Mineral compositional constraints on the petrogenesis of gabbroic and monzodioritic rocks in Rangpur District, NW Bangladesh

• Published in: Journal of Asian Earth Sciences: X 2023-06, Vol.9, p.100134, Article 100134
• Main Authors: Hossain, Ismail, Tsunogae, Toshiaki, Jannatun, Nura, Sazzadur Rahman, Md, Nahar, Mowsumi, Mehedi Hasan, A.S.M., Momotaz Khatun, Most
• Format: Article
• Language: English
• Subjects: Bangladesh; Geothermobarometry; Mineral chemistry; Oxygen fugacity; Plagioclase-amphibole; Two-pyroxenes
• ISSN: 2590-0560; 2590-0560
• DOI: 10.1016/j.jaesx.2022.100134

[Display omitted] •The rocks are dominantly gabbroic in Barapaharpur and monzodioritic types in Voktipur.•It is the first report on two-pyroxene-rich (CPX and OPX) gabbro in Bangladesh.•Gabbroic rocks yielded the highest temperature (850–957 °C) and pressure (5.8–9.3 kbar).•Rocks show oxygen fugacity ranging from −11.6 to −13.5 and −10.6 to −12.0.•The data provide tholeiitic and calc-alkaline basaltic and dioritic equivalents suites.•Demarcate island arc type suprasubduction and subduction-related environments. This research deals with the petrography and mineral chemistry of gabbroic and monzodioritic rocks to characterize the minerals, including their genesis and tectonic evolution. The rocks are dominated by gabbro in Barapaharpur and monzodiorite in Voktipur, and they contain plagioclase, hornblende, biotite, and magnetite, with few epidote and apatite, as well as clinopyroxene and orthopyroxene in gabbroic member. Chemical analyses of minerals were carried out by an electron microprobe analyzer. The chemistry of plagioclases is compositionally oligoclase-andesine and andesine, amphiboles are dominantly magnesiohornblende, biotites are Mg-biotites (phlogopite), and pyroxenes are diopside and enstatite types. The calculated P-T conditions of the gabbroic rocks yielded about 850–957 °C and 5.8–9.3 kbar, which are the highest geothermobarometric values in the region. However, the monzodioritic rocks yielded about 696–723 °C and 5.6–6.2 kbar. The estimated emplacement depths from the calculated highest pressure values equalize at ∼ 20–33 km and at ∼ 19–22 km, respectively. Biotite chemistry provides quantitative fO2 within QFM and HM buffers ranging from −10.6 to −12.0 in gabbroic rocks and −11.6 to −13.5 in quartz monzodiorite, respectively. These data are also echoed by amphibole and pyroxene mineral chemistry, which collectively demonstrate that the source materials were in a relatively higher oxidation state within arc magmatism. The mineral data also offer calc-alkaline I-type suites formed within subduction-related environments. However, gabbroic rocks show a dominantly island arc signature substantiating the suprasubduction zone, and also have the affinity of the island arc tholeiitic and calc-alkaline basaltic equivalents within orogenic environments and marginally boninitic affinities.