magmatic rocks are sparse and contradictory. Pre- vious Rb – Sr and Sm – Nd dating of LH granitic
and mafic rocks yielded whole rock ages ranging from 2510 to 1220 Ma Frank et al., 1977; Bhanot
et al., 1982; Bhat and LeFort, 1992. Better con- straints on the age and provenance of the Protero-
zoic supracrustal sequences, associated igneous suites and their basement, however, are essential
for reconstructions of the relationship between HH and LH before the onset of the Himalayan
orogeny.
We have sought to characterize the age and distribution of the Proterozoic igneous suites in
the different units and we have used the Sm – Nd technique to compare their protolith characteris-
tics. Another key objective of our study was to determine zircon
207
Pb
206
Pb ages from a metarhy- odacite and a metabasalt that are associated with
clastic metasediments in the basal sedimentary succession Rampur formation of the LH. For
the metabasalts, our new zircon data do not confirm the Archean Sm – Nd whole rock age in
the literature. In addition, we present geochemical data on the Rampur metabasalts and on grani-
toids from the LH and the MCT zone which provide constraints on their source characteristics
and the crustal evolution of the former passive margin of northern India.
2. Regional geology
In the Himalaya of NW India three major tectonic units can be distinguished from south to
north Gansser,
1964: i
the Subhimalaya
Miocene – Pleistocene molasse deposits which are overthrusted along the Main Boundary Thrust by
the ii LH, which consists primarily of Protero- zoic to Cambrian low-grade metasedimentary
rocks, metagranitoids and metavolcanics. iii The HH, thrusted over the LH along the MCT, com-
prises greenschist to amphibolite facies Precam- brian
to Cambrian
metasediments High
Himalaya Crystalline and the Paleozoic to Meso- zoic Tethyan Zone Fig. 1. Neogene nappe stack-
ing of these major units, differential uplift and erosion have exposed the LH in two tectonic
windows — the Kishtwar Window in the NW Fuchs, 1975; Sta¨ubli, 1988; Guntli, 1993 and the
Larji – Kullu – Rampur Window LKRW in the SE Auden, 1948; Berthelsen, 1951; Jhingran et
al., 1952; Frank et al., 1973; Tho¨ni, 1977.
In the LKRW and in the Simla area two super- posed Proterozoic supracrustal sequences can be
distinguished, each ending with redbeds and a shallow-water carbonate platform: i Rampur
Chail-Khaira-Shali and
ii the
younger, Neoproterozoic
Simla – Blaini – Krol sequences
e.g. Frank et al., 1995. Stromatolites in the Shali Group carbonates at the top of the lower se-
quence suggest a lower to mid-Riphean age Ash- girei et al., 1975. The basal unit of this lower
sequence is the Rampur formation Jhingran et al., 1952, consisting of massive beds of white
quartzarenites alternating with thin layers of seriz- ite and chlorite schists, locally associated with
metarhyolites and metabasaltic dikes and lava flows Rampur metabasalts. Where preserved,
sedimentary structures like cross bedding and rip- ple marks indicate a normal polarity for these
series. At present, it is not clear whether the Rampur formation quartzites are unconformably
overlying the Bandal granitoid complex Srikantia and Bhargava, 1998 or whether the granitoids
intrude these quartzites Sharma, 1977. This im- portant issue can only be resolved through further
studies. Rb – Sr whole rock data suggest a Pale- oproterozoic age for the Bandal granitoids Frank
et al., 1977 which are remarkably similar to the Jeori – Wangtu granitoid gneiss complex JWGC,
Fig. 1, except for the higher metamorphic grade of the latter.
The JWGC is an integral part of the LKRW Bhargava, 1982; Srikantia and Bhargava, 1998
and consists of partly mylonitic augen gneisses Wangtu gneiss, paragneisses, mica schists and
minor concordant sheets of metabasites. It is characterized by an inverted metamorphic zona-
tion Vannay and Grasemann, 1998. Near Jhakri the JWGC is faulted along a W dipping thrust
overlying the quartzites of the Rampur formation. Preliminary
40
Ar
39
Ar geochronology
results Frank, pers. comm. as well as young cooling
ages from a similar thrust further E in Garhwal Metcalfe, 1993 suggest that this thrusting has
been active in the last few Ma, indicating an
out-of-sequence thrust. The JWGC probably rep- resents the basement for the Rampur formation
and has been faulted onto its own cover during the last few Ma along the above mentioned W
directed out-of-sequence thrust Vannay et al., 1999.
In the NW Himalayas, the MCT zone consists of highly deformed mylonitic orthogneisses and
paragneisses i.e. metamorphic equivalents of the Haimanta Group metasediments revealing an in-
verted metamorphic gradient. This greenschist to amphibolite facies metamorphic sequence in the
hanging wall is distinct from the footwall which comprises low-grade metamorphic Precambrian
sedimentary rocks of the LH along a brittle thrust zone. Field observations suggest that the pro-
toliths of the mylonitic orthogneisses did not in- trude into the Haimanta Group, but rather
represent the basement of these Proterozoic metasediments Trivedi et al., 1984; Srivastava
and Mitra, 1996; Grasemann et al., 1999. This interpretation is supported by the large lateral
extent of the orthogneisses and the fact that they always have the same litho-tectonic position at the
base of the Haimanta Group. The characteristic association of the MCT orthogneiss mylonites
with carbonaceous-graphitic schists and phyllites has been assigned different names in different
Fig. 1. Simplified tectonic cartoon map of the NW Himalaya India after Frank et al. 1995 and Vannay and Grasemann 1998. Labelled black squares correspond to sample locations. KW = Kishtwar window; LKRW = Larji – Kullu – Rampur window,
JWGC = Jeori – Wangtu Gneiss Complex.
Fig. 2. Zircons from a metabasalt HB6596 and b metarhy- olite HF4990, Larji – Kullu – Rampur window.
Bhuntar and Manikaran. These metabasalts have also been studied by Bhat and LeFort 1992. The
original mineralogy and textures of these volcanic rocks have been destroyed by Tertiary deformation
and greenschist facies metamorphism. The meta- morphic assemblage is Amp ferri – potassian –
tschermakitic hornblende+Bio [MgMg+Fe =
0.47]+Plg Ab
80 – 82
+Qtz+sphene+ilmenite.
3
.
2
. Geochronology of the Rampur metabasalts Knowing the age of the Rampur metabasalts is
of major importance for the stratigraphic correla- tions in the LH. Analysis of subtype S24 and S25
Fig. 2a; Pupin and Turco, 1972 zircons from metabasalt HB6596 by the single grain evapora-
tion technique Kober, 1987; Klo¨tzli, 1997 yielded an age of 1800 9 13 Ma 1 s without
evidence of an older component Table 1. This age is interpreted as dating magmatic zircon
growth. However, if this zircon is inherited, this would represent a maximum crystallization age.
In contrast, Bhat and LeFort 1992 published a whole rock Sm – Nd isochron age of 2510 9 90
Ma 2 s, also from the Rampur metabasalts. In addition, Bhat et al. 1998 studied the Sm – Nd
systematics of the Garhwal and Bhowali mafic volcanic rocks of the Kumaun Lesser Himalaya
and interpreted the composite whole rock Sm – Nd reference line as indicating an age of 2510 9
80 Ma. Dating metabasaltic rocks by means of whole rock Sm – Nd isochrons, however, is highly
problematic. Several published Sm – Nd isochrons on old mafic rocks have subsequently been shown
by other methods to represent mixing arrays without any chronological meaning e.g. Chauvel
et al., 1985; Compston et al., 1986; Gruau et al., 1990.
Our zircon
207
Pb
206
Pb date implies that the 2.5 Ga age may be geologically meaningless and can-
not be correlated with the magma-forming event. Depleted mantle Nd model ages for the Rampur
metavolcanic rocks, including the data of Bhat and LeFort 1992, fall in the range 1.40 – 2.17 Ga
Table 2. These ages are younger than their Sm – Nd ‘isochron’ age, again indicating that the
Archean age is not meaningful. areas e.g. Bajaura Nappe: Frank et al., 1973;
Outer Granite Band: Bhatia and Kanwar, 1973; Baragaon Gneiss: Bhanot et al., 1978; Seawa
Paragneiss: Sharma et al., 1973; Gahr-Manjrot Formation: Bassi, 1989. The age of the Precam-
brian Haimanta Group metasediments is not well constrained, but several lines of evidence suggest a
Neoproterozoic age Frank et al., 1994; Draganits et al., 1998.
3. Lesser Himalaya: Rampur metabasalts
