Biotite compositional variations – a physicochemical approach to investigate crustal involvement and ore potential in Middle Jurassic plutonic rocks from the Malayer-Boroujerd Plutonic complex, W Iran

Biotite is the most frequent ferromagnesian phase in granitoids from the Malayer-Boroujerd Plutonic complex (MBPC), Zagros Orogen. The abundance of biotite decreases from granitoids towards dioritic intrusions, dioritic and gabbroic dykes to gabbroic intrusions coincident with increase in hornblende/biotite ratio. Primary magmatic biotite composition in the MBPC granitoids (Fe-rich type) changes from annite to siderophyllite, while Fe-depleted secondary biotites from the altered gabbros and gabbroic-dioritic dykes approximate to the phlogopite-eastonite series, and those from least-altered gabbros display average composition of these groups, however, extend towards high-Al end-members. Using Ti-in-biotite geothermometer, the crystallization temperature varies between 550 °C to 750 °C in MBPC granitoids (P < 5 kb), consistent with the results for the well-calibrated garnet-biotite thermometer applied in S-type granites (~721 °C). The temperature decreases from I-type toward S-type granitoids, as the Al2O3/TiO2ratios increase and Mg # decrease, indicating Ti-deficiency in magmas originating at lower temperatures. Coupled with existing data, new geochemical and P-T determinationsimply three modes of oxygen fugacity f (O2) and crustal involvement during the Middle Jurassic magmatic activities. Mode-I: primary biotite plotting below the NNO buffer, with high alumina and iron content (low-Mg # ; high Fe # and AlTvalues), at low f (O2) (10–18 to 10–16 bars) found in S-type granites. Mode-II: biotites crystallized at higher f (O 2) in I-type granitoids (f (O2) > 10–15 bars). Mode-III: biotite crystallized at higher oxidizing conditions (f(O2) > 10–10 bars) in gabbroic and dioritic dykes as well as gabbroic intrusions. The progressive decreases in f (O 2)–values and oxidizing conditions from I-type to S-type granites (above HM-to below QFM-buffer) reflect increasing incorporation of less-oxidized upper crustal materials during magmagenesis. In addition, the highest f (O2) in MBPC mafic intrusive rocks is consistent with a modest crustal contamination, if any. There is lack of evidence for porphyry type alteration zoning and subvolcanic porphyritic intrusions associated with porphyry copper system. The small patches of calc-silicate rocks from the NW-MBPC formed in contact with less oxidized and highly fractionated, crustal-derived granular granitoids and are less predisposed to develop Cu-Au mineraliza- tion. However, given the relatively higher f (O2)-values, the Middle Jurassic mafic intrusive rocks originated from the meta- somatised mantle-wedge beneath SaSiZ, appears to be the best candidate for further ore exploration programs in the area