GEOCHRONOLOGY 6.1 Introduction -...

92

GEOCHRONOLOGY

6.1 Introduction

The nuclides of an element that differ in the numbers of neutrons their nuclei

contain are called its ‘isotopes’. Not all combinations of neutrons and protons form

stable nuclei. Hence, some isotopes are stable but some are radioactive and eventually

changes to a stable element.

One of the earliest applications of the phenomenon of radioactivity was to date

the time of crystallization of minerals using the decay of uranium to lead (Patterson,

1956; Russel and Farquhar, 1960). The activity of a sample of any radioactive nuclide

is the rate at which the nuclei of its constituent atoms decay. The rate of decay of a

radioactive parent nuclide to a stable daughter product is proportional to the number

of atoms, n, present any time t (Rutherford and Soddy, 1902)

- dN / dt = λn

Where, λ is the constant of proportionality, which is characteristic of the radionuclide

in question and is called the decay constant, expressed in units of reciprocal time.

Measurement of the activities of radioactive samples shows that, in every case,

they fall of exponentially with time. Hence, the time variation of activity follows the

formula

N = N0e-λt

this formula gives the number N of undecayed nuclei at the time t in terms of the

decay probability per unit time λ of the nuclide involved and the number N0 of

undecayed nuclei when t = 0.

i.e.

N0 = Neλt

93

The number of radiogenic daughter atoms formed, D*, is equal to the number

of parent atoms consumed:

D* = N0 - N

D* = Neλt – N

D* = N (eλt – 1)

If the number of daughter atoms at time t = 0 is Di, then the total number of

daughter atoms after time t is given as

D = Di + N (eλt – 1)

This equation is the fundamental basis of most geochronological dating tools.

The Rb-Sr geochronology rests on the radioactivity of one of the two isotopes

of Rb, 87Rb, which decays by beta decay mechanism to stable 87Sr which can be

expressed in the form of the following equation:

87Rb →87Sr + β- + v- + Q where, Q = 0.275 MeV/atom

The decay constant used for 87Rb is 1.42 x 10-11 y-1 (Steiger and Jager, 1977).

Rb is an alkali metal with an ionic radius similar to that of K and can

substitute the latter in all K-bearing minerals such as muscovite, biotite, K-feldspar,

clay minerals and evaporite minerals. The isotopic composition of Sr in rock/minerals

containing Rb can be expressed in mathematical form

87Srp = 87Sri + 87Rb (eλt – 1)

Where p indicates the abundance of a given nuclide at the present and i indicates the

initial abundance of that nuclide.

The precise measurement of absolute isotope concentration is difficult and

instead, isotope ratios are normally determined. A stable isotope, not involved in the

radioactive decay scheme (therefore remains constant with time), is used as the

divisor isotope. In the Rb-Sr isotope system the stable isotope used is 86Sr and the

above equation can therefore be rewritten in the form

94

(87Sr/86Sr) p = (87Sr/86Sr) i + (87Rb/86Sr) (eλt – 1)

Sm and Nd are rare earth elements that are found in majority of minerals in

silicate rocks. One of the radioactive isotopes of Sm, 147Sm, decays by alpha emission

to stable 143Nd. Sm and Nd are less mobile in various geochemical environments than

Rb and Sr and hence the Sm-Nd isotope system is considered more robust (Anand and

Balakrishnan, 2010). The decay of 147Sm to 143Nd can be expressed in the form of an

equation as follows:

147Sm → 143Nd + α + Q where, Q = 4.32 MeV/atom

The decay constant used for 87Sm is 6.54 x 10-12 y-1 (Lugmair and Marti, 1978).

The isotopic composition of Nd in rock/minerals can be expressed in

mathematical form

143Ndp = 143Ndi + 147Sm (eλt – 1)


initial abundance of that nuclide.

In this case of the Sm-Nd isotope system the above equation is divided by the

stable isotope 144Nd and can be rewritten in the form

(143Nd/144Nd) p = (143Nd/144Nd) i + (147Sm/144Nd) (eλt – 1)

The principle of geochronology using Pb is based on the decay of U isotope to

stable isotopes of Pb. Of the four stable isotope of lead, only 204Pb is non-radiogenic.

The other lead isotopes are the final decay products of three complex decay chains

from uranium (U) and thorium (Th). The isotopic composition of Pb in rock/minerals

containing U and Th can be expressed in the form of three equations:

206Pbp = 206Pbi + 238U (eλ238t – 1) --- (1)

207Pbp = 207Pbi + 235U (eλ235t – 1) --- (2)

208Pbp= 208Pbi + 232Th (eλ232t – 1) --- (3)

95


initial abundance of that nuclide. All the above equations can be represented as

isotope ratios with the stable isotope, 204Pb, in the denominator.

By dividing the above equation (2) by (1) and rearranging, we get the following

equation:

(207Pbp - 207Pbi) / (206Pbp - 206Pbi) = 235U (eλ235t – 1) / 238U (eλ238t – 1)

207Pb* / 206Pb* = (235U/238U) x (eλ235t – 1)/ (eλ238t – 1)

The 235U/238U is a constant and by knowing the radiogenic 207Pb/206Pb isotope

ratio the time ‘t’ can be determined using the above equation. Recent Pb loss will not

affect the 207Pb/206Pb age because to solve the above equation for t, concentrations

of Pb and U are not required.

Radiogenic isotopes have been used to determine the time of crystallization of

igneous rocks and peak metamorphism and rates of cooling of metamorphic terrains.

More precisely it refers to the most recent time at which the rock or a mineral cooled

below a certain temperature. There are two approaches while dating rocks of igneous

origin formed billions of years ago using long-lived radioactive isotope systems. They

are: a) isochron approach using whole rock samples of an igneous rock or a cogenetic

suite of rocks or a whole rock sample and its constituent mineral; and b) using

minerals with very high ratios of parent / daughter elements.

The most important assumption in the case of whole-rock isochron

geochronology is that each whole rock sample must have crystallized from the same

source at the same time and remained as closed system with respect to the parent-

daughter isotope system. On the other hand, a mineral isochron is valid if only all the

minerals isotopically closed at same time.

The atoms of daughter isotope produced by the decay of parent isotope occupy

unstable lattice sites in parent isotope rich minerals and tend to migrate out of the

crystal if subjected to a thermal pulse. However, if fluids in the whole-rock remain

static, daughter isotope released from parent rich minerals will tend to be taken up by

the nearest mineral in which the daughter isotope is compatible. Thus the whole-rock

96

system may remain closed, even though the system is open at mineral scale. Hence,

the whole-rock isochron is considered to date the timing of crystallization of an

igneous rock (Rollinson, 1995; Dickin, 2005).

The principal control on the retentively of a radiogenic daughter product in

minerals is temperature. For most minerals, at or near the temperatures of their

igneous crystallization, the daughter isotope will diffuse out of the mineral as fast as it

is produced and so cannot be accumulated. On cooling to below a critical temperature

the rate of escape of the daughter isotope would be negligible compared to its

production. This temperature of a mineral is termed as its closure temperature for a

particular parent-daughter isotope system (Jäger et al, 1967; Faure, 1986; Dickin,

2005).

Different minerals will close at different temperatures and different isotopic

systems in the same mineral also close at different temperatures. Biotite had a closure

temperature of 300 ± 50º C for the Rb-Sr system (Jäger et al., 1967). The closure

temperature of muscovite is constrained to 500 ± 50º C for Rb-Sr system. Sphene has

a closure temperature of > 650°C for U-Pb system (Scott and St-Onge, 1995; Zhang

and Schärer, 1996; Pidgeon et al., 1996). For the same system, zircon has a higher

closure temperature of > 900°C (Mezger 1990; Pidgeon et al., 1996). Larger plutons

emplaced at deeper crustal levels will cool slowly. Hence, the minerals with higher

closure temperature will preserve comparably higher igneous cooling age than

minerals with lower closure temperature in case of larger plutons.

Granitoid rocks around the Gadag area of the Chitradurga schist belt from

locations marked in Fig. 2.3 were considered for Rb-Sr, Sm-Nd and Pb-Pb isotopic

studies. The procedure for column separation and analysis of Rb, Sr, Sm, Nd and Pb

are discussed in section 4.1.3 of chapter 4.

6.2 Results

The Rb-Sr, Sm-Nd and Pb-Pb isotopic ratios of the granitoid rocks are plotted

in separate isotopic evolution diagrams using Isoplot 3.7 version of Ludwig (2008) for

evaluation of their respective ages.

97

Ages of Eastern granitoid rocks (Lakkundi domain)

Detailed geochemistry and petrogenesis discussed in Chapter 4 and 5 show

that three different phases of granitoid rocks are present in Lakkundi domain. These

are, granodiorites with LREE enriched fractionated REE pattern, granodiorite with

less REE abundance and positive Eu anomaly and granite with HREE enriched REE

pattern.

In the Rb-Sr isotope evolution diagram (Fig. 6.1) the granite and granodiorites

of Lakkundi domain together define an array whose slope corresponds to unusually

high age of 3659 ± 160 Ma (MSWD = 109) and y-axis intercept, the initial 87Sr/86Sr

ratio of 0.6915 ± 0.0079 which is less than the Earth’s initial value, termed as Basaltic

Achondrite Best Initial (87Sr/86Sr ratio of BABI = 0.69899 ± 5, Papanastassiou et al,

1969). However, trace element and isotope modeling of these granitoid rocks have

shown that their magmas have derived from different sources with varying degrees of

crustal contamination. Hence, this age does not have any geological significance.

When the granodiorites alone were plotted in the Rb-Sr isotope evolution

diagram (Fig. 6.2), it defined a collinear array with substantial scatter, the slope of

which corresponds to an age of 2932 ± 300 Ma (MSWD = 7.0). In this case also the

granodiorites may have had different initial ratios and hence give scatter and the age

given may not represent any geological event. When Sm-Nd ratios of granitoid rocks

of this domain were plotted in a Sm-Nd isotope evolution diagram, they do not define

any collinear array as spread in 147Sm/144Nd ratio was very small.

Pb-Pb isotope evolution diagram was constructed by using K-feldsaprs,

whole-rock and sphenes. Care was taken to pick the clearest sphenes and K-feldspars

free of visible inclusions under a microscope as discussed in detail in section 4.1.3 of

Chapter 4. As the U isotope tracer solution (U spike) was not available during the

course of this work, U/Pb ratio could not be determined and the data could not be

plotted in the concordia diagram (Wetherill, 1956a) to know about the concordance of

the U-Pb isotope ages. It has been observed that the sphenes from granitoid rocks are

usually concordant and their 207Pb/206Pb ages represent the time of cooling of sphenes

to below its closure temperature. Krogstad et al. (1988, 1991), Balakrishnan et al

(1990) and Anand and Balakrishnan (2010) have observed that most of the sphene

98

ages of granitoid rocks of eastern Dharwar craton are close to zircon ages which

represent the time of crystallization of granitoid magma. Hence, in the absence of U-

Pb zircon ages, Pb-Pb sphene ages would be considered to represent either theeither

the age of crystallization of these granitoid magmas or the minimum age of

crystallization.

In the Pb-Pb isotope evolution diagram (Fig. 6.3) for sphene, K- feldspar and

whole-rock of the granodiorite sample Gad 006 define a collinear array corresponding

to an age of2520.9±7.7 Ma (MSWD = 0.13). It also gives a 207Pb/206Pb age of 2521

Ma. In similar Pb-Pb isochron diagram, sphene, K-feldspar and whole-rock of

granodiorite sample G 28 (Fig. 6.4) define collinear array with a slope which

corresponds to an age of 2523.5±8.4 Ma (MSWD = 0.15) 207Pb/206Pb age of this

sample is 2531 Ma. These two granodiorite samples have yielded similar ages which

cannot be distinguished from each other within the uncertainty.

These sphene ages indicate the time when the sphene cooled and reached

temperature of 650º C which is the closure temperature of sphene for U-Pb system.

Igneous zircons have a higher closure temperature of > 900°C. Sphene Pb-Pb isotope

age may represent the time when igneous rock cooled to < 650º C or time when

metamorphic rock cooled below this temperature. However, there is no evidence of

rocks in the Gadag area having undergone high grade metamorphism with

temperature exceeding 650º C. The rocks of the greenstone belt have been

metamorphosed only to greenschist to lower amphibolites facies (Raase et al., 1986;

Harris and Jayaram, 1982). Hence, the Pb-Pb ages of the sphene may date the time of

igneous cooling to ca. 650º C.

The time of crystallization of the granodiorite magma will be nearly same as

the closure temperature of zircons for the U-Pb system (> 900°C). The time interval

between the sphene age and actual time of crystallization of magma may depend upon

the rate of cooling. Relatively smaller plutons emplaced at shallow crustal levels will

cool faster and the 207Pb/ 206Pb age may nearly approach the time of crystallization.

According to petrogenetic models, Gadag area is comprised of a number of smaller

plutones crystallized at medium to shallower depth. Hence, the time of crystallization

of the granodiorites of the Lakundi domain may be same as or somewhat higher than

the 207Pb/ 206Pb sphene age. .

99

The less REE abundant granodiorite sample Gad 007, define a collinear array

of sphene, K-feldsapar and whole-rock (Fig. 6.5), with a slope corresponding to an

age of 2612±26 Ma (MSWD = 1.14). 207Pb/ 206Pb age of this granodiorite is 2605 Ma.

In the Pb-Pb isotope evolution diagram for sphene, K-feldspar and whole-rock

of granite sample Gad 004/1 (Fig. 6.6) characterize a collinear array with a slope

equivalent to an age of 2696 ± 33 Ma (MSWD = 1.13). This granite gives a 207Pb/ 206Pb age of 2693 Ma.

Hence, the Lakkundi domain consists of three distinct phases of granitoid

rocks. Among these, granite phase is the oldest and the REE depleted granodiorites is

older than the other granodiorites. Hence, this domain could be considered as an

amalgamation of three different phases of granitoid intrusions that formed over a time

span of at least132 Ma.

0.6

0.8

1.0

1.2

1.4

0 2 4 6 8 10 1287Rb/86Sr

87Sr

/86Sr

Age = 3659±160 MaInitial 87Sr/86Sr =0.6915±0.0079

MSWD = 109

Lakkundi domain (granites + granodiorites)

Figure 6.1 Rb- Sr isotopic evolution diagram for the granitoid rocks of the Lakkundi domain of Gadag area of Chitradurga greenstone belt. The high uncertainty in age indicates that granitoid phases had different initial 87Sr/86Sr ratio at the time of crystallization. Note that 87Sr/86Sr initial ratio of the array is less than BABI.

100

0.72

0.74

0.76

0.78

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

87Rb/86Sr

87Sr

/86Sr


MSWD = 7.0

Lakkundi domain (granodiorites)

Figure 6.2Rb- Sr isotopic evolution diagram for the granodiorites of the Lakkundi domain of Gadag area of Chitradurga greenstone belt.

0

40

80

120

160

200

240

280

0 400 800 1200 1600

206Pb/204Pb

207 Pb

/204 Pb

Age = 2520.9±7.7 MaMSWD = 0.13

Gad 006 + K-Feldspar + Sphene

Figure 6.3 Pb-Pb isotopic evolution diagram for the granodiorite sample Gad 006 and its K-feldspar and sphene separates of the Lakkundi domain of Gadag area of Chitradurga greenstone belt.

101

0

20

40

60

80

100

120

140

0 200 400 600 800206Pb/204Pb

207 Pb

/204 Pb

Age = 2523.5±8.4 MaMSWD = 0.15

G 28 + K-Feldspar + Sphene

Figure 6.4 Pb-Pb isotopic evolution diagram for K-feldspar, sphene and whole-rock of the granodiorite sample G 28 of the Lakkundi domain of Gadag area of Chitradurga greenstone belt.

0

14.2

14.6

15.0

15.4

15.8

16.2

16.6

17.0

10 14 18 22 26206Pb/204Pb

207 Pb

/204 Pb

Age = 2612±26 MaMSWD = 1.14

Growth Curve intercepts at -344 and 2733 Ma


Figure 6.5 Pb-Pb isotopic evolution diagram for K-feldspar, sphene, and whole-rock of the granodiorite sample Gad 007 of the Lakkundi domain of Gadag area of Chitradurga greenstone belt. Pb-Pb growth curve intercept at 2733 Ma. This granodiorite sample has low REE abundance compared to other granodiorites of this domain.

102

13

15

17

19

21

23

25

0 20 40 60 80206Pb/204Pb

207 Pb

/204 Pb

Age = 2696±33 MaMSWD = 1.13

Gad 004/1 + K-Feldspar +K-Feldspar(L) + Sphene

Figure 6.6 Pb-Pb isotopic evolution diagram for whole-rock, sphene, K-feldspar and its leachete of the granite sample Gad 004/1 of the Lakkundi domain of Gadag area of Chitradurga greenstone belt. This granite sample has HREE enriched REE pattern.

Ages of western granitoid rocks (Srimant Gudda domain)

Srimant Gudda domain is comprised of both granodiorites and granites. In the

Rb-Sr isotopic evolution diagram, granodiorites and granites of this domain (Fig. 6.7),

together define a collinear array with considerable scatter, which corresponds to an

age of 2630±61 Ma (MSWD = 138). However, trace element and isotope modeling of

their petrogenesis show that their magmas were derived from distinct sources. Hence,

these grantioid phases might have had different 87Sr/86Sr initial ratios at the time of

crystallization. Hence, the age does not have any geological significance.

In the Sm-Nd isotope evolution diagram (Fig. 6.8) granodiorites and granites

of this domain, together delineate a collinear array whose slope corresponds to an age

of 2527 ± 13 Ma (MSWD = 0.71). Although this Sm-Nd age has low error and better

MSWD values, this age also may not represent any geological event as they are

petrogenetically two different phases, i.e. granite and granodiorite could have had

different initial 143Nd/144Nd initial ratios and ages. This collinear array may rather

correspond to a mixing relationship between the crustally contaminated granite

magma and more juvenile granodiorite magma. When the granodiorites of this

domain alone were plotted in Sm-Nd isotope evolution diagram (Fig. 6.9) they

103

defined a collinear array with considerable scatter, corresponds to an age of 2543 ± 93

Ma (MSWD = 0.82).

In the Pb-Pb isotope evolution diagram (Fig. 6.10) for sphene, K- feldspar and

whole-rock of the granodiorite sample Gad 014 define a collinear array corresponding

to an age of 2506.5±4.5 Ma (MSWD = 0.0053). In the similar diagram, sphene, K-

feldspar and whole-rock of the granodiorite sample Gad 016 (Fig. 6.11) define a

colliner array with a slope which corresponds to an age of 2501.5±7.1 Ma (MSWD =

1.01). The 207Pb/ 206Pb ages of these samples are exactly same as the Pb-Pb isochron

ages.

Granite sample Gad 017 has little sphene but has garnet. Pb-Pb isochron

diagram for whole-rock, K-feldspar and two garnet separates define a colliner array

(Fig. 6.12) which corresponds to an age of 2709±53 Ma (MSWD = 2.3). This sample

has a 207Pb/ 206Pb age of 2729 Ma.

Hence, Srimant Gudda domain comprises of two distinct phases of granitoids.

Granite phase whose magma was derived from partial melting of continental crustal

sources is older than granodiorite phase.

0

1

2

3

4

5

0 20 40 60 80 100 12087Rb/86Sr

87Sr

/86Sr


MSWD = 138

Srimant Gudda Domain(grnodiorite + granites)

Figure 6.7 Rb-Sr isotopic evolution diagram for the granites and granodiorites of the Srimant Gudda domain of Gadag area of Chitradurga greenstone belt. These grantioid phases might have had different 87Sr/86Sr initial ratios at the time of crystallization. Hence, the age does not have any geological significance.

104

0.509

0.511

0.513

0.515

0.517

0.519

0.521

0.0 0.2 0.4 0.6147Sm/144Nd

143 N

d/14

4 Nd

Age = 2527±13 MaInitial 143Nd/144Nd =0.509360±0.000020

MSWD = 0.71

Gad 016

Gad 017 Gad 017_R

Gad 012 Gad 013

Gad 014

G 15

Srimant Gudda domain

Figure 6.8 Sm-Nd isotopic evolution diagram for the granites and granodiorites of the Sreemant Gudda domain of Gadag area of Chitradurga greenstone belt. Although tightly fitting isochron with a MSWD = 0.71 has less uncertainity, this age also may not represent any geological event as two different phases, i.e. granite and granodiorite could have had different initial 143Nd/144Nd initial ratios and ages. This collinear array may rather correspond to a mixing relationship between the crustally contaminated granite magma and more juvenile granodiorite magma (Gad 017_R indicates replicate analysis).

0.5107

0.5109

0.5111

0.5113

0.5115

0.5117

0.5119

0.5121

0.08 0.10 0.12 0.14 0.16147Sm/144Nd

143 N

d/14

4 Nd

Age = 2543±93 MaInitial 143Nd/144Nd =0.509346±0.000076

MSWD = 0.82

Granodiorites of Srimant Gudda domain

Figure 6.9. Sm-Nd isotopic evolution diagram for the granodiorites of the Srimant Gudda domain of Gadag area of Chitradurga greenstone belt.

105

12

16

20

24

28

32

0 20 40 60 80 100 120206Pb/204Pb

207 Pb

/204 Pb

Age = 2506.5±4.5 MaMSWD = 0.0053


Figure 6.10 Pb-Pb isotopic evolution diagram for whole-rock, K-feldspar and sphene of the granodiorite sample Gad 014 of the Srimant Gudda domain of Gadag area of Chitradurga greenstone belt.

13

15

17

19

21

23

25

0 20 40 60 80206Pb/204Pb

207 Pb

/204 Pb

Age = 2501.5±7.1 MaMSWD = 1.01


Figure 6.11 Pb-Pb isotopic evolution diagram for whole-rock, K-feldspar and sphene of the granodiorite sample Gad 016 of the Srimant Gudda domain of Gadag area of Chitradurga greenstone belt.

106

010002000

3000

13

15

17

19

21

5 15 25 35 45206Pb/204Pb

207 Pb

/204 Pb

Age = 2709±53 MaMSWD = 2.3

growth curve intercepts at -786 and 2953 Ma

Gad 017 + K-Feldspar + Gt 1 + Gt 2

Figure 6.12 Pb-Pb isotopic evolution diagram for whole-rock, K-feldspar and two garnet separates of the granodiorite sample Gad 017 of the Srimant Gudda domain of Gadag area of Chitradurga greenstone belt.

207Pb/ 206Pb isotope ratio in the sphene is not affected by recent loss or gain of

Pb or U. Hence, the ages derived from 207Pb/ 206Pb represents minimum estimate of

time of crystallization of the sphenes. For sphenes analyzed from Gadag area Pb-Pb

isochron ages are agreeing with 207Pb/ 206Pb ages within the analytical and isochron

uncertainties and therefore the Pb-Pb ages are considered robust.

Age of Harpanahalli granite

Harpanahlli domain is comprised of granites with light REE enriched and

heavy REE depleted chondrite normalized REE patterns and one sample with light

REE depleted and heavy REE enriched chondrite normalized REE pattern.

In the Rb-Sr isotopic evolution diagram the granite samples define a collinear

array with considerable scatter, which corresponds to an age of 3392±530 Ma

(MSWD = 4.5). Initial 87Sr/86Sr ratio, 0.679 ± 0.031, is less than that of BABI. Hence

this age may not have any geological significance.

However, In the Rb-Sr isotopic evolution diagram for the minerals of sample

Gad 009/1, Plagioclase feldspar + K- feldspar + Biotite + whole-rock, define a

collinear array with a slope which corresponds to an age of 2808 ± 39 Ma

107

0.80

0.82

0.84

0.86

0.88

0.90

0.92

0.94

2.4 2.8 3.2 3.6 4.0 4.4 4.8 5.287Rb/86Sr

87Sr

/86Sr


MSWD = 4.5

Harpanahalli domain

Figure 6.13 Rb- Sr isotopic evolution diagram for the granites of the Harpanahalli domain of Gadagarea of Chitradurga greenstone belt.

0

2

4

6

8

10

12

14

0 100 200 30087Rb/86Sr

87Sr

/86Sr


MSWD = 1.20

Harpanahalli domain(Gad 009/1 + Plagioclase feldspar + K- feldspar + Biotite)

Figure 6.14 Rb-Sr isotopic evolution diagram for the granites of the Harpanahalli domain of Gadag area of Chitradurga greenstone belt.

6.3 Discussion

The Kolar and Ramagiri Schist belts and surrounding granitoids in the eastern

Dharwar craton have been extensively studied by Balakrishnan & Rajamani 1987;

108

Krogstad et. al. 1989, 1991, 1995; Balakrishnan et al. 1990, 1999; Zachariah et al.

1995, 1996. Tholeiitic and komatitic metabasalts and felsic volcanics are the major

rock types that make up these schist belts. Unlike the Kolar and Ramagiri areas, the

schist belts of the western Dharwar craton are dominated by metasediments. The

rocks of the schist belts in the eastern Dharwar craton are of 2700 Ma age and

distinctly older than the surrounding granitoids which intruded 2630 to 2500 Ma ago

(Balakrishnan et al. 1999).

Reportedly, oldest dated mineral on the eastern Dharwar craton is from

western quartz-monzonite to the south of the Kolar schist belt. It contains inherited

zircons with 207Pb/206Pb ages of 3277 ± 5 Ma, which is interpreted as a xenocrysts

incorporated into a younger magma (Chardon et al., 2002). In the western margin of

the Kolar schist belt, 207Pb/206Pb zircon age data provides a minimum age of 3140 Ma

for the TTG basement (Krogstad et al., 1991). However, these are indirect evidences

for occurrence of Mesoarchean TTG rocks and outcrops of such old basement rocks

are seldom present in the eastern Dharwar craton. The ages of granitoid rocks range

between 2.63 and 2.5 Ga.

Among the different thin elongated greenstone belts all the dated ones show

more or less similar ages of 2700 Ma. That are Kolar (Sm/Nd age of 2732 ± 155 Ma,

Balakrishnan et al., 1990), Ramagiri (2706±29 Ma, Balakrishnan et al., 1999 as well

as Pb-Pb age of 2746 ± 64 by Zacharaiah et al., 1995), Hutti-Maski (Sm-Nd age of

2664 ± 86 Ma; Anand, 2007, Anand and Balakrishnan, 2010.) and Sandur (2691 ± 18

Ma, Nutman et al., 1996).

These granitoid rocks surrounding the schist belts are not genetically related to

the amphibolites of the belt. The available age information indicates that they are all

juvenile additions to the continental crust during the 2650-2500 Ma periods. Whereas,

Chadwick et al. (2000) grouped the entire plutonic complex under the term ‘Dharwar

batholith’ due to the close lithological similarity, structural coherence and

emplacement of imperative lithologies between ca. 2700 and 2500 Ma.

In the western Dharwar craton, TTG form the major part of the crust, which

were formed between 3.4 and 2.6 Ga. Beckinsale et al. (1980) suggested an age of

3315 ± 54 Ma for composite gneisses, based on Rb-Sr isotope studies, which include

109

amphibolite bands occurring near Hasan, ca 200 km west of Banglore. Taylor et al.

(1984) reported a Rb-Sr and Pb-Pb isochron age of 3175 ± 45 Ma for the basement

gneisses from the Western Dharwar craton. U-Pb dating performed on detrital zircons

from Sargur supracrustal rocks yields ages ranging from 3.0 to 3.3 Ga (Nutman et al.,

1992), whereas magmatic zircons coeval with belt formation (Holenasrsipur area)

have been dated at 3.3 Ga (Peucat et al., 1993). Sm-Nd whole rock isochron age of

3352 ± 110 Ma is reported recently for komatiites from Sargur group greenstone belts

by Jayananda et al. (2008). A number of K-rich granites of younger ages around 2.6

Ga are also reported from the eastern-south eastern part of the western Dharwar

craton. Using the original data of Venkatasubramanian and Narayanaswamy (1974),

Rogers (1988) calculated a Rb-Sr whole rock isochron age of 2563 ± 49 Ma for the

Arsikere pluton. Jayananda et al. (2006) reported a more precise SIMS U-Pb zircon

age of 2617 ± 3 for the same pluton. Another K-feldspar- granites pluton from the

nearby area is Chitradurga pluton which has Pb-Pb whole-rock isochron age of 2605

± 18 Ma, (Taylor et al. 1984); Rb-Sr whole-rock isochron age of 2603 ± 28 Ma; mean 207Pb/206Pb age of 2614 ± 9.7 Ma using SHRIMP (Jayananda et al. 2006).

The geochemical data (Fig 6.16) of the eastern Dharwar craton indicate that

these granitoid rocks are juvenile additions to the continental crust during a period of

2650-2500 Ma. However, large scale magmatic additions have had happened during

late Archean between ca. 2550 Ma and 2500 Ma ago. Available geochemical and

isotopic modeling shows that they were derived from metasomatized mantle wedge-

sources and some had experienced small extents of continental crustal contamination.

(Balakrishnan and Rajamani, 1987, Krogstad et al., 1991; Jayananda et al., 2000;

Chardon et al., 2002)

On the other hand granitoid rocks of the western Dharwar craton show a wide

range in age of their formation (Fig 6.16). This may be indicative of continuous

magmatic additions during the cratonization of western Dharwar craton. Among the

younger granitoid rocks of WDC, most of the 2.7 to 2.6 Ga granitoid rocks are

derived from distinctly enriched mantle sources with different degrees of crustal

contamination (Jayananda et al., 2006). Whalen et al., (2002) also have reported

crustal origin for tonalitic plutonic rocks in the western Superior Province, Canada.

110

Hyderabad

Bangalore

Granulite Terrane

Ramagiri

Sandur

Hutti

100 km

N

Deccan Traps

Kolar

Chitradurga

Gadag

Clospet

Granitoid gniess & Intrusives

Clospet granitoids

Eastern Dharwar supracrastals

Western Dharwar supracrastals

Cuddapah Basin

3277 ± 5 Ma (d)3140 Ma (c)

2732 ± 155 Ma (f)

2706±29 Ma (g)

2664 ± 86 Ma (i)

2691 ± 18 Ma (h)

3315 ± 54 Ma (k)

Sargur

Hassan

3352 ± 110 Ma (j)

2617 ± 3 (l) Arsikere

2614 ± 9.7 Ma (l)

2553 ± 2 Ma (e)2553 ± 3 Ma (e)

2516 ± 3 Ma (g)

2528 ± 1 Ma (g) 2545 ± 1 Ma (g)

2528 ± 18 Ma (i)

2559 ± 13 Ma (i) 2539 ± 14 Ma (i)

2911 ± .04 Ma (m)

3028 ± 28 Ma (n)

2523 ± 8 Ma (p)

2612 ± 26 Ma (p)2696 ± 33 Ma (p)2506 ± 5 Ma (p)

2709 ± 53 Ma (p) 2808 ± 39 Ma (p)

2513 ±5 Ma (a)

2518 ±5 Ma (b)

2605 ± 18 (n)

Figure 6.15 Schematic geological map of Dharwar craton modified after Chadwick et al, 1996 and 2000. Geochronological data recorded on both eastern as well as western Dharwar cratons are added. (a) Friend and Nutman, 1991; (b) Jayananda et al, 1992; (c) Krogstad et al, 1991; (d) Chadron et al, 2002; (e) Krogstad et al, 1988; (f) Balakrishnan et al, 1990; (g) Balakrishnan et al, 1999; (h) Nutman et al, 1996; (i) Anand, 2007 (j) Jayananda et al, 2008; (k) Beckinsale et al, 1980; (l) Jayananda et al, 2006); (m) Anilkumar et al, 1996; (n) Taylor et al, 1984.(p) present study.

111

EDC

Age

2600 2800 3000 3200 3400

No

of d

etec

tion

0

2

4

6

8

10

WDC

Age (Ma)

2600 2800 3000 3200 34000

1

2

3

4

5

6

(a)

(b)

Figure 6.16 Distribution of available ages of granitoid rocks expressed as probabilistic histograms with bin widths of 20 My. Uncertainties within the 20 My (± 10) gap are grouped together. Note that most of the granitoid rocks of the eastern Dharwar craton (EDC) show an age peak at 2550 Ma and 2530 Ma. On the other hand western Dharwar craton (WDC) has a continuous spectrum of age distribution. Geochronological datas are from, Friend and Nutman, 1991; Bhul 1987; Meen et al, 1992; Peucat et al, 1993; Bhaskar Rao et al, 1983; Monrad 1983; Jayananda et al, 1992; Krogstad et al, 1991; Chadron et al, 2002; Krogstad et al, 1988; Balakrishnan et al, 1990; Balakrishnan et al, 1999; Nutman et al, 1996; Anand and Balakrishnan 2010; Dhoundial et al, 1987; Jayananda et al, 2008; Beckinsale et al, 1980; Jayananda et al, 2006); Anilkumar et al, 1996; Taylor et al, 1984 and also from present study.

The tectonic contact between the eastern and western Dharwar craton have

been reported close to or all along the eastern margin of the Chitradurga greenstone

belt (Chadwick et al. 1996). In the northern part of the Chitradurga greenstone belt in

Gadag area (present study area), the tectonic contact is represented by a mylonitic

112

zone occurring between the eastern chlorite actinolite schist of the greenstone belt and

the Lakkundi granitoids (Chadwick et al. 2003). Therefore, the Lakkundi granitoids

may be considered as part of eastern Dharwar craton if this mylonitized zone

represents the boundary between the WDC and EDC.

However, the findings of the present study show that Lakkundi granitoid rocks

present to east of the Chitradurga greenstone belt in the Gadag area differ from the

granitoid rocks present in the eastern Dharwar craton. Dominant granite and

granodiorite phases has older 207Pb/206Pb sphene ages of 2696 ± 33 Ma and 2612±26

Ma than the granitoids of the eastern Dharwar Craton which may be considered as the

minimum age because, the age of crystallization of these granitoid rocks may be

similar or older than sphene ages. Their Sm-Nd model ages are also significantly

older and similar old Sm-Nd model ages are not reported from the granitoid rocks of

eastern Dharwar craton.

Furthermore, the trace element and isotope modeling of their petrogenesis

show that the magmas representing the granodiorite rocks of Gadag area were derived

by partial melting of basaltic sources and magma representing the granite was derived

by partial melting of Mesoarchan TTG rocks. Evolution of Nd with time for granitoid

rock samples of Lakkundi assuming 147Sm/144Nd value of 0.10 is modeled and given

in (Fig. 6.17). Modeling calculations show that protolith of the granite sample has Nd

isotope composition similar to CHUR at 481 Ma prior to the time of crystallization of

this sample. This sample would have separated from depleted mantle (DM) at 3335

Ma ago. Nd evolution with time for granitoid rocks samples of Srimant Gudda

domain show (Fig. 6.18) that protolith of the granite sample from the domain has Nd

composition similar to CHUR at 3460 Ma ago. This sample would have separated

from depleted mantle (DM) at 3627 Ma ago. Similar Nd evolution with time for

granite samples of Harpanahalli domain show (Fig. 6.19) that protolith of the granite

sample Gad 011 from the domain has Nd isotope composition similar to CHUR at

3062 Ma. This sample could have been separated from depleted mantle (DM) at 3288

Ma. From these Nd isotope evolution diagrams (Fig. 6.17, 6.18 and 6.19) it can be

inferred that in each of the domain of the magmas representing the protoliths of these

granitoid rocks separated from depleted mantle sources between 3000 to 3600 Ma

ago.

113

Age (Ma)

2000 2500 3000 3500 4000

-12

-8

-4

0

4

8

Nd

DM (Goldstein et al., 1984)

DM (DePaolo, 1981)

2696 3117

Gad 004/1

Gad 007Gad 006

Figure 6.17 Evolution of Nd isotope with time for granitoid rocks samples of Lakkundi assuming Sm/Nd value of 0.10, modeling calculations show that protolith of the granite sample has Nd composition similar to CHUR at 481 Ma prior to the time of crystallization of this sample. This sample would have separated from depleted mantle (DM) at 3335 Ma.

Age (Ma)

2000 2500 3000 3500 4000

-12

-8

-4

0

4

8

Nd


DM (DePaolo, 1981)

2709 3627

Gad 017

Gad 014

Srimant Gudda domain

Figure 6.18 Nd isotope evolution with time for granitoid rock samples of Srimant Gudda domain show that protolith of the granite sample from the domain has Nd composition similar to CHUR at 3460 Ma. This sample would have separated from depleted mantle (DM) at 3627 Ma.

114

Age (Ma)

2000 2500 3000 3500 4000

-12

-8

-4

0

4

8

Nd


DM (DePaolo, 1981)

2808 3288

Gad 011

Gad 009/1

Harpanahalli domain

Figure 6.19 Nd isotope evolution with time for granite samples of Harpanahalli domain show that protolith of the granite sample Gad 011 from the domain has Nd composition similar to CHUR at 3062 Ma. This sample could have been separated from depleted mantle (DM) at 3288 Ma.

Whereas granitoid rocks of eastern Dharwar craton predominantly represent

juvenile additions to the continental crust formed by partial melting of metasomatized

mantle wedge during Late Archean. Thus this raises questions about the mylonitized

zone along the eastern margin of the Chitradurga greenstone belt, whether it

represents the boundary between the eastern and western Dharwar cratons or not.

Perhaps the suture between WDC and EDC may occur further to east of the eastern

margin of the Chitradurga greenstone belt. However, further detailed geochemical and

isotope geochronological studies are needed to understand the significance of the

mylonitized high strain zone occurring along the eastern margin of the Chitradurga

greenstone belt.

GEOCHRONOLOGY 6.1 Introduction -...

Documents

Transcript of GEOCHRONOLOGY 6.1 Introduction -...