crystallization conditions and petrogenesis of the paleoproterozoic basement rocks in bangladesh: an...

Journal of Earth Science, Vol. 25, No. 1, p. 87–97, February 2014 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-014-0402-1

Hossain, I., Tsunogae, T., 2014. Crystallization Conditions and Petrogenesis of the Paleoproterozoic Basement Rocks in Bangladesh: An Evaluation of Biotite and Coexisting Amphibole Mineral Chemistry. Journal of Earth Science, 25(1): 87–97, doi: 10.1007/s12583-014-0402-1

Crystallization Conditions and Petrogenesis of the Paleoproterozoic Basement Rocks in Bangladesh: An Evaluation of Biotite and Coexisting Amphibole

Mineral Chemistry

Ismail Hossain*1, Toshiaki Tsunogae2, 3

1. Department of Geology and Mining, University of Rajshahi, Rajshahi 6205, Bangladesh

2. Faculty of Life and Environmental Sciences (Earth Evolution Sciences), University of Tsukuba, Ibaraki 305-8572, Japan 3. Department of Geology, University of Johannesburg, Auckland Park 2006, Johannesburg, South Africa

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 ferro-magnesian minerals, have been analyzed with an electron microprobe. The biotite, Mg-dominant trioc-tahedral 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 amphi-boles, 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 metal-logenic 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. KEY WORDS: oxygen fugacity, biotite, Paleoproterozoic, basement rock, Columbia supercontinent, Bangladesh.

1 INTRODUCTION Biotite is a common ferromagnesian phase in calc-alkaline

rocks and is characterized by considerable chemical and struc-tural variations. Structural and compositional data for biotite can be used as a petrogenetic indicator of different magmatic suites (Lalonde and Bernard, 1993; Solie and Su, 1987; Speer, 1987). Biotites generally occur over nearly the entire spectrum of igne-ous rocks, from ultramafics to felsic rocks (Speer, 1984). The occurrences of biotites in the Paleoproterozoic basement rocks from Bangladesh are very common (3%–8%). The mineralogy

*Corresponding author: [email protected] © China University of Geosciences and Springer-Verlag Berlin Heidelberg 2014 Manuscript received December 7, 2012. Manuscript accepted March 12, 2013.

of the host rocks has been focuson very few studies, except our recent publication on amphibole geothermobarometry and fluid inclusions (Hossain et al., 2009). In that study, the crystallization temperature and pressure conditions of the dioritic rocks have been estimated as 680–725 C and 4.9–6.4 kbar by hornblende-plagioclase geothermometer and Al-in-hornblende geobarometer, respectively (Hossain et al., 2009). Moreover, these dioritic rocks dated Paleoproterozoic (1.73 Ga) age, regarded as a con-tinuation of the Central Indian tectonic zone (CITZ) with a rem-nant of Columbia supercontinent (Hossain et al., 2007). Al-though the studies of bulk geochemistry of calc-alkaline rocks in the CITZ are available (e.g., Dwivedi et al., 2011; Hossain et al., 2008), mineralogical study on biotite or amphibole in igneous lithologies is very limited (e.g., Hossain et al., 2009; Kumar and Rino, 2006). In general, the determination of certain boundary conditions of temperature, pressure, oxygen fugacity, fluid, and the nature of whole rock compositions, knowledge of mineral

Ismail Hossain and Toshiaki Tsunogae

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stabilities and phase equilibria are incredibly significant. As biotite compositions generally diverge, which provide important clues to evaluate mineral chemical variation between different phases in Paleoproterozoic dioritic rocks, as well as to measure intensive parameters of crystallization. As amphiboles represent a very significant phase of the studied dioritic rocks, which might be involved in magmatic differentiation and recording

conditions of crystallization (Allen and Boettcher, 1978). It also appears fruitful to revisit their composition.

This article describes the mineral chemistry of biotite and coexisting amphibole with new light on qualitative and quantita-tive estimation of oxygen fugacity during crystallization and tectonic conditions of the Paleoproterozoic dioritic rocks from Maddhapara, Bangladesh (Fig. 1).

Figure 1. Location map of the Maddhapara basement rocks in Bangladesh showing probable depth of the rocks (modified from Hossain et al., 2009). Contour index indicates depth variations of basement rocks from the surface.

Crystallization Conditions and Petrogenesis of the Paleoproterozoic Basement Rocks in Bangladesh

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2 GEOLOGICAL SETTING Detailed geological investigations on Paleoproterozoic

(1.73 Ga) basement rocks in the northwestern part of Bangladesh indicate that the pluton regarded as a continuation of the CITZ in the Columbia supercontinent configuration (Hossain et al., 2007). In between the exposed Peninsular shield and Shillong massif in India lays the Garo-Rajmahal gap corresponding to a shallow basement ridge (Desikachar, 1974) known as platform flank zone. The Paleoproterozoic magmatism shaped the platform flank which was underlain by Permian Gondwana sediments. The north-south trending Maldah-Purnea Basin and Ghatal-Burdwan Basin forms a segment of the continental rifted basin zone (Khan and Chouhan, 1996). Maddhapara, the study area is the shallowest part within an area of the platform known as Rangpur saddle (Fig. 1). It is in the form of a dome bounded by N-S trending faults in the east and the west and slope in the north known as northern slope (Dinajpur slope) and another in the south known as southern slope (Bogra slope). Several drillings in Bangladesh showed that the thickness of the sedimentary succes-sion decreases updip from about 5 000 m at the shelf edge to about 150 m in the area of the Rangpur saddle. The basement rocks in Maddhapara are unconformably overlain by thin sedi-mentary cover of Permian Gondwana sediments, which is as-sumed to be of Lopingian (Late Permian) age (0.26 Ga). Above this sequence, a thick Late Cretaceous to Pleistocene sedimen-tary sequence with recent alluvial cover present in the study area (Reimann, 1993). The minimum average rate of exhumation of the Maddhapara basement rocks was estimated to be 1215 m/Ma, suggesting relatively slow exhumation during Paleopro-terozoic to Lopingian time (Hossain et al., 2009). The dioritic rocks occur in Maddhapara area within Rangpur saddle at a shal-low depth (128 m) from the surface and the estimation of mini-mum emplacement depth of those dioritic rocks at 1722 km (Hossain et al., 2009), and consistent paleodepth in these regions are also available (Hossain and Tsunogae, 2008; Mishra et al., 2007). The rocks are mostly fresh and show no significant effect of later hydrothermal alteration, although some altered granitic rocks occur locally. They are sometimes cut by later granitic pegmatite, aplite, and quartz veins.

3 LITHOLOGICAL UNITS The main plutonic body at Maddhapara consists of diorite,

quartz diorite, monzodiorite, quartz monzonite and granite with SiO2 ranges from ~50 wt.% to ~75 wt.% (Hossain et al., 2008). Dioritic rocks are occasionally cut by granitic-pegmatite, aplite and quartz veins. Most of the plutonic rocks are pheneritic with few mylonitic lithologies.

Diorite: Diorites have, on average, 48% plagioclase, 43% amphibole, 4% quartz, 3% biotite, 1% K-feldspar with epidote, titanite, zircon, magnetite, apatite, and minor alteration products such as sericite. The rocks type is generally mesocratic to melanocratic, very coarse to medium grained (62 mm). Euhedral to subhedral plagioclase shows oscillatory zoning, mostly poly-synthetically twinned with albite twinning and prismatic-cellular growths. Composition of plagioclase in diorites shows higher anorthite content of An26–53 (Hossain et al., 2009). Compositional zoning is obvious for plagioclase in some diorites, showing anorthite-rich core (An53–40) and albite-rich rim (An32–35). Compo-sition of K-feldspar in diorite has relatively high orthoclase (Or91–95–Ab4–9). Large plagioclase grains (4 mm) sporadically surrounded by hornblende, quartz and biotite (Fig. 2) indicate porphyritic texture. On the other hand fine-grained biotite en-closed in plagioclase indicates poikilitic texture. Some large pla-gioclase crystals may contain small quartz, apatite, and/or horn-blende inclusions (e.g., Sample 12.1). Large euhedral hornblende crystals (6 mm) are common. The studied biotites occur as sub-hedral to euhedral crystals in close association with plagioclase, hornblende, K-feldspar, quartz, epidote, titanite and apatite.

Quartz diorite: Dominant quartz diorite hold, on average, 57% plagioclase, 26% amphibole, 7% quartz, 7% biotite and 1% K-feldspar with epidote, titanite, zircon, magnetite and apatite. Rarely occurred monzodiorite and quartz monzonite show similar petrographic features (Fig. 2). Minor alteration products of these rocks are sericite and chlorite. Composition of plagioclase in quartz diorite varies from An21 to An36. Tex-turally quartz diorite is very similar to diorite and optically biotites also show similar pattern.

Monzodiorite: Rarely occurred monzodiorite contains, on average, 47% plagioclase, 19% amphibole, 13% K-feldspar,

Figure 2. Petrographic photographs showing representative textures and internal relationship among biotite (Bt), hornblende (Hbl), plagioclase (Pl), quartz (Qtz) and epidote (Ep) of dioritic rocks (samples P10 and SL1).


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10% quartz and 8% biotite with minor titanite, epidote, magnet-ite, zircon and apatite. Medium to coarse grained (25 mm), subhedral K-feldspar(orthoclase) is common. K-feldspar in monzodiorite has relatively consistent composition of Or91–95–Ab4–9. Texturally biotites do not vary with diorite and quartz diorite, other than its abundant comparatively high.

Quartz monzonite: Rarely occurred quartz monzonite contains, on average, 40% K-feldspar, 25% amphibole, 18% plagioclase, 10% quartz and 6% biotite with minor titanite, epidote and zircon. Composition of plagioclase in quartz mon-zonite shows anorthite content at An21–36. Medium to coarse grained (25 mm), subhedral K-feldspar (orthoclase and micro-cline) is common. Large euhedral hornblende crystals are com-mon too. Nature of biotites authenticate similar pattern with mon-zodiorite.

Granite: Quantitatively very few granites contain, on aver-age, 29% quartz, 51% K-feldspar, 9% plagioclase and 5% biotite with minor amphibole, titanite, epidote, chlorite, zircon and apa-tite. Minor alteration products of these rocks are sericite, chlorite and calcite. Quartz occurs as isolated grains and/or aggregates. Rare quartz intergrowths with K-feldspar and plagioclase formed micrographic and/or granophyric textures. Myrmekitic textures are also present. The K-feldspar is both microcline and orthoclase forms the majority of phenocrysts. Coarse grained (>5 mm) sub-hedral orthoclase grains are relatively fresh.

4 MINERAL CHEMISTRY 4.1 Analytical Methods

Chemical analyses of minerals in dioritic rocks were carried out by electron microprobe analyzer (JEOL JXA-8621) at the Chemical Analysis Division of the Research Facility Center for Science and Technology, the University of Tsukuba, Japan. The analyses were performed under conditions of 20 kV accelerating voltage and 10 nA sample current, and the data were regressed using an oxide-ZAF correction program supplied by JEOL. The results of representative analysis of biotites and their structural formulae are given in Table 1. The BIOTERM (Yavuz and Öztaş, 1997) computer program is used for calculations of the minera-logical structural formulae of mica analyses and considers calcu-lation uncertainties of F3+ with better confidence. 4.2 Results 4.2.1 Biotite

Chemically, micas can be given the general formula X2Y4–6Z8O20(OH,F)4 in which X is K (0.89–0.97 pfu), Na (0.00–0.02 pfu), and Ca (0.00–0.01 pfu); Y is AlVI (0.13–0.25 pfu), Mg (1.33–1.53 pfu), Fe2+ (0.95–1.19 pfu), Fe3+ (0.02–0.14 pfu) with less commonly Mn (0.01–0.02 pfu), Cr (0.00–0.01 pfu), Ti (0.09–0.15 pfu), Zn (0.00–0.01 pfu); Z is chiefly Si (2.80–2.84 pfu) and AlIV (1.16–1.20 pfu) in studied biotites. Structurally, micas can be classified as dioctahedral (Y=4) and trioctahedral (Y=6). The aver-age formula of biotite is (K0.93Na0.01)(Mg1.45Fe2+

1.05Fe3+0.06

Ti0.12AlVI0.19Mn0.02)(Si2.82AlIV1.18)O10.00(OH)2.00. Hence the studied

biotites demonstrate the trioctahedral common mica. Structural formulae of biotites shows that Si (2.80–2.84 pfu)

and AlIV (1.16–1.20 pfu) cations fill the tetrahedral sites (Table 1). The octahedral sites, however, display slightly more variability between 2.85 to 2.92 cations pfu (average 2.89 pfu). The 12-fold

co-ordination sites range between 0.91 to 0.99 cations pfu (aver-age 0.95 pfu). All these suggest that biotites are close to the ideal stoichiometric values (Yavuz et al., 2002). Very consistent stoichiometric values are reported for biotite in calc-alkaline Ma-lanjkhand granitoids, India (Kumar and Rino, 2006). Biotite in diorite and monzodiorite has almost consistent chemical composi-tion. Its FeO*/(FeO*+MgO) ratio varies only slightly from 0.55 to 0.61 (*, total). TiO2 content is also nearly consistent, having the range in quartz diorite (2.1%–2.6%) is slightly higher than that in diorite (1.6%–2.5%) and monzodiorite (1.8%). Analyzed biotite samples have Mg# [Mg/(Mg+Fe)]=0.52–0.59. The most pro-nounced variations in studied biotite are in AlVI contents (0.13 to 0.25 pfu) and Fe2+/(Fe2++Mg) values (0.39 to 0.47). Interestingly, the values are very similar to those of biotite in Malanjkhand granitoids, India, Mg#=0.45–0.59, AlVI=0.17–0.44 pfu and Fe2+/(Fe2++Mg)=0.37–0.54. The studied biotite is classified as phlogopitic in the biotite quadrilateral (annite-siderophyllite-phlogopite-eastonite) (Fig. 3). In the Mg-(AlVI+Fe3++Ti)-(Fe2++Mn) ternary diagram for the classification of trioctahe-dral micas by chemical and lithological affinity, most biotites from basement rocks in Bangladesh plot in the “Mg biotites” field, which includes Mg dominant trioctahedral micas (Fig. 4). It is noted that the Malanjkhand granitoids and microgranular enclaves in India also show consistent classification as phlogopitic with trioctahedral Mg-biotites (Kumar and Rino, 2006). The studied biotites show relatively high Mg (1.33–1.53 pfu). It is noted that the decrease in Fe3+ and increase in Mg in the biotite of more evolved phases suggest very minor change in oxygen fugacity. Solidification index of basement rocks show clear progressive crystallization (Fig. 5). Similar progres-sive crystallization trend also show biotites from Malanjkhand granitoids, India. Although there is no significance correlation between MgO and FeO* in biotites of dioritic rocks whereas negative correlation between MgO and FeO* suggests that Mg=Fe substitution in calc-alkaline Malanjkhand granitoids, India. Although it displays consistent petrogenesis of the stud-ied minerals, making regional correlation is that crystallization conditions of granite plutons could be irrespective of their ages and different mode of tectonic settings.

4.2.2 Amphibole

Ca-amphibole in the examined samples has a wide composi-tional variation in XMg=Mg/(Fe+Mg)=0.50–0.66, Si=6.35–6.71 pfu, and Fe3+/(Fe2++Fe3+)=0.10–0.45. The most amphiboles are compositionally magnesiohornblende with some pargasite, mag-nesiohastingsite, edenite and tschermakite (Hossain et al., 2009). Compositions of amphiboles in diorite, quartz diorite, and mon-zodiorite are generally indistinguishable, as for example, coarse-grained amphibole in diorite shows almost identical XMg (0.52–0.66) with those in quartz diorite and monzodiorite XMg (0.50–0.63). However, in terms of XMg, the core and rim value do not show any remarkable variation. For the purpose of the present study, it is very important to revisit amphibole chemistry based on the implausible research work by Ridolfi et al. (2010), where they performed the overall themobarometric calculations from the pre-eruptive conditions of amphibole bearing calc-alkaline magmas in both the oceanic and continental settings (<40 km). In addition, working with independent components


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Table 1 Results of electron-microprobe analyses of representative biotites from dioritic rocks in Bangladesh

Analysis 3.1 3.2 3.3 11.1 11.2 12.1 12.2 13.1 28.2 28.3 28.1 14.1 14.2SiO2 (wt.%) 36.40 36.71 36.85 36.84 36.92 36.51 37.31 36.44 35.72 36.01 35.61 36.74 36.59Al2O3 (wt.%) 15.32 15.20 15.15 14.75 14.77 14.84 15.32 14.75 15.42 15.12 15.02 15.09 15.32TiO2 (wt.%) 2.10 2.05 2.57 2.51 2.50 1.59 1.64 2.18 1.76 2.41 1.93 1.79 1.77Cr2O3 (wt.%) 0.05 0.00 0.11 0.03 0.00 0.05 0.05 0.06 0.03 0.04 0.04 0.00 0.04FeO* (wt.%) 17.20 16.98 17.03 17.33 17.84 16.66 16.80 17.97 17.20 17.88 18.40 16.41 16.10MnO (wt.%) 0.25 0.26 0.27 0.23 0.23 0.33 0.32 0.28 0.19 0.35 0.27 0.25 0.29MgO (wt.%) 12.84 12.95 12.92 12.83 12.96 13.08 13.48 12.56 12.16 11.45 11.28 12.94 12.68CaO (wt.%) 0.00 0.00 0.02 0.00 0.00 0.01 0.01 0.03 0.11 0.01 0.04 0.06 0.01Na2O (wt.%) 0.07 0.06 0.02 0.04 0.09 0.03 0.08 0.12 0.07 0.12 0.14 0.09 0.10K2O (wt.%) 9.69 9.55 9.45 9.72 9.85 9.50 9.46 9.28 8.91 9.62 9.69 9.46 9.23ZnO (wt.%) 0.08 0.02 0.11 0.06 0.02 0.06 0.08 0.04 0.08 0.07 0.03 0.05 0.00Total 93.98 93.78 94.50 94.32 95.19 92.65 94.54 93.71 91.65 93.07 92.45 92.88 92.13Si 2.80 2.82 2.81 2.82 2.81 2.84 2.83 2.82 2.81 2.81 2.81 2.84 2.84AlIV 1.20 1.18 1.19 1.18 1.19 1.16 1.17 1.18 1.19 1.19 1.19 1.16 1.16Fe3+ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Sum (Z) 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00AlVI 0.18 0.19 0.17 0.15 0.13 0.20 0.21 0.16 0.24 0.20 0.20 0.22 0.25Ti 0.12 0.12 0.15 0.14 0.14 0.09 0.09 0.13 0.10 0.14 0.11 0.10 0.10Cr 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Fe3+ 0.08 0.06 0.14 0.10 0.13 0.02 0.04 0.13 0.05 0.02 0.02 0.02 0.03Mg 1.47 1.48 1.47 1.46 1.47 1.51 1.53 1.45 1.42 1.33 1.33 1.49 1.47Fe2+ 1.02 1.03 0.95 1.00 1.01 1.06 1.03 1.03 1.08 1.14 1.19 1.04 1.01Mn 0.02 0.02 0.02 0.01 0.01 0.02 0.02 0.02 0.01 0.02 0.02 0.02 0.02Zn 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Sum (Y) 2.89 2.90 2.92 2.86 2.89 2.90 2.92 2.92 2.90 2.85 2.87 2.89 2.88Ca 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00Na 0.01 0.01 0.00 0.01 0.01 0.00 0.01 0.02 0.01 0.02 0.02 0.01 0.02K 0.95 0.94 0.92 0.95 0.96 0.94 0.92 0.91 0.89 0.96 0.97 0.93 0.91Sum (X) 0.96 0.95 0.92 0.96 0.97 0.94 0.93 0.93 0.91 0.98 0.99 0.94 0.93OH 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00F 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Cl 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Sum (A) 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00Xph 0.51 0.51 0.51 0.51 0.51 0.52 0.52 0.50 0.49 0.46 0.46 0.52 0.51Xan 0.35 0.35 0.33 0.35 0.35 0.36 0.35 0.35 0.37 0.40 0.41 0.36 0.35Xmn 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.01 0.01 0.01Xal 0.06 0.07 0.06 0.05 0.05 0.07 0.07 0.05 0.08 0.07 0.07 0.07 0.09Xti 0.04 0.04 0.05 0.05 0.05 0.03 0.03 0.04 0.04 0.05 0.04 0.04 0.04I.E. 0.43 0.43 0.43 0.43 0.44 0.42 0.42 0.45 0.45 0.47 0.48 0.42 0.42Mg# 0.57 0.58 0.57 0.57 0.56 0.58 0.59 0.55 0.56 0.53 0.52 0.58 0.58Fe2+/(Fe2++Fe3+) 0.93 0.94 0.87 0.91 0.89 0.98 0.96 0.89 0.96 0.98 0.98 0.98 0.97Fe2+/(Fe2++Mg) 0.41 0.41 0.39 0.41 0.41 0.41 0.40 0.42 0.43 0.46 0.47 0.41 0.41Talc 3.97 5.57 7.68 4.47 3.04 5.30 7.08 6.50 8.69 2.40 0.08 4.87 6.93Ti-phlogopite 12.14 11.84 14.73 14.45 14.31 9.29 9.37 12.66 10.40 14.13 11.44 10.41 10.34Ferri-eastonite 8.19 6.40 13.78 10.50 12.81 2.24 4.16 12.83 5.04 2.16 2.30 1.75 3.42Muscovite 1.36 1.07 2.30 1.75 2.13 0.39 0.69 2.14 0.84 0.36 1.92 1.46 2.85Eastonite 16.05 17.29 12.93 11.88 9.23 19.15 19.45 11.90 22.02 19.33 16.70 18.60 19.13Phlogopite 58.29 57.84 48.58 56.96 58.49 63.53 59.25 53.96 53.01 61.62 67.56 62.93 57.33

Note: Z, Y, X, and A are abbreviations for tetrahedral cations, octahedral cations, interlayer cations, and anions proposed by the IMA nomenclature for micas (Rieder, 2001). Xph, Xan, Xmn, Xal, Xti=Mole fractions of phlogopite, annite, manganobiotite, aluminobiotite and titanobiotite determined on basis of all octahedral ions (calculations from Jacobs and Parry 1979). Iron-enrichment index (I.E.)=(Fe+Mn)/(Fe+Mn+Mg). Magnesium number (Mg#)=Mg/(Mg+Fe). Ferric and ferrous iron separations were obtained by the Bioterm software (Yavuz and Öztaş, 1997). Mica end-member calculations (wt.%) as talc, Ti-phlogopite, ferri-eastonite, muscovite, eastonite, and phlogopite are taken from Dymek (1983).


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Siderophyllite Eastonite

PhlogopiteAnnite

1

2

0

1.0

0.8

0.6

0.4

0.2

0.00.5 0.4 0.3 0.2 0.1 0.0

Fe /(Fe +Mg)2+ 2+

Al

in f

orm

ula

VI

0.00.51.0

Figure 3. Biotite classification of dioritic rocks in Bangla-desh in the binary diagram AlVI vs. Fe2+/(Fe2++Mg).

Al +Fe +TiVI 3+

Fe +Mn2+

Mg-Biotite

Fe-Biotite

Mg

Figure 4. Classification of biotites by chemical and lithological affinity (adapted from Foster, 1960). (AlT, Si*, Mg*, VIAl*) of a single equilibrium phase (i.e., am-phibole), those empirical formulations are relevant to all calc-alkaline lithologies (Table 2). Amphiboles in the studied calc-alkaline rocks show a silicon index (Si*) range from 7.62 to 7.89 (average 7.74), which is significantly correlated to tem-perature. Whereas, magnesium index (Mg*) varies from 2.57 to 2.87 (average 2.70), which is the best correlative indicator with oxygen fugacity. On the other hand, octahedral aluminium index (VIAl*) in amphibole ranges from -1.28 to -0.53, which is mainly sensitive to water content in the melt. Other inde-pendent components like AlT (1.57–1.93 pfu) are applicable for confining pressure of crustal depth. These compositional indi-ces, obtained by summing fractions of the amphibole major cations, are used as input variables in order to improve the

0.75

0.65

0.55

Solidification index

FeO

*/(

FeO

*+

Mg

O)

Biotites

( =-97)r

35 30 25 20 15 10 5 0

0.85

Figure 5. Relation between the FeO*/(FeO*+MgO) ratio of biotites and the solidification of the rock. The solidification index is 100 MgO/(MgO+FeO*+Na2O+K2O), where the oxides are in weight percent (Speer, 1984). performances of the T, fO2 and H2Omelt calculations (Ridolfi et al., 2010). 4.2.3 Plagioclase

The composition of plagioclase in diorite, quartz diorite, monzodiorite, and quartz monzonite varies from An21 (oligo-clase) to An53 (labradorite). Plagioclase in diorite shows higher anorthite content of An26–53 than that in quartz diorite, monzo-diorite, and quartz monzonite (An21–36). Compositional zoning is obvious for plagioclase in some diorites, showing anorthite-rich core (An53–40) and albite-rich rim (An32–35). During the differentiation of silicate liquids from basaltic rocks to quartz monzonite in the studied pluton, plagioclase evolved from anorthite-rich to albite-rich species increasing in SiO2 (58.11 wt.%–60.58 wt.%) and decreasing in CaO (8.31 wt.%–7.02 wt.%). This is followed by the reduction of plagioclase Al2O3 content (25.75 wt.%–24.22 wt.%), which always remains higher than that of calc-alkaline liquids (Ridolfi et al., 2010).

4.2.4 K-feldspar

Medium to coarse grained, subhedral K-feldspars (ortho-clase) are common. K-feldspar in diorite and monzodiorite has relatively consistent composition of Or91–95–Ab4–9. 5 OXYGEN FUGACITY

Biotite is a very good sensor for the oxidation state of the magma from which it crystallized. Wones and Eugster (1965) applied the composition of biotite solid solutions in the ternary system KFe3

2+AlSi3O10(OH)2–KMg3AlSi3O10(OH)2–KFe3

3+AlSi3O12(H-1) for approximation of oxygen fugacity that are stable at a variety of oxygen buffers. The compositions of the buffered biotites in the ternary system for Fe3O4-Fe2O3, Ni-NiO, and FeSiO4-SiO2-Fe3O4 buffers (Fig. 6). The biotites generally fall between the QFM (SiO2-Fe2SiO4-Fe3O4) and NNO (Ni-NiO) buffers in the Fe2+-Fe3+-Mg ternary diagram (Fig. 6). From this diagram, it can be seen that Fe3+/(Fe2++Fe3+) is about 0.25 for Fe3O4-Fe2O3 buffer conditions; 0.10 for Ni-NiO; 0.05 for SiO2-Fe2SiO4-Fe3O4; and <0.02 for Fe1–xO-Fe3O4. As the studied bio-tites show the Fe3+/(Fe2++Fe3+) range from 0.02 to 0.13, they are clearly coincide with QFM and NNO buffers. The applicability of these compositional projections depends on the Fe3+ content of the studied biotites, which offers only a qualitative idea of the oxygen


93

Fe O3 4-Fe O2 3

Ni-NiO

Fe SiOO

2 42 3 4

-SiO -Fe

Fe2+ Mg

Fe3+

Figure 6. Compositions of biotites from basement rocks pro-jected onto KFe3

2+AlSi3O10(OH)2-KMg3AlSi3O10(OH)2-KFe3

3+AlSi3O12(H-1) ternary system. Lines labeled with the solid oxygen buffers are compositions of the buffered biotites from Wones and Eugster (1965).

fugacity. A quantitative estimation of the oxygen fugacity or temperature can be obtained using the experimental work of Wones and Eugster (1965) if the biotite coexists with magnetite+ alkali feldspar and an independent estimation can be made of either temperature or oxygen fugacity (Speer, 1984). The pres-ence of coexisting biotite, alkali feldspar and iron-titanium oxide minerals in the studied samples from dioritic rocks provide the base for tentatively estimating oxygen fugacity. The oxygen fuga-city can also be evaluated from the calibrated curves of Wones and Eugster (1965) in fO2-T space (Fig. 7). The dioritic rocks equilibrated at an oxygen fugacity between -12.35 and -12.46, which shows the conditions between NNO and QFM buffers for the temperatures of crystallization interval between 880 and 910 °C. Generally biotites in quartz dioritic-rich rocks crystallized over range of temperatures (Dodge and Moore, 1968) and Mg-rich biotites are more stable and can contain much more halogens than Fe-rich biotites at high temperature (Mueller, 1972). In that case, the chemistry of biotite can yield more fluid compositions, which may accelerate increasing temperature (Speer, 1984). It is therefore noted that equilibrated crystallization temperature in this study is remarkably higher than previously measure by hornblende-plagioclase thermometer (680–725 °C at 4.5–6.4 kbar) (Hossain et al., 2009). However, the oxygen fugacity evaluation from the calibrated curves of Wones and Eugster (1965) in fO2–T space were within low pressure (2 070 bar) than the present study. It is very interesting that calibrated oxygen fugacity from Ma-lanjkhand granitoids, India show almost consistent values ranging from -13.07 to -12.94 within temperature 850 to 910 °C, respec-tively and its pressure conditions also about 3.5 kbar (Kumar and Rino, 2006). It is important to note that some of granitoids were probably buffered during crystallization and their oxygen fugacity increases with decreasing temperature, e.g., the Ben Nevis com-plex, Scotland (Haslam, 1968) and the Baie-des-Moutons syenitic complex, Quebec (LaLonde and Martin, 1983). If we consider the studied biotites crystallization temperature is relatively high, it means the calculated results are, at least, the minimum value of oxygen fugacity and decreasing temperature formulates it more higher oxygen fugacity. Walch (1975) stated that the composi-tional trend in the mafic rocks is believed to reflect a decrease in oxygen fugacity with failing temperatures, excluding the more magnesian trend in the intermediate rocks indicates an increase in the oxygen fugacity.

However, coexisting amphibole mineral chemistry shows

clear coincidence with biotite, especially oxygen fugacity values. These values range from -11.7 to -12.3 (Table 2), which are al-most consistent with oxygen fugacity measured from biotite mineral chemistry. These calibrations (like biotite) also show comparatively higher temperature (846–887 °C) and lower pres-sure (195–304 MPa or 2–3 kbar) than previous measure tem-perature and pressure (Hossain et al., 2009). From re-examination of amphibole chemistry based on Ridolfi et al. (2010), only very few data of present study admit the validity level and their uncertainty listed in Table 2. This study also dis-plays (Fig. 8) the decreasing temperature with increasing oxygen fugacity, where NNO and NNO+2 curves were taken from O’Neill and Pownceby (1993). The relative oxygen fugacity is also exponentially correlated to the increase of Mg/(Mg+Fe) in the C-site (Ridolfi et al., 2008; Scaillet and Evans, 1999). There-fore, it means the results are, at least, the minimum value, de-creasing temperature also formulate it higher oxygen fugacity, which is also dependent on the Mg*. Czamanske and Dillet

Temperature ( C)o

Lo

g o

xy

gen f

ug

acit

y (

bars)

0

-10

-20

-30

30

40

50

6070

80

90

100

70

400 600 800 1 000

Ni-NiO

Fe SiO-SiO

-Fe O

2

4

2

3

Biotite+gas Kalsilite+leucite+olivine+gas

Ptotal=2 070 bars Samples

Sanidine+hematite+gas

Figure 7. Position of biotites from basement rocks in Bangla-desh of differing Fe/(Fe+Mg) compositions as a function of temperature and oxygen fugacity, after Wones and Eugster (1965).

T ( C)o

log

f O2

NN

O

NN

O+2

700 800 900 1 000 1 100

-14

-13

-12

-11

-10

-9

-8

Figure 8. The logfO2-T diagram for the selected studied amphiboles. Error bars represent the expected maximum logfO2 errors (0.4 log unit) and the NNO and NNO+2 curves are taken from O’Neill and Pownceby (1993).


94

Table 2 Results of electron-microprobe analyses of selected amphiboles from dioritic rocks in Bangladesh

Sample 3.1 3.2 3.3 12.1 11.1 11.2 10.1 10.2

Spot 1 3 5 19 1 2 13 15

SiO2 (wt.%) 43.62 42.83 42.78 43.26 44.60 45.16 43.25 43.69

TiO2 (wt.%) 0.82 1.00 0.89 0.65 1.13 1.15 0.82 0.71

Al2O3 (wt.%) 10.42 10.61 10.90 10.97 9.10 9.04 10.39 10.07

Cr2O3 (wt.%) 0.03 0.02 0.00 0.04 0.00 0.02 0.03 0.04

FeO* (wt.%) 17.13 17.35 17.60 17.40 15.81 15.92 17.52 16.96

MnO (wt.%) 0.46 0.38 0.34 0.42 0.23 0.32 0.35 0.35

MgO (wt.%) 10.59 10.25 10.47 10.29 11.39 11.86 10.16 10.49

CaO (wt.%) 11.70 11.69 11.64 11.32 11.75 11.60 11.70 11.73

Na2O (wt.%) 1.32 1.34 1.16 1.24 1.31 1.39 1.06 0.90

K2O (wt.%) 1.21 1.36 0.11 1.29 1.02 0.97 1.09 1.03

H2Oamp (wt.%) 1.87 1.85 1.85 1.86 1.87 1.89 1.85 1.85

Fe2O3 (wt.%) 5.27 4.73 8.85 6.66 3.38 5.06 5.47 5.52

FeO (wt.%) 12.38 13.09 9.64 11.41 12.77 11.36 12.59 11.99

O=F, Cl (wt.%) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Total (wt.%) 99.71 99.15 98.63 99.39 98.53 99.81 98.78 98.36

Al# 0.19 0.18 0.17 0.21 0.19 0.16 0.20 0.21

Si* 7.69 7.62 7.66 7.68 7.86 7.89 7.73 7.80

Mg* 2.66 2.57 2.72 2.68 2.77 2.87 2.63 2.73 VIAl* -1.13 -1.19 -0.53 -1.05 -1.23 -1.28 -0.99 -0.97

AlT 1.83 1.89 1.92 1.93 1.61 1.57 1.85 1.79

Species Mg-Hbl Mg-Hst Tsch-Prg Tsch-Prg Mg-Hbl Mg-Hbl Mg-Hbl Mg-Hbl

Physical-chemical conditions

T (°C) 876 887 881 877 850 846 871 860

Uncertainty (σest) 22 22 22 22 22 22 22 22

P (MPa) 268 289 304 309 195 185 273 252

Uncertainty (Max error) 67 72 76 77 49 20 68 63

ΔNNO 0.4 0.2 0.5 0.4 0.5 0.7 0.3 0.5

logfO2 -11.9 -11.9 -11.7 -11.9 -12.3 -12.2 -12.1 -12.1

Uncertainty (σest) 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4

H2Omelt (wt.%) 6.4 6.1 9.5 6.8 5.8 5.6 7.1 7.2

Uncertainty 0.4 0.9 1.4 1.0 0.4 0.4 0.4 0.4

Note: Al#. the aluminium number of amphibole; Si*. silicon Index; Mg*. magnesium index; VIAl*. octahedral aluminium index; AlT. total aluminium; Mg-Hbl. Magnesiohorblende; Mg-Hst. Magnesiohastingsite; Tsch-Prg. tschermakitic pargasite (after Ridolfi et al., 2010).

(1988) among others, have clearly shown that, during mag-matic crystallization, trends in amphibole compositions may reflect either progressively oxidizing or reducing conditions. Moreover, the water content in the melt (H2Omelt) (Table 2), calculated with the solubility models of Moore et al. (1998) and Newman and Lowenstern (2002), did not show any good correlation with amphibole composition. 6 PETROGENETIC CONSIDERATIONS

Application of various models from Abdel-Rahman (1994), biotite mineral chemistry demonstrates the nature of magma types of various sources and distinct petrogenetic his-tories. He introduced discrimination diagrams on the basis of statistical approach of major element composition of biotite

minerals in the entire spectrum of igneous rocks crystallized from three distinct magma types. The chemical composition of biotites from Paleoproterozoic basement rocks on the ternary discrimination diagram of FeO*-MgO-Al2O3 (Fig. 9) and oth-ers distinct diagrams (not shown) Al2O3-FeO*, Al2O3-MgO and MgO-FeO* suggest third types, which indicate biotites in calc-alkaline orogenic complexes; mostly I-type suite formed within subduction-related environments. The FeO*/MgO ratio of the studied biotites ranges between 1.27 and 1.56 (average 1.38) and biotites of Paleoproterozoic Malanjkhand granitoids and microgranular enclaves (Kumar and Rino, 2006) ranges from 1.43 to 2.20 (average 1.67), which are similar or close to Mg-biotites (FeO*/MgO=1.76) typically associated with calcic amphibole commonly found in calc-alkaline (mostly orogenic


95

MgO

FeO* Al O2 3

Calc alkaline

orogenic suites

-

Peraluminous

(including S-type)

suites

Anorogenic

alkaline

suites

Figure 9. Plot of biotites from basement rocks in Bangla-desh on FeO*-MgO-Al2O3 ternary biotite discrimination diagram (from Abdel-Rahman, 1994). and subduction-related), I-type granitoid suites (Abdel-Rahman, 1994). The results prove undoubted the studied suite is calc-alkaline orogenic complexes. Generally, calcic amphiboles play a key role in the petrogenesis of calc-alkaline suites (Martin, 2007). The studied calcic amphibole compositions display rela-tively parental (primitive) members of arc-related igneous suites (Table 2). Owing to the well-established K-h (potash-depth) relationship developed perpendicular to the trend of an arc, espe-cially a continental arc, primitive calc-alkaline magmas may vary in K content from 0.2 wt.% to more than 2 wt.% K2O (Mar-tin, 2007). The studied amphiboles contain 0.11 wt.% to 1.36 wt.% of K2O (avg. 1.01 wt.%) and this certainly reflect in the level of K in the primary amphibole (Table 2). Kawakatsu and Yamaguchi (1987) reported the close inter-relationship between magmatic and deuteric products of crystallization in relatively shallow calc-alkaline plutons of dioritic bulk-composition. 7 DISCUSSIONS AND CONCLUSIONS

The biotite compositions in the Paleoproterozoic dioritic rocks suggest phlogopitic nature with trioctahedral Mg-biotites, which display Mg#=0.52–0.59, AlVI=0.13 to 0.25 pfu and Fe2+/(Fe2++Mg)=0.39 to 0.47. These biotites having relatively high Mg and low AlVI contents reflect slightly fractionated magma (Hecht, 1994), which might be a relative indicator for the origin of the parental magma (Aydin et al., 2003; Burkhard, 1993; Lalonde and Bernard, 1993). However, the enrichment of Fe in ferromagnesian minerals is not a necessary conse-quence of crystallization differentiation but is highly dependant on the oxygen and water fugacity (Mueller, 1972). The mole fractions of octahedrally coordinated Mg2+, Fe2+, and Fe3+ in these biotites suggest that their nature is similar to the primary biotite unaffected by hydrothermal alteration (Beane, 1974). Hence the chemical compositions of these studied biotites, amphiboles and plagioclase offer to believe their crystallization from a melt (Speer, 1984). The result is consistently reasonable according to the bulk geochemistry of the dioritic rocks, where major and trace element modeling, higher levels of incompati-

ble elements and comparison with experimental melt composi-tions elucidate that the rocks were derived from a basaltic source, with assimilation fractional crystallization (Hossain et al., 2008). So, this is a typical of the I-type dioritic rocks, where a contribution of mantle material to melt and mixing process is assumed.

Although the qualitative estimate of oxygen fugacity of the studied biotites generally fall between the QFM and NNO buffers, the quantitative oxygen fugacity equilibrated at about -12.35 and -12.46, which exhibit the source materials were relatively higher oxidation state during crystallization and re-lated to arc magmatism. Amphibole mineral chemistry also delineates consistent quantitative oxygen fugacity ranges from -11.7 to -12.3. Low Al# (0.16–0.21) amphiboles are found in equilibrium with H2Omelt of 5.6 wt.%–9.5 wt.% (avg. 6.8 wt.%) and were calibrated oxygen fugacity between NNO and NNO+2, consistent with the typical values of calc-alkaline magma crystallization (Behrens and Gaillard, 2006; Martel et al., 1999). These results are also a very consistent with other calc-alkaline rocks (Tahmasbi et al., 2009; Kumar and Rino, 2006). The variable oxygen fugacity values indicated by vari-able Fe3+ contents suggest the primary differences in redox state of the host magmas. However, some inferences on the oxidation state of the magma can be made using the rock min-eral assemblage and mineral chemistry. These results are also reliable in attendance of Mg-rich amphiboles with Mg* (2.57–2.87) in host rocks, which suggest a relatively high oxidized magma (Wones, 1989). It is noted that oxygen fugacity greatly influences numerous physical and chemical properties, includ-ing phase equilibria, element partitioning, and diffusion and rheological properties. Moreover, igneous micas are used as metallogenic indicator. As for example from Selby and Nesbitt (2000), where they stated that the major and trace element contents of biotites from mineralized porphyry copper deposits have been examined as indicators of the economic potential and evolution of the ore deposits.

Biotite composition is rather reliable indicator of the tec-tonic setting of the rocks. In this context, application of biotite chemistry in different discrimination diagrams and chemical index (Yavuz et al., 2002; Abdel-Rahman, 1994), which reveal the host dioritic rocks were a calc-alkaline orogenic complex; mostly I-type suite formed within subduction-related environ-ments. In general, I-type lithologies are relatively oxidized then sedimentary-derived granitic magmas, which is the consistent observation of the study. The contemporary outcome is also supported from different papers (Tahmasbi et al., 2009; Helmy et al., 2004; Wones, 1989), where reasonably stated that the intrinsic oxygen fugacity of magma is related to its source material, which in turn depends on tectonic setting. Accord-ingly the biotites compositions illustrate an apparent calc-alkaline trend of differentiation. The occurrences of Mg-rich magnesiohornblende, pargasite and Fe2+ biotite in dioritic rocks suggest relatively higher oxidized magma (Tahmasbi et al., 2009). The study suggests that the trends of highly oxidized magmas are commonly associated with compressive tectonic and convergent plate boundaries (Ewart, 1979). The results would be useful in further understanding of mineralization as well as crystallization of magma and the tectonic activities in


96

CITZ and adjoining areas in India.

ACKNOWLEDGMENTS We wish to thank the University of Tsukuba for allowing

us to use their facilities. We also wish to thank Chairman, Petrobangla and Managing Director, Maddhapara Granite Min-ing Company Ltd. for their kind permission for sampling and supports. Special thanks are due to Dr. N Nishida for his assis-tance on microprobe analyses and Prof. F Yavuz for his soft-ware support. We are acknowledged anonymous reviewers for their constructive review and detailed comments. REFERENCES CITED Abdel-Rahman, A. F. M., 1994. Nature of Biotites from Alkaline,

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