Amphibole chemistry of quartzdiorites from Boroujerd granitoid complex (Western Iran)

Esmaeily D., Maghdour-Mashhour R.

Faculty of Geology, College of Science, University of Tehran, Iran

esmaili@khayam.ut.ac.ir

Boroujerd Granitoid Complex of the Sanandaj-Sirjan Zone in western Iran mainly consists of three units: granodiorite, quartzdiorite and monzogranite. Amphibole, the dark-colored rock-forming inosilicate mineral, from quartzdiorites has been analyzed with electron microprobe. Composition of Amphiboles used to describe the nature of the granitic magma and estimate the pressure at which quartzdiorite unit of Boroujerd granitoid complex is emplaced. Structural formulae for hornblende were calculated on the basis of 13 oxygens. Based on the nomenclature of Leake et al. [1], they are classified as magnesiohornblende to actinolitic hornblende. Good correlation between Al(IV) and total Al in amphiboles from metaluminous I-type quartzdioritic unit of Boroujerd complex demonstrates that Al content is controlled by crystallization pressure. Among several calibrations for aluminum-in-hornblende barometry, Anderson & Smith [2], a temperature-corrected Al-in hornblende barometer, are used. This geobarometry indicate that the quartzdiorite of Boroujerd granitoid complex was emplaced at pressure of 1.04±0.6 to 1.48±0.6 Kbar. Temperature was calculated based on coexisting hornblende and plagioclase using the thermometry of Blundy and Holland [3]. Calculated Tempratures of emplacement are in the range of 750 °C to 790 °C. log(XF/XOH) versus log(XMg/XFe2+) plot of Ague and Brimhall [4] is also used in order to shed light on processes contributed in genesis of the host rock e.g. degree of contamination. log(XMg/ XFe2+) and log(XF/XOH) ranges from 0.1 to 0.3 and -2 to -0.5 respectively for the analyzed amphiboles and most of samples from qaurtzdioritic unit of Boroujerd complex fall within I-MC (Moderately contaminated I-type) field. Noteworthy that, quiet a few samples classified as I-SCR and I-WC types. These observations are consistent with previous studies on biotites and whole rock geochemistry of Boroujerd complex. In general calc-alkaline quartzdiorite magma of Boroujerd complex is emplaced at depth of of 3.64±1.8 to 5.18±1.8 Km and interpreted as having formed with moderate contributions from aluminous metasedimentary material or magmas from aluminous supracrustal material, either by assimilation or cotamination, during its petrogenesis.

References:

1. Leake, B.E., Woolley, A.R., Arps, C.E.S., Birch, W.D., Gilbert, M.C., Grice, J.D., Hawthorne, F.C., Kato, A., Kisch, H.J., Krivovichev, V.G., Linthout, K., Laird, J., Mandarino, J.A, Maresch, W.V, Nickel, E.H., Rock, N.M.S., Schumacher, J.C., Smith, D.C., Stephenson, N.C.N., Ungaretti, L., Whittaker, E.J.W. and Guo, Y. (1997) Nomenclature of amphiboles: Report of the subcommittee on amphiboles of the International Mineralogical Association, Commission on New Minerals and Mineral Names. American Mineralogist, 82, 1019.1037.

2. Anderson, J.L. and Smith, D.R. (1995) The effects of temperature and .O2 on the Al-in-hornblende barometer. American Mineralogist, 80, 549.559.

3. Blundy, J.D. and Holland, T.J.B. (1990) Calcic amphibole equilibria and a new amphibole-plagioclase geothermometer. Contribution to Mineralogy and Petrology, 104, 208.224.

4. Ague, J., and Brimhall, G. H, (1988), Regional variations in bulk chemistry, mineralogy, and the compositions of mafic and accessory minerals in the batholiths of California, Geol. Soc. Amer. Bull., 100, 891-911.