Crystallization conditions and petrogenesis of the paleoproterozoic basement rocks in Bangladesh: An evaluation of biotite and coexisting amphibole mineral chemistry

Abstract

The Paleoproterozoic (∼1.73 Ga) basement rocks from Maddhapara, Bangladesh show a large range of chemical variations including diorite, quartz diorite, monzodiorite, quartz monzonite and granite. These are composed of varying proportions of quartz+plagioclase+K-feldspar+biotite+ hornblende±epidote+titanite+magnetite+apatite and zircon. Amphibole and biotite, dominant ferromagnesian minerals, have been analyzed with an electron microprobe. The biotite, Mg-dominant trioctahedral micas, is classified as phlogopitic nature. Relatively high Mg (1.33–1.53 pfu), Mg# (0.52–0.59) and low AlVI (0.13–0.25 pfu) contents in the biotite reflect slightly fractionated magma, which might be a relative indicator for the origin of the parental magma. Biotite is also a very good sensor of oxidation state of the parental magma. Oxygen fugacity of the studied biotites estimate within the QFM and HM buffers and equilibrate at about −12.35 and −12.46, which exhibit the source materials were relatively higher oxidation state during crystallization and related to arc magmatism. Whereas, calcic amphiboles, a parental member of arc-related igneous suite, display consistent oxygen fugacity values (−11.7 to −12.3), low Al# (0.16–0.21) with H2Omelt (5.6 wt.%–9.5 wt.%) suggest their reliability with the typical values of calc-alkaline magma crystallization. The oxygen fugacity of magma is related to its source material, which in turn depends on tectonic setting. Discrimination diagrams and chemical indices of both biotite and amphibole of dioritic rocks reveal calc-alkaline orogenic complexes; mostly I-type suite formed within subduction-related environments. Moreover, igneous micas are used as metallogenic indicator. The biotites with coexisting amphibole compositions show an apparent calc-alkaline trend of differentiation. The study suggests that the trend of oxidized magmas is commonly associated with compressive tectonic and convergent plate boundaries.