Chemical characteristics of biotite from Boroujerd Granitoid complex (Middle Jurassic), Western Iran

Document Type : Research Paper


1 Department of Geology, University College of Sciences, University of Tehran, Tehran, Iran

2 Faculty of Earth Sciences, Kharazmi University, Tehran, Iran


Biotite samples from different units of Boroujerd Granitoid Complex (BGC) of the Sanandaj-Sirjan Zone, western Iran, have been
analyzed by electron microprobe for major elements. Biotite analyses from three units of quartzdiorite, granodiorite and monzogranite
of BGC have their own distinct non-overlapping compositional fields in the annite – siderophyllite – phlogopite – eastonite
quadrilateral (ASPE), reflecting their host rock compositions. Biotite from each rock unit has an increasing trend of Al contents at
almost fixed Fe/(Fe+Mg) values. In quartzdiorite it shows an approximately constant range of Fe/(Fe+Mg) with a low to moderate Al
content from 2.5 to 3 atoms per formula unit (apfu). Biotite from granodiorite exhibits a fairly wide range of Al values reaching up to
3.32 apfu, at Fe/(Fe+Mg) from 0.6 to 0.7, whereas biotite from monzogranite have a relatively narrow range of Fe/(Fe+Mg) and total
Al values of limited range of 3.1 to 3.3 apfu. Biotite compositions from these two latter units considered to be derived entirely from
crustal material, characterized by a remarkable increase in total Al at relatively high Fe contents. Biotite samples of quartzdiorites
define a distinct and non-overlapping trend from those of granidiorites and monzogranites and hence interpreted to be derived from a
parental magma with different composition. Calculation of log(XMg/XFe) ranges from -0.09 to -0.02 and most of samples from
quartzdiorite fall within weakly and moderately contaminated I-type field of log(XF/XOH) versus log(XMg/XFe) diagram, whereas
the other two units, containing biotites with log(XMg/ XFe)< -0.21, classified as strongly contaminated reduced I-type. Oxygen
fugacity (log ƒO2) of -15.4 to -17.5 bars and ƒH2O of 200 to 560 bars were calculated for quartzdiorite. Likewise, log (ƒO2) of –17.66
bars and water fugacity (ƒH2O) of 400 and 700 bars were also calculated for granodiorite and monzogranites respectively. In the
FeO*–MgO–Al2O3 biotite discrimination diagram, biotite compositions from BGC are distributed between the calc-alkaline and
peraluminous fields, i.e., biotite from the qaurtzdioritic rocks fall principally in the calc-alkaline field, whereas those from the
granodioritic and monzogranitic units plot almost exclusively in the peraluminous field consistent with their host rock nature