U-Pb Isotope Studies on Titanites and Zircons

Transcription

1 CHAPTER -4 U-Pb Isotope Studies on Titanites and Zircons from the Granitoids surrounding the Hutti Schist Belt of the Eastern Dharwar Craton

2 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

3 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 The sample descriptions are given in section 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~ir 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 ( 238~, and ~ 234~) are radioactive is ~ an intermediate daughter in the decay series of which ~ finally ends in stable '06pb. The end product of 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= x lo-'' ;1235~ = x 10~'~ 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 ~eflon@ 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.

4 - Table 4.1 U-Pb data for zircon and mineral separates from different granitoids surrounding the Hutti Schist Belt. 1:ractionS Concentrations (ppm) Atomic Ratios sample u T" pb 206pb 206pb, Ages ma) I[-1 Golapalli Granodiorite Zircon Zircon , Zircon Titanite Titanite I[-7 Yelagatti Granodiorite Zircon Zircon Zircon Titanitel Titanite Western Granitoids Zircon I Zircon Zircon Titanite Titanite Titanite Watgal Granodiorite Zircon Zircon Zircon Titanite Titanite [I-9 Gajalagatta Granodiorite Zircon Zircon Zircon Titanite Titanite A-3 Kasan~doddi Granite Zircon

5 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 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 00 microns in size, pink and translucent. They have given 207~b/206~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 Ma. Titanites were about 100 microns in size, honey brown in colour. The titanite 207~b/206~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 206~b/204~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

6 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 207~b/206~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 207~b/206~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 * 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 A 3 Ma (Fig. 4.2~) 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 207~b/206~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.

7 Chapter I Lr-Pb Isotope Studies ,. 800~".,.,. Lower Intercept: 80 * 1700 Ma 7'.,. Upper Intercept: 2519 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 % 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.

8 /-, Chapter r n &!a 04 0 N 2400,/ -, ,,/22,,, , Lower Intercept Ma Upper lntercept Ma -%- -,-. MSWD = Fig. 4.2 U-Pb Concordia diagrams for Yelagatti granodiorite. a) discordia line defined by three fractions of zircons. The points arc ca % 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 Ma.

9 Chapter I I..' -c----, Upper Intercept 2559 * I3 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.

10 U-Pb Isotope Stztdies 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. 207~b/206~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 207~b/20G~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 207~b/206~b ages of 2512, 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 207~b/206~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 207~b/206~b age of 2173 Ma and is highly discordant (Table 4.1).

11 Chapter 4 LT-Pb Isotope Studies...,. / _.' / /", Lower Intercept: Ma -,/'..,,. MSWD /'..>2, = 38.';* '....,",."'.,...A,. Upper Intercept: 2474 * I80 Ma,, #, #, #,,,,,,, - 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.

12 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.

13 Cfzczpter 4 U-Pb Isotope Studies 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., Also refer Tables 2.1 and 4.1).

14 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 207~b/206~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 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 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, 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

15 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 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 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 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

16 Chapter 4 U-Pb isotope Studies... however older than the 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 Ma ago substantial addition to the continental crust had taken place in the Hutti area.