegang's rings , metasomatic diffusion during granitization and sodium metasomatosis mafic Rocks.

One of the more modern explanatory models of crystallization dynamics assumes a strong magma undercooling (English supercooling ) as a starting point , as it is realized on the edge of intrusions. If there is also a lack of crystallization nuclei - characteristic of very hot magmas - the onset of crystallization is delayed. The reason for the delay is the reduced temperature gradient between the growing crystal aggregate and the surrounding melt - the temperature gradient is, besides the concentration gradient, the crystallization (N) and the growth rate (G), the main driver in the crystallization process. Once started, however, the crystallization proceeds quite quickly and radial-ray minerals grow. The further deposition of minerals ultimately depends on the speed with which material can diffuse from the surrounding melt to the crystallization front (diffusion rate D). However, the incorporation of minerals on the surface of the orbit causes material depletion in the immediate vicinity of the growing orbit - which in turn initiates the deposition of new minerals.

Ron Vernon (1985) attributes the supercooling of the magma to an increased water content in the magma. An increased water concentration lowers the liquidus temperature and also transfers potential crystallization nuclei into solution. Due to the delay in the start of crystallization, the actual crystallization must then take place suddenly, as a result of which elongated, needle-like crystals grow on existing crystals of cold interfaces - such as on intrusion margins with the formation of comb textures or on solid inclusions with the formation of orbicular textures.

More recent studies postulate the following steps, which follow one another very quickly, as an explanatory model:

  • Overheating of the magma or lowering of the liquidus due to a sudden supply of water - which causes a depolymerization of the melt
  • Volatilization of dissolved gases in the melt leading to crystal fragments of the host rock
  • Adiabatic undercooling of the melt due to the gas evaporation
  • Geochemical fractionation as a result of the onset of crystallization on cold seeds
  • Column crystallization in the core at a high crystallization rate followed by shell formation
  • Poikilitic crystallization (growth of small crystals in large host crystals with the same axis orientation) in the vicinity of the solidus in the outer shell area and in the adjacent matrix.

Pegmatite areas (with megacrystals of alkali feldspar) in the matrix can be explained by a mixing of the matrix melt with the water-rich residual melt that formed during the shell formation.

Postmagmatic impacts through metasomatosis and metamorphosis are often of importance . Also recrystallization of the core have been observed.

Conclusion

Orbicular textures occur in intrusive, volcanic and also metamorphic rocks. However, their structure is not characteristic of any of these geological milieus. Because very similar orbicules can be found in very different rocks and, conversely, completely different orbicules appear in very similar rocks.

The rarity of orbicular textures suggests very rare educational conditions. Melting areas that were initially overheated and later supercooled should also only rarely be found in plutons. In particular, magmas with a granitic composition have sufficient nuclei to ensure normal crystallization. Since orbicular textures appear to be attached to tubes and similar heat-transferring structures (to higher, cooler sections of an intrusion), they can be attributed to convective instabilities in the terminal stages of plutons.

The deformed and partially digested orbicules suggest that their rarity may also be due to a general destruction and resorption of their previously formed textures. Their rhythmic shell structure is a sign of a very stable educational environment, as disturbed conditions in turn result in a disturbed shell structure.

Orbicular textures are probably more common and more complicated than previously assumed.

use

Orbicular textures can be sanded and processed into table tops, decorative jewelry or art objects.

Occurrence

Orbicular texture from Nuuksionpää near Espoo in Finland
Orbicular texture in the granite of Concordia in South Africa
Rapakivi boulder with orbicular texture from Neuenkirchen

Almost a third of the orbicular textures occurring at over 300 sites worldwide come from Finland (with 83 occurrences, 29 of which are in the pending area and 54 as bed debris):

France also has some sources:

The United States has numerous sites, especially in the batholith of the Sierra Nevada California, the occurrences of orbicularites are increasing:

Other known occurrences are:

In Germany, orbicular textures have been found in various bedloads from Scandinavia . Locations include Buxtehude , Hohensaaten and Neuenkirchen .

See also

literature

  • JN Elliston: Orbicules: an indication of the crystallization of hydrosilicates . In: I. Earth-Sci. Rev. Band 20 , 1984, pp. 265-344 .
  • David. J. Leveson: Orbicular rocks - a review . In: Geol. Soc. Amer. Bull. Band 77 , 1966, pp. 409-426 , doi : 10.1130 / 0016-7606 (1966) 77 [409: ORAR] 2.0.CO; 2 .
  • Seppo Lahti: Orbicular rocks in Finland. With contributions by Paula Raivio and Ilkka Laitakari . Geological Survey of Finland, 2005, p. 177 .
  • Hans-Peter Meyer: On the petrology of orbiculites. Dissertation. Karlsruhe 1989.
  • Hans-Peter Meyer and R. Altherr: A model for the genesis of orbicular granitoid rocks . In: Terra Abstracts . tape 3 , 1991, pp. 426 .
  • JG Moore and JP Lockwood: Origin of comb layering and orbicular structure, Sierra Nevada Batholith . In: Bulletin of the Geological Society of America . tape 84 , 1973, pp. 1-20 .
  • RH Vernon: Possible role of superheated magma in the formation of orbicular granitoids . In: Geology . tape 13 , 1985, pp. 843-845 , doi : 10.1130 / 0091-7613 (1985) 13 <843: PROSMI> 2.0.CO; 2 .

Individual evidence

  1. ^ JJ Sederholm: On orbicular granites . In: Bull. Comm. geol. Finland . tape 83 , 1928, pp. 105 .
  2. ↑ David. J. Leveson: Orbicular rocks - a review . In: Geol. Soc. Amer. Bull. Band 77 , 1966, pp. 409-426 , doi : 10.1130 / 0016-7606 (1966) 77 [409: ORAR] 2.0.CO; 2 .
  3. ^ Arthur Gibbs Sylvester: The nature and polygenetic origin of orbicular granodiorite in the Lower Castle Creek pluton, northern Sierra Nevada batholith, California . In: Geosphere . tape 7 (5) , 2011, pp. 1134–1142 , doi : 10.1130 / GES00664.1 .
  4. ↑ Hans-Peter Meyer: On the petrology of orbiculites. Dissertation. Karlsruhe 1989.
  5. ^ RH Vernon: Possible role of superheated magma in the formation of orbicular granitoids . In: Geology . tape 13 , 1985, pp. 843-845 .
  6. ↑ Juan Díaz-Alvarado, Natalia Rodríguez, Carmen Rodríguez, Carlos Fernández and Ítalo Constanzo: Petrology and geochemistry of the orbicular granitoid of Caldera, northern Chile. Models and hypotheses on the formation of radial orbicular textures. In: Lithos . Volumes 284–285, 2017, pp. 327-346 , doi : 10.1016 / j.lithos.2017.04.017 .
  7. ↑ a b M. Piboule, L. Soden, J. Amossé and B. Briand: Le massif basique de Loreto di Tallano (Corse du Sud): mise en évidence du contrôle de la surfusion adiabatique dans la genèse des faciés orbiculaires . In: CR Acad. Sci. Paris . 309, II, 1989, pp. 713-718 .
  8. ^ SR Paterson: Magmatic tubes, pipes, troughs, diapirs, and plumes: Late-stage convective instabilities in compositional diversity and permeable networks in crystal-rich magmas of the Tuolumne batholith, Sierra Nevada, California . In: Geosphere . Vol. 5, No. 6 , 2009, p. 496-527 , doi : 10.1130 / GES00214.1 .
  9. ^ Seppo Lahti: Orbicular rocks in Finland. With contributions by Paula Raivio and Ilkka Laitakari . Geological Survey of Finland, 2005, p. 177 .
  10. ↑ A. Simonen: Orbicular rock in Kuru, Finland . In: Commission Géologique de Finlande, Bulletin . tape 222 , 1966, pp. 93-107 .
  11. ↑ Mi Jung Lee, D. Garcia, J. Moutte, CT Williams and F. Wall: Carbonatites and phoscorites from the Sokli Complex, Finland . In: Mineralogical Society Series, 10th Mineralogical Society . London 2004, ISBN 0-903056-22-4 , pp. 129-158 .
  12. ↑ M. Barrière, L. Chauris and J. Cotten: Premières données sur un faciès orbiculaire dans le massif granitique de 1'Aber-Ildut (Finistère, France) . In: Bull. Soc. fr. Mineral. Cristallogr. tape 94 , 1971, p. 402-410 .
  13. ↑ D. Besse, A. Fabre, D. Ponsignon and JC Goujou: Le granite orbiculaire de Jainallat . In: Le règne minéral . No. 58 , 2004.
  14. ↑ Sylvie Decitre, Dominique Gasquet and Christian Marignac: Genesis of orbicular granitic rocks from the Ploumanac'h plutonic complex (Brittany, France): petrographical, mineralogical and geochemical constraints . In: Eur. J. Mineral . tape 14 , 2002, p. 715-731 .
  15. ↑ M. Barriere: Le gabbro orbiculaire of Alharisses (massif de Néouvielle, Pyrénées Françaises) . In: Bull. Soc. Ms. Minéral. Cristallogr. tape 95 , 1972, pp. 495-506 .
  16. ↑ JP Couturié: Un nouveau gisement de granite orbiculaire dans le Massif Central français: le granite du Signal de Randon (Lozère) . In: Contrib. Mineral. Petrol. tape 42 , 1973, pp. 305-312 .
  17. ^ DJ Leveson: Orbicular rocks of the Lonesome Mountain area, Beartooth Mountains, Montana and Wyoming . In: Bull. Geol. Soc. At the. tape 74 , 1963, pp. 1015-1040 .
  18. ^ GE Goodspeed: Orbicular rocks from Buffalo Hump, Idaho . In: Am. Mineral. tape 27 , 1942, pp. 37-47 .
  19. ↑ BB Van Diver: Origin of the Jove Peak orbiculite in Wenatchee Ridge area, northern Cascades, Washington . In: Am. J. Sci. tape 266 , 1968, pp. 110-123 .
  20. ^ JG Moore and JP Lockwood: Origin of comb-layering and orbicular structure, Sierra Nevada Batholith, California . In: Bull. Geol. Soc. At the. tape 84 , 1973, pp. 4007-4010 .
  21. ^ Robert D. Enz, Albert M. Kudo and Douglas G. Brookins: Igneous origin of the orbicular rocks of the Sandia Mountains, New Mexico . In: Geological Society of America Bulletin . Vol. 90, 1979, pp. 138-140 , doi : 10.1130 / 0016-7606 (1979) 90 <138: IOOTOR> 2.0.CO; 2 .
  22. ↑ - TonalitL. Aguirre, F. Hervé and M. Del Campo: An orbicular tonalite from Caldera, Chile . In: Journal of the Faculty of Science, Hokkaido University, Japan . Vol. 17 (2), 1976, pp. 231-259 .
  23. ↑ Hans Niemeyer Rubilar: La granodiorita orbicular del Cordón de Lila, región de Antofagasta, Chile . In: Andean Geology (in Spanish) . tape 45 (1) , 2018, doi : 10.5027 / andgeoV45n1-3114 .
  24. ↑ RF Symes, JC Bevan and M. Qasim Jan: The nature and origin of orbicular rocks from near Deshai, Swat Kohistan, Pakistan . In: Mineralogical Magazine . Vol. 51, 1987, pp. 635-647 .
  25. ^ DL Reid, DG Bailey and DH French: Orbicular texture in boulders from the Glenroy Valley (S39) South-East Nelson, New Zealand . In: Journal of Geology and Geophysics . tape 15: 4 , 1972, pp. 643-648 , doi : 10.1080 / 00288306.1972.10423989 .
  26. ↑ Anders Lindh and Helena Näsström: Crystallization of orbicular rocks exemplified by the Slättemossa occurrence, southeastern Sweden . In: Geol. Mag. Band 143 (5) . Cambridge University Press, 2006, pp. 713-722 .
  27. ^ DF Palmer, J. Bradley and WM Prebble: Orbicular granodiorite from Taylor Valley, South Victoria Land, Antarctica . In: Geol. Soc. Amer Bull. Volume 78 , 1967, p. 1423-1428 .
  28. ↑ Margarida C. Simões: Ocorrência de granito orbicular em Couto do Osso, Serra da Peneda . Sociedade Geológica de Portugal, Lisbon 1981, p. 125-128 .
  29. ^ Adhir Kumar Basu: Role of the Bundelkhand Granite Massif and the Son-Narmada megafault in precambrian crustal evolution and tectonism in Central and Western India . In: Journal of the Geological Society of India . tape 70 , 2007, p. 745-770 .
  30. ^ M. Maggetti, BB Van Diver, G. Galetti and J. Sommerauer: P / T conditions of orbicular gabbro from Reichenbach, West Germany . In: New Year Mineral. Depending on the band 134 , 1978, pp. 52-75 .
  31. ↑ I. Bryhni and JA Dons: An orbicular lamprophyre from Vestby, Norway . In: Lithos . tape 8 , 1975, p. 113-122 .
  32. ^ AV Lapin and H. Vartiainen: Orbicular and spherulitic carbonatites from Sokli and Vuorijärvi . In: Lithos . Volume 16, Issue 1, 1983, pp. 53-60 , doi : 10.1016 / 0024-4937 (83) 90034-8 .