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
Yeganehfar, Hadi; Deevsalar, Reza; Ahadnejad, Vahid; Shinjo, Ryuichi; Neill, Iain
Neues Jahrbuch für Mineralogie - Abhandlungen Band 196 Heft 1 (2019), p. 19 - 42
77 references
published: Jun 11, 2019
published online: Mar 26, 2019
manuscript accepted: Feb 12, 2019
final revised version received: Feb 11, 2019
manuscript revision requested: Oct 8, 2018
manuscript received: Jun 8, 2018
ArtNo. ESP154019601001, Price: 29.00 €
Abstract
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/TiO2 ratios increase and Mg# decrease, indicating Ti-deficiency in magmas originating at lower temperatures. Coupled with existing data, new geochemical and P-T determinations imply 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 AlT values), at low f(O2)(10– 18 to 10– 16 bars) found in S-type granites. Mode-II: biotites crystallized at higher f(O2)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(O2)–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 mineralization. However, given the relatively higher f(O2)-values, the Middle Jurassic mafic intrusive rocks originated from the metasomatised mantle-wedge beneath SaSiZ, appears to be the best candidate for further ore exploration programs in the area. Key words: biotite; major element geochemistry; geothermometry; oxygen fugacity; ore formation, Sanandaj-Sirjan Zone; Zagros Orogen