Chemical characteristics of biotite from Boroujerd Granitoid complex (Middle Jurassic), Western Iran
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Abstract:
Biotite samples from different units of Boroujerd Granitoid Complex (BGC) of the Sanandaj-Sirjan Zone, western Iran, have beenanalyzed by electron microprobe for major elements. Biotite analyses from three units of quartzdiorite, granodiorite and monzograniteof BGC have their own distinct non-overlapping compositional fields in the annite – siderophyllite – phlogopite – eastonitequadrilateral (ASPE), reflecting their host rock compositions. Biotite from each rock unit has an increasing trend of Al contents atalmost fixed Fe/(Fe+Mg) values. In quartzdiorite it shows an approximately constant range of Fe/(Fe+Mg) with a low to moderate Alcontent from 2.5 to 3 atoms per formula unit (apfu). Biotite from granodiorite exhibits a fairly wide range of Al values reaching up to3.32 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 totalAl values of limited range of 3.1 to 3.3 apfu. Biotite compositions from these two latter units considered to be derived entirely fromcrustal material, characterized by a remarkable increase in total Al at relatively high Fe contents. Biotite samples of quartzdioritesdefine a distinct and non-overlapping trend from those of granidiorites and monzogranites and hence interpreted to be derived from aparental magma with different composition. Calculation of log(XMg/XFe) ranges from -0.09 to -0.02 and most of samples fromquartzdiorite fall within weakly and moderately contaminated I-type field of log(XF/XOH) versus log(XMg/XFe) diagram, whereasthe other two units, containing biotites with log(XMg/ XFe)< -0.21, classified as strongly contaminated reduced I-type. Oxygenfugacity (log ƒO2) of -15.4 to -17.5 bars and ƒH2O of 200 to 560 bars were calculated for quartzdiorite. Likewise, log (ƒO2) of –17.66bars and water fugacity (ƒH2O) of 400 and 700 bars were also calculated for granodiorite and monzogranites respectively. In theFeO*–MgO–Al2O3 biotite discrimination diagram, biotite compositions from BGC are distributed between the calc-alkaline andperaluminous fields, i.e., biotite from the qaurtzdioritic rocks fall principally in the calc-alkaline field, whereas those from thegranodioritic and monzogranitic units plot almost exclusively in the peraluminous field consistent with their host rock natureKeywords:
Formula unit
Phlogopite
Hornblende
Ilmenite
Mineral redox buffer
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Alkali feldspar
Fractional crystallization (geology)
Peralkaline rock
Microcline
Amphibole
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Diorite
Petrogenesis
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Allanite
Titanite
Phlogopite
Hornblende
Fluorapatite
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Biotite, the dominant ferromagnesian mineral in granitoid rocks, can be used to discriminate tectonic setting, magma types and magnetite- ilmenite series. In this study, we analyzed biotite with an electron microprobe (wavelength dispersion) from different granitoids. Intrusive rocks from Maherabad porphyry Cu-Au prospecting are meta-aluminous. Biotite from Maherabad are Mg-rich type and the ratio of Fe/(Fe+Mg) is 0.286-0.309. Maherabad biotite compositions fall in the field of (sub-alkaline) calc-alkaline orogenic suites. Based on High TiO2 and low Al2O3 in biotites, Maherabad also belongs to magnetite series. Intrusive rocks from Najmabad, Dehnow, Gheshlagh and Khajehmourad (NDGK) are classified as belonging to the ilmenite-series of reduced S-type granitoids. Biotite from NDGK areas are Fe-rich and the Fe/(Fe+Mg) ratio in Najmabad is 0.491-0.511, in Dehnow-Kuhsangi 0.583-0.675, Gheshlagh 0.56-0.58, and Khajehmourad 0.705-0.720. respectively NDGK biotite compositions fall in the field of peraluminous granite (P) suites (S-type). Based on low TiO 2 and high Al2O3 in biotites, Najmabad, Dehnow and Gheshlagh biotite belong to ilmenite series.
Ilmenite
Petrogenesis
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Cassiterite
Amphibole
Mineral redox buffer
Hornblende
Fractional crystallization (geology)
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