Open Session - SWISS GEOSCIENCE MEETINGs

82
Symposium 2: Mineralogy-Petrology-Geochemistry
In order to determination of pressure (depth) and temperature condition of the plutons, some rock-forming minerals are
studied by Electron Micro Probe Analysis method. All amphiboles have Ca+Na contents greater than 2.35 p.f.u., indicating
that they are calcic amphiboles according to the classification of Leake et al. (1997). Amphibole compositions occupy a narrow
compositional range with a Mg/(Mg +Fe 2+ ) ranging from 0.39 to 0.44 and a Si content of 6.31 to 6.51 atoms per formula
unit (afu) and can be classified in the IMA nomenclature (Leak et al. 1997) as hastingsite, ferrihastingsite, ferrotschermakie
and ferrohornblende. In general composition of plagioclase is Or 0.6-3.0 Ab 70.5- 83.7 An 14.2-27.8 .
Understanding the evolution of a granitoid pluton requires knowledge of the depth at which the various minerals crystallized
and the amount of post-crystallization upward movement. The pressure of emplacement of a granite pluton can be
constrained by geologic and petrologic criteria. Based either on cation exchange or solvus relations, there are several gethermobarometer
applicable to granitoid rocks. Several studies revealed that Al content of hornblende in calc-alkaline granitoids
varies linearly with pressure of crystallization, thereby providing means of determining the depth of pluton emplacement.
Values about 5.9 – 6.5 kbar for the plutons were determined, using the calibration of Anderson and Smith (1995). These
pressures are corresponding with 21 to 23 km depths. Temperatures derived from amphibole-plagioclase thermometry
(Blundy and Holland, 1994) suggest crystallization at about 700- 720 °C.
Figure 1. Geological map of the north of Shahrekord showing the distribution of granitoid plutons.
REFERENCES
Anderson, J. L. & Smith, D. R. 1995: The effect of temperature and oxygen fugacity on Al-in-hornblende Barometry. American
Mineralogist, 80, 549–559.
Davoudian, A. R., Genser, J., Dachs, E. & Shabanian, N. 2008: Petrology of eclogites from north of Shahrekord, Sanandaj-
Sirjan zone, Iran. Mineralogy and Petrology, 92, 393–413.
Dewey, J.F., Pitman, W.C. III, Ryan, W.B.F. & Bonnin, J. 1973: Plate tectonics and the evolution of the Alpine System.
Geological Society of America Bulletin, 84, 3137–3180.
Holland T.J.B. & Blundy J.D. 1994: Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase
thermometry. Contributions to Mineralogy and Petrology, 116, 433–447.
Leake, B. E. & the IMA Commission 1997: Nomenclature of amphiboles. Report of the subcommittee on amphiboles of the
International Mineralogical Association on new minerals and mineral names. European Journal of Mineralogy, 9, 623–
651.