Gondwana Research, V 8, No. 3, p p . 421 -432. 0 2005 International Association for Gondwana Research, Japan. ISSN: 1342-937X

Gondwana Res ea YC h

Geochemistry and Origin of Neoproterozoic Granitoids of Meghalaya, Northeast India: Implications for Linkage with Amalgamation of Gondwana Supercontinent

Subhasish Ghoshl, A.E. Fallick2, D.K. Paul3* and P.J. Potts4

' Geochronology and Isotope Geology Division, Geological Survey of India, 15A 13 B, Kyd Street, Kolkata - 700 016, India Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 OQF, Scotland, E-mail: T.Fallick@ suerc.gla.ac.uk. Department of Geology, Presidency College, College Street, Kolkata-700073, India, E-mail: dalimpaul@yahoo.co.in

* Department of Earth Sciences, The Open University, Milton Keynes MK76AA, U.K., E-mail: P.J.Potts@open.ac.uk. * Corresponding author

(Manuscript received June 1,2004; accepted May 19,2005)

Ab sir act

Many granitic bodies intrude the basement gneisses in Meghalaya Plateau, Northeast India. Rb-Sr whole-rock isotopic ages of the granitoids range from 881 to 479 Ma while the ages of the basement orthogneisses vary from 1714 to 1150 Ma. All the plutons are dominantly metaluminous and show geochemical variation. Oxygen isotopic compositions in the granitoids and gneisses are concordant (FPO: + 5.78%0 to + 8.70%0). However, the gneisses from high-grade terrain have low P O value of +2.52%0 to +5.31%0. Initial a7Sr/86Sr (I,) ratios of the plutons vary from 0.70459 to 0.71487 and tend to increase with progressive younging in age. The geochemical characters suggest derivation of the granites from lower crustal source. The fractionated rare earth patterns observed in the granitoids can be obtained by partial melting of gneisses or diorites. Some gneiss samples have experienced interaction with hydrothermal fluids resulting in lowering VaO. The isotopic ages of granite plutonism in Meghalaya are similar to the plutonic and tectonothermal events in other parts of India, southwestern Australia and document final amalgamation events of the Gondwana Supercontinent.

Key words: Neoproterozoic granite, oxygen isotope, petrogenesis, Meghalaya, Northeast India.

Introduction A suite of granite plutons intrudes the Proterozoic

orthogneiss and metasediments of the Meghalaya Plateau, NE, India. Rb-Sr whole-rock isotopic ages of the granite plutons range from 881 to 479 Ma and contrast with the 1714-1150 Ma ages of the basement gneisses (Ghosh et al., 1991; 1994a; Chimote et al., 1988; Selvan et al., 1995). These isotopic ages are similar to those of the granite plutons of Kerala (740-550 Ma, Santosh and Drury, 1988); Tamil Nadu (637 to 395 Ma, Nathan et al., 2001; Santosh et al., 2005) and acid volcanic activity in Rajasthan (780 to 680 Ma, Rathore et al., 1999) and tectonothermal events in Eastern Ghats, Orissa (553 to 550 Ma, Aftalion et al., 2000). The magmatic history of Meghalaya is important as the plateau forms the northeastern margin of Neoproterozoic India and records evidence of the evolving plate boundary at this time. On the ongoing debate on the evolution of the supercontinent Rodinia,

current views favour its formation during the Grenvillian event (1300-1000 Ma ago), subsequent disintegration and final amalgamation around 550 Ma ago (see Kroner and Cordani, 2003 for a review). On a regional scale, the East African Orogen (Stern, 1994) consisting of deformed and metamorphosed Precambrian rocks has been widely regarded as the principal collision zone during late Neoproterozoic amalgamation of Gondwana (Collins and Windley, 2002; Collins et al., 2003 and references therein). Fitzsimons (2000) recognised continent-scale sutures of similar age in other parts of Gondwana. Long et al. (2005) discussed the age and origin of a granite pluton from Brazil in the light of amalgamation of western Gondwana supercontinent. Reconstruction of the continental assembly in the Neoproterozoic period juxtaposes southwestern Australia against northeastern India (Collins, 2003; Harris and Beeson, 1993; Fitzsimons, 2000; Rogers and Santosh, 2002). The evolutionary history of Meghalaya

422 S. GHOSH ET AL. ~

in northeastern India thus assumes special significance in inter-continental correlation.

Models of growth of continental crust invoke episodic addition of juvenile material onto cratonic areas. The chronology, petrology and geochemistry of magmatic bodies document origin and growth of continental crust. This paper presents oxygen and strontium isotopic data, rare ear th element contents and major element composition of the granite plutons and basement gneisses of Meghalaya and discusses their petrogenetic evolution. The relevance of the granite to understand crustal evolution during Neoproterozoic in the context of dispersal and reassembly of Gondwana crustal fragments is also discussed.

Geological Set-up The Meghalaya Plateau forms the northeastern

extension of the Indian Peninsular Shield. It is an E-W trending oblong horst block elevated about 600 to 1800 m above the Bangladesh plains in the south and separated from Peninsular India by the Rajmahal-Gar0 gap (inset map in Fig. 1). The Proterozoic metasedimentary Shillong Group and the basement Gneissic Complex make up most of the plateau. The southern part of the plateau is covered by Cretaceous Sylhet basalt and Tertiary shelf sediments (Fig. 1).

The Shillong Group of rocks, occurring in a 240 km long NE-SW trending intracratonic basin, was metamorphosed to greenschist facies and rests unconformably as indicated by a basal conglomerate (Nandy, 2001) on an assortment of rock types including sillimanite-bearing gneisses, amphibolite, banded iron formation, granulites and granite gneisses collectively known as the Gneissic Complex. Basic eruptives occur as concordant and discordant bodies within the Shillong Group metasediments and are referred to as Khasi greenstones (Mazumder, 1986). Syn- to late tectonic granites occur as discordant plutons cross-cutting the Shillong metasediments (Kyrdem, Mylliem plutons) and basement gneiss (Rongjeng, Sindhuli plutons) (Fig.1). In Nongpoh and South Khasi areas the granite plutons cut across both the Shillong metasediments and the gneissic complex (Fig.1). For convenience, the granites and gneisses are described by the locality names. The mineralogical data for the basement gneisses suggest medium- to upper- amphibolite - grade metamorphism. Around Sonapahar, in the central part of the plateau (Fig. l), the peak metamorphism reaches granulite grade (La1 et al., 1978). The common rock types include medium- to coarse grained granitic gneisses, cordierite-garnet-biotite-sillimanite bearing granulitic gneisses and pyroxene-hornblende granulites intruded at places by quartzo-feldspathic veins (La1 et al., 1978).

Noritic/dioritic enclaves are present in the granite plutons. Such enclaves are particularly common in the South Khasi and Mylliem plutons and attain sizes up to 10 to 12 km2. The contact between the enclaves and the younger porphyritic granite is both sharp and intermingled at different localities. A number of prominent lineaments trending NE-SW and E-W are present in the plateau; the most prominent structural feature is the E-W- trending Dauki fault, which marks the southern border of the plateau (Fig. 1). The granite plutons occur in greater number in the eastern part of the plateau compared to the western part. Some salient petrological and mineralogical descriptions of the intrusive plutons are given in table 1.

Analytical Methods Major element analyses of representative samples were

carried out using an automated sequential X-ray fluorescence spectrometer (PW 1400 of M/s. Philips, Holland) with side window Rhodium tube on fused discs, while Rb, Sr concentrations of the whole-rock samples were determined on pressed powder pellets using a 3 KW Mo side window anode tube, LIF220 crystal and scintillation detector at 75KV, 40mA settings. The conversion of measured X-ray intensities into element concentration is based on calibration of the spectrometer by measuring the peak intensities for a series of international whole-rock standards. Sr was extracted by conventional ion exchange chromatography and Sr isotopic analyses were carried out using a single collector VG 54R thermal ionisation mass spectrometer. Measured s7Sr/s6Sr ratios were normalised to 86Sr/ssSr value of 0.1194. The experimental uncertainty in 87Rb/s6Sr ratio is 2% and about 0.02% in s7Sr/86Sr ratio. The isochron parameters were calculated by the linear regression method of York (1969) and using 87Rb decay constant of 1.42 x lo-" a-l. All errors are quoted at 20 level.

The rare earth elements as well as Hf, Th, U, Sc, Cs, Cr were determined by instrumental neutron activation (INAA) at the Open University, U.K. following techniques described by Potts et al. (1985). Analyses were performed on 300 mg sample aliquots; activity data were corrected for sample mass, radioactive decay, and neutron flux variations. INAA detection limits generally lie in the range 0.03-1 ppm depending on elemental sensitivity.

Oxygen isotope analyses of silicates were performed at SUERC, East Kilbride by the fluorination method of Clayton and Mayeda (1963) as modified for ClF, rather than BrF, by Borthwick and Harmon (1982). Prior to oxygen extraction, -10mg samples were heated under vacuum at 200°C for a minimum of one hour and briefly prefluorinated. Oxygen was released by reaction at 650"

Gondwanu Research, V. 8, No. 3,2005

NEOPROTEROZOIC GRANITOIDS OF MEGHALAYA, NORTHEAST INDIA 423

overnight, and after purification was converted to CO, by reaction with platinised carbon. Reaction yields were measured manometrically and isotope ratios measured on a dual inlet, triple collector mass spectrometer. The oxygen isotope ratios are reported in the conventional

notation relative to Standard Mean Ocean Water (V-SMOW); NBS 28 gives F1*0=9.6%o and precision is +0.2%0 (lo).

Geochemistry Detailed chemical data on the granite and gneisses of

Meghalaya have been presented earlier (Ghosh et al., 1991; 1994a). The average chemical compositions of the granite and gneisses are given in table 2. The granite plutons have variable SiO,, maximum variation is shown by the Kyrdem (63.0 to 74.1% SiO,) and Rongjeng (64.1 to 77.0% SiO,) plutons. These also show variation in Na,O, KzO, MgO and Fe(T). All the granite samples display a calc-alkaline trend in the AFM diagram. In the normative Ab-Or-An diagram (Barker, 1979), the data points straddle the fields of granite and quartz monzonite. Based on the geochemical data (WCNKvalues) the Rongjeng

pluton is peraluminous with highest Sr content while the other plutons are dominantly metaluminous. In chemical composition, the different granitoid plutons show considerable overlap. As a result, no distinct field can be demarcated for these in the chemical variation diagrams. When the chemical constituents of Kyrdem pluton are plotted against the Differentiation Index (Ghosh et al., 1991), a positive correlation with SiO,, Na,O+K,O, Rb, Rb/Sr and negative correlation with CaO, MgO, Fe (T) and TiO, is noticed. These trends suggest crystal fractionation. For other plutons, only three samples have been analysed from each of them. Granitic rocks have been classified into I and S type (Chappell and White, 1974). In terms of mineralogy and major element characteristics, the Meghalaya granites (Kyrdem, Nongpoh, Sindhuli, South Khasi) have characters such as predominance of biotite, presence of sphene as accessory, normative diopside, metaluminous nature (WNK = 1.11 to 1.67, WCNK = 0.77 to 1.15, %O/Na,O = 1.68 to 2.18) and igneous xenoliths. These indicate similarity to I-type granite. However, the plutons have high K,O, Th/U ratio and evolved 1,. The Rongjeng granite have S-type characteristics (WNK = 1.35 to 2.09, N C N K = 1.17 to 1.55, %O/Na,O = 1.15 to 2.29).

Sediments (Quaternary)

Sediments ( Cret-Tertiary)

Sylhet Traps

Granite plutons

Metasediments (Shillong Group)

Amphibolite

Granite Gneiss

Granulite

Augen Gneiss

Rb-Sr isochron age in Ma

K-Ar age in Ma

Fig. 1. Geological map of Meghalaya showing the granite plutons with isotopic ages (after Chakraborty, 1990). Note the Predominance of the granite plutons in the eastern part of the plateau. Insert (in box) shows the position of Meghalaya plateau which is at the extreme notheastern part of the Peninsular India and is separated by Rajmahal-Garo gap.

Gondwana Research, V. 8, No. 3,2005

Tabl

e 1.

Petro

logi

cal,

min

eral

ogic

al a

nd R

b-Sr

who

le-r

ock

isoc

hron

age

s of

gra

nite

plu

tons

and

gne

isse

s fr

om M

egha

laya

.

Are

a C

ount

ry ro

ck

Petro

logy

M

iner

alog

ical

ass

embl

age

Rb-

Sr a

ge (

Ma)

and

Isr

Gra

nite

plu

ton

s K

yrde

m

Met

ased

imen

ts o

f Sh

illon

g G

roup

Myl

liem

M

etas

edim

ents

of

Shill

ong

Gro

up

Non

gpoh

G

rani

teG

neiW

B

iotit

e G

neis

s/

Am

phib

olite

of S

hillo

ng G

roup

So

uth

Kha

si

Gne

iss a

nd m

etas

edim

ents

Ron

gjen

g G

rani

te G

neis

s

Sind

huli

Gra

nite

Gne

iss

Gra

nite

Gn

eiss

Pa

thar

hkan

g So

uth

of N

ongp

oh

-

111 F

ine-

grai

ned

biot

ite g

rani

te

I1 P

orph

yriti

c gr

anite

/Coa

rse-

grai

ned/

Qua

trz

mon

zoni

te/Q

uartz

sye

nite

I H

ornb

lend

e bi

otite

gra

nite

/Dio

rite

111

Fine

-gra

ined

gra

nite

I1

Coa

rse-

grai

ned/

Porp

hyrit

ic g

rani

te

I Dio

rite

(occ

ur a

s en

clav

es)

111 F

ine-

grai

ned

gran

ite

I1 C

oars

e-gr

aine

d/Po

rphy

ritic

gra

nite

I D

iorit

e (o

ccur

as

encl

aves

) I11

Pin

k eq

uigr

anul

ar

gran

ite

I1 Coarse-grained/Porphyritic g

rani

te

I Dio

rite/

Nor

ite

I1 M

ediu

m-g

rain

ed/e

quig

ranu

lar

pink

gra

nite

I1

Coa

rse-

grai

ned/

Porp

hyri

tic g

rani

te

I Dio

rite

(occ

ur as

encl

aves

) M

ediu

m- t

o co

arse

-gra

ined

gra

nite

Gra

nite

gne

iss

Gra

nite

gne

iss

K-f

elds

par-

Qtz

-Pla

g (A

n,-A

,,)

K-f

elds

par-

Qtz

-Bio

t-Pla

g (A

n,-A

n,,)

Plag

(An,

-A

n,,)-

Bio

t-Hb-

Pyx-

Qtz

K

-fel

dspa

r-Q

tz-P

lag (

An,-

A,,)

K

-fel

dspa

r-Q

tz-B

iot-P

lag (

An,

-An,

,) 60

7213

, 0.7

1187

247

Plag

(An,-An,,)-Biot-Hb-Pyx-Qtz

Qtz

-K-f

elds

par-

Bio

t-Pla

g (A

n,-A

,,)

K-f

elds

par-

Qtz

-Bio

t-Pla

g (An,

-An,

,) 55

0215

, 0.7

0948

247

Plag

(An,-An,,)-Qtz-K-feldspar-Hb-Biot-Pyx

K-f

elds

par-

Qtz

-Pla

g (A

n,-A

,,)

K-f

elds

par-

Qtz

-Bio

t-Pla

g (A

ns-A

nlo)

69

0219

, 0.7

1074

229

Plag

- Bio

t-Hb-

Pyx -

Qtz

K

-fel

dspa

r-Q

tz-B

iot

K-f

elds

par-

Qtz

-Bio

t-Pla

g (A

ns-A

n,,)

7882

22, 0

.706

9922

0 Pl

ag-Q

tz-K

-fel

dspa

r-H

b-B

iot-P

yx

8082

32, 0

.725

9426

4 K

-fel

dspa

r-Q

tz-P

lag-

Hb

881+

-39,

0.70

517?

68

4792

26,

0.7

1482

2 72

Qtz

- K-f

elds

par-

Plag

-Bio

t-Gt

Qtz

- K-f

elds

par-

Plag

-Bio

t 17

1424

4, 0

.705

4624

83

1150

226,

0.7

0681

+49

~~

Rb-

Sr a

ge d

ata

are

from

Gho

sh e

t al.,

199

1, 1

994a

and

Myl

liem

gra

nite

dat

a fr

om C

him

ote

et a

l., 1

988.

Gondwana Research, V . 8, No. 3,2005

S. GHOSH ET AL.

NEOPROTEROZOIC GRANITOIDS OF M E G W Y A , NORTHEAST INDIA 425

Table 2. Average major element composition (wt%) of granite plutons and gneisses from Meghalaya.

(11 (21 (31 141 (5) (61 (7) (81 (91

n = 16 n = 3 n = 2 n = 3 n = 5 n = 3 n = 3 n = l n = 3

SiO, TiO, '4'203

Fe 0 MnO MgO CaO Na,O K,O p20, K,O/Na,O K,O/MgO CaO/ Na,O+K,O FeO/ FeO(T)/MgO A/NK A/CNK

68.53(3.56) 0.60 (0.3)

14.05(1.60) 1.94(1.34) 1.84(0.92) O.OS(0.04) 1.30(0.86) 2.29(0.97) 2.54(0.47) 5.38 (1.10) 0.31(0.34) 2.18(0.63)

8.56 (1.01) 0.30(0.15)

72.46(2.15) 0.45(0.11)

13.76(0.57) 2.23(0.53) 0.99(0.09) 0.05(0.01) 0.32(0.30) 2.08(0.58) 2.08(0.58) 4.55 (1.04) 0.13(0.06) 1.68(0.44)

2 1.641 13.75) 0.29(0.08)

0.75(0.07) 0.92(0.06)

1.42(0.20) 1.46(0.10) l.O(O.12) l.OS(0.07)

62.95(2.1) 0.91 (0.21)

15.65 (0.04) l.l(O.95)

3.54(0.12) 0.12(0.03) 1.88(0.58) 3.55(0.87) 3.29(0.41) S.ZO(0.96) 0.45(0.11) 1.64(0.50) 2.94f1.80) 0.43(0.13)

0.71(0.02)

1.41(0.05) 0.89(0.06)

67.58(0.87) 0.83(0.04)

15.26 (1.56) 2.09(0.92) 1.92(0.57) 0.06(0.01) 0.67(0.03) 2.7(0.35)

2.73(0.17) 4.94(0.19) 0.20(0.02) 1.81(0.11) 7.33(0.27) 0.35(0.04)

0.84(0.06)

1.55 (0.12) 1.04(0.10)

68.79(5.3) O.SS(0.53)

12.73(1.66) 2.52 ( 1.48) 2.05(0.76) 0.23(0.02) 1.09(0.76) 2.48 (1.45) 2.59(0.26) 4.63(0.82) 0.21(0.13) 1.82(0.45) 8.23(7.92) 0.36(0.24)

0.82(0.06)

1.77(0.30) 1.32(0.16)

68.14(0.32) 0.77(0.17)

14.91(2.11) 2.72(0.69) 1.80(0.24) O.OS(0.02) O.SO(0.19) 2.42(0.37)

2.81(0.35)) 4.83(0.53) 0.18(0.12) 1.75 (0.38)

10.45(3.36) 0.32(0.05)

0.89(0.02)

1.51 (0.21) l.OS(0.20)

56.21(2.60) 73.58 1.80(0.09) 0.31

15.32(1.14) 13.81 4.88(0.19) 1.43 4.46(0.28) 1.08 0.12(0.01) 0.04 3.40(1.15) 0.56 6.45(1.15) 1.7 1.48(0.25) 3.58 3.77(0.28) 3.73 0.67(0.04) 0.06 2.61(0.57) 1.04 1.19(0.38) 6.66 1.23(0.20) 0.23

0.724(0.08) 0.81

2.35(0.04) 1.39 0.85(0.09) 1.06

75.90(1.12) 0.34(0.11)

12.05(0.58) 1.42(0.58) 0.75(0.27)

0.013(0.005) 0.15(0.0)

1.07(0.24) 3.36(0.09) 4.20(1.01) 0.06(0.02) 1.26(0.33)

28.01(6.75) 0.14(0.04)

0.93(0.02)

1.21(0.18) l.O(O.13)

(1) Kyrdem granite, (2) Nongpoh granite, (3) Mylliem granite (Source: Majumdar, 1986), (4) South Khasi granite, (5) Rongjeng granite, (6) Sindhuli granite, (7) Diorite from South Khasi pluton, (8) Granite gneiss from south of Nongpoh, (9) Granite gneiss from Patharkhang. Major oxide data are from Ghosh et al., 1991, 1994a. Numbers in the parentheses indicate the standard deviation (20). n = Number of samples from granites and gneisses.

Amongst the trace elements (Table 3) Sc, Cr, Hf show inhomogeneous distribution in the granite plutons. Th is consistently high. This reflects inhomogeneous accessory restite minerals (Ghosh et al., 1991).

The dioritic enclaves in the South Khasi batholith are chemically distinct from the host porphyritic granite in having a lower SiO,, Na,O, higher Fe(T), MgO and CaO (Table 2). These enclaves show an increase in SiO,, Fe(T), Rb/Sr ratios but decrease in CaO, K,O, MgO with differentiation index. These are also distinctly metaluminous with a mild tholeiitic affinity. All granite gneisses from Nongpoh, Sonapur, Patharkhang and Rongieng have SiO, exceeding 70%. However, Patharkhang gneisses have highest SiO, content (74.65 to 76.82%) and lowest N,O, (11.54 to 12.68%). The Nongpoh gneisses have high Al,O, (13.37 to 15.38%) compared to the other gneisses where Al,O, ranges from 10.10 to 12.68%. Rb varies from 64 to 325 ppm in Patharkhang, 62 to 169 ppm in Rongjeng and 76 to 212 ppm in Nongpoh gneisses. Rb/Sr ratios in Patharkhang are considerably high (1.22 to 9.77) compared to Rongjeng (0.08 to 0.88) and Nongpoh gneiss (0.14 to 2.34). The gneisses are metaluminous and show a calc-alkaline trend in A-F-M diagram. In a normative An-Ab-Or diagram the gneissic samples from Sonapur fall in granodiorite field, Rongjeng and Patharkhang gneisses fall in the granite field. With increasing SiO,, 50 increases while MgO and CaO decrease reflecting crystal fractionation.

Low U content and very high Th/U ratio (4 to 30.04, Table 3) compared to an estimated crustal average of -3.8, suggest that the source material was strongly depleted in U relative to Th.

Rare Earth Elements Concentrations of rare earth elements in the whole-rock

samples of granite and granite gneiss are given in table 3. Chondrite normalised REE patterns (using normalising values of Nakamura et al., 1974) for the gneisses and granitoids are shown in figure 2A to 2D. A11 the gneisses and granites show fractionated REE patterns with enrichment of light rare earth elements (LREE). The granites have higher CREE contents than the gneisses. The REE patterns in all the granites are similar (Fig. 2A to 2C). The REE patterns of the gneisses are also generally similar (Fig. 2D) with the exception that two samples of Patharkhang gneisses have conspicuous negative Eu anomalies (Eu/Eu* = 0.22 to 0.25) and CREE (205 to 214 ppm). CREE contents vary from 224 to 937 ppm for the granites and 145 ppm to 443 ppm for the gneisses. ( L O ) , ratios vary from 6.19 to 47.09. All the granite samples are characterised by conspicuous negative Eu anomaly (Eu/Eu*= 0.22 to 0.65). The most conspicuous negative Eu anomaly (Eu/Eu* = 0.22) found in the fine-grained granite of Kyrdem is consistenf with very low Sr contents indicating removal of plagioclase. It is

Gondwnna Research, V. 8, No. 3,2005

426 S. GHOSH ET AL.

interesting to note that the REE patterns in the dioritic enclaves and the host South Khasi pluton are similar except that the diorites have a less conspicuous Eu anomaly (Fig. 2C).

(Fig. 3). The ISr ratios of the Meghalaya granites are higher than that found in normal mantle-derived rocks (Faure, 2001). Although not so clear, the gneisses also indicate an increase in ISr with younging of age.

Strontium Isotopic Composition Oxygen Isotope Composition The measured 87Sr/86Sr ratios and the concentrations

of Rb and Sr of the granites and gneisses are given in table 4. The Isr values are comparatively low (Sindhuli: 0.70517, Rongjeng: 0.70699) in the plutons that intrude in the gneisses cpmpared to the plutons (Kyrdem: 0.71482) that intrude the metasediments. Amongst the granite plutons, the initial 87Sr/86Sr ratio (Is,> increases linearly with progressive younging of Rb-Sr age from the Sindhuli pluton (881 Ma) to the Kyrdem pluton (479 Ma)

1000

+ Fine grained Granite (260iRJ) X Porphyritic Granite (28iRJ)

0 Porphyritic Granite (137DiNP) 0 Porphyritic Granite (1 15hVP)

a2

5 h a c 2 100 W

E

. c a2 a Q in

10

5 L a c e Nd SmEu Tb Yb Lu

1 0 0 0 ~ 1 1 I I I 1 I I I I I I I I l j

a2 d q

10

South Khasi 0 Fine grained Granite (219iSK) A Porphyritic Granite (128iSK) + Noritic Enclave (227iSK)

-

-

The oxygen isotopic composition of two samples from each of the granitoid plutons and basement gneisses are given in table 4. The P O values are uniform and restricted in the range + 5.78 to + 8.70%0, if the gneissic samples of Patharkhang-Sonapahar are excluded. Duplicate analyses did not generate reproducible oxygen isotope data on the two samples, 162/SP and 197A/SP, of granite gneiss from the Patharkhang region. The whole-rock 6l8O data for these two samples are very low, +2.52 to +5.31%0.

1000 I I I I I I I I I I I I I I I Sindhuli

Q) 3 8 1 rA

10

0 Granite (21 USG) + Granite (1 82iSG)

Kyrdem

0 Porphyritic Granite (8IKM) A Hb- Biotite Granite (SIKM)

X Fine grained Granite

1 0 0 0 - 1 I I 1 I I I I I I I I I I I -

Granite Gneiss

+ Nongpoh (135SN) .cI 0 Rongjeng (258AiRJG)

+ Patharkhang (1 97A/SP) x Patharkhang (162/SP)

W .I a 3 6 100 = -

R \

CI W

a rA

10 T

Fig. 2. Chondrite normalised (after Nakamura, 1974) REE plots for granites (2A to C) and gneisses (2D). The field of Patharkhang gneiss is shaded. The gneisses have similar REE pattern but the 1714 Ma old Patharkhang gneisses have more pronounced Eu anamoly.

Gondwana Reseauch, V. 8, No. 3,2005

Tabl

e 3.

REE

abu

ndan

ces

(in p

pm) i

n gr

anite

s and g

neiss

es o

f Meg

hala

ya.

Sam

ple.

51

81

12

1 11

51

137D

22

7t

1281

21

91

281

2601

18

21

2111

25

8Al

135S

l 16

21

197A

l N

o.+

KM

KM

KM

NP

/NP

SK

SK

SK

RJ

RJ

SG

SG

RJG

N

N

P SP

La

47.2

Ce

12

0 N

d 60

.8

Sm

11

Eu

1.75

Tb

1.

55

Yb

5.14

Lu

0.

8 sc

18

.1

Cr

130

cs

3 H

f 8.

89

Th

41.7

U

4.

6 X

REE

248.

24

L@uN

6.

12

LaJY

b,

6.19

L%

JSm

, 2.

7 C

eJY

b,

6.04

E

mu

*

0.5 1

Th

/U

9.06

60.6

11

1 40.9

6.

33

0.99

1.

05

2.72

0.

46

3.7

9 9.74

6.

77

68.3

17

22

4.05

13.6

7 15

.04

6.02

10

.56

0.47

4

104

123

207

244

79.1

93

.7

13.6

14

.5

0.94

1.

02

2.06

1.

64

4.5

3.52

0.

63

0.53

3.

7 5.

7 6

7 6.

74

5.1

6.35

9.

38

97.7

66

.2

15.7

3.

93

411.

83

481.

91

17.1

4 24

.09

15.6

23

.58

4.81

5.

34

11.9

1 17

.94

0.22

0.

25

6.22

16

.84

102

208 86

.7

13.4

1.

5 1.

76

5.03

0.

85

7 13 1.59

9.

56

82.4

11

.8

419.

24

12.4

2 13

.69

4.79

0.34

6.

98

10.7

117

208

245

391

115

142

18.3

21

.6

3.32

2.

43

1.93

2.

63

4.17

5.

51

0.62

0.

74

23.1

9.

7 84

22

12.5

13

.7

6.45

62

.4

1.05

3.

29

505.

34

773.

91

1.54

19.5

8 29

.17

18.9

3 25

.48

4.02

6.

06

15.2

18

.37

0.65

0.

38

6.14

18

.96

57.3

22

0 10

5 42

6 38

.9

169

6.31

25

.5

0.88

4.

11

0.9

2.92

1.

8 7.

74

0.26

1.

15

2.7

13.3

7

15

1.58

3.

16

5.58

10

.9

4.38

3.

57

21 1.

35

856.

42

43.4

55

22.8

7 19

.86

21.4

9 19

.18

5.72

5.

43

15.1

14

.24

0.39

0.

54

9.91

15

.41

254

483

169 23.7

2.

26

2.51

3.

64

0.49

3.

7 10

1.9

14.2

13

6 6.25

93

8.6

53.8

3 47

.09

6.74

34

.34

0.35

21

.76

126

243 89

.6

14.6

1.

36

2.62

4.

39

0.64

6.

4 I5

3.1

8.72

76

6.92

48

2.21

20.4

3 19

.37

5.43

14

.32

0.27

10

.98

173

34 1

133 20.9

2.

53

2.46

5.

06

0.72

9.

4

0.68

13

17.3

33

.4

1.11

67

8.67

34.7

67

.5

31.5

5.

69

1.56

0.

86

2.49

0.

37

8 1.22

6.

97

9.5 1.3

144.

67

110

218 91.5

14

.6

2.87

1.

52

3.3

0.49

19

.9

53 3.03

9.

03

3.26

44

2.28

25.4

24.9

5 9.

74

23.3

1 23

.07

9.41

22

.51

5.21

3.

84

4.74

17

.44

7.02

17

.1

0.41

0.

85

0.67

30

.09

7.31

7.

79

49.9

99

.4

39.9

7.

65

0.57

1.

41

5.29

0.

72

2.9

8 0.43

6.

94

3.17

20

4.84

7.19

6.

36

4.1

4.86

0.

22

8.07

25.6

45.1

80

.7

61.6

14

.1

1.28

3.

02

7.29

1 3.

6

7.17

2.93

21

4.09

4.68

4.

17

2.01

2.

86

0.25

7.

03

20.6

Gondwana Research, V . 8, No. 3,2005

NEOPROTEROZOIC GRANITOIDS OF M E G W Y A , NORTHEAST INDIA

428 S. GHOSH ET AL.

Table 4. Strontium and oxygen isotope and REE data of granitoids and gneisses of Meghalaya.

Sample No. Aredrock suite Rb Sr Rb/Sr e7Rb/86Sr 87Sr/86Sr K,O SiO, 6*a0 1 REE (ppm) (ppm) YO YO %o (ppm)

5/KM 8/KM 115/NP 137D/NP 128/SK 131/SK 28/RJ 39/RJ 182/SG 211/SG 227/SK 228/SK 268G/NG 268E/NG 135S/N 135H/N 162/NP 197A/SP 258MRJG 2587/RJG

Kyrdem granite Kyrdem granite Nongpoh granite Nongpoh granite South Khasi granite South Khasi granite Rongjeng granite Rongjeng granite Sindhuli granite Sindhuli granite Norite from South Khasi Diorite from South Khasi Granite gneiss from Sonapur Granite gneiss from Sonapur Granite gneiss from South of Nongpoh Granite gneiss from South of Nongpoh Granite gneiss from Patharkhang Granite gneiss from Patharkhang Granite gneiss from Rongjeng Granite gneiss from Rongjeng

219 296 323 223 216 164 198 291 166 188 119 125 94 107 131 116 154 64 93 93

455 290 116 253 374 560 1098 160 685 313 891 382 218 205 901 144 29.6 52 133 152

No systematic difference in 61s0 values between the granites and gneisses is noticed. There is a reasonable concordance of 6l8O between the granitoids and basement gneisses for Nongpoh (8.0+0.7 and 8.2+0.5%0) and for Rongjeng (7.2k0.4 and 6.6+0.9%0). For the South Khasi pluton, the granite data (7.150.3 %o) and the dioritic enclave (6.9+0.8%0) are similarly indistinguishable. No regional distribution pattern is apparent in 6l80 values.

Petrogenesis

The granitoids are coarse-grained with a dominant potash-rich porphyritic phase and megacrysts of K-feldspar except for the Sindhuli granite, which is homophanous. Approximate depth of generation of granitic magmas can be estimated, based on normative quartz, albite, orthoclase composition and experimental results on the granite system (Anderson and Bender, 1989). One essential assumption is that the rock should represent a minimum or near-minimum melt. On the normative quartz-albite- orthoclase diagram (Fig. 9 of Anderson and Bender, 1989) the Meghalaya plutons fall near the 6 kbar minima, corresponding to about 18 km depth of melt generation. Combined P80 and Sr isotopic studies have been found useful in understanding the origin of granitic rocks (Harmon and Halliday, 1980). These are based on the observation that the crust is enriched in both 6l80 and 87Sr compared to the mantle. Magmas derived from the mantle have characteristic isotopic composition, initial s7Sr/66Sr = 0.703+0.001; 6l8O = 6.0+0.5%0. Therefore granites, which originated in the crust and mantle have

0.4812 1.0178 2.7841 0.8837 0.5768 0.2925 0.1803 1.1818 0.2428 0.6005 0.1338 0.3273 0.4301 0.5229 0.1455 0.8048 5.2027 1.2286 0.7012 0.2637

1.3944 2.9526 8.1089 2.5625 1.6716 0.8472 0.522 5.2922 0.7028 1.7405 0.3874 0.8486 1.2479 1.5176 0.4212 2.3368 15.6664 3.5865 2.0371 0.7649

0.7246 0.73608 0.77404 0.72894 0.72585 0.71926 0.7125 0.76437 0.71402 0.72665 0.71256 0.72408 0.73483 0.73801 0.71387 0.74503 1.12545 0.7975 0.75082 0.73523

4.32 4.47 5.25 3.36 5.11 4.97 5.04 5.4

4.23 5.19 4.01 3.46 3.27 3.48

3.73 5.02 3.07 3.32

64.64 72.9

73.38 74

67.26 66.91 68.58 69.99 67.79 68.23 53.85

59 71.56 69.56

73.58 76.24 76.82 75.35

8.55 8.16 8.65 7.31 6.84 7.34 6.84 7.63 7.98 6.89 6.01 7.7

6.92 7.38 8.7

7.63 5.31,4.77 4.53,2.52

7.51 5.78

248 224 482 419 774

856

482

505

442

205 214 145

distinctive 6l80 and Sr isotopic signatures. The observed P O values and s7Sr/86Sr ratios for the Meghalaya granites are higher than normally found in mantle derived rocks. This is most pronounced in the Kyrdem pluton (P80 8.16 and 8.55 %o: ISr = 0.71482). The 6l8O values are, however, on the lower side of the normally suggested boundary (10%0) between S and I type granites of Australia (ONeil and Chappell, 1977). Taylor and Sheppard (1986), in a review of stable isotope systematics of igneous rocks, concluded that magmas with P O > 7.5%0 must have been derived from or have assimilated crustal material, and ruled out the possibility that fractional crystallization of basic magma (with 6ls0 -+6%0), could produce granites with P O > +7.5%0. It is thus concluded that the Meghalaya granites could have originated by partial melting of crustal material. The high Th/U ratio of granites also suggests the involvement of a lower crustal component in the generation of granites.

The initial 87Sr/s6Sr (IS> ratios show an increase with progressive younging in Rb-Sr age as seen from the I,, versus time diagram (Fig. 3).

Among the samples of the present study, granite gneisses of Patharkhang are anomalously low in 6I8O. Igneous rocks with low 6lSO values are believed to have interacted with heated, surface-derived fluids (Cartwright and Valley, 1992). Demonstration of oxygen isotope disequilibrium among rock forming minerals in the low 6lSO granites would provide useful confirmation of the conclusion. However, the large range in the whole-rock 6I8O values in the samples of the Patharkhang gneiss rules out a high temperature (magmatic) origin for the low

Gondwana Research, V . 8, No. 3,2005

NEOPROTEROZOIC GRANITOIDS OF MEGHALAYA, NORTHEAST INDIA 429

f K

f M GG t R

+S .D ion

:

value. The Patharkhang gneisses possibly experienced oxygen isotope exchange with low 6180 fluids at elevated temperature. In view of restricted nature of low 6 W rocks, it appears that large volumes of high temperature fluid rich in SiO, locally infiltrated the area (La1 et al., 1978).

Another significant feature is the occurrence of noritic/ dioritic enclaves in the granite plutons particularly in the South Khasi batholith. The broadly similar mineralogy of the plutons (Ghosh et al., 1991) implies that their source rocks were also similar. It is thus considered possible that the source rock of these plutons could either be gneissic or noritic/dioritic rocks. From geochemical and isotopic consideration either of the rock types could be the source rock for the granites. The differences in modal composition between the two have been shown in table 1. Many experimental works have shown that granitic rocks can be formed by crustal melting. Petrogenetic models have related variation of trace element abundances to source rock composition and degree of melting. Such calculations have been attempted here for the granitic plutons following the formulation of Shaw (1970). A dioritic rock with Qtz,,K-Feld,,Plag,,Biot, is assumed to undergo melting in the ratio 10:20:60: 10. Distribution coefficients have been taken from Anderson and Cullers (1978). The abundances of certain elements e.g., Ce, Lu, Rb, Sr in the predicted melt at 60% melt are generally similar to those observed in the granite. When similar petrogenetic calculations are attempted with a gneissic source

0.72

- 0.71 65 % (I) m a I=

- .- .- c - - 0.7

r 0.69

composition (Qtz,,K-Feld,,Plag,,Biotlo) undergoing melting in the ratio, 10:30:40:20, reasonable agreement is obtained at 30% partial melting. The moderately fractionated REE patterns indicate that garnet may have been a residual phase in the source region. The presence of negative Eu anomalies and low Sr contents indicates that plagioclase has been removed during the evolution. The geochemical variation including initial strontium isotopic composition of the granite plutons cannot be explained by crystal fractionation alone. Several sources possibly contributed to the melt generation of the different plutons.

The similarity of chondrite normalised REE patterns of the noritic enclaves and host South Khasi pluton suggest derivation of the plutonic magma from a noritic source composition.

Discussion

Linkage of Meghalaya granite with amalgamation of Gondwana supercontinent

In the Meghalaya Plateau, granite plutons intrude the basement orthogneisses and the overlying Shillong Group of metasediments The granite plutons yield Rb-Sr whole- rock ages ranging from 479 Ma to 881 Ma. The basement gneisses are distinctly older at 1714 Ma and 1150 Ma. Considering the low metamorphic grade of these gneisses

Gondwana Research, V. 8, No. 3, 2005

Strontium evolution diagram showing the isochron ages (in Ma) and initial 87Sr/86Sr ratio of granitoids and gneisses of Meghalaya in relation to mantle evolution. Horizontal and vertical bars are the errors in the age and ISr,respectively, Symbols used: D-Noritic enclaves in South Khasi batholith, G-Goalpara, K-Kyrdem, M-Mylliem (Chimote et a1.,1988), R-Rongjeng, S-Sindhuli and SK-South Khasi.

430 S. GHOSH ET AL.

it is believed that the Rb-Sr whole-rock ages represent their protolith ages. Formation of the Rodinia supercontinent during the 1300-1000 Ma Grenvillian event is believed to have been followed by disintegration and dispersal of the crustal fragments around 750 Ma ago (Kroner and Cordani, 2003). Most of these dispersed terranes reassembled around 550 Ma ago in the Gondwana superccntinent (Meert and Van der Voo, 1997). These events left signatures in the crustal rocks (Windley, 1995). In this context the isotopic ages of basement gneisses and high-grade metamorphic rocks of Meghalaya suggest involvement of the Meghalaya region in an accretion event. The Late Neoproterozoic-Cambrian event of tectonothermal metamorphism and plutonism, termed earlier as Pan-African orogeny (Kennedy, 1964) is quite widespread in Africa, Asia, Antarctrica and western Australia (Fitzsimons, 2000; Rogers and Santosh, 2002).

This event is recorded in India (Fig. 4 ) as: (i) Major granulite metamorphism in the Southern

Granulite Belt (550 Ma, Holzl et al., 1994; Miller et al., 1996; Santosh et al., 2003; Ghosh et al., 2004; Braun and Brocker, 2004), (ii) widespread igneous, thermal and metamorphic activity from 540 to 670 Ma in the Kerala Khondalite Belt (Ravindra Kumar et al., 1990), (iii) granite plutonism (395 to 637 Ma) in Tamil Nadu

(Nathan et al., 1994; 2001; Santosh et al., 2005), (iv) Erinpura granite emplacement (740 to 955 Ma) and Malani volcanic activity (680 to 780 Ma, Rathore et al., 1999) in northwestern India, (v) thermal metamorphism in the Eastern Ghats (553 to 550 Ma, Aftalion et al., 2000; Dobmeier and Raith, 2003), (vi) pegmatite activity in Nellore schist belt (Ghosh et al., 1994b).

Crustal fragments of the eastern Gondwana in India, Madagascar, Sri Lanka and East Antarctica are overprinted by the Late Neoproterozoic-Cambrian events. These events are represented by different orogenies in south western Australia (Myers and Nelson, 1997; Collins, 2003). In a reconstructed continental assembly (Fig. 4) southwestern Australia juxtaposes with northeastern India (Fig. 4). Both of them encircle the northern fringe of Antarctica. The Pinjarra orogen of southwestern Australia has been described as a collision zone between India and Australia representing the Late Neoproterozoic-Cambrian assembly of the Gondwana supercontinent (Collins, 2003). From a detailed structural and geochronological study of the Leeuwin Complex of southwestern Australia (Collins, 2003) suggested that the high-grade metamorphism at 522rt5 Ma is close to the estimates of the final amalgamation of the Gondwana supercontinent. Recently Santosh et al., (2005) reported U-Pb electron microprobe ages of zircon

Fig. 4. Reconstruction of part of Gondwanaland at the end of Neoproterozoic (updated from Myers and Nelson, 1997; Collins, 2003) showing Neoproterozoic events/orogenies in parts of India, Madagascar, Antarctica and southwestern Australia. Inclined hatchet-marked areas demarcate the extent of the East African orogen (650-520 Ma, Stern et al., 1994) in the Gondwana supercontinent.

Gondwana Research, V r 8, No. 3,2005

NEOPROTEROZOIC GRANITOIDS OF MEGHALAYA, NORTHEAST INDIA 431

and monazite from two granite plutons of Kerala and Tamilnadu in southern India in the range 500-520 Ma. Santosh et al. (op. cit.) believed that these ages represent Late Neoproterozoic-Cambrian felsic magmatism during collisional- extensional episodes of the Gondwana supercontinent.

The isotopic ages for the Meghalaya granite plutons of the present study are similar to those of Kerala and Tamilnadu. Granites of both the regions are devoid of mineralisation. Whereas the Kerala plutons were classified as A-type granites with abnormally high Ba and Sr contents (Santosh and Drury, 1988), the Meghalaya plutons have S-type characteristics. The Kerala plutons intrude the basement charnockites and migmatites but the relations between granite plutonism and granulite metamorphism is not clearly understood. However, tensional stress and thermal upheaval associated with break-up and reassembly of the Gondwana supercontinent could have caused melting in the lithosphere. The coeval and broadly similar granitic activity in Meghalaya, Kerala and elsewhere in India appears to be manifestation of the collisional-extensional episodes of the Gondwana supercontinent.

Conclusions Geochemically, the Meghalaya granites of Northeast

India are metaluminous with significant europium anomalies, fractionated REE pattern with enrichment of LREE. Oxygen and strontium isotopic data indicate derivation from lower crustal sources with moderate Rb/Sr ratios. These granitoids originated due to tensional stress and thermal upwell related to the collisional- extensional episodes of the Gondwana supercontinent. Crustal fragments with magmatic events in the range 850 Ma to 450 Ma are scattered in parts of eastern Gondwana (Fig. 4).

Acknowledgments The paper is published with the kind permission of the

Director General, Geological Survey of India. Comments of G. Sarkar were useful in improving the paper. We thank B. Das for help in drafting the figures. A.S. Collins and an anonymous reviewer are thanked for constructive journal reviews and M. Santosh for helpful editorial comments. D.K. Paul is grateful to Council of Scientific and Industrial Research, India for support during preparation of this paper.

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