Date

2007-03-24

Author

Köksal, Serhat
Toksoy-köksal, Fatma
Göncüoğlu, M. Cemal

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Zircon typology, proposed by Pupin (1980), is a method based on examination of zircon crystal types in granitic rocks and their statistical evaluation. While providing data for the source of granitoids (e.g., crustal, hybrid, or mantle), zircon typology also suggests the evolution temperatures. In this method, at least 100-150 zircon crystals enriched from a granitoid are examined under the microscope. Crystal faces are determined and zircon crystals are described based on Pupin (1980) classification according to relative development of the pyramidal and prismal faces. Zircon crystals determined are put into the zircon typology diagram. According to the distribution of the crystal types on the typology diagram, temperature index (I.T.) can be determined, besides typological evolution trend which is used for granitoid classification. Based on this method temperature index depends on the relative change of the (110) and (100) zircon prism faces. The use of this method for petrologic and geothermometric discussions is questioned by various authors (e.g., Vavra, 1990; Benisek & Finger, 1993) and it is stated that the zircon morphologies can only characterize the physico-chemical conditions at the latest stages of granitoid evolution. Besides zircon typology, also by geothermometric studies concerning zircon, apatite and monazite saturation hypothetical approaches can be made on the emplacement temperatures of the magmatic rocks (Watson & Harrison, 1983; Harrison & Watson, 1984; Montel, 1993; Piccoli et al., 1999). In the scope of this study, by conducting zircon and apatite saturation thermometry study it is aimed to correlate obtained results with the zircon typology results, and investigation of applicability of these methods. In this scope, zircon and apatite geothermometry studies were performed on the I-type Baranadağ and Terlemez, S-type Hisarkaya and A-type Çamsarı granitoids from central Anatolia, and correlated with the zircon typology data. Zircon and apatite geothermometry data on the I-type Baranadağ granitoid ranges between 741.7 to 754.4ºC, and 877.1 to 903.4ºC, respectively (Table 1). I-type Baranadağ granitoid zircon crystals, show distribution in the range of 750-900ºC on the zircon typology diagram, and temperature index is determined as I.T.= 680 (Köksal et al., submitted), which corresponds to 850ºC. Additionally, zircon and apatite geothermometry studies on the I-type Terlemez granitoid yielded 769.6ºC, and 892.9ºC, respectively. Zircon typology study on the I-type Terlemez granitoid on the other hand, points out crystal types of 750-850ºC interval and I.T. corresponding to the 850ºC (I.T.=731; Köksal et al., submitted). When zircon and apatite geothermometry studies are compared with the zircon typology data, it is observed for these two granitoids that zircon geothermometry data correspond approximately to lowest value represented by crystal types revealed from zircon typology study, whereas apatite geothermometry data match up about the highest value, temperature index values are in between of the zircon and apatite geothermometry data. Samples Hisarkaya-a and Hisarkaya-b representing the S-type Hisarkaya granitoid are granitic rocks having low temperature indexes according to zircon typology studies. Zircon crystals from Hisarkaya-a sample distribute in the range of 600-800ºC with a I.T.=376 (Köksal et al., submitted) representing 700ºC (Table 1). Alternatively, for Hisarkaya-a sample zircon and apatite geothermometry studies give 827.9ºC and 771.3ºC, respectively. The same relationship is valid for Hisarkaya-b sample of which zircon types disclose range of 600-750ºC, and I.T.=333 (Köksal et al., submitted) corresponding to 650ºC. For the Hisarkaya-b sample zircon and apatite geothermometry results are 842.9ºC and 759.4ºC, respectively. In this granitoid I.T. values revealed from zircon typology data is lower than the data of other two methods. While zircon geothermometry presents higher results than other methods, apatite geothermometry results are nearly the highest temperature levels of zircon crystal types in this granitoid. Moreover, A-type Çamsarı granitoid show rather narrow zircon crystal distribution with high temperature index. I.T. for this rock is 795 (Köksal et al., submitted) corresponding to 900ºC, and zircon types on the typology diagram are around 850-900ºC field (Tablo 1). On the other hand, zircon and apatite geothermometry data are rather low: zircon geothermometry gives 712.8-771.0ºC, whereas apatite geothermometry presents 635.6-713.1ºC. While evaluation of the zircon typology data it should be noted that the evolution of zircon crystals within granitoids can be controlled by many other factors besides temperature. For instance during solidification of a granitoid zircon crystals may be experienced serious morphological alteration by taking trace elements within magma chamber (Vavra, 1994) or late-zircon growth in the water-rich magmas may results in the lowering of temperature index (Pupin, 1980). Accordingly, in the previous studies (e.g., Köksal et al., 2006; Köksal et al., in review) abrupt typological changes, especially in the outer zones, in zircon crystals of the rocks concerned in this study are detected. Liew & McCulloch (1985) show by empirical studies that the heat of I-type granite melt (800- 900 °C) needs higher rates than those of S-type one (≤ 700 °C). If the source characteristics are considered it seems that the zircon typology data are comparable with the apatite geothermometry data for I-tipi Terlemez and Baranadağ granitoids. Similarly for S-type Hisarkaya granitoid apatite geothermometry data are comparable with the zircon typology data. However for A-type Çamsarı granitoid zircon and apatite geothermometry results are similar but much lower than zircon typology data. Zircon geothermometry values are lower than apatite geothermometry results probably because of zones and structures or inherited zircon cores belong to previous phases those characterizing the probable primary phase in the samples. Differences between the zircon typology data and zircon/apatite geothermometry values are possibly result from inherited cores and zircon growths of primary phases. Indeed in the I-type Baranadağ and Terlemez granitoids presence of distinct magmatic episodes related with the magma mixing were suggested based on the cathodoluminescence images from zircon crystals (Köksal et al., submitted). Correspondingly while in the A-type Çamsarı granitoid zircon growth belong to the distinct magmatic phases are detected, in the S-type Hisarkaya granitoid inherited zircon cores were noticed (Köksal et al., submitted). Consequently, in the Central Anatolian Granitoids, geothermometric assessments based on the zircon typologies are comparable with zircon and apatite geothermometry studies for some samples (e.g., I-type granitoids), they show commonly differences, which maybe originated from properties, even occasionally handicaps, of the methods, they may also because of the individual properties of the rocks like source and generation processes. Therefore it would be beneficial to consider each granitoid as a detailed case study.

Subject Keywords

Zirkon, Apatit, Jeotermometre, Tipoloji, Granitoyid, Orta Anadolu, Zircon, Apatite, Geothermometry, Typology, Granitoid, Central Anatolia

URI

https://hdl.handle.net/11511/75007

Collections

Department of Geological Engineering, Conference / Seminar