Chapter-7

Geochemistry of Granitic rocks

Selected samples of medium and coarse grained GG, SG and Barotiya rocks were

analysed to determine major, minor and trace element concentration including REE. The

location of analysed samples is marked in the geological map of study area, given in Fig.

7.1.

7.1 Analytical methods

7.1.1 Sample Preparation for XRF analysis

The samples were processed in three different stages. (i) The rock samples were broken

into small chips of 5cm size. The chips were cleaned with water and air dried on a sheet of

paper in a dust free environment. (ii) The rock chips were crushed using hardened steel

mortar and pestle, the entire sample powder was homogenized, and by coning and quartering

about 200 g of sample was selected. (iii) the sample was the reduced to -200 mesh size using

agate ball mill. In all the above steps, care was taken to avoid cross contamination.

Major and selected trace elements were analysed by X-ray Fluorescence (XRF)

spectrometry, Li by Atomic Absorption Spectrometry (AAS), W by ICP-Atomic Emission

Spectrometry, and Rare Earth Elements also by ICP-AES after pre-concentration of REE by

column chemistry.

For XRF analysis, about 5 g of homogenized powdered rock samples were mixed with

5 to 6 drops of polyvinyl alcohol as binding medium and kept under pressure (5 kglcrn" for

about 2 to 3 minutes to form pellets. 23 samples of Govindgarh granite [13 medium grained

(MGG) and 10 coarse grained (CGG)], 7 samples of Sewariya granite. 4 samples of Barotiya

rocks (2 each of mica schist and meta-volcanics) and 1 sample of pseudotachylite were

analysed for 10 major (Si, Ti. Al, Fe, Mn, Mg, Ca, Na, K and P) and six trace elements (Rb,

Sr, Y, Zr, Nb and Ba) using SIEMENS SRS 3000 sequential X-ray Fluorescence

Spectrometer at Wadia Institute of Himalayan Geology, Dehradun.

Samples were analysed at accelerating voltage of 20140 kV (for major elements) and

55/60 kV (for trace elements) using Rh X-ray tube. International Geostandards for granitic

Scale - ,

i

Fig. 7.1: Geological map of Govindgarh-Sewariya area showing location of analysed

samples.

rock, including GSN. MA-N, GH and DG-H (Govindaraju, 1989, Saini et a]., 1998) were

used as reference standards. Loss on ignition (LOI) was measured after heating the samples

in platinum crucible under blue flame of Bunsen burner for 15 minutes. The data obtained in

the above analyses are presented in Table. 7.1.

7.1.2 Preparation of 'B' Solution for Li analysis using .4AS

'B' solution was prepared for 20 wmples of granites (10 medium-grained GG, 7

coarse-grained GG and 3 SG) following the procedure of Shapiro and Brannock (1 962). 0.5 g

of sample was taken in 70 ml Teflon beaker. 15 rnl of HF, 10 ml of HN03 and 2.5 rnl of

HCIo4 were added to the Teflon beaker. It \+as covered and kept on hot plate at 80-100 OC

for 4 hours. Then the beakers were uncovered and evaporated to dryness. Then 15 rnl of

HN03, 10 ml of HF and 1.5 ml of HCIOl were added and evaporated completely. Further 10

ml of HN03 was added and evaporated to complete dryness. When the fumes stopped, 10 ml

of 2N HCl and 20 ml of distilled water were added to the residue in the Teflon beaker and

gently warmed till the solution became clear. It was found that the solutions contained un-

dissolved grains of tourmaline, which was separated with ashless filter paper.

The residual tourmaline was fused with sodium peroxide and sodium hydroxide in

nickel crucible at about 750 T for 20 minutes. After cooling to room temperature, distilled

water added to the flex, left overnight, and then the contents of the crucible were transferred

to beaker with 1:1 HCl and heated to obtain clear solution. This solution (prepared by

dissolving tourmaline) was added to the previously dissolved counterpart which was

prepared by acid digestion. and made up to 100 ml in volumetric flask. This solution was

used for Li analysis by AAS at Wadia Institute of Himalayan Geology, Dehra Dun. Due to

non availability of boron hollow cathode lamp, B could not be analysed. Results of lithium

analysis are presented in Table:7.1.

7.1.3 Preparation of 'B' Solution for W analysis using ICP-AES

Very recently, an ICP-AES (Jobin Yvon Ultima 2) facility has been developed in the

Department of Earth Sciences, Pondicherry University. An attempt was made to determine

the concentration of tungsten in selected rock samples using ICP-AES. Two different

procedures of solution preparation were tried, by alkali fusion and acid digestion. The result

r r f tungsten analysis done on the aulerion $%hick i~.ii, prcp:ir.pti h> 8lk;ili fusiorl ft;i,jlo\ie~l by

pre-ccincentration of tungsten u as not \atisf"ai'tilr>. H C I ~ C \ C P , an;l$?\E\ of. bungs;cn in B

solution przpared by acid digesiion ?ir.ldcd attcpt;1F31~' ~ c s ~ ~ l b . Both tht"\c prc~cedures of

solution preparation are described bclow.

First, the solution was prepared by aiksli fu<ion method usial,g .;c?diurn p;rt)xide and

sodium hydroxide in the proportion of 7:1?.ii [nixed l i i th (1.5 g of sample iil nickel crucible.

The rr-iixture was heated in a Bunsen burner for about 20 minutes iili the nickel crucible

turned red and a homogenous melt formed. The melt v;ss iuirlcd rcpc.a:rtdly to get the

san-rpie and flex homogeni~ed and subsecjuently it was ailokseii tcr cod. 5 0 ml cpf distilled

uater was added to the crucible and left overnight. The next day contents of the crucibles

were transferred by using 6 N HC3 to 500 rnl beaker, and heated ar "31-1IKI "C ti l l viscous

silica gel formed (H4Si04).

The silica gel was filtered by subsequent cleaning with 6 N HC1 follo~.vcd by Milli-Q

water till no trace of acid remained (filter paper becomes completely white) and the filtrate

was collected. Then the filtrate was transferred, by repeatedly washing ?he filtering flask

with 6N HCI, into glass beaker. The filtrate bras dried completely by heating at about 108 "C

and dissolved with 30 ml IN HC1. Then this solution was transferred to centrifuge bottle by

repeated washing of beaker with 3N HCI. 10-15 drops of phenol red (pH indicator) was

added to the above solution when it becomes orange in colour. Subsequently, 1 : 1 ammonia

solution (NH40H) (about 80 ml) was added to the solution till the colour changes from

orange to pink. pH at this point is about 6.5 - 8.5.

The solution was centrifuged for about 30 minutes at 6000 rpm and supernatant

solution containing mono and divalent ions was decanted and stored separately (first set of

analytical solution. which is not expected to contain tungsten). The precipitate was dissolved

in 2N HCl and evaporated to complete dryness. Subsequently, it was dissolved in 30 rnl of

IN HN03 and made up to 50 ml using distilled water. These were stored separately as the

second set of analytical solution, which is expected to contain tungsten. When the two sets of

solutions were analysed for tungsten using ICP-AES, significant values of tungsten were

obtained in both the sets of solutions, which showed that pre-concentration of tungsten could

not be achieved by this procedure. These analytical data were therefore discarded.

Alternatively, B-solution was prepared for 15 samples (6 samples from medium

grained GG, 5 from coarse grained GG and 2 each from Sewariya granite and Barotiya mica

schist) by acid digestion using HF-FINO3-E3CEO:. as descsikcd in the previous section cpf this

chapter. It was found that tourmaline was not co~npletely digehted. Published data o n

chemical composition of tourmaline show that tungsten concentration is below the detection

limit (Neiva, 1974). Therefore. assuming that tourmaline does not contain detectable

concentration of W, the tourmaline grains were filtered and discarded. and the clear solution

was made up to volume in 100 ml flask and used for analysis by ICP-AES (I:ltirna-2 o f Jobin

Yvon, France). The instrument was calibrated for W concentration by diluting the 10.0660

ppm W standard solution provided by Jobin Yvon, France to 0.25 pprn, 0.5 ppm, 1 ppm. 2.5

ppm and 5 ppm. Accuracy of the analytical measurement was checked by using the granite

standard, DG-H (standard from Wadia Institute of Himalayan Geology. Dehra Dun, India].

The DG-H standard yielded W concentration of 7 pprn by our analysis, kvhich matches well

with the prescribed value 8 ppm. W concentration measured for the rocks of study area is

listed in Table 7.1.

7.2 Geochemistry of Granitic rock

7.2.1 Classification

In total alkalis vs. silica diagram (Cox et al., 1979 and Wilson, 1989). SG falls in the

field of granite whereas the two varieties of GG fall in both alkali granite and granite fields

(Fig. 7.2). It is also observed that SG has a wider range of silica content compared to GG;

whereas there is a narrow range of total alkalis content in SG and MGG while it varies

widely in GG.

Govindgarh granite can also be called as tourmaline leucogranite because by visual

estimate it contains < I 0 volume percent mafic minerals (tourmaline +garnet). Biotite is t h e

dominant mafic mineral in Sewariya granite and it is appropriate to name this rock as biotite

granite.

The Sewariya granite and both varieties of Govindgarh granite are peraluminous (Fig

7.3) with AICNK ratio (molecular proportion of Al203/[CaO+Na~O+K20] 21). All samples

of GG contain normative corundum (ranging 0.71-2.55 and 0.20-3.06 wt.% in MGG and

CGG respectively) with N C N K ratio >1.4 (except one sample of CGG, G-259, with a v a l u e

of 1.24). AICNK ratio is in the range of 1.41- 1.66 in medium grained GG and 1.24- 1.7 1 i n

coarse grained GG. SG has N C N K ratio in the r a n F of 1

1.53-3.57 wt.%. The relatively wider range of A/CNK in G

101

the presence of nrore number of alumiiao-silicate ntlinerals ~tounnaline and garnet. in addition

to feldspars and muscovite) and minor variation in their relative proportion.

.-

-~ 2 Mec tw grained g.a?!?e 2 a Caarse paned g.i i ilr

Fig. 7.2: Plot of granites from the study area in silica vs. total alkalis diagram (after Wilson,

1989)

1 Meraluminous I

Fig. 7.3: Plot of GG and SG in alumina-saturation diagram. (from Shand, 1947). [filled

triangle- MGG; filled square- CGG; open circle- SG].

7.2.2 Major and trace element geochemistry

The composition of medium grained GG and coarse grained GG are comparable and

while both these are different from SG in many elemental abundances. Si02 content of

Sewariya granite shows a wider range from 68.55 to 74.56, as compared with Govindgarh

granite (70.29 to 74.87 and 71.41 to 74.66 wt.% Si02in MGG and CGG respectively).

GG is enriched in NazO compared to KzO (mean NazO 5.26, 5.68 and K20 3.64, 3.00

wt.% respectively in MGG and CGG), whereas SG contains more K?O than Na20 (mean

K 2 0 4.92 and Na20 2.71 wt.7~). This is related to the dominance of sodic plagioclase over K-

feldspar in GG, and vice-versa in SG. While plotting KzO versus NazO wt.%, it is observed

that both the varieties of GG define a inverse relation between these two alkalis while no

such trend is discernible in SG (Fig. 7.4).

Fig. 7.4: Plot of GG and SG in NazO vs. K 2 0 wt.% diagram Symbol are same as in Fig. 7.3.

The A1203 content of GG (ranges 14.15-15.26 wt.% and 13.70-15.95 wt.% in MGG

and CGG respectively) and SG (ranges 13.55 - 14.74 wt.%) are comparable. In GG, CaO is

in the range of 0.46-1.18 and 0.36- 1.59 wt.% in MGG and CGG respectively; while it ranges

from 0.64 to 1.94 wt.% in SG. The Fez03 content of GG is relatively low, 0.24-0.86 wt..% in

medium grained GG and 0.0.5-1.04 wt.% in coarse grained GG: as compared to Sewariya

granite which contains 1.62 - 3.01 wt.% Fe203.

No significant variation is observed in MnO content of GG and SG. In SG, P20S

content ranges from 0.14 - 0.24 wt.%. whereas in GG it shows wider range from 0.07 - 0.39

wt.7~ and 0.06 - 0.46 wt.% in medium grained and coarse grained GG respectively, though

the average concentration of P,Oj is comparable in all these rocks. Nearly comparable

concentrations of A1203 and PzOj in all these granites indicates that P205 solubility in the

parent magmas is controlled by their peraluminosity (Pichavant, 198 1,1987; Manning, 198 1)

Occasional higher values of P20j in the GGs may be due to presence of apatite, which is

commonly observed in outcrops of GG.

Among trace elements, Ba concentration is in the range of 1 I - 206 ppm and 6- 422

ppm, Rb content 70 - 328 pprn and 49 to 304 ppm, and Sr concentration ranges 20 -197 pprn

and 33- 168 pprn in M G G and CGG respectively. Zr content varies from 2 1 to 56 pprn and 21

to 54 pprn in MGG and CGG respectively; h% (except one sample in MGG) is <6 pprn

[except 6245 (12ppm)l in GG, whereas Y is <10 pprn in GG (except 3 samples in MGG and

one sample in CGG).

In SG, the Ba concentration is in the range of 14 - 670 pprn and Sr 18 - 100 ppm.

Two samples of SG (SG-147 & SG-221) have high Ba (456 & 670 ppm) and Sr (72 &lo0

ppm) concentrations compared to other samples. Rb concentration varies from 2 12 to 58 1

ppm, and Zr 66 to 69 pprn in SG, whose Y and Nb contents are higher (28 to 37 pprn and 9

to 14 pprn respectively) than that of GG (<lo ppm of Y and Nb).

The composition of pseudotachylite (which occurs as irregular veins within SG) is

comparable with SG in all elements, except for its relatively high Y and Nb content, and

therefore it is inferred to have formed by near-complete melting of adjoining SG during

shearing event.

7.2.3 Contrasting Geochemical Characteristics of GG and SG

There are several contrasts in the geochemical characteristics of older Sewariya

granite and the younger Govindgarh granite. GG has a narrow range of SiOz and a wide

range of alkalis, whereas SG has a wide range of Si02 and a narrow range of alkalis. The

relative proportion of mafic components Ti02+Fe203+Mg0 is generally low in GG (ranges

from 0.1 to 1.23 wt.% with a mean of 0.63 wt.% in CGG and from 0.39 to 1.06 wt. % with a

mean of 0.74 wt. %). and the corresponding values are higher in SG (ranges from 1.9 to 3.98

wt.% with a mean of 2.67 ~1.5%). Higher total iron oxide and TiO, wt.% in SG is due to

presence of biotite as essential mineral. Presence of euhedral tounnaline crystals in the

Govindgarh granite reflects their early crystallisation from the parent magma which has

minimized the formation of other ferromagnesian minerals.

CaO content of SG decreases with increasing silica content (Fig. 7.5) which could be

related to in-situ crystal fractionation of plagioclase. Similarly, total iron (expressed as

Fe.03) and MgO contents of SG also decrease with increasing silica content which could be

related to in-situ crystal fractionation of biotite. Relatively small variation of alumina

content with silica in SG indicate that A1203 in the parent magma could have been buffered

by phases in which Al is an essential constituent (Hanson and Langmuir, 1978j.The

concentration of K 2 0 in SG is higher than that of GG and shows slight increase with

increasing Si02 (Fig. 7.5).

Compared to SG, NalO and A1203 contents are higher in GG phases and show wide

range of these value. Compared to SG, Fez03 content of GG is much less compared to SG

(1.5 to 3 wt.%) and ranges from 0.1 to 1 wt.%.

Large differences exist in the concentration of Rb, Nb, Zr and Y between GG and SG,

with all these 4 elements enriched in SG as compared to GG. The range of concentration of

Rb, Sr, Ba, Y, Nb and Zr is much wider in SG, and in many bivariate plots of these trace

elements there is either positive correlation (in Nb-Y, Y-Zr, Sr-Zr, Sr-Baj or inverse relation

(in Rb-Sr) between pairs of trace elements (Fig. 7.6). In the two varieties of GG, the range of

concentration of these trace elements is relatively narrow, and no correlation is discernible in

the bivariate plots. The concentration of Li is abnormally low in the two varieties of GG

(average of 12 ppm in MGG and 4 ppm in CGG) as compared to SG (average 33 ppm).

The relatively high Na20 and Sr concentration and positive Eu anomaly (presented in

next chapter) in MGG and CGG are due to the higher modal percent of sodic plagioclase in

these leucogranites. GGs also have lower concentration of trace elements (eg.:Zr,Y,REE) that

mainly reside in accessory minerals of the source rock, which indicates that a significant

proportion of these elements remained in the restite during anatectic generation of the

granitic magma.

Fig. 7.0: contci: Trace clt.rnent variation i n GG and SG. Values are plotted in ppm.

Symbols arc. sanrc as in Fig 7.3.

7.2.4. Quartz-rilbite-Orthaclase Diagram of Tuttle and Bowen (1958)

Tuttlc and Bou cn ( 1958) ohser~ ed that ( 1 ) hqdrous silicate melts that occur at the lowest

ternpratirre4 contain clojt. t o equal proportions of quart^, orthoclase and albite, and (2) granites

contalnlng more than 80 rtt.(.i. 01 yuartc. orthoclase and albite components. have compositions

that art. close tto thts\r lo*est temperature melts. These experiments are consistent with granites

forming a\ end prtduct of fractional crqstallisatiun of ~nafic melts, or as primary granitic melts

fornicd during panial meltirlg, a\ is no^, %idely thought to bt: the case.

A11 the \ar-t~ple\ crf SG and GGs contain more than 80% nornlative quart^+ orthoclase

+albite. These ratios are plotted in Fig. 7.7 after Tuttle and Bowen (1958). Samples of Sewariya

granite Pall clvsc to the scrnarq ~ninima (haplogranite filed) corresponding to water pressures of

5 0 to 3U(N bars. Some samples of GG lie closc to ternarj minlma at relatively higher water

pressures betctccn 3000 and 5000 bars, while many samples of GG are scattered away frarn

ternary mir~ima for different water pressures.

Since the samples of GG <fa not show any definite differentiation trend in ~i l ica vs. other

elements variation diagrams, it is inferred that the scattered disposition of GG in Fig. 7.7 indicates

the variation inhe r i~d from heterogenous source rack. The relative pos~tions of SG and GG in Fig.

7.7 also indicates that generation of SG melt took place at lower water pressures as compared to

that of GG melt.

. ""* i F r - ' r m i ?tr '* . U>* . ,r*. .~-l

Flg 7 7 f'1t)t of SG .ind C X ; III 7'tlttlc' ,lnd Bo\ccn'\ (19.58) quartt-alb~te-orthocla~t' phase

di,t~r,iir~ ('trit 'itli ' t'ur\e"\ fi>r vtater-\ctturattd I~yuid\ In equil~hrium ~ i l t h quartL and

a1A;il: t'ctif4par ; i t tiif'krcnt conlinlng pres$urc\ t i n bar) and the hdplogranite field are

7.25 IlIulti Elenlent tbriatisn Diagram

The cctntcpr of ydtr d~agrann u u i put fornard by Wocd et al ( I979 ). Sun ( 19XO),

Thompicxn r IOS2r anit 'I'ht~r~tp>on el ti1 ( 19841. ?*hi$ diagram helps to define the behaviour of

~nccrn~patihic clir'~;~cna\ during piinial rnclting and fractional crystallisat~on. A range of

incomprt;rihlc r.tcnscnt\ arc plttired Irl the X-axi\ in the docreasing order of incompatibility and in

k"-a?& t31e:r ahuacl~ncci arc nr>nnaliicif ru t\timatcd r~hundances in the primordial earth (Fig. 7.8).

h4urlii elrrmtcrlr tilagram, reflect d~fkrent behat iors of d~f'ftrtnt groups of trace elements. For

exampla I,I1., elcrt%cnrg. arc mart rllakile (Ce. Rh, K, Ba, Sr, Eu), i n contrast to the less mobile HFS

eleraent\ {S, Exlf, 23, lt, Nb, 'EL) (Railinion, 1993). The LIL elemtnts concentrations may be a

function d rhit &$laliar of a fluid phase, svherc~q the WFS element4 coszcentrations are oantrelled

by the rkrntisrry c , l the ~irun.c: ilnd the crybttaVmclt pr(xesses which have taken place during the

evaluriosr of the rock. The: hetemgeneous 811ix of these trace elements produces peaks and troughs,

sod the clmc.; &frn& by diffe~nt elements may provide usefut petroeentic infornlation

Concerning crysat-iiquid quili$fia.

Fig. 7.8: himir i te nnmtle nonnaiized spider diagram showing the granites fro111 srudy area

(afier Tay k ~ r anstel Mchnnan, 198 1). (a1 McBi~irn grained GG, (b) SG .

The incoinpatihle elements of GG and SG of the analysed samples of are plotted in n~ulti

element variation diagram (Fig. 7.8), after normalizing with primordial mantle values

(h.lcDonough et a!.. 1992. cf. Rollinson, 1993; Sun. 1980). It is observed that SG and GG show

different patterns in this diagmm, with Inany peaks and troughs in SG samples, and relatively flat

pattern produced hq GC. All samples of SG show trough for Sr, P and Ti, whereas GG shows

trough for P and Ti tind only one sample shows trough for Sr. The patterns produced by GG and

SG are comp;zrnble tvith upper continental crust values (Taylor and bIcLennan,l981).

I n Fig. ?.9. the. conct.ntrntions of Zr in the granites (SG and GG) are compared to the

propi>rtion\ of %r Ill:tt can bc dissolved in granitoici melts of various compositions at different

tempera1urt.s (W;fticir~ ttrlii Harrin~n, 19X.3). The temperature line marked in the diagram are

from KaIskck, 3 X E I Ifor 7IX)"C) and Wzitson and Harrison. 1983 (for 750°C - 800°C) with

extrapolations o f t!lt wmperature line. ar the lower end form 0.5 to 0.8 of cation ratio. It is

okscned that thc s;irnplcs ofC;ovindgarh granite fall at lower tenlperatures (< 750 "C) cornpared

to those. of Scuar i i r . granite (> 750°C). I t also observed [flat there is a difference between early

fom~eii mitdiunl , p i n e d GC; (mostly ahove 7Ot) "C) and late ibnned course grained GG (many

bc10~ ' 7 0 ~ ~ "i.) i n rhl5 diii~ram.

Fig. 7.9: Zn rlitnc'prntr;itia,rI irt [he. grxtnires plottcd against cation ratio (Na+K+ZCa,/(AI 5).

iaN4br Watson and I+;nrri\un, 19831. Sjr~; thI \ are .;anle as in Fig 7.3.

(:happel1 17(X)J! hits also observed that higher Zr and Y concentrations are characteristic

features o f high rcrnpcratiirt. granites in which tnelting continued to higher temperatures before

the 111agr113 COLIICI 11cp;ilf its source, and that high temperature granites owe their origin to

jnsufiicier~t It)%+ t~1lij)Cr;iftlrC tllelt cor1Ipt)nL'nts (quart/., ortho~lase, albite and water) in the source

r q i o r a . IIigher c.~~rtc~~nrratic,n of Zr and k' present in SG as cornpared to GG, therefore indicates

[ha( SG nwlt $ecir.rarcti at rclarivcly higher temperature from a source rock which consisted

of in.;ntl'ficiit~~t rl:i;srt/. ;lrtIrcicl:nt., :tlhitc and Lvatcr.

7.2.6 ?tletallogcnetic Specialization of t;ranite - - I t ic t o ~ i r : ~ ~ , i i ~ ~ i ~ * Ictzc~,gr;inl~~\ iC;ti\! irii~ti the \tud> area have yielded the following

i,ilut. crf :ung\ren 1 t t ? - [tprll i+ it11 an a\crltgi' ot 1 ~ p f n 111 %1G(;. ;il~d 7 to 1 1 ppm with average

of 5 pp111 $81 ('( ;( i S( ; ,iriii i rI l t ; i \chi\[ \hrm 11\cr~gc tilng\tL'n concentration of 5 pptn and 8 ppm

T L L I I $ W C ~ : \ ~ P I > . I /:c \It ~ o f ~ i . ~ f ~ l r ; i t i o ~ in t%,th .inJ SG I \ abnor111dl cotnpared to average granlte

idi:?c { I ? 1.5 j 1 ~ v i ~ K " i i n i s : i ~ h ~ ~ ~ 3 t , I'.)?l)\, 'fhc alczrrgc: ctrmpa>\rtion of the rnet~llupenetically

q~i';:.diii.~l F L ~ ~ P . rt*pi,fieJ h) 7 l~chrnd~x t (1')77\ ~ i l , t t tk clo\cl> a i t h GG as well as SG for

1113111 cte : r i t sn r i ?ti i i i , t l l i~p t u n p t c n c'lahlr*. 7 2 ) Iiighcr concc~ntriitron o f W in GGs) compared to

ntrr:ml gr,arr::i\ ;\ : ~ j f ~ + r i ~ ~ i t ( $ r~*fl&*c[ thctl I I ! I T ~ ; I ~ ~ O ~ C ~ C I ~ C \pciali~tdt~on. Howeber, SG and the

f?l:r,r \cII:%E , i ~ % , ififcbifi'nj tc h ' s ' ~ ~ ' aicj:i:ecd ,ihncrniliil cuncrrritrarion of M' during infiltration

i"ict;i~~~ila~;;-irr ,~~,ta::;j\.rtl>~fip tit2 j \ \ t ~ i i g ~ < t C L C ~ I I of Iei~c~grar~ite niagmittis~~~ (which produced

CIC;\ j i* a21c *IU~!> J X L * ~ l :* ~I\ilit1pj~lljllt~J h j tht* f ; ~ t that SG a i ~ i IXIICIT\ s ch i~ t art: not far fiorn

thC It3itgltel? jifgtl.,jwi!\ ~ ~ , l ~ i ) r ~ * r l \ aflc %rnai!\~d C;C;s, arc w\cr;iI k m w a y from nearest tungsten

B)n>*lw;i I

7.3 h S a ~ f ~ t & l $ i ~ QPEXeSSCX,

i'aa~nsi ~ ~ s : l t a a : , s ;In$ fr,gL,egi,u,il i f ) \~*iEli\;llion ;ire the t ~ 0 important rnngn~atic prwe5ces

uhch giv,ksn~ r t : ~ k=3qtjk:~ii~n tdr~et? of DgfhCP)U'l rock, frt%m different xmrces. Out of different

hinds, rat r!icjajil;, i*,31ci~ aircjrmn~ d i f j rqeril;t>nur:~ fusion ;ire the %implest nlelting model?. Dunng

batch i t w l ~ t l y asj wiijap: the csw~ctilrs:miaa of 31~ efcn~etlt in the rc l t car) be defined h> the

foFle,ii an:! a;E;irrii,a r ZE~a\tisr, EVyb r.

7.3.5 Tract? E.:fr.mar~b %$&ding rtf %iuree ub" Granitic 3lagms

Cinlalaic nM&:rsrla rtmld be pe.c~~crz:;iicd Iry p~aial fyie,'lting of sliffererlt ~ourc't.s rind the

~ ~ m ~ ~ ~ i t i o ~ ~ a$f ~ ~ ~ t g t ~ t s to gcncri$led <ou%~ be fx~i.lifilicd b) jdfc'r procc\sca. With rhe atailable

d8&, an elfa~n hzx !wen dn&e E81 mlrZeI IIX stx~n 'e r r f Scws i j~ ) granife iind (;~)airldg;trh granite.

Differenr rnndt*I~rlg ,I~P!o:IC.~C\ k! cri' .itIcinpted h) considering wurce rock\ like ba\altlc and pre-

existing rclch\ ( 4 ~ P I I I I I ~ I L ~ ' O I I I [ ) O \ I ~ I O I ~ to ~di'ntif> the 5ource and proce.ise\ by i ~ h ~ c h these

granltc\ forrncd

The S'tcu,ls~>,i grdrnte .inti (;cn~iidgarh granite ,Ire more e ~ o l ~ c d , hence nzagmas

representril$ 1hCii' ~ r , i l l l t r ~ i i I \ c,lrlilr>t 'i7c a product of partial melting of mantle jource The

baqaltic wurzc. M.I \ firit con~iilcrcd tor tflc traci' clement rnodellng hut thl4 co'~ld not catisfy the

;tbundni~cc of tr,rt.c cie.nlalf\ to :~nJ 111 ,lnJ SC; Thesi. I I I I C ~ ,ch~jt (of Rdrotikd group of the

Dttlhi Supr.rgrottpi u a i corlsltictc.ci ,ts .III~ISCC rock ~ i l d thi\ :tl\o dld IIOI s ,~ t~ \ f> the genenltton of

the gr,inirx s n ~ i r fw.\cilI ir: the i!tiJ> ,irt;t l'hcrcforc.. allernate wurce \uch quart/ diorlte /

\anulitt?l\i. rcpr) i?c. i i f r o i i ~ tilt I3,1iitlcti ( ~ E I ~ I \ \ I C ( ' t~~~iplcx IRGC) occurring in the M L I \ u ~ ~ area in

A ~ I I I L " ~ I ) i \ t r i c t . I L111 'it% d:, fro:^^ tht- \tild> .~IL"I, h<l\ hec"11 c o ~ ~ \ ~ J e r e d The BGC con\t~tutes the

ba\enli.nt l i l r /).ethl hli[%Lb~g~<)i~[? ( 3 f soi.Lt !I ~rhin uh1i.h SI; dnd GC; arc crnpi,~ced

Kb. Sr ,tzlii tl.1 ~.crrlccrlrr:tiron ~n S ( i ;ind \\ere ~i\cii to illode1 the wurce for S G and GG

~lldgrl~di. !11 i)nc trf t l rc {'l,ifti,d I I I ~ ' [ [ I I I ~ ~l~odcl\ , the ioliri'e. rc\ldt~i' ;i\\c~nhldgc \%as a\sunicd a\

20rc K-titlri\g.i,rr i 3rr i pl.r~i<~l,trc + 25'; aniphihoIc +7SC8i kiorite KdHi, I \ 0 330. 0 048. 0 014

arid 3.2tr0, Ksf, I* 3 X-0, 2 h30, 0.032 and O.f?O: Kcit+., (3 170. 0360. 0.044 clnd 6360

rc.\pcct:\c,!? t o r K - t ~ l d i p ~ i r . pI'igitrcI;i\i', iii~iphih~lc 'ind h i ~ t i t ~ ( A r ~ h , 1976). C,, of Kb. Sr and Ba

.ire I 10, I5iJ ;83t11 5/31) re\pect~i c i .

T!EC c.,r!c:if~tt.rl ('! \<ilu;.\ ,ire pltrttcti \\ 8th conc~ntration of' trace elcnlent of G(; and SG

i F ~ p 7 IOI 1111s fni~tlcl ~ n l c ~ l ~ l n g p.irtadl r~lclttng lealing amph~kolirt. re\iduc .iati\fic\ 111e trace

elemern$ ahuiici,inicr r ~ t (;I>\ indgaris gr;initc, hut nor the Se\%ari).a granite

I.3;msai zllclting ~~nr$Jr.i ciln,lriznng a iitffercnt re\idut' as<emblnge of 5 0 3 p las i~ lase+ 30%

cl~ntrp>rt,\enr. + I t ) ' ; ilrri~n~,rtrrcnc + 10% g ~ w c t wa\ al\o attcn~ptcd. The Kd~h I \ O.()lS, 0.0.32,

O.IK13 ,inti 0 (BElcl: Kdi, r \ 2.X.JO. 0.5 16, 0 009 and 0.0 15: KciH, 0.360, 0.13 1. O.(XIS and 0.017

~s[x~!r%rI) for pidgnrbcld\c, clnnap)rtr-\cn~.. onhapyroxcnc and garnet iArth, I9761 C,, of Rb, Sr

and 8 3 arc E 1'. 479 dnd 7fXJ ~ ~ * \ p ~ \ i t ~ \ e l j (S;iki(w 19961 The rewlt\ arc ploticd in (kg . 7.1 1 )

and rt 1% ins"irrr.24 ~Raf the n?;;rgir;la ~pre'~enting S e ~ w i y ; . ~ grCinitc \bas tlot formed by partial

trn%lting alone. fience,., mrxdeling of frtlctionixl cryctaflisation was a l w attempted. Variable degrees

of frnczbrul crj\tdli.;atann of an assemblage ccor~sistiag of liO% Plagioclase + 40% K-feldspar

frc)m a p ~ n t u y

Fig. 7.10: Tract. element m&eiing for Govindgarh granite. Calculated CL values for partial

melting of a mica schist of Barotiya group leaving a residue containing 20% K-

FeIdspar + 30% Plagioclase + 25% Amphibole + 25% Biotite. Symbols are same as

in Fig. 1.3.

I * Paniol melting trend o Fractional crystallisalion trend I i

204 melt . A . * * * 4 . * .

Fig. 7.1 1: Trace element modeling for Sewariya granite. Calculated CL values for partial melting

of a sanukitoid leaving a residue containing 50% Plagioclase + 30% Clinopyroxene + 10% orthopyroxene + 10% Garnet. Symbols are same as in Fig. 7.3.

magma representing 20% partial melt of above source satisfies the trace element abundances in

most of the SG samples (Fig. 7.1 1 ).

The above trace element modeling indicates that partial melting of mica schist at

amphibolite facies could generate magmas similar to GG, while dehydration melting of

intermediate rocks (quartz diorite 1 sanukitoid) sources leaving granulite residue could produce

magmas similar to that of SG.

7.4 Tectonic Discrimination Diagrams

Granites can be grouped into two broad categories, orogenic and anorogenic, based on

whether or not the rocks are associated with an orogenic event. Moniar and Piccoli (1989)

showed that the variation of the A1203 against Si02 indicates different tectonic setting and can

distinguish orogenic (higher values of A1103) from anorogenic setting. The values of GG and SG

are plotted in silica vs. alumina diagram (Fig. 7.12), wherein both the granites fall in orogenic

field. Pearce et a1 ( 1 984) discriminated granites formed in different tectonic settings based on

the concentration of Y, Nb and Ta, and identified the fields of oceanic ridge granites (ORG).

within-plate granite (WPG), volcanic arc granite (VAG) and syn-collisional granites (syn-

CLOG) in bivariate plots Nb vs. Y, and (Y+Nb) vs. Rb.

In the bivariate diagram of Nb vs. Y (Fig. 7.13 A) it is observed that all the samples of SG

and GG fall in VAG + syn-COLG field. However, SG forms a small cluster clearly separated

from GG and occur more towards the boundary of WPG. The two varieties of GG have a large

spread of values in Y and Nb as compared to SG.

In another bivariate plot of Y+Nb vs. Rb (Fig. 7.13 B) the samples of SG and GG fall

partly in VAG and remaining in syn-COLG fields. In this plot also, SG forms a small cluster

clearly separated from GG and occur more towards the boundary of WPG. The two varieties of

GG have a large spread of values in Y+Nb as compared to SG.

Orogenic (IAG+CAG+CCG) a I

b 8 Q

00

Anorogenic

I I I I 1 I - 68 70 72 74 76 78 80

SiO,

Fig. 7.12: Tectonic variation diagram of SiOz vs. All03 (after Moniar and Piccoli, 1989). IAG-

Island arc granite, CAG-Continental arc granite, CGG-Continental collision granite.

Symbols are same as in Fig. 7.3.

Pearce et al (1984) have observed that crystal fractionation of plagioclase in granites can

shift their composition from WPG and ORG fields into the VAG in Nb-Y and Rb-(Y+Nb) plots.

Since fractional crystallisation of plagioclase is significant in SG magma (discussed in the next

chapter on REE modeling), it is possible that such a shift could have influenced the occurrence

of SG samples in VAG and VAG+ syn-COLG fields but close to WPG field in Nb vs. Y (Fig.

7.13 A) and in Y+Nb vs. Rb (Fig. 7.13 B) bivariate plots.

10 100 1000

Y+Nb ppm

Fig. 7.13: Plot of granites from study are in the Nb vs. Y (A) and Rb vs. (Y+Nb) (B)

discrimination diagram for granites (after Pearce et a1 1984) showing the fields of

syn-coIlisional (Syn-COLG), within-plate granite (WPG), Volcanic-are granite

(VAG) and ocean-ridge granite (ORG). Symbols are same as in Fig 7.3.