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JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.80, October 2012 pp.481-492

Mineralogical Study of Gabbro-Anorthosite from Dumka, Chhotanagpur Gneissic Complex, Eastern Indian Shield NILADRI BHATTACHARJEE, JYOTISANKAR RAY, SOHINI GANGULY and ABHISHEK SAHA Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata - 700 019 Email: [email protected] Abstract: The Chhotanagpur Gneissic Complex (CGC), bearing imprints of widespread high grade metamorphic and magmatic history since Palaeoproterozoic, represents an integral crustal segment of Eastern Indian Shield. The gabbroanorthosite intrusives constitute a part of mafic-ultramafic magmatism in the CGC. The study area around Dumka (24°16' to 24°20'N: 87°13' to 87°22'E) predominantly comprises of granite gneiss and charnockitic country rocks within which gabbro-anorthosite intrusions occur as lenses. Field relations and structural studies reveal that the country rocks of Dumka have suffered three phases of deformation represented by F1, F2 and F3 folds. The gabbro-anorthosite intrusives maintain a sharp contact with the host rocks, deformed and metamorphosed. Relict igneous layering or primary igneous foliation (Sig) is recorded where metamorphic overprint is minimal. Mineral phases of gabbro-anorthosite rocks suggest that clinopyroxene compositions from gabrro correspond to diopside and clinoferrosilite, while those from anorthosite are clinoferrosilite. Amphiboles from the gabbro-anorthosite rocks are calcic, and range from ferroan pargasite in gabbro to ferroan pargasitic hornblende in anorthosite. Plagioclase from gabbro and anorthosite belong to bytownite and andesine respectively. Chemical composition of garnet in gabbro is almandine. Thermobarometric estimates for Dumka gabbroanorthosites correspond to 511°C to 915°C and 5.0-7.5 kb pressure, comparable to that estimated for Bengal Anorthosite (593-795°C, 4.1-7.3 kb). Fractionation trend of plagioclase substantiates a single parental magma in the evolution of Dumka gabbro-anorthosite intrusives. Keywords: Proterozoic, Gabbro-Anorthosite, Granulite facies, Chhotanagpur Gneissic Complex INTRODUCTION

Anorthosite occurs in close spatial and genetic association with gabbro/norite and the entire spectrum of rock composition has been classified on the basis of increasing plagioclase content as gabbro/norite-anorthositic gabbro /norite-gabbroic /noritic anorthosite- anorthosite (Buddington, 1939; Bose, 1997). Proterozoic gabbroanorthositic rocks in the ancient cratons all over the world have spatially and temporally limited occurrences but they occur in close association with Anorthosite-MangeriteCharnockite-Granite (AMCG) suites and constitute important components for crustal growth and represent a significant proportion of Proterozoic crust in several continents (Ashwal, 1993; Corfu, 2004, Bogdanova et al. 2004; Larin et al. 2006). Proterozoic gabbro-anorthositic rocks are concentrated within the crustal belts which have been affected by amphibolite-granulite facies metamorphism and tectonic activitites related to Grenville orogenies. The Chhotanagpur Gneissic Complex (CGC) represents a segment of Precambrian high- grade metamorphic terrain

of the Central Indian Tectonic Zone (CITZ) in the eastern part of the Indian Peninsular Shield (Chatterjee and Ghose, 2011) and associated with a number of gabbro-anorthosite occurrences. The CGC presents litho-tectonic assemblages bearing imprints of polyphase deformation, metamorphism and magmatism spanning over ~2 Ga since the Palaeoproterozoic (Ghose, 1983; Mukherjee and Ghose, 1999; Ghose and Mukherjee, 2000). It consists of basement gneisses, migmatites, khondalite, leptynite, granulite and meta-igneous rocks with supracrustal metasedimentary enclaves. These have been intruded by mafic-ultramafic rocks, gabbro-anorthosite bodies, dyke swarms and granite plutons ranging in composition from ultramafic to acidic and sodic alkaline to ultrapotassic (Ghose and Mukherjee, 2000). These magmatic episodes are grouped into three distinct phases: (i) Pre-tectonic mafic -ultramafic intrusive, (ii) Syn-tectonic basic intrusive and (iii) Syn-to late tectonic granitic intrusions (Ghose and Chatterjee, 2008). In the CGC bimodal gabbro-anorthosite magmatism is contemporaneous with the mafic-ultramafic magmatism. These occurrences within the Chhotanagpur Gneissic

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NILADRI BHATTACHARJEE AND OTHERS

Complex are located at Saltora in Bankura district of West Bengal, Bela in Gaya; Biwabathan and Kauria in Daltonganj; Hizla in Dumka and Hazaribagh (Mukherjee and Ghose, 1992; Ghose et al., 2008). The area around Dumka (24°16' to 24°20'N: 87°13' to 87°22' E) comprises gabbro-anorthosite rocks which occur as lenses and enclaves within granite gneisses and charnockite country rocks (Bhattacharyya, 1975; 1976). We present new field, petrographic, and mineral-chemical data of Dumka gabbro-anorthosite to evaluate the P-T conditions of constituent mineral phases and crystallization history of these rocks. REGIONAL GEOLOGY

The Proterozoic Chhotanagpur Gneissic Complex (CGC) occurs as a mobile belt to the north of the Archaean Singhbhum-Orissa Craton and constitutes an integral crustal segment of the Eastern Indian Shield. It covers an area of about 1,00,000 sq. km. in parts of West Bengal, Bihar, Jharkhand, Uttar Pradesh, Madhya Pradesh, Chhatisgarh and Orissa (Ghose 1983; 1992; Ghose and Mukherjee, 2000). CGC is a high-grade gneissic terrain dominantly characterized by amphibolite to granulite facies rocks juxtaposed between the Mesoproterozoic lowgrade metasediments and metavolcanic rocks of North Singhbhum Mobile Belt (NSMB) in the south (Saha, 1994; Ghose and Chatterjee, 2008) and Proterozoic metasediments, granitoids, mafic-ultramafic rocks partly covered by Mahakoshal Mobile Belt (MMB) and Bihar Mica Belt (BMB) in the north (Roy and Devrajan, 2000; Chatterjee and Ghose, 2011). The eastern margin of the CGC is covered

by the Gangetic alluvium (Fig.1) which separates it from the Shillong Meghalaya Gneissic Complex (SMGC) (Chatterjee et al. 2007; 2008; Ghose et al. 2008) (Fig. 1). In the west, the CGC is separated from the Central Indian Tectonic Zone (CITZ) by the younger Gondwana sediments of Mahanadi graben (Fig.1). The Chhotanagpur Gneissic Complex (CGC) has undergone three phases of deformation giving rise to distinct fold patterns (F1, F2 and F3) and related linear fabric ( Sarkar, 1982, 1988; Ghose, 1983). The structural evolution of the area is marked by a first generation isoclinal folds followed by second phase folding forming antiformal and synformal structures. The axial trend in the central and northern part of the area is dominantly N-S but the southwestern part exhibits an SW axial trend (Ghosh and Mukherjee, 2000). This second phase of folding is characterized by several syntectonic intrusions of basic igneous rocks that have shared the high-grade metamorphism with the host rocks (Bhattacharyya, 1975; Kumar and Ahmad, 2007). The third generation folding with NW-trending axial trace has affected the earlier folds producing several macroscopic structural domes and basins. The CGC comprises dominantly of migmatites and granite gneisses varying from granodiorites to tonalite, trondhjemite and diorite (Ghose, 1983). The supracrustals consist of metamorphosed pelitic, calcareous and psammitic sediments occurring as enclaves within the gneiss. The metasediments and the gneisses have been profusely intruded by mafic rocks presently exposed as amphibolite, metadolerite, metagabbro, metanorite and pyroxene granulite (Mahadevan, 1992, 2008; Ghose et al. 2005). These meta- igneous rocks are generally concordant

Fig.1. (A) Geological map of the Chhotanagpur Gneissic Complex showing the location of Dumka (after Ghose et al., 2008) EGB Eastern Ghats Belt, SC – Singhbhum-craton, CGC - Chotanagpur Gneissic Complex, CITZ - Central Indian Tectonic Zone, dashed line Singhbhum shear zone, stippled area - Gondwana Basins: M-Mahanadi Basin, G-Godavari Basin. (B) A simplified map of the CGC showing location of different anorthosite bodies (including Dumka anothosite) JOUR.GEOL.SOC.INDIA, VOL.80, OCT. 2012

MINERALOGICAL STUDY OF GABBRO-ANORTHOSITE FROM DUMKA, CHHOTANAGPUR GNEISSIC COMPLEX

with the foliation of the host rocks. These mafic-ultramafic intrusives preceded the major granitic activity at 1600 Ma (Pandey et al. 1986). Anorthosites are mainly exposed towards north of Adra, at and around Saltora (known as Bengal anorthosite), Bela, Daltonganj and Dumka (Ghose, 1992). The anorthosites and associated gabbroic rocks bear strong imprints of metamorphism and deformation. Several intrusions of anorthosite, both massif and differentiated types have been encountered in the CGC. U-Pb dating of massif anorthosite around Saltora (known as the Bengal anorthosite emplaced at 1550±12 Ma in the eastern part), gives imprints of granulite facies Grenvillian metamorphism at 947 Ma (Chatterjee et al. 2008). The emplacement of gabbro and anorthosite in CGC was largely controlled by second generation deformation (D2), correlatable with the massif anorthosite in Eastern Ghats (Ghose et al. 2005). In the Dumka region, high- grade metamorphic rocks including granite gneiss, augen gneiss, tightly folded migmatitic gneiss, charnockite, khondalite, garnetiferous pyroxene granulite, ortho/para amphibolites and calc-silicate gneisses are exposed as country rocks (Bhattacharyya, 1976; Chatterjee et al. 2008).

FIELD RELATIONS AND STRUCTURE

High-grade metamorphic country rocks predominantly comprise of granite gneiss and charnockite around Dumka (Fig.2). They show polyphase deformations. Well-developed gneissic foliation is most distinct and may be designated as S1. A planar structural feature (S0) is found to be developed on original composition layers of the country rocks. Linear structure in the country rocks is recognized by parallel arrangement of elongated biotite grains defining distinct mineral lineation. Three generations of folding in the country rocks corroborate that they had been subjected to multiple episodes of deformations. Characteristic features of each of the deformational phases are described below: F1 fold: F1 fold records the earliest phase of deformation in the area and has developed on the original composition layers (S0) of the country rocks. F1 folds occur locally and are represented by relict, rootless, isoclinal folds (Fig.3a). The overall pervasive foliation (S1) in country rocks is axial planar to F1 fold (Fig.3a). F2 fold: F2 folds have developed on mesoscopic scale

Fig.2. Geological map of the study area. JOUR.GEOL.SOC.INDIA, VOL.80, OCT. 2012

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Fig.3. (a) Rootless isoclinal fold (F1) representing the earliest phase of deformation in granite gneiss country rocks. (b) Stereopole diagram showing poles to the foliation plane of country rocks. The orientation β axis is 45°→273°. (c) Gabbro-anorthosite occurring as intrusives within charnockite. (d) Field photograph showing primary igneous foliation (Sig) developed in gabbro. (e) Stereopole diagram showing the poles to the primary igneous foliation of gabbro-anorthosite. (f) Photomicrograph showing kinked twin lamellae of plagioclase in gabbro

on the S1 planes (delineating pervasive gneissic foliation) of country rocks and these are characterized by S- and Z type fold patterns. F3 fold: F3 folds have developed on a regional and megascopic scale representing the latest phase of deformation. The distribution of plots of poles to foliation

plane (S1) of country rocks defines a great circle girdle in stereopole diagram (Fig.3b). The controlling β axis plunges almost towards west with the deduced plunge amount 45°→273° (Fig.3b). Gabbro-anorthosite intrusives occur as lenses within granite gneiss and charnockite, maintaining a sharp contact JOUR.GEOL.SOC.INDIA, VOL.80, OCT. 2012


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pyroxene, amphibole, and garnet. Biotite, alkali feldspar, quartz, zircon and opaque occur as accessory phases. Plagioclase grains display prominent lamellar twinning. At places untwinned plagioclase shows undulose extinction and in some grains the twin planes are curved (Fig.3f). Plagioclase occasionally develops myrmekitic intergrowth with quartz. Pale green clinopyroxenes are partially or wholly altered to amphibole. Light yellowish green to green coloured amphibole occurs as fibrous grains around pyroxene indicating their development after clinopyroxene. Subhedral grains of orthopyroxene in shades of pink in gabbro, show characteristic pleochroism. Garnets are pale brown in colour. Minor flakes of light brown to dark brown biotite, alkali feldspar and quartz are present. Both primary and secondary opaque grains are present as accessory phases. Primary opaques are distinctly euhedral, while subhedral grains of opaques are formed from pyroxene. Zircon occurs as inclusion within plagioclase, showing anomalous interference colour. Variations in modal compositions (Table 1) of gabbro and anorthosite are shown in Pl-Opx-Cpx and Pl-Px-Hbl triangular diagrams (Fig.4a and b).

with the host rocks (Fig.3c). These bodies are deformed, feebly metamorphosed and maintain igneous layering (Sig) (Fig.3d). The plots of poles to igneous layering of gabbroanorthosite define a sub vertical girdle axis (Fig.3e), suggesting a forceful injection of the melt into the country rock. The structure and field relations of the rocks as summarized by Bhattacharya (1976), is as follows: (a) Deposition of pelitic, psammitic and calcareous sediments. (b) Development of rootless, isoclinals F 1 folds, metamorphism of original sedimentary layers and development of S1 (coaxial planar to F1) pervasive gneissic foliation. (c) Development of mesoscopic F2 fold on S1 (represented by S and Z replicas). (d) Intrusion of gabbro-anorthosite bodies into the country rocks. (e) F3 fold causing regional warps and development of space joints. [Younging direction: From (a) to (e)]. PETROGRAPHY OF GABBRO-ANORTHOSITE

ANALYTICAL TECHNIQUES

Megascopically, gabbro-anorthosite rocks are mesocratic, coarse-grained and more or less equigranular. Plagioclase grains are prismatic, white to off white in colour, while dark coloured, subhedral, mafic minerals are represented by pyroxene, amphibole and very rarely biotite. They exhibit crude to moderately developed planar fabric defining primary igneous layering (Sig) (Fig.3d). Under microscope, the gabbro-anorthosites show hypidiomorphic granular texture transgressing to ophitic and sub-ophitic. They are essentially composed of plagioclase and clinopyroxene with subordinate amounts of ortho-

Electron microprobe analyses (EPMA) of constituent mineral phases from gabbro-anorthosite were conducted at CISAG, University of Napoli Federico II, utilizing an Oxford Instruments Microanalysis Unit. This unit is equipped with an INCA X-act detector and a JEOL JSM-5310 microscope operating at 15 kV primary beam voltage, 50-100 mA filament current, a 15-17mm spot size and a net acquisitiontime of 50s. The analyses were obtained with energydispersive spectrometry (EDS) and back-scattered electron (BSE) images and measurements were done with an INCA

Table 1. Modal analyses (vol %) of Dumka gabbro-anorthosite Serial No. Sp. No. Rock type Plagioclase K-feldspar Clinopyroxene Orthopyroxene Amphibole Quartz Opaque Garnet Biotite Zircon An% of plagioclase

1

2

3

4

5

6

7

8

9

10

11

12

N73 GAB 59.53 15.87 0.67 19.93 2.07 1.67 0.34 38.00

N1 GAB 67.50 17.70 6.20 1.80 5.40 1.20 0.07 15.00

N72 GAB 55.60 23.20 11.00 3.40 2.80 3.60 44.00

N70 GAB 59.93 26.73 10.07 0.53 2.67 0.07 46.00

N17 GAB 62.13 11.87 0.27 21.33 1.93 2.33 0.13 44.00

N18 GAB 40.80 16.60 0.30 36.50 2.10 3.40 50.00

N40 GAB 40.80 33.30 23.30 2.10 44.00

N71 GAB 38.80 1.60 10.90 2.00 41.00 3.80 1.50 46.00

N76 GAB 88.90 5.10 4.30 1.20 42.00

N50 ANS 96.92 2.40 0.07 0.33 0.27 54.00

N57 ANS 91.20 7.80 1.00 44.00

N66 ANS 91.60 1.10 5.10 1.60 0.40 41.00

GAB: Gabbro and ANS: Anorthosite JOUR.GEOL.SOC.INDIA, VOL.80, OCT. 2012

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Amphibole Mineral chemistry of amphibole from gabbro-anorthosite rocks have been furnished in Table 2. The Na (B) vs Ca + Na (B) variations of the analyzed amphiboles indicate these to be calcic type (Leake, 1978) (Fig. 6a).When amphibole compositions are plotted in Mg/ (Mg + Fe2+) vs TSi diagram, those present in gabbro (sample no. N71) are ferroan pargasite, and in anorthosite (sample no.N66) it is ferroan pargasitic hornblende (Fig. 6b). Plagioclase

Plagioclase compositions (Table 2) range from An41 to An44 (andesine) and An79 to An81 (bytownite) in anorthosite and gabbro respectively in plots of Or-Ab-An (Fig.7). Alkali Feldspar

Alkali feldspar from anorthosite (sample no. N66) ranges 2.0 Quad

Q

1.5

1.0 Ca-Na

Fig.4. (a) Modal composition of Dumka gabbro-anorthosite plots in Plagioclase- Orthopyroxene-Clinopyroxene composition diagram (after Streckeisen, 1976). (b) Modal composition of Dumka gabbro-anorthosite plots in PlagioclasePyroxene-Hornblende composition diagram (after Streckeisen, 1976)

0.5 Na

0.0 0.0

0.5

1.0

1.5

2.0

J

Wo

X-stream pulse processor. The following standards were used for calibration: diopside (Mg), wollastonite (Ca), albite (Al,Si, Na), almandine (Fe), orthoclase (K). Chemical analyses of mineral are presented in Tables 2 and 3. MINERAL CHEMISTRY Diopside

Pyroxene

Chemical analyses of pyroxene in gabbro-anorthosite rocks are presented in Table 2. Q-J relations (Morimoto, 1989) indicate all the analyzed pyroxenes as Quad pyroxenes (Fig.5a). Pyroxene compositions of gabbro (Sample no. N71) as per IMA recommended Wo-En-Fs diagram (Morimoto et al. 1988), plot in the fields of diopside and clinoferrosilite, whereas, pyroxenes from anorthosite (sample no. N50 and N66) correspond to clinoferrosilite (Fig. 5b).

Hedenbergite Augite

Pigeonite Clinoenstatite

En

Clinoferrosillite

Fs

Fig.5. (a) Pyroxene compositions of Dumka gabbro-anorthosite in Q (Ca+Mg+Fe) – J(2Na) diagram (Morimoto, 1989). (b) Pyroxene compositions of Dumka gabbro-anorthosite in Wo-En-Fs diagram (Morimoto et al. 1988). JOUR.GEOL.SOC.INDIA, VOL.80, OCT. 2012


487

Table 2. Chemical analyses of pyroxenes*, amphiboles* and feldspars* (wt%) Sp. No.

N50

N66

N71

Mineral

Pyroxene

Pyroxene

Pyroxene

ANS

ANS

GAB

ANS

GAB

ANS

ANS

GAB

3

3

6

4

4

4

3

3

48.98 0.66 35.22 0.79 13.26 0.55 0.03 99.48

48.03 0.06 1.11 36.59 0.12 0.92 12.47 0.57 0.02 99.88

51.64 0.14 1.52 12.71 0.02 0.26 11.39 22.00 0.26 99.93

39.96 2.54 11.63 21.72 0.27 7.02 10.88 1.42 1.78 97.22

42.36 2.27 11.54 16.94 0.07 0.26 10.28 11.59 2.15 1.40 98.86

SiO2 Al2O3 FeO BaO CaO Na2O K2O Total

57.53 26.67 0.49 0.02 8.83 6.21 0.01 99.76

64.04 18.63 0.82 0.88 14.92 99.29

47.87 32.95 0.55 0.31 15.75 2.19 99.62

Si Al Fe2 Ba Ca Na K Ab An Or

32 10.335 5.643 0.074 0.001 1.699 2.162 0.003 56.0 44.0 0.1

32 11.934 4.090 0.060 0.318 3.547 8.2 91.8

32 8.825 7.153 0.084 0.022 3.112 0.783 20.1 79.9 -

Rock Type No. of Anal(n) SiO2 TiO2 Al2O3 FeO Cr2O3 MnO MgO CaO Na2O Total

N66

N71

N66

Amphibole Amphibole

SiO2 TiO2 Al2O3 FeO Cr2O3 MnO MgO CaO Na2O K2O Total

N66

N71

Plagioclase K-Feldspar Plagioclase

Structural formula No of oxygens TSi TAl TFe3 M1Al M1Ti M1Fe3 M1Fe2 M1Cr M1Mg M2Fe2 M2Mn M2Ca M2Na Q,J WO EN Fs

6 1.954 0.031 0.015 0.048 0.164 0.788 0.948 0.027 0.023 0.002 1.92,0 1.16 39.16 59.68

6 1.919 0.052 0.028 0.002 0.075 0.177 0.004 0.743 0.943 0.031 0.025 0.002 1.89,0 1.22 36.76 62.03

6 1.959 0.041 0.027 0.004 0.024 0.300 0.001 0.644 0.079 0.008 0.894 0.019 1.92,0.04 45.85 33.03 21.12

TSi TAl Sum_T CAl CCr CFe3 CTi CMg CFe2 CMn Sum_C BCa BNa Sum_B ANa AK Sum_A

23 6.153 1.847 8 0.262 0.632 0.294 1.611 2.165 0.036 5 1.795 0.205 2 0.219 0.35 0.569

23 6.299 1.701 8 0.319 0.008 0.287 0.254 2.279 1.820 0.033 5 1.847 0.153 2 0.467 0.265 0.732

NaB

For amphibole, cation-proportions calculated on the basis of 15 cationic total, excluding Na and K. Fe 3+ calculated on the basis of charge balance equation. Rock type :-ANS : Anorthosite, GAB : Gabbro. *Analysis indicate average data, detailed analysis are available with the authors.

(Ca + Na)B

Fig.6. (a) Plot of amphibole compositions from Dumka gabbro-anorthosite (after Leake 1978). (b) Compositions of amphiboles from Dumka gabbro-anorthosite (after Leake, 1978). Explanations of symbols are same as in Fig.6a. JOUR.GEOL.SOC.INDIA, VOL.80, OCT. 2012

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NILADRI BHATTACHARJEE AND OTHERS Table 3. Chemical analyses of biotite*, garnet8 and ilmenite* (wt%) Sp.No

N66

Mineral Rock type

N71

N50

N66

N71

Biotite

Garnet

Ilmenite

Ilmenite

Ilmenite

Anorthosite

Gabbro

Anorthosite

Anorthosite

Gabbro

2

2

2

1

3

TiO2 Al2O3 FeO MnO MgO Total

44.18 0.06 45.40 0.63 0.29 90.56

43.04 0.15 50.85 0.39 0.02 94.45

50.30 0.10 48.06 0.83 0.30 99.59

Al Ti Fe2 Mn Mg

3 0.002 0.945 1.080 0.015 0.012

3 0.005 0.900 1.182 0.009 0.001

3 0.003 0.969 1.029 0.018 0.011

No. of anal (n) SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO BaO CaO Na2O K2O Total

34.13 4.24 13.88 0.41 22.76 0.39 8.68 0.70 0.42 0.13 8.28 94.02

oxygen atoms Si AlIV AlVI Ti Fe2 Cr Mn Mg Ba Ca Na K Fe_FeMg Mg_FeMg

24 5.684 2.316 0.406 0.531 3.170 0.054 0.055 2.155 0.046 0.075 0.042 1.759 0.6 0.4

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO BaO CaO Na2O

36.73 0.09 22.29 29.17 1.72 4.04 6.66 -

Total

100.71

Structural formula TSi TAl AlVI Ti Fe2 Mg Mn Ca Na Sum_Cat Alm Gross Pyrope Spess

24 2.879 0.121 1.936 0.006 1.912 0.473 0.114 0.559 8.000 53.05 22.91 19.35 4.69

*Analysis indicate average data, detailed analysis are available with the authors

in composition from Or87-Or92 (Table 2) and corresponds to orthoclase in Or-Ab-An diagram (Fig.7).

Or

Biotite

nid

ine

Biotite compositions (Table 3), indicate them to be ironrich biotite.

cla se/

Sa

Garnet

Or

tho

Garnet compositions (Table 3) mainly correspond to almandine. Opaque Opaque phases of gabbro (N71) are identified as ilmenite, while those from anorthosite (N50 and N66) correspond to titanomagnetite (Table 3).

Anorthoclase

Albite

Ab

Oligoclase

Andesine

Labradorite

Bytownite

Anorthite

An

Fig.7. Composition of feldspars from Dumka gabbro-anorthosite in Or-Ab-An diagram

GEOTHERMOBAROMETRY Geothermometry

Thermometric method using Fe-Mg exchange between JOUR.GEOL.SOC.INDIA, VOL.80, OCT. 2012


co-existing garnet-clinopyroxene from gabbro yields temperature ranging from 511°C to 789°C (Table 4).The lower range of temperature substantiates the role of cooling/ re-equilibration or metamorphic reconstitution. Similarly, different methods adopted for mineral pairs: garnetamphibole (Graham and Powell, 1984) gives a temperature range of 523° to 537°C for gabbro; amphibole-plagioclase (after Blundy and Holland, 1990) using AlIV content of amphibole and plagioclase records temperature ranging from 871°C to 915°C; and Fe-Mn exchange between co-existing garnet and ilmenite (Pownceby et al. 1991) yields temperature ranging from 603°C to 860°C (Table 4). Geobarometry

Geobarometric data for the Dumka gabbro-anorthosites are presented in Table 5. The empirical calibrations of reactions involving co-existing garnet, amphibole and plagioclase (Kohn and Spear, 1989) in gabbro records

pressure ranging from 5.0 to 6.3 kb. Similarly, estimation of pressure using aluminium(total) content of amphibole (Hammarstrom and Zen, 1986; Hollister et al., 1987; Johnson and Rutherford, 1989) yields a range of 5 to 7.5 kb for the gabbro-anorthosite rocks. DISCUSSION

Plagioclase compositions in the Dumka anorthosites correspond to andesine. Petrographic observation of gabbro (N71) indicates that orthopyroxene grains have formed through dehydration melting of hornblende (Fig.8). Garnet is formed through reaction between orthopyroxene and plagioclase during granulite facies metamorphism similar to those observed in gabbro-anorthosite rocks of Bengal anorthosite (Chatterjee et al., 2008). Distinct petrographic features like kinked twin lamellae (Fig.3f) and peripheral granulation in plagioclase with small, recrystallized

Table 4. Geothermometric data of Dumka gabbro-anorthosite Rock type

Thermometric method Garnet-Cpx thermometry Garnet-Cpx thermometry Garnet-Cpx thermometry Garnet-Cpx thermometry Garnet-Cpx thermometry Garnet-Cpx thermometry

Gabbro (N71) Garnet-Cpx thermometry Garnet-Cpx thermometry Garnet-Cpx thermometry Garnet-Cpx thermometry

Temp. (°C)

Average Temp. (°C)

Reference

589 629 593 619 580 615 638 672 761 789 644 693 616 651 569 667 511 566 738 760

609

Mysen and Heier (1972)

606

Raheim and Green (1974)

598

Mori and Green (1978)

655

Ellis and Green (1979)

775

Ganguly (1979)

669

Dahl (1980)

634

Powell (1985)

618

Krough (1988)

539

Pattison and Newton (1989)

749

Sengupta et al. (1989)

Gabbro (N 71)

Garnet-Amphibole thermometry

523 537

530

Graham and Powell (1984)

Anorthosite (N 66)

Amphibole –Plagioclase thermometry

871 911 876 874

883

Blundy and Holland (1990)

Gabbro (N 71)

Amphibole –Plagioclase thermometry

915 881 894

897

Blundy and Holland (1990)

Garnet-Ilmenite thermometry

630 870 603 860

741

Gabbro (N 71)

Garnet-Ilmenite thermometry

JOUR.GEOL.SOC.INDIA, VOL.80, OCT. 2012

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Pownceby et al. (1991)

490

NILADRI BHATTACHARJEE AND OTHERS Table 5. Geobarometric data of Dumka gabbro-anorthosite Barometric method

Gabbro (N 71)

Garnet-Amphibole-Plagioclase

Anorthosite (N 66)

Al content in Amphibole



Gabbro (N 71)




plagioclase around larger grains provide evidence in favour of post-magmatic deformation. Thermobarometric estimates for Dumka gabbro-anorthosites with temperature ranging from 511°C to 915°C and 5.0-7.5 kb pressure suggest a P-T condition comparable with that estimated for Bengal

Pressure pressure (Kb)

Average pressure (Kb)

6.30 5.95 4.99 6.93 6.72 7.02 6.69 7.41 7.17 7.50 7.13 5.66 5.49 5.74 5.46 6.62 6.24 6.15 6.24 7.06 6.63 6.53 6.63 5.41 5.08 5.00 5.08

5.75

Kohn and Spear (1989)

6.84

Hammarstrom and Zen (1986)

7.3

Hollister et al. (1987)

5.59

Johnson and Rutherford (1989)

6.31

Hammarstrom and Zen (1986)

6.71

Hollister et al. (1987)

5.14

Johnson and Rutherford (1989)

2

Reference

a

1.5

log C.I.

Rock type

1 0.5 0 0

20

40

60

80

100

Plagioclase

log C.I.

2 1.5 1

0.5 0

b 10

20

30

40

Plagioclase/ Pyroxene Fig.8. BSE image showing development of orthopyroxene (opx) from amphibole (amp) and occurrence of garnet (grt), zircon (zrn) and ilmenite (ilm) in gabbro

Fig.9. (a) Plot of log C.I. vs plagioclase for Dumka gabbroanorthosite. (b) Plot of log C.I. vs plagioclase/pyroxene for Dumka gabbro-anorthosite. JOUR.GEOL.SOC.INDIA, VOL.80, OCT. 2012


anorthosite (593-795°C, 4.1-7.3kb) (Ghose et al. 2008). PT conditions during the last phase of deformation are attributed to be higher — 7.0 to 10.5 kb and 775-825°C deduced from the metapelitic granulite of Dumka (Chatterjee et al. 2008). Gabbro-anorthosite magmatism in shield areas is often related to mega-fault formed either due to Late Palaeoproterozoic collision or post-collisional deformations (Larin et al. 2006). Such anorthosite bodies are formed by H2O- fluxed anatexis of gabbroic host rocks in many instances (Selbekk et al. 2000). Gabbro-anorthosite melts are also produced by repeated impulses of crystal-charged melts from basaltic magma (e.g. Bengal anorthosite)

491

(Mukherjee et al. 2005). A gradual decrease in colour index (log C.I.) with increase of modal contents of plagioclase and plagioclase/pyroxene ratio in the gabbro-anorthosite of Dumka (Figs. 9a and b), suggests differentiation from a single parental magma. Acknowledgements: The authors are thankful to L. Melluso, Dipartimento di Scienze della Terra, Università di Napoli for providing EMP analyses. The authors thank Mousumi Banerjee for her active co-operation during the field work. The authors gratefully acknowledge an anonymous reviewer for his valuable comments and fruitful suggestions.

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(Received: 27 December 2010; Revised form accepted: 26 December 2011)

JOUR.GEOL.SOC.INDIA, VOL.80, OCT. 2012