Myrmecite

Myrmekite is a microscopic growth of worm-shaped quartz in plagioclase . The diameter of the worm-like quartz stems or quartz tubes is usually well under a millimeter. This intergrowth structure of myrmecite with plagioclase is usually in contact with alkali feldspar . Myrmekite forms under metasomatic conditions in conjunction with tectonic deformations . Under no circumstances should it be confused with micrographic adhesions such as writing granite or with granophyric adhesions, as these are of magmatic origin.

etymology

The name Myrmekit is derived from the Greek μὑρμηχἰα (wart) or from μὑρμηξ (ant). It was used for the first time in 1899 by Jakob Johannes Sederholm in a scientific description of this structure.

Emergence

A total of four different ways of creating myrmekite can be distinguished:

• Potassium metasomatosis of primary, zoned plagioclases
• Calcium metasomatosis on primary, albite-rich plagioclases (in anorthosites)
• Sodium and calcium metasomatosis in primary alkali feldspars
• Calcium and sodium removal from cataclastically stressed, primary, zoned plagioclases

The last mode was described recently (2018) by Lorence G. Collins .

Formation of myrmecite during potassium metasomatosis

Different types of myrmecite can form during potassium metasomatosis:

• Edge myrm kit
• Wart myrm kit
• Ghost myrm kit

Edge myrm kit

Edge myrmkite on zoned plagioclase surrounded by interstitial microcline (gray and black)

Randmyrmekit , engl. rim myrmekite , arises during the initial stage of potassium metasomatosis in cataclastically relatively weakly deformed magmatites. The fractures mainly start at the grain boundaries. This enables, for example, potassium metasomatosis with hot potassium solutions to penetrate the edges of zoned plagioclase crystals. As a consequence, marginal myrmite forms on the plagioclase crystal as well as interstitial alkali feldspar. Due to the low anorthite content of the plagioclase margins, the quartz stems are only very thin.

The chemical changes remain limited to the sub-millimeter range at this stage and are therefore relatively small. However, there are transitions to the next stage of the wart myrm kit.

Examples of marginal myrmecite are found in plutons in the Sierra Nevada .

Wart myrm kit

Wart myrmite in a megacrystal-bearing quartz monzonite from Twentynine Palms , California

With increasing tectonic tensions, the cataclasis is also intensified. Fractures can now spread to the inside of the crystal and crystals can be bent. As a result, potassium metasomatosis can also progress further. Ultimately, the plagioclase is almost completely or completely displaced by alkali feldspar. At locations with incomplete displacement forms Warzenmyrmekit (Engl. Wartlike myrmekite ).

There are transitions between rocks that only lead to marginal myrmecite to those with both marginal myrmecite and warty myrmect and finally to those with only warty myrmecite. A very important observation is the correlation between the quartz stem thickness (diameter of the worm-shaped tubes) and the calcium content of the original plagioclase in the unchanged igneous parent rock. The thickest stems occur in rocks with the highest calcium content in plagioclase.

An example of the occurrence of wart myrmite is the quartz monzonite from Twentynine Palms in California .

Ghost myrm kit

Ghost myrmite in the Rubidoux Mountain Leuco granite in California

This is the third type of quartz-plagioclase adhesions during potassium metasomatosis in granitoids. This type is also dependent on tectonic deformations. With him there is an unbalanced removal of calcium, sodium and aluminum from the deformed plagioclase grid. This in turn creates an imbalance in the relative ratio of the remaining aluminum and silicon . There is an excess of silicon which cannot be built into the crystal structure by the alkali feldspar that displaces the plagioclase. This silicon excess then in turn forms the Geistermyrmekit (engl. Ghost myrmekite ) - there are formed either tiny Quarzovoide in leftover Albitinseln in alkali or independent, group arranged in the shape Quarzovoide (without Albitinseln) in the alkali feldspar (see figure for clarity).

The Rubidoux Mountain Leukogranite and several granodiorites in the Sierra Nevada have this structure.

Myrmecite formation during calcium metasomatosis

Spectacular Myrmecite from Alastaro, Finland

Also during calcium metasomatosis, myrmecite formation occurs under various circumstances:

• Calcium metasomatosis from deformed alkali feldspar in igneous rocks
• Calcium metasomatosis from deformed alkali feldspar in Charnockites
• Calcium metasomatosis from deformed plagioclase in anorthosites

Calcium metasomatosis from deformed alkali feldspar in igneous rocks

Cracks in alkali feldspar , filled with central quartz and marginal myrmecite during calcium metasomatosis

In this type of metasomatosis, hot, calcium-containing solutions penetrate through tectonically caused cracks into the primary alkali feldspar and react with the crystal lattice. As a result, central quartz and peripheral myrmecite form in the cracks. These displacement reactions can cover large parts of the primary alkali feldspar (up to over 60%). A characteristic feature of this type is the constant thickness of the quartz tubes; In potassium metasomatosis, however, the thickness depends on the calcium content of the plagioclase, and the tubes bend towards the alkali feldspar.

As an example of this type of metasomatosis, the megacrystal-leading granite at Alastaro in Finland can be mentioned.

Calcium metasomatosis from deformed alkali feldspar in Charnockites

The process is the same as before, the difference lies in the parent rock on which the calcium solutions act. Charnockites differ from common granitoids by the appearance of orthopyroxes ( hypersthen ) and are often of metamorphic origin or metamorphic overprint.

Examples can be found in Sri Lanka .

Calcium metasomatosis from deformed plagioclase in anorthosites

Anorthosites have practically no alkali feldspar, which is why plagioclase of this type is attacked by calcium solutions instead of alkali feldspar. The resulting myrmecite also has stems of constant thickness. In contrast to the first type, however, the quartz stems can tend towards the primary, quartz-free plagioclase. This behavior is explained by the simultaneous incorporation of sodium, which leads to an increased SiO ₂ content in the feldspar lattice.

Examples can be found in anorthosites of the so-called "layered igneous complexes" (layered intrusive bodies).

Formation of myrmecite during sodium-calcium metasomatosis

Myrmekite replaces alkali feldspar perthite during sodium-calcium metasomatosis; the worm-shaped quartz is irregular. Lyon Mountain Granite Gneiss, Ausable Forks, New York

In its first variant, this metasomatosis only affects rock inclusions in granitoids. Hot, sodium-rich liquids from the host rock in the temperature range from 450 ° C to 650 ° C react with the alkali feldspar in the inclusions to form myrmekite. During this process, there is a balancing reaction (re-equilibration) with the plagioclases in the inclusions that are poor in sodium. As a consequence, calcium is released in the plagioclase, which in turn can act on the alkali feldspar with the formation of myrmekite. Basically, this process is comparable to the calcium metasomatosis of deformed alkali feldspars described above, only that in this case the sodium-containing solutions act as a reaction trigger.

The Velay granite in the north-eastern Massif Central serves as an example of this type of metasomatosis .

In the second variant, sodium and calcium react together. The primary alkali feldspar (perthitic and common microcline) is replaced, plagioclase (albite or oligoclase) and, in some places, myrmecite are formed. This time the Myrmekit is neither worm-shaped nor curved, rather it remains limited to the interior of the plagioclase and consists of irregular spindles, ovals and arched structures.

Calcium must be present in sufficient quantities for this process to take place. This is the only way to create a relatively calcium-rich plagioclase, which in turn can provide enough SiO ₂ for myrmecite formation. If only sodium is present, there is no myrmecite formation.

An example of this process can be found in the Lyon Mountain Granite Gneiss near Ausable Forks in New York State .

Formation of myrmecite with progressive deformation

As deformation progresses in mylonitic , ductile shear zones , myrmekite is usually concentrated in the margins of sigmoidal alkali feldspar crystals. It occurs more frequently in the two sectors under tension (ie shortening). Simpson and Wintsch (1989) explain this asymmetrical arrangement of myrmekite with a dissolution reaction of the alkali feldspar, which occurs during retrograde metamorphosis, preferably at points with high differential stress. Internally, a monoclinic symmetry can be observed not only in the alkali feldspar eyes, but also in the myrmekite . The latter can then also be used as an indicator for the sense of shear independently of the alkali feldspar eyes . The asymmetrically arranged myrmekite is thus a so-called quadrant structure (engl. Quarter structure ).

For Lorence G. Collins, however, the alkali feldspar is not of primary, igneous origin, but rather through a metasomatic displacement reaction of the primary plagioclase (potassium metasomatosis in contrast to the sodium-calcium metasomatosis suggested by Simpson and Wintsch). For him, the actual deformation process goes back much further than is immediately apparent. Another conclusion, therefore, is that the shear zone mylonites emerged from primordial magmatites.

Explanations on genesis

Myrmecite adhesions find different explanations among petrologists:

• Castle & Lindsley try to interpret it with a so-called “exsolution silica-pump model”. This model is based on chemical segregation during cooling. As a result, plagioclase separated from the alkali feldspar when quartz was still mobile. The presence of magma for the formation of myrmecite is not required in their view.
• LG Collins explains Myrmekit purely metasomatically , i. H. the formation took place below the eutectic :
  • Through an exchange of tectonically deformed, primary plagioclase with secondary potassium feldspar during a potassium-accentuated metasomatosis process.
  • Through several Ca and Na-Ca metasomatosis types, which mainly affect tectonically deformed, primary alkali feldspar; an exception to this is the metasomatic change in anorthosites, in which primary plagioclase is replaced.

Occurrence

Myrmecite can occur in a large number of rocks of different origins. It is commonly found in granitoids and similar igneous rocks such as diorite and gabbro . It also occurs in metamorphic rocks, for example in gneiss of granitic composition, in anorthosites and in the orthopyroxene-rich Charnockites .

Examples

• Australia :
  • Broken Hill
  • Lachlan fold belt in New South Wales
    • Cooma Granite Complex - Silurian, 435 to 423 million years BP
    • Vologorong batholith .
• China :
  • Langshan Granite , Inner Mongolia
  • Lantian granite , Shaanxi
  • Liudaogou Granite , Fengming , Hebei
• Germany :
  • Hailbach gneiss in the Spessart - Lower Devonian , 410 million years old BP
  • Mitterteich granite in the Upper Palatinate - Upper Carboniferous, 310 million years old BP
  • Tittling granite in the Bavarian Forest - Upper Carboniferous, 324 to 321 million years old BP
  • Two-mica granite of the Fürstenstein intrusive complex in the Bavarian Forest
• Finland :
  • Alastaro granite at Alastaro
• France :
  • Ax granite in the northern axial zone of the Pyrenees - Upper Carboniferous, 301 million years BP
  • Estivaux granite in the Thiviers-Payzac unit - Lower Carboniferous, 346 million years BP
  • Velay granite in the northeastern Massif Central
• Greece :
  • Maronia-Pluton , East Macedonia and Thrace
• Iran :
  • Ghooshchi Granite and Neighboring Rocks , West Azerbaijan
• Italy :
  • Interest rate (Cima di Vila)
• Austria :
  • Karawanken Granite Pluton - Middle Triassic , 221 million years BP. Myrmecite occurs in monzonite and biotite granite.
  • Karawanken Tonalitic Pluton - Oligocene , 28 million years BP
• Slovakia :
  • Older granites (orthogneiss) of the High Tatras south of Rohác - Lower Devon, 405 million years BP, metamorphosed in the Lower Carboniferous, 363 to 347 million years BP
• Spain :
  • Corredoiras-Orthogneiss , allochthonous Ordenes complex in Galicia - metamorphosed in the Lower Devonian , 406 to 385 million years old BP
• United States of America :
  • California :
    • La Quinta quartz diorite in the San Jacinto Mountains
    • Rubidoux Mountain leuco granite in the Sierra Nevada
    • Twentnine Palms quartz monzonite at Twentynine Palms
    • Woodson Mountain Granodiorite near Temecula , Peninsular Ranges Batholith - Lower Cretaceous ( Albian ), 105 million years BP
  • Massachusetts :
    • Cape Ann granite near Cape Ann - Silurian , 435 to 415 million years BP
  • New York :
    • Lyon Mountain Granite Gneiss at Ausable Forks

See also

• Eutectic
• structure
• granite
• Granophyr
• Solidus

Web links

• ( Page no longer available , search in web archives: exchange of primary plagioclase with secondary alkali feldspar and myrmecite )

swell

• LG Collins website entirely dedicated to the Myrmekite phenomenon

Individual evidence

1. ↑ LG Collins: Hydrothermal Differentiation And Myrmekite - A Clue To Many Geologic Puzzles . Theophrastus Publications, Athens 1988.
2. ↑ LL Perchuk, TV Gerya, K. Korsman: A model for charnockitization of gneissic complexes. In: Petrology. Volume 2, 1994 pp. 395-423.
3. ^ LR Wager, GM Brown: Layered Igneous Rocks. Freeman and Company, San Francisco 1967.
4. ↑ D. Garcia, ML. Pascal, J. Roux: Hydrothermal replacement of feldspars in igneous enclaves of the Velay granite and the genesis of myrmekite. In: European Journal of Mineralogy. Volume 8, 1996, pp. 703-711.
5. ^ C. Simpson, RP Wintsch: Evidence for deformation-induced K-feldspar replacement by myrmekite . In: J. Metam. Geol. Band 7 , 1989, pp. 261-275 .
6. ↑ D. Shelley: Igneous and metamorphic rocks under the microscope . Chapman and Hall, London 1993.
7. ↑ LG Collins: K-, Na-, and Ca-metasomatism - characteristics of replacement textures associated with feldspars and ferromagnesian silicates and the formation of coexisting rim, wartlike, or ghost myrmekite . In: Geosphere, in preparation . 2013.
8. ^ RO Castle, DH Lindsley: An exsolution silica-pump model for the origin of myrmekite. In: Contributions to Mineralogy and Petrology. Volume 115, 1993, pp. 58-65.
9. ^ LG Collins: ( Page no longer available , search in web archives: Replacement of primary plagioclase by secondary K-feldspar and myrmekite. ) 1996.
10. ↑ A. Dombrowski et al .: Orthogneisses in the Spessart Crystalline Complex, north-west Bavaria: Silurian granitoid magmatism at an active continental margin . In: Geologische Rundschau . tape 84 , 1995, pp. 399-411 .