Comb texture

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As comb texture in is Petrology a manifestation of igneous rocks called. This anisotropic texture represents a concentrated collection of single crystals . It is created by unidirectional solidification and is bound to discontinuity surfaces.

history

The comb texture was first scientifically defined in 1973 by JG Moore and JP Lockwood. In 1982, Shannon and colleagues introduced the metallurgy technical term Unidirectional Solidification Texture , abbreviated UST , for comb texture . However, a first description comes from Russian authors from the Transbaikal region from 1957. Meanwhile, quite a lot of occurrences of crest textures are known.

description

Orbicular granite from Mount Magnet. The shells of the central orbicle show a crest texture.
The spinifex structure, closely related to the comb texture, in a komatiite

The term comb texture , English comb layering , comb structure or also comb texture , French litage en peignes , is derived from the comb-like or lawn-like growth of minerals on a solid discontinuity surface in the rock. The individual individuals grow up at an angle of 60 to 90 degrees. They are thin-plate or long prismatic and can at the same time be skeletal, curved, feathery, widening or dendritically branched. Because comb textures repeat mostly multiple and often rhythmic, they may as able texture are considered.

The individual comb layers are usually separated from isotropically granular areas. Their thickness ranges from centimeters to meters. They differ both in their mineral composition and in the direction in which the crystals grow. A growth cycle is built up from two different individual layers.

In the English literature, the terms crescumulate layering , harrisitic texture , spinifex texture ( German Spinifexgefüge ) and Willow Lake layering are practically used synonymously. There is also an affinity for orbicular textures , in which crest textures can often be observed.

Donaldson (1977) describes the following sequence within a comb texture: at the edge of the intrusion there is a harrisitic texture with needle-shaped crystals arranged in a comb-like manner, which branch out and often show an irregular extinction . The crystals can be bent. However, their deformation is not due to any tectonic stress. Towards the inside of the intrusion, the crystals then increasingly take on a granular, cumulative texture . The growth side of a cycle is very clear, almost sharp, whereas the inside is much less clear and marks the gradual end of the crystal growth. This fuzziness is based on locally changing physical / chemical conditions.

Constructive minerals

The following minerals can be involved in the structure of comb textures: olivine (also serpentinized), pyroxene ( orthopyroxene and clinopyroxene ), amphibole ( hornblende ) and, above all, plagioclase as well as quartz and alkali feldspar . Feldspar representatives like nepheline are also rare . The crystals are often zoned - signs of a crystallization out of balance.

Occurrence

The walls of magma chambers , inclusions , orbicules , megacrystals, corridors or other internal interfaces function as discontinuities . The occurrences of comb textures are therefore linked to these structures. Comb textures are often associated with cumulative textures on the bottom or on the edge areas of magma chambers. The two textures then usually merge seamlessly.

Comb textures are generally relatively rare. They can be found in a range of igneous rocks, ranging from gabbros or basalts to diorites and granitoids . Dolerite and lamprophyres should be mentioned among the gangue rocks . They can also be seen in the rather exotic carbonatites . They also occur in aplites and pegmatites .

use

Since the crystals scatter in their growth direction, the resulting V-shaped opening angle can be used to determine the sense of direction. It points in the direction of magma / magma chamber.

Deposits

Comb textures or UST are occasionally strongly mineralized and can consequently be involved in the formation of significant deposits .

Emergence

Comb textures are now clearly viewed as of magmatic origin, with the crystallizing magma being strongly oversaturated and undercooled . The grainy intermediate areas, on the other hand, are viewed as only slightly undercooled or oversaturated. A high level of hypothermia and rhythmic supersaturation are responsible for the growth of long-rayed and dendritic crystals.

Slow crystal growth in an igneous melt, on the other hand, leads to the formation of well-formed, idiomorphic crystals . With rapid growth, depending on the rate of undercooling, either elongated crystals or so-called hopper crystals (crystals with a hollow interior) form. The rapid growth in the case of the ridge texture should not have been influenced by the supercooling rate, since ridge textures grew on the edge areas of larger, only slowly cooling plutonic bodies. A high level of undercooling generally delays the start of homogeneous crystallization (see delayed boiling ). If the crystallization process finally starts, it is preferably carried out heterogeneously on crystals, mineral or rock fragments that have already formed. The crystals grow very quickly and form elongated, dendritic crystals in layers or shells.

The water content of the melt undoubtedly also plays a major role. In addition to the viscosity , a high water content primarily lowers the liquidus temperature (ie the temperature of melting or initial crystallization) and therefore also has a retarding effect on crystallization, which in turn results in very high supercooling.

Consequently, ridge textures often arise in the contact area with orbicular magmas. These magmas are low-viscosity, fast-flowing, low- density melts that convectively rise up in tube-like structures. Comb textures can also be found in pegmatites , which represent an environment very rich in fluids. Also gabbroische location intrusions lead comb textures as so-called Harrisitische textures to floors and walls.

Conclusion

The formation conditions of comb textures have arisen from a complex interplay of several factors and depend on the one hand on external physical environmental parameters and on the other hand on the internal chemical properties of the molten liquid.

Externally, a generally high crystal growth rate (G), as indicated by branched and unidirectional crystals, and strong overheating , which prevents crystallization, although the temperature actually required for this would be given. A high geothermal gradient and a high cooling rate are also considered to be important factors. Internally, crystal growth depends directly on the concentration of the elements involved. The oversaturation therefore plays a very important role in this context. The preferred growth direction is a direct consequence of the competition for the required elementary building blocks. If their concentration decreases, then there is a gradual stoppage of growth. Only with a renewed supply can further layers grow again.

Comb textures can only emerge from melts that do not have any crystals or whose nucleation rate (N) is zero or below the amount of crystals removed. The nucleation is therefore essentially heterogeneous and starts from the walls. Strong chemical changes in the structure of the layer minerals indicate that either very rapid physical / chemical changes took place in the melt, or that the latter was renewed very quickly. Certain magmatic crest textures likely emerged from molten liquids that were wedged between the magma chamber and its wall and concentrated there.

In the UST in aplit / pegmatite associations, models currently predominate which advocate a very rapid, multiple successive crystallization. The melt is saturated with water, but is in imbalance with the resulting crystals. The repetition of the position is also explained with an oscillating eutectic or cotectic - based on an abrupt adiabatic pressure drop due to loss of fluid or degassing, and a subsequent return to normal conditions. Such conditions are realized in the overpressure dome area of intrusions .

In the sub-volcanic area, McCarthy and Müntener (2016) highlight the following two points:

  • Ridge layers (and also orbicular textures) grow in sub-volcanic melt formation zones. Their crystallization occurs as a result of the depressurization of superheated melts.
  • The growth of ridge layers and orbicular textures is controlled by the volatile components of the melt (dissolved liquids and gases) as well as by adiabatic ascent routes.

The melt formation can take place very quickly - over a period of months to years.

Two model concepts are now being discussed as general formation mechanisms for comb textures:

  • Oscillating comb layers as an expression of a self-organizing system . Rapid crystal growth leads to the formation of a boundary layer , which in turn induces the growth of further layers.
  • External parameter changes. These can be caused by melting pulses, with each individual pulse representing a new layer growth.

References

Since comb textures occur worldwide, only a few striking examples are selected here:

See also

literature

  • R. Gill: Igneous Rocks and Processes, a practical guide . Wiley-Blackwell, 2010 (English).
  • JG Moore and JP Lockwood: Origin of comb layering and orbicular structure, Sierra Nevada batholith, California . In: Geological Society of America Bulletin . tape 84 , 1973, pp. 1-20 (English).
  • Ron H. Vernon: A practical guide to Rock Microstructure . Cambridge University Press, 2004 (English).

Individual evidence

  1. ^ A b J. G. Moore and JP Lockwood JP: Origin of comb layering and orbicular structure, Sierra Nevada batholith, California . In: Geological Society of America Bulletin . tape 84 , 1973, pp. 1-20 .
  2. ↑ a b J. R. Shannon, BM Walker, RB Carten and EP Geraghty: Unidirectional solidification textures and their significance in determining relative ages of intrusions at the Henderson Mine, Colorado . In: Geology . tape 10 , 1982, pp. 293-297 .
  3. ↑ a b V. S. Kormilitsyn and MM Manuilova: Rhythmic banded quartz porphyry, Bugdai Mountain, southeast Transbaykal region (in Russian) . In: Zapiski Vsesoyouz Mineral Obsch . tape 86 , 1957, pp. 355-364 .
  4. ^ A b C. H. Donaldson: Laboratory duplication of comb layering in the Rhum pluton . In: Mineralogical Magazine . tape 41 , 1977, pp. 323-336 .
  5. ↑ GE Lofgren and CH Donaldson: Curved branching crystals and differentiation in comb-layered rocks . In: Contrib. Mineral. Petrol. tape 49 , 1975, pp. 309-319 .
  6. ↑ D. London: The application of experimental petrology to the genesis and crystallization of granitic pegmatites . In: Canad Mineral . tape 30 , 1992, pp. 499-540 .
  7. ^ DG Durant and AD Fowler: Origin of reverse zoning in branching orthopyroxene and acicular plagioclase in orbicular diorite, Fisher Lake, California . In: Mineral. Mag. Band 66 (6) , 2002, pp. 1003-1019 , doi : 10.1180 / 0026461026660073 .
  8. ↑ F. Faure, G. Trolliard, C. Nicollet and J.-M. Montel: A developmental model of olivine morphology as a function of the cooling rate and the degree of undercooling . In: Contribution to Mineralogy and Petrolog . tape 145 , 2003, p. 251-263 .
  9. ^ Ron H. Vernon: Possible role of superheated magma in the formation of orbicular granitoids . In: Geology . tape 13 , 1985, pp. 843-845 .
  10. ^ R. Gill: Igneous Rocks and Processes, a practical guide . Wiley-Blackwell, 2010.
  11. ^ K. Breiter, A. Müller, J. Leichmann and A. Gabašová: Textural and chemical evolution of a fractionated granitic system: the Podlesí stock, Czech Republic . In: Lithos . tape 80 , 2005, pp. 323-345 .
  12. ↑ VN Balashov, GP Zaraisky and R. Setlmann: Fluid-magma interaction and oscillatory phenomena during crystallization of granitic melt by accumulation and escape of water and fluorine . In: Petrology . tape 8 , 2000, pp. 505-524 .
  13. ↑ Douglas John Kirwin: Unidirectional solidification surfaces associated with intrusion-related Mongolian mineral deposits . In: IAGOD Guidebook Series . tape 11 . London 2005, p. 63-84 ( [1] ).
  14. ↑ a b Anders McCarthy and Thomas Müntener: Comb layering monitors decompressing and fractionating hydrous mafic magmas in subvolcanic plumbing systems (Fisher Lake, Sierra Nevada, USA) . In: Journal of Geophysical Research: Solid Earth . tape 121 , 2016, doi : 10.1002 / 2016JB013489 .
  15. ^ DG Durant: Orbicular Diorite of Fisher Lake, California: Reverse Zoning and Oscillatory Precipitation Mechanisms . Univ. of Michigan, Ann Arbor 2001, pp. 224 .
  16. ↑ RW Smillie and RE Turnbull: Field and petrographical insight into the formation of orbicular granitoids from the Bonney Pluton, southern Victoria Land, Antarctica . In: Geol. Mag. Band 151 (03) , May 2014, pp. 534-549 , doi : 10.1017 / S0016756813000484 .
  17. ↑ L. Aguirre, FA Hervé and M. Del Campo: An orbicular tonalite from Caldera, Chile . In: J. Fac. Sci. Hokkaido Univ. tape 17 (2) , 1976, pp. 231-260 .
  18. ^ Arthur Gibbs Sylvester: The nature and polygenetic origin of orbicular granodiorite in the Lower Castle Creek pluton, northern Sierra Nevada batholith, California . In: Geosphere . Vol. 7, No. 5 , 2011, p. 1134–1142 , doi : 10.1130 / GES00664.1 .
  19. ↑ K. Katz and J. Keller: Comb-layering in carbonatite dykes . In: Nature . Vol. 294, 1981, pp. 350-352 .
  20. ^ JS Petersen: Columnar-dendritic feldspars in the lardalite intrusion, Oslo Region, Norway: 1. Implications for unilateral solidification of a stagnant boundary-layer . In: J. Petrol. tape 26 (1) , 1985, pp. 223-252 , doi : 10.1093 / petrology / 26.1.223 .
  21. ^ Jacob B. Lowenstern and W. David Sinclair: Exsolved magmatic fluid and its role in the formation of comb-layered quartz at the Cretaceous Logtung W-Mo deposit, Yukon Territory, Canada . In: Transactions of the Royal Society of Edinburgh: Earth Sciences . tape 87 , 1996, pp. 291-303 ( [2] ).
  22. ↑ Sven Hönig, Jaromír Leichmann and Milan Novák: Unidirectional solidification textures and garnet layering in Y-enriched garnet-bearing aplite-pegmatites in the Cadomian Brno Batholith, Czech Republic . In: Journal of Geosciences . tape 55 , 2010, p. 113-129 , doi : 10.3190 / jgeosci.065 .
  23. ↑ K. Breiter: From explosive breccia to unidirectional solidification textures: magmatic evolution of a phosphorus and fluorine-rich granite system (Podlesí, Krušné Hory Mts., Czech Republic) . In: Bull Czech Geol Survey . tape 77 , 2002, p. 67-92 .
  24. ↑ D. Bosch et al .: Lithospheric origin for Neogene-Quaternary Middle Atlas Lavas (Morocco): Clues from trace elements and Sr-Nd-Pb-Hf isotopes . In: Lithos . Vol. 205. Elsevier, 2014, p. 247-265 .
  25. ↑ WH Taubeneck and A. Poldervaart: Geology of the Elkhorn Mountains, northeastern Oregon: Part2. Willow Lake intrusion . In: Geol.Soc. At the. Bull. Band 71 (9) , 1960, pp. 1295-1322 , doi : 10.1130 / 0016-7606 (1960) 71 [1295: gotemn] 2.0.co; 2 .
  26. ↑ D. Jargalsaihan: Metallic Mineral Deposits . Ed .: D. Jargalsaihan et al., Guide to the Geology and mineral resources of Mongolia. Ulaan Baatar 1996, p. 167 .