ISSN 1028-6845

BGJ BANGLADESH GEOSCIENCE JOURNAL, VOL. 7, P. 1-18 , 2001

Petrogenetic Study of Precambrian Basement Rocks from Maddhapara, Dinajpur, Bangladesh

MD. ZAFRUL KABIR, KHAL°IL R. CHOWDHURY, EUNUSE AKON,

AZIZUL ISLAM KAZI & S.M . MAHBUBUL AMEEN

Abstract

Petrogenetic study of Maddhapara crystalline basement rocks has been ca rried out based on mineralogical and geochemical investigation of some core and tunnel samp les . The rocks are predominantly composed of tonalites and granodiorites with minor adamellite granites and gneisses. Gneiss is the on ly metamorphic li tho-unit encountered in the borehole showing gneissic as we ll as xenomorphic granu lar texture . The tonalites and granodiori tes are characterized by low sili ca and higher iron, magnes ium, ca lc ium ox ide. Their characteristic mafic mineral phases, presence of only magnetite, meta luminous nature, higher CaO and Na20, low K bear testimony that they be long to I-type granito id pluton originated from lower most crust or from upper mantle source. Mineral ogy and chemistry also suggest that the M addh apara basement rocks were formed under orogeni c tectoni c env ironments. The average K20/Na20 ratio of the tonalites-granodiorites of M add hapara is comparab le with the Archean rocks of South India and Singhbhum. It is assumed that the main rock tona lities and granodiorites were evolved through the fractiona l crystal lizati on and differenti ation of a common primary basa lti c magma orig inated from the deepest crust or upper mantle. The adamellites occupying fissures and fractures of the main rocks might have formed from the residual portion of the same melt after crysta lli zation of tonalites-granodi-orites. The granodioritic gneisses indicate a limited scale metamorphi c activ ity in the Maddhapara basement.

Key words: X-ray diffraction, petrochemistry, I-type & S-type, petrogenesis, · Maddhapara.

Introduction Precambrian crysta ll ine basement rocks have been encountered in different

drill ho les at shal lower depths in the Western Fore land Shelf of the Bengal Basin . In other parts of the Basin these are buried under thousands of metres of Tertiary and Quaternary sedimentary deposits. In Maddhapara, the Precambrian crystalline basement rocks are present at depths varying from 128 m to 160 m. Maddhapara mine area lies between the longitudes 89°03'30"· and 89°04'53" and the latitudes 25°33'15" and 25°34'15", and belongs to the tectonic element - Rangpur Sadd le -of the Bengal Basin (Fig. 1 ). So far a very few studies, mainly on megascopi c and petrographic characteristics, have been carried out on these rocks . Taking thi s in

Authors' addresses: M R. M o. ZAFRUL KABIR, PROF. D R. KHALIL R. CHOWDHURY & MR. S.M. MAHBUBUL AMEEN, Department of Geo logica l Sc iences, Jahangirnagar Univers ity, Savar, Dhaka 1342, Bangladesh . D R. EUNUSE A KON, Nuclear Mineral Unit, Atomic Energy Research Establishment, Ganokbari , Savar, Dhaka 1342, Bangladesh . DR. Az1zuL ISLAM. KAZI, Bangladesh Council for Scientifi c and Industrial Research (BCSIR), Dr. Kudrat-e-Khuda Road, Dhaka 1000, Bangladesh. E-mail: khalilrc@juniv.edu

2 M o. ZAFRUL KABIR, KHALIL R. CHOWDHURY, EUNUSE A KON, A.I. KAZI & S.M. M AHBUBUL AMEEN

66°

HIMALAYAS

90°

Legend

- ·-·- -International \ Boundary

~/ --- -Tectonic · · Division { · ~:::;::; . -River

·-.'--..._ ·--rr.-·---/ .

// //

/o~Y sy 100 200Km

I I

Scale

90°

Fig. 1. Locati on of the study area.

mind, petrological, mineralogical , X-ray diffraction and geochemical studies have been carri ed out on six core samples from the depth between 69.5 m and 338.5 m of the dril l hole BH-18 and four tunnel samples of two different levels, e.g., sublevel (-246 m msl) and production level (-270 m msl) of Maddhapara mine site (KABIR 2001 ). The purpose of the study was to classify and elucidate the petrogenesis of these rocks.

Methodology Petrography, X-ray diffraction and geochemical analyses were applied for this

work. For petrographic studies forty thin sections were prepared in the 'Pet-Min Laboratory' of the Geological Survey of Bangladesh (GSB), Dhaka. The bulk powder X-ray study of the rock samples was carried out in a JOEL-JOX-49D X-ray diffractometer using CuKcx radiation in the same laboratory. The samples were run from 2°28 to 70°28 with a scanning speed of 2°28 per minute. The major element analyses of th e representative samples were done in the "Analytical Reserach Divis ion" of Bangladesh Council for Scientific and Industrial Research (BCSIR), D haka . Silica content was determined following gravimetric method (VOGEL 1962). The rest of the elements was determined by Atomic Absorption Spectrophotometry

Petrogenetic Study of Precambrian Basement Rocks 3

(AAS), where SHIMADZU Atomic Absorption Flame Emission Spectrophotometer­AA 640 1 F was used.

Petrography

The petrographi c study has revealed that the tonalites and the granodiori tes are the main litho-units in the drill hole as well as in the tunnels. Gneisses and adamellite granites are the other litho-units. The modal composition of all the litho­units is given in the Table 1.

Table 1. Modes of the litho units encountered in the drill hole and in the tunnel.

Minerals Tonalite Granodiorite Adamellite Gne iss

18/5 18/6 T/8 T/9 18/2 18/3 T/10 18/4 T/7 18/1

Quartz 20.54 19.53 18.70 17.54 25.33 22.26 24.25 30.54 29.20 27.2 7

Plagioclase 46.51 38.20 45.32 43.20 44.80 46.80 43 .75 25.52 27.15 35 .62

K-feldspar 4.78 6.50 5.60 6.22 12.20 12.12 19.25 37.32 40.12 14.76

Hornb lende 14.23 15.66 12.87 14.92 5.80 8.60 2.25 0. 76 1.0 2.2 7

Biotite 5.85 7.86 4.65 . 9.45 4.77 4.16 3.85 1.52 0.5 5 8.46

Ch lorite 4.50 5.40 7. 19 4.45 4.11 1.26 3.75 1.92 1.73 6.92

Pyroxene 2.87 3.66 2.3 0.92 1.87 3.86 1.25 t 0.08 3.07

Sphene 1.37 1.90 2.08 1.25 1.73 2.06 1.21 0.14 0.25 1.02

Opaques 0.12 0.52 0.65 1.02 0.72 0.23 1.02 t t 0.80

Un identified t 0.21 0.10 t 0.64 t t t 0.23 t

18/5 =sample no. 5 of dri ll ho le 18; T/9 =sample no. 9 of tunnel; t =trace

Tonalite

Tonalites are leucocratic to mesocratic, hypidiomorphic, med ium to coarse grai ned rocks having plagioclase, quartz, hornblende, biotite, k-fe ldspa r as major mineral s, while chlorite and sphene are considered as accessory mi nera ls. According to the albite and albite-carlsbad twin law euhedral to subhedral , coarse to medium grained plagioclases usually show polysynthetic and antiperthite texture (Plate 1 ). A number of grains are found in some plagiocl ases containing inclusions of apatite needles, biotite laths and fine opaque minerals. Orthoclase is the dominant potassi um feldspar, which appears as med ium to coarse, colourl ess grains having less alteration and low relief than plagioclase. The platy an hedral microclines are observed with imperfectly developed crossed-hatch twinning. Fine to medium, anhedral, equant quartz grains are easi ly distinguishable with thei r fresh appearance and undulose extinction. Chloritization of hornblende and bi oti te is common. Medium to coarse grained platy hornblendes represent the main mafi c phase in the tonalites and show typical amphibolitic cleavage and strong pleochroism from dark green to brownish green. Their extinction angle vari es from 20° to 28°. Very strongly pleochroic tabular to prismatic biotites occur in close association with hornblende and fibrous, patchy chlorite (Plate 2). Subhedral prismatic pyroxenes are fractured and show reaction texture as wel I as zoning. Very high relieved wedge shaped sphene exists within the interstices of other gra ins.

.14 MD. ZAFRUL KABIR, KHALIL R. CHOWDHURY, EUNUSE AKON, A.I. KAZI & S.M. MAHBUBUL AMEEN

Table 4. Average K20/Na20 ratio of granitoids compared to Rb-Sr radiometric ages and in itia l Sr-ratio.

SI. No. Area K20/Na20 Type of Granitoid Radiometric Initial

(Av.) Age (M yr.) 87Sr/86 Sr

1 Hassan 0.25 Tona lite 3,358 0.700

3,315 0.700

2 Shimoga-Honnali 0.36 Tona lite 3,035 0.7025

3 Tarikere (Kaldu rga) 0.39 Tona lite 3,250 0.702

4 Holenarsipur 0.63 Tona I ite-Granodiorite 3,070 0.702

5 Chitradurga 0.88 Granodiorite 2,475 0.706 2,385 0.704

6 Champion 0.90 Adamellite-Granodiorite 2,820 0.703

gneiss(Kolar) 2,866 0.702

7 Closepet granite 1.02 Granite-Adamellite 2,380 0 .705

2,000 0.711

8 Pod i Ii-Kan igi ri 1.71 Granite-Adame! lite 1,060 -

9 Chipurupalle- 2.92 Gran ite-Adamel I ite 816 0.735

Razam

10 Maddhapara 0.64 Tonalite-Granodiorite - -

Data . 1 to 9 = CRAWFORD (1969), VENKATASUBRAMANIAN (1974), VENKATASUBRAMANIAN et al. (1971 ), VENKATASUBRAMANIAN & NARAYANASWAMI (1974); BHALLA et al. (1977); BHALLA (1979); PERRAJU et al. (1979); BECKINSALE et al. (1980), RAJAGOPALAN et al. (1980) and MONRAD et al. (1982). Data 10 = present study.

present in the deeper part an d the granodiorites are in the upper part of the borehole. These vertical zonations support the fractional crystallization of the ea rlier basaltic magma. The adamellitic granite, having higher quantity of plagioclase than normal granite, may be genetically linked with the tonalites and granodiorites. They might have formed from the residual portion of the melts after crystallization of tonalites-granodiorites. At the latest stage of fractional crystallization the residual silicate melt rich in volatile components has given rise these coarse-grained adamellite granites, which occupied fissures and fractures forming veins in the main rocks. The gneiss is similar in composition to granodiorite. The presence of gneiss indicates that the Maddhapara basement rock has experienced metamorphic activity to some extent.

Based on mineralogical and petrochemica l characteristics the Maddhapara crystal I ine basement rocks comprising predominantly of tonal ites-granodiorites have been classified as I-type granitoid. I-type suits tend to be more ti ghtly constrained in composition than S-type suits (CHAPPELL et al. 1988), probably because I-type igneous protol iths are more homogenous than those of S-type. The S-type granitoids are inferred to have been derived from the partial melting of large volume of metasedimentary rocks in the middle crust (BLEVIN & CHAPPELL 1995). As the Maddhapara crystalline basement rocks are composed of I-type materials the input of metasedimentary material in the source are unlikely. The I-type nature and lower Rb/Sr ratio of Maddhapara tonalites-granodiorites support that the rocks might have been derived by melting of igneous rocks whose source was lowermost crust or upper mantle. The minralogical and geochemical characteristics of the