Chemistry Reference
In-Depth Information
feldspar. Biotite often begin to crystallize in the same time as amphibole and
plagioclase, of intermediate composition. Muscovite appears at the end of
crystallization with quartz and feldspar.
The biotite is thus a very common constituent of intermediate and
acidic igneous rocks, volcanic and plutonic, silica saturated or not, alumina
saturated or not. It is particularly characteristic of the intermediate meta-
aluminous rocks. Its composition depends, of course, on the composition of
the host rock, in which is often the major ferro-magnesian mineral.
In a series diorite - granodiorite - granite - pegmatite, biotite varies
from more magnesian and titanium-richer terms in ibasic, to intermediate
rocks, to iron-richer terms in acidic rocks. Alumina contents also increases:
in the case of fractional crystallization under the influence of minerals such
as pyroxene, amphibole (or less aluminous biotites), differentiated magmas
are enriched in aluminum, which leads to the crystallization in the final
terms of aluminous biotite and muscovite.
The composition of biotite also reflects the type of magmatic series.
Compare, for instance, primary magmatic biotites in various magmatic
suites:
Sain.-Arnac granite (Pyrenees Orientales, France): calc-alkaline granite
massif emplaced at an higher level of the crust (roof in the chlorite zone)
(Touil, 1994):
Ansignan charnockitic granite (Pyrenees Orientales, France): granulite
facies (Touil, 1994).
Laouzas peraluminous magnesio-potassic granite (Montagne Noire,
France) emplaced in K-feldspar
+
sillimanite zone (Demange, 1982 and
unpublished data).
Sierra dos Orgaõs garnet granite tholeiitic (Rio de Janeiro, Brazil) lately
(post-metamorphism) emplaced in rocks in amphibolite facies at the
base and of greenschist facies rocks at the roof (Demange and Mach-
ado, 1998).
Variations in biotite composition in each series is compatible with the
model of fractional crystallization: decreases of titanium content, increases
of Fe/Fe
Mg ratio with differentiation. The comparison between these
sequences shows:
+
The influence of the level of emplacement: titanium content is increased
as the level of emplacement is deeper, that is the temperature of the host
rocks is higher. In Ansigan charnockitic granite alone, the only stable
biotites, stable at the magmatic stage, are magnesian biotites;
the magnesian (Laouzas) or ferriferous (Orgaõs) character of the series;
note that the presence of iron-rich garnet in the Orgaõs series is more
the consequence of the high level of iron in this series than any peralu-
minous character;
