In this method an arrangement of
given prismatic and pyramidal crystal faces constitute a population type. The
abundance of zircon crystals related to certain types led to the setting up, in
those particular cases, of more or less numerous subtypes. So, that rocks with
comparable percentages of the same zircon type can then be easily separated -significantly-
provided that their subtype distribution is known (Pupin, 1980).
MAIN
ASPECTS OF THE TYPOLOGY METHOD
The
typologic study of zircon populations from granitic rocks lead to the
proposition of a genetic classification with three main divisions: (1) granites
of crustal or mainly crustal origin [(sub) autochthonous and aluminous granites)];
(2) granites of crustal + mantle origin, hybrid granites (calc-alkaline and sub-alkaline
series granites); (3) granites of mantle or mainly mantle origin (alkaline and
tholeiitic series granites). In detail, there are many petrogenetic variants of
each of the following granitic rocks: granodiorite, monzogranite and alkaline
granite. The variations observed with zircon typology are accompanied
petrographically by modifications of associations of other main and accessory
minerals, and on the field by the presence or absence of basic microgranular
xenoliths, associated microgranites, rhyolites or basic rocks. In the typologic
diagram, some endogenous non granitic rocks (i.e. migmatites, tonalites,
rhyolites ...) show a logical distribution with regard to different genetic
stocks of granitic rocks (Pupin, 1980).
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According to Pupin (1980), the main
types show 0, 1 or 2 prisms, i.e. {100},
{110}
in combination with either one of the three pyramids {101},
{211}
or {301},
or the arrangement {101}
+ {211}.
Main types and subtypes can be related on a square board with two variables,
depending upon the relative development of the crystalline faces (Figure 3). The
initial typologic diagram design was entirely based on the relative natural
abundances of zircon types. S-type, which is the most abundant habitus that can
be found in endogenous and exogenous rocks, forms, then, the diagram’s main
point, the other maintypes logically arranged around. Some main types stay
purely theoretical (i.e. : C, I, R, F) : their emplacement was suggested as T
and K types are mainly linked with the P and D types in alkaline rocks.
The secondary types have been derived from the above main types from which
they can be deduced by adding one or more extra pyramids, with usually a minor
development.
Regarding the more complex cases where
a crystal shows a great number of different pyramidal forms, the nomenclature
can yet be used. A bipyramidal crystal {100}
{110}
with four pyramids {101}
{211}
{112}
{311}
would be labelled SWZ, the W and Z letters listed in alphabetical order (corresponding
to the supplementary pyramids {311}
and {112}
with respect to the S principal type; the relative developments of {100}
{110}
prisms and {101}
{211}
pyramids allow the subtype characterization (SWZ1 to SW25)
as for a common S type. He also stated that among the thirty natural types eight
types are only frequent (D, G, J, L, N, P, S, U). He also stated that that the
crystal elongaton is not at all taken into account in their proposed typology.
Pupin (1980) stated that the chemical characteristics of the
crystallization medium play a leading role in the origin and relative growth of
zircon pyramids. Zircon originating in an hyperaluminous or hyperalkaline medium
show well developed {211}
pyramid whereas those grown in an hyperalkaline or hyperaluminous medium have
well developed {101}
pyramid; the Al/alkaline ratio is therefore believed to control the zircon
population index Ā, an observation which is confirmed when considering the
plutonic rocks distribution in the (I.A., I.T.) diagram (The Fig.3). The {301}
pyramid shows a distinct spreading propensity in an alkaline medium where it
occurs in the T
[{100},
{110},
{101},
{301}]
and K [{100},
{101},
{301}]
types in potassium-rich alkaline medium. On the other hand, the {112}
pyramid preferentially appears in hyperaluminous media where it can sometimes
develop prominently. As the leading factor governing the relative development of
the zircon prisms really is the temperature of the crystallization medium, they
have therefore proposed that mineral as a geothermometer.
Reference:
Pupin,
J. P., 1980. Zircon and granite petrology. Contribution to Mineralogy and
Petrology, vol. 73, p. 207-220. |
