CHAPTER -6. Geochemistry of Titanites and Sm-Nd Isotope Analysis on Mineral Separates from the Granitoids of the Eastern Dhanvar Craton

Transcription

1 CHAPTER -6 Geochemistry of Titanites and Sm-Nd Isotope Analysis on Mineral Separates from the Granitoids of the Eastern Dhanvar Craton

2 Geochemistry of Titanites and Srn-Nd Isotope Analysis on Mineral Separates from the Granitoids of the Eastern Dharwar Craton 6.1 Introduction: Titanite, zircon, allanite, apatite and rutile are common accessory minerals found in granitoid rocks. These minerals are known to concentrate trace elements like Nb, U, Th, Hf and lanthanoid group of elements (rare earth elements, shortly REE). Allanite can hold much of the light REE and control their distribution, but the modal abundance of allanite in granitoid rocks is much less than 1%. Titanite is known to contain total rare earth element content ranging from to wt. % (Green and Pearson, 1986). In comparison to other accessory minerals, titanite is significantly larger in size and has a typical wedge shaped habit. Gromet and Silver (1983) have shown that more than 80% to 90% of the total REE abundances of granodiorites are found in titanite and allanite. The decrease in overall REE.abundances in rocks representing residual melts can be attributed to the presence of titanite or apatite in the cumulate assemblage formed from monzodiorite or granodiorite parent mammas (Stevenson et al., 1999, Basir and Balakrishnan, 1999). Thus titanite has the potential to explain the distribution of REE among the petrogenetically related phases of granitoid rocks. Titanite also concentrates important trace elements, such as, Nb, Ta, Y and Zr which are used in geochemical studies. Gromet and Silver (1983) reported that the whole rock-normalized pattern of titanite in granodiorite was enriched in heavy and middle REE relative to light REE. Stem and Hanson (1991) showed that the whole rock-normalized REE pattern of titanite was sub-parallel to that of the whole rock. Basir and Balakrishnan (1999) showed both convex-up and sub-parallel patterns relative to the whole rock from the titanites of granitoids around the Hutti-Maski Schist Belt, in the eastern Dhanvar craton. Titanite also crystallizes during metamorphism or migrnatization. When a rock with titanite is subjected to upper amphibolite facies or higher grade of metamorphism there may be changes in the trace element abundance in titanite. It is important to understand the difference in REE abundance between igneous and metamorphic titanites. A comprehensive study of trace element (including RE'E) geochemistry of titanites fi-om different types of granitoid rocks is essential to detennine the relative

3 Chapter 6 Geoche~?listv of Titanites... enrichment of trace elements such as Nb and REE (a) in compositionally different ganitoids like quartz-monz~diorites, granodiorites and granites and (b) in titanites formed or equilibrated during memorization of granodiorites. Titanite and hornblende have similar closure temperatures for Sm and Nd but different Srn/Nd ratios. Hence, these minerals can be used for Sm-Nd isotope dating using isochron method. Two samples, a granodiorite and a quartz-monzodiorite surrounding Kolar and Ramagiri schist belts respectively, whose ages were precisely determined by U-Pb geochronology, were chosen for this exercise. 6.2 Samples and Analytical Methods: The samples considered for the trace element (including REE) studies were from the granitoids surrounding the Kolar and Ramagiri schist belts. Two samples from the Dosa granodiorites (L-1 & K-2), one sample from the Dod quartz-diorites (K-1) and one granodiorite (K-4) from Patna area were collected east of the Kolar Schist Belt and one sample each from Satu (L-3), Tippanapalli (K-5) and Gutapalli (K-3) areas were collected west of the Kolar Schist Belt (sample locations marked in Fig. 2.4). In the Ramagiri area, a quartz-monzodiorite (R-1) and a granodiorite (R-3) were collected from the Gangam Complex occumng to the west of the Ramagiri Schist Belt. A quartz-diorite (R-4) from the central Ramagiri Complex and a sample from the eastern Chenna Gneisses (R-5) were collected (sample locations marked in Fig. 2.5). The methodology adopted for the REE and other trace element analyses and major element analyses on titanites has been discussed earlier in Chapter 3. The REE from the titanite mineral separates were analyzed using Inductively Coupled Plasma - Atomic Emission Spectrometer (ICP-AES, Model: Ultima 2, Jobin Yvon) at the National Facility for Geochemical Research (NFGR) in the School of Environmental Sciences, Jawaharlal Nehru University, New Delhi. The working standards VM9, MBH and DGH of NFGR were used. The titanite-ree elutes were analyzed after calibrating the ICP-AES with the above working standards. The accuracy of the analysis is therefore considered high. Most of the samples were duplicated and were analyzed up to three times to ensure high precision of the analysis (about 5%). The averages of the analyses are given in Table 6.1. The whole rock REE data are from Balakrishnan and Rajamani (1987) and Mohanta (1998). Most of the whole rock REE data were generated using Isotope Dilution technique at

4 Stony Brook, USA with analytical precision of less than 2% while for a few samples the data were generated using ICP-AES (Labtam). The trace elements Ba, Th, U, Nb, Ta, Pb, Sr, Sc, Zr and Y were analyzed on 'B' solutions prepared from titanite mineral separates using ICP-AES (Department of Earth Sciences, Pondicherry University). The major elements in the titanites from the granitoids surrounding Ramagiri and Kolar Schist Belts were analyzed from thin sections using Electron Probe Micro Analysis technique at the Department of Geology, University of Mysore. For the Sm-Nd mineral isochron study two samples, a granodiorite from the Patna granite from Kolar area and a quartz-monzodiorite from the Gangam Complex from Ramagiri area were considered. Precise U-Pb and Sm-Nd isotope studies on the granodiorite from Patna granite has been carried out by Krogstad et 01. (1991, 1995). Detailed geochemical studies on the quartz-monzodiorite from Rarnagiri area has been carried out by Mohanta (1998) and U-Pb isotope study aiso has been carried on this sample by Balakrishnan et al. (1999). Titanite and hornblende minerals were separated from these rocks and purified following the standard techniques described in section 3.2, Chapter 3. The separation of Sm and Nd were camed out following the procedure described in section 3.4 of Chapter Analytical Results: Titnnite Geochemistry The analyzed titanites have high abundances of REE. The REE values in pglg of the titanites from the IColar and Ramagiri granitoids are shown in Table 6.1. The whole rock-normalized plots are useful in understanding the distribution of REE between titanite and the host rock (Fig. 6.1 and 6.2). The whole rock-normalized plots for the REE of titanites from the granitoids west and east of the Icolar Schist Belt are shown in Fig. 6.1 a and 6. lb. From figure 6.1 a it could be seen that the titanites from the rocks west of the Kolar Schist Belt show distinct REE patterns from that of east. The titanites are invariably enriched in the MREE relative to their host rocks, with slight negative Eu anomaly. The sample K-4 from Patna granite shows lower whole rock-normalized REE values than K-1 a quartz-diorite. Titanites from granodiorites L-1 and K-2 show higher I& values than K-1.

5 Chapter 6 Geoclze~?ztsfry of Tifurz;ics... Table 6.1 Trace element abundances (in pglg) in Titanites from Kolar and Ramagiri Granitoids of Dharwar Craton, Southern India. The analytical uncertainty is -5%. 1 Total REE / ' Note: Total REE also includes La abundance in pg/g

6 Chapter 6 Geochenzist~ of Titanites L-1 Dosa Granodiorite / K-1 Dod Quartz-D~or~te K-2 Dosa Granod~orite - 6.," / K-4 Patna Granite --e L-3 Satu Granodiorite 1...+I... K-3 Gutapaili Granodiorite 1 f" -7- K-5 Tippanapaili Granodiorite Fig. 6.1 a) Whole rock normalized plot for the REE of titanites from the granitoids occurring west of the Kolar Schist Belt. These granodiorites have shown nearly parallel REE patterns for the titanites. The titanite/whole rock REE values for the titanites from granodiorites are significantly higher than those for the titanites from granites. b) Whole rock normalized plot for the REE of titanites from the granitoids occurring east of the Kolar Schist Belt. The titanites have shown lower titanite/whole rock values for the LREE relative to the HREE with well pronounced negative Eu anomaly.

7 Chapter 6 Geochenzistry of Titanites... The titanitelwhole rock plots for the rocks east of the Kolar Schist Belt are similar and show an enrichment of WREE relative to the host granitoids. The Kd values of the eastern gneisses are relatively higher than that of the western granitoids. The negative Eu anomaly is more pronounced in the eastern granitoids. Figure 6.2 shows the whole rock-normalized REE patterns for the titanites from the granitoids surrounding the Ramagiri Schist Belt. The titanite from the western Gangam quartz-monzodiorite (R-I) has almost a flat pattern for the REE relative to the host rock. But the titanite from a granodiorite (R-3) that occurs along with the Gangam quartz-monzodiorite shows a HREE enriched pattern with high negative Eu anomaly. The granodiorite from the Central Ramagiri Complex (R-4) has a pattern similar to that of the Gangam granodiorite (R-3). The titanites from the Chenna Gneisses east of the Ramagiri Schist Belt have higher titanitelwhole rock Kd values showing a progressive increase in the HREE content. I0 R-I v--- - I Quartz-Monzodiorite - Gangam Complex &+... R-3 Granodiorite - Gangarn Complex ~ R-4 Quartz-Diorite - Rarnagiri Complex -..* R-5 - Granodiorite Gneiss - Chenna Gneisses I Fig. 6.2 Whole rock normalized plots for the REE of titanites from the granitoids surrounding the Ramagiri Schist Belt. The titanites from the quartz-monzodiorite from the western Gangam Complex have shown very low titanitelwhole rock REE values with a near flat pattern, where as, the titanites from the granodiorite from the Gangam Complex and- quartz-diorite from the Ramagiri ~om~lex show a LREE depleted, HREE enriched titanitelwhole rock REE pattern with negative Eu anomaly. The Chenna Gneisses show very high titanitelwhole rock REE values with little or no Eu anomaly.

8 Chapter 6 Geochernishy of Titanires... The elemental data determined as oxide percentage using EPMA technique and the number of cations calculated on the basis of 20 oxygen atoms are shown in Table 6.2. Trace element abundances of Ba, Nb, Sr, Zr and Y in titanites (Table 6.1) were normalized to their abundances in the respective whole rocks and are shown in Fig The titanites are highly enriched in Nb, Y and Zr while they have lower abundances of Ba and Sr relative to the whole rock. Two of the titanites (from R-3 and R-4) have nluch higher abundances of Zr compared to other samples. The trace element abundances of Ba, Th, U, Nb, Ta, Pb, Sr, Sc, Zr and Y were also normalized to their abundances in bulk crust and shown in Fig Table 6.2 Electron Probe Micro Analysis Data for elements determined as oxide % on titanites from IColar and Ramagiri Granitoids of eastern Dhanvar craton. Area Sample L-1 Kolar Rarnagiri L-3 K-4 K-5 R-4 R-5 FeO MnO CaO Na20 RE203* ) i 0.14, Total / Number of Cations calculated on the basis of 20 0 atoms Si / / *Includes oxides of La, Ce, Nd, Sm, Eu, Gd, Dy, Er and Yb

9 Chapter 6 Geochernistv of Titunites. Fig. 6.3 The abundances of trace elements in titanite mineral separates from the granitoids surrounding the Kolar and Ramagiri schist belts normalized to their respective whole rocks. a) Kolar Granitoids: The titanites have higher Kd values for High Field Strength Elements (HFSE) such as Nb, Y and REE while the Kd values for Ba and Sr are low. The Kd value for Zr is less than that for the REE but greater than one. b) Ramagiri Granitoids: The titanites have higher Kd values for High Field Strength Elements (HFSE) such as Y and REE while the Kd values for Ba and Sr are low. Except the quartz-monzodiorite, R-1, the samples have higher Kd value for Zr.

10 Chapter 6 Geochemistry of Titanites.. o. 0 t 1, ~ Ba Th U Nb Ta La Ce Pb Sr Nd Zr Srn Eu Gd Dy Y Er Yb Fig. 6.4b Fig. 6.4 The trace element abundances of Ba, Th, U, Nb, Ta, Pb, Sr, Zr, Y and REE in titanite mineral separates from a) Kolar and b) Ramagiri granitoids normalized to their abundances in bulk crust. Titanite has higher partition co-efficient for most of these trace elements except for Ba and Sr. The data for the bulk crust are from Wedepohl, (1995).

11 Chapter 6 Geochemistty of Titanites Sm-Nd Isotope Analysis of Mineral Separates The Sm-Nd isotope data obtained for the titanite and hornblende mineral fractions and the whole rock for the two samples (Patna Granite and Gangam quartzmonzodiorite) are given in Table 6.3. Sm-Nd isochron age (Fig. 6.5) obtained for the Patna Granite considering the Sm-Nd data of the present study for the mineral separates and the Sm-Nd isotope data for the whole rock obtained by Krogstad et al. (1995) is Ma (MSWD = 1.3) which agrees well within their analytical uncertainties with the reported U-Pb zircon age of Ma (Krogstad et al., 1991) for this sample. The whole rock-mineral isochron age (Fig. 6.6) obtained for the Gangam quartz-monzodiorite from Ramagiri area is Ma (MSWD = 0.83) which agrees well with the U-Pb titanite age of 2510 * 2 Ma obtained by Balakrishnan et al. (1999) for this sample. Table 6.3 Nd and Sr isotopic data for mineral separates and whole rock samples of Patna Granite from Kolar area and Gangam Quartz-monzodiorite from Ramagiri area. Whole Rock(Krogstad) / / ( i Ramagiri Quartz-monzodiorite from Ramagiri area Titanite Hornblende Whole Rock , & * *

12 Chapter 6 Geochemistry of Titanites... Titanite 1' /+' / ] Age : hole ROC^ [ f / A J',., / ' / ' / = 2608 r 120 Ma -. Hornblend%,' Initial '43~d/'hl~d = MSWD = 1.3 U-Pb titanite age: Ma / U-Pb zircon age: Ma / 4 '." "A, -, Fig. 6.5 Sm-Nd mineral isochron diagram for titanite and hornblende mineral separates along with the whole rock (Krogstad et al. 1991) from Patna Granite occurring to the west of the Kolar Schist Belt. The age obtained is indistinguishable from the more precise U-Pb zircon and titanite ages of h 2.5 Ma and I Ma, respectively (Krogstad et al ). Whole ROC$*', Titanite,*" Fig. 6.6 Sm-Nd mineral isochron diagram for titanite and hornblende mineral separates and whole rock from Gangam Quartz-monzodiorite occurring to the west of the Ramagiri Schist Belt. The age obtained is indistinguishable from the more precise U-Pb titanite age of h 2 Ma (Balakrishnan et al. 1999).

13 Chapter 6 Geochemistry of Titanites Discussion: Three types of granitoid rocks with distinct cooling histories can be identified on the basis of U-Pb isotope studies from Kolar and Ramagiri areas. (a) (b) They are: Rocks with identical zircon and titanite ages indicating faster cooling rate Rocks where titanites are younger by - 10 Ma due to slower cooling rate (c) Rocks where titanites are younger by >80 Ma, indicating that these were affected by post-crystallization thermal events wherein the temperature exceeded the titanite closure temperature for U-Pb isotope system but zircons were not reset. The rare earth elements are accommodated in the seven fold coordinated ca2+ site in titanite. The ~i~~ site could be substituted by AI~+ and ~ e as ~ they + have ionic radii some what similar to ~i~'. Therefore, the possible substitution mechanisms as suggested by Green and Pearson (1986) are: ca" + +i4+ t+ R.EE3+ + A13+ (1) ca2' + si4+ t+ REE3+ + A13+ (2) or ca" + Ti4+ ++ REE3+ + Fe3+ (3) Much worlc has been done on the U-Pb content of titanite and their behaviour in different environments (Pan et nl., 1993, Frost et a]., 2000). In order to understand the substitution mechanism in titanite the major cations determined in titanite (Table 6.2) were plotted versus total REE (Fig. 6.7 a to d). From the plots it could be observed that the REE show a positive correlation with Si and a negative correlation with Ca content. To be consistent with the equations (1 & 2), A1 content should increase with increase in REE, whereas, a negative correlation between REE and A13+ is observed (Fig. 6.7b) indicating that the above suggested s~~bstitution mechanisms are not relevant for the titanites studied. Also, a slight positive correlation is sl~own in the plot Ti vs. REE (Fig. 6.7~). Therefore, to account for the observed correlation between REE and other major cations an alternative substitution mechanism is suggested. The following equation represents the substitution of REE for Ca in titanites: 2ca2' + AI~+ o REE~' + si4'

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15 The distribution of trace elements between a mineral and magma is denoted by the arti it ion coefficient Kd, The partition coefficient is dependent on several factors such as temperature, composition, oxygen fugacity and to a lesser extent on pressure (Rollinson, 1993). Temperature seems to be the major controller of the partition coefficient, (&) with which it has an inverse relationship. Compositional effect on the partition coefficient of REE is also important. The igneous titanites from the Patna Granite occurring to the west of Kolar Schist Belt show low I(d values for the REE. The REE pattem is MREE enriched with a small negative Eu anomaly. The titanites from the granodiorite gneisses Dod and Dosa, occurring to the west of the Kolar Schist Belt that were affected by thermal metamorphism have shown similar REE patterns but with higher I(d values for the REE than the Patna Granite. The granitoid gneisses to the east of the Kolar Schist Belt (Kambha Gneisses) are younger compared to the Dod and Dosa gneisses that crop out to the west of the Kolar Schist Belt (Table 2.1). The titanite ages of Kambha Gneisses are only 10 to 20 Ma younger than the zircon ages (Table 2.1). These gneisses show higher I(d values for the REE relative to those of the western granitoids. The Kambha Gneisses show structures and textures indicative of migmatization and partial anatexis soon after emplacement of their precursors. Low temperature crystallization of megacrystic titanites during migmatization could have caused the enrichment of REE relative to the host rock in the titanites. The high negative Eu anomaly of the Kambha Gneisses can be attributed to the equilibration of metamorphic fluids with plagioclase feldspars under low foz conditions. This must have resulted in considerable enrichment of REE (except Eu) and U in these titanites. In the Ramagiri area, the titanites from the quartz-diorite of the Ramagiri Complex (R-4) and the granodiorite of the Gangam Complex (R-3) have identical REE patterns (Fig. 6.2). The quartz-monzodiorite from the Gangarn Complex shows a more or less flat titanitelwhole-rock pattem with very little variations in the REE. The I(d values are very low for the REE in these titanites. A higher temperature of crystallization and a lower cooling rate could be the reasons for the lower REE contents in the quartz- monzodiorite relative to the coexisting and coeval granodiorite. However, the titanites from the Chenna Gneisses, whicl~ have yielded U-Pb ages that are about 100 Ma younger than the zircons, have higher REE abundance and show higher REE I& values. The metamorphic/migmatitic event has probably brought about an enrichment of the REE in

16 Chapter 6 Geochenzis fry of Titanites.. the titanites relative to their respective wholie rocks. Therefore. it is suggested that the REE, U and Pb in the titanites have re-equilibrated during migmatization of both Kambha and Chenna gneisses from Kolar and Ramagiri areas respectively. Titanites have high density (3.45 to 3.55 gm/cm') and a wedge shaped habit which together facilitate their gravity settling in magmas. Titanite crystals could therefore be effectively removed from granodioritc magma by fractional crystallization (Basir and Balakrishnan, 1999). Since the Kd values for REE in titanites is variable, fractional crystallization of titanites would give rise to residual magmas with different REE patterns. Granodioritic compositions were considered for modeling of the abundances of REE and Nb in granodioritic magmas on fractional crystallization of titanite. Assuming that the titanite crystals were effectively removed from the melt at the instant they were formed, Rayleigh's fractionation equation, C, /C, = F'~-') (where, CL is the concentration of the trace element in the residual liquid, Co is the initial concentration of the trace element in the primary magma, F is the fraction of melt remaining and D is the bulk partition coefficient of the fractionating mineral phase), was considered for the modeling. Based on such modeling it was observed that as low as 2% fractional crystallization of the titanites is sufficient to deplete the REE and Nb and introduce significant positive Eu anomaly in the residual granodioritic magmas (Fig. 6.8). Fig. 6.8 Fractional crystallization modeling involving removal of 0.5%, 1% and 2% titanites from the granodioritic magma. As low as 2% fractional crystallization of the titanites is sufficient to deplete the REE and Nb and introduce significant positive Eu anomaly in the residual granodioritic magmas.

17 Chapter 6 Geochemistry of Titanites... Based on phase equilibria, trace element and isotope studies it has been suggested that the granitic and granodioritic magmas could be derived by partial melting of pre- existed granitoids (Balakrishnan and Rajamani, 1987; Jayananda et al., 1992; Krogstad er al., 1995; Basir and Balakrishnan, 1999). Partial melting of granodiorites leaving a residue consisting of 63-58% plagioclase, 12% clinopyroxene, 5% orthopyroxene, 15% quartz and 5% amphibole with or without titanite (0-5s) was modeled for different extents of partial melting. It was observed that the REE concentration depends chiefly on the residual mineralogy involving titanite and the effect of different extents of partial melting is negligible. The equation for batch melting, also known as equilibrium partial melting, CL/CO = ~/[DRs + F(1 - DR~)] (where, CL is the concentration of the trace element in the melt, Co is the concentration in the unmelted source, DRS is the bulk partition coefficient of the residual solid and F is the weight fraction of melt produced), was considered for the modeling. The REE and Nb abundances in the melt formed by 10% partial melting of granodiorites have been shown in Fig It is inferred from these plots that the melt formed by leaving a residue without titanite has higher REE and Nb abundances with more negative Eu anomaly. The presence of even 1% titanite in the residue significantly lowers the abundances of REE and Nb wcthout tctanlte s U - ui w~th 3% t~tan~te Fig. 6.9 Partial melting modeling of granodiorites formed by 10% partial melting leaving a residue consisting of 6358% plagioclase, 12% clinopyroxene, 5% orthopyroxene, 15% quartz and 5% amphibole with or without titanite (0-5%). Partial melting of granodiorites leaving even very low concentrations of titanite in the resultant residue significantly depletes the REE and Nb when compared to partial melting without leaving titanite in the residue.

18 Chapter 6 Geochemistry of Titunites... The significance of REE geochemistry of titanite in the petrogenesis of granodiorites is hence demonstrated by modeling fractional crystallization involving fractionation of titanite from representative granodioritic magmas and partial melting of pranodiorites presuming resultant residues with or without titanite. The trace element analyses on the titanite mineral separates from Kolar and Ramagiri granitoids have shown that the titanites have higher Kd values for High Field Strength Elements (HFSE) such as Nb and Y (Fig. 6.3). The higher I& values for Nb in titanites is coherent with its substitution for Ti. The fractional crystallization of titanites will alter the Nb concentration in the residual melts (Fig. 6.8). If titanite is present as one of the residual phases during partial melting of intermediate rocks then also Nb will be depleted in the resulting maomas (Fig. 6.9). The Sm-Nd mineral isochron study on mineral separates from Patna Granite from Kolar area and Gangam Quartz-monzodiorite from Ramagiri area has given ages that are indistinguishable from the precise U-Pb ages (Table 2.1) obtained on these rocks (Krogstad et al and Balakrishnan et al. 1999). The uncertainty on mineral isochron ages is high which needs to be reduced by including minerals that were closed to Sm and Nd at similar temperatures and with different SmNd ratios.