Geochemical Constraints on Anorogenic Felsic Plutonism in North Delhi Fold Belt, Western India

Gondwctnci Research, % I , N o . 2, pp. 247-255. 0 1998 Internntionul Associntian,ti,r Gondwunn Resecirch, Jqmn ISSN: 1342-937X

Gondwana Research

Geochemical Constraints on Anorogenic Felsic Plutonism in North Delhi Fold Belt, Western India M. K. Pandit' and M. K. Khatatneh2 'Department of Geology, University of Rajusthan, Juipur -302004, India. 2Nutional Petroleum Company, Amman, Jordan (Manuscript received November 27, 1996; accepted May 19, 1997)

Abstract Ajitgarh pluton, along the northwestern fringe of Indian peninsular shield, is a composite trondhjemite-alkali granite body. Both the granitoid types, intrusive into metasediments of Delhi Supergroup, represent different pulses of anorogenic magmatism. The granitoids, post-dating the major deformationel phase, demonstrate a time relationship with each other. Subtle variations in mineralogy and geochemistry indicate distinctive petrogenetic lineage for both the granitoids. Their tectonic setting, mineralogical compositions and geochemical signatures are consistent with A-type granites and an igneous source. Ajitgarh granitoids are quite distinct from other syn-orogenic, S-type granite massifs ( 1 700-1500 Ma) of North Delhi Fold Belt and appear to be related to the late Proterozoic anorogenic magmatism associated with rejuvenation of the rift system that controlled the Delhi sedimentation. All the granitoid bodies within North Delhi Fold Belt have so far been presumed to be syn-orogenic and coeval at 1700-1500 Ma.

Key words: Delhi Supergroup, trondhjemite, alkali granite, anorogenic.

Introduction The Aravalli Mountain Region (AMR), situated along the northwestern periphery of Indian peninsular shield, has evolved through two Proterozoic accretionary cycles represented by the Aravalli (Early Proterozoic) and Delhi (Middle to Late Proterozoic) Fold Belts. The rocks of Delhi Supergroup are contained in two domains, the North Delhi Fold Belt (NDFB) and South Delhi Fold Belt (SDFB). It is still amatter of debate whether NDFB and SDFB represent diachronous Delhi sedimentation (Heron, 1953) or represent two independent basins, subsequently juxtaposed along a suture ( Bose, 1989; Volpe and Macdougal, 1990).Widespread acid magmatism is a characteristic feature of the Delhi Fold Belt. The older magmatic activity (1 700 - 1500Ma) is restricted to the NDFB and the younger and more extensive one (850 Ma) is encountered within and along the western margin of SDFB (Erinpura Granite of Heron, 1953). Distinct ages for the granitoids of axial section of Delhi Supergroup (SDFB) and in the NDFB (Chaudhary et al., 1984;Anjaneya Sastri, 1992) have been considered as conclusive evidence for diachronous Delhi sedimentation (Sinha Roy, 1988). The granitoids of NDFB have been reported to be syn-orogenic, related to a single thermal event (Bose, I98 1 ; Anjaneya Sastri, 1992). In the present work the anorogenic nature of Ajitgarh granitoids

has been discussed in light of relevant petrochemical data. These granitoids appear to be related to a distinctly younger thermal event as compared to other NDFB granitoids.

Occurrence and Geology The Ajitgarh pluton (27"26' N : 7550' E) intrudes the metasediments of Delhi Supergroup (Das, 1988). The contact relationship between the intrusive granitoids and host metasediments is concealed under alluvial sand. The intrusive nature of the pluton is, however, manifested by its emplacement athwart to the regional structural grain. The position of Ajitgarh granitoids within regional lithostratigraphic framework is shown in Fig. 1. Roy and Das (1985) have reported three phases of deformation in the region. The first deformation (DF,) has resulted in a series of NE - SW trending isoclinal folds, coaxially refolded during subsequent (DF,) deformation.A less pervasive, third deformation (DF3) has resulted in broad warps with roughly E - W trending axial orientations. Roy and Das (1985) have also considered the granite emplacement, syntectonic with the DF, deformation.The generalized geological setting of the area is summarized as: Intrusives

Quartz pegmatite veins Granite Trondhjemite

M. K . PANDIT AND M.K. KHATATNEH

248

D

C

^I

7f150'

1

270

_.

2 5'

t

t

+ \

\

t

t

t

t t +

\

?,

I

Fig. 1.

75'

50

A. Synoptic map ofAravalli Mountain region (AMR) showing distribution of Delhi Fold Belt. NDFB - North Delhi Fold belt, SDFB - South Delhi Fold belt.B.Geologica1 map of NDFB showing location of Ajitgarh. C. Ajitgarh pluton in the regional stratigraphic framework. D.Lithological map of Ajitgarh pluton showing distribution of trondhjemite and alkali granite. 1. Conglomerate; 2. Sericite quartzite; 3. Ainphibolite; 4. Trondhjemite 5. Alkali granite; 6 . Alluvium

Country Rock

Sericite Quartzite (Delhi Supergroup).

Ajitgarh pluton has been reported to be a homogeneous alkali granite body (Roy and Das, 1985; Das; 1988, Chaudhary et al., 1984). However, Pandit et al. (1996) have identified a trondhjemite phase (in addition to alkali granite), on the basis of field relations, distinct mineralogy and geochemistry. Trondhjemite, representing an earlier pulse of acid magmatism, occupies the southeastern part of the pluton, in sharp contact with alkali granite. The younger age of alkali granite is established by its tongues as seen protruding into trondhjemite. Absence of any chilled margin suggests a small time difference

between their emplacement. Both the granitoids are further cut across by younger quartz- pegmatite veins.

Petrography and Mineralogy Trondhjemite and granite are medium to coarse grained, holocrystalline, non-foliated, massive and compact rocks with predominant hypidiomorphic granular texture. They are characterised by well preserved magmatic texture and absence of any planar fabric. Trondhjemites are off white to grey whereas the alkali granites are pink in colour. The distinction Gondwana Research, V I , No. 2, 1998

ANOROGENlC PLUTONISM, RAJ ASTHAN

between both the granitoids is further reflected in their mineral compositions. Granite has predominant potash feldspar (microcline microperthite) and quartz with minor plagioclase (albitic). Biotite and hornblende, together constitute about 10% of total rock volume. An increase in biotite abundance, in relation to hornblende, is noticed within the pluton from SW to NE direction. Euhedral zircon, sphene, opaques and allanite are the accessory minerals, in order of abundance. Feldspars exhibit moderate sericitization, and inclusions of quartz within large perthite grains. Hornblende shows sieve like appearance due to inclusions of minute quartz grains. Relict clinopyroxene core, mantled by hornblende is also observed in some grains. In few cases, biotite demonstratesreplacement relationship with hornblende and is seen growing along the grain margins and cleavage planes of the latter. The trondhjemite is characterized by predominant quartz and plagioclase (albite - oligoclase), minor potash feldspar and hornblende. Biotite is conspicuously absent. Sphene is the main accessory mineral besides zircon, epidote and opaques. The trondhjemite mineralogy has already been discussed in detail elsewhere (Pandit et a]., 1996).

Geochemistry Whole rock, major and trace elements (Zn, Ga, Th, Rb, Sr, Y, Zr, Nb) were determined by XRF method using pressed powdered pellets, on a Siemens SRS 3000 Sequential X-Ray

249

spectrometer at Wadia Institute of Himalayan Geology, Dehradun. Rare Earth Elements were determined by wet chemical method, using ICP MS (Plasma Quad PQI), at National Geophysical Research Institute, Hyderabad. The major and trace element data for Ajitgarh granitoids is given in Table 1. W E data on representative samples is given in Table 2. The geochemical data indicate sub-alkalic affinity of the pluton. The granites are meta-aluminous (A/CNK <1) whereas the trondhjemites are peraluminous (A/CNK > I). Peraluminous nature of trondhjemite is due to extreme depletion in CaO and K,O and does not indicate any alumina oversaturation. A high A/CNK ratio is a common feature of trondhjemitic rocks (Barker, 1979). Silica, showing a wide range from 67.4 to 78.65%, can discriminate the two granitoid types into moderately silicic, alkali granite and highly silicic, trondhjemite. The alkali granites have a restricted silica range of 67.40 to 69.56%, (barring three samples) and narrow alumina variation between 13.33 to 13.87%. Preponderance of K,O over Na,O is also seen in alkali granite with Na20/K20ratio varying from 0.78 to 0.90. Highly silicic (73.27 to 78.65%) trondhjemites are extremely depleted in CaO (0.51 to 1.41%) and MgO (0.01 to 0.54%). Low K,O (<2%) and high Na20/K,0 ratio (2.9 to 3.3) are diagnostic of trondhjemites. Distinction between trondhjemite and alkali granite is also underlined in the Ab-

Table I . Major (wt %) and trace element (ppm) composition of Ajitgarh granitoids. S.No. I SampleNo.ML13

2 MLII

3 ML7

4 ML18

5 ML14

6 ML17

7 ML20

8 9 ML24 ML22

10 ML15

11 MZI

12 13 MZ16 MZ13

14 MZ10

MAJOR ELEMENT SiO, 73.27 73.84 0.34 TiO, 0.38 12.56 A1,0, 12.96 0.59 Fe,O, 0.85 FeO I .97 I .43 MnO 0.05 0.04 0.53 MgO 0.34 CaO 1.36 I .36 4.40 Na,O 3.96 K,O I .43 1.59 0.00 P,OS 0.00

75.02 0.37 12.65 0.46 1.15 0.04 0.54 1.40 4.80 1.66 0.00

75.74 0.4 I 12.60 0.47 1.16 0.03 0.25 0.92 4.60 1.38 0.00

76.47 0.30 12.21 0.43 1.04 0.03 0.20 0.43 4.11 1.51 0.00

77.45 0.29 12.23 0.26 0.66 0.03 0.24 0.51 4.01 1.53 0.00

78.04 0.35 12.23 0.36 0.88 0.04 0.01 0.39 4.08 1 .so 0.00

78.15 0.35 12.48 0.39 0.99 0.02 0.01 0.55 4.63 1.51 0.00

78.32 0.35 12.38 0.24 0.57 0.03 0.01 0.08 4.42 1.53 0.00

78.65 0.34 12.17 0.13 0.3 I 0.02 0.08 0.02 4.24 1.58 0.00

67.40 0.39 11.70 1.47 3.56 0.06 0.77 1.36 3.69 4.36 0.53

67.54 0.41 13.69 3.49 0.07 1.12 2.05 3.59 4.52 0.00

67.58 0.40 13.50 1.50 3.64 0.07 1.06 2.02 3.77 4.34 0.00

67.64 0.43 13.67 1.55 3.77 0.08 1.15 2.12 3.54 4.53 0.00

68.10 0.36 13.46 1.32 3.19 0.06 1.09 I .89 3.18 4.56 0.00

1.04 1.32

1.20 I .38

1.30 I .46

1.30 I .48

1.40 I .48

1.20 1.35

1.40 1.39

1.40 I .40

0.89 1.10

0.93 1.24

0.95 1.28

1.00 1.28

0.99 1.21

31 24 37 21 39 65 443 34 0.54 6.82 1.91 I I .97

38 26 59 14 26 57 509 31 0.54 8.93 1.84 8.63

26 25 43 16 24 67 504 39 0.67 7.52 I .72 11.72

27 24 46 14 22 67 524 38 0.64 7.82 1.76 11.39

25 27 57 7 21 57 559 40 0.33 9.81 I .43 9.81

25 27 67 11 23 63 559 45 0.48 8.87 1.40 8.34

50 38 19 27 1 56 168 16 66 21 48 77 40 1 558 22 44 2.55 0.76 7.25 8.35 2.18 1.75 9.96 40 I .OO

47 49 19 21 6 3 170 206 79 66 51 58 403 381 25 27 2.15 3.12 7.90 6.57 2.04 2.15 67.17 127.00

49 18 7 171 65 51 507 25 2.63 9.94 2.04 72.43

33 19 14 193 65 51 38 I 25 2.97 7.47 2.04 27.21

AICNK 1.20 I .09 1.39 I .60 AINK TRACE ELEMENT Zn 50 38 24 Ga 25 45 Th 46 Rb 9 5 56 Sr 45 Y 60 65 Zr 529 466 Nb 39 33 RbISr 0.20 0.09 ZrIY 8.82 7.17 I .54 I .97 YINb ZrRh 11.50 10.36

Gondwanir Research, K l , No.2, 1998

1.44

15 MZ2l

M. K. PANDIT AND M.K. KHATATNEH

250 Table I , (continued) 17 MZ18

18 MZ31

19 MZ7

20 ML9

21 M03

22 ML2

23 MZ4

24 MZ27

25 MZ35

26 MZ30

27 ML5

28 ML23

29 ML26

30 ML6

68.10 0.36 13.65 1.46 3.55 0.06 0.94 2.18 3.76 4.45 0.00

68.12 0.40 13.49 1.49 3.62 0.06 0.99 1.98 3.60 4.39 0.00

68.36 0.4 I 13.56 1.41 3.50 0.08 I .23 2.27 3.14 4.50 0.00

68.42 0.38 13.57 I .45 3.53 0.06 0.80 2.04 3.66 4.4 1 0.00

68.50 0.39 13.89 I .46 3.54 0.07 1.06 2.07 3.16 4.54 0.00

68.50 0.38 13.87 1.41 3.50 0.07 0.96 I .98 3.19 4.53 0.00

68.55 0.39 13.68 1.41 3.44 0.07 1.03 2.04 3.98 4.4 I 0.00

68.58 0.38 13.62 1.42 3.46 0.08 0.90 2.03 3.63 4.50 0.00

68.64 0.37 13.52 1.37 3.35 0.06 1.07 2.07 3.73 4.50 0.00

68.95 0.39 13.80 1.46 3.53 0.06 0.86 1.92 3.22 4.39 0.00

69.56 0.35 13.33 1.19 2.91 0.05 I .oo 2.03 3.16 4.32 0.00

70.1 1 0.35 13.85 1.18 2.86 0.05 0.65 1.88 3.95 4.61 0.00

71.45 0.30 13.60 0.95 2.30 0.03 0.66 2.13 3.94 4.48 0.00

7 1.82 0.33 13.49 0.8 I I .96 0.02 0.62 1.38 3.83 4.5 1 0.00

71.93 0.3 1 13.55 0.67 1.64 0.03 0.58 I .66 3.86 4.26 0.00

0.91 1.25

0.94 1.26

0.96 I .34

0.94 1.25

I .00 1.37

I .00 1.36

0.9 I 1.20

0.94 1.27

0.91 1.22

1.03 1.36

0.98 1.35

I . 10 1.20

0.90 1.21

0.98 1.20

0.97 I .24

32 17 2 I67 65 49 371 23

34 18 8 I65 68 49 431 24

40 18 I6 177 73 50 398 26

43 18 9 I62 64 49 403 23

42

409 23

37 18 21 168 62 48 408 24

51 19 22 158 70 48 40 I 25

49 18 6 166 62 47 384 23

31 18 6 167 66 50 406 23

36 19 19 I66 80 50 41 5 26

30 17 10 I34 65 51 403 23

26 18 9 106 45 52 419 22

18 17 6 125 51 57 462 25

14 18 6 101 27 57 398 23

32 19 10 80 35 80 394 2s

2.57 7.57 2.13 185.50

2.43 8.80 2.04 53.88

2.42 7.96 I .92 24.88

2.53 8.22 2.13 44.78

2.60 8.18 2. I7 3 I .46

2.7 I 8.50 2.00 19.23

2.26 8.35 I .92 18.23

2.68 8.17 2.04 64.00

2.53 8.12 2.17 67.67

2.08 8.30 1.92 21.84

2.06 7.90 2.22 40.30

2.36 8.06 2.36 46.56

2.45 8.11 2.28 77.00

3.74 6.98 2.48 66.33

2.29 4.93 3.20 39.40

S.No. I6 SampleNo.MZ25 MAJOR ELEMENT SiO, TiO, AI?O,

Fe,O, FCi) MnO MgO CaO Nap K20 PI05

A/CNK AlNK

TRACE ELEMENT

zn Ga Th Rb Sr Y Zr Nb Rb/Sr ZrlY YlNh Zr//Th

18

13 169 65

so

S. No. 1-10: Trondhjemite, 11-30: Alkali granite

Table 2. REE composition of Ajitgarh granitoids. Sample No.

ML 23

MZ 4

MO 3

ML 15

La

76.03 155.73 11.38 54.18 12.21 2.31 10.25 13.15 7.66 8.81 1.13

75.34 148.46

Pr Nd Sm ELI Gd DY Er Yb L 11

107.74 197.88 4.00 67.57 19.8I 2.95 12.70 17.17 9.42 12.46 1.54

55.06 13.91 2.46 11.28 14.33 7.8 9.94 I .26

46.69 109.62 0.66 6 I .24 17.84 2.04 13.66 18.1 9.72 1I .63 1.26

CREE

453.7 I

353.92

352. I1

296.12

4.04 7.20 0.53 3.34

4.50 6.93 0.6 I 3.83

3.79 6.16 0.58 3.33

2.39 4.06 0.38 1.71

Ce

Cc/Yb LdLu Eu/Eu* LdSm

11.1

Or-An diagram (Barker, 1979) wherein both plot i n trondhjemite and granite fields, respectively (figure not given). The geochcmical discrimination between both granitoid types is better defined in trace elemental abundances. The trondhjemites are extremely enriched in HFSE (Zr, Nb, Th, Y ) and depleted in Rb and Sr (corresponding with low K,O and

low CaO, respectively). Alkali granites have moderate HFSE concentration, moderate Sr and high Rb. The major elemental behaviour of alkali granites and trondhjemites shows an apparent continuity of trends (Fig. 2) exccpt in case of K,O wherein the trends are subparallel. A closer scrutiny, however, reveals colinear trends and systematic variation in trondhjemites and complexly clustered data points for alkali granites. Trondhjemites are low alumina type and have a close similarity with Webb Canyon gneisses (Barker, 1979). Other NDFB granitoids do not correspond with either trondhjemite or alkali granite (Fig. 2). The distinction is clearly brought out in case of alumina and alkalis although granitic rocks are not expected to demonstrate large scale major element variations among themselves. The discrimination between trondhjemite and alkali granite is better explained by the trace element behaviour (Fig. 3) where well defined, linear, trace element correlations with silica in trondhjemites (Th, Sr, Nb, Zr, Ga etc) indicate their evolution through fractional crystallization. In contrast, a complex evolution for alkali granites is suggested by irregular inter-element correlations. A negative slope of Rb (in alkali granite) with increasing silica is at variance with fractional crystallization. Low CaO, high total alkalis, high FeONgO ratio, high G d A1 ratio and enrichment in LREE and HFSE are diagnostic geochemical signatures of A-type granites (Collins et al., 1982; Gondwana Research, % I , No. 2, 1998

ANOROGENIC PLUTONISM, RAJASTHAN

m

o

0 0

25 1

o

0

15 65 70

6.5

75

w

250

t

75

65

80

70

SiOz

sioz

s102

70

75

00

sio2

1

I

I

1 0 65

70

sioz

7.5

so

& ~

65

70

75

A

A

A

80

~

0

I+ -I 65

70

SiOz

so2

75

80

75

80

Si02

I I

" I m l

I

3

& AAA plpp 65

711

7.5

w

Slot Fig. 2. Comparison of major element behaviour of Ajitgarh granitoids with other NDFB granitoids. Open triangles - Trondhjemite; open circles - alkali granite, crosses - other NDFB granitoids. S a m symbols have been used in subsequent diagrams (other NDFB granitoids data from Anjaneya Sastri, I992 and Goswami and Gangopadhyay, 197I ).

Whalen et al., 1987) and non-compressive/tcnsional tectonic regime.

Tectonic Discrimination In the tectonic discrimination scheme of Maniar and Piccoli ( I989), the Ajitgarh granitoids demonstrate anorogcnic affinity

and plot in thc RRG+CEUG fields in the C-F and F-M diagrams (Fig. 4) (valucs recalculated aftcr Maniar and Piccoli, 1989). In the Na,O - K,O diagram (Fig. S), these granitoids show bimodal distribution, A-type nature (alkali granite) and an igneous source (Barton and Sidle, 1994; Divakara Rao et al., 1995). Other NDFB granitoids plot in a field of their own and define a distinct S-type affinity as compared to I-type, Ajitgarh granitoids. In the trace element discrimination scheme of Pearce et al. ( 1 984), Ajitgarh granitoids define a non-orogenic tectonic environment and plot in the WPG + RRG fields (Fig. 6). In the Rb - SiO, diagram the trondhjemites plot in the field oforogenic granites, on acount of low Rb content. In the Y - Nb diagram and Rb - Y + Nb diagrams also (Fig. 7), these plot in the WPG field (Pearce et al., 1984). A high Ga/Al ratio has been considered as a distinctive feature of A-type granites and the Gondwnno Resenrch, V. I , No. 2, 1998

65

70

75

SiG

00

65

70

sioz

Fig. 3. Hacker diagrams showing distinct trace element behaviour of alkali granite and trondhjemite.

ratio has been successfully employed to discriminate the Atype granites from others (Collins et al., 1982; Whalen et al., 1987).Preferential retention ofAn rich plagioclase in the source region during partial melting could result in a higher GdAl ratio due to preferential exclusion of Ga in anorthite structure, as compared to Al. In production of A-type liquid, Ga would partition more readily into the melt than Al. A high GdAl ratio (2.5 to 4.5) of Ajitgarh granitoids is a conclusive evidence of their A-type affinity. The Primordial Mantle normalized trace element spiderdiagram patterns (normalized after Sun and McDonough, 1989) of Ajitgarh granite (Figs. 8,9) and trondhjemite bring out inherent distinction in the behaviour of LILE (K, Rb, Ba). The deviation may be attributed to low abundance of these elements in trondhjemites. Moderate to strong anomalies in Nb, Sr, P and Ti are noticeable in both the granitoids. The Ajitgarh granitoid patterns also compare well with other A-type granites (Collins et al., 1982). The REE patterns for Ajitgarh granitoids underline inherent differences between alkali granite and trondhjemite although both define a predominant LREE fractionation.Moderately enriched HREE showing almost horizontal slope (for both granitoids) suggest HREE enriched source and absence of garnet as residual phase. The trondhjemite has lower total REE, strong Pr anomaly

252

M. K. PANDIT AND M.K. KHATATNEH

lo00

25

WPG

RRG + CEUG

10

A

A

5

A

A

ORG

A

A A

A

A

IAG + CAG + CCG

I

4

0

I I

1 65

70

8

I

Si02

75

a0

C 25

RRG + CEUG

0

20

0

ooooo Q

0

15

O 0 0 0

0

L

A

10

0

0

0

l !

0 o A

A A

VAG + COLG

65

A

I I

I

70

75

AA

5

A I I

I

2

1

0

80

A

A

0

Sia

I 1

I

3

4

I

100 WPO + om3

5

M Fig. 4. F - C, and F - M diagrams showing anorogenic nature of Ajitgarh granitoids.The F, C and M values have been recalculated after Maniar and Piccolli (1989).

1

$lo

7

I

I

6l

xl:

5

X

Ix

A-type

I

VAG + COLG + ORG

l ! 65

I

I

70

I

Si02

75

ao

Fig. 6. Trace element discrimination diagram showing anorogenic nature of Ajitgarh granitoids (fields after Pearce et al., 1984)

*I/ 1

0

1

2

3

4

5

N120 Fig. 5. NaZO - K,O diagram showing different clusters for alkali granite, trondhjemite and other NDFB granitoids. Note A-type nature of alkali granite, I-type source for both granite and trondhjemite and distinct S-type nature of other NDFB granitoids. The ‘1’ and ‘S’fields after Barton and Sidley (1994) and A-type after Divakara Rao et al., (1996).

and a relatively more pronounced Eu anomaly, as compared to alkali granite (Table 2). Marked difference in LREE fractionationbetween alkali granite ((LdSm), = 3.33 to 3.83) and trondhjemite ((LdSm), = 1.71) is seen while HREE fractionation shows a remarkable similarity in both the granitoids. The REE patterns of Ajitgarh granitoids are also comparable with other A-type granites (Collins et al., 1982). Moderate to strong anomalies in Nb, Sr, P, Ti, general depletion in HREE and highly positive Zr anomaly support a crystallizationfrom a high temperature liquid in which Zr was soluble. Thus the post-tectonic nature of the pluton, indicated by field disposition, textural and mineralogical characters is also substantiated by the geochemical signatures. Gondwunn Research. W, No. 2, 1998

253

ANOROGENIC PLUTONISM, KAJASTHAN

2000 400

1000

100

10

- VAG

1

'

'

1

ORG ' 1 1 1 1 1 1

' ''

gllil

I

100

10

'

4

-

llilll'

loCn

Y+Nb

5

'

I

I

I

I

I

I

I

I

I

I

I

(

(

La Pr Eu Tb H o Tm Lu Ce Nd Sm Gd Tly Er Yb Fig 9 Chondnte normalized REE patterns ofAjitgarh granitoids Note minor differences i n LREE behaviour between alkali granite and tiondhjemite

Discussion and Conclusion

ORG I

1 1

I 1 1 1 1 1 1 1

L

,

111111

10

100

I

I 1 1 1 1 1 1 I

1081,

Y Fig. 7. Trace element discrimination diagram showing WPG character of Ajitgarh granitoids (fields after Pearce et al., 1984).

600

g.1 a

g

0.01

0.003

Ba Th Nb La Sr Zr Ti Yb Rb K Ce Nd Sm Y Fig. 8. Primitive mantle norinalized trace element patterns showing minor variation between alkali granite and trondhjemite in Ba, Rb and Th.

Gondwunn Research, V l , No. 2, 1998

All other NDFB granitoids (Udaipur, Seoli, Dadikar, Saladipura, Bassi-Bairat), related to orogeny, are emplaced along the cores of antiformal structures of Dclhi metasediments. Their syn-tectonic nature is further manifested in development of planar fabric, in perfect harmony with regional structure and associated host rocks (Bose, 1981;Anjaneya Sastri, 1992). The Ajitgarh pluton has been described as a homogeneous alkali granite body, emplaced coeval with the DF, de€ormation (Roy and Das, 1985). Pandit et al. (1996) have demonstrated the compositc (trondhjemite - granite) nature of the pluton. Both the granitoids are ascribed to represent different pulses of anorogenic magmatism. Comparison of othcr NDFB granitoids with Ajitgarh granitoids brings out well defined differences in tectonic cnvironment and petrogenetic history. Ajitgarh granitoids have a well preserved magmatic fabric (unaffected by any deformation). Diagnostic mineral assemblage (hornblende as significantmafic phase, absence of muscovite, sphene as major accessory mineral) and geochemical parameters (metaaluminous to mildly peraluminous nature, moderate to high Na,O/K,O ratio) are characteristic of non-orogenic emplacement and I-type nature. Other NDFB granitoids showing well developed gneissosity (Bose, 1981), are syntectonic with S-type geochemical signatures (per-ahminous nature, potash enrichment, preponderance of K,O over Na,O a high K,O/Na,O ratio). Their syn-tectonic nature is also supported by retention of the planar and linear elements of first deformation with partial superposition of second one (Bose, 1981).A high IqI(0.7289 to 0.713 1) is also a conclusive evidence of their derivation from a metasedimentary source (Table 3). Noticeable differences in major element hehaviour between Ajitgarh granitoids and other NDFB granitoids are the manifestation of their contrasting sources and distinct

M. K. PANDIT AND M.K.KHATATNEH

254

petrogcnctic lineage. Although non availability oftraceelement dala on other N D F B granitoids does not allow the comparison ofpetrogenetically significant trace elements, their distinction with Ajitgarh granitoids can be conclusively established on the basis of availablc evidence. Table 3. Geochemistry of other NDFB granitoids.

S.No.

I

2

3

66.17 0.86 15.06 0.78 4.70 0.07 0.5 1 2.32 2.55 4.99 0.45 I .96 I .09 1.57 0.7131

74.32 0.28 13.10 0.78 I .42 0.03 0 0.84 2.44 5.94 0.04 2.43 1.09 1.25 0.7131

74.33 0.18 12.57 0.45 2.27 0.03 0 0.87 3.16 5.28 0.03 I .67 I .01 1.15 0.7289

4

Dr. S. Agrawal and Dr. Anil M a h e s w a r i h a v e helped in improving the manuscript. We also extend our thanks to Mr. Rakesh Singhal for his assistance in the computation work. Help rendercd by Mr. Rakesh Saxena during the preparation of the paper is also thankfully acknowledged. We are thankful to Dr. V. Balram (NGRI, Hyderabad) and Dr. N.K. Saini (WIHG, Dehradun) for whole rock analysis.

5

72.14 0.47 14.23 0.63 I .74 0.00 0.83 I .20 2.68 6.10

74.54 0.12 13.46 0.56 I .78 0.00 0.72 I .42 2.61 5.83

1.25 0.95 1.26

0.35 1.24 I .44

ISaladipura Granite 2. Udaipur Granite 3. Average of Dadikar Granite 4. Average of Bairat granite 5. Average of Basi granite [ 1-3 after Anjaneya Sastri (1992);4,s after Goswami and Gangopadliyay,( I 97 I)]

T h e whole rock R b - S r data, outlining two discrete thermoteclonic events ( I 700 - 1500 Ma and 850 Ma) in the Delhi Fold Belt, provides a broad geochronological framework of the acid magmatism in northwestern Indian shield (Chaudhary et al., 1984;Anjaneya Sastri, 1992). Recent S m - N d data (Volpe and Macdougal, 1990; Tobisch, et al., 1994) identifying an older magmatic event at 100 M a in the SDFB contests the presumption that all SDFB granitoids are related to a single thermal event (850 Ma). Similarly, a younger, post-tectonic, anorogenic magmatism, the ‘Albitite line’ of Ray (1990) has been identified in the NDFB. I t has been tentatively correlated with 800 M a intraplate thermal event (Roy e t al., 1995). T h e 800 - 750 Ma mineral ages, noted in s o m e N D F B granitoids (Gopalan et al., 1979) also indicate a younger thermal event. Distinct geodynamic setting and geochemical signatures of Ajitgarh granitoids preclude their derivation from the same thermal event that generated other NDFB granitoids (1700 1500 Ma). Ajitgarh granitoids appear to be related to a much younger (800 Ma), intraplate maginatism (Roy el al., 1995) indicating post-orogenic rejuvenation of the rift system that controlled the Delhi sedimentation (Ray, 1990). The inferences need to b e substantiated by authentic isotope data.

Acknowledgement We are thankful to the Convenors of IGCP 368 for providing t h e opportunity to present the work. T h e paper has been immensely benefited by comments offered by Dr. V. Divakar Rao and an anonymous referee. Fruitful discussions with

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