Monchiquit

from Wikipedia, the free encyclopedia

The Monchiquit is a dark to black, sodium stressed alkaline rocks igneous origin and belongs to the group of alkaline lamprophyre .

Etymology and history

The name Monchiquit derives from its type locality , the Serra de Monchique the same Monchique in southern Portugal ( Algarve ), from. The rock was scientifically described for the first time by M. Hunter and Karl Heinrich Rosenbusch in 1890.

Outward appearance

Monchiquit warehouse aisle at Sainte Dorothée, Quebec, Canada. Interspersed with ocelli layers. Petrological Province of the Montérégie Hills

Monchiquit is a fine-grained, gray to almost black, mesotype to melanocrates rock . It generally has a pan- idiomorphic and at the same time porphyry , granular, often amygdaloid or ocellar texture with a dense, mostly glassy base. As phenocrysts act amphibole , clinopyroxene and olivine (including conversion products), sometimes biotite .

classification

Like all other lamprophyres, the monchiquit cannot be defined with the help of the stretch iron or the TAS diagram . The differentiation of the individual lamprophyres is based on their actual mineral inventory, whereby a distinction is made between light and dark minerals.

The monchiquit is characterized in its leucocratic component by the fact that there is no feldspar in the basic mass and only glass or foids appear. It has brown amphibole , titanium nugite , olivine and biotite as melanocrate components . It is therefore very similar to camptonite and sannaite , but differs in its lack of feldspar and its often glassy formation. Chemically and mineralogically, it is also very close to Limburgite , but has a different structural structure and increased water content.

composition

Selected monchiquit compositions in the TAS diagram

The SiO 2 content of Monchiquite is generally between 39 and 42% (but can rise to 47%), its content of the alkalis Na 2 O + K 2 O is between 4 and 9%, with the sodium content outweighing the potassium content. They therefore mainly fall into the TAS field U1 of the basanites or tephrites , but can also occasionally switch to the foidite field and the basalt field.

mineralogy

By definition, Monchiquite have the following mineral inventory:

As light-colored components, foids take the place of feldspars. This expresses the undersaturation of the monchiquite in SiO 2 . As Foid usually comes Analcim before, but it can also Hauyn , leucite , nepheline or Nosean back in its place. In contrast to other lamprophyren, there is also a colored or colorless glass component in the base material. It has not yet been clarified whether the analcime was primarily formed or whether it is a secondary conversion product from the glass component. Also Ocelli and Amygdalen may be present, but are not as common as in Camptoniten. They are mostly filled with calcite . Zeolites also occur as conversion products of the basic mass . If there is a deep brown biotite and there is no olivine, this indicates the Fourchit variety , named after the Fourche Mountains in Arkansas .

Accessories are apatite , magnetite , spinel , titanite and titanomagnetite .

Chemical composition

The following table shows averaged rock analyzes of Monchiquit veins from the type locality in the Serra de Monchique, from Williston in Vermont ( Lake Champlain Province) and from the Montérégie Hills in Québec .

oxide Type locality (wt.%) Vermont (wt%) Montérégie hill (wt.%) Trace elements Type locality (ppm) Vermont (ppm) Montérégie Hill (ppm)
SiO 2 43.05 40.69 42.32 Pb 6th
TiO 2 3.80 2.81 3.89 Cu 35 95
Al 2 O 3 14.56 12.33 15.03 Ni 46 205
Fe 2 O 3 (dead) 11.69 14.37 6.95 Cr 134 226
FeO 6.42 V 272 276
MnO 0.21 0.21 0.22 Zr 359 314 410
MgO 7.46 9.99 5.86 Y 37 38
CaO 10.45 13.56 11.88 Sr 1700 1605
Na 2 O 4.80 2.77 4.30 Ba 997 1188 765
K 2 O 2.83 1.94 2.17 Rb 89 55 46
P 2 O 5 1.06 1.32 0.88 Nb 111 127

Note: The analyzes listed do not contain any volatiles such as water and carbon dioxide ; the latter is usually between 4 and 8% in monchiquites.

The ultrabasic character of Monchiquite (SiO 2 less than 45%), the predominance of sodium in alkalis, increased contents of Mg, Fe and Ca and a strong enrichment in the incompatible elements Nb, P, Sr, V and Zr can be clearly recognized .

Emergence

Thin section of the monchiquit storage corridor of Sainte Dorthée, Quebec, Canada. Crossed polarizers.

Due to their ultramafic to mafic character, as evidenced by low SiO 2 values ​​and fractional crystallization of olivine and clinopyroxene, it can be assumed that monchiquites emerged from the shell material through partial melting . Their negative REE pattern , their enrichment in LREE (light rare earths) and incompatible elements point to enriched mantle rock, very likely garnet peridotite or garnet lherzolite as parent rock . Enriched mantle rock is found either in the lithosphere in a depth range of 100 to 200 kilometers or in deeper mantle areas below 660 kilometers (below the asthenosphere ).

The ascent mechanism is possibly coupled to a hotspot mantle plume, but deep-reaching tectonic disturbances are also conceivable, which cause melting with a decrease in pressure.

Occurrence and locations

The monchiquite is a typical gangue rock in alkali rock complexes ( alkali isenites , alkali gabbros ). But it can also form lava flows , for example in the Hopi Buttes in Arizona , or small intrusion bodies . In addition to Alnöites , Polzenites and ultra- basic alkaline gangue rocks, it occurs in nephelinitic - carbonatitic associations. In fact , it occurs relatively frequently in carbonatites .

Monchiquit is mostly associated with Camptonites , occasionally also with Limburgites .

The only occurrences of monchiquit in Germany are sub-volcanic passages on the Horberig near Oberbergen in the Kaiserstuhl .

Monchiquit is found in the aforementioned type locality in Portugal , Australia ( Victoria ), Brazil ( Fernando de Noronha , Minas Gerais ), Germany ( Kaiserstuhl ), England ( Jersey ), France ( Haute-Garonne ), India ( Kathiawar , Meghalaya ) , Iran , Cameroon (upper reaches of the Benue ), Canada ( Ontario , Québec ), Morocco ( Taourirt ), Poland ( Outer Western Carpathians ), Scotland ( Caithness , Inverness-shire ), Czech Republic ( Ústí nad Labem ), Turkey , Hungary , the USA ( Arkansas , Arizona - Hopi-Buttes-Volcanic-Field , Montana , New England States , New Mexico - Navajo-Volcanic-Field ) and Wales ( Monmouthshire ).

Individual evidence

  1. ↑ M. Hunter and H. Rosenbusch (1890): About Monchiquit, a Camptonite gangue from the retinue of the Eläolithsyenite. Tschermaks Mineralogical and Petrographic Communications. Vienna. Vol. 11, 2nd Ser., Pp. 445-466.

swell

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

Web links