experimental studies to constrain parental magma of malangtoli volcanics from singhbhum craton of...

Experimental Studies to Constrain Parental Magma of MalangtoliVolcanics from Singhbhum Craton of the Eastern Indian Shield

MOUSUMI BANERJEE1, JYOTISANKAR RAY

1*, NUPUR ADHIKARY1, SANDIP NANDY

2,C. MANIKYAMBA

3, MADHUPARNA PAUL1, DOLA CHAKRABORTY

1 and ALIREZA ESLAMI4

1Department of Geology, University of Calcutta, Kolkata - 700 019, India2Geological Survey of India, Central Petrology Laboratory, Kolkata - 700 016, India

3CSIR-National Geophysical Research Institute, Hyderabad- 500 007, India4Department of Econmic Geology, Faculty of Sciences,Tarbiat Modares University, Tehran 14115-175, Iran

*Email: [email protected]

Abstract: Malangtoli volcanics of the Singhbhum craton of the eastern Indian shield is one of the important Proterozoiclava suites. Experimental studies on 1 atmosphere pressure constrain the parental magma type and temperature range ofcrystallization of the parent magma (deduced to be in the range of 15000C to 12000C). The experimental studies showthat at 15000C, plagioclase is the first phase to crystallize, followed by few opaques which join along with plagioclase at14500C. At subsequent lower temperature (14000C-13000C), plagioclase and opaque continue to crystallize. At 12500Cplagioclase and opaque still persist while pyroxene appears first and liquid (glass) still remains. Appearance of opaqueminerals (magnetite and illmenite) at both ~14000C and ~13000C indicate oscillation of oxygen fugacity in the parentmagma, petrographically documented by coarser phenocrysts as well as finer or peripheral tiny grains. Use of tectonicdiscrimination diagrams (based on discrimination factors F1-F2 and FeOt/MgO vs. TiO2) shows an island arc tholeiiticaffinity for Malangtoli volcanic, suggests that the role of proto-plate convergence in Singhbhum architecture played animportant role to build up Malangtoli volcanics during Proterozoic.

Keywords: Malangtoli volcanics, Phase chemistry, Singhbhum Craton, Discriminant function, Island- arc setting

INTRODUCTION

The Singhbhum craton represents mafic magmatismspanning from early Archaean through Proterozoic [~0.1Ga: represented by newer dolerites (Saha, 1994)]. Theearliest phase of magmatic activity in the Singhbhum cratonwas marked by mafic enclaves within Older MetamorphicGroup (Saha et at., 1980). The next phase of maficmagmatism was perhaps more dominant at c. 3.1 Ga andrepresented by varieties of mafic lavas which constitutespart of Iron-Ore Group (Saha, 1994). It was followed bythe dominant mafic-ultramafic activities and manifested byspatially separated, but contemporaneous (?) lava flowsdesignated in the literature as unclassified Dalma-Dhanjori-Jagannathpur-Simplipal-Malangtoli volcanics, eitheryounger or almost coeval with the Singhbhum Group ofrocks [c. 2.2 Ga (Saha, 1994)]. However, portion of theunclassified lavas occurring between Malangtoli (21045’N:85015’E) in the north and Pala Lahara (21028’N: 85015’E)in the south designated as Malangtoli lavas (Saha, 1994).These undeformed basaltic lavas are associated with the

western BIF bearing sequences in the iron ore basin(Dunn, 1940; Banerjee, 1982; Iyenger and Murthy, 1982;Mahadevan, 2002; Ray et al., 2006). The Malangtolivolcanics have been assigned stratigraphic equivalence toJagannathpur volcanics (Saha, 1994), however, it lacks field,petrographic and geochemical data.

The experimental study at 1 atmospheric pressure onbasalts is of great importance to understand the sequenceof appearance of phases below the liquidus (Green andRingwood, 1967). The exact characterization of appearedphases deduced through good quality electron micro probedata helps us to understand prevalent nature of the parentmagma composition. The experimental and mineralanalytical data are useful tool to determine the direction offractionation of basaltic magma. Generation of magma bypartial melting and subsequent fractionation of magmaneed to be considered in relation to abundances of majorand trace elements especially incompatible elements (Greenand Ringwood, 1967). The experiments at 1 atmosphericpressure were conducted on a synthetic glass (obtained

0016-7622/2016-88-2-245/$ 1.00 © GEOL. SOC. INDIA

JOURNAL GEOLOGICAL SOCIETY OF INDIAVol.88, August 2016, pp.245-255

JOUR.GEOL.SOC.INDIA, VOL.88, AUGUST 2016

246 MOUSUMI BANERJEE AND OTHERS

from basaltic rocks) in an attempt to define liquid lines ofdescent from the parent magma (e.g. Byerly et al., 1976;Juster et al., 1989) as well as temperature-range ofcrystallization. Critical considerations of major andincompatible trace elements on experimentally obtainedglass, therefore, serve as useful tool to constrain parentalmagma type. The present study thus aims to focus onMalangtoli volcanics by concentrating on field andexperimental aspects. One-atmospheric pressureexperimental studies have been carried out for the first timeon Malangtoli volcanics. The experiment was carried outon two extremely homogenous, massive, fine grained andamygdule-free (and alteration-free) samples of Malangtolivolcanics.

GEOLOGICAL BACKGROUND

The Singhbhum granitoid platform is surrounded byseveral major crustal provinces peripherally and sedimentarysequences with BIF, and mafic lava flows erupted along thewestern and eastern margins of the granitoid platform (Bose,2009). The major rock types in both the zones aremetabasalts and minor peridotitic komatiite with occasionalpresence of gabbroic cumulates. The large volume of thebasaltic rocks precludes the possibility of their being derivedfrom fractional crystallization of the high-MgO components(Sengupta et al., 1997). The western volcano-sedimentarysequence forms a relatively open basin viz. Koira-Gua Basinor simply “Iron ore basin” (Bose, 2000; Bose, 2009). Thebasalt-quartzite package is comparable to platformalgreenstone belt (Thurston, 1990; Condie, 1990). Malangtolivolcanics have been reportedly undergone low grademetamorphism. However, undeformed basic volcanism(located in the western and south-western parts of theSinghbhum craton) from Malangtoli volcanics have beenreported by several workers (Dunn, 1940; Iyengar andMurthy, 1982; Saha, 1994; Sengupta et al., 1997; Misra andJohnson, 2005). These lavas are post-Singhbhum Granitein age and overlain by undeformed Kolhan Group orequivalent sediments (Saha, 1994). Saha and co-workers(cited in Saha, 1994) stated that Malangtoli lava covers anarea of 800 sq. km to west of Keonjhargarh in betweenMalangtoli (21°45'N:85°15'E) in the north and Pala Lahara(21°28'N:85°15'E) in the south. Saha (1994) describes thislava as vesicular in most of the places with local shearingand chloritization. Along the southern boundary of thelava flows, gently southward dipping quartzitic sandstonehas been reported which was designated as MankarhachuaGroup (Sarkar et al., 1990). Saha (1994) had assignedbroadly a uniform silica oversaturated tholeiitic basaltic

composition for Malangtoli. However, there is no unanimityregarding the tectonic setting: whether it is really continentalor oceanic (Sahoo and Das, 1993; Saha, 1994). Thus, thereis ample scope to characterize these Malangtoli lavas ongeochemical basis and tectonic framework. The presentcontribution provides an idea about the temperature rangeof crystallization, parental magma type and control oftectonic setting on the Malangtoli magmatism.

FIELD STUDY

Field work was carried out in two sectors: (1) Kanjipanisector lies within the latitudes 21°30'00" - 21°35'00" N andlongitudes 85°22.4'00" - 85°30'53.6" E. The area consistsof mafic lavas and associated quartzite, ironstone,conglomerate and talc schist (Fig. 1a). (2) Nuakot sectorlies within the latitudes 21°40'00" - 21°48'00" N andlongitudes 85°15'00" - 85025'00" E. The area consists ofmafic lavas, quartzite and conglomerate (Fig. 1b). ForKanjipani sector, a close association of massive basaltinterlayered with sediment has been observed (Fig. 1c).Sometimes, fresh massive basalts are well exposed in thissector with the development of spaced joints (Fig. 1d). InNuakot sector, massive basalts are ubiquitously found(Fig. 1e). Laterites form common cap rock (Fig. 1f).

EXPERIMENTAL STUDY

Rock specimens are fine to medium grained, darkcoloured with moderately high specific gravity (Fig. 2a and2b). In general, both the specimens contain plagioclase,pyroxene (both clino and ortho), ± opaque; occasionallyuralitic amphibole and rarely olivine. In most of the cases,pyroxenes are partially uralitized giving an overall greenishappearance to the rock. The rocks overall show relictporphyritic texture. The thin-section petrographic charactersof the samples have been presented as photomicrograph(Fig. 2c and 2d).

The fresh representative specimens (free from vesiclesand amygdules) were crushed to fragments (max dimension6mm) using rust free iron mortar. Care was taken to avoidiron contamination. After every grinding operation in theiron mortar, as a precautionary measure, the samplefragments were subjected to magnetic separation to removeiron particles if any. The fragmentary particles so obtainedwere further grinded by Micro Mill. The speed of the millwas kept at optimum level for the best yield. For betterpreparation of pulverized materials, wet grinding methodwas preferred The addition of acetone enhanced the rateof pulverization and also decreased the amount of heat


EXPERIMENTAL STUDIES TO CONSTRAIN PARENTAL MAGMA OF MALANGTOLI VOLCANICS 247

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Fig.1. Showing (a) detailed geological map of the Malangtoli volcanics around Kanjipani sector, Keonjhar district, Orissa (prepared bypresent authors), (b) detailed geological map of the Malangtoli volcanics around Nuakot sector, Keonjhar district, Orissa (preparedby present authors), (c) field photograph of massive basalt (B) with associated sediment (Sed) {Kanjipani sector}, (d) massivebasalts traverse by two sets of joint planes (JP) {Kanjipani sector}, (e) exposure of massive basalts (B) with local presence ofquartz amygdules (Qa) {Nuakot sector} and (f) field photograph of lateritic cappings (Lat) {Nuakot sector}.

a

b



generated due to friction of agate balls against the samplein the bowl. After wet grinding, the powdered sampleswere taken out from the mill and transferred to awatch glass and kept inside an oven for about 15-20 minutesfor drying. This heating in the oven helped the samples todrive out all acetone used in the pulverization process. Thesize of powdered samples was -200 mesh. About 5-6 gm ofthe powdered sample was transferred from the watch glassinto a previously cleaned and dried platinum crucible forpreparation of starting materials. The crucible was nextplaced inside a silicon carbide high temperature furnace(OKAY-make, model 45C 1Y SW and model 50R 1Y CB).Temperature inside the furnace was made to increase till itreached 1430°C. At 1430°C, the samples were placed forfour hours to ensure total melting. After four hours, thecrucible containing the molten sample was immersed in abeaker containing cold distilled water. As a result of quickoutpouring of the samples in the cold water, the sampleswere quenched and converted to a tough solid refractoryglass. The quenched samples were transferred into awatch glass by continuous gentle tapping on the back of thecrucible using a light metal hammer. After transferreing all

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Fig.2 (a and b) Photographs of Malangtoli samples for specimen no. K-523 and K-540 respectively and (c and d) Photomicrographs ofMalangtoli basalts, (c) in plane polarized light and (d) between crossed polars {Qa – Quartz amygdules, Qmac –Quartz microamygdules clustures, Pr – Pyroclasts, U – Uralites, Plag – Plagioclase, Opq – Opaque and Px – Pyroxenes}.

the materials into the watch glass, they were put into theplanetary micro mill for fine crushing and powdering.Glass samples were checked under the microscope forensuring optical homogeneity. Details pertaining to run-products (Tables 1 and 2) and representative photo-micrographs (Figs. 3a to 3d) of the run-products arediscussed in the paper.

PHASE CHEMISTRY

The run-products of both the samples (K-523 and K-540) at 12500 C were analyzed at the Geological Survey ofIndia, Kolkata, using a Cameca SX-100 microprobe atEPMA laboratory. Electron probe micro analyses wereperformed on epoxy mounted well polished samples, havingstandard thickness. The thin sections were carbon coatedfor charging purposes. The analyses were done in point modeinvolving macro volume of samples. Both synthetic andnatural standards were used. The details of those standardsused as well as their analyses have been given in Table 3.The analyses were performed with 12nA (nano-Ampere)current at 15 KV voltage using 1µm beam size. The EPMA



data is precise and the oxide total is accurate in the range of±1.5%. The crystalline phases analyzed were mainlypyroxene, very few plagioclase and large amount of glass.Representative chemical compositions of plagioclase andpyroxene have been given in Table 4-5.

Pyroxene

Mineral chemistry data obtained from thirteen analysesof pyroxene (Table 4) of the run-products have beenplotted in pyroxene composition diagrams. The analyzedpyroxene compositions have been plotted in a Q-J diagramin order to classify them in a systematic manner on the basisof “Q” and “J” relations (Morimoto et al., 1988). As per“Q” and “J” relations (where Q= Ca+Mg+Fe and J= 2Na)(expressed in terms of a.p.f.u), the pyroxenes are designatedas “QUAD” pyroxenes (Morimoto, 1989) (Fig. 4a). Thepyroxene data derived from the present study fall within thediopside-augite field (Fig. 4b). This deduced pyroxene

composition is in conformity with the petro-mineralogicalcharacter of Malangtoli basalt as given by Saha (1994).

Plagioclase

Analyzed plagioclase compositions (Table 5) areprojected on Or-Ab-An triangular diagram (Fig. 4c). Theplots represent oligoclase composition. This deducedoligoclase composition is consistant with the overall rockchemistry (see TAS diagram, Fig.5a).

WHOLE-ROCK CHEMISTRY

The whole-rock geochemistry of two Malangtoli lavasamples, selected for experimental studies has been givenin Table 6. The analyses were undertaken for (homogeneous)powdered glasses (experimentally obtained) of the samplesby electron micro probe (EMP) analysis at the GeologicalSurvey of India, Kolkata using a Cameca SX-100 machine.

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Fig.3. (a) Few to moderate opaque in subhedral to euhedral shape, lots of glass are observed after reaching the temperature 1400°Cunder plane polarized light (Specimen no: K-523), (b) Few pyroxene grains first observed at the temperature 1250°C betweencrossed polars (Specimen no: K-523), (c) Lots of tiny plagioclase and pyroxene are observed at temperature 1400°C betweencrossed polars (Specimen no: K-540) and (d) Prominent association of plagioclase, pyroxene and glass at the temperature1250°C between crossed polars (Specimen no: K-540) {Plag – Plagioclase and Px – Pyroxenes}.



The analytical conditions of EMP during the glass analysiswere the same as that of mineral phase analysis describedearlier.

The CIPW normative analyses (Table 6) show that thesamples are either ‘andesite’ or ‘basaltic andesite’ or ‘basalt’(Fig 5a) and all of them contain normative-quartz. The glassphases (Table 6) obtained from phase chemistry studies are

plotted in TAS diagram, where the plots are in accordancewith that of the whole-rock data. In AFM diagram, samplesshow a relative depletion of iron contents and enrichmentof alkalies representing a calc-alkaline character of the rocks(Fig. 5b). In (FeOt+TiO2)-Al 2O3-MgO diagram (Jensen,1976) (Fig. 5c), the samples fall in HFT (High Iron Tholeiite)field with affinity towards tholeiitic andesite.

Table 1. Experimental constraints and optical characters of run products obtained after the experiments. (Specimen No.K-523)

Serial Temperatures Duration of Optical ImplicationsNo. (0C) experiments Characters

1 1500 6 hrs Consist entirely of glass; few tiny flakes Parent magma has suffered liquid immiscibility. Highof plagioclase are present. oxygen fugacity prevails at 1450°C. At 1500°C

2 1450 6 hrs 50 mins Lots of tiny plagioclase and lots of glass plagioclase is the first phase to crystallize. With loweringare present; Opaques are present in of temperature few opaque joins with plagioclase atmoderate amount. 1450°C. At subsequent lower temperature (1400°C -

3 1400 4 hrs Lots of tiny plagioclase, few to moderate 1300°C) plagioclase and opaque continue to crystallize.

opaque (subhedral to euhedral shape) At 1250°C plagioclase and opaque still persist and at this

and lots of glass (Fig.3a). temperature pyroxene appears for the first time.

4 1360 8 hrs 20 mins Lots of glass and moderate amount of Characteristically liquid (glass) still remains at thisplagioclase, opaque also present. temperature. These experimental findings suggest that

5 1330 3 hrs Lots of opaque and few tiny plagioclase the crystallized rock should show intergranular

observed. and intersertal texture and this has been verified by

6 1300 8 hrs Lots of glass, moderate amount of petrographic studies as well.

plagioclase and few opaque.

7 1250 5 hrs Sufficient amount of glass, moderateamount of plagioclase and pyroxene inform of micro-crystalline aggregates;plagioclase > pyroxene, very fewopaque (Fig.3b).

Table 2. Experimental constraints and optical characters of run products obtained after the experiments (Specimen No.K-540)

Serial Temperatures Duration of Optical ImplicationsNo. (0C) experiments Characters

1 1500 6 hrs Consist entirely of glass; few tiny flakes Plagioclase is the first phase to crystallize at 1500°C.of plagioclase are also present. On reaching 1400°C, few opaque minerals join with

2 1450 6 hrs 50 mins Lots of tiny plagioclase and lots of glass.plagioclase and cease to crystallize immediately thereafter.

3 1400 4 hrs Moderate amount of tiny plagioclase andFrom 1330°C- 1300°C plagioclase and pyroxene continuelots of glass are present. Opaque mineralsto crystallize while at 1300°C opaque again appears for aare present in moderate amount as short spell. At 1250°C plagioclase and pyroxene continuesubhedral crystals (Fig.3c). to crystallize and liquid still remains. Appearance of

4 1360 8 hrs 20 mins Lots of glass, moderate amount of opaque at 1400°C and 1300°C indicate oscillation ofplagioclase and very few tiny oxygen- fugacity. This should give rise to intergranularpyroxenes are present. and intersertal texture and this has been verified through

5 1330 3 hrs Lots of plagioclase, very few clino- petrography. Thin section studies further indicatepyroxene and tiny amount of glass development of corona structure, marked by presence ofare present. pyroxene grains encircling olivine due to reaction of

6 1300 8 hrs Moderate amount of glass, fairly large olivine with silica at still higher temperature.amount of plagioclase, few opaqueand very few amount of pyroxeneare present.

7 1250 5 hrs Moderate amount of glass, lots ofplagioclase and lots of pyroxene arepresent (Fig. 3d).



DISCUSSION

The discriminant functions as suggested by Pearce(1976) F1= +0.0088SiO2 -0.0774TiO2 +0.0102Al2O3

+0.0066FeO -0.0017MgO -0.0143CaO -0.0155Na2O-0.0007K2O and F2= -0.0130SiO2 -0.0185TiO2

-0.0129Al2O3 -0.0134FeO -0.0300MgO -0.0204CaO-0.0481Na2O +0.0715K2O. when plotted on the diagram(based on discrimination factors) show that the andesite

samples fall in the shoshonite field where as the basaltsamples fall in the calc-alkaline basalts and island arctholeiite field (Fig. 6a). On the FeOt/MgO vs. TiO2 diagram(after Jelinek et al. 1980) Malangtoli volcanics plot in“island arc tholeiite field” (Fig. 6b).

Geochemical characterization of basaltic provinces ofSinghbhum craton are scanty. Alvi and Raza (1992)suggested that the Dhanjori volcanics have basalt to

Table 3. Elemental wt% in EPMA Standards (supplies by BRGM, France)

Std. Std. Used forName Type determin - Element wt% in EPMA Std.

ation ofNa Mg Al Si K Ca Fe P F Cl Mn Cr Ti O

Albite Natural SiO2 & 0.0848 0.0848 0.1008 0.3181 0.0018 0.0045 0.0005 0.4886Na2O

Al2O

3Natural Al

2O

30.5293 0.4707

Fe2O3 Natural FeO 0.6994 0.3006

Orthoclase Natural K2O 0.0031 0.0918 0.3018 0.1348 0.0100 0.4585

Apatite Natural CaO & 0.3902 0.1832 0.0340 0.0035 0.3891P2O5

Cr2O

3Natural Cr

2O

30.6842 0.3158

Olivine Natural MgO 0.3040 0.1902 0.0668 0.4390

MnTiO3

Synthetic MnO & 0.3642 0.3176 0.3182TiO

2

Table 4. EPM analyses of pyroxene

Sample K540

Rock Basalt

Mineral Px Px Px Px Px Px Px Px Px Px Px Px Px

Major Oxides

SiO2 50.71 48.77 49.2 49.43 50.42 49.63 49.27 49.3 49.78 49.73 48.94 49.52 49.78TiO2 0.75 0.76 0.76 0.81 0.86 0.76 0.79 0.83 0.72 0.66 0.78 0.77 0.78Al

2O

314.53 14.26 14.62 14.47 14.61 14.47 14.49 14.51 14.51 14.67 14.26 14.69 14.74

FeO 10.71 11.03 11.31 10.43 10.92 10.58 10.87 10.73 10.51 10.34 10.86 10.87 10.39Cr

2O

30.02 0.03 0.07 0.12 0.15 0.03 0.04 0.04 0.03 – 0.04 0.08 0.09

MnO 0.24 0.22 0.22 0.26 0.22 0.19 0.2 0.16 0.14 0.23 0.18 0.18 0.19MgO 6.92 6.89 6.89 6.96 7.23 6.9 6.7 7.07 6.93 6.91 7.01 7.15 6.9CaO 13.94 13.97 13.53 13.84 13.79 13.92 13.89 13.75 13.76 13.52 14.0814.0 14.02Na

2O 1.5 1.73 1.59 1.62 1.76 1.73 1.71 1.68 1.7 1.36 1.64 1.7 1.81

K2O 0.16 0.19 0.19 0.2 0.19 0.21 0.22 0.21 0.26 0.16 0.22 0.21 0.21

Total 99.5 97.85 98.38 98.12 100.16 98.42 98.19 98.28 98.34 97.58 98.01 99.17 98.91

O=6 (cation calculated as 6 oxygen basis)

TSi 1.915 1.868 1.878 1.888 1.886 1.889 1.883 1.879 1.896 1.913 1.872 1.870 1.883TAl 0.085 0.132 0.122 0.112 0.114 0.111 0.117 0.121 0.104 0.087 0.128 0.130 0.117M1Al 0.561 0.512 0.535 0.539 0.530 0.537 0.535 0.530 0.546 0.578 0.514 0.523 0.540M1Ti 0.021 0.022 0.022 0.023 0.024 0.022 0.023 0.024 0.021 0.019 0.022 0.022 0.022M1Fe2+ 0.027 0.072 0.049 0.038 0.038 0.048 0.023 0.043 0.039 0.007 0.063 0.050 0.046M1Cr 0.001 0.001 0.002 0.004 0.004 0.001 0.001 0.001 0.001 – 0.001 0.002 0.003M1Mg 0.390 0.394 0.392 0.396 0.403 0.391 0.382 0.402 0.393 0.396 0.400 0.402 0.389M2 Fe2+ 0.311 0.282 0.313 0.295 0.304 0.288 0.287 0.299 0.296 0.326 0.285 0.293 0.283M2Mn 0.008 0.007 0.007 0.008 0.007 0.006 0.006 0.005 0.005 0.007 0.006 0.006 0.006M2Ca 0.564 0.573 0.553 0.566 0.553 0.568 0.569 0.561 0.561 0.557 0.577 0.566 0.568M2Na 0.110 0.129 0.118 0.120 0.128 0.128 0.127 0.124 0.126 0.101 0.122 0.124 0.133M2K 0.008 0.009 0.009 0.010 0.009 0.010 0.011 0.010 0.013 0.008 0.011 0.010 0.010Sum_cat 3.992 3.991 3.991 3.990 3.991 3.990 3.989 3.990 3.987 3.992 3.989 3.990 3.990Wo 43.40 43.2 42.12 43.42 42.37 43.60 43.60 42.85 43.38 43.07 43.39 42.98 43.98En 29.98 29.64 29.85 30.39 30.91 30.07 29.27 30.66 30.40 30.63 30.05 30.5430.11Fs 26.62 27.16 28.03 26.19 26.72 26.33 27.13 26.49 26.21 26.29 26.56 26.48 25.91



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�Fig.4. (a) Pyroxene compositions from experimental run-products

of Malangtoli volcanics in Q (Ca+Mg+Fe) – J (2Na)diagram (Morimoto 1988), (b) Pyroxene compositions fromexperimental run-products of Malangtoli volcanics in Wo-En-Fs diagram (Morimoto 1989) and (c) Composition offeldspars from experimental run-products of Malangtolivolcanics in Or-Ab-An diagram.

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Fig.5. (a) Total alkali vs. Silica diagram with plots of Malangtolisamples from present study area, (b) AFM diagram withplots of Malangtoli volcanics from present study area.Tholeiitic-calc alkaline boundary (Irvine and Baragar 1971)has also been depicted and (c) Jensen’s Cation – diagram(Jensen 1976) with plots of Malangtoli volcanics frompresent study area.

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b b

c

c



andesite compositional range with a strong affinity towardsisland arc tholeiites. For Dalma basalts, however, a highmagnesian character has been inferred being controlled byProterozoic marginal basin situation (Chakraborti and Bose,

1985). Blackburn and Srivastava (1994) suggested that theOngarbira basalt of Singhbhum craton was generated in anextensional environment. Saha (1994), on the basis of fewpublished major elements data, suggests continental floodbasalt affinity. This hypothesis, of course, (as Saha, 1994suggested) needs further corroboration. The present study(see previous paragraph) puts forward an island arc tectonicaffinity for Malangtoli. Our observation is in conformitywith the recent findings by Singh et al. (2015). On the basisof major, trace and PGE chemistry Singh et al. (2015)inferred that the Malangtoli rocks correspond to basalt andandesite displaying tholeiitic to calc-alkaline trend in atransitional arc to rift controlled back arc tectonic milieuproximal to an active continental convergent margin. Thus,proto-plate convergence in Singhbhum played an importantrole in the eruption of Malangtoli volcanics duringProterozoic.

CONCLUSION

1. Use of relevant tectonic discrimination diagramssuggest an island arc tholeiitic affinity for Malangtolivolcanics which was controlled by proto-plateconvergence during Proterozoic.

2. Experimental studies on Malangtoli volcanics at1 atmosphere pressure indicate the crystallizationrange of the parental magma to be in the range of1500°C to 1200°C.

Table 6. Whole rock analyses of selected samples for experimental petrology and their corresponding EPM analyses of glass phases

K523 K540

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

SiO2

57.23 59.23 55.45 56.3 58.63 60.25 59.99 59.01 57.24 58.86 56.5 57.21 57.65 57.25 49.96 49.22 49.49 48.64TiO

20.9 0.86 0.93 0.79 0.91 0.94 0.96 1.05 0.87 1 0.98 0.81 0.96 1.04 0.74 0.75 0.76 0.79

Al 2O3 14.08 14.6 13.88 14.06 14.7 14.14 13.87 13.88 13.85 14.61 13.99 13.82 13.48 14.03 14.57 14.37 14.57 14.51MnO 0.16 0.01 0.1 0.04 0.04 0.16 0.14 0.18 0.07 0.1 0.12 0.11 0.22 0.02 0.22 0.31 0.13 0.3FeO 11.73 8.82 10.65 10.48 9.03 8.3 8.4 8.97 9.97 9.25 10.63 10.83 10.41 10.22 11.79 11.71 11.41 10.55CaO 7.57 8.04 7.9 8.13 7.99 7.8 7.45 7.79 7.84 8.09 8.1 7.56 7.46 7.72 13.8 14.11 13.95 13.65MgO 4.39 4.25 4.16 4.39 4.21 3.92 3.96 3.87 4.08 4.11 4.12 3.93 4.06 4.1 7.06 7.02 7.12 6.79Na

2O 1.61 1.48 2.28 2.34 1.57 1.66 1.67 1.39 2.13 1.85 2.2 2.17 2.3 2.19 1.02 1.71 1.66 1.46

K2O 1.54 0.8 1.63 1.61 0.82 0.99 0.94 0.84 1.59 0.77 1.61 1.66 1.71 1.69 0.18 0.21 0.18 0.22Cr2O3 0.13 0.01 0.04 0.03 – – 0.03 0.08 – 0.02 0.03 – 0.09 1.04 0.08 0.18 – 0.55Total 99.35 98.09 97.01 98.18 97.89 98.17 97.41 97.05 97.63 98.66 98.28 98.08 98.33 98.43 99.42 99.59 99.27 97.48

CIPW Norms

Qz 15.36 21.88 10.83 10.68 20.73 22.9 23.2 23.34 14.13 19.8 12.19 13.36 13.7 14.09 3.98 – 0.19 1.88Or 9.12 4.73 9.63 9.51 4.85 5.85 5.55 4.96 9.4 4.55 9.51 9.81 10.11 9.99 1.06 1.24 1.06 1.3Ab 13.57 12.52 19.29 19.8 13.28 14.05 14.13 11.76 18.02 15.65 18.62 18.36 19.46 18.53 8.59 14.47 14.05 12.35An 26.6 30.83 22.82 23.11 30.64 28.21 27.57 29.15 23.53 29.29 23.54 23.07 21.41 23.46 34.61 30.91 31.77 32.39Di 8.46 7.45 13.62 14.31 7.41 8.62 7.71 7.83 12.75 8.95 13.84 12.06 12.98 12.26 27.44 32.18 30.76 28.94Hy 20.92 15.86 15.72 16.13 16.01 13.45 14.05 14.47 14.96 15.12 15.32 16.78 15.42 14.02 18.77 14.21 16.9515.2Mt 2.34 3.42 3.52 3.32 3.49 3.54 3.57 3.7 3.44 3.62 3.6 3.35 3.57 3.68 2.36 3.26 3.28 3.32Il 1.7 1.63 1.77 1.5 1.73 1.79 1.82 1.99 1.65 1.9 1.86 1.54 1.82 1.98 1.4 1.42 1.44 1.5Cm 0.3 0.01 0.06 0.04 – – 0.04 – – 0.03 0.04 – 0.13 1.53 0.17 0.27 – 0.81Ol – – – – – – – – – – – – – – – 1.85 – –Name A A BA BA A A A A A A BA A A A B B B BFig.5a

Table 5. EPM analyses of plagioclase.

Sample K540

Rock Basalt

Mineral Plag Plag

Major Oxides

SiO2

62.3 62.33TiO

20.14 0.19

Al 2O3 23.38 23.63FeO 0.16 0.27MnO 0.05 –MgO 0.01 0.01CaO 4.66 4.73Na

2O 9.2 9.12

K2O 0.1 0.13Total 100.00 100.41

O = 32(cation calculated as 32 oxygen basis)

Si 11.056 11.022Al 4.886 4.921Ti 0.019 0.025Fe2 0.024 0.040Mn 0.008 –Mg 0.003 0.003Ca 0.886 0.896Na 3.166 3.127K 0.023 0.029Cations 20.071 20.063Ab 77.69 77.17An 21.74 22.11Or 0.005 0.007



3. The experiments constrain the sequence of appearanceof different phases, namely plagioclase, opaque,pyroxene, rare coronal olivine and glass. Furthermore,when the parent magma was crystallizing, there wasoscillation of oxygen fugacity conditions as evidencedby appearance of opaque mineral (magnetite andillmenite) at ~1400°C and at ~1300°C.

Acknowledgements: Authors are thankful to University

Grants Commission for providing special assistance forestablishing one atmosphere experimental set-upat Department of Geology, University of Calcutta. Dr.S. Sengupta, GSI, provided ample support during EPMAstudies. Appropriate authorities of University of Calcuttaand CSIR-NGRI provided necessary infrastructure to carryout this work. The authors are thankful to anonymousreviewer for providing very exhaustive and helpful reviewto improve the presentation of this paper.

�

�Fig.6. (a) Tectonic discrimination diagram based on discrimination factors namely F1 and F2 (Pearce 1976) and (b) Tectonic discriminationdiagram based on FeOt/MgO vs. TiO2 (Jelinek et al. 1980). This diagram shows that the investigated samples for the Malangtolioccupied island-arc tholeiite field. Shaded area in this diagram denotes field for the island-arc basalt (Wilson, 1989).

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(Received: 31 October 2014; Revised form accepted: 14 December 2015)

experimental studies to constrain parental magma of malangtoli volcanics from singhbhum craton of...

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