Geochemical clues to elucidate the tectonic environment of the Chamoli Volcanics, Lesser Himalayas, Uttar Pradesh, India

Geochemical clues to elucidate the tectonic environment of the Chamoli Volcanics, Lesser Himalayas, Uttar Pradesh, India M. RAZA

L1THOS

Raza, M. 1981 10 15: Geochemical clues to elucidate the tectonic environment of the Chamoli ~/'oicanics, Lesser Himalayas;, Uttar Pradesh, India. Lithos, Vol. 14, pp. 295--303. Oslo. ISSN 0024.4937° Geochemical data on the Chamoli Voleanics of the Garhwal Group suggest the-;r strong affinity with oceanic tholeiites. However, the field relations and o~her geological information do not support tki~ conclusion and indicate an epicontinental rather than eugeosynclinal environment of eruption. The lack of correlation of chemical data with geological setting precludes the possibility that these basalts are true ocean floor basall:s. It is inferred that the Chamoli Volcanics with an oceanic tholeiite affinity were probably erupted as a result of initial rifting in the Proto-Tethys, which at that time was an intercontinental sea. The rifting was started during the depositional regimes of this intercontinental sea in which shallow water sediments were being deposited. Ocean type tholeiRic magma, guided by the rift zone, disrupted the cycle of sedimentatior~ and gave rise to the development of the quartzite-lava sequence of the Chamoli Formation.

M. Raza, Department of Geology, Aligarh Muslim University, Aligarh 202 001, India; February 1981.

Over the last two decades or so a close relationship between tectonic environment and the composition of volcanic rocks has been c,bserved by many workers (Chayes 1964; Engel et at. 1965; Dickinson & Hatherton 1.e67; Jakes & 7¢hite 1972). In view of this relationship the geochemical data have been widely used to establish the palaeotectonic setting of volcanic episodes (Pearce & Cann 1971; Bickle & Pearce 1975; Coleman 1977). In such studies, it is obvious that any interpretation based on geochemical clues must be reconciled with the geological setting of the area. In the Lesser'Himalayan region the basic igneous rocks of the Chamoli Formation of the Garhwal Group (Mehdi et at. 1972) show a close geochemical affinity with oceanic tholeiite (R~Lza 1980). However, the field relations and the regional geology of the area do n a t seem ro be consistent with this interpretation. This inconsistency in geochemical and geological data appears to be a most interesting feature which merits particular attention to resolve some fundamental concepts on tectonomagmatic processes. In the present paper, with the help of" detailed geochemical and geological data, the author has made an attempt to develop a model which explains this unusual occurrence of oceanic type tholeiites.

Geological setting The Garhwal Group of rocks (Jain 19"/I) represents an autochthonous zone in the Lesser Himalayas. It forms an important stratigraphic unit, occurring in an elongated sedimentary belt extending from the Jamuna fiver in the west to the Kali fiver in the east (Fig. 1). It is bounded by well defined tectonic boundaries (i.e. the North Almora Thrust in the south and the Main Central Thrust in the north). The age of the Garhwal Group is placed somewhere in the time interval from Precambrian to Middle Palaeozoic on the basis of stromatolites and some bryozoan fossils (Vaidiya 1969; Agarwal 1974). Because of the scarcity of fossils, the upper and lower age limits of the Garhwal Group have not been well defined. In the Alaknand Valley section the rocks of this group have been divided into five lithologic units which represent the following general sequence (Mehdi et al. 1972):

Garhwai Group

Patroli Formation Gwanagarh Formation Chamoli Formation Lameri Formation Rudraprayag Formation

The basic rocks are associated with the quart-

296 M. Raza

LITHOS 14 (198D rs" ~s' S C A

I 0

2

L

3o'd

E

4

6kt~

..m.--

•

o

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i • .

o .

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V , MACHNI ~ .'% ~ V v

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I,NAPRAYAG

~k''.

o

•

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°

m

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SHRINAGAR

V

KA R NAP RAYAG 4

V

V

l----i L~-----I~.=-I ~ 1

2

3

~,

l^V^l 5

6

'7"

Fig. !. Si~f,~:+~ified geological map o f the area (after K u m a r & Agarwal 19751. 1 - D u d a t o l i G r o u p , 2 - Granite and gneiss, _+ - Rudraprayag [:grmation, 4 - Lameri F o m a a 6 o n , 5 : - Chamoli Formation, 6 - Dhari Volcanic,, 7 - K a m a p r a y a g Volcanic, 8 - Bhekuna V+~lcanic. 9 -- Basic instrusives, 10 - G w a n a g a r h Formation, 11 - Patroli Formation, 12 - R i v e r teiTaces.

zite-lava sequence constit~'~ting the Chamoli Formation. Kumar & Aga]rwal (1975) carried out detailed stratig:r~Lphical work in the Alakanand Valley ~tnd subdivided the Chamoli Formation as follows:

Chamoli Formation

Bhekuna Metavolcardcs Nagnath Quartzite Karnaprayag Metavoicanics Haryali Quartzite Dhari Metavolcamcs

in addilion to basic lava flows there are sills and dykes of basic compos:tion belonging to more or less the sarae period intruded into the whole sequence• Becat~se of the complex structure of the area it is veery difficult to determine the exact thicknesses of different lithologic units. The whole sequence, including basic flows and ir:+trusives, has been :folded, faulted ;md metamorphosed into green schist f~cies. Because of their interbedding with as,+~ciated metasedimentary ~ock~, the basic volcanics have generally been conside~'ed as penecontemporaneous 'with sedimentation (Mehdi e,: a~. 1972; Kumar & Agarwal 1975).

Petrography Petrographically the volcanic flows and associated basic intrusi~es of the Chamoli Formation display simple and uniform mineral assemblages and textural relations• They are greenish grey coloured, massive to foliated fine grained rocks• The vesicularity of the eruptive rocks varies from about 10% to over 50%. The size', of vesicles ranges from a millimeter to over 5 centimeters and they are rounded, oval to elliptical in shape. Mineralogically the rocks consist predominantly of plagioclase and pyroxene. However, plagioclase constitutes more than 5()% o,f the rock volume forming phenocrysts as well as the ground mass. It is generally albitic in composition (An2o to Ano) as determined on the universal stage by the method of Slemmons (1962). Augite is the predominant variety of pyroxene. Olivine is always absent, though occasional pseudomorphs indicate its earlier existence. Other minerals like chlorite, epidote, calcite and opaques appear as alteration products. Vesicles are filled with chlorite, calcite, albite, quar~:z and chalcedony in any combination or alone. Though

Chamoli Volcanics

LITHOS 14(1981)

the primary mineralogy has generally been obliterated by alteration processes, the igneous textures are preserved in many samples,

Analytical procedures The samples for this study were collected from scattered exposures of the Chamoli Volcanics. After a detailed thin section study, 59 of the freshest samples were sele~:ted for chemical analyses. Samples with probable secondary mineral assemblages were rejected. After crushing the rock to 30 mesh, all fragments which were free from any secondary mineral association were hand picked. The 30 mesh size fragments were then powdered to minus 200 mesh size. The major elements were determined by the rapid methods of Shapiro & Brannock (1!962). Na20, K20 and the trace elements were determined by atomic absorption spectrophotometer. To evaluate the effectiveness of this technique double runs were made taking unsorted and sorted samples (30 mesh) from the stone specimen. This led to the conclusion that the unsorted sample has suffered slightly greater Sllfilitization (Table 1). All the data were standardized against USGS standards BCR-I, W-1 and Sy. 1. The data are presented in Table 2. The average chemical composition of these volconics is compared with those of other volcanics of the Himalayan region in Table 3.

Magma type discrimination Chemical methods have been widely used to d:iscriminate the volcanic rocks from different tectonic environments (Chayes 1964; Dickinson & Hatherton 1967; Dickinson 1968). However, i~~ recent years some workers have used various wariation diagrams which effectively differentia~:e the basic magmas of different tectonic environments. Pearce, T. H. et al. (1975, 1977) and Pearce, J. A. (1976) proposed such diagrams which are principally based on major element compositions. On the other hand, Pearce & Cann (1971, 1973), Pearce (1975) and Pearce & Norry (1979) made a strong case for using immobile minor and trace elements like Ti, Zr and Y. These methods are more effective because Ti, Zr, and Y, like the REE, appear immobile dunng various alteration processes. In the present study the author has used both

297

Table 1. Geochemical comparison of unsorted and sorted rocka(at 30 mesh stage). Major oxide m wt% and trace elements in ppm. SIP., TiP2 Al,_O3 Fe.,O3 FoP MgO CaP Na,O K,O P205 MnO H20 Ga Zr

Sorted rock

Unsorted rock

50.31 0.32 14.72 2,82 8.00 9.58 6.3 ! 3.67 0.81 0.13 0.13 ! .80 5 50

50.96 0.3 ! 14.55 2.56 8.31 9.61 6.01 4. ! 5 0.70 0.13 0.12 i .90 5 48

the major and trace element compositions to discriminate the magma type of the Chamoli Volcanics, and has tried to use only those elements which appear relatively little affected by many alteration processes. Major e l e m e n t s

Miyashiro & Shido (1975) devised a series of variation ¢~iagrams whereby the ratios FeO*/ MgO (FeO* = FeO + Fe203 x 0.9) were plotted against various major elements to distinguish the magma type. Following the same methods the constituents of the Chamoli Volcanics are plotted in Fig. 2. Aad an abyssal tholeiite (ocean floor basalt) affinity of these volcanics is revealed by FeO*-FeO*/MgO and TiO2-FeO*/ MgO diagrams which show consistent results. The MgO, FeO (total) and AI203 plot also appears fruitful to provide better discrimination between ocean island basalts, ocean ridge a~d floor basalts, continental basalts, spreading centre basalts and orogenic basalts. The method has been proposed by Pearce et al. (1977), who used only those samples which had silica contents (anhydrous) ranging between 51% and 56%. In the present study only 38 samples passed the silica screen. Their plots in this ternary diagram (Fig. 3) occupy mostly the field of ocean ridge and floor basalts. Only a few samples fall in the adjacent field of ocean island basalts but lie very near to the bo~:tlndary between the fields of ocean floor basalts and ocean island basalts. The othLer important characteristic of the Chamoli~ Voicani,cs which places them in the care-

298

L I T H O S 14(1981)

M. Raza

Table 2. Major (wt.%) and trace (ppm) element compositions of the Chamoli Volcanics. D5

D6

D8

D20

D22

'

D25

D30

D32

D35

D38

SiO, rio. Al~O~ Fe:O~ FeO MgO CaO Na_,O K,O P.,O~ MnO H.,O

48.79 0.82 14.64 3.48 6.88 ! 1.66 6.21 3.88 1.62 2.12 0.14 2.33

49.81 o. ! 3 14.98 6.44 6.60 10.24 4.51 2.54 0.72 0.24 0.23 3.15

47.52 2.0 ! 13.04 2.48 7.20 ! !.00 6.82 4.28 1.68 0.10 0.10 2.35

,49.13 o. 14 15.23 2.52 7.48 10.62 6.82 4.35 !.56 0.22 0.11 2.57

49.32 0.22 14.28 3.51 7.60 8.85 6.16 5.15 1.50 0.40 0.17 2.9!

49.60 1.27 15.00 3.84 6.08 9.93 5.94 5.36 0.78 0.33 0.10 2. I I

51.41 o. 16 13.17 2.57 7.84 10.08 5.50 4.82 !.20 0.06 0.10 2.95

49.53 1.20 15.12 3.58 9.82 10.93 7.23 4.21 1.50 2.12 0.13 2.50

49.21 0.88 14.22 3.53 6.98 10.23 6.25 2.87 1.67 0.25 0.14 2.11

50.31 0,32 14,72 2.82 8.00 9.58 6.31 3.67 0.81 0.13 0.13 !.80

Total

100.57

99.59

98.58

!t~.57

100.06

99.99

99.81

98.64

98.25

98.60

9 60

5 40

5 140

16 60

I! 80

15 125

16 50

18 125

-

5 50

Ga Zr D39

D40

SiO,. TiO_, AI20~ Fe,O3 FeO MgO CaO Na_,O K,O P,O, MnO H,O

49.22 0.50 15.23 3.21 8.23 9.23 7. ! 2 3.45 0.72 0.28 0.23 1.90

49.2!! 0.12 14.50 3.2t 8.81 9.36 6.31 3.21 i.22 0.11 0.22 2.20

53.07 0.90 15.74 4.15 6.40 7.06 6.05 4.56 1.80 0.19 0.23 0.22

Total

99.32

98.30

-

i5 40

Ga Zr K32

KI0

K37

Ki5

K18

K23

K25

50.88 0.84 I 1.90 6.60 6.06 7.96 9.83 5.69 ! .50 1.33 0.19 1.35

48.83 1.73 12.66 !.36 10.16 7.25 7.70 5.15 ! .88 0.7 ! 0.69 0.46

45.35 2.11 13.73 2.15 9.68 1 !.06 6.99 5.12 0.38 0.3,6 0.32 0.88

52.71 !.52 12.09, 2.14 9.76, 6.14 6.66 5.76 0.45 0.63 0.26 !.76

48.60 !.19 13.23 2.38 9.44 7.96 ! 2.98 2.11 0.15 0.35 0.28 !. 10

44.80 2.68 12.58 5.15 7.00 2.43 8.65 4.55 ~.58 0.43 0.31 2.76

50.99 1.31 13.81 3.46 8.40 7.37 7.64 5.69 0.72 0..47 0.25 !.03

100.47

100.18

98.33

98.13

99.79

100.46

98.50

100.54

5 60

nd 55

-

5 120

-

5 110

-

6 130

K38

K40

K42

K27

K28

K30

K45

K50

K52

K60

K70

SiO, "riO., AI_,O ~ F e_,(L FeO big() CaO Na,O KrO P:O, MnO H,()

51.72 i.90 ! !.08 7.06 3.04 8.95 5.83 3.41 2.29 0.22 0.25 !.65

54.13 3.15 10.50 !.29 9.64 7.89 6.88 3.35 !.20 0.37 0.28 !.09

53.77 1.08 14.64 3.25 6.80 7.79 6.27 5.62 0.30 0.28 0.17 0.50

53.61 2.55 14.42 3.97 6.08 6.53 5.93 4.69 i.02 0.19 0.18 !.46

55.07 !.28 13.95 3.26 6.24 6.73 6.38 5.36 0.78 0.23 0.24 !.34

54.13 i.10 14.34 2.77 6.40 7.10 5.03 5.70 0.12 1.37 0.15 0.98

45.21 2.69 14.08 3.55 8.16 9.29 9.10 3.68 0.30 0.67 0.22 !.66

53.31 i.15 12.80 3.90 5.80 7.70 8.24 4.02 0.30 0.15 0.20 1.34

48.27 1.72 14.43 2.08 9.36 6.65 8.88 4.82 0.30 0.58 0.21 1.26

50.42 !.17 I4.00 3.85 7.20 8.53 6.04 5.70 0.54 0.21 0.26 ~.24

Total

98.75

100.1 i

100.47

100.78

100.16

99.28

98.61

98.99

98.54

99. t6

5 120

-

!I 70

-

5 100

5 90

21 130

-

28 100

-

Ga Zr

Chamofi Voicanics 299

LITHOS 14 (1981)

K75 SiO2

KI01

AI203 Fe20,~ FeO MgO CaO Na20 K,O P205 MnO H20

13.99 3.00 8.00 7,37 6.48 5,42 0.30 0.18 0.23 !.30

50.83 ! ,09 ~2.31 2.84 7.80 7;66 6.60 4.35 1.92 0.24 0.10 4.88

Total

98.60

100.58

TiO-

51.25 1, I 0

GR Zr

K110 51,15 0.83 12.64 3.06 7,52 8.90 6.05 6,03 0.24 0.93 0.17 1.78 99.66

K I i;5 54.71 1.46 15.59 3.77 6.60 4.22 6.45 4.62 0.90 0. t8 0.27 ! ,62 100.39

K120

BI

P$

BI 1

"47.07 i.23 16.78 5.50 4.60 7.25 7.70 5.02 2,40 0.22 0.27 2,02

48.05 ! .43 13.72 ! .83 10.04 .7.43 8.65 4.55 1.58 0.43 0.03 0,76

57.88 1.95 12.60 3.87 6.60 6.08 5.50 4.35 0.42 0.42 0,30 0,72

100,02

98.50

100.69

B32

52.16 1,42 12.6{* 4.05 8.40 8.22 5.06 4,82 1.20 0.29 0.20 1.51

47.04 1.20 14.89 1.37 9.64 10.49 6.15 4.35 0.10 0.13 0.11 3.24

44.26

100.03

98.71

99.85

17

-

-

-

-

80

-

-

-

B35

B40

B42

B46

B47

B48

BS0

B52

B55

CaO Na,O K,O P.,O~ MnO H.,O

45.10 0.33 16.48 3.03 10.68 5.20 7.81 4.35 i .38 0.24 0. ! ! 3.54

53.47 0.19 12.72 1.82 6.00 8.57 7.68 5.42 0.43 0.24 0.38 1.69

52.30 0.16 11.54 4.55 6.68 8.22 7.92 5.69 0.60 0.88 0.27 0.77

51.86 0.23 14.23 2.41 6.60 10.34 6.59 5.09 0.43 0.39 0.15 0.16

51.92 C~.86 12.92 2.81 6.68 9.85 6.16 5.82 0.72 0.37 0.32 n.08

50.22 0.84 11.65 3.10 10.00 7.69 8.10 4.59 0.75 0.44 0.09 1.38

48.42 0.92 16.37 2. i I 8.52 7.25 7.70 ,~,.48 ! .92 0.19 0.27 0.39

51.91 0.85 12.12 3.54 7.12 11.28 5.06 4.35 2.40 0.10 0.23 0.62

49.58 !.18 14.96 2.25 7.96 8.06 6.88 6.13 0.60 0.61 0.20 0.50

Total

98.25

98.61

99.58

98.48

99.11

98.80

98.54

99.58

98.91

12 70

5 50

5 60

20 50

6 62

5 60

6 65

5 55

-

MgO

Ga Zr I2

19

SiO2 TiO2 AI203 Fe~O3 FeO MgO CaO Na:O KzO P205 MnO H20

51.89 2.12 12.76 3.96 6.84 8.14 7.03 4.95 0.48 0.26 0.35 !. 1!

52.71 ~.38 13.93 t .71 9.80 8.06 5.50 4.35 1.20 0.27 0.19 0.67

54.85 0.88 12.77 3.89 7.60 6.45 6.15 5.02 1.26 0.33 0.29 1.33

Total

98.89

98.78

100.82

5 130

-

Ga Zr

120

130

8 62

130

-

-

-

-

-

-

1.1 !

14.23 2.17 10.80 12.87 6.16 4.95 1.44 ! .20 0.16 0.68

-

SiO2 TiO2 Ai203 Fe:03 FeO

12

B30

I42

133

138

140

52.40 i.42 15.16 4.29 8.24 7.41 7.26 4.95 0.90 0.26 0.25 1.35

48.13 2.00 13.43 1.67 10.64 7.31 I 1.65 2.68 0.15 0.36 0.26 0.86

47.50 1.23 15.00 3.24 8.60 10.09 8.47 2.27 1.20 0.17 0.33 1.60

50.00 !.57 12.63 ! .80 10.16 7.25 7.77 4.82 0.45 0.39 0.32 1.40

50.44 0.14 14.17 4.19 8.52 7.90 5.94 4.69 1.26 ~.21 9.27 2.28

53.55 !.38 13.12 2.20 7.60 7.45 6.15 4.69 1.02 0.18 0.20 1.40

54.01 !.43 12.77 ! .59 9.80 8.06 5.50 4.42 i.50 0.43 0. i 7 0.52

100.89

99.14

99.70

98.56

lf~9.01

98.94

100.20

13 95

15 125

6 80

14 85

8 180

14 125

7 66

148

5

125

149

•

i •

i! ¸

L I T H O S 14 ( 1 9 8 I )

300 M. Raza

Table 3. Average major elements (%) of the Chamoli Volcanics and other basaltic provinces of the Himalayas. Partial Volcanies

SiO-, TiO_, AI.,O:~ Fe:O3 FeO MgO CaO Na,O K:O P,Os MnO H~O

Mandi Volcanics

Chamoli Volcan[cs

Bhowali Volcanics

i

2

3

4

5

6

46.61 1.71 14.38 3.47 9.93 8.09 %98 i .50 0.7 i 0.16 -

51.19 1.74 14.38 ! .77 8.85 5.44 9.57 339 1.1t7 0.2!0 0.13 2.14

53.21 1.14 12.28 2.17 8.06 6.97 3.02 3.50 2.03 0.02 0.31 4.38

50.51 1.11 13.61 3.07 8.01 8.55 6.89 4.49 1.02 0.42 0.21 1.55

50.89 1.00 14.75 2.17 8.75 8.92 5.40 4.64 1.14 0.36 0.13 -

50.87 1.76 15.98 2.04 16.03 6.12 4.80 4.55 1.24 -

(I) Pareek (1973); (2) Bhat, M. 1. (personal communication); (3) Patwardhan & Bhandari (1974); (4) Present work; (5) Divakara Rao et ai, (1974); (6) Shah & Merh (1976),

(Fig. 5) also classifies the Chamoli Volcanics as ocean ridge tholeiites, as most of their analyses plot in the appropriate field. The other Himalayan basics, with differing geological ages, also show a close affinity with ocean floor basallts as evident from plots of their average compositions in various variation diagrams (Figs. 2, 3 & 5).

gory of ocean ridge tholeiite is their FeO*/MgO ratio. Miyashiro (1975) considered the FeO*/ MgO ]ratio of basaltic rocks to be a usefull indicator in the recognition of ocean floor and island arc tholeiites. He observed that in ocean floor tholeiites the FeO*/MgO ratio is always less than 2%. In the Chamoli Volcanics more than 77% of analyses show a FeO*/MgO ratio less than 2% (Fig. 4), which again supports their ocean floor origin. A TiOz--SiO2 plot {Macdonald & Katsura 1964; Whitehead & Goodfellow 1978) also provides a clue for distinguishing between the basalts of differing tectonic environments. This diagram

Immobile minor and trace elements The above methods, which are principally based on major elements, indicate an ocean floor tholeiite affinity for the Chamoli Volcanics, but

,10.0 eCHAMOL! VOLCANICS

14 I-~ // ~ ® t o| " ~" ®t ® • I

•

®*

12

& AVERAGE PANJAL VOLCANICS •

FIELD OF ABYSSAL THOLEIIT~

• •• % . . .

FIELD OF .,~BYSS~L THOLEIITE

4. AVERAGE

MANOI

OAVERAGE

BHOWALI

C A - C A L ¢ ALKALI

VOLCANICS VOLCANIGS

BASALT

Th . T H O L E I t T E ®®o

.,.o

*o

10

/ -7."0~ .:.

- I

,

/.,: , |

6

®

I t "

s "°

°

°

- ' , '. . . -0.'o'*" ,,"

/

-.-4

1.0

:

"--"

/ 0 !

%....1

0

°-' "L,° oe~,~

1

~

2 3 FeO~/MqO

4

0

1

•

:2 3 FeO*/MgO

4

Fig. 2. FeO*-FeO*/MgO [a] and TiO-,-FeO*/MgO [hi var~qation diagrams (Miyashiro & Shido !975) showing abyssal tholeiite affir.dtv of the Chamoli Volcanics. Data on other Himalayan basics from Table 3.

LITHOS 14(1981)

C h a m o l i Volcanicz

/'

/

/

/

/'OCEAN

/

|

":

"d," ~.

"& \

~'"~l

\

\

%

OCEAN~;GE ANDI ~-~p FLOOR' J OROGENIC'~ .,

J so

I" Z I.IJ 0 4O O~ la 0,.

\M O k ,O,

I

~

56

FeO/~

/

/

ISt.A~t~= I

"

/ /

/

~

I

\

301

>¢J 2 4 I,ZLI

\ ~ '~ J zo

0

ILl at tl.

\

Fig. 3. MgO - FeO* - AI2Osvariation diagram (Pearce et al. 1977) showing an ocean floor basalt affinity of the Chamoli Voleanics. Data on other Himalayan basics from Table 3. S~mplledesignationsas in Fig. 2.

m

8-

o F i g . 4.

~

0.5 t.o

I

I

I

!

t.5 2.o 2.5 3.0 F, 0¢~1Hg 0

Histogram showing range in FeO*/MgO ratios in the

Chamoli Volcanics. as the major element contents of altered basic rocks is totally dependent upon their mobility during the alteration, this conclusion requires corroborative evidence. The confirmation of the ocean floor tholeiite affinity of the Chamoli Velcanics is provided by TiO2-Zr/P2Os, SiOz-Zr/TiO2, Zr/TiO2--Ga and Ti-Zr variation diagrams (Fig. 5). The Chamoli V olcanics would be classed as ocean floor tholeiites on all these diagrams which show consistent results.

Tectonic significance The results obtained from both major and in~mobile trace elements indicate that the Chamoli Volcanics possibly originated at a mid oceanic ridge. However, because of the general absence of ultramafic variants or their derivatives and the

association with shallow water sediments, the Chamoli Volcanics cannot be called ophiolites either according to the definition of Benson (1926) or its modification by Thayer (1967). On the other band, not only in the present area but in almost "all the localities of the Lesser Himalayas, the basic volcanics and shallow water metasediments are interbedded, contemporaneously folded and epimetamorphosed. These features indicate a pre-orogenic volcanic activity that was mainly related to tensional t~ctonic conditions. Pearce e~ al. (1977), after a smwey of thousands of basaltic analyses from various known tectonic environments, observed that the basaltic rc,cks which erupt at the time of active breaking tap of a continent always have a somewhat oceanic character. This oceanic character of basalt suggests the breaking to be an attempt

H

~3

Fig. 5. TiO2-SiO2variation diagram (Macdonald& Katsura 1964;Whitehead& Goodfe!!low1978)for the Chamoli Volcanics. Data on other Himalayanbasics from Table 3. Sampledesignations as in Fig. 2.

P'2 !.

C

C ~0

~

L

B

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(successful or not) t o create new oceanic floor along a rift in a continental p~ate. If the above observationa are acceptable, the Chamoli Volcanics with an ocean floor tholeiite affinity may be considered to have been erupted in a rifting environment. Mehdi et ai. (1972) suggested that the rocks of the Garhwal Group were deposited in a rifted basin bounded by the North Almora Thrust in the ,south and the Main Central Thrust in the North. Ahmad & Ahmad (!976) considered that, ur~tii the end of the Carboniferous period, Angaraland was a part of Gondwanaland and Tethys at that time was an intercontinental sea I Proto-Tethys). In the carboniferous period, the Proto-Tethys opened up. with the formation of a mid oceanic ridge, the remnants of which are represent,:d by the basic rocks of the Himalayan region and ~he extensive lava flows of Siberia, which was a part of the Himalayas in that period. Burrett (1972) and Kamen Kaye (1972) also agreed that Tethys was opened up as an ocean by a 'rift' e×tend[ng f~'om west to east. In the Himalayan region the development of a rift zone in the t~pper Palaeozoic period has also been suggested by Burke & Dewey (1973), and Sinha Roy ( 1977K The ocean floor tholeihe affinity of basic rocks of varying ages from different

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Fig. 6, Plots of the Chamoli Volcanics in various discrimina,tion diagrams principally based on immobile minor and t'race elements. The classification boundaries are as suggested by Winchester & Fl,:)yd (1977) for [al Floyd & Winchester (1978) for [bi, [c]~ and Pearce & Cana (1973) for [dl. In [b] and Icl: AB =: alkali basalts; B + TB + N = basaaites, trachy basa~fites, nephelinites; A = andesites; D + RD = dacites and rhyodacites: R = rhyo!ites; TA = trachy andesites: T = trachytes; Ph = phonolites; C + P = comendites and pantellerites, in Idl: A & B = island arc basalts B & C = calc-alkalic basalts, B & D = ocean floor basalts.

parts of ~the Himalayas indicates that in the Pre-cambrian period the rifting started first with eruption of Mandi and assocaated volcanics of Himachal Pradesh and continued into the upper Palaeozoic period when it extended into Kashmir ir~ the west and Assam in the east. The oceamc type tholeiite magma guided by the rift zone disrupted the cycle of sedimemation resulting in the formation of the quartzite-lava sequence of the Cha~oli Formation and possibly produced a number of other such formations which are found in different parts of the Lesser Himalayas. Acknowled~,ments. - The author thank~ Prof. N. Ahmad for guidance. Prof. S. H. Rasul for providing facilities in the department and Dr. S. M. Naqvi. N.G.R.I. Hyderabad for helpful comments on the manuscript. The a,uthcr is also graleful to Dr. M. !. Bhat for all his help d,LJring the course of this work.

References Agarwal, N. C. ~974: Discovery of bryozoan fossils in the calcareous horizon of Garhwal Group, Pauri Garhwai distrier, U.P. Him. Geol. 4. 600-618. Ahmad. F. & Ahmad. Z. S. ~976: The genesis of the Himalayas - A ne~ approach (Abstract). Him. Geol. Sere. New Delhi.

Chamofi Volcan~rcs .~03

LITHOS 14 0981) Benson, 1926: The tectonic conditions accompanying the intrusion of basic and ult~basic igneous rocks. U.S. Nat. Acad. Sci. Mere. i, !-90. Bickle, M. J. & Pearce, J. A. 1975: Ocean marie rocks in Eastern Alps. Contrib. Min. Fetrol. 49, 17"7-189. Burke, K. & Dewey. J. F. 1973: Plume generated triple junctions: key indicators ~.n applying plate tectonics of old rocks. J. Geol. 81,406-433. Burrett, C. F. 1972: Plate ~ectonic and the fusion of Asia. Earth Planet. Sci. Lett. 21, 181-189. Chayes, F. 1964: A petrographic distinction between cenozoic volcanics in and around the open oceans. J. Geophys. Res. 69, 1573-1580. Coleman, R. G. 1977: Ophiolites, Springer Verlag. Heidelberg. 229 pp. Dickinson, W. R. 1968: Circum-Pac:,fic andesite types. J. Geophys. Res. 73, 2261-2269. D~ckinson, W. R. & Hatherton, T. 1967: Andesite Volcanism and seismicity around the Pacific. Science, N.Y. 157. 801803.

D~vakara Rao, K., Satayanarayana, K., Naqvi, S. M., Hussain, S. M. & Dileep $~umar, 1974: Geochemistry and petrogenesis of basic rock~ from Bhowali, U.P. Geophysical Res. Bull. 12, ~53-74. Eagel, A. E. J., Engel, C G. L & Havens, R. G. 1965: Chemical characteristics ~f oceanic basalt and the upper mantle. Bull. Geol. Soc. Am. 76, 719-734. Floyd, P. A. & Winchester, J. A. 1978: Identification and discrimination of altered a~d metamorphosed volcanic rocks using immobile elements. Chem. Geol. 21. 291-306. Jain, A. K. 1971: Stratigrapt~y and tectonics of Lesser Himalayan region of Uttar Kashi, Garhwal Himalaya. Him. Geol. I. 25-28. Jakes~ P. & White, A. J. 1972: Major and trace elements abundances in volcanic rocks of orogenic area. Bull. Geol. Soc. Am. 83.29--40. Kamen Kaye, M. 1972: Permo-Carboniferous Tethys and lndian Ocean. Bull. Am. Assoc. P~'trol. Geol. 56 (IV), 1984-1999. Kumar, G. & Agarwal, N. C. 1975: Geology of the SrinagarNandprayag area (Alaknanda Valley), Chamoli Garhwal ~nd Tehri Garhwal districts, Kurnaon Himalaya (U.P.). H m . Geol. 5~ 29-59. Macdonald, G. A. & Katsura, T. 1964: Chemical composition of Hawaiian iavas. J. Petrol. 82, 82-133. Mehdi, S. H., Kumar, G. & Prakash, G. 1972: Tectonic evolution of Eastern Kumaon: A new approach. Him. Geol. 2, 481-501. Miyashiro, A. 1975: Origin of the Troodos and other Ophiolites: A reply to Hynes. Earth Planet. Sci. Lett. 25, 217-222. Miyashiro, A. & Shido, F. 1975: Tholeiitic and calc-alkalic series in relation to the behaviour ef titanium, vanadium, chromium and nickle. Am. J. Sci. 275. 265-277. Pareek, H. S. 1973: Geological setting, petrography and pe trochemistry of the Darla traps and its comparative study with the Mandi and Panjal Traps. J. Geol. Soc. India. 14, 355-368.

Patwardhan, A. M. & Bhandari. A. M. ]974: Petrogenesis of spilites occurring at Mandi, HimachaD Pradesh, India, pp. 175-189 in Amstutz, G. C. (ed.): Spilite and Spilitic Rocks, Springer Verlag, Heidelberg. Pearce, J. A. 1975: Basalt geochemistry used to investigate past tectonic e~vironraents on Cyprus. Tectonophysics 25. 41-67. Pearce, J. A. 1976: Statistical analysis of major element patterns in basalts. J. Pe~,rol. 17. 15-43. Pearce, J. A. & Cann, J. R. 1971: Ophiolite origin investigated by discriminant anaq,,ses using Ti, Zr and Y. Earth Planet. Sci. Lett, 12, 339-349. Pearce, J. A. & Cann, J. R. 1973: Tectonic setting of basic volcanic rocks determined by using trace element analyses. Earth Planet. Sci. Lett. 19, 29~--300. Pearce, J. A. & Norry~ M. J.. 1979: Petrogenetic implications of Ti, Zr, Y and Nb variation~ in volcanic rocks. Contrib. Min. Petrol. 69, 33-47. Pearce, T. H.. Gorman, B. E. & Birkett, T. C. 1975: The TiO,, K20, P20.~ diagram: A raethod of discriminating between oceanic and non oceanic basalts. Earth Planet. Sci. Lett. 24. 419-426. Pearce, T. H., Gorman, B. E. & Birkett, T. C. 1977: The relationship between major element chemistry and tectonic environment of basic and intermediate volcanic rocks, Earth Planet. Sci. Lett. 36, 12 Ir ! ~2. Raza, M. 1980: On the na~are of magma t~,pe of Chamoli metavoicanics using mino~ and trace elemen~,~.Ind. J. Earth Sci. 7, i 19-130. Shah, O. K. & Merh, S. S. ~976: Spilites of 13himtaI-Bhowali area, district Nainital, U.P. Him. Geol. 6. 423-448. Shapiro, L. & Brannock, ~V. W. 1962: Rapid analysis of silicate, carbonate and phc,sphate rocks, U.S. Ge~l. Surv. Bull. 1144 p. 56 p. Slemmons, D. B. 1962: Determination of volcanic and plutonic plagioclases using a ~hree - or four - axes universal stage. Geol. Soc. Am. Spec. Pap. 69. Sinha Roy, S. 1977: Gondwana and related rocks in the Himalayas and adjacent area and their bearing on Palaeozoic-Mesozoic plate tectonics in the eastern Tethys region. Proc. 4th Int. Gondwalta Syrup. Calcatta. Thayer, T. P. 1967: Chemical and structural relations of ultramafic and feldspathic rocks in Alpine intrusive complexes, pp. 222-238 in Willey. P. J. (ed.): Ultramafic and Related Rocks. Valdiya, K. 1969: Stromatolites of Lesser H,;malayan carbonate formations and the Vindh)ans. J. Geol. Soc. India I0, !-25.

Whitehead, R. F. S. & Goodfellow, W. D. 1978: Geochemistry of volcanic rocks from the "latagouche Group, Bathurst, New Brunswick, Canada. Can. J. Earth Sci. 15, 207-219. Winchester, J. A. & Floyd, P. A. 1977: Geochemical discriminatior~ of different magma series and their differer~tiation products using immobile elements. Chem. Geol. 20, 325344.