Minette

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The minette is a potassium-rich alkaline rock of igneous origin from the group of the Shoshonite lamprophyres .

Etymology and history

An oolithic iron ore that was mined in Luxembourg and Lorraine was originally referred to as minette . Léonce Élie de Beaumont used the term for the first time in 1822, but to describe a species of lamprophyrus.

Outward appearance and demeanor

Minette in thin section

The minette is a predominantly greenish-gray to black, sometimes also brownish, mesotypical , more rarely melanocratic duct rock with a porphyry texture. Larger intruders are minerals of the phlogopite - biotite series, and occasionally amphibole . Diopside augite and olivine pseudomorphoses function as smaller insects . The dense basic mass consists of the aforementioned minerals as well as sanidine and subordinate plagioclase . The structure of the basic mass is variable; its panallotriomorphic to pan idiomorphic feldspar sub-structure can be developed like an ice flower. Also globular and spherulitic textures are often present. The mica inmates sometimes show fluidic regulation.

There are also some examples of minette lava flows such as in the Navajo Volcanic Field . These are of particular interest for petrology , since dike rocks are mostly subject to hydrothermal changes and are therefore less informative.

Minettes can also be present as inclusions in foreign rock, inclusions in syenite and Rapakivi granite are mentioned .

The calculated pressure and temperature ratios of minette magmas before the start of the eruption are 1138 ° C (Si-rich) to 1144 ° C (Si-poor) and 12 to 16 ± 4 kilobars.

classification

As a lamprophyrus, the minette is not suitable for a classic classification based on the QAPF or TAS diagram . The differentiation of the individual lamprophyres takes place with the help of the actual mineral inventory. In its light component, the minette has feldspars , whereby the predominance of alkali feldspar over plagioclase is decisive. The dark component is dominated by biotite / phlogopite, followed by hornblende . Diopside Augite and possibly olivine can be added.

The minette is very similar to the kersantite in terms of its mineral composition , but the latter has a plagioclase predominance as a distinguishing criterion. Also for Vogesit are similarities, however, does not carry Vogesit biotite / phlogopite. With kersantite, vogesite and spessartite , the minette forms the group of the relatively SiO 2 -rich Shoshonite lamprophyres .

composition

The SiO 2 content of the minettes can vary in a relatively wide range between 50 and 60%; H. Minettes have a mafic to intermediate composition. They can therefore be both undersaturated and saturated with SiO 2 . The most striking feature, however, is their usually very high K 2 O content of 5 to 7.5%, sometimes even up to over 9%. Their total alkaline content is around 7 to 9%, but can increase up to 12%. The minettes therefore mostly fall into TAS fields S2 and S3 and are therefore equivalent to Shoshonites and Latites due to their potassium predominance . You can also switch to fields U2 and U3 ( Phonotephrite and Tephriphonolite ).

Most of the minettes are hypo- to hyperaluminous , relatively rare representatives also peralkaline .

mineralogy

According to their definition, minettes have the following mineral composition:

Dark component:

Light component:

In addition to biotite, sanidine and plagioclase, quartz (can reach up to 20 percent by volume) and accessories such as apatite , barite , chromite , rutile , spinel and zircon are found in the base mass . Furthermore, the secondary minerals calcite , chlorite , dolomite , magnesite and sericite as well as finely divided ore minerals, mostly as hematitic pigment, but also as ilmenite or pyrite .

The clear zonal structure in mica suggests imbalances during crystallization.

Conversion processes usually include the mica, which is partially or completely chloritized, the olivine, which is usually completely pseudomorphically replaced by actinolite , chlorite, carbonates, quartz and talc , and the diopside augite, which is converted into chlorite and / or calcite.

Chemical composition

The following table contains an average of 66 global Minetten analyzes, an average of 18 analyzes from Jersey and an average of 2 analyzes from Jáchimov; the averaged trace elements come from 10 analyzes from Queen Maud Land in Antarctica , one analysis from Jersey and 2 analyzes from Jáchimov:

oxide Average value (wt.%) Jersey (wt.%) Jáchymov (wt.%) Trace elements Antarctica (ppm) Jersey (ppm) Jáchymov
SiO 2 53.14 53.79 57.32 Pb 27.5
TiO 2 1.41 1.12 1.02 Cu 40.6 50 14.5
Al 2 O 3 14.40 12.53 15.37 Ni 189 414 77.5
Fe 2 O 3 3.39 3.06 3.16 Cr 673 574 246
FeO 4.32 3.86 3.67 V 299 140 122.5
MnO 0.13 Zr 149 647 430.5
MgO 7.17 8.34 5.60 Y 26.5 93 31
CaO 6.83 7.20 4.20 Sr 812 2034 450
Na 2 O 2.20 1.63 2.10 Ba 3073 6027 1683.5
K 2 O 5.70 6.94 5.65 Rb 59 301.5
P 2 O 5 1.49 1.02 Nb 9.5 21st 15th

Note: The analysis values ​​are free from volatiles . In minettes, water and carbon dioxide are between 2 and 8 percent by weight.

Clearly recognizable is the predominance of potassium in the minettes, as well as a high to extremely high accumulation of incompatible elements such as barium , phosphorus , strontium , vanadium , zircon and also generally of rare earths .

Origin and Association

The mostly mafic character of the minettes suggests an origin in the earth's mantle of the lithosphere . The very high content of incompatible elements and in particular LILE with simultaneous depletion of HFSE allows only a very low degree (<1%) of partial melting . The parent rock is probably garnet peridotite or a mantle peridotite metasomatosed by liquids saturated with potassium . In the latter case, this allows a somewhat higher melting rate. The metasomatizing fluids themselves come from recycled continental crust . A connection to previous subduction processes is also being considered in this context. The melting process very likely took place under a high H 2 O / CO 2 ratio. This assumption is also supported by the enrichment of the minettes in LREE (light rare earths ), which in turn indicates enriched mantle rock.

Hybridization of Dacite and Lamproitic magma is also possible.

Minette tunnels mostly appear as late phases in granitoids ( granites , granodiorites , monzonites and monzogranites ).

Varieties

Occurrence and locations

In Germany there are minette walks in the Alb valley of the southern Black Forest and in the Upper Palatinate near the KTB .

In addition to its type locality in France ( Vosges ), minettes can be found in the following regions:

Individual evidence

  1. ↑ LVS Nardi, J. Pla Cid, MF Bitencourt: Minette mafic microgranular enclaves and their relationship to host syenites in systems formed at mantle pressures: major and trace element evidence from the Piquiri Syenite Massif, southernmost Brazil. In: Mineralogy and Petrology. Vol. 91, No. 1-2, 2007, pp. 101-116.
  2. ^ VT McLemore, OT Rämö, MA Hamilton, PJ Kosunen, M. Heizler: Geochemistry and geochronology of Proterozoic comingled Jack Creek rapakivi granite and minette in the northern Burro Mountains, Grant County, New Mexico. In: 2002 Denver Annual Meeting (October 27-30, 2002).
  3. ^ SN Feldstein, Lange. R .: Pliocene Potassic Magmas from the Kings River Region, Sierra Nevada, California: Evidence for Melting of a Subduction-Modified Mantle. In: Journal of Petrology. Volume 40, Issue 8, 1999.
  4. ↑ D. Metais, F. Chayes: Varieties of lamprophyre. In: Carnegie Inst. Wash. Year Book. 62, 1963, pp. 156-157.
  5. ↑ AM Dayal, SM Hussain: Sr – Nd isotopic composition of lamprophyre dykes from Queen Maud Land, East Antarctica. NGRI, Hyderabad 1999.
  6. ^ RH Mitchell, SC Bergman: Petrology of Lamproites. Plenum Press, 1991, ISBN 0-306-43556-X .
  7. ↑ SW Bachinski, RB Scott: Geochim. Cosmochim. Acta. 43, 1979, pp. 93-100.
  8. ^ D. Prelevic, SF Foley, V. Cvetkovi, RL Romer: Origin of Minette by Mixing of Lamproite and Dacite Magmas in Veliki Majdan, Serbia. In: Journal of Petrology. Volume 45, Number 4, 2004, pp. 759-792.

swell

  • W. Wimmenauer: Petrography of igneous and metamorphic rocks. Ferdinand Enke Verlag, Stuttgart 1985, ISBN 3-432-94671-6 .

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