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CHAPTER -4
U-Pb Isotope Studies on Titanites and Zircons from the Granitoids surrounding the Hutti Schist Belt of the Eastern Dharwar Craton
U-Pb ISOTOPE STUDIES ON TITANITES AND ZIRCONS FROM
THE GRANITOIDS SURROUNDING THE RUTTI SCHIST BELT
OF THE EASTERV DNARWAR CRATON
4.1 Introduction:
Titanite (sphene) and zircon are accessory minerals commonly found in
granitoid rocks and syenites. Titanite crystallizes in monoclinic system. It varies in
colour from green, brown to black or colourless. It is a calcium titanium silicate
(CaTiSiOj), in which one of 02- can be replaced by OH- or F ions. ca2+ is substituted
by larger ions including REE, U, Th, Mn and Pb (Higgins and Ribbe, 1976), though U
is preferred over Pb. This property renders titanite as an eligible candidate for U-Pb
isotope geochronology (Pamsh, 1989).
Zircon crystallizes in the tetragonal system and may vary in colour from
colourless to reddish brown. It is a zirconium silicate with the composition ZrSiOa.
The Zr site accommodates U, Th and Hf and largely excludes Pb making zircon an
important geochronometer (Parrish, 1989; Mezger and Krogstad, 1997). Zircon has a
higher closure temperature for U-Pb isotope system than titanite and hence considered
to record the time of its crystallization in granitoid magmas.
Titanite on the other hand can crystallize or re-equilibrate in a variety of
conditions and hence records the time when it was last closed for U-Pb. The closure
temperature for U-Pb isotope system in titanite is still debated though 600°C to 712OC
is considered as the most appropriate range for closure temperature (Scott and St-
Onge, 1995; Zhang and Scharer, 1996). Zhang and Schiirer (1996) and Pidgeon et al.
(1996) have shown that the closure temperature increases with the grain size of
titanites. Nevertheless, zircon seems to lose Pb more readily than titanite at low
temperatures leading to discordance in the U-Pb ages (Mezger and Krogstad, 1997).
This may be due to the high U content of zircon which resclts in metamictization
leading to excessive Pb loss and certain other reasons such as volume diffusion
(Watson and Harrison, 1983). By and large, zircon and titanite, though present in
accessory amounts, chiefly control the distribution of U and Th in most granitoid
rocks along with apatite and allanite.
The granitoids surrounding the Hutti Schist Belt, in the eastern Dharwar craton
are predominantly granodiorites having titanite and zircon as common accessory
minerals. Six granodiorite samples from different locations in the Hutti area were
taken for the U-Pb isotope analysis on zircons and titanites separated from them. Tile
sample locations are marked in Fig. 2.2. The sample descriptions are given in section
3.1.1 ofChapter3.
Feldspar mineral grains were also separated following conventional mineral
separation techniques (described in Chapter 3, section 3.2) and used for estimation of
common Pb isotopic composition. Microscopic handpicking, column chromatography
for the separation of U and Pb and mass spectrometry were performed at the
Zentrallabor fiir Geochronologie (ZLG), Institut f~ i r Mineralogie, Universitat Miinster,
'Germany. The sample preparation, column chromatography and mass spectrometry
procedures are described in the section 3.3 of Chapter 3.
The principle of geochronology using U and Pb is based on the decay of U
isotopes to stable isotopes of Pb. All the three naturally occumng isotopes of U ( 2 3 8 ~ ,
2 3 5 ~ and 2 3 4 ~ ) are radioactive. 2 3 4 ~ is an intermediate daughter in the decay series of
2 3 8 ~ which finally ends in stable '06pb. The end product of 2 3 5 ~ is '07pb. The decay of 238 U and ' 3 5 ~ can be summarized as follows:
2:! u_j2::~b +8: ~e + 6,B- + Q , where Q = 47.4 MeVIatorn
2:i ~ 3 ~ : : Pb +7: ~e + 4P- + Q , where Q = 45.2 MeVIatom
The decay constants (Steiger and Jager, 1977) used are
A * ~ ~ u = 1.55125 x lo-''
; 1235~ = 9.8485 x 1 0 ~ ' ~
4.2 Analytical Results:
Care was taken to pick the clearest zircon grains (15-to 30 grains) free of
visible cores under a binocular microscope. These mineral grains were abraded,
spiked and digested in ~eflon ' lined pressure vessel and taken for U-Pb separation.
Titanite mineral separates (1 A fraction) were also handpicked (50 to 60 grains)
carefully without any visible inclusions under a microscope. These grains were
abraded, washed, weighed, spiked and digested in ~ e f l o n @ lined pressure vessel and
taken for U-Pb separation. The column cheniistry and mass spectrometry were
performed as described in Section 3.3, Chapter 3. The U-Pb data obtained from zircon
and titanite mineral separates from the granitoids surrounding the Hutti Schist Belt
area are given in Table 4.1.
Table 4.1 U-Pb data for zircon and mineral separates from different granitoids surrounding the Hutti Schist Belt.
- Concentrations ,
1:ractionS Atomic Ratios (ppm) Ages ma)
sample u T" ---- 201pb 2 0 6 p b 2 0 6 p b
I[-1 Golapalli Granodiorite Zircon 1 155.78 57.14 1194.40 0.1639 0.1443 0.3130 7.0725 Zircon2 47.22 18.87 227.21 0,1651 0.1535 0.2869 6.5331 Zircon 3 132.14 42.58 396.10 0.1629 0.1370 0.2552 5.7306 Titanite 1 77.26 143.43 47.25 0.1720 1.1229 0.5068 12.0180 Titanite2 17.68 21.10 268.56 0.1716 1.4421 0.4825 11.4133
I[-7 Yelagatti Granodiorite Zircon 1 105.21 36.44 Zircon2 37.58 14.22 Zircon 3 71.04 30.08 Titanitel 110.54 111.77 Titanite 2 13.75 16.01
11-2 Western Granitoids Zircon I 60.80 26.25 Zircon2 49.85 16.63
Zircon 3 54.46 33.72 Titanite 1 90.66 99.65 Titanite 2 80.19 100.65 Titanite 3 20.60 27.43
11-5 Watgal Granodiorite Zircon 1 61.53 26.58
Zircon 2 247.27 16.34 Zircon 3 76.09 32.17 Titanite 1 118.71 119.38 Titanite 2 17.49 19.05
[I-9 Gajalagatta Granodiorite Zircon 1 35.68 15.48 Zircon 2 18.27 8.53 Zircon3 37.17 15.36 Titanite 1 130.96 190.81 Titanite 2 8.50 7.64
A-3 Kasan~doddi Granite Zircon 80.10 13.31
Chczpter 4 U-Pb Isotope Stztdies..
The U-Pb Concordia plots for zircon and titanite are given in figures 4.1, 4.2,
4.3, 4.4 and 4.5. Isoplot 2.49 version of Ludwig (2001) was used for generating the
concordia plots (Wetherill, 1956). Initial-Pb correction was performed on the samples
after determination of Pb isotopic composition on feldspar separates (Table 4.2).
Zircons are more discordant than the titanites. Pb blanks were less than 30 pg and U
blanks were less than 5 pg during the entire analysis. All age errors reported are
estimated using Isoplot 2.49 version on the basis of analytical uncertainties.
Table 4.2 Pb ratios for Feldspar separates from the Hutti granitoids.
4.2.1 Nortlzern Granitoids
Golapalli Granodiorite
Three fractions of zircons and two fractions of titanites were separated from
the Golapalli Granodiorite (location H-1, Fig. 2.2). The zircons were 4 0 0 microns in
size, pink and translucent. They have given 2 0 7 ~ b / 2 0 6 ~ b ages of.2496, 2509 and 2486
Ma for the three different fractions, 1, 2 and 3 respectively. From the concordia plots
(Fig. 4.la) the upper intercept age obtained is 2519 * 500 Ma. Because the zircons are
highly discordant (ca. 50%) the above age could be considered as a minimum one and
the actual age of crystallization of zircon is higher than 25 19 Ma.
Titanites were about 100 microns in size, honey brown in colour. The titanite
2 0 7 ~ b / 2 0 6 ~ b ages for the Golapalli granodiorite are 2577 and 2573 Ma for the two
different fractions. The titanite U-Pb concordia (upper intercept) age is 2574 * 8 Ma
(Fig. 4.lb). One of the titanite samples (TI) is reversely discordant plotting above the
concordia curve. This could happen due to U loss or Pb gain. However, the
2 0 6 ~ b / 2 0 4 ~ b ratio for this sample is low (206~b /204~b = 47.25, Table 4.1) and therefore,
these titanites must have had significant amount of common Pb. Due to uncertainties
in knowing the precise isotope composition of the common Pb, the correction for it
Qlupter 4 U-Pb Isotope Stzddies.. .
could have made this sample to plot reversely discordant. The initial-Pb correction
was performed using the Pb isotope compositions measured on K-feldspar separates
from this sample (Table 4.2).
Yelagatti Granitoid
Three fractions of zircons and two fractions of titanites were separated from
the Yelagatti Granitoid (H-7 in Fig. 2.2). The zircons were moderate in size, pink and
translucent. They have given 2 0 7 ~ b / 2 0 6 ~ b age of 25 10,2523 and 2514 Ma and an upper
intercept discordia age of 2555 h 210 Ma (Fig. 4.2a). The zircons are -40%
discordant and hence the age obtained could be considered as a minimum age of
crystallization of the zircons.
The titanites were > 100 microns in size, brown coloured and subhedral with
broken edges. They have given 2 0 7 ~ b / 2 0 6 ~ b age of 2532 and 2530 Ma for the different
fractions. The titanite upper intercept age on the concordia curve is 2528 h 18 and the
lower intercept age of -1 19 * 800 Ma (Fig. 4.2b). As the lower intercept obtained is
meaningless, the discordia was forced through zero and the upper intercept age thus
obtained is 253 1 A 3 Ma (Fig. 4 . 2 ~ ) .
4.2.2 Western Granitoids
A sample of granodiorite was collected from the western part of the Hutti
Schist Belt near Kardikal at location H-2 (Fig. 2.2). Three fractions each of zircons
and titanites were separated for U-Pb analysis. The zircons are pink, moderate in size
and translucent. They are discordant (35 - 45%) and give an upper intercept discordia
age of 2559 zt 13 Ma (Fig. 4.3a). As they define a tightly fit collinear array (MSWD =
0.81) the above date could closely represent the crystallization age of the zircons and
the actual age of crystallization of the zircons could be older.
The titanites were brown and sub-angular. Two fractions were collected at 1A
and one fraction was collected at O.8A on the isodynamic separator. They plot very
close to the concordia (Fig. 4.3b) giving an age of 2574 * 43 Ma. Their 2 0 7 ~ b / 2 0 6 ~ b
ages of 2555, 2545 and 2557 Ma are indistinguishable from each other and they could
be considered as the time of closure of these titanites to U-Pb isotope system.
Chapter I Lr-Pb Isotope Studies.. .
-.- ,. 8 0 0 ~ " . ,. , . Lower Intercept: 80 * 1700 Ma
7' .,. Upper Intercept: 251 9 i: 500 Ma - 7' ,:-' P,: MSWD = 37 .I. .... , ,... . - -... .. .., .... " . .. . .. ... ,.. . .
Fig. 4.1 U-Pb Concordia diagrams for Golapalli granodiorite. a) discordia line defined by three fractions of zircons. The points are ca. 35-50% discordant due to partial Pb loss. The Pb loss is variable for these zircon fractions; b) discordia line defined by two fractions of titanites separated from this granodiorite. TI is reversely discordant due to inaccurate correction for common Pb.
Chapter 4
Fig. 4.2 U-Pb Concordia diagrams for Yelagatti granodiorite. a) discordia line defined by three fractions of zircons. The points arc ca. 35-45% discordant due to partial Pb loss. The Pb loss is variabic for these zircon fractions; b) discordia line defined by two fractions of titanites separated from this sample. The titanite fractions are close to the concordia curve and hence the discordia thus defined intercepts the concordia curve on the lower side below zcro giving a negative lower intercept age; c) discordia line defined by two fractions of titanites separated from this granodiorite and forccd through zero. The upper intercept age thus obtained is 2530.9 + 3.4 Ma.
3 - 0
r n &!a 04 0 N
, . 2400 , / - , /- ,
- - 2000 - , ,/22, ,
1200 02-- - -- - - - - -
800 , Lower Intercept 163 + 800 Ma Upper lntercept 2555 + 210 Ma
MSWD = 3 0 -%--- -,-. -7-
00 7
Chapter I
I . .' -c----
,
Upper Intercept 2559 * I 3 Ma
Fig. 4.3 U-Pb Concordia diagrams for Kardikal granodiorite. a) discordia line defined by three fractions of zircons which show 35-45% discordance. These points define a tightly fit collinear array and the upper intercept age could represent minimum age of crystallization age of the zircons; b) three fractions of titanites separated from this sample plot close to the concordia curve. The discordia line has a very high lower intercept. Hence, the discordia line is not shown.
U-Pb Isotope Stztdies ...
4.2.3. Eastern Granitoids
Watgal Granodiorite
Zircons and titanites were separated from the granodiorite collected from
location H-5 (Fig. 2.2). The clearest of the zircon grains (three fractions) were
selected for U-Pb analysis. They were 60 to 80 microns in size, pale pink in colour
and translucent. The following are the age data obtained from the three fractions.
2 0 7 ~ b / 2 0 6 ~ b ages are 2483, 2462 and 2458 Ma for the different fractions. The zircons
are discordant and give an upper intercept discordia age of 2474 * 180 Ma (Fig. 4.4a)
which could be the minimum age of crystallization of zircons.
The titanites were pale brown and moderately sized, discordant and give an
upper intercept age of 2548 * 3 Ma (Fig. 4.4b). Because the lower intercept age is -78
=t 280, it is inferred that the Pb loss might have occurred recently due to weathering.
The 2 0 7 ~ b / 2 0 G ~ b ages for the two titanite fractions are 2549 and 2547 Ma.
Gaialaaatta Granodiorite
The sample H-9 from Gajalagatta (Fig. 2.2) immediately east of the Hutti
Schist Belt gives a zircon upper intercept discordia age of 2522 i 210 Ma (Fig. 4.5a).
The zircons were small in size, pink and translucent. They give 2 0 7 ~ b / 2 0 6 ~ b ages of
2512, 251 1 and 2507 Ma for the three different fractions which could be considered
as minimum age of their crystallization.
The titanites were sub-angular, moderate in size and honey brown in colour.
The upper intercept titanite age on the concordia curve is 2539 i 14 Ma (Fig. 4.5b).
The 2 0 7 ~ b / 2 0 6 ~ b ages for the two fractions of titanites are 2544 and 2514 Ma.
Kasamdoddi Granite
One fraction of zircon was separated from the granite sample from
ICasamdoddi. The zircons were pale pink, small in size and anhedral with broken
edges. This fraction has given a 2 0 7 ~ b / 2 0 6 ~ b age of 2173 Ma and is highly discordant
(Table 4.1).
Chapter 4 LT-Pb Isotope Studies ...
_.' /
,.
1200
800/ /" Lower Intercept: 8 + 41 0 Ma , ,. Upper Intercept: 2474 * I80 Ma - ,/' ..,,. /' ..>2, MSWD = 38 .';* '. . ..,",."' ., . . . A -
, , # , # , # , , , , , , ,
Fig. 4.4 U-Pb Concordia diagrams for Watgal granodiorite. a) discordia line defined by three fractions of zircons. One of the points is ca. 90% discordant. The upper intercept age could represent the minimum age of crystallization of zircons; b) Concordia diagram for Watgai granodiorite showing a discordia line defined by two fractions of titanite. The points plot very close to the concordia and hence the discordia defines a negative lower intercept age.
Chapter I C7- Pb Isotope Studies.
, .- Lower Intercept: 405 r 200 Ma ,; Upper Intercept: 2539 i 14 Ma
Fig. 4.5 U-Pb Concordia diagrams for Gajalagatta granodiorite. a) discordia line defined by three fractions of zircons which show 25-30% discordance. Two of the fractions are very close to each other and hence the discordia defined has a higher uncertainty on the age; b) discordia line defined by two fractions of titanites separated from this sample. One of the titanite fractions is slightly reversely discordant due to common Pb correction.
Cfzczpter 4 U-Pb Isotope Studies ...
4.3 Discussion:
The granitoids occurring to the north of the Hutti Schist Belt are distinct from
the rest of the granitoid rocks surrounding the schist belt. The Golapalli granitoid has
yielded a titanite age of 2574 * 8 Ma which is distinct from the titanite ages of the
other granitoids. This appears to be the oldest of the granitoids surrounding the Hutti
Schist Belt based on titanite ages. The titanite age obtained for the Yelagatti
granodiorite is 2531 * 3 Ma. From this it is inferred that these granodiorite plutons
cooled to less than 650°C, the blocking temperature for U-Pb in titanites, at distinct
dates with a minimum difference of 32 million years. The difference in the ages could
be argued as due to different rates of cooling for these plutons.
These plutons show noticeable similarities in mineralogy and texture
(porphyritic nature with I<-feldspar megacrysts) and are exposed in an elliptical zone
north of the Hutti Schist Belt. They were probably emplaced at relatively
upper/shallower crustal levels. The rate of cooling hence must have been faster and
could not have taken more than 10 Ma to cool to ca. 600°C from the temperature of
crystallization (<900°C) of these plutons. Both these plutons, with similar texture,
mineralogy, composition and size would be expected to have similar cooling rates. In
view of this, the difference in titanite ages may indicate their emplacement and
cooling to <6S0°C at different times. The similarity in the rock types can be attributed
to a singular source and petrogenetic process that was responsible for the
emplacement of these plutons. Thus the difference in titanite ages could be due to the
emplacement of these plutons in different time periods.
Prolonged granitoid magmatism lasting tens of million years have been
observed along the active plate margins such as the North American Cordillera
(Wemicke et nl., 1987; Liu, 2001) and the Peruvian Andes (Petford and Atherton,
1992). Occurrences of subduction related granitoid plutons that were emplaced over a
period of 60 Ma (starting from ca. 100 to 40 Ma ago) have been reported from
Transhimalayas in the Ladakh region (Scharer et nl. 1990).
The titanite age of 2531 rt 3 Ma for the Yelagatti granitoids is not recorded in
the granitoids from any other part of the Hutti area. This age for Yelagatti granitoid is
indistinguishable from the zircon ages of 2532 =k 3 Ma and 2528 rt 1 Ma for the
eastern Kambha Gneisses of the Kolar area and the western Gangam Complex of the
Ramagiri area respectively (Krogstad et al., 1991 and Balakrishnan et nl., 1999. Also
refer Tables 2.1 and 4.1).
Chapter 4 U-Pb Isotope Studies ...
The sample (H-2) from the granodiorites occurring towards west of the Hutti
Schist Belt near Icardikal has given indistinguishable ages for titanite and zircon
within the analytical uncertainty. The zircon discordia upper intercept age of 2559 * 13 Ma can be considered as the minimum age of crystallization of zircons from
granitoid magma. The 2 0 7 ~ b / 2 0 6 ~ b age for the titanite fractions are 2555 could
represent the time when the pluton has cooled to < 650°C. Therefore, based on similar
zircon and titanite ages it is suggested that the western granodiorite was emplaced
32559 Ma ago and cooled to < 650°C within a few million years. Furthermore, it did
not undergo a thermal event that could disturb the U-Pb isotope system in titanites
after their emplacement.
Two samples of granodiorites were considered for the U-Pb analysis on
titanites from the eastern granitoids of the Hutti area. One fraction of zircon from a
granite sample from location H-3 near Kasarndoddi (Fig. 2.2) was also used for U-Pb
analysis (no titanite separates from this sample). The samples from Watgal and
Gajalagatta, locations H-5 and H-9 (Fig 2.2), are about 8 km apart. These eastern
granitoid outcrops are characterized by their occurrence as linear ridges running
approximately northwest-southeast. The Gajalagatta pluton (sample H-9) has intrusive
contact relationship with the schist belt (Plate 3.2b), which indicates that the
metavolcanics are older than this pluton.
The titanite upper intercept ages for both Watgal and Gajalagatta samples
(2539 i 14 Ma and 2548 * 3 Ma) are indistinguishable within their analytical
uncertainties. The more precise titanite upper intercept age for sample H-9
(Gajalagatta pluton) of 2548 i 3 Ma can be considered as the time when titanite
closed to the U-Pb isotope system at both the locations occumng to the east of the
Hutti Schist Belt. The similarity in these ages suggests that a series of granodioritic
plutons were emplaced and cooled to < 650°C ca. 2548 Ma ago which occur to east of
Hutti Schist Belt. Further this age also places constraints on the age of the rocks of the
schist belt, which must be older than 2548 Ma.
Precise U-Pb studies on titanites and zircons have been carried out on the
granitoid rocks surrounding Kolar and Ramagiri schist belts of the eastern Dhanvar
craton (Krogstad et al., 1989, Krogstad et al., 1991 and Balakrishnan et al., 1999).
Krogstad et al. (1991) have reported zircon ages of 2631 =i 6.5 Ma, 2610 10 Ma and
2551 * 2.5 Ma and titanite age of 2552 =k 1 Ma for the granitoids occumng west
(Dod, Dosa and Patna plutons) of the Kolar Schist Belt. The Chenna Gneisses
Chapter 4 U-Pb Isotope Studies .. .
occuning east of the Ramagiri Schist Belt has given a zircon age of >2650 * 7 Ma
and a titanite age of 2545 + 1 Ma (Balakrishnan et nl. 1999) representing intrusive and
cooling ages respectively (Table 2.1). Balakrishnan et al. (1999) noticed the
similarities between the Chenna Gneisses that are granodioritic and migmatized and
the western granitoids of the Kolar area that are dioritic to granitic and are not
migmatized.
The zircon age of 2554 k 13 Ma obtained on the western granitoids fi-om Hutti
area is the minimum age for crystallization of zircons. The same sample has yielded a
titanite age of 2557 k 4 Ma. These ages are similar to the titanite ages for Dod and
Dosa gneisses and Patna granite of the Kolar area and the titanite.age for Chenna
gneiss of the Ramagiri area (Table 2.1). In the present study zircons older than 2600
hla are not found. Based on the titanite ages it is evident that the granitoids occuning
to the east and west of the Hutti Schist Belt probably cooled to below 650°C at around
the same time (ca. 2550 Ma) as the western granitoids of the Kolar area and eastern
granitoids of the Ramagiri area.
In the case of Hutti, unlike Kolar and Rarnagiri, the eastern granitoids show
intrusive relationship with the schist belt rocks. Extensive outcrops of migmatites
observed in the Kolar and Ramagiri areas are not encountered in the Hutti area. There
is a general increase in the grade of metamorphism from lower green schist facies to
the upper amphibolite and granulite facies observed fi-om north to south in the
Dhanvar craton (Pichamuthu, 1965; Raase et al., 1986). The signature of a thermal
event that could have affected the rocks of the Kolar and Ramagiri areas between 500
and 800 Ma, as evidenced by discordia lower intercept ages (Krogstad et al., 1991;
Balakrishnan et al., 1999), is absent in the Hutti granitoids. This Pb-loss during Pan-
African tectono-thermal event reported in the granitoids of the Kolar and Rarnagiri
areas may be attributed to their proximity to the Southern Granulite Terrain which has
numerous ca. 550 Ma old granite intrusions (Hansen et al., 1985, Barlett et al., 1995,
Jayananda et al., 1995, Jayananda and Peucat, 1996).
All the zircon and titanite ages obtained for the granitoid rocks of the Hutti
area are less than 2600 Ma old. These rocks are younger than the significant phases of
granitoid gneisses of the western Dhanvar craton, whose ages are in the range of ca.
2900-3300 Ma (Beckinsale et al., 1980; Taylor et al., 1984; Bhaskar Rao et al., 1991;
Naha et nl., 1993; Peucat et al., 1993; Chadwick et al., 1997). The Hutti granitoids are
Chapter 4 U-Pb isotope Studies ...
however older than the 25 13 Ma age reported for the Closepet Granitoids (Friend and
Nutman, 1991).
Based on this geochronological study it is evident that different phases of
granitoid maamatism have taken place during the late Archean around the Hutti area.
The northern granitoids represent the oldest and the youngest of the granitoids in the
Hutti area on the basis of their titanite ages. The western granitoids could be relatively
older than the eastern granitoids, though their' ages are indistinguishable within the
analytical uncertainties. Thus between 2600 - 2530 Ma ago substantial addition to the
continental crust had taken place in the Hutti area.