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Late Cretaceous emplacement of the Indus suture zone ophiolitic mélanges and an Eocene-Oligocene magmatic arc on the northern edge of the Indian plate
Earth and Planetary Science Letters, 55 (1981) 157-162
157
Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
[41
Late Cretaceous emplacement of the Indus suture zone ophiolitic mrlanges and an Eocene-Oligocene magmatic arc on the northern edge of the Indian plate M.E. Brookfield Department of Land Resource Science, Guelph University, Guelph, Ont. NI G 2 W1 (Canada)
and
P.H. Reynolds Department of Physics and Geology, Dalhousie University, Halifax, N.S. (Canada) Received February 18, 1981 Revised version received May 11, 1981
We report three 4°Ar/agAr dates (on stratigraphically located samples) of 82 ___6 Ma from a syenite cutting the Indus suture zone ophiolitic mrlange and about 39 and about 45 Ma from granodiorite intrusions north of the suture zone. Sedimentological observations indicate Eocene to Miocene deposition of coarse clastics by very large braided and meandering streams in a continental back-arc setting. These observations suggest that the ophiolitic mrlanges of the Indus suture zone were emplaced in the late Cretaceous, shortly after a major change in plate motions in the Indian Ocean: they further suggest that an Andean-type magmatic arc d~veloped on the northern edge of the Indian plate during the Eocene and Oligocene.
1. Introduction
In view of the recent interest in the Tertiary collision zone between India and Tibet and the rarity of stratigraphically located radiometric dates from the suture zone, we wish to summarize observations made by M.E.B. during five months of field study in 1978 and 1979, and report 4°Ar/39Ar dates on stratigraphically located samples. Though few, these dates, and sedimentological observations have a bearing on two critical factors in the interpretation of the India-Tibet collision zone. These factors are: (1) the age of emplacement of the Indus suture ophiolitic mrlanges and associated island arc volcanics and sediments, and (2) the age of the Ladakh-Deosai batholith complex to the north (see Fig. 1). Previous workers [1,2] have suggested that ophiolite emplacement took place in the early Ter-
tiary during collision of the Indian plate with a southward-facing late Cretaceous to early Tertiary island arc represented by the Indus suture volcanics (Dras volcanics) and the Ladakh-Deosai batholith complex (Fig. 2A). Our results suggest a late Cretaceous age for emplacement of the ophiolitic m~langes which occurred during collision of the Indian plate with a southward-facing middle Cretaceous island arc (represented by the Dras volcanics). This was followed, after a tectonically quiet period in the Paleocene to early Eocene, by the development of a northward-facing Andeantype magmatic arc on the northern edge of the Indian plate during the late Eocene and Oligocene (Ladakh-Deosai batholith complex) (Fig. 2B). Ophiolite emplacement thus took place in a lull between the activity of two separate magmatic arcs of differing polarity.
0012-821X/81/0000-0000/$02.50 © 1981 Elsevier Scientific Publishing Company
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Fig. 1. Sketch m a p of northern Kashmir, showing tectonic zones and sample locations. I = Tibetan plateau; H = K a r a k o r n m batholith zone; llI=Shyok m61ange zone (shaded); I V = " K o h i s t a n sequence" zone (A = L a d a k h - D e o s a i batholith zone; B - - I n d u s suture zone); V = Zanskar zone. S = Satpura village, L = Lamayuru. Crosses mark radiometric dating sample locations, with ages and sample n u m b e r s close by.
2. I n d u s suture z o n e
East of Kargil, the Indus suture zone consists of Triassic Atlantic-type turbidites (Lamayuru flysch) derived from a stable continental margin, overlain by undated volcaniclastic turbidites which pass gradually upwards into basic to intermediate volcanics and volcaniclastic sediments [3], the Dras volcanics, of mid-Cretaceous (Barremian to Turonian) age [4,5]. The Dras volcanics form an island arc suite evolving from tholeiitic at the base to shoshonitic at the top indicating a steeply dipping subduction zone [6]. Ophiolitic m61ange zones occur below the volcanics and within the turbidite sequences [3]. These m61anges contain Permian shelf limestones, Triassic-Jurassic pelagic limestone and sandstone, and mid- to late Cretaceous
cherts [7,8]. There is no evidence in the Indus suture zone for late Cretaceous or early Tertiary volcanism. South of Lamayuru (Fig. 1), the carbonate sequence of the Zanskar zone is overlain by thrust sheets consisting of Mesozoic turbidites and volcanics, with ophiolitic m61anges and large exotic carbonate blocks; overlain by a hartzburgite thrust sheet [8,9]; the whole forming the Spongtang Klippe of Fuchs [9]. Relationships in the Spong Valley and westwards suggest that the thrust sheets were initially emplaced southwards onto shelf carbonates during the late Cretaceous ([2,10,11], M.E.B., unpublished data) in' postTuronian [4] probably post-upper Campanian [12], pre-upper Maastrichtian times. Apart from the hartzburgite thrust sheet, the Spongtang sequence
159
INDIA
DRASVOLCANICARC d~
Sea Level
1 .MID - CRETACEOUS
ALTERNATIVES 2. LATE CRETACEOUS
B DRASARC INACTIVE
A
EVOLVING.~L LADKH- DEOSAI tr~ ~ BATHOLITHCOMPLEX
LADAKH-DEOSAI .'~*%" ~ ' * ~ ~ B A T H O L ICOMPLEX ITH 3.
EOCENE
KARAKORUM ~ SHYOKMELANGE
KARAKORUM 4.
EARLYMIOCENE
Fig. 2. Alternative views of the Cretaceous-Tertiary evolution of the collision zone: (A) with continuously evolving magmatic arc (based on Frank et al. [2]); (B) with two arcs of different ages and polarity (based on Andrews-Speed and Brookfield [3]).
is identical to that of the Indus suture zone. Were the Indus suture units initially emplaced towards the end of the Cretaceous, as was the Spongtang Klippe, or during the early Tertiary? An undeformed hornblende syenite intrudes the metamorphosed ophiolitic m~lange and Dras volcanics at Kargil (Fig. 1) and is cut by the post-Miocene northward motion along the Kargil thrust. The generation of the syenite is possibly related to partial melting during emplacement of the m61ange and Dras volcanics. Hornblende from the syenite has yielded an age spectrum (Fig. 3) which has an apparent age plateau comprising about 90% of the total 39Ar released. The inferred age is 82 _ 6 Ma (the absolute uncertainty at the 95% confidence level). This Coniacian-Santonian [13] estimated age of emplacement is consistent with the pre-Maastrichtian age of emplacement of the Spongtang Klippe and with conventional K-Ar ages, of 67 Ma from an amphibolite from the
Kohistan ( = Dras and of 7 7 _ 1 M a type not specified) Chiktan nala, near
volcanic unit) in Pakistan [14] from "Dras volcanics" (rock from the ophiolitic m61ange a t Kargil, Ladakh [15].
m
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160 3. Ladakh-Deosai batholith
The Ladakh-Deosai batholith complex to the north of the Indus suture zone consists of tonalitic to granodioritic intrusives, with minor grabbroic phases, intruded into the mid-Cretaceous Dras volcanics and associated sediments and in places unconformably overlain by late Eocene to Miocene coarse continental elastics [2,10,11]. Few records of volcanic equivalents of the intrusions have been noted, except north of Kargil and around Satpura (Pakistan) (M.E.B., personal observations); but parautochthonous conglomerates west of Leh contain abundant rhyolitic and andesitic clasts presumably derived from volcanics now eroded away [21. Previously published radiometric dates on the Ladakh-Deosai batholith complex are: a conventional K-Ar date of 48.4 +-1.7 Ma from biotite gneiss near Kargil [16] and five fission track ages ranging from 15 to 43 Ma on "granites" from the Kargil area [17], though these have large errors [18]. Sharma et al. [15] reported K / A r dates of 27.8 __+0.6 Ma from a pink porphyritic granite, and 38 __+2 Ma from associated Shyok volcanics north of Leh, Ladakh. Desio et al. [19] have reported a Rb-Sr date of about 49.6 Ma (recalculated to X = 1.42 × 10-11 y r - 1 ) from a granodiorite from Satpura. Were the Ladakh-Deosai batholiths emplaced as a continuation of the Cretaceous Dras volcanic island arc [2] or were they emplaced during a separate younger phase of arc development? The lack of pre-Eocene ages above, and the additional dates below suggest the second alternative. Sample SK 2a is from a fresh dacite dyke intruding folded metasediments (amphibolite facies) of the Kohistan (= Dras volcanic unit) sequence at Satpura (see Fig. 1). The dacite is marginal to a late-stage, high-level granodiorite intrusion which has a thin contact aureole. The dacite and granodiorite post-date deformation and metamorphism of the Kohistan unit which probably occurred during the activity of the midCretaceous island arc and its later southward emplacement over the Indian shield. That the Kohistan sequence was already emplaced when the granodiorite was intruded is shown by the contrast
between the amphibolite facies metasediments and the fresh, high-level intrusion. Completely fresh muscovite extracted from the dacite has yielded a spectrum (Fig. 4) which in a strict sense does not have an apparent age plateau. However, the three steps in the middle of the spectrum comprising about 50% of the total 39Ar released and which have the lowest atmospheric correction define a plateau at an age of about 42 Ma. Sample SK 12b is from a slightly deformed hornblende biotite granodiorite intruding foliated orthogneiss at Khaplu (Fig. 1). Both units are cut by a swarm of basaltic dykes related to the emplacement of the Shyok m61ange to the north. Biotite from the granodiorite has yielded an age spectrum (Fig. 4) with a very well-defined plateau over greater than 90% of the gas released. The plateau age is about 39 Ma. While both samples from the Ladakh-Deosai batholith complex, have yielded essentially undisturbed age spectra, there does appear to be relatively small but nevertheless analytically distinguishable difference in apparent age between the two. There may have been only one event greater than about 42 Ma ago, the argon clock recording the subsequent cooling of the batholith rocks to muscovite and biotite blocking temperatures: the existing Rb-Sr data (above) and the older date given by the higher-level Satpura intrusion support this interpretation. 40-30-20:~ ~O-m ¢ ®
1
SK12b Biotite
30-'~
SK 2a Muscovite
t___ 1" 10
! 20
] 30
4
/
~
0
5
0
6
0 807
090
100
Cumulative% agAr Released
Fig. 4. Age spectra for SK 12b biotiteand SK 2a muscovite(as in Fig.3).
161
4. Tertiary clastics Petrographic and palaeocurrent data from the continental clastics overlying the Ladakh-Deosai batholith complex in Ladakh suggest that these clastics were deposited by ephemeral streams and large braided and meandering streams flowing eastwards and that these deposits were derived from the north, west and south, and filled a depression between the rising Ladakh-Deosai batholith complex to the north and the Zanskar zone and Indus suture zone to the south [20]. A northern autochthonous continental clastic belt unconformably overlies the Ladakh-Deosai batholith complex and consists of arkosic conglomerates passing upwards into finer-grained arkosic sandstones, siltstones and clays: these sediments represent an alluvial complex, derived from the north, interfingering with deposits of easterly flowing braided streams [20]. Only Permian to early Cretaceous fossils are represented in the limestone pebbles found [2]. At Kargil, these autochthonous clastics yielded Eocene non-marine faunas and are overlain by a thick braided stream/meandering stream complex, in which the current directions are predominantly easterly, and which contain Oligocene-Miocene non-marine faunas [20]. A southern parautochthonous belt, called the I-Iemis Conglomerate by Frank et al. [2], consists of thick arkosic conglomerates interbedded with arkosic sandstones, siltstones and minor clays; the whole forming a braided stream complex with current direction to the east and north [20]. The conglomerates contain clasts derived predominantly from the Dras volcanics and associated rocks of the Indus suture zone, with abundant acid to intermediate volcanic pebbles presumably derived from the volcanic equivalents of the Ladakh-Deosai batholith complex. The conglomerates also contain Permian to lower Middle Eocene limestone pebbles, derived from the Zanskar zone [2].
5. Discussion
The absence of Paleocene to early Eocene oceanic or arc-derived sediment or igneous activity and the presence of late Maastrichtian to early
Eocene (Ypresian) shelf limestones in the Zanskar zone [21,22] suggest a lack of tectonic activity on the northern edge of the Indian plate during this time period. The presence of late Eocene to Miocene continental clastics, with abundant igneous clasts from both the Dras Volcanics and Ladakh-Deosai batholith complex, and evidence for deposition in very large easterly flowing streams with both northerly and southerly sources, suggests a continental back-arc basin. Thus the Ladakh-Deosai batholith complex was apparently an Andean-type magmatic arc developed on the northern edge of the Indian shield during Eocene to Oligocene times (Fig. 2B). The results reported here have some wider implications regarding the evolution of the collision zone between India and Tibet. The late Cretaceous (Senonian, probably Campanian) age of emplacement of the Indus suture ophiolitic mrlanges is the same as that of the belt of ophiolite nappes and mrlanges running from Cyprus eastward, along the southern edge of the Tethys, to the Andaman Islands off Burma [23]. This ophiolite zone forms the inner ophiolite sub-belt of Strcklin [24]. The sc~lrcity or absence of contemporary volcanism associated with the late Cretaceous ophiolite emplacement and the existence of mid-Cretaceous island arc volcanics developed over steeply dipping subduction zones, suggests that the ophiolites were emplaced along predominantly transform faults, albeit with some thrust component [23]. The time of emplacement of the ophiolites closely follows a major change in plate motions in the Indian Ocean between 90 and 80 Ma [25,26], followed by rapid spreading during the Paleocene [27,28]. The absence of Paleocene to Lower Eocene tectonic or magmatic activity on the Indian plate during this rapid northward motion of India, and the presence of Paleocene to mid-Eocene shelf limestones in the Zanskar region [21], suggest that subduction was taken up along the southern Asian boundary; possibly in the Karakorum Mountains [29]. Spreading slowed in the Indian Ocean from early Eocene to mid-Oligocene times [27] with the development of the Ladakh-Deosai batholith complex, an Andeantype margin along the northern edge of the Indian plate. The model developed also implies that at least
162
in the northwest, the northern margin of India has not been significantly displaced southwards during the Himalayan orogeny, and that there has been little underthrusting of areas to the north by Indian continental crust.
Acknowledgements Financial support was provided by the Natural Sciences and Engineering Research Council of Canada and the National Geographic Society. Sample irradiations were performed in the McMaster University nuclear reactor.
References l P. Le Fort, Himalayas: the collided range. Present knowledge of the continental arc, Am. J. Sci. 275A (1975) 1-44. 2 W. Frank et al., Geological observations in the Ladakh area (Himalaya): a preliminary report, Schweiz. Mineral. Petrogr. Mitt. 57 (1977) 89-113. 3 C.P. Andrews-Speed and M.E. Brookfield, Middle Palaeozoic to Cenozoic geology and tectonic evolution of the northwestern Himalaya, Tectonophysics (in press). 4 C. Rossi Ronchetti, Molluscs from the Upper Cretaceous at Burji-La (Baltisan, Central Asia), R.iv. Ital. Paleontol. 73 (1967) 811-832. 5 V.D. Mamgain and B.R.J. Rao, Orbitolines from the limestone intercalations of Dras Volcanics, J. and K. State, J. Geol. Soc. India 6 (1965) 122- 129. 6 C. Klootwijk et al., The extent of Greater India, 2. Palaeomagnetic data from the Ladakh intrusives at Kargil, northwestern Himalayas, Earth Planet. Sci, Lett. 44 (1979) 47- 64. 7 S.K. Shah and M.L. Sharma, A preliminary report on the fauna in radiolarites of the ophiolite-mtlange zone around Mulbelda, Ladakh, Curr. Sci. 46 (1977) 817. 8 J.P. Bassoullet et al., Permien terminal neritique, Scythian pelagique et volcanisme sous-marin, indices de processus tectono-sedimentaire distensifs a la limite Permien-Trias dans tm bloc exotique de la suture de l'Indus (Himalaya du Ladakh), C.R. Acad. Sci. Paris, Ser. D, 287 (1978) 675-678. 9 G. Fuchs, On the geology of western Ladakh, Jahrb. Geol. Bundesanst. (Austria) 122 (1979) 513-540. 10 V.J. Gupta and S. Kumar, Geology of Ladakh, Lahoul and Spiti regions of Himalaya with special reference to the stratigraphic position of flysch deposits, Geol. Rundsch. 64 (1975) 540-563. 11 S.K. Shah et al., Stratigraphy and structure of the western part of the Indus suture belt, Ladakh, northwest India, Himalayan Geol. 6 (1976) 543-556. 12 J.-P. Bassoullet et al., D~couverte de Cr~tac~ sup~rieur
ca/caire ptlagique darts le Zanskar (Himalaya du Ladakh). Bull Soc. Gtol. Fr. 20 (1978) 961-964. 13 J.E. Van Hinte, A Cretaceous time scale, Am. Assoc. Pet. Geol. Bull. 60 (1976) 498-516. 14 M.Q. Jan and D.R.C. Kempe, The petrology of the basic and intermediate rocks of Upper Swat, Pakistan, Geol. Mag. 10 (1973) 285-300. 15 K.K. Sharma et al., Potassium-argon dating of Dras volcanics, Shyok volcanics and Ladakh granite, Ladakh, northwestern Himalaya, Himalayan Geol. 8 (1978) 288- 295. 16 M.N. Saxena and J.A. Miller, Metamorphism, magmatism and orogeny in the light of radiometric dates in northwestern Himalayas, Bull. Ind. Geol. Assoc. 5 (1972) 63-69. 17 N. Lal and K.K. Nagpaul, Fission track geochronology of some Himalayan rocks, Himalayan Geol. 5 (1975) 104-114. 18 P.K. Mehta, Tectonic significance of the young mineral dates and the rates of cooling and uplift in the Himalaya, Tectonophysics 62 (1980) 205- 217. 19 A. Desio et al., On the geological age of some granites of the Karakorum, Hindukush and Badakhshan (Central Asia), Proc. 22nd Int. Geol. Congr., 11 (1964) 479-496. 20 M.E. Brookfield and C.P. Andrews-Speed, Sedimentology of the Indus flysch and molasse (in preparation). 21 M. Gaetani et al., Uppermost Cretaceous and Paleocene in the Zanskar Range (Ladakh--Himalaya), Riv. Ital. Paleontol. 86 (1980) 127-166. 22 S. Metzeltin and A. Nicora, A new Eocene outcrop from Zanskar Range, Himalaya, Riv. Ital. Paleontol. 83 (1977) 803 - 824. 23 M.E. Brookfield, The emplacement of giant ophiolite nappes, Tectonophysics 37 (1977) 247-303. 24 1. St0ck/in, Structural correlation of the Alpine ranges between Iran and Central Asia, Soc. Geol. Fr., Mem. h.s. 8 (1977) 333-353. 25 B.D. Johnson et al., Spreading history of the eastern Indian Ocean and Greater India's northward flight from Antarctica and Australia, Geol. Soc. Am. Bull. 87 (1976) 1560-1566. 26 B.D. Johnson et al., Early spreading history of the Indian Ocean between India and Australia, Earth Planet. Sci. Lett. 47 (1980) 131-143. 27 C.McA. Powell, A speculative tectonic history of Pakistan and surroundings: some constraints from the Indian Ocean, in: Geodynamies of Pakistan, A. Farah and K.A. De Jong, eds. (Geological Survey of Pakistan, Quetta, 1979) 5-24. 28 D. McKenzie and J.G. Sclater, The evolution of the Indian Ocean since the late Cretaceous, Geophys. J. R. Astron. Soc. 25 (1971) 437-528. 29 A. Desio, Geologic evolution of the Karakorum, in: Geodynamics of Pakistan, A. Farah and K.A. De Jong, eds. (Geological Survey of Pakistan, Quetta, 1979) 111-124. 30 R.H. Steiger and E. Jager, Subcommission on Geochronology: convention on the use of decay constants in geo- and cosmochronology, Earth Planet. Sci. Lett. 36 (1977) 359362. 31 V. Stukas and P.H. Reynolds, 4°Ar/39Ar dating of the Long Range Dikes, Newfoundland, Earth Planet. Sci. Lett. 22 (1974) 256-266.
157
Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
[41
Late Cretaceous emplacement of the Indus suture zone ophiolitic mrlanges and an Eocene-Oligocene magmatic arc on the northern edge of the Indian plate M.E. Brookfield Department of Land Resource Science, Guelph University, Guelph, Ont. NI G 2 W1 (Canada)
and
P.H. Reynolds Department of Physics and Geology, Dalhousie University, Halifax, N.S. (Canada) Received February 18, 1981 Revised version received May 11, 1981
We report three 4°Ar/agAr dates (on stratigraphically located samples) of 82 ___6 Ma from a syenite cutting the Indus suture zone ophiolitic mrlange and about 39 and about 45 Ma from granodiorite intrusions north of the suture zone. Sedimentological observations indicate Eocene to Miocene deposition of coarse clastics by very large braided and meandering streams in a continental back-arc setting. These observations suggest that the ophiolitic mrlanges of the Indus suture zone were emplaced in the late Cretaceous, shortly after a major change in plate motions in the Indian Ocean: they further suggest that an Andean-type magmatic arc d~veloped on the northern edge of the Indian plate during the Eocene and Oligocene.
1. Introduction
In view of the recent interest in the Tertiary collision zone between India and Tibet and the rarity of stratigraphically located radiometric dates from the suture zone, we wish to summarize observations made by M.E.B. during five months of field study in 1978 and 1979, and report 4°Ar/39Ar dates on stratigraphically located samples. Though few, these dates, and sedimentological observations have a bearing on two critical factors in the interpretation of the India-Tibet collision zone. These factors are: (1) the age of emplacement of the Indus suture ophiolitic mrlanges and associated island arc volcanics and sediments, and (2) the age of the Ladakh-Deosai batholith complex to the north (see Fig. 1). Previous workers [1,2] have suggested that ophiolite emplacement took place in the early Ter-
tiary during collision of the Indian plate with a southward-facing late Cretaceous to early Tertiary island arc represented by the Indus suture volcanics (Dras volcanics) and the Ladakh-Deosai batholith complex (Fig. 2A). Our results suggest a late Cretaceous age for emplacement of the ophiolitic m~langes which occurred during collision of the Indian plate with a southward-facing middle Cretaceous island arc (represented by the Dras volcanics). This was followed, after a tectonically quiet period in the Paleocene to early Eocene, by the development of a northward-facing Andeantype magmatic arc on the northern edge of the Indian plate during the late Eocene and Oligocene (Ladakh-Deosai batholith complex) (Fig. 2B). Ophiolite emplacement thus took place in a lull between the activity of two separate magmatic arcs of differing polarity.
0012-821X/81/0000-0000/$02.50 © 1981 Elsevier Scientific Publishing Company
158
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,
1 •
'
OPHIOUTIC MELANGES
I O'FLO , MESOZO'+
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SED,MENTS
,~ ~ "-", | i~l TERTIARY CLASTICS
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IV SKARDUQ "'v:..,..
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IV
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•
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" ~ ' ° SP O N G T A N G ~ ' . ~ KLIPPE + ' "
" "
Fig. 1. Sketch m a p of northern Kashmir, showing tectonic zones and sample locations. I = Tibetan plateau; H = K a r a k o r n m batholith zone; llI=Shyok m61ange zone (shaded); I V = " K o h i s t a n sequence" zone (A = L a d a k h - D e o s a i batholith zone; B - - I n d u s suture zone); V = Zanskar zone. S = Satpura village, L = Lamayuru. Crosses mark radiometric dating sample locations, with ages and sample n u m b e r s close by.
2. I n d u s suture z o n e
East of Kargil, the Indus suture zone consists of Triassic Atlantic-type turbidites (Lamayuru flysch) derived from a stable continental margin, overlain by undated volcaniclastic turbidites which pass gradually upwards into basic to intermediate volcanics and volcaniclastic sediments [3], the Dras volcanics, of mid-Cretaceous (Barremian to Turonian) age [4,5]. The Dras volcanics form an island arc suite evolving from tholeiitic at the base to shoshonitic at the top indicating a steeply dipping subduction zone [6]. Ophiolitic m61ange zones occur below the volcanics and within the turbidite sequences [3]. These m61anges contain Permian shelf limestones, Triassic-Jurassic pelagic limestone and sandstone, and mid- to late Cretaceous
cherts [7,8]. There is no evidence in the Indus suture zone for late Cretaceous or early Tertiary volcanism. South of Lamayuru (Fig. 1), the carbonate sequence of the Zanskar zone is overlain by thrust sheets consisting of Mesozoic turbidites and volcanics, with ophiolitic m61anges and large exotic carbonate blocks; overlain by a hartzburgite thrust sheet [8,9]; the whole forming the Spongtang Klippe of Fuchs [9]. Relationships in the Spong Valley and westwards suggest that the thrust sheets were initially emplaced southwards onto shelf carbonates during the late Cretaceous ([2,10,11], M.E.B., unpublished data) in' postTuronian [4] probably post-upper Campanian [12], pre-upper Maastrichtian times. Apart from the hartzburgite thrust sheet, the Spongtang sequence
159
INDIA
DRASVOLCANICARC d~
Sea Level
1 .MID - CRETACEOUS
ALTERNATIVES 2. LATE CRETACEOUS
B DRASARC INACTIVE
A
EVOLVING.~L LADKH- DEOSAI tr~ ~ BATHOLITHCOMPLEX
LADAKH-DEOSAI .'~*%" ~ ' * ~ ~ B A T H O L ICOMPLEX ITH 3.
EOCENE
KARAKORUM ~ SHYOKMELANGE
KARAKORUM 4.
EARLYMIOCENE
Fig. 2. Alternative views of the Cretaceous-Tertiary evolution of the collision zone: (A) with continuously evolving magmatic arc (based on Frank et al. [2]); (B) with two arcs of different ages and polarity (based on Andrews-Speed and Brookfield [3]).
is identical to that of the Indus suture zone. Were the Indus suture units initially emplaced towards the end of the Cretaceous, as was the Spongtang Klippe, or during the early Tertiary? An undeformed hornblende syenite intrudes the metamorphosed ophiolitic m~lange and Dras volcanics at Kargil (Fig. 1) and is cut by the post-Miocene northward motion along the Kargil thrust. The generation of the syenite is possibly related to partial melting during emplacement of the m61ange and Dras volcanics. Hornblende from the syenite has yielded an age spectrum (Fig. 3) which has an apparent age plateau comprising about 90% of the total 39Ar released. The inferred age is 82 _ 6 Ma (the absolute uncertainty at the 95% confidence level). This Coniacian-Santonian [13] estimated age of emplacement is consistent with the pre-Maastrichtian age of emplacement of the Spongtang Klippe and with conventional K-Ar ages, of 67 Ma from an amphibolite from the
Kohistan ( = Dras and of 7 7 _ 1 M a type not specified) Chiktan nala, near
volcanic unit) in Pakistan [14] from "Dras volcanics" (rock from the ophiolitic m61ange a t Kargil, Ladakh [15].
m
P
LE 23b
I 26
~o ," Cumulative%
s"
6"
Hornblende
,"
8'o
'
90
I 100
39Ar Released
Fig. 3. 4°Ar/39Ar age spectrum for sample LE 23b hornblende. Half heights of black bars represent the 20 relative (between step) uncertainties: the small diagon',d slash separates two gas fractions. Decay and isotopic constants are those suggested by Steiger and J~iger[30]. Analytic procedures have been described elsewhere [31].
160 3. Ladakh-Deosai batholith
The Ladakh-Deosai batholith complex to the north of the Indus suture zone consists of tonalitic to granodioritic intrusives, with minor grabbroic phases, intruded into the mid-Cretaceous Dras volcanics and associated sediments and in places unconformably overlain by late Eocene to Miocene coarse continental elastics [2,10,11]. Few records of volcanic equivalents of the intrusions have been noted, except north of Kargil and around Satpura (Pakistan) (M.E.B., personal observations); but parautochthonous conglomerates west of Leh contain abundant rhyolitic and andesitic clasts presumably derived from volcanics now eroded away [21. Previously published radiometric dates on the Ladakh-Deosai batholith complex are: a conventional K-Ar date of 48.4 +-1.7 Ma from biotite gneiss near Kargil [16] and five fission track ages ranging from 15 to 43 Ma on "granites" from the Kargil area [17], though these have large errors [18]. Sharma et al. [15] reported K / A r dates of 27.8 __+0.6 Ma from a pink porphyritic granite, and 38 __+2 Ma from associated Shyok volcanics north of Leh, Ladakh. Desio et al. [19] have reported a Rb-Sr date of about 49.6 Ma (recalculated to X = 1.42 × 10-11 y r - 1 ) from a granodiorite from Satpura. Were the Ladakh-Deosai batholiths emplaced as a continuation of the Cretaceous Dras volcanic island arc [2] or were they emplaced during a separate younger phase of arc development? The lack of pre-Eocene ages above, and the additional dates below suggest the second alternative. Sample SK 2a is from a fresh dacite dyke intruding folded metasediments (amphibolite facies) of the Kohistan (= Dras volcanic unit) sequence at Satpura (see Fig. 1). The dacite is marginal to a late-stage, high-level granodiorite intrusion which has a thin contact aureole. The dacite and granodiorite post-date deformation and metamorphism of the Kohistan unit which probably occurred during the activity of the midCretaceous island arc and its later southward emplacement over the Indian shield. That the Kohistan sequence was already emplaced when the granodiorite was intruded is shown by the contrast
between the amphibolite facies metasediments and the fresh, high-level intrusion. Completely fresh muscovite extracted from the dacite has yielded a spectrum (Fig. 4) which in a strict sense does not have an apparent age plateau. However, the three steps in the middle of the spectrum comprising about 50% of the total 39Ar released and which have the lowest atmospheric correction define a plateau at an age of about 42 Ma. Sample SK 12b is from a slightly deformed hornblende biotite granodiorite intruding foliated orthogneiss at Khaplu (Fig. 1). Both units are cut by a swarm of basaltic dykes related to the emplacement of the Shyok m61ange to the north. Biotite from the granodiorite has yielded an age spectrum (Fig. 4) with a very well-defined plateau over greater than 90% of the gas released. The plateau age is about 39 Ma. While both samples from the Ladakh-Deosai batholith complex, have yielded essentially undisturbed age spectra, there does appear to be relatively small but nevertheless analytically distinguishable difference in apparent age between the two. There may have been only one event greater than about 42 Ma ago, the argon clock recording the subsequent cooling of the batholith rocks to muscovite and biotite blocking temperatures: the existing Rb-Sr data (above) and the older date given by the higher-level Satpura intrusion support this interpretation. 40-30-20:~ ~O-m ¢ ®
1
SK12b Biotite
30-'~
SK 2a Muscovite
t___ 1" 10
! 20
] 30
4
/
~
0
5
0
6
0 807
090
100
Cumulative% agAr Released
Fig. 4. Age spectra for SK 12b biotiteand SK 2a muscovite(as in Fig.3).
161
4. Tertiary clastics Petrographic and palaeocurrent data from the continental clastics overlying the Ladakh-Deosai batholith complex in Ladakh suggest that these clastics were deposited by ephemeral streams and large braided and meandering streams flowing eastwards and that these deposits were derived from the north, west and south, and filled a depression between the rising Ladakh-Deosai batholith complex to the north and the Zanskar zone and Indus suture zone to the south [20]. A northern autochthonous continental clastic belt unconformably overlies the Ladakh-Deosai batholith complex and consists of arkosic conglomerates passing upwards into finer-grained arkosic sandstones, siltstones and clays: these sediments represent an alluvial complex, derived from the north, interfingering with deposits of easterly flowing braided streams [20]. Only Permian to early Cretaceous fossils are represented in the limestone pebbles found [2]. At Kargil, these autochthonous clastics yielded Eocene non-marine faunas and are overlain by a thick braided stream/meandering stream complex, in which the current directions are predominantly easterly, and which contain Oligocene-Miocene non-marine faunas [20]. A southern parautochthonous belt, called the I-Iemis Conglomerate by Frank et al. [2], consists of thick arkosic conglomerates interbedded with arkosic sandstones, siltstones and minor clays; the whole forming a braided stream complex with current direction to the east and north [20]. The conglomerates contain clasts derived predominantly from the Dras volcanics and associated rocks of the Indus suture zone, with abundant acid to intermediate volcanic pebbles presumably derived from the volcanic equivalents of the Ladakh-Deosai batholith complex. The conglomerates also contain Permian to lower Middle Eocene limestone pebbles, derived from the Zanskar zone [2].
5. Discussion
The absence of Paleocene to early Eocene oceanic or arc-derived sediment or igneous activity and the presence of late Maastrichtian to early
Eocene (Ypresian) shelf limestones in the Zanskar zone [21,22] suggest a lack of tectonic activity on the northern edge of the Indian plate during this time period. The presence of late Eocene to Miocene continental clastics, with abundant igneous clasts from both the Dras Volcanics and Ladakh-Deosai batholith complex, and evidence for deposition in very large easterly flowing streams with both northerly and southerly sources, suggests a continental back-arc basin. Thus the Ladakh-Deosai batholith complex was apparently an Andean-type magmatic arc developed on the northern edge of the Indian shield during Eocene to Oligocene times (Fig. 2B). The results reported here have some wider implications regarding the evolution of the collision zone between India and Tibet. The late Cretaceous (Senonian, probably Campanian) age of emplacement of the Indus suture ophiolitic mrlanges is the same as that of the belt of ophiolite nappes and mrlanges running from Cyprus eastward, along the southern edge of the Tethys, to the Andaman Islands off Burma [23]. This ophiolite zone forms the inner ophiolite sub-belt of Strcklin [24]. The sc~lrcity or absence of contemporary volcanism associated with the late Cretaceous ophiolite emplacement and the existence of mid-Cretaceous island arc volcanics developed over steeply dipping subduction zones, suggests that the ophiolites were emplaced along predominantly transform faults, albeit with some thrust component [23]. The time of emplacement of the ophiolites closely follows a major change in plate motions in the Indian Ocean between 90 and 80 Ma [25,26], followed by rapid spreading during the Paleocene [27,28]. The absence of Paleocene to Lower Eocene tectonic or magmatic activity on the Indian plate during this rapid northward motion of India, and the presence of Paleocene to mid-Eocene shelf limestones in the Zanskar region [21], suggest that subduction was taken up along the southern Asian boundary; possibly in the Karakorum Mountains [29]. Spreading slowed in the Indian Ocean from early Eocene to mid-Oligocene times [27] with the development of the Ladakh-Deosai batholith complex, an Andeantype margin along the northern edge of the Indian plate. The model developed also implies that at least
162
in the northwest, the northern margin of India has not been significantly displaced southwards during the Himalayan orogeny, and that there has been little underthrusting of areas to the north by Indian continental crust.
Acknowledgements Financial support was provided by the Natural Sciences and Engineering Research Council of Canada and the National Geographic Society. Sample irradiations were performed in the McMaster University nuclear reactor.
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