Schlieren (Petrology)

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Schlieren are planar, petrological anisotropies and, according to primary layer textures, very important igneous large structures in granites and related rocks. It is a question of inhomogeneities in the rock structure that emerge in a vague and diffuse manner as dark, elongated or striped, zonal bands and garlands. Compared to the host pluton , they differ in their very different chemical / mineralogical composition, but also in their different structure and color. Often, streaks also delimit more complex geometric structures such as inclusions . The way they originate is generally based on magmatic flow movements that they trace.

etymology

Igneous streaks in the coarse-grained facies of the Piégut-Pluviers-Granodiorite near La Chapoulie, Nontron

The German word Schliere originally referred to an impurity or cloudiness in glasses . The German term was first introduced into English in 1906 by Grove Karl Gilbert to denote layered mineral secretions.

description

Schlieren are an expression of foliation in granitoids. In the most extensive to ribbon-uneven, stretched in the gate and moved appearing Srukturen accumulate during magmatic viscous flow coarse-grained mafic minerals to - mainly biotite and hornblende , but also accessory minerals with rare earths such as Allanit , titanite and apatite and zircon . Megacrystals of alkali feldspar can also accumulate. The iron-magnesium minerals are available as flat disks, flat to linear blades or as purely linear pencil shapes. They can be enriched in streaks up to a factor of 10 and more. Biotite and hornblende usually have no more than 4 percent by volume in granites in Modus, but can reach an average of 40 or even up to 60 percent by volume in Schlieren.

The length dimension of Schlieren ranges from meters to kilometers. Your contact with the host rock is sharp in the majority of cases, but can also take place as a gradual transition. An asymmetry can also often be observed, with a sharp boundary on one side and a continuous transition on the other. The thickness of streak bands or streak packages is usually in the decimeter range.

Addressing Schlieren should be done carefully, as compositional banding can sometimes look very similar.

Structures

Streaks in the coarse-grained facies of the Saint-Mathieu-Leukogranite

In addition to flat-lying, sometimes very incident steep, flat belts, other geometries are known - see, for example zwiebelschalig bands arranged sequences tubes (so-called ladder programs , English ladder dikes or streak tubes - were of Ernst Cloos called bow streaks), rings ( English schlieren rings ), ellipses or ellipsoids, spirals, snail-like structures and also spider-shaped arrangements. Other streak structures reminiscent of sediments are scouring and backfilling (English scour-and-fill ), curved streak packages that are cut off by higher lying packages and landslides (English slumps ).

Individual streaks and streak packages are often curved in their horizontal extension. They can split up, reunite and also form very irregularly composed structures.

Once formed, streak structures can later be penetrated by new magma pulses or displaced in synmagmatic disturbances by settlement movements within the solidifying magma chamber. Sometimes mafic streaks are also associated with locally limited secondary enamel secretions from the host pluton. In this case they are to be interpreted as restite or melanosome.

A Schlieren arch (English schlieren arch ) marks an intrusive body, whose streaks, which trace the flow movement, are concentrated at the edge of the intrusion, but which are practically absent inside. In the Schlierendom (English schlieren dome ) the Schlieren layers are concentrated in a vertex inside the intrusion. Schlieren troughs (English schlieren troughs ) are arch-like, trough-shaped depressions / depressions filled with streaks, which can often have gradations within the streaks.

Emergence

The way in which streaks develop has not been fully clarified in all cases, but they can generally be interpreted as expressions of the movements inside a magma chamber . They therefore reflect viscous deformations that took place before the final freezing of the granitic crystal pulp (with 50 and more percent crystals).

Considered in detail, possible causes are primarily differential magmatic flow movements (especially convective in nature - the movements are not always laminar, but occasionally also turbulent - i.e. curling, vortex-shaped or tubular.) Further causes are the viscous shearing of xenolithic inclusions , the Sinking of inclusions in the magma slurry and the rising of bubbles during the degassing phase. According to the Bagnold effect, the flow movements lead to sorting according to grain size and density (English flow sorting ), but no plastic deformation of the crystals that are formed (note: Bagnold's results are, however, now being questioned). The resulting Schlieren layers can show graded stratification, reverse graded stratification and also oblique stratification (a very similar effect can also be achieved by the gravitational sinking movement of the growing crystals). Conceivable mechanisms of streak accumulation are also filter-like pressing of the melt (English filter pressing ) and also diffusion .

Concentric Schlieren forms are interpreted as vortex structures that were formed by ascending or descending currents. The snail shapes probably document the movement of large bubbles through the crystal slurry.

Conclusion

As a result, there are a number of processes that contribute to the formation of streaks. These include above all the flow movements of the solidifying magma mush, gravitational sinking movements, repeated magma injections and magma pulses, penetration of small magma diapirs and insufficient homogenization in advanced anatexis .

In spite of a relatively intensive scientific examination of the way streaks originate, there is still a lack of fully developed studies that relate their nature to the growth and thermal development of the magma chamber.

meaning

The importance of streaks lies in the fact that they are often the last physical process that acted on the magma. They are therefore decisive for determining the magma chamber dynamics. In addition, they can provide very helpful pointers for determining the time of intrusion and crystallization in comparison to the regional deformation history.

Occurrence

Streaks appear worldwide in igneous intrusive rocks , especially in granitoids and their relatives. They also occur in migmatites , which are then referred to as streak migmatites . In them, biotite-rich relics of the paleosome - amphibolites or calcium silicate rocks - have dissolved into twisted streaks.

Fine examples of streaks can be found in Cathedral Peak granodiorite in the Sierra Nevada Mountains of the United States . Steep streaks associated with inclusions characterize the Vinalhaven Intrusion in Maine .

Annular streaks can be seen in the South Mountain Batholith on Nova Scotia . Interesting streaks associated with orbicular textures appear in the Ploumanac'h granite in Brittany and in the migmatites of the French migmatite terran of Saint-Malo . The Tavares pluton in Brazil contains numerous ladder passages. Ellipsoidal streaks mark the Flores stick in the Borborema province of Brazil.

The intrusive complex around the Fürstensteiner Diorit in the Bavarian Forest houses tube-like streaks.

Individual evidence

  1. ↑ Ernst Cloos: The Sierra Nevada Pluton in California . In: New Yearbook for Mineralogy, Geology, and Paleontology . 76, part B, 1936, p. 355-450 .
  2. ↑ M. Barrière: On curved laminae, graded layers, convection currents and dynamic crystal sorting in the Ploumanac'h (Brittany) subalkaline granite . In: Contributions to Mineralogy and Petrology . tape 77 , 1981, pp. 214-224 .
  3. ↑ C. Dietl, H. de Wall and F. Finger: Tube-like schlieren structures in the Fürstenstein Intrusive Complex (Bavarian Forest, Germany): evidence for melt segregation and magma flow at intraplutonic contacts . In: Lithos . tape 16 , 2010, p. 321-339 .
  4. ↑ DB Clarke, D. Grujic, KI McCuish, JCP Sykes and FM Tweedale: Ring streak: description and interpretation of field relations in the Halifax pluton, South Mountain Batholith, Nova Scotia . In: Journal of Structural Geology . tape 51 , 2013, p. 193-205 .
  5. ^ RA Bagnold: Experiments on a gravity-free dispersion of large solid spheres in a Newtonian fluid under shear . In: Proceedings of the Royal Society of London . A225, 1954, pp. 49-63 .
  6. ↑ MI Hunt et al: Revisiting the 1954 suspension experiments by RA Bagnold . In: Journal of Fluid Mechanics . tape 452 , 2002, p. 1-24 .
  7. Jump up ↑ SR Paterson: Magmatic tubes, pipes, troughs, diapirs, and plumes: late-stage convective instabilities resulting in compositional diversity and permeable networks in crystal-rich magmas of the Tuolumne Batholith, Sierra Nevada, California . In: Geosphere . tape 5 , 2009, p. 496-527 .
  8. ↑ J. Žák, SR Paterson, V. Janoušek and P. Kabele: The Mammoth Peak sheeted complex, Tuolumne batholith, Sierra Nevada, California: a record of initial growth or late thermal contraction in a magma chamber? In: Contrib. Mineral. Petrol. tape 158 (4) , 2009, pp. 447-470 .
  9. ^ Ron H. Vernon and Scott R. Paterson: Mesoscopic structures resulting from crystal accumulation and melt movement in granites . In: Transactions of the Royal Society of Edinburgh: Earth Sciences . 97 Issue 04, 2006, p. 369-381 , doi : 10.1017 / S0263593300001516 .
  10. ^ RA Wiebe et al .: Steep schlieren and associated enclaves in the Vinalhaven granite, Maine: possible indicators for granite rheology . In: Contributions to Mineralogy and Petrology . 2007, doi : 10.1007 / s00410-006-0142-z .
  11. ^ Fergus Tweedale: Occurrence and origin of ring schlieren in the South Mountain Batholith, Meguma zone, Nova Scotia . Dalhousie University, Halifax, Nova Scotia 2012.
  12. ↑ Sylvie Decitre, Dominique Gasquet and Christian Marignap: Genesis of orbicular granitic rocks from the Ploumanac'h Plutonic Complex (Brittany, France): petrographical, mineralogical and geochemical constraints . In: European Journal of Mineralogy . tape 14 , 2002, p. 715-731 .
  13. ^ I. Milord and EW Sawyer: Schlieren formation in diatexite migmatite: examples from the St Malo migmatite terrane, France . In: Journal of Metamorphic Geology . tape 21 , 2003, p. 347-362 .
  14. ^ RF Weinberg, AN Sial and RR Pessoa: Magma flow within the Tavares pluton, Northeastern Brazil: Compositional and thermal convection . In: Geological Society of America Bulletin . tape 113 , 2001, p. 508-520 .
  15. ↑ Viviane Oliveira de Souza, Antônio Carlos Galindo and Fernando César Alves de Silva: O Stock Flores: Exemplo de magmatismo granítico tipo-A no Domínio Rio Piranhas-Seridó, NE da Província Borborema . In: Pesquisas em Geociências . tape 44 (2) , 2017, pp. 345-366 .
  16. ^ Carlo Dietl, Hemin A. Koyi, Helga de Wall and Mark Gößmann: Centrifuge modeling of plutons intruding shear zones: application to the Fürstenstein Intrusive Complex (Bavarian Forest, Germany) . In: Geodinamica Acta . tape 19 / 3-4 , 2006, pp. 165-184 .