JOURNAL OF INDIA YAMUNAGEOLOGICAL SINGH AND SOCIETY VEENA KRISHNA Vol.74, August 2009, pp.200-208
200
Rb-Sr Geochronology and Petrogenesis of Granitoids from the Chhotanagpur Granite Gneiss Complex of Raikera-Kunkuri Region, Central India YAMUNA SINGH and VEENA KRISHNA Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Begumpet, Hyderabad - 500 016 Email:
[email protected] Abstract: The Precambrian Chhotanagpur granite gneiss complex (CGGC) terrain covers more than 80,000 sq km area, and is dominated by granitoid gneisses and migmatites. Recent geochronological data indicate that the CGGC terrain has witnessed five tectonomagmatic thermal events at: (i) 2.5-2.4 Ga, (ii) 2.2-2.0 Ga, (iii) 1.6-1.4 Ga, (iv) 1.2-1.0 Ga, and (v) 0.9-0.8 Ga. Of these, the third and the fourth events are widespread. The whole-rock Rb-Sr isotopic analysis of twenty granite samples from the CGGC of Raikera-Kunkuri region, Jashpur district, Chhattisgarh, Central India, yields two distinct isochrons. The eleven samples of grey granites define an isochron age of 1005±51 Ma with moderate initial 87 Sr/86Sr ratio of 0.7047±0.0065, which corresponds to the fourth tectonomagmatic event. On the other hand, the nine samples of pink granites indicate younger isochron age of 815±47 Ma with a higher initial 87Sr/86Sr ratio of 0.7539±0.0066 that matches with the fifth phase of the thermal event. The data suggest emplacement of large bodies of grey granite at ~1005 Ma that evolved possibly from precursors of tonalitic-granodioritic composition. Furthermore, the younger age (~815 Ma) suggests the age of metasomatism, involving isotopic resetting, that resulted in genesis of pink granite bodies of limited areal extent. By analogy, the age of metasomatism (~815 Ma) may also be taken to represent the age of Y-mineralisation in the Raikera-Kunkuri region of the CGGC terrain. Keywords: Rb-Sr Geochronology, Petrogenesis, Grey and Pink granitoids, Precambrian, Chhotanagpur granite gneiss complex, Central India.
INTRODUCTION
The Precambrian Chhotanagpur granite gneiss complex (CGGC) terrain covers >80,000 sq km area in parts of West Bengal, Bihar, Jharkhand, Uttar Pradesh, Madhya Pradesh, Chhattisgarh, and Orissa states. It is dominated by granitoid gneisses and migmatite; and enclaves of different sizes, compositions, and metamorphic grades occur within the gneisses. A major part of the terrain has attained upper amphibolite facies metamorphism and the rest greenschist facies. The gneissic rocks are interpreted to have formed by ultrametamorphism, whereas, the granitoids are believed to have formed from deformation and extensive regional metamorphism and palingenetic melts (Sarkar, 1982, 1988; Majumdar, 1988). The available data indicate that the atomic minerals in the CGGC terrain are hosted mostly by grey and pink granitoids, and related pegmatoids (Saxena et al. 1992; Sinha et al. 1992; Singh and Rai, 1992; Singh and Singh, 1996; Singh et al. 1999). Of the two types, the grey granitoids are
widespread and form large-scale bodies, whereas, the pinktype forms small-scale bodies of limited areal extent. Due to their widespread nature, the grey granitoids have been studied in more detail in various parts of the CGGC terrain, as compared to the pink-type. Even though the pink-type is considered to represent the youngest phase in CGGC (Saxena et al. 1992; Mahadevan, 1992, 2002), the exact time-gap between the two types is not well-defined. It is likely that the grey and pink varieties of granitoids represent a single episode of felsic magmatism; and pink colour of granitoids has been caused by flushing of post-magmatic fluids. The Rb-Sr isotopic systematic is known to be sensitive to secondary processes like metamorphism, hydrothermal alteration, and chemical weathering (e.g. Page, 1978; Weis and Wasserburg, 1984; Singh and Chabria, 2002). It can therefore serve as a sensitive indicator of secondary and local thermal events. On the other hand, studies on polymetamorphic gneisses and granites (Black, 1988;
0016-7622/2009-74-2-200/$ 1.00 © GEOL. SOC. JOUR.GEOL.SOC.INDIA, INDIA VOL.74, AUGUST 2009
Rb-Sr GEOCHRONOLOGY AND PETROGENESIS OF GRANITOIDS FROM CGGC, CENTRAL INDIA
Kamineni et al. 1990) have indicated that a proper sampling strategy, considering the scale of isotopic resetting, could help recognising the times of primary crystallisation and secondary (resetting) events. The Rb-Sr systematic of grey and pink granites can therefore be expected to help delineating events and contribute to an understanding of the Proterozoic crustal evolution of the CGGC. In the light of the fact that the signatures of magmatic to post-magmatic changes would be reflected in Rb-Sr systematics, the present study is undertaken, and results of the same are presented and discussed in this contribution.
and myrmekitic intergrowths. Feldspars show alteration to sericite, with alkali feldspars (microcline and orthoclase) relatively less altered, in comparison to plagioclase. In addition, chloritisation of biotite occurs. Locally, the pink granites are dominated by alkali feldspars, some grains of which are fresh, whereas, others are moderately altered to sericite and kaolin. Locally, alkali feldspathisation of plagioclase is pronounced in pink granite. Also, plagioclase feldspars are partially replaced by quartz and white mica in pink granite. In such cases, quartz is normally interstitial and anhedral, and occurs as rounded inclusions in feldspar. Such alterations are the manifestations of metasomatic/ hydrothermal activity. The modal compositions of the investigated grey and pink granites are given in Table 1. The data indicate that there is no significant mineralogical variation between the two types. However, the average modal abundance of biotite (6.4%) is more than muscovite + sericite together (4.4%) in grey granite, whereas, both tend to be in equal proportion in pink granite (Table 1). Furthermore, cloudy plagioclase and relatively higher abundance of sericite characterize the pink granite. Based on IUGS recommended parameters (Streckeisen, 1976), the granites could be named as biotite granite, or two-mica (biotite-muscovite) granite. However, to keep a distinction between the two types in the field, the terms ‘grey’ and ‘pink’ granites have been retained. Similar to modal compositions, the average chemical compositions of both grey and pink types, with narrow compositional range, also show minor variations (Table 2). Compared to grey granite, pink granite shows subtle, but distinctly elevated concentrations of Na (2.36-2.90,
GEOLOGICAL SETTING
The regional geological framework of the CGGC is shown in Fig.1 (after Rai et al. 1991; Ghose, 1983, 1992). The metasedimentary sequence, comprising cross-bedded, pebbly-, and fine-grained-quartzites and sericite-, chlorite-, biotite-, and hornblende-schists, contains the oldest lithologies in the area. This sequence also contains interbedded (and metamorphosed) mafic lava flows including amphibolites and dolerite dykes. These are further intruded by texturally different types of granites, displaying grey and pink colours and related pegmatites and quartz veins. At places, like Kunkuri, Raikera, a gradual transition from granites to pegmatitic granites, and further normal and zoned pegmatites are distinctly seen (Singh, 1988). The granites show textural variations. They are either biotite granite or two-mica granite, and are characterised by hypidiomorphic granular texture together with perthitic
84° 00'
ga
R
nR
an
.
Gondwana Supergroup
PermoCarboniferous
Vindhyan Supergroup
Middle upper Proterozoic
Bijawar Group
Lower Proterozoic
Chhotanagpur granite gneiss complex
Precambrian
Bhagalpur
So
G
.
80° 00'
Varanasi
Bihar Mica Belt (BMB)
Gaya
Panna
Sidhi
Renukut
24°
Hazaribagh
00'
Rihand R. Jajawal Simdega Jabalpur
Raikera-Kunkuri area Ranchi
Ambikapur
STB Deo
Siri
Purulia
Kanyaluka KOLKATA
Sundargarh
Bilaspur
0
100
200 Km
Delhi Kolkata
Mumbai
Study area Chennai
80° 00'
201
84° 00'
88° 00'
Fig.1. Regional geological map of CGGC (after Rai et al. 1991; Ghose, 1983, 1992). JOUR.GEOL.SOC.INDIA, VOL.74, AUGUST 2009
202
YAMUNA SINGH AND VEENA KRISHNA Table 1. Modal (Volume%) composition of the representative grey and pink granites from CGGC of the study area Grey granite
Modal composition Quartz K-feldspar Plagioclase Biotite Muscovite+ sericite Others
1
2
3
4
Pink granite
5
6
7
8
Av.
28.6 31.3 28.2 27.1 27.4 27.2 29.9 27.3 28.4 34.9 32.4 32.1 33.5 38.9 36.0 31.7 33.9 34.2 24.6 25.7 25.4 26.4 22.5 23.5 24.8 24.4 24.7 4.8 5.7 5.6 6.6 6.2 7.4 7.5 7.7 6.4
1
2
3
4
5
6
7
8
9
Av.
29.3 30.2 26.5 28.3 24.3 27.9 20.1 27.5 38.3 28.0 27.6 31.9 36.6 32.4 37.2 38.7 33.2 33.3 30.0 33.4 30.3 18.6 24.8 24.8 30.4 25.7 32.2 25.9 20.0 25.9 5.5 5.1 7.2 6.9 3.3 4.6 10.1 4.9 5.0 5.8
5.7
3.1
7.5
3.8
3.0
3.8
3.7
4.5
4.4
5.8
11.5
3.4
5.0
3.7
1.9
1.2
6.6
6.0
5.0
1.4
1.8
1.2
2.6
2.0
2.1
2.4
2.2
2.0
1.5
2.7
1.5
2.6
1.1
1.2
3.2
1.8
0.7
1.8
Analysts: P.K. Gupta and L.S.R. Reddy.
METHOD OF STUDY
av. 2.65% Na2O), K (5 – 6%, av. 5.50% K2O), variable but comparatively high K2O/Na2O ratio (1.79-2.36, av. 2.08), and relatively low average molar A/CNK ratio of 1.22; highfield strength elements like Y (53-76, av. 61 ppm) and Zr (302-518, av. 384 ppm) also exhibit elevated concentration (Table 2). Such geochemical variations in pink granite of the Raikera-Kunkuri region of CGGC, compared to grey-type, are considered to be due to metasomatism, accompanied by elevated concentrations of Y and Zr in the former (Singh and Singh, 1996).
Fresh samples were collected from granite quarries (Singh, 1988) for dating by the Rb-Sr whole-rock isochron (WRI) method. The sample analysis was carried out in Geochronology and Isotope Geochemistry (GIG) Laboratory of the Atomic Minerals Directorate for Exploration and Research (AMD), Hyderabad. Samples were digested, using concentrated HF and HNO3 in Teflon digestion bombs at 130° C for 48 hours. This was followed by dissolution in HCl. Separation of Rb and Sr from dissolved rock-solutions
Table 2. Chemical compositions (Oxides in wt%, others in ppm) of the representative grey and pink granites from CGGC of the study area Grey granites Oxide/Ratio/ Element
1
2
3
4
Pink granites 5
6
7
8
Av.
1
2
3
4
5
6
7
8
9
Av.
SiO2
71.59 72.24 72.87 71.90 72.06 72.88 72.06 71.75 72.17
72.58 72.72 71.50 72.06 71.89 72.37 71.06 72.06 72.56 72.08
Al2O3
14.96 14.96 14.96 14.56 14.56 13.97 14.56 14.56 14.64
16.24 14.96 13.83 14.17 13.83 14.56 13.63 14.56 13.63 14.37
Fe2O3
0.95
0.92
0.89
0.91
0.85
1.32
0.89
0.76
0.94
1.03
0.91
1.22
0.96
0.90
0.78
1.25
0.71
0.58
0.92
FeO
0.85
1.01
0.94
0.79
1.04
1.26
0.86
1.12
0.98
1.16
0.76
1.47
1.04
1.37
0.65
1.36
0.97
1.04
1.09
MgO
0.26
0.26
0.26
0.34
0.34
0.26
0.26
0.34
0.29
0.37
0.26
0.46
0.34
0.38
0.26
0.45
0.34
0.48
0.37
CaO
1.58
1.09
1.09
0.97
1.09
1.21
1.09
2.55
1.3
0.08
1.09
1.09
1.09
0.83
1.09
1.40
1.09
1.01
0.97
Na2O
2.36
2.36
2.53
2.53
2.53
2.69
2.69
2.69
2.55
2.80
2.36
2.67
2.53
2.67
2.69
2.90
2.69
2.53
2.65
K2O
5.25
5.10
5.10
5.25
5.25
5.10
5.10
5.10
5.15
5.00
5.56
5.64
5.25
5.64
5.25
6.00
5.25
5.93
5.50
TiO2
0.18
0.27
0.29
0.21
0.23
0.27
0.16
0.18
0.22
0.24
0.20
0.26
0.21
0.25
0.16
0.31
0.18
0.27
0.23
P2O5
0.36
0.43
0.30
0.30
0.30
0.19
0.19
0.26
0.29
0.37
0.38
0.15
0.19
0.12
0.30
0.18
0.30
0.07
0.23
MnO H2O Total
0.004 0.003 0.004 0.004 0.004 0.006 0.004 0.004 0.004 0.08
0.12
0.08
0.06
0.03
0.04
0.09
0.03
0.07
99.42 98.76 98.31 97.82 98.28 99.20 97.95 99.34 98.60
0.05 0.003
0.05 0.003
0.04 0.003
0.05 0.004
0.02 0.024
0.06
0.02
0.05
0.14
0.20
0.13
0.06
0.09
0.04
0.08
99.98 99.33 98.36 97.90 97.97 98.20 98.73 98.19 98.32 98.45
K2O/Na2O
2.22
2.16
2.02
2.08
2.08
1.90
1.90
1.90
2.03
1.79
2.36
2.11
2.08
2.11
1.95
2.07
1.95
2.34
A/CNK
1.23
1.35
1.30
1.29
1.27
1.17
1.26
1.02
1.24
1.60
1.29
1.11
1.23
1.15
1.24
0.99
1.24
1.10
1.22
20
25
22
17
20
19
22
20
20.6
47
17
40
20
55
17
47
21
57
35.7
Cr
2.08
Cu
4
4
3
3
3
2
5
2
3.2
7
3
7
3
7
2
6
2
6
4.8
Ga
20
21
11
15
21
16
15
18
17.1
18
13
17
18
18
15
19
21
16
17.2
Ni
6
6
7
