Fig. 6. Rb versus Nb + Y plot for granitic rocks from the Larji – Kullu – Rampur window LKRW, Kishtwar window
KW, MCT zone and Jeori – Wangtu Granitic Gneiss complex JWGC, NW Himalaya. Field boundaries from Pearce et al.
1984. COLG = collisional granites, VAG = volcanic arc granites, WPG = within-plate granites, ORG = oceanic ridge
granites.
5. Lithostratigraphic and paleogeographic correlations
It has been suggested Parrish and Hodges, 1996; Whittington et al., 1999 that the lithologi-
cal sequences of the HH Crystalline and LH can be identified along strike of the Himalayan orogen
on the basis of their distinct Nd model ages and that the MCT marks a terrane boundary. Accord-
ing to Whittington et al. 1999, the HH is charac- terized by depleted mantle model ages in the
range of 1.2 – 2.0 Ga and distinct from the LH where Nd model ages range from 2.3 – 3.4 Ga.
Our Sm – Nd data Table 2 suggest that this could merely reflect an inadequate database. The
analyzed Paleoproterozoic LH granitic rocks and the MCT orthogneiss mylonite have model ages
ranging between 2.4 and 2.6 Ga. However, in the NW Himalaya, the Paleozoic granites within the
HH Crystalline Fig. 1 yield model ages ranging from 1.6 to 2.9 Ga Table 2 and unpublished
data and thus overlap data reported for the LH and the MCT zone.
The tholeiitic composition of the Rampur meta- basalts indicates a lherzolitic source and relatively
high degrees of partial melting. In addition, the lack of a residual garnet signature CeYb : 9
suggests shallow depth of melting and, possibly, passive crustal extension. The tectonic setting of
the Rampur metabasalts remains ambiguous, but at present there is no clear geochemical evidence
for the involvement of plume material in their petrogenesis. The suggestion by Bhat et al. 1998
that they were generated above the center of an Archean mantle plume is based on the interpreta-
tion of an apparently erroneous Sm – Nd isochron age. It fails to explain the absence of a plume
signature in the basalt trace element chemistry, the absence of komatiites in the LH and the low
MgO – high Na
2
O chemical composition of these mafic volcanic rocks. According to White and
McKenzie 1989, central plumes of rising mantle are characterized by abnormally high tempera-
tures. As the potential asthenosphere temperature is increased up to 1480°C, the percentage of MgO
increases systematically from about 10 to 18, and the percentage of Na
2
O simultaneously decreases.
HFSE-rich restite phases from crustal source rocks during partial melting. The observed REE
distributions and the negative Sr and Eu anoma- lies indicate fractionation by mineral phases con-
taining these elements at some stage in the evolution of the source or of the magma e.g.
residual plagioclase combined with plagioclase fractionation. These compositional features, the
range in initial
87
Sr
86
Sr ratios and the negative initial oNd values Table 2 clearly point to a
major role of older continental crust in the forma- tion of these granitoids. The depleted mantle
model ages Table 2 suggest a contribution of Archean crust. A recycled Archean component is
also documented by the \ 2.9 Ga domain detected in grain D of sample HF4990.
In the NW Himalaya, a period of silicic volcan- ism and granite emplacement occurred around
1.86 – 1.84 Ga. These Paleoproterozoic granitoids are not restricted to the LH tectonic windows.
From S of Kishtwar to Nepal, they form a nearly continuous zone of mylonites and augengneisses
at the base of the MCT. The geochemical and isotopic similarities between the MCT mylonites
and the Lesser Himalaya granitic rocks are consis- tent with the notion that the MCT cuts down-sec-
tion into the hanging-wall. The considerable areal extent and the range in initial
87
Sr
86
Sr and initial o
Nd of the Paleoproterozoic granitoids clearly suggest processes of large-scale reworking of sialic
crust. The geochemical data Fig. 6 apparently rule out subduction-related magmatism, but can-
not distinguish between i anatexis of existing crust during a period of syn- to post-collisional
crustal thickening; ii crustal melting in a post- collisional extensional regime or iii melting in
response to post-collisional lithospheric delamina- tion and mafic underplating. As there is no clear
evidence for collision-related crustal thickening andor deformation, an alternative thermal cause
for crustal melting must be found. An attractive mechanism has been proposed by Sandiford et al.
1998. Their thermal-isostatic model predicts sig- nificant crustal melting as a consequence of burial
of basement sequences enriched in heat producing elements during thermal subsidence.
Proterozoic rocks are also present in the Ar- avalli domain south of the Himalayas. Unfortu-
nately, possible connections are not only obscured by the alluvium of the intervening Indo-Gangetic
plain but also by the fact that an unknown amount of Proterozoic material has been de-
stroyed in the Himalayan orogeny. Available geochronological data suggest that several periods
of acid magmatism have affected the Aravalli domain between c. 3.3 and 0.8 Ga e.g. Tobisch et
al., 1994; Wiedenbeck et al., 1996. In addition, a Paleoproterozoic c. 2.5 – 2.4 Ga mafic magmatic
event is recorded at the base of the Aravalli Supergroup e.g. Verma and Greiling, 1995.
These lavas are LREE enriched and range from magnesian komatiites to Fe-rich tholeiites. They
have been interpreted as products of the interac- tion of a deep mantle plume with subcontinental
lithosphere Ahmad and Tarney, 1994. The avail- able data, however, are still too sparse and too
imprecise to allow any firm assessment of the continuation of the LH into the northwestern part
of the Indian shield. Still, it is interesting to note that the NE – SW trending Aravalli mountain
range is characterized by a complex history of multiple tectonic events e.g. Verma and Greiling,
1995 that affected both, the sialic Archean base- ment and the Proterozoic sedimentary cover se-
quences Aravalli and Delhi Supergroups. In contrast, there is no evidence for a distinct post-
depositional deformation and metamorphic re- crystallization in the Proterozoic domains of the
LH prior to the Himalayan orogeny and no evi- dence for the Early Paleozoic granitic magmatism
that is widespread in the HH Crystalline. This is illustrated by the U – Pb data for the Wangtu
granitoid: an upper concordia intercept of 1866 9 10 Ma was obtained from the regression of five
bulk zircon fractions Singh et al., 1994, whereas the lower intercept of 48 9 28 Ma suggests Ter-
tiary Pb loss or zircon overgrowth.
6. Conclusions
