SHORT COMMUNICATION
Biotite Rb-Sr
Ages:
Constraints on Exhumation of
the
Karakoram
Metamorphic Complex,
Eastern Ladakh
SANDEEP
SINGH*',B~SWAJAYEE
A.
PATRA',A.R.
V I J A N ~and A.K.
JAIN' 'Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee - 247 6672~eochronology Division, KDMIPE, ONGC, Kaulagarh Road, Dehradun - 248 001
Email: [email protected]
Abstract: Biotite and K-feldspar Rb-Sr ages from three granitoid intrusives withln the Karakoram Metarnorph~c Belt
(KMB), eastern Ladakh yield indistinguishable ages between 12-10 Ma. Since the closure temperature of biotite for Sr diffusion is 30&50° C, we interpret these ages due to exhumation at 10 to 7 km depth within the chst. This implies that
the Pangong Metamorphic Belt of the KMB, located at the southern termmation of the Eurasian Plate, was undergoing a fast exhumation in a transpressional regime during Mlddle Miocene. These data also indicates that the southern edge of the Eurasian Plate has exhumed rapidly in comparison to the Higher Himalayan Crystallines within the Indian Plate segment.
Keywords: Rb-ST ages, Granitoids, Karakoram complex, Eastern Ladakh.
The application of thermochronology is a key tool to constrain the tectonic processes and the temporal evolution of the Himalayan orogen. Long-term denudation processes in the NW Himalaya might be related to exhumation, in its true sense, which describes the return of deep-seated metamorphics to present day Earth's surface by various processes like tectonics, erosion or combination of both (Stuwe and Barr, 1998; Ring et al. 1999; Theide et al. 2004).
For a better insight into timing of rock uplift and/or unroofing of the Himalaya, radiometric data are significant because they provide the temporal constraint of the cooling history of the terrain. Rb-Sr mineral ages have often been used to constrain the time-marker based on its closure temperature, the temperature range below which a mineral becomes closed to parent-daughter migration. Various minerals have different closure temperatures in different radiometric system (Dodson, 1973; Wagner et al. 1977; Harrison and McDougall, 1980; Dodson and McClellad- Brown, 1985). The closure (blocking) temperature of biotite for Rb-Sr is 300*50° C (Dodson, 1973; Dodson and McClellad-Brown, 1985). This depends upon isotopic exchange ( 8 7 ~ r diffusion coefficient) and chemical exchange (Rb and Sr diffusion coefficient and their partition coeficjents) of Rb and Sr isotopes in the minerals with their surroundings.
The junction between the Indian and Eurasian Plates is
represented by two significant sutures in the Trans-Himalaya, namely the Indus Tsangpo Suture Zone (ITSZ) and the Shyok Suture Zone (SSZ). To the north of Shyok Suture Zone are the Karakoram Mountains - a rapidly exhumed terrain as a consequence of the collision tectonics of the southern edge of the Eurasian Plate. The southern part of the Eurasian Plate has also been subjected to deformation, metamorphism, melting, and widespread fluid infiltration with rapid and pronounced exhumation due to colIisional tectonics (Jain et al. 2002,2003).
In this work, we report new biotite Rb-Sr ages from three granitoid bodies from Karakoram Mountains, eastern Ladakh and interpret them in terms of exhumation and unroofing history of southern edge of the Eurasian Plate.
To the north of the SSZ, the Karakoram Tectonic Unit, contains the Karakoram Shear Zone (KSZ), the Karakoram Batholithic Complex (KBC) and the Karakorarn Metamorphic Complex (KMC) (Fig. 1 ; Jain et al. 2003).
The KSZ is a narrow, about 2-5 krn wide and 200 krn
long belt of mylonitized and imbricated sequence of granite gneisses, volcanics, conglomerates, slate-phyllite, limestone and serpentinite. These rocks are intensely mylonitized along the NubraShyok Valleys (Jajn et aI. 2003 and references therein). Further southeast, this sequence contains highly m yloni tized granite gneiss and amphiboli te; along the Tangtse-Pangong-Chushul sector, the mylonites being
28 SHORT COMMUNICATION
derived from the granitoids of the K M B and the granodiorite-diorite suite of rocks of the SSZ. Mylonite is best exposed along the Tangtse Valley and intruded by relatively less deformed muscovite hornblende-bearing granitoids. A zircon U-Pb SHRIMP age of 18.0+0.6 Ma (Searle et al. 1998) from these intrusives suggest a minimum age for ductile shearing within the KSZ. The granites also have older zircon components with U-Pb SHRIMP age of 63.020.8 Ma.
The KBC is composed of biotite-muscovite granite and does not appear to extend beyond the Y-shaped bend in the Shyok Valley as a monolithic mass (Gregan and Pant, 1983; Jain et al. 2003). Two distinct generations of granites with I-type and S-type of affinities have been identified in the KBC (Srimal, 1986; Srimal et al. 1987). Along the southern margin, the batholith is thr6st onto the KSZ rocks (the Nubra Formation - Weinberg et al. 2000). The crystallization age of this batholith ranges between 130 and SO Ma (Srimal et al. 1987; Searle, 1991; Debon and Khan, 1996; Trivedi et al. 1997; Weinberg and Searle, 1998) with some phases as young as 25 to 17 Ma (Parrish and Tirrul, 1989; Searle et al. 1998).
The KMC has been subdivided into two distinct metamorphic belts in Shyok-Pangong Mountain and
Phobrang area of the Karakorarn Mountains, and represents Fig.1. Geological m a p of Trans-Himalaya arid Karakorarn Mountains, Tangtse-Pangong Tsso rcgion, KRC: Karakorarn the deformed and extensively metamorphosed leading edge
Batholithic Complex ( u f i ~ r Sain et al. 2003). of the overriding Eurasian Plate. From south to north, these
have been classified as the Tangtse Group and Pangong Group (Jain et al. 2003). The Tangtse Group is exposed as the outer metamorphic belt in the immediate vicinity of the KSZ and is characterized by high grade sillimanite-bearing garnetiferous schist and gneiss, amphibolite, hornblende granite gneiss and leucogranite (Fig. 1). Mica schist and gneiss have undergone prolific migrnatization and melting, which has led to the generation of insitu injection granite, called as the Pangong Injection Complex (Weinberg and Searle, 1998). Numerous dome-shaped and elongated lenses of these granitoids reveal passive melt injection in the country rocks which are extensively migmatized. Searle et al. (1998) obtained a U-Pb zircon SHRIMP crystallization age between 20 and 17 Ma for the leucogranite. Weinberg et al. (2000) also dated the Karakoram leucogranite near Sati in the Shyok-Nubra sector and obtained U-Pb zircon rim SHRIMP age of 15.0k0.4 Ma with zircon cores showing ages from 1437 to 84 Ma. However, Phillips et al. (2004) used the same sample along with four others and generated U-Pb ID-TIMS data from five shear zone samples between
15.68-cO.52 and 13.73k0.28 Ma.
Further eastwards, an inner belt of the KMC, known as the Pangong Group, comprises mainly of phyllite, marble,
Chibrn diorilcgnnodioritr
Volcanics- gahhro
greenschist/amphiboli te, different varieties of mica schist, calc-silicate and a mylonite granite gneiss band (Jain et al, 2003 and references therein), This belt has been variously classified as the ophiolite suite belonging to the Pangong suture zone (Searle, l996), Pangong Tso Group (Thakur, 1993), or Karakoram Schist (Dunlap and Wysoczanki, 2002): Within this belt, metamorphism varies from biotite grade of middle greenschist facies to siilimanite-muscovite grade of middle amphibolite facies. Mylonitized granite gneiss, demarcating the northern splay of the KSZ (cJ, Searle et al. 1998), separates this sequence from the outer Tangtse Group (Jain et al. 2003).
Samples for the Rb-Sr thermochronology were collected from the outer Tangtse Group metamorphics containing the migmatite sequence and the Darbuk Granite (Fig.1). Unweathered samples were crushed, sieved and separated following standard heavy liquid and magnetic separation. Pure biotite and feldspar grains were hand-picked under it
binocular microscope. Rb and Sr were collected following standard cation-exchange method. Rb and Sr isotopic analyses were carried out on VG 354 Thermal Ionization
Massspectn~nit*tl-y at KDMIPE, ONGC. A mixed " ~ b - ~ ~ ~ r
SHORT COMMUNICATlON 29
spike was used for isotopic dilutxon measurements. The Table 1. Rb and Sr analyttcal data of whole rock and mineral separates standard SRM 987 gave a mean 87Sr/8%r ratlo of from sample SY 25/65, Karakoram Metamorph~c Complex 0.71022-~0.00006 (20) over the duration of this work. The sj sample Rb Sr s 7 ~ b / 8 6 ~ r * 87Sr/86Sr~2a" * age of each sample was calculated from the regression of " C P P ~ ) ( P P ~ ) (atomc) (atom~c) whole rock, biotite and feldspardata, follow~ng the two error I Whole Rock 109 26 34 25 9 258 0 7237l+O 00019 regression of Williamson (1968) uslng the complete 2 Feldspar 8998 7840 3326 0723I2+000014 algorithm of the Provost (1990) with errors on age and 3 Rlotlte 80 1 77 I 5 97 145 792 0 74373kO 00028 inrt~al 8 7 ~ r / 8 6 ~ r ratios are quoted at 2 0 level. The errors on Age: 10.21k0.29 Ma 8 7 ~ b / 8 % r ratios are taken at 2%. * error +2 % ,
**
2 standard devtatlon of meanThe sample SY 25/65 (34" 2.319'
N
: 78" 12 920' E) represents a leucocrat~c gran~te as a part of rnigrnatite and ~njection complex wlthin the Tangtse Group metarnorphics. Rb and S,r ~sotoplc analytical data of the whole rock and minerals are glven In Table 1. The sample yields a regresslonllnecorrespond~ng to an age of 10.21~0 29 M a with an initial 8 ' ~ d 8 6 ~ r ratlo of 0.7225k0.0001 (F1g.2). The MSWD for this regression IS 1.24.
SY 25/65
a74 L
z
;i07-35,C
Age = 10 2t i 029 Ma ST, 0 7225t 0 0001 (20)
an %
o s o a ~ o x n m m i m
''R~?'s r
Fig.2. Whole rock
-
K-Feldspar - biotite regress~on line for leucocratic grankte sample.Table 2. Rb and Sr analytical data of whole rock and m~neral separates from sample SY 26/66, Karakoram Metamorph~c Complex
Sl Sample Rb Sr 87~b/86Sr" 87Sr/86Srk2a** No No ( P P ~ ) ( P P ~ ) (atomc) (atom~c) I Whole Rock 159 28 103 95 4 434 0 70859+0 00007 2 Feldspar 123 44 449 22 0 795 0 70809k0 0001 4 3 Blotlte 90040 25 24 103 358 072378+000029
Age: 10.77k0.30 Ma
* error +2 % ,** 2 standard dev~atron of mean
Table 3. Rb and Sr analyt~cal data of whole rock and mineral separates from sample SY 461122 of the Darbuk Gran~te, Karakoram Metamorph~c Complex
S1 Sample Rb Sr 8 7 ~ b / 8 6 ~ r ' 8 7 ~ r / 8 6 ~ r & 2 a S z No No (ppm) (ppm) (atormc) ' (atom~c)
1 Whole Rock 77 45 116 65 1 922 0 71 27520 0001 I 2 Feldspar I46 72 400 97 1 059 0 7 123 1+0 00024 3 B~otlte 1071 98 44 82 69 319 0 72408k0 00032 Age: 11,90+0.48 M a * error 42 % ,
**
2 standard devtatlon of meanThe sample SY 26/66 (34" 2.13' N and 78" 12.676' E) is with an initial 8 7 ~ r / 8 6 ~ r ratio of 0.7080+0.0001 (MSWD a part of leucocratic biotite granite in this belt, and is about =0.29) (Fig.3). This age is in concordance wlth the age of 100 rn away from SY 25/65. This body also forms a part of leucocratic granite within the error
the cupola rnjection withrn the rnigrnatlte body. Rb and Sr The third sample SY 46/122 (34'7.858' N : 78O6.559' E) isotopic analyticaj data are presented in Table 2 and define has been collected from the Darbuk Granite, which could a regression line corresponding to an age of 10.77k0.30 Ma be a product of rnlgrnatization of the Pangong metamorphic
SY 26/66
073
;j on-
Age =1077t030Ma
81 = 0 7080 f 0 0001 (20)
a705 A
iD 40 83 eo 10J la
8 7blB6sr ~
Fig.3. Whole rock - K-Feldspar
-
brotlte regresslon llne for leucocrat~c blot1 te granxte sample./
Age =1190f048MaSrl = 0 7124 * 0 0001 (2a)
I
a71 1 I
0 1 0 2 D 3 D 4 0 f 1 3 8 3 7 0 8 3
"R bP6sr
Fig.4. Whole rock - K-Feldspar- biotite regression Ilne for Darbuk Granite sample.
30 SHORT COMMUNICATlON complex and presently lylng as a stock with intrus~ve contact
wlth both Pangong Metamorphlc Complex and Shyok
Tectonic Zone The Rb and Sr lsotoplc data are presented In Table 3 The segresslon of the data yield an ~sochron age of 11 90k1 28 M a w ~ t h a n i n ~ t l a l 8 7 ~ r / 8 6 ~ r ratio of
0 7 12420 0001 (MSWD = 1 28) (Fig 4)
The results from all the three samples are concordant wrthln the error and rndicate that the system rerna~ned closed w ~ t h respect to Rb and Sr since the closure temperature of biotr te The cooling and exhumation rates can be calculated based on the closure temperature 30&50° C for biotlte and present day surface temperature 10+,5O C Uslng these data the coolrng rate for sample S Y 25/65 IS 28 4*C/Ma, for SY 26/66 ~t IS 26 g°C/Ma whrch IS within error 11m1t of each other, and for the Darbuk Granrte it rs 24 4"C/Ma However, the exhumation rates appears to be 0 9;0 4 m d y r , 0 940 3 mm/yr and 0 820 3 mrnlyr for SY 25/65, SY 26/66 and Darbuk body for 3O0C/km geotherm respectively wlth average exhumatron of about 0 9 mm/yr However, ~t turns out to be about 0 7 m d y r for 40°C/km geotherm
The data from Rb-Sr Wr-Feld-BIO also indlcate that the Pangong Injectron Complex body was between about 10-7 krn at around 11 M a during magma ascent in a transpressional regime However, Ar-Ar data by Searle
rocks The emplacement mechan~sm can be attributed to happen w~thrn the d u c t ~ l e deformat~on field In response to VISCOUS injectron and the growth of cupola (McLellan, 1984, Rubin, 1993, Brown, 1994, Wanberg, 1996) The cupola structure also indicates that the dlaplrlsm of felslc magma and sohdification was s~multaneous wlthin the transpressional reglrne
The present data indrcate that the rocks of southern Euraslan Plate exhumed very fast from a depth of 10-7 km around 12- 10 Ma to the present level @ 0 9 - 0 7 rnm/yr However, t h e d a t a p u b l ~ s h e d from the Hlrnalayan Metamorphlc Belt of the Indtan Plate sector lndlcate older WR - biotite ages e g Mandr Gramte - 18 to 20 M a (Jager e t a1 197 1, Mehta, 1977), Rohatang Metamorph~cs
- 17
Ma ( M e h t a , 1977), J u t o g h Metarnorphics a t Kulu - 13 5 Ma (Mehta, 1977), Zanskar Shear Zone - 15 4 to 16 3 Ma (Inger, 1998) and somewhat slower exhumat~on ratesWith this new data it appears that the Karakoram Metamorphic Complex and rts assoc~ated d r a p ~ r ~ c magmatic bod~es have been affected by transpresslonal tecton~cs w~thln the Karakoram Shear Zone at a very late stage of the coll~slon tectonics and raprd exhumat~on around 12-10 Ma
et a1 (1998) on amphibole from nearby yleld a plateau age
13 8+0 1 Ma lndlcatlng the tlmlng of coollng through work was funded the
of Sclence and Technology (DST) of India under its HlMPROBE
-
5000C between 17-15 km The Of transport of program We are grateful to the Director. KDMIPE, Dehradun forfelslc magma ls ln d'ap'rlc manner and Sear'e9 extend~ng the analytlcal facll~ty RM Man~ckavasagam and lgg8) structure the Tangtse gorge A K Choudhary are thanked for helpful drscuss~ons at vanous T h e C P P O ~ ~ structure 1s due to Pervasive lntruslon of stages Comments made by K Gopalan, Rasrnus Theide and two
leucocratlc granrte magma Into hlgh-grade and migrnatitlc anonymous reviewers have vastly unproved th~s rnanuscnpt
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(Received 29 September 2004, Revrsed form accepted 23 September 200.5)
