The development of a distinct crystal shape of an accessory zircon is strongly influenced by the chemical composition of the generating melt (e.g. Caruba, 1975; Pupin, 1980; Vavra, 1990; Benisek and Finger, 1993). Therefore zircon morphology and typology studies are commonly used for petrogenetic investigations. Pupin and Turco (1972) established a method for the statistical evaluation of distinct crystal shape types. This method is based mainly upon the relative sizes of two prism faces ({110} and {100}) and two pyramidal faces ({211] and {101}) respectively. The distinct crystal shape types are plotted in a typological grid commonly known as the "Pupin-diagram" (Pupin and Turco, 1972; Pupin, 1980).
Using the statistical evaluation of the external shape types of zircon populations, however, only the latest stage of zircon crystallisation is observable. Using the cathodoluminescence technique the internal structures (growth zoning) of zircons can be studied. Combined cathodoluminescence investigation of sections oriented parallel and perpendicular to the zircon-c-axis through the center of the crystals, enables the reconstruction of internal evolutionary trends of the crystal shape. Plotted as "tracks" in the common PUPIN-diagram of zircon typology these trends can help to elucidate the magma evolution during zircon crystallisation.
The described method was applied to several magmatic rock types from Austria and Germany. One example is discussed more in detail here: The "K1-Gneiss" of the Felbertal tungsten deposit (Tauern Window, Austria) displays high concentrations of Rb, Cs, U, W, and other elements. According to Eichhorn (1995) these geochemical characteristics can be modelled to be presumably due to extensive contamination of the original K1-magma. Our zircon investigations reveal a jump in the internal evolutionary trend from {100}-dominated to {110}-dominated zircon shape types, while there is no evolution existent regarding the pyramidal faces (both stages are dominated by the {101}-pyramid). These characteristics can be attributed to a significant increase of the U-content according to Benisek and Finger (1993). This points to a quite spontaneous change in melt composition and therefore is a hint for a rather important event in the petrogenetic evolution of the K1-Gneiss protolith. These data do strongly support the model of Eichhorn (1995).
As shown in the example of the K1-Gneiss the internal evolutionary trends of the zircon crystal shape display information about the magma evolution during zircon growth. Events which modify the primary melt composition like contamination, assimilation, fractionation and magma-mixing can become "visible" by zircon investigations. Due to these possibilities zircon investigations can serve as a tool to supplement geochemical studies.
Benisek, A. & Finger, F., Contr. Min. Petr. 114, 441-451 (1993).
Caruba, R., Thesis, 143 pp., Univ. Nice (1975).
Eichhorn, R., Münchner Geologische Hefte 15, 1-78 (1995).
Pupin, J.-P., Contr. Min. Petr. 73, 207-220 (1980).
Pupin, J.-P. & Turco, G., C. R. Acad. Sc. Paris 275, 799-801 (1972).
Vavra, G., Contr. Min. Petr. 106, 90-99 (1990).