Paleo-uplift and cooling rates from various orogenic belts of India, as revealed by radiometric ages

Tectonophysics, 70 (1980) 135-158 Elsevier Scientific Publishing Company,

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P~EO-UPLIFT AND COOLING RATES FROM VARIOUS OROGENIC BELTS OF INDIA, AS REVEALED BY RADIOMETRIC AGES

KEWAL K. SHARMA NAGPAUL 2

l, K.D. BAL 2 RAJINDER

PARSHAD

2, NAND LAL 2 and K.K.

i Wadia Institute of Himalayan Geology, Deh~dun-248001 (India) 2 Department of Physics, Kurukshetra University, Kurukshetra-132119 {Received

November

14,197s;

revised version

accepted

(India)

May 7,198O)

ABSTRACT Sharma, K.K., Bal, K.D., Parshad, R., Nand Lal and Nagpaul, K.K., 1980. Paleo-uplift and cooling rates from various erogenic belts of India, as revealed by radiometric ages. Tectonophysics, 70: 135-158. The significant discordance of the radiometric (Rb-Sr, Pb-U, K-Ar and fission track) ages from various erogenic cycles of the Dharwar, Satpura, Aravalli and Himalayan orogenie belts in India, coupled with their corresponding blocking temperatures for various radiometric clocks in whole rocks and minerals, has been used to evaluate the cooling and the uplift histories of the respective erogenic belts. The blocking temperatures used in the present study of various Rb-Sr (isotopic homogenization at 6OO*C, muscovite at 5OO*C and biotite at 300°C), Pb-U (monazite at 530°C), K-Ar (muscovite at 35O*C and biotite at 3OO’C) and fission-track clock (zircon at 350°C, sphene at 300°C, garnet at 28O*C, muscovite at 130°C, hornblende at 120°C and apatite at lOO*C for the cooling rate l*C/Ma) have been found suitable to explain the differences in mineral ages by different radiometric techniques. The nature of the cooling curves drawn using the temperature versus age data for various erogenic cycles in India has also been discussed. The cooling and the uplift patterns determined for various erogenic cycles of India, suggest comparatively slow cooling (5.0-0.2’C/Ma) and uplift (180-2 m/Ma) for the Peninsular regions and rapid cooling (25.0--l.O’C/Ma) and fast uplift (800-30 m/Ma) during the Himalayan Orogenic Cycle (Upper Cretaceous~ertia~) in the Extra-Peninsular region.

INTRODUCTION

A large number of fission-track mineral ages from granites, pegmatites and metamorphites from various erogenic belts of India have been reported by various workers (Nagpaul et al., 1975; Sharma et al., 1975, 1977; Koul and Virk, 1976; Nand Lal et al., 1976c; Virk and Koul, 1977; Saini et al., 1978; Singh and Virk, 1978; Parshad et al., 1979). The fission-track mineral ages are generally much younger than those -determined by other radiometric methods. Even the cogenetic and coexisting minerals such as zircon, sphene, garnet, muscovite, biotite, apatite, etc. from the same region give different OO~O-~951~80/000~0000/$02.50

@ 1980 Elsevier Scientific

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136

fission-track ages, as also the ages of apatites collected from different levels of a batholithic body show variation at times. Studies on thermal annealing of tracks on a large variety of minerals carried out in our laboratory (Nagpaul et al., 1974; Nand La1 et al., 1974,1976b, 1977; Nand Lal and Nagpaul, 1975; Saini et al., 1975; Saini and Nagpaul, 1979) and by Fleischer et al. (1965); Naeser and Faul(1969); Haack and Potts (1972); Wagner (1972) and Haack (1977) have given satisfactory explanations for the above-mentioned age differences. In fact, these age differences in fission track ages of cogenetic minerals and the same mineral collected from different levels of the same batho~thic body have proved to be useful in the calculations of cooling and uplift rates (Naeser and Dodge, 1969; Naeser and Brookins, 1975; Nagpaul and Mehta, 1975; Sharma et al., 1975; Nand La1 et al., 1976; Wagner et al., 1977; Mattinson, 1978). This gives the fission-track technique an added advantage over other radiometric methods. In the present paper the fission-track, Rb-Sr, Pb-U and K-Ar mineral age data, coupled with their respective temperatures of daughter retention (block~g/closing temperature), have been used to infer tectonic and thermal histories of various regions of India. The synthesis of the available radiometric age data, although limited, should not only give information on the cooling history of various erogenic belts but it should also provide a check on the different blocking temperatures adopted in this paper, in relation to each other. An attempt has also been made to illustrate some of the similarities and differences between the older poly-phase complexes of the Indian Shield (later consolidated into a craton) and the younger Himalayan Orogenie Belt. ANALYTICAL

AND EXPERIMENTAL

PROCEDURES

The salient features of the experimental procedure adopted by the authors for fission-track dating and the annealing experiments of minerals used in the present paper are given below.

For dating of micaceous minerals (muscovite) tight books of 1 X 2 cm surface area and 0.5 cm thickness were selected and cleaved into three parts. The inner surfaces of the two outer parts were used for fossil-track density measurements (p,) and the middle part for induced-track density measurements (pi) after irradiation. For dating of non-mi~a~eous minerals, viz. zircon, sphene, garnet, hornblende and apatite, the “population method” for track-density measurements was used. In this technique surfaceswith identical track registration and track etching characteristics for ps and pi measurements were used. For the present study large-sized crystals were hand-picked from their host rocks. Polished sections were etched in their respective etchants for ps measure-

137

ments. These were then heated to wipe off fossil tracks and sent for irradiation. To avoid tracks which may result because of contamination during handling of the polished samples and irradiation and to use 47r geometry for pi measurements, the upper 20-30 pm surfaces of the irradiated samples were polished off, before taking pi measurements. In fine-grained rocks, where apatite occurs as small crystals, an “in situ” method was used for dating (Nagpaul et al., 1973). According to this technique thin sections of rocks are prepared, polished, etched and scanned for ps measurements, sent for irradiation, polished to remove 20-40 I.crnthickness, etched and scanned again for pS + pi measurements on the grains used for pS measurements. Grains having large size (>lOO pm) and homogeneous uranium concentration were only used for age calculation in a number of thin-sections to minimize statistical error. Annealing experiments in minerals The influence of various environmental factors such as temperature, pressure, shock, plastic deformation, etc. on track fading, has been studied by various workers (Fleischer et al., 1975 and references therein) and only temperature has been found to be effective. All minerals and glasses so far investigated (Wagner, 1972; Saini and Nagpaul, 1979 and references therein) show track fading with increasing temperature, however, the sensitivity of different minerals to thermal annealing of tracks varies greatly. The results of annealing experiments are presented in the form of Arrhenius plots, for various degrees of track fading, in which the relationship between annealing time and temperature is linear, expressed as: lnt = ha + (E/KT) where t = annealing time, a = material specific constant which depends on the degree of annealing, E = activation energy, K = Boltzman constant, T = annealing temperature. Arrhenius diagrams drawn on the basis of the experimental data carried out in our laboratory were extrapolated to geological time scales for the calculation of paleo-temperatures (Saini and Nagpaul, 1979). MINERAL AGES - THE CONCEPT OF BLOCKING TEMPERATURE

The radiometric ages of undisturbed volcanics and high level intrusives with a simple geological history are generally related to an identifiable geological event. However, in an erogenic belt, the complex cycles of deformation, metamorphism, intrusion and uplift often preclude such a relationship. In such a complex geological environment, not only is the disagreement between two or more mineral ages by same technique common, but also the different techniques used for radiometric dating produce discordant mineral age patterns and the scatter of dates may range up to hundreds of million

138

years. With ever-increasing radiometric mineral ages from various erogenic belts, the problems involved in the interpre~t~on of the data have become more clearly focussed. There are two distinct schools of thought regarding the interpretation of ages: (i) that the radiometric daughters are retained fully by the host crystals shortly after the time of crystallization, suggesting thereby that the age corresponds to the time of crystallization, within the accuracy of measurement and (ii) that rocks formed during orogeny at great depth, take many years to cool to a temperature at which the daughter retention is perfect (critical/blocking temperature) and the age therefore is a cooling age. The slow-cooling hypothesis has received considerable support in the recent years (Armstrong, 1966; Moorbath, 1967; Brown and Miller, 1969 and Wagner et al., 1977) and has given a more realistic interpretation of the radiomet~c data. It now appears that radiometric data from an erogenic belt may not equivocally be assigned to a specific geological event, but record the time when a rock cooled down to some critical temperature. The critical temperature for a particular radiometric system in a mineral is that temperature below which essentially the whole of the daughter isotope is retained in the mineral and it is usually lower than the temperature at which the homogenization of isotopes takes place in the whole rock. Recently, Wagner et al. (1977) reported blocking temperatures for Rb-Sr, Pb-U, K-Ar and fissiontrack clocks in minerals and used this data for the calculation of cooling and uplift rates in the Alps. Mattinson (1978) also worked out the thermal history of Salinian plutonic rocks (U.S.A.) using the blocking temperatures for different minerals. The blocking temperatures for fission-track clocks in various minerals TABLE I Closing temperatures for various minerals by different radiometric clocks Sr. No.

Mineral name

By other radiometric methods

By fission track method for cooling rates O.l’C/Ma

l.O’/Ma

10.O°C/Ma

1. 2. 3.

Apatite Hornblende Muscovite

80 * 100 * 110 *

100 * 110 * 130 *

120 * 135 * 150 *

4. 5. 6.

Garnet Sphene Biotite

260 * 280 * I

280 * 300 * -

7. 8.

Monazite Whole rock

-

-

300 * 320 * -

* Data taken from Sharma et al. (1979). ** Data taken from Wagner et al. (1977).

method

(“C)

-.

500 ** 350 ** -

Rb-Sr K-Ar .-

300 300 530 600

Rb-Sr K-Ar Pb-U Rb-Sr

** ** ** **

139

were calculated by earlier workers (Wagner et al., 1977; Nand La1 et al., 1979) by extrapolating laboratory annealing experiments to geological time of the order of lo6 years and taking the temperature at which 50% of the tracks are stable. Recently, Haack (1977) suggested that the fission-track blocking temperature is a function of cooling rate and this concept has now been widely accepted. In the present paper the fission-track blocking temperatures for sphene, garnet, hornblende, muscovite and apatite have been calculated (Table I) based on the annealing experiments conducted in our laboratory and the procedure suggested by Haack (1977). The blocking temperatures at various cooling rates for zircon could not be calculated as its annealing characteristics are available only for 0% and 100% track retention (Krishnaswami et al., 1973); however, it is likely to be higher than that of the sphene (3OO’C at a cooling rate of l’C!/Ma) as indicated by the higher ages for zircon than the coexisting sphene (see Table II). From the Arrhenius plots of zircon given by Krishnaswami et al. (1973) an annealing temperature of about 350°C for 50% track retention for the period 108-lo9 years may be calculated, and this has been used as the blocking temperature for zircon in the present paper. The blocking temperatures used in the present paper for monazite (Pb-U), muscovite (Rb-Sr, K-AI-) and biotite (Rb-Sr, K-Ar) are as suggested by Wagner et al. (1977). The uplift rates in the present paper have been derived according to the equation: cooling rate uplift rate = geothermal gradient Where cooling rates are calculated from the blocking temperatures and ages of the various minerals and an average geothermal gradient of the order of 30”C/km has been adopted. GEOCHRONOLOGY BELTS

AND

COOLING-UPLIFT

RATES

OF

VARIOUS

OROGENIC

Although the available geochronological data, particularly the mineral ages, for the peninsular and the extra-peninsular India are for too meagre for arriving at the final conclusion regarding the cooling and the uplift rates of different erogenic cycles, an attempt has been made to synthesize and correlate the available information from various erogenic belts such as Dharwar, Satpura, Aravalli and Himalaya (Fig, 1). The various cycles recognized in each erogenic belt have also been discussed. DHARWAR

OROGENIC

BELT

Older Metamorphic Cycle Some of the oldest rocks (hornblende schist) dated from the Hatti gold mine, Dharwar Orogenic Belt, by the K-Ar method gave an age of 3,295 +

Fig. 1. Sketch map showing various erogenic belb of India.

200 Ma (Sarkar, 1968). Venketasubramanian and Narayanaswami (1974) reported a Rb-Sr isochron age of 3,250 ?r 150 Ma for the gneissic pebbles from Kaldurga conglomerate, Karnataka and suggested that gneissic pebbles possibly represent the relicts of Dharwar basement. Crawford (1969) also reported an isochron age of 3,065 Ma for the gneisses from peninsular India. These ages suggest the presence of an older metamorphic cycle in the Dharwar Orogenic Belt.

Rock type and locality

Rb-Sr isotopic homogenization in magmatic rocks

Detrital monazite Satbhaya

Pegmatite, Tamilnadu

Pegmatite, Visha~apatnam

Pegmatite, Salem

Pegmatite, Khamam

Pegmatite, Kanyakumari

Pegmatite, Madurai

Sr. No.

1.

2.

3.

4.

5.

6.

7.

8.

zircon apatite zircon apatite

zircon apatite

monazite zircon sphene apatite

monazite zircon apatite

monazite zircon sphene apatite

monazite

whole rock

Mineral/rock dated

480 430 510 450

570 520

1570 1406 1280 510

1570 1320 530

1570 1330 1210 620

1570

1600

Age (Ma)

F-T F-T F-T F-T

F-T F-T

Pb-U F-T F-T F-T

Pb-U F-T F-T

W-U F-T F-T F-T

Pb-U

Rb-Sr

Method

350 100 350 100

350 100

530 350 300 100

530 350 100

530 350 300 100

530

600

Blocking temperature (“C)

Radiometric age data and uplift and cooling rates for Dharwar Orogenic Belt

TABLE II

510-

480-

450

430

570-520

1570-1406 1406-1280 1280510

1570-1320 1320530

1570-1330 1330-1210 1210620

1600-1570

Age span (Ma)

138

166

166

36 13 8

24 10

25 13 11

76

Uplift rate (m/Ma)

4.2

5.0

5.0

1.1 0.4 0.3

0.7 0.3

0.8 0.4 0.3

2.3

Cooling rate (‘C/Ma)

5

142

Based on K-Ar ages, Sarkar (1968) suggested the closing of the earliest phase of metamorphism (phase I) and granitization in the Lower Dharwar times during the period 3,000-2,800 Ma. A lead isochron age of 2,900 rt 200 Ma for galena from Chitaldrug and amphibolite from Kolar by Vinogradov et al. (1964) also support the above observations. The main Dharwarian Orogeny, metamorphism (phase II} and granitization {pen~sul~ gneiss), a~~ord~g to Sarkar (1968), closed around 2,7002,600 Ma as supported by concordant Rb-Sr, lead isochron and K-Ar ages. He also suggested that the metamorphism and granitization (phase III) which affected the Dharwar Orogenic Belt, closed at about 2,000 Ma as represented by 2,380-2,000 Ma age of the Closepet granite. Crawford (1970) reported the age of the Closepet granite as 2,560 + 420 Ma, while Venkatasubramanian et al. (1971) determined an isochron age of 2,000 + 80 Ma for the granite occurring near Closepet, Magedi and Savandurga. Eastern

Ghats Cycle

The mineral ages from the Nellore Mica Belt and the adjoining regions of Eastern Ghats cluster around 1,600 Ma and represent the Eastern Ghats Cycle. In the Vishakhapatnam area charnokites and khondalites have been dated about 1,650 Ma. Detrital monazite from Satbhaya (1,570 Ma), allanite from Anakapai (1,585 Ma) and Samarskite from Nellore (1,550 Ma) also date the Eastern Ghats Cycle at around 1,600 Ma. Indian Ocean Cycle The evidence of younger erogenic-metamorphic events and associated anorthositic, granitic and pegmatitic activities in the part of South India has been recognized and is named the Indian Ocean Cycle. The Indian Ocean Cycle, or the Ballangoda Cycle of Holmes (1955), is quite widespread and it caused metamorphism and magmatism around 500 Ma; it has been widely recognized in the Eastern Coast, South India and Ceylon. Cooling and uplift rates From the Dharwar Orogenic Belt a large number of whole-rock and mineral ages by various methods and of different rock types are available. The data on the whole rocks and the minerals representing the older cycles (including Dharwar Cycle) from this belt are inadequate to calculate the cooling and the uplift rates for earlier phases; however, there is sufficient information available for the Eastern Chats and the younger Indian Ocean cycles (Table II). Presuming that the pegmatites of Nellore Mica Belt crystallized at 6OO’C

143

around 1,600 Ma and the monazite with the blocking temperature of 530°C dated 1,570 Ma, the cooling and uplift rates for the period between 1,6001,570 Ma may be calculated as 2.3’C/Ma and 76 m/Ma, respectively. Similarly, the uplift and the cooling rates for the pegmatitic bodies of Tamilnadu, Vishakhapatnam and Salem calculated for various time spans using fissiontrack ages of zircon, sphene and apatite are given in Table II. It is evident from the table that the regions around Tamilnadu, Vishakhapatnam and Salem gradually cooled subsequent to the 1,600 Ma metamorphic event during the Eastern Ghat Cycle. Another thermal event (- 500 Ma) in South India is represented by fissiontrack zircon and apatite ages (Table II). The narrow age gap between zircon and apatite, that have a large difference of blocking temperatures (-250°C), possibly suggests that the Indian Ocean Cycle magmatism took place under shallow depths (Table II). SATPURA OROGENIC BELT

On the basis of extensive structural and stratigraphic studies in parts of Bihar and Orissa, supplemented by radiometric (K-Ar) age data, Sarkar and Saha (1962) and Sarkar et al. (1969) have established three distinct erogenic cycles, each with accompanying granitic activity in this region; these are given below. Older Me ta~~rp~i~ Cycle

Deposition, folding and cross folding of talc-magnesian me&sediments, argillites and arenites of Older Metamorphic groups, their metamorphism under almandine-amphibolite conditions, intrusion by basic igneous bodies of various sizes and granitization closed around 3,200 Ma. Iron Ore Cycle

Deposition of Iron Ore Group sediments, pyroclastics and volcanics, isoclinal folding around NNE-NNW-trending axis, low-grade metamorphism (mainly greenschist facies) followed by the emplacement of the extensive Singhbhum granite complex in the central and eastern parts of the region took place around 2,700 Ma. This was succeeded by Gangapur and Singhbhum Group sediments, and eruption of Dalsma, Dhanjori and Jagannathpur basic lavas between 1,70~1,600 Ma. Singhbhum Orogenic Cycle

Folding, regional metamorphism and local cross-folding of the Gangapur Group, Singhbhum Group and Dalma lavas occurred in several phases between 1,550 and 850 Ma. Sarkar et al: (1964) correlated the orogeny,

me~mo~hism and unitization of the Mm-i belt (890-970 Ma) with one of the phases of Singhbhum (850 Ma), G~gapur (846-946 Ma), Ranchi (980 Ma), Gaya (955 Ma), Dhanbad (893-1086 Ma) and Gurpa (917-934 Ma) and considered all these to belong to the Satpura cycle. The Bihar Mica Belt lies along the northern fringe of the Chotanagpur plateau and covers parts of Gaya, Hazaribagh, Monghyr and Bhagalpur districts of Bihar. This belt is mostly covered by rocks of Precambrian age intruded by granites, metadolorites and pegmatites. The granite complex, which is one of the predominant rock units of the region, is included in the Cho~na~ur granite-gneiss. Dunn (1929) and Iyer (1932) believed these rocks to be the product of assimilation of the older rocks by the intrusive granite (Chotanagpur granite-gneiss) a part of which has been dated at 1,000 Ma (Crawford, 1968). Bagchi (1957) assigned a syn- to post-kinematic metasomatic origin to these granites, contemplating in-situ emplacement of the metamorphites by granitic fluids under deep-seated conditions. Mahadevan (1965) considered that the granites and the associated pegmatites of this region are genetically associated with the igneous activity and were emplaced into the metamorphic rocks of the upper amphibolite facies during the Satpura Cycle. Cooling and uplift rates For the vast area of the Satpura Orogenic Belt, covering parts of Bihar and Orissa, the geochronolo~c~ data representing three main erogenic cycles are very limited. Only a few ages reflect the older Me~mo~hic and the Iron-Ore cycles, while age data on different minerals by different techniques representing the Satpura cycle in Bihar Mica Belt are adequate. The cooling and the uplift rates calculated for various age spans on the basis of the averaged mineral age data on monazites, muscovites, garnets and apatites from the Bihar Mica Belt are shown in Table III. These data would possibly represent regional cooling and uplift rates for this belt and are not much different from the cooling and the uplift calculated for a single pe~atite (Domch~~h) body. The present calculations of uplift and cooling rates are not much different from the earlier determined rates for the Bihar Mica Belt by Nand La1 et al. (1976~). ARAVALLI

OROGENIC

BELT

The Bundelkhand granite and the Berach granite (2,550 Ma) form the craton for the Aravalli. Orogenic Belt which is represented by the rocks of the Aravalli Group, Delhi Group, Upper Vindhyans and Malani suite. The age of 3,500 Ma determined for the detrital zircon present in the schistose rocks of the Aravalli Group near Udaipur by Vinogradov et al. (1964) suggest the presence of still older basement than the Bundelkhand granite in this region. The Aravalli Orogenic Belt has undergone at least four distinct cycles of

Average of four analyses from Ranchi, Pitchtabli, Wazaribagh and Monghyr (Averaged data from different pegmatitas, Bihar Mica Belt)

Domchanch pegmatite, Bihar Mica Belt

1.

2. 970 830 455

830 590

garnet apatite monazite garnet apatite

970 920

Age (Ma)

monazite muscovite

Mineral/rock dated

Pb-U F-T F-T

F-T F-T

W-U K-Ar

Method

530 280 100

280 100

530 350

Blocking temperature (“C)

whole rock garnet hornblende muscovite garnet muscovite apatite whole rock biotite

Metamorphic rocks, Bhilwara

Pegmatite (Danta Mica Mine) Bhunas, Bhilwata

Godra granite, Gujrat

1.

2.

3.

Mineral/rock dated

Rock type and locality

Sr. No.

K-Ar F-T F-T F-T Rb-Sr Rb-Sr

900

Rb-Sr F-T F-T

Method

1024 990 810 430 955

X500 1370 970

Age (Ma)

300

350 280 130 100 600

600 280 120

Blocking ;;rture

Radiometric age data and uplift and cooling rates for Aravalli Orogenic Belt

TABLE IV

Rock type/locality

Sr. No.

Radiometric age data and uplift and cooling rates for Satpura Orogenic Belt

TABLE III

955-

1024990810-

900

990 810 430

1500-1370 1370970

Age span (Ma)

970-830 830-455

920-830 830-590

970-920

Age span (Ma)

1.8 0.4

0.8 0.8

3.6

181

68 27 2

82 13

5.5

2.1 0.8 0.1

2.5 0.4

Cooling rate (“C/Ma)

Cooling rate (“C/Ma)

Uplift rate (m/Ma)

59 13

25 25

120

Uplift rate (m/Ma)

g

146

deformation, metamorphism and magmatism which have been described by Sharma (1978) and have been designated as below.

recently

Araualli Urogenic Cycle The first phase of folding and syn-kinematic migmatization in the Aravalli rocks is reflected by an age of 2,060 Ma for migmatites from Govaliya near Udaipur (Crawford, 1970; Naha and Halyburton, 1974). Crawford (1970) also assigned an age of 2,500-2,000 Ma for the Aravalli Group of rocks and suggested the closing phase of deformation and magmatism around 2,000 Ma. Delhi Orogenic Cycle The second cycle of deposition (Delhi Group of rocks), folding, metamorphism and magmatism, possibly lasted between 2,000-1,500 Ma. The closing stage of this cycle was marked by regional and intensive deformation and metamorphism of Delhi and Aravalli rocks and emplacement of granites. The Udaipur and Saladipura granites from the Khetri Copper Belt have a Rb-Sr isochron age of 1,480 f 40 Ma (Gopalan et al., 1978a) that probably corresponds to the magmatism associated with the closing stage of the Delhi Orogenic Cycle around 1,500 Ma. Sharma et al. (1975) and Sharma (1978), on the basis of the fission track garnet age of 1,370 + 130 Ma and epidote age of 1,486 It 89 Ma, suggested folding and metamorphism of the Aravalli and Delhi rocks around 1,500 Ma. Erinpura Orogenic Cycle The third cycle in the Aravalli Orogenic Belt closed around 950 Ma and represen~d by mineral ages such as 948 Ma and 958 Ma for the muscovites (K-Ar) from the Aravalli mica schist (Sarkar et al., 1964), 1,024 Ma age of muscovite (K-Ar, Mehta, 1976) from Bhunas Pegmatite (Danta Mica Mine) and 990 Ma age of garnet (fission track, Sharma et al., 1975; Nand Lal et al., 1976a). This cycle is also represented by the Erinpura granite emplacements along the axial zone of the Aravalli range. Gopalan et al. (1978b) reported a Rb-Sr isochron age of 955 ? 20 Ma for Godra and related granites of Gujrat. Granites of similar age have also been reported from Ajmer (935 Ma), Chhapoly (1,010 Ma) and Untala (955 Ma) by Crawford (1970). has been

Makmi Orogenic Cycle The last phase of deformation in the Aravalli Orogenic Belt is correlated with the broad and open cross-folds present on various scales (Sharma, 1978). Even on a regional scale the Aravalli orographic strike has also been cross-

147

folded and Sharma (1978) suggested an age of cross-fold~g movement at around 750 Ma. The age of 735 Ma for the Mount Abu granite (Crawford, 1975) and the Malani suite of rocks (Crawford, 1970) suggest association of magmatic activity with the last phase of deformation in the Aravalli Orogenie Belt. The reported biotite age of 700 Ma from Saladipura and Udaipur granites (Khetri Copper Belt) by Gopalan et al. (1978a) possibly indicate local mobilization of Sr-isotopes during a later metamorphic event coinciding with the last phase of deformation in the region.

A number of mineral and whole-rock radiometric ages by different techniques (Rb-Sr, Pb-U, K-Ar and fission track) are available from the Aravalli Orogenic Belt. While the whole-rock isochron ages indicate the time of isotopic homogenization and possibly crystallization of magmatic rocks, the mineral ages reflect the cooling periods and therefore help in calculating the cooling and tectonic uplift rates. The cooling rates of 2.5 and 0.4*C/Ma and the uplift rates of 82 and 13 m/Ma (Table IV) have been calculated between the period 1,500-1,370 Ma and 1,370-967 Ma, respectively, for the region around Bhilwara after the second phase (Delhi Orogenic Cycle) of deformation, metamorphism and magmatism in the Aravalli Orogenic Belt. Similar calculations for the Bhunas Pegmatite body from a closeby region suggest cooling rates of 2.1, 0.8 and O.l”C/Ma and uplift rates of 6827 and 2 m/Ma for the age spans 1,024-990 Ma, 990-810 Ma and 810-430 Ma represent~g the Erinpura Orogenic Cycle. This cycle is more clearly broughtout by the whole-rock and the biotite ages from Godra granite, Gujrat (Gopalan et al., 1978b). Based on Rb-Sr whole-rock and biotite ages a cooling rate of 5.5’C/Ma and an uplift rate of 181 m/Ma may be calculated for the period 955-900 Ma (Table IV). HIMALAYAN OROGENIC BELT This erogenic belt, unlike the other erogenic belts of India, is geologically and geochronologically less understood. The various phases of deformation, metamorphism and magmatism in the evolution of the Himalayan Orogenic Belt are not clear so far, however, Valdiya (1964) attempted to review and compare various diastrophic movements during the evolutionary history of Himalaya with the Alps. The presence of Precambrian phases of deformation, metamorphism and magmatism in the crystalline basement such as Salkhalas, Vaikritas, Jutoghs and Chails have been suggested by Pande and Gupta (1970), Saxena (1976) and Mehta (1976). The occurrence of Precambrian gneisses as old as 2,030 Ma (Wangtu gneiss), 1,890 Ma (Munsiari gneiss), 1,430 Ma (Baragaon gneiss) and 1,220 Ma (Bandal gneiss) in the crystalline basement suggest some possible magmatic correlation with the adjoining regions of the Aravalli Orogenic Belt (Bhanot et al., 1976; Bhanot et al., 19781979).

148

The presence of a pronounced unconformity marked between the Martolis and the Ralams in Kumaun (Heim and Gansser 1939), the Haimantas and the Jadha in Garhwal (Auden, 1949) and the Dogras and the Tanawals in the Vihi, Sindh and Banihal areas of Kashmir (Wadia, 1957) suggest diastrophic movements towards the end of the Purana Era. The dating of granitic rocks from Mandi, Manali and Rohtang regions around 500-600 Ma (Jager et al., 1971; Bhanot et al., 1976; Mehta, 1977) suggest magmatism associated with the closing phase of this episode; however, the information on metamorphism and deformation related to this event is not clear so far. The presence of Caledonian and Hercynian epiorogenic movements in the Himalaya has also been suggested by Valdiya (1964), besides the Tertiary Orogeny which brought about evolution of the Himalaya in at least four successive phases. The first diastrophic phase of the Himalayan Orogeny took place towards the Upper Cretaceous during which period marine conditions in Karakoram and Tibet Plateau, marine transgression by the Nummulitic sea in the lesser Himalayan region and emplacement of ophiolites in the Indus-Suture zone took place. This phase has been designated the “Karakoram Orogeny” by De Terra (1933) and Valdiya (1964). Valdiya (1964) considers that the second phase, known as the “Post Kirthar Orogeny”, took place towards the end of the Eocene and culminated in the Oligocene period. As a result of this the Tibetan Plateau emerged from the cradle of the sea, the crystalline basement started ridging up along the Indo-Tibetan border, followed by possible development of exotic nappes of the Kumaun-Tibet border and emplacement of tou~~~e-being granites of batholithic dimensions. During the third phase, which took place towards the Middle Miocene period, the Himalayan Orogenic Belt witnessed the strongest upheaval; this brought about tight isoclinal folding, overturning and thrusting and was designated the “‘Sirmurarian Orogeny”. Recently, Sharma and Nagpaul (1980) have dated the thrusting event in the Satluj Valley at around 15 Ma. The fourth phase in the tectonic evolution ‘of the Himalaya commenced in the Pliocene and continued down to Middle Pleistocene time. This phase, called the “Siwalik Orogeny”, brought about folding and faulting of the Siwaliks, formation of the Main Boundary Fault, reactivation of the lesser Himalayan thrusts, elevation and tilting of Karewas of Kashmir and development of the Indo-Gangetic depression. Cooling and uplift ru tes The geodetic surveys carried out by the Survey of India (1975) have revealed that the Himalaya has been rising at a rate of 0.8 mm/year (800 m/Ma during the past 75 years). Sharma et %l. (1978), using fission-track techniques, calculated a tectonic uplift of 550 m/Ma for the past 8 Ma from the Mandi region of the northwest Himalaya. Saini et al. (1979) have sug gested uplift rates of 0.03 mm/year, 0.08 mm/year and 0.15 mm/year for

149

the period 200-46 Ma; 31-20 Ma and 20-11 Ma, respectively for the Kinnaur region of the Higher Himalaya. These authors also suggested that this region uplifted at a comparatively much faster rate of 0.8 mm/year between 15-7 Ma ago during Miocene thrusting. Mehta (pers. commun., 1979), using Rb-Sr muscovite and biotite ages from Kulu-Manali area, has calculated an uplift rate of 800 m/Ma during the period 25-10 Ma. The information on the cooling rates from the Himalayan Orogenic Belt is limited to the fissiontrack data reported by Sharma et al. (1978) and Saini et al. (1979). Sharma et al. (1978) suggested a cooling rate of 2-3”C/Ma for the period 32-10 Ma and 3-6’CfMa between 16-8 Ma for the Mandi area. Saini et al. (1979) calculated the cooling rate of l’C/Ma, Z’C/Ma and 4”C/Ma for the periods between 200-46 Ma; 31-20 Ma and 20-11 Ma, respectively for the Kinnaur region. They also suggested rapid cooling (-25’C/Ma) subsequent to thrusting in the area. Frank et al. (1977), using Rb-Sr age data, also suggested rapid cooling in the vicinity of the thrust zone in the Kulu area. The above data obviously suggest faster uplift and rapid cooling after the Middle Miocene (S~mu~an Orogeny) event which brought about folding, overtuming and thrusting in the Himalaya. DISCUSSION

The available radiometric age data from four erogenic belts of India viz. Dharwar, Satpura, Aravalli and Himalaya discussed in the foregoing pages have been illustrated with the help of histo~ms in Fig. 2. In the erogenic belts of Peninsular India, the older me~morphic cycles (3,50~3,000 Ma) are depicted by K-Ar, Pb-U, and all-ages-combined histograms, whereas the Dharwar cycle, Iron Ore cycle and Bundelkhand granite episode (2,9002,500 Ma) are very well brought out by Rb-Sr, Pb-U, and all-ages-combined histograms. The Aravalli cycle (-2,000 Ma), the Eastern Chats cycle (-1600 Ma) and the Delhi cycle (-1500 Ma) are well represented in all the radiometric methods including fission track. The younger erogenic cycles of Peninsular India, i.e., the Erinpura cycles (-950 Ma), the Satpura cycle (-950 Ma), the Malani cycle (-750 Ma) and the Indian Ocean Cycle (-500 Ma) have left their imprints either in the recrystallized minerals (such as micas) of the older rocks or the minerals formed during the metamorphism and magmatism associated with these cycles. On the basis of the mineral ages available by different radiometric methods for the erogenic cycles younger than 1,600 Ma, it has been possible to calculate the cooling and the uplift rates as discussed earlier. ~though the radiometric age data for the Himalayan Orogenic Belt is limited as compared to its spread, still the available ages suggest at least granitic activities around 2,000 Ma, 1,500-1,400 Ma, 1200-1,100 Ma and 600-500 Ma, and are more or less in conformity with the adjoining regions of the Aravalli and Satpura Orogenic belts. The Himalayan Orogenic Belt also witnessed younger diastrophic phases around 350 Ma, 200 Ma and 75

PENINSULAR

INDIA.

30

Y---

EXTRA

PENINSULAR

lh3lA

151

600-

0

I

200

400

I

600

!

800

1000

1200

1400

1600

,

le.00

AGE IN MILLION YEARS Fig. 3. Curves showing cooling pattern during various erogenic cycles of the Indian Peninsula.

Ma in comparison to the nearly stabilised Peninsular region (Fig. 2). The Upper Cretaceous-Tertiary Orogeny of the Himalayan Orogenic Belt shows various phases within it as evinced by clustering of radiometric ages around 60 Ma and 15 Ma (Fig. 2). The radiometric ages of the minerals, the blocking temperatures used and the cooling and uplift rates calculated for various erogenic belts are given in Tables I-V. Table VI gives a comparative picture of the cooling and uplift pattern for different erogenic cycles, based on averaged mineral ages, and these are illustrated in Fig. 3 and 4, respectively. The cooling history of various erogenic cycles in the Peninsular region has many features in common. The cooling in most of the regions starts with a rapid rate of about 3-@C/Ma after each erogenic and metamorphic cycle and gradually falls to a slower rate after a few million years. Erinpura and Satpura cycles show more or less similar cooling curves. It is interesting to note that the Satpura and Aravalli Orogenic Belts have a more or less identical cooling history between 800-460 Ma. Another interesting observation regarding the erogenic belts of Peninsular India is the convergence of the cooling curves to Fig. 2. Histograms showing radiometric ages of Peninsular (Modified after Balasundram and Balasubrahmanyan, 19731, and Extra-peninsular India.

TABLE V Uplift -

Sr.

and cooling

rates of Himalyan

Orogenic

Belt

Locality/region

Method

Age span

Uplift rate (m/Ma)

1.

Outer Himalayan

geodetic

past 75 years

800

-.

2.

region Kulu

800

2.0-3.0 3.0-6.0 -

No.

Rb-Sr mineral

25-10

Ma

Cooling ?C/Ma)

rate

ages

3.

Mandi

fission track

32-16Ma 16- 8Ma past 8 Ma

550

4.

Kinnaur

fission track

200-46 Ma 31-20 Ma 20-11 Ma 15- 7Ma

30 80 150 800

1.0 2.0 4.0 25.0

a common point, suggesting uniform thermal conditions being attained around 450 Ma. It further suggests that Peninsular India did not witness any tempe~ture rise after the closing of the Purana Era and had attained near surface conditions by then. The suitability of the various blocking temperatures used in the present paper may, on the basis of the uniform cooling pattern observed for various erogenic cycles in India, be suggested to explain the differences in mineral ages obtained by the same or different radiometric techniques. The uplift pattern for the erogenic belts of Peninsular India is also more or less similar. After a phase of deformation and me~morphism, the region underwent quick uplift, fast erosion and fast cooling. The uplift and the cooling rates gradually fall, till the time that the region was again affected by the subsequent phase of deformation and metamorphism. The Indian Ocean cycle is the youngest erogenic cycle that affected the Eastern Ghats, South India and Ceylon. The pegmatite bodies emplaced during this phase show a faster rate of cooling than in the other cycles. This observation supported by smaller age difference between the zircon and apatite (Table II), with a large difference in their blocking temperatures, suggest a shallow depth of emplacement for these bodies. The faster cooling rate for these bodies may not be due to faster uplift but may be attributed to rapid temperature loss of the pegmatite bodies due to their nearness to the surface. However, some diastrophic effects (tectonic uplift) in the Peninsular region during the closing of the Purana Era, which could bring about stability of the Indian Shield and a large-scale unconformity in the Himalayan region, cannot be ruled out. The Himalayan region, where punctuated deposition continue from the Cambrian to the Jurassic in the Tethyan Sea, witnessed a major change with

* In years.

Indian Ocean Cycle Himalayan Orogenic Cycle

Satpura Orogenic Cycle Erinpura Orogenic Cycle

Delhi Orogenic Cycle

Eastern Ghats Cycle

1600-I570

.

Grogenic cycle

Age span (Ma)

-

-

-

166

-

-

-

-

-

5.0

-

-

-

0.4 0.3 I.3 9

30

1.0 -

76

2.3

-

-

0.1 -

0.8 -

0.8

-

3.6

-

-

-

-

-

2 -

25 25

120

-

-

(m/Ma)

(OC/Ma)

(‘C/Ma)

(m/Ma)

Satpura Orogenic Belt

Dharwar Orogenic Belt

Comparative cooling and uplift rates from various erogenic belts of India

TABLE VI

-

-

-

-

5.5

2.1 0.8

0.4 -

-

-

-

-

181

68 27

13 -

82

2.5

-

(m/Ma)

-

(°C/Ma)

Aravalli Orogenic Belt

-

4.0 3.6-6.0 25.0 -

2.0-3.0 2.0 -

1.0

-

-

-

-

CWMa)

800 550 800

80 800 150 -

-

30

-

-

-

(m/W

Himalayan Grogenic Belt

:

154 90Or

30

25 600,

HIMALAYAN F

20

OROCENIC

CYCLE

ERINPURA INDIAN

OCEAN

6O

c\ 1609

.

*i 4EASTERN

z d

2

ii? 2 8 ”

,.;___.. . . . . . . . . ..__._..\ ,,... “‘;,,’

0

0

i*l_(_

1

”

2

c

4

I_. 8

%..

10

3

AGE

Fig. 4. Curves showing

6 (YEARS

uplift and cooling

~10'1

,;.

12

14

16

18

e3

rates during various erogenic

cycles of India.

the northward drift of India, after its separation from Antarctica and Australia, towards the beginning of the Cretaceous, i.e. 130 Ma ago (Johnson et al., 1976). This change brought about deepening of the Tethyan Sea in the first instance, followed by compression, deformation, metamorphism and mag matism (Ladakh Granite complex) in the Himalayan region towards the Late Mesozoic-Early Tertiary period. This change in the mantle-crustal stability of the Indian Plate also resulted in large-scale eruption of Deccan Trap lavas in the Peninsular region through tensional fractures. Similarly in the Extrapeninsular region, the junction between the Indian and the Eurasian plates witnessed volcanic eruptions (Indus and Shyok volcanics) (Sharma and Kumar, 1978) and the emplacements of ophiolitic rocks with the continued northward movement of India. The compression in the Himalayan region resulted in folding and thrusting, while in the Peninsular region tensional fractures modified the coastline of India and also developed a series of parallel faults. The major phase of thrusting in the Himalayan region took place around the Middle Miocene (Valdiya, 1964; Gansser, 1974; Sharma and Nagpaul, 1980). The southward movement of nappes brought up rocks from the deeper region which started cooling at a rapid rate of about 2O”C/ Ma in the vicinity of the thrust zones (Frank et al., 1977; Sharma and NagPaul, 1980). Another fact that emerges from the above-discussed cooling and uplift

155

histories of various erogenic cycles of India is that the Himalayan Orogenic Cycle (Upper Cretaceous-Tertiary) witnessed the highest uplift and cooling rates in comparison to older cycles of the Peninsular India and may be attributed to large-scale horizontal shift of the Indian plate from 40’S latitude to 8”N latitude in the past 71 Ma as suggested by Molnar and Tapponier (1975) at an average rate of about 50 mm/ye~ (Le Pichon, 1968; ~inster et al., 1974). REFERENCES

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