Chapter-$

REE Geochemistry of Granitic Rocks

8.1 Introduction

Rare earth elements all have very similar chemical and physical properties. This arises

from the fact that they all form stable 3' ions of similar size. Small differences in chemical

behavior are a consequence of the small but steady decrease in ionic size with increasing atomic

number. These small differences in size and behavior are exploited by a number of petrological

processes causing the REE series to become fractionated relative to each other. It is this

phenomenon which is used by geochemists to probe into the genesis of rock suites and unravel

petrological process. Trends on REE diagrams are usually referred to as REE pattern and the

shapes of the REE pattern is of considerable petrological interest.

We are interested in hnowing what process or processes are responsible for the production

of suite of rocks. This usually includes various proportion of given mineral phase or suite of

mineral assemblage can be used to place constraints on the temperature, pressure and

composition of the system. The rare earth elements (REE) play an important role because the

changes in the shapes of the REE patterns within a suite of rocks or melts are directly related to

the shape of the mineral-melt distributions coefficient patterns of the mineral involved.

8.2 Determination of Rare Earth Elements

Rare earth elements occur in ppb and ppm levels in the rocks. Many workers have

suggested different procedures for sample digestion and effective determination of REEs by ICP-

AES after pre-concentration (Walsh et al., 1981; Crock et al., 1984; Satynarayan et al., 1989;

Rathi et al., 1991 ; Zackariah, 1992; Sahoo and Balakrishnan, 1994) in different laboratories.

However, a suitable procedure for REE analysis needs to be developed taking into account one's

own laboratory and instrumental conditions and nature of samples to be analyzed. The present

REE analyses were carried out at the School of Environmental sciences, Jawaharlal Nehru

University, New Delhi.

Complete sample digestion is important to obtain the true abundances of the REEs

present in the rack. Because, refractory minerals such as mtile, zircon, monazite contain high

concentration of REEs (Gromet and Silver, 1983). The separation of REE as a group adapting

two columns (HNO? and HCI columns) procedure was necessary. Presence of matrix elements

(such as Fe, Ca and Sr) can affect the REE analysis by ICP-AES. After the columns were

calibrated they were used for the separation of REEs as a group from the unknown samples.

6 samples of Govindgarh granite (4 MGG and 2 CGG), 3 samples of Sewariya granite

and 3 samples of Barotiya rocks (one each of mica schist, felsic and mafic volcanics) were

analysed usi~lg Ulti~lla 2 (Jobin Yvon, France) ICP-AES at the School of Environmental

Sciences, Jawaharlal Nehru University, New Delhi.

For REE analysis the samples were fused using a mixture of NaOH and Na202. 0.5 gm of

rock powder was taken in a 751111 nickel crucible with the fluxes NaOH and Na20z in the

proportion of 2:2.5. The ~nixture was heated in a Bunsen burner for about 30 minutes in an

oxidizing flame till the nickel crucible turned red and a homogenous melt formed. The melt was

swirled repeatedly to get the sample and flux homogenized and subsequently it was allowed to

cool down. 50 ml of double distilled water was added to the crucible and left overnight. The next

day contents of the crucibles were transferred by using 6N HCI to 500 ml beaker, and heated at

90-100 "C till viscous silica gel formed (H4Si04).

The silica gel was filtered by subsequent cleaning with 6N HCl followed 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 the filtering flask with 6N

HC1, into glass beaker. The filtrate was dried completely by heating at about 100 "C and

dissolved with 30 nil IN HCl. Then this solution was transferred to centrifuge bottle by repeated

washing of beaker with 3 3 HCI. 10-15 drops of phenol red (pH indicator) was added to the

above solution when it becomes orange in colour. Subsequently, 1:l ammonia solution (NkOH)

(about 80 rnl) 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 4 "C and the

supernatant solution containing mono and divalent ions was decanted through filter paper. The

precipitate was transferred to Teflon beaker by repeated washing by 6N HCI and evaporated to

complete dryness. Subsequently, it was dissolved in 30 ml of 1N HN03 which was used for

group separation of REEs using cation exchange resin columns.

8.2.1 Column calibration

The separation of REEs as a group was done using cation exchange resins (Bio-Rad AG

50 X-8,100-200 mesh size) in quartz columns (24 cm long x 1 em inner diameter). The columns

were calibrated using multi-element solution containing matrix elements (Fe, Ca, Sr, Al, Mn, Na,

K, Zr and Ba) and Rare Earth Elements. 30 ml of the multi elements solution dissolved in I N

HNOl was initially loaded into the (cation exchange resin) 1HNO3 column. To elute the matrix

elements, 1.78N HNO3 was passed through column which was collected at 10 ml intervals and

subsequently analysed by ICP-AES. From the analytical data it was noted that by passing 70 mi

of 1.78N HIV02, all the matrix elements were eluted except Fe and all REEs were eluted using

200 ml of 6 N HN03.

Similar type of column calibration exercise was followed for HCI column also. By

passing 70 ml of 1.78N HCI through the HCl column, Fe and other matrix elements were

removed and REEs were eluted by passing 220 ml of 6 N HCl. The procedural details of pre

concentration of REEs for determination by ICP-AES is provided in the form of a flow chart

Table 8.1. Each column was used after regeneration. The result of REE analysis of rock samples

from the study area is listed in Table 8.2.

8.3 Petrogenesis

The petrogenetic study of igneous rocks attempts to characterize the sources of their

magmas in terms of chemical composition and mineralogy, extent of partial melting required to

generate the parental magmas, extent of fractional crystallisation that the parental magmas might

have undergone to give rise to magmas represented by the igneous rocks.

Petrogenetic modeling of granitic rocks is very complex. Because they can be formed by

a variety of petrogenetic processes such as, partial melting, fractional crystallisation, magma

mixing, liquid inlmiscibility and crustal contamination and also from a variety of sources, such

as, by partial melting of basaltic rocks leaving amphibolite, granulite or eclogite residues, by

partial melting of ultramafic rocks of upper mantle and by fractional

crystallisation of mantle derived magmas, anatexis of crustal rocks or combination of these

(Skjerlie and Johnston, 1996).

The granites occuning in the Govindgarh-Sewariya area can be classified as alkali granite

and granite. Geochemical modeling is carried out using the least mobile elements, such as, the

REEs. For quantitative modeling of partial melting and fractional crystallisation processes,

Table 8.1: Preconcentration procedure adopted for REE analysis

Sample 0.5 gm

u 2g NaOH + 2.5g Na202 fusion

U Dissol\~e in H?O & 6N HCI

u Evaporate i t to complete

dryness

U Precipitate silica

u. Filter out the silica precipitate

U Evaporate to dryness the

filtrate and re-dissolve in

30ml 1N HCl

u Add N b O H till pH=7

u NH40H precipitation

u Centrifuge the solution

u Decant the supernatant liquid

u Dissolve the precipitates in

1 N HN03 about 30ml

Column chemistry

HNOj column

u Load the solution 30 ml (IN m 0 3 )

Matrix elements elution with 70mI

1.78N HNO3

u Collection of REE with 200 ml

I Evaporate to dryness& dissolve in

I 1N HCI

I HCl Column

I Load the solution 30 rnl in to the HCl

I column

I Matrix elements elution with 70 rnl

i 1.78N HCI

I Collection of REE with 220 rnl6N HCI

U Evaporate to dryness

/ Pick up in 1 Om1 of 1 N HNO3 prior

to analysis

appropriate Kd values for different source and melt systems, as suggested by Hanson (1980),

Nash and Crecraft (1985), Arth (19761, have been used. The REE values of chondrite given by

Hanson (1980) have been used to normalize the data generated in the present work.

8.3.1 REE modeling of Govindgarh granite

Govindgarh granite is enriched in LREE and HREE, and depleted in the middle REE

concentration, showing bowl shape with positive Eu anomaly [(EuIEu*) > I and ranges fronl 0.83

to 14.141 in normalized REE plot (Fig. 8.la, b). Two samples of GG show slight negative

Eu/Eu* values. (Ce/YbIs ranges 1.08-4.70 in GG with HREE enrichment f ron~ Dy to Yb and

(Dy/Yb)s in the range 0.58- 1.25. The LREE of GG is 7.26 to 34.84 ppm with only 5 to 40 times

greater than chondrite values.

The SREE value is indicating that LI significant proportion of these elements (REE) were

retained in the source during anatectic generation of the granitic magma. The positive Eu

anomalies in the chondrite normalized plot are invariably associated with relatively high

concentration of Sr (,and in some cases Ba) and vice-versa for the low concentration.

REE modeling was attempted in order to infer the magmatic processes and the source of

granites in the Govindgarh-Sewariya area. It was considered that partial nlelting of peridotite

mantle source can not generate granudiorite lnagnlas (Green, 1973, 1976). Further. experimental

investigations by Helz (1976) and Rapp and Watson (1988, 1995) indicate that granodiorite

magmas could be generated by partial melting of mantle derived basaltic rocks and /or its

differentiaties.

The high concentration of boron in Govindgarh granite revealed by rhe ubiquitous

presence of tourmaline, together with high alumina/alkalies ratio of this rock shows that the

source could be pelitic sediment. In the study area, Govindgarh granite is closely associated with

mica schist of Barotiya Group. Therefore it is possible that the parent rnagmn of Govindgarh

granite could have been generated by partial melting of meta-sedimentary rocks of Barotiya

group. The REE pattern of the Barotiya rocks, including mica schist, mafic and felsic voicanics,

is shown in Fig. 8. Id.

Trace element abundances have been calculated assuming a mica schist as source which

had undergone 5 to 20% partial melting leaving 10% K-feldspar + 16% plagioclase + 45%

hornblende +20% biotite +5% apatite + 2% zircon in the residue.

La Ce Nd Sm Eu Gd Dy Er Yb

REE

L a CC Nd Sm 1 : ~ <;cl D) 1:r Yh

REE

Fig. 8.1: REE pattern ol'dil'l'crent rocks from the study area.

A:MGG; B: CGG; C: SG and D: Barotiya rocks

The calculated REE abundances are plotted along with the chondrite normalized REE

abutdances of mica schist parent (Fig. 8.2). I t is evident from this plot that the calculated REE

abundances are low with no Eu anomaly.

1000 1 - -

i MGG -3- CGG ! - 0 - Mica schist

i ...+... Panial meit trend 100 a_ - . - . -- ---- - .- -.- .

' 0 .

Ce Nd Sm Eu Gd Dy Er Yb

REE

Fig. 8.2: Cdlculared ohondritc normalized REE patterns for 5, 10, 15 and 20% partial melting

of mica schist (open circle) leaving residue of 10% K-feldspar+ 18% plagioclase+

459 hcx-nblende+ 20% biotite+ 5% apatite+ I C/o allanite+ 1 % zircon. Calculated

patterns (dotted lines) have flat HREE nbundances and no Eu anomaly. Symbols:

Filled triangle aith solid lines - medium grained GG, Open square vcith solid lines -

coarse grained GC and plus with dotted lines -calculated melt.

Similarly, various other possibilities were attempted by changing major as well as

accessory phases in residue mineral composition. Partid melting models with mica schist as

source rock and leaving various proportions of fallowing restits mineral assemblages were also

attempted, but none of these models march with the REE abundance of GG.

1. 10% K-feldspsr + 18% plagioclsse + 45% hornblende +209 biotite +5% apatite + 1%

~ircon + 19 alIanite

2. 10%- K-feldspar + 169 plagioclse + 45% hornblende +20% biotite +5% apatite + 2%

zircon +. 2 9 slianite

3. 10% K-feldspar + 15% plagioclase + 45% hornblende +20% biotite +5% apatite + 2%

zircon + 3% allanite

4. 20% K-feldspar + 30%- plagioclase + 25% hornblende +20% biotite +5% apat~te + 20% K-

feldspar + 30% plagloclase + 25% hornblende +25% biotite

The trace element modeling using Rb, Sr and Ba of GG suggests that the GG magma

could be generated from hydrous sources possibly in amphibolite condition. Moreover, the REE

pattern of GG is con~plementary to that of an~phiboles, which are enriched in middle REE than

LREE and HREE. The HREE abundances in the GG sample shows that accessory phases such as

garnet, zircon, apatite, allanite ~ : i t h amphibole could have played major role in the generation of

GG magma. With this idea, partial melting of mica schist source leaving 90% amphibole + 5%

biotite + 5 9 apatite as residue was considered.

5 ro 30 9; ptutial melting of a source similar to mica schist in the study area, leaving an

amphibolite residue (90% hornblende +5% biotite +5% apatite) would produce a melt having

LREE enriched REE pattern with a positive Eu anomaly (Fig. 8.3). The calculated pattern shows

that garnet and allaniti. fractionation played major role in the generation of the GG magma. The

calculated mica schist sarnple has higher REE abundance than GG samples. However, in a

heterogenous mrta-sedimentary rock like mica schist it is expected that the range in REE

abundance. may be large enough to he con~parable with the REE abundance of GG.

Presence of the tourmaline, garnet and high A1203 content in GG shows it can be derived

from S-type Inagma. Furtlier, titi111 the partial melting models of mica schist source leaving

amphiboIite residues, it is evident that such process could produce a magma having REE

abundance similar to that of Govindgarh granite. However, it does not satisfy Eu abundance in

few samples of Got i11dgart.l granite. This suggests the possibility of fractionation of plagioclase

from the melt derived by above model. Then, Gavindgarh granite samples G-101 and G-241B

were chosen on the basis of their Sr concentration being close to the average Sr content of all the

analysed Govindgarh granite samples. Another reason for considering these two GG samples (G-

101 and G-2418) is that their KEE abundmces are less than those of other GG samples (G- 105,

G-141A, G-245 & G-2591, and therefore the composition of these two samples would be closer

to that of parent magma.

0 1 - -- " . - - - - - - - 7- -7- 7--T 1

Ce Nd Sm Eu Gd DY Er Yb

REE

Fig. 8.3: Calculated chondrite normalized REE patterns for 5 , 10, 20 and 30% partial melting

of mica schist leaving residue of 90% honrblende+5% biotite+5% apatite. Calculated

pattcrns (dotted lines) arc analogous with GG sainples except for very small negative

Eu ancjmaiq in one sample (G-245). Symbols are same as in Fig. 8.2.

The fractional crqsrallisation of plagloclase from sample G-211 B would also not satisfy

REE abundance prt)ducing slight positive Eu anomaly in 5% fractional crystallisation but with

lo, 15, 20 5 fractionid crystallisation shows negative Eu anomaly (Fig. 8 . h ) . In the same

methad, the calcuIated REE abundance of 5 to 208 fractional crystallisation of plagioclase for

sample G-101 is plotted in Fig. 8.4h. The obtained pattern shows positive Eu anornaly which is

not comparable with REE abundances in GG. These features in~ply that the REE pattern of

Govindgarh granite is nut due to frrtcrionation of plagioclase feldspar.

The REE abundance< and the results of REE modeling for Ciovindgarh granite shows that

partial melting of a hcterogencous source leaving amphibolite residue woulci satisfy the REE

abundances of GG.

Fig. 8.48: Calculated chondritt. normalized REE patterns for 5, 10, 15 and 20% fractional

crystallisation of 1009 plagic~lase from G-241. Calculated patterns (dotted lines)

rue not analo~ous with REE abundances and Eu anomaly of GG samples. Symbols

arc same as in Fig. 8.2.

Fig. 8.4b: Cdculated ehsndritc: nomnlized REE patterns for 5, 10, 15 and 208 fractional

crystdlisation of 100% plagioclase fmm G-101. Calculated patterns (dotted lines) are

not analogoos with REE abundances and Eu anomaly of GG samples. Symbols are

same as in Fig. 8.2

8.3.2 REE modeling for Sewariya granite

REE abundance of SG shows depleted negative slope pattern from LREE to HREE with

strong negative Eu anomaly [03dEu*) <I, ranges from 0.19 to 0.621. The high (Ce/Yb)N \lalue of

6.57 - 8.58 together with HREE depleted (DylYb)~ value ranging from 1.51 to 3.54 in SG

shows high degree of differentiation (Fig. 8 . 1 ~ ) . The CREE is 50.91 to 187.49 ppm in SG with

LREE and HREE 100 and 10 times greater than chondrite respectively.

REE abundances were calculated assuming mica schist (dominant country rock in the

study area) as source, which had undergone 5 to 30% partial melting leaving 90% hornblende + 5% bioitite +5% apatite in the residue. The calculated REE abundances are plotted along with the

chondrite normalized REE abundances of mica schist parent (Fig. 8.5). The REE abundance of

calculated melts is not similar to that of SG. The derived melts show HREE enrichment in 20 to

30% partial melts than in 5 to 10% melt and there is progressive decrease in rock/chondrite

LREE values fro111 5 to 30%- melts with no Eu anomaly. It is evident that SG melt can not be

derived from partial melting of mica schist under amphibolite facies condition.

Fig. 8.5: Calculated chondrite nomai i~ed REE patterns for 5. 10, 15 and 20% partial melting of

mica schist (G-119) leaving residue of 90% amphibole +5% biotite+% apatite.

CalcuIated patterns (dotted lines) have higher LREE abundances and no Eu anomaly.

Symbols: upen circle-Sewariya granite, filled square- Parental rock and filled triangle-

calculated melt.

Partial melting models with various other proportions of different restite mineral

assemblages considering mica schist as source rock, were also attempted, but none of these

models match with the REE abundance of S(;.

1, 50% plapioclasc + 27%- CPX t 9% OPX + 10% garnet + I % zircon + 3% apatite

2. 55% plagioclase + 28% CPX + 10% OPX + 5% gEarnet + 2% zircon

The suitability of sanukitnid of Masuda area as source for Sewariya granite melt was

tested with various proportions of different restite mineral assemblages are as follows

1. 505: plagiociasc + 17% CPX + 9% OPX + 10% garnet + 1% zircon + 3% apatite

2. 55% plagioclase + 78% CPX + 10% OPX + 5% garnet + 2% zircon

3. 50% plagic,ciasc + 33C;i CPX + 10% OPX + 5% garnet -t 2% apatite

4. 585; plagioclasc +. .30(2: CPX + 10% OPX + 2% garnet

Thc mclt dcrivcd from sar~ukitoids leaving the residue assemblage of 50% plagioclase + 33% CPX + 10'; OI'S + 59; garnet + 206 apatite are considered. The LREE pattern of calculated

melts for 5 to 3 0 ( i melt is cornparable with SG, but HREE abundance is much lower than that of

SG (Fig. 8.6). ,000 ...- " ..

1

r & - - - , l

REE

Fig. 8.6: Ca1cuttttc.d chandrite normafized REE patterns for 5,10,20 and 30% of partial melting of

smukitoid source lav ing residue of 50% plagioclase+33% CPX +lo% OPX+5%

gdrnet +29 apatite. Derived melts have depleted HREE pattern compared to SG.

Symbls ;tre smurfe as in Fig. 8.5.

Alternately, for the sanukitoid source, 5 to 30% partial melting leaving 58% plagioclase + 20% CPX + 10%'- OPX +2% garnet residue was considered. The REE abundance in the

calculated melts are plotted in Fig. 8.7. REE abundances in these melts are comparable to some

extent with SG.

Fig. 8.7: Calculated chorrdrite nonnalired REE patterns for 5,10,20 and 309 of partial melting

of \anuhitoid wurce leaving residue of 58%. plagioclaset30% CPX + l O % OPX+2%

garnet. Symbols are sarrle as in Fig. 8.5

Fractional cr).italli\ation of certain rnineral phases from the initial melt was modeled in order

to examine whether \uch prtxesws could produce the REE pattern observed in SG. The higher

abundance5 crf REF, in .;anlplt: SG-221 \ h o ~ c the characteristic< of u\*olved phase in SG. Sample

SG-22 1 u as con.;idered a\ original nlelr (Co) to find out, whether fractional crystallisation have

its imprints in the other \timples of the SG suituc (\uch iis SG-138 and SG-150). The following

nlodels of fractional cr)staliisation from a melt having the composition of SG-221 were

attempted.

1. 50% plligiwlase i 49.5% biotite +0.5% ~ircon was considered as fractionated mineral

phmes. For 5 to 30% fractio~~al crystallisation of this rnineral assemblage, the calculated

melts do not correspond with SG (Fig. %.$a).

2. 50% plagiwlase i- 19.59 hornblende 4.5% zircan was assumed as fractionated mineral

assemblage. Far 5 to 30% fractional crystallisation of this mineral assemblage, the

derived melts do not show much variation in LREE, and there is large deviation from SG

1 35

in HREE (Fig. 8.8b). From this pattern it is believed that zircon content in the

fractionated mineral phase would lower the HREE abundance for calculated melt.

3. 50% plagioclase + 49.5% hornblende +0.4% zircon +0. I % allanite was fractionated from

the melt of SG-221 composition (Fig. 8 . 8 ~ ) . The REE abundances of the calculated melts

are comparable with the REE pattern of Sewariya granite.

By and large. partial melting of a source similar to sanukitoid in the BGC of Masuda area,

leaving a granulite residue of 58% Plagioclase + 30% CPX + 10% OPX + 2% Garnet could have

produced the initial granitic magma. Fractional crystallisation of 50% Plagioclase + 49.58

Hornblende + 0.4% Zircon + 0.1% Allanite from this initial magma could have produced the

magma from which SG was formed.

Fig. 8.8a: Calculated chcmdritc. nomthi~ed REE patterns for 5,10,10 and 30% fractional

erqst;~ili\atisn of 50Q plrvgioclasc +49.58 biotite +0.5% ~ircon from parent SG 221.

CrilcuIated KEE patterns ;we not comparable with SG samples. Synlbols are same as

in Fig. 8.5

Fig. 8.Sb: Calculated chondrite normalizzd REE patterns for 5,10,20 and 3 0 9 fractional

crystallisation of' 50% plagioclase +49.5%. hornblende +0.5% zircon from parent SG

22 1. Calculated melts have depleted HREE pattern than LREE as compared with SG

sa~npies. Synlbols are same as in Fig. 8.5

Fig. 8.8~: Calculated chondrite nonnalizd REE patterns for 5,10,20 and 30% fractional

cr;2"aliisation of 50% plagioclase 49.5% biotite +0.4% zirco~~+O.lC/callanite from

prmnt SC 221. Calculated REE patterns are comparable with SG samples. Sgmboli

are. same as iil Fig. 8.5

It is concluded from REE modeling that the parent magma of the two types of granites (GG

and SG) of Sewariya-Govindgarh area were derived from partial melting of different source

rocks within the crust and at different crustal levels. GG fonned by partial melting of mica schist

leaving amphibolite residue in the mid crustal level, whereas SG formed by partial melting of a

source similar to sanuktoid leaving granulite residue in the lower crustal level and followed by

fractionation.