Sm–Nd age of the Garhwal–Bhowali volcanics, westernHimalayas: vestiges of the Late Archaean Rampur
flood basalt Province of the northern Indian Craton
M.I. Bhat a,*, S. Claesson b, A.K. Dubey a, Kanchan Pande ca Wadia Institute of Himalayan Geology, 33 G M S Road, Dehra Dun 248 001, India
b Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50007, Stockholm, S-104-5, Swedenc Physical Research Laboratory, Ahmadabad 390 009, India
Received 21 May 1997; received in revised form 1 December 1997; accepted 1 December 1997
Keywords: Garhwal volcanics; Late Archaean flood basalt province; Northern Indian Craton; Sm–Nd age
1. Introduction north and the Main Boundary Thrust in the south(Fig. 1). The predominantly unfossiliferous nature
A major hurdle usually faced in the geological of the rocks together with the scacity of thestudies of the Himalayas is the lack of age control radiometric age data has always hampered reliableon most of its rock units. The situation is particu- stratigraphy and a correlation scheme for thelarly problematic for the east–west litho-tectonic Lesser Himalayas. In order to help at least partiallysegment called the Lesser Himalayas, tectonically resolve this problem, Bhat and Le Fort (1992)demarcated by the Main Central Thrust in the suggested dating the mafic volcanic rocks, which
occur as interbedded flows within metasedimentaryrocks. The volcanic rocks crop out along two
* Corresponding author. roughly linear belts and are a persistent and conpi-
218 M.I. Bhat et al. / Precambrian Research 87 (1998) 217–231
Fig. 1. Major locations of mafic volcanics in the Himalayas. Base map after Gansser (1964). Enclosed area is shown enlarged in Fig. 2.
cuous feature particularly of the western part of nearby Mandi volcanics (Fig. 1) are coeval withthe Rampur volcanics. More recently, Bhat andthe Lesser Himalayas (Fig. 1). Along with their
associated meta-sediments, these volcano-sedi- Claesson (1995) reported a whole rock Sm–Ndage of 1.53±0.04 Ga for another volcanic occur-mentary sequences have been given various names
in different areas. Thus the Rampur volcanics, rence, the Bafliaz volcanics exposed farther westin the Kashmir Himalayas (Fig. 1). It may beexposed in the Kulu–Rampur tectonic window,
are associated with the Banjar Formation, the noted that the Late Palaeozoic–Early MesozoicPanjal Traps of the Kashmir Himalayas are theMandi–Darla volcanics (hereafter referred to
simply as Mandi volcanics) with the Sundarnagar only volcanics in the whole Himalayan belt whoselower and upper age limits are well defined by theFormation, the Garhwal volcanics (also known as
Berinag-, Karanpryag volcanics) with the Berinag underlying and overlying fossiliferous sedimentarybeds. As the age results are very different from theFormation and the Bhowali volcanics with the
Nagthat Formation. Because of the lack of age presumed ages of these volcanics and associatedsedimentary rocks, they have consequently inducedcontrol, these volcano-sedimentary formations
have been assigned different ages in the available the reassessment of the correlation and tectonicschemes for the region (Thakur, 1992; Valdiya,correlation schemes (Valdiya, 1980, 1995; Virdi,
1988; Thakur, 1992). Bhat and Le Fort (1992) 1995; Virdi, in press).In this contribution the whole rock Sm–Ndmade a start at dating these mafic volcanics, report-
ing a Late Archaean (2.51±0.09 Ga) whole rock isotope data on the Garhwal and Bhowali volcan-ics, western Lesser Himalayas is reported (Fig. 1).Sm–Nd age and an e(T)Nd value of +5.3±1.7 for the
Rampur volcanics (Fig. 1). This date suggested a The Garhwal volcanics, though generally consid-ered to be coeval with the Rampur volcanics, arevery old rifting and sedimentation history of the
Himalayas comparable to that of the Aravalli belt yet to be dated and their age relationship with theBhowali volcanics remains uncertain. Their rela-of the Indian Peninsular Shield (cf. Sugden et al.,
1990; Roy, 1990; Verma and Greiling, 1995). tionship and tectonic significance will also bediscussed.Later, Bhat and Le Fort (1993) argued that the
219M.I. Bhat et al. / Precambrian Research 87 (1998) 217–231
2. Geological background the rocks to the west in Kumaun region ( Valdiya,1986). This will help the present discussion aboutthe possible eastward extent of the volcanics.As in other parts of the Himalayas, different
geological maps of the Kumaun Lesser Himalayas The Berinag Formation (Garhwal volcanicsincluded) is dominantly comprised of coarsepresent a different litho-tectonic and correlation
framework with different group/formation names. grained quartzite and interbedded mafic volcanics.The volcano-sedimentary succession is consideredThe map and description published by Valdiya
(1980) was opted for because of the detailed, if to be a tectonic allochthon in thrust contact withdifferent litho-tectonic units both at its top andnot entirely undisputed, description of lithological
units, their tectonic relations and correlation with base (Fig. 2). Valdiya (1980) mapped a separatelitho-tectonic unit, the shale dominated Rautgaraother lithological units. Moreover, Valdiya also
published a geological map of the Nepal Lesser Formation (upper part of the Damtha Group),under-thrust by the Berinag Formation (Fig. 2),Himalayas and gave lithological correlation with
Fig. 2. Geological map of the Kumaun Himalayas (Valdiya, 1980). Most of the samples analysed for Sm–Nd isotopic compositioncome from the enclosed area (rectangle), shown enlarged in the inset. The three solid circles outside the rectangle (one to the rightbetween Talwari and Gwaldam and two towards the lower right side near Bhowali) are the locations of three samples collectedoutside the main area of sampling and not shown in the inset. R, Ratighat.
220 M.I. Bhat et al. / Precambrian Research 87 (1998) 217–231
whereas others (Srivastava and Ahmad, 1979; to the west (Ahmad and Bhat, 1987; Bhat and LeFort, 1992).Thakur, 1992) consider the two as a single strati-
graphic unit, the Berinag Formation. In the latter A detailed account of the geochemistry of theGarhwal volcanics and associated dykes fromcase, the two formations would seem to indicate
an upsection depth-controlled lithological varia- Karanprayag and surrounding area (Fig. 2) waspublished by Ahmad and Tarney (1991). Like thetion from dominantly argillaceous sediments and
volcanic rocks to dominantly quartzite and volcan- Rampur- and Mandi volcanics, the Garhwal vol-canics have a limited compositional range, cluster-ics, and the isotopic composition of the volcanics
in association with argillaceous sediments and ing across Mg-tholeiite–Fe-tholeiite boundary in acation % Al–(Fe+Ti)–Mg diagram (Jensen,quartzites should then be similar.
In the southern part of the Kumaun Lesser 1976). They are typical low-Ti tholeiites with cross-ing rare earth element (REE) patterns andHimalayas, Valdiya (1980) correlated the volcano-
sedimentary Nagthat Formation (Fig. 2) with the enriched incompatible element patterns relativeto primordial mantle and with characteristicBerinag Formation. However, pointing to the neg-
ligible presence of volcanics in the Nagthat (Nb/La)N≤1 (Fig. 3a). Their incompatible ele-ment patterns (typically characterized by troughsFormation west of Nainital, Virdi (1988) considers
it unlikely to be a part of the more voluminous at Nb, Sr, P and Ti) match those of low-Titholeiites of any age and geographic distributionvolcanics-bearing Berinag Formation, and places
it at a much younger stratigraphic level relative to (Fig. 3b). While fractional crystallization is sug-gested to have played a role in the petrogenesis ofthe latter. East of Nainital he considers it as
Berinag Formation (s.s.); the two having come in these lavas, the major control on bulk chemistryis inferred to be by variable degrees of melting ofcontact along a fault (F–F: Fig. 2). Inherent in
this argument is the unlikely assumption that an enriched source. Of significance to the presentstudy, particularly for interpretation of the Sm–Nderuption and spread of lava should be uniform
throughout. Obviously, as was the case earlier with data in terms of the crystallization age of thevolcanics are:the Rampur volcanics vis-a-vis the Mandi volcan-
ics, controversy about the age relationship of the (1) the crossing REE patterns, which do notfavour two component mixing origin for theGarhwal volcanics with the Bhowali volcanics
remains. volcanics; and(2) the conclusion of Ahmad and Tarney (1991)
that despite strong ‘continental signature’reflected by decoupling between large-ion-3. Sampling, analytical techniques and isotopic
data lithophile elements (LILE) and high-field-strength elements (HFSE), that is, negativeNb, Sr, P, Eu and Ti anomalies, in theirThe Garhwal volcanics show variable effects of
metamorphism, with individual flows having devel- incompatible element patterns, detailed petro-genetic considerations rule out assimilation ofoped schistose margins but have retained coarse
centres where the igneous mineralogy and textures crustal material by the Garhwal volcanics.They reach this conclusion mainly on the basis ofare largely preserved. Plagioclase and clinopyro-
xene, the two primary phases, are partially replaced two considerations. First, the magnitude of thenegative Nb anomaly in the volcanics is too lowby secondary minerals, which are mostly albite,
amphibole, chlorite and epidote. Quartz and calcite (Nb/La and Nb/Ce average at 0.36 and 0.19,respectively) not only relative to primordial mantleoccur usually as vesicle infillings. Compared with
the Garhwal volcanics, the Bhowali volcanics are [1.04 and 0.4: Sun and McDonough (1989)] butalso for any realistic crustal component (for lowerless metamorphosed and with a rather poorly
developed schistosity. This N–S variation in the crust: Nb/La and Nb/Ce=0.54 and 0.26 and forupper crust 0.83 and 0.39, respectively) to havedegree of metamorphism is similar to the variation
seen in Rampur- versus Mandi volcanics occurring acted as an end member in any magma-crust
221M.I. Bhat et al. / Precambrian Research 87 (1998) 217–231
Fig. 3. (a) Primordial mantle (PM) normalized incompatible element patterns for Himalayan low-Ti tholeiites. Data source: $,Rampur volcanics, Bhat and Le Fort (1992); #, Mandi volcanics, Ahmad and Bhat (1987);6, Bhowali volcanics, Bhat and Ahmad(1987); ×, Garhwal volcanics, Ahmad and Tarney, 1991. PM values from Sun and McDonough (1989). (b) PM normalizedincompatible element patterns for worldwide low-Ti tholeiites of different ages shown for comparison with the Himalayan low-Titholeiites (a). The patterns are marked by troughs at Nb, Sr, P and Ti, thus emphasizing close similarity in chemical compositionwith the Himalayan volcanics (a). Data source: Late Archaean flood basalts from Pilbara Craton (n) and Kaapvaal Craton ($),Nelson et al. (1992); Late Archaean Aravalli mafic volcanics, India (&), Ahmad and Tarney (1994); Late Proterozoic volcanics,New Brunswick, Canada (+), Dostal and McCutcheon (1990); Proterozoic Scourie dolerites (#), Weaver and Tarney (1981); Jurassicdolerites, Tasmania (+), Hergt et al. (1989); and Diabase (W1), Virginia, USA (×), Govindaraju (1989). PM values from Sun andMcDonough (1989).
mixing. Even the Archaean crust, though with a these authors show that Ce–Nd data for the vol-canics plot along a linear trend which passesvery low negative Nb anomaly [Nb/La=0.44 and
Nb/Ce=0.22: Weaver and Tarney (1985)] does not through the origin and, following Horan et al.(1987), argue that the trend reflects the sourceprovide any helpful contaminant as its more fusible
silicic components have, in contrast to the Garhwal ratio and therefore uncontaminated nature of themagma. However, while theoretically thesevolcanics, positive Eu and Sr anomalies. Second,
222 M.I. Bhat et al. / Precambrian Research 87 (1998) 217–231
arguments may be valid for bulk assimilation, from the Bhowali volcanics near Bhowali. All thesesamples are from flows and were collected alongthey may not hold if the assimilation was selective.
For instance, a LREE-bearing fluid derived from road sections. In addition, one sample (G2) comesfrom a mafic body in the Higher Himalayancrustal rocks with highly non-radiogenic
Nd-isotopic composition could significantly affect Crystalline nappe exposed between Tilwari andGwaldam (Fig. 2). Because of partly unexposedthe Nd isotopic composition of a mantle derived
magma without leaving any significant effect on and partly disturbed contact relations, there isuncertainty whether this body is intrusiveits bulk elemental chemistry. Nevertheless, using
combined isotope–chemical relations, Bhat and Le (dyke/sill ) or extrusive (flow). Furthermore, inview of the greater possibility of magma-rock/fluidFort (1992, 1993) also concluded that there was a
general absence of a crustal component in the interaction in plutonic conditions, dyke sampleswere not included in the study.Rampur- and Mandi low-Ti tholeiites, although a
minor crustal influence in some Mandi volcanics The Sm–Nd isotope analyses were done at theLaboratory for Isotope Geology, Swedish Museumwas possible. To explain the paradox of a highly
positive initial eNd and a progressive develop- of Natural History, Stockholm. The analyticalprocedures essentially followed the standard tech-ment of a ‘crustal signature’ upsection in the
Rampur–Mandi volcanic flow sequence, Bhat and niques described in Claesson (1987), with theexception that Nd isotopic compositions were ana-Le Fort (1992, 1993) favoured a progressive
re-enrichment of asthenospheric source contempo- lysed on spiked samples. The analyses were per-formed in a static mode on a Finnigan MAT 261raneous with melt extraction.
Given the background of available geochemical multicollector mass spectrometer. During thecourse of this study, measurements of a La Jollastudy, of the 14 samples analysed in the present
study, ten (G5, G8, G12, G13, GV2, GV4, GV11, Nd standard showed that the 143Nd/144Nd ratiosmeasured on this instrument were 195 ppm tooGV16B, GV18E and GV20E) were collected from
Karanpryag area (Fig. 2). One sample (A3) was low. Based on a combination of the authors’previous results from the same instrument and acollected from the Rautgara Formation at
Adibadri, and two samples (BH1 and BH2) are compilation of published values from other
Table 1Sm–Nd isotopic data for the Garhwal–Bhowali volcanics
Sample No. Sm (ppm) Nd (ppm) 147Sm/144Nda 143Nd/144Ndb,c
aUncertainity in 147Sm/144Nd measurements=0.5%.b143Nd/144Nd normalized to 146Nd/144Nd=0.7219 and corrected for a bias of 195 ppm, which has been added to the normalized values.cErrors quoted are within-run internal precision (2sm). However, for isochron calculation, reproducibility is estimated to 0.000030(i.e. ~0.006%). This is a conservative estimate to accommodate possible extra uncertainity introduced by the bias correction.
223M.I. Bhat et al. / Precambrian Research 87 (1998) 217–231
laboratories, including the original values by constant. The reason for this instrumental bias ismost likely to be an aging effect in the FaradayLugmair and Carlson (1978), the best estimate of
the 143Nd/144Nd ratio for the La Jolla Nd standard cups. The isotopic ratios given in Table 1 havebeen corrected for this bias. Replicate analyses ofis 0.511854, while a series of 20 measurements of
the La Jolla standard during this study gave a any of the Garhwal volcanic samples were notmade. However, replicate analyses of a samplevalue of 0.511754±7. The frequent analyses of the
standard demonstrated that the bias remained (AV74) from Abor volcanics, eastern Himalayas
Fig. 4. (a) Sm–Nd conventional isochron diagram for Garhwal volcanics. Sample locations are shown on Fig. 2. Samples fromKaranpryag area ($) only are included in the regression (see text and Table 2). Regression analyses of the data were carried out asper the method of York (1969); the precision in e(T)Nd calculated following the procedure of Fletcher and Rosman (1982), and theerrors quoted for age and initial eNd are 2s. #, Sample from Adibadri; ×, sample collected near Gwaldam; ), samples from Bhowaliarea. (b) e(T)Nd versus time trajectories for Garhwal–Bhowali volcanics. Also shown for comparison is the evolutionary path for depletedmantle defined by Nd-isotopic data whose average upper and lower values lead to present day eNd values of +11 and +9, respectively(cf Jahn, 1990).
224 M.I. Bhat et al. / Precambrian Research 87 (1998) 217–231
Table 2Regression results of Garhwal volcanics data in different sample combinations
Samples used in regression Points in regression Age (2s) (Ma) e(T)aNd MSW
aFor calculation of e(T)Nd , 147Sm/144Nd and 143Nd/144Nd of CHUR are 0.1967 and 0.512638, respectively. The precision in e(T)Nd valueshave been calculated following the procedure of Fletcher and Rosman (1982).
(Fig. 1) analysed along with the Garhwal samples Mandi volcanics have been noted (Fig. 3a).gave the following values: 147Sm/144Nd=0.1928 Chemical similarity alone or geographic proximityand 0.1925 and 143Nd/144Nd=0.512513±7 and cannot, however, be taken as firm evidence for0.512514±6 (Bhat and Claesson, unpublished their coeval relationship as low-Ti tholeiites showdata). a close chemical similarity irrespective of their age
Table 1 lists the whole rock Sm–Nd isotope data and spatial distribution (Fig. 3b). It is onlyfor the analysed samples, and Fig. 4a gives a through an independent determination of age thatconventional isochron plot of the data while relationship between these low-Ti tholeiite occur-Fig. 4b shows isotopic evolution paths for indivi- rences could be established. Comparison of thedual samples. Despite their disparate locations in Garhwal–Bhowali volcanics with the Rampur–the field, the samples plot along a linear trend in Mandi volcanics shows an extra-ordinary sim-Fig. 4a, with a reasonable spread in 147Sm/144Nd. ilarity in their ratios, age and initial eNd. Such anRegression of samples from Karanprayag area identical variation in 147Sm/144Nd and present dayalone defines a good isochron (MSWD=1.21) 143Nd/144Nd ratios may not be fortuitous. It isof 2508±78 Ma (2s) with an initial 143Nd/ worth mentioning that given over 200 km distance144Nd=0.509644±80 (2s), which corresponds to in the sampling sites of the Rampur–Mandi andeNd(T)=5.1±0.5 as calculated following the pro- Garhwal–Bhowali volcanics, it is unlikely that anycedure of Fletcher and Rosman (1982). As is
post-eruption alteration could have resulted inexpected from their linear disposition, inclusion in
such an identical resetting of the Sm–Nd isotopicthe regression analysis of samples from Adibadriratios. When samples from all the four locations:(A3) or Bhowali (BH1 and BH2) or GwaldamRampur, Mandi, Garhwal and Bhowali (except(G2) did not produce a significantly different agetwo samples, M4 and M37, from Mandi volcanics);and initial eNd (Table 2). The similarity in age andare regressed (Fig. 5), the age (2486±69 Ma) andinitial eNd yielded in any sample combination there-initial eNd (5.0±0.5) yielded, though less precisefore most likely suggests their isochronous(MSWD=2.11), remain within the analyticalrelationship.uncertainty of the age and initial eNd yielded indi-vidually by the Rampur volcanics and the Garhwalvolcanics. The reason for eliminating M4 and M374. Discussionfrom the regression is the inferred influence ofminor crustal component in these two samples4.1. Relationship of Garhwal–Bhowali volcanics(Bhat and Le Fort, 1993).with Rampur–Mandi volcanics
Several studies (Cattell et al., 1984; Chauvelet al., 1985; Wilson and Carlson, 1989; GruauThe close chemical similarity of the Garhwal-
and Bhowali volcanics with the Rampur- and et al., 1990) have demonstrated that some pub-
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Fig. 5. Sm–Nd conventional isochron diagram for combined data on Rampur–Mandi–Garhwal–Bhowali volcanics, the RFBs (seetext). (, Rampur volcanics; 6, Mandi volcanics, symbols for Garhwal and Bhowali samples remain the same as in Fig. 4a. Twosamples of Mandi volcanics, M4 and M37 are not included in the regression (see text). Regression analyses of the data were donewith the method of York (1969); the precision in e(T)Nd has been calculated following the procedure of Fletcher and Rosman (1982),and the errors quoted for age and initial eNd are 2s. The age yielded by the combined data, though less precise (note higher MSWD),is similar to that of the Garhwal (Fig. 4a) or Rampur volcanics (Bhat and Le Fort, 1992).
lished Sm–Nd ‘isochrons’ are in fact mixing lines. through the study of Rampur volcanics from theKulu–Rampur window that the existence of theA common feature of such ‘isochrons’ is that they
are based on non-cogenetic sample sets which late Archaean volcanics in the Himalayas was firstrecorded (Bhat and Le Fort, 1992), and becauserepresented either a large compositional spectrum
[ultrabasic komatiites to felsic intrusives/volcanics: of their chemical similarity with most of the floodbasalts ( low-Ti tholeiites: Fig. 3), it is preferableHamilton et al. (1979); ultramafic-mafic volcanics
to sodic granite and felsic porphyry: McCulloch to call these volcanics the Rampur flood basalts(RFB).and Compston (1984)] or came from different
geological formations (Hamilton et al., 1979) or The stratigraphic relationship of the sample G2in relation to other samples remains problematicgeographic localities (Hamilton et al., 1977). The
initial eNd value yielded by these apparent ‘isoch- because of its uncertain relationship with the sur-rounding metasedimentary rock. If the Almorarons’ is usually low. On the contrary, as already
noted, the samples represent a very limited bulk Group (Fig. 2) is older than the BerinagFormation, as is currently accepted, then G2 couldcompositional variation. Moreover, the same age
and a highly positive initial eNd value were obtained represent an intrusive part of the magma in thebasement.from two different sets of samples, with each set
collected in two widely separate locations and In contrast to the LREE-enriched compositionof the RFB, the retention of highly positive initialanalysed in two different laboratories.
In summary, the similarity in chemical and eNd value of +5 suggests that their mantle sourcehad a pre-history of LREE-depletion for a con-isotopic composition, and consequently the age
and the initial eNd of the Rampur–Mandi volcanics siderable period of time before being re-enrichedin LREE closely before the melt generation. Aand the Garhwal–Bhowali volcanics suggest that
these volcanics are most likely the vestiges of a comparison of RFB initial eNd with the worldwidePrecambrian mafic volcanics, including those fromsingle, late Archaean episode of magma eruption.
Their present distribution could be attributed to the Indian Shield, shows that the RBF plot closeto the upper limit of the evolutionary path definedpost-eruption tectonic deformation. Since it was
226 M.I. Bhat et al. / Precambrian Research 87 (1998) 217–231
Fig. 6. Nd-isotopic evolution of RFBs compared with the Precambrian rocks of India and rest of the world. Also shown for comparisonis the evolutionary path for depleted mantle (DM: see caption to Fig. 4b). RFB datum plots close to the upper limit of the DMpath, corroborating the asthenospheric derivation of the rocks (Bhat and Le Fort, 1992, 1993). While the source of majority of theworld data set remains the same as listed in Shirey and Hanson (1986) and Jahn et al. (1987), a few are from recent literature; datasource on the Indian rocks are: 1, Basu et al. (1981); 2 and 5, Gopalan et al. (1990); 3, Kumar et al. (1996); 4, Bhaskar Rao et al.(1996); 6, Balakrishna et al. (1990); 7, Bernard-Griffiths et al. (1987); and 8, Bhat and Claesson (1995).
by the depleted mantle (Fig. 6). The depleted west as the Kishtwar window in the KashmirHimalayas, and assign it to the Berinag Formationnature of source for these volcanics fits well with
the global data on mantle derived Precambrian throughout. Eastwards, while Berinag–Nagthatformations are recognized in Nepal, the equivalentrocks, which suggest the establishment of depleted
mantle very early in Earth history (Shirey and of Damtha Group of the Kumaun LesserHimalayas (Fig. 2) is the Lower NawakotHanson, 1986; Jahn et al., 1987).Formation [Fig. 7: Valdiya (1986)]; farther east,in Sikkim and Bhutan, Phuntsholing Formation4.2. Spatial extent of the RFBconstitutes the equivalent lithology (Valdiya,1986). Thus, the repeated appearance of this vol-Given the involvement of the Himalayan rocks
in the Tertiary compressional tectonics, and conse- cano-sedimentary sequence from Kishtwar in theKashmir Himalayas through the Kulu–Rampurquent horizontal translation and juxtaposition of
different lithologies as well as burial beneath thrust window, Kumaun and Nepal to the Bhutan LesserHimalayas in the east makes it evident that thesheets, it is not possible to determine the spatial
distribution of these Late Archaean volcanics volcanism affected a fairly vast area, some 1700 kmalong the strike.activity. Similarly, estimation of their southward
extension is hampered by the presence of the Taking the present distance between the twomost extreme exposures, between Jipti in northTertiary sedimentary deposits and the recent allu-
vium south the Lesser Himalayas. However, if the and Chukka in the south along the Indo–Nepalborder, marked by Kali River (Fig. 2), as thestrike extension of the Berinag–Nagthat forma-
tions is any guide, it appears that the volcanic minimum width for the spread of lava flows, thetotal area of lava extent in the Lesser Himalayasactivity covered a very large area. For example,
Thakur (1992) and Thakur and Rawat (1992) alone is at least 170 000 km2, although in reality ithas to be greater by several orders of magnitude.trace this volcano-sedimentary succession west-
ward through the Kulu–Rampur window to as far Again, wherever these volcanics are better exposed,
227M.I. Bhat et al. / Precambrian Research 87 (1998) 217–231
Fig
.7.
Age
olog
ical
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228 M.I. Bhat et al. / Precambrian Research 87 (1998) 217–231
for example along the Rampur–Jakri road in Himalayan region experienced a volcanic phase offlood basalt proportions.Himachal (to the west of the map area in Fig. 2),
in and around Karanprayag and along The true magnitude and tectonic setting of theRFB episode should be seen in the light of sugges-Bhowali–Ratigarh road in Kumaun, the thickness
of flow succession with intercalated minor sedi- tions made by several authors for southward con-tinuation of the Lesser Himalayan litho-units intomentary beds reaches up to ca 1000 m. Although
this crude estimate does not make the aerial spread the northwestern part of the Indian PeninsularShield (Holland, 1908; Auden, 1935; Gansser,of these Late Archaean volcanics in the Himalayas
comparable to those of the well known 1964; Valdiya, 1976; Azmi, 1993; Brookfield, 1993;Virdi, in press; Parrish and Hodges, 1996).Phanerozoic flood basalt provinces (Deccan,
Siberian, Parana), it does suggest that the Another oft-repeated feature referred to in favour
Fig. 8. Map showing the Aravalli fold belt in relation to the Himalayan belt. In its present disposition, the Aravalli belt is demarcatedby the Western Marginal Fault ( WMF) on the west and Easterm Marginal Fault or the Great Boundary Fault (GBF) on the east.Gravity studies have shown that while the WMF marks the western side of the Delhi–Hardwar gravity high, the GBF extends furthernorth and joins the Datagang–Tilkar Fault (DTF) under the Gangetic alluvium (Sivaji et al., 1992). Allowing for the horizontaltranslation and constriction of rocks in the Aravalli and Himalayas during their Precambrian and Tertiary compression phase,respectively, the orientation of the two regions relative to each other suggests rather typical triple-arm disposition of rifting in thetwo regions. Furthermore, the presence of high density, basic material has been suggested under both the Himalayas and the Aravallifold belt (Qureshi, 1969, 1971; Verma et al., 1986); the Aravalli gravity high extends through Delhi and joins Himalayas in theGarhwal region, forming the well-knowm Delhi–Hardwar gravity high/ridge. If rifting and basin formation in the two regions begansimultaneously, as this study suggets, the most likely tectonic scenario would be that of mantle plume (inset), resulting in upwellingof high density basaltic magma into crustal levels (cf White et al., 1987; White, 1988).
229M.I. Bhat et al. / Precambrian Research 87 (1998) 217–231
of commonality of the Lesser Himalayan and the the Aravalli regions has never been in dispute. Theorientation of the two rifts therefore points to aShield region geology is the extension of Aravalli
structural (NNE–SSW ) trend into the Himalayas mantle plume origin for the contemporaneousrifting and associated magmatism in the two(Valdiya, 1976). In geophysical terms this has been
interpreted as the Delhi–Hardwar gravity-high- regions (Fig. 8). On speculative terms, this raisesan interesting possibility of offering a commonridge (Qureshi, 1971; Verma et al., 1986), which is
suggested to extend from the Aravalli fold belt origin and tectonic relationship for the NNE–SSWdirected Delhi–Hardwar gravity-high-ridge withinto the Himalayas. A similar linear belt of high
gravity, ascribed to magma underplating, was the Himalayan gravity high (Qureshi, 1969, 1971;Verma et al., 1986) by basic magma injection andearlier suggested under the Higher Himalayas
(Qureshi, 1969). The Late Archaean age of upwelling [Qureshi (1971); see also Dubey andBhat (1991)] which now appears to have begunRampur volcanics (Bhat and Le Fort, 1992) pro-
vided the first positive evidence in support of the with the Late Archaean rifting.stratigraphic and tectonic unity between the LesserHimalayan and the Aravalli region as volcanics ofsimilar age and stratigraphic order are known at Acknowledgmentthe base of the Aravalli Supergroup rocks[2.5–2.0 Ga: Verma and Greiling (1995)] of the Isotopic data presented here is a part of thelatter region (Fig. 8). Sugden et al. (1990) sug- work carried out on the Himalayan rocks at LIG,gested that the eruption of the 2.5 Ga old basal Stockholm under a Visiting Scientist fellowship toAravalli volcanics marked the first rifting event MIB by the Swedish Institute, Sweden. His sincereafter cratonization of the region. The same conclu- thanks go to each member of the Lab for theirsion was reached by Verma and Greiling (1995) great help throughout his stay there. The authorswho, after considering geological, geophysical and in particular want to thank Hans Schoberg forgeochronological data, concluded that the depos- help with the Nd isotope analyses. Frequent discus-ition of Aravalli Supergroup took place on the sions with N.S. Virdi and N.S. Gururajan helpedpassive margin of the Aravalli basin. Thus, if the in clearing up some confusing points regarding theage of the oldest superacrustal sediments in the stratigraphy of the Himalayas. Comments by M.E.Himalayan and Aravalli belts is the same, it follows Brookfield were very encouraging. The authors arethat the basin formation in the two regions initi- grateful to the reviewer Bor-ming Jahn for pointingated by contemporaneous lithospheric rifting and out some errors in their calculations.magmatism.
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