Fig. 4. oNd versus 1Nd diagram at the time of eruption 1.8 Ga with fractional crystallization FC and AFC DePaolo,
1981 trends for the metabasic rocks from the LKRW includ- ing samples analyzed by Bhat and LeFort, 1992. The AFC
vectors represent fractional crystallization of metabasalt Rn6 with assimiliation of granitoid HF5090 r = 0.3. Note that
this is not a unique solution, just a possible explanation for the crustal contamination model.
high initial
87
Sr
86
Sr ratios that are far outside the variation range of the mantle trend.
Essentially two options are available for the introduction of a crustal component: either i
magmas were derived from the mantle and be- came contaminated during ascent through the
crust crustal-level contamination or ii recycling of older crust mantle source contamination. The
evidence at the moment is far from clear-cut although the correlation between Nd content and
o tNd Fig. 4 would be consistent with increas-
ing degrees of contamination of the least evolved mafic lavas by LREE-enriched crust such as the
Proterozoic granitoids. The correlation R = 0.93 between 1Nd and
143
Nd
144
Nd in the Rampur metabasalts also indicates that the 2.5 Ga Sm – Nd
array Bhat and LeFort, 1992 is an artifact of mixing between depleted mantle melts generated
at 1.8 Ga and an older enriched lithospheric component.
4. Lesser Himalaya and MCT zone: felsic magmatic rocks
4
.
1
. Geochronology of felsic igneous rocks Frank et al. 1977 published a Rb – Sr whole
age of 1840 9 70 Ma 2 s based on four granitic WR samples from Bandal and Sainj LKRW,
documenting the existence of Proterozoic acid magmatism in the Himalayas. Our 24 granitic
samples from the Kishtwar Window, LKRW, JWGC and the MCT orthogneiss mylonites
Table 2 also plot on this regression, suggesting the widespread presence of Paleoproterozoic felsic
plutons in the LH and in the MCT zone, in agreement with radiometric evidence in the litera-
ture. In the JWGC, the Wangtu granitoid gave a U – Pb zircon age of 1866 9 10 Ma Singh et al.,
1994, and Rb – Sr whole rock ages of 2025 9 86 Ma Kwatra et al., 1986 and 1866 9 64 Ramesh-
war et al., 1995. The MCT mylonitic or- thogneisses yielded a Rb – Sr whole rock age of
1865 9 60 Ma Trivedi et al., 1984. Although Rb – Sr systematics do not resolve the Proterozoic
magmatic activity in detail due to the possibility of open-system behavior in such complex terrains
CFB. TiV ratios in the range of 20 – 50 are also consistent with an origin similar to CFB Sher-
vais, 1982, i.e. moderately high degrees of partial melting of shallow mantle and fractionation pro-
cesses under reducing conditions. The trace ele- ment characteristics of the Rampur samples are
clearly different from those typical of ocean island basalts OIB; Fig. 3b, but they share some chem-
ical features high large ion lithophile elements, low TiO
2
and negative Ta – Nb anomalies with subduction zone magmas. Whereas the trough at
Sr is probably a consequence of low-pressure plagioclase fractionation as indicated by the nega-
tive Eu anomalies, the negative Nb – Ta anomaly could reflect the existence of a residual Nb – Ta
phase during the partial melting process in the mantle e.g. Foley and Wheller, 1990. Alterna-
tively, the Nb – Ta anomaly could be a conse- quence of crustal contamination. A continental
crustal signature is indeed suggested by several geochemical parameters such as the distinctly low
CePb 4.4 – 11.9 ratios relative to OIB and N- MORB, and the combination of high ThYb
0.4 – 1.5 ratios with low TaYb : 0.2. In addi- tion, two out of four samples are displaced to
it is striking that in the Lesser Himalaya we did not find any evidence for the Early Paleozoic
magmatic event that is widespread in the HH Crystalline e.g. LeFort et al., 1986.
Metarhyodacite HF4990 is associated with the basal metasediments of the Rampur formation in
which K-feldspar forms conspicuous ovoid phe- nocrysts. Quartz phenocrysts are blue, frequently
embayed and may show signs of incipient dy- namic recrystallization. Together with bookshelf-
type
plagioclase porphyroclasts
the quartz
phenocrysts are set in a sheared groundmass of fine-grained feldspar, biotite and quartz. Zircons
Fig. 2b plot in subtype fields J4 and J5 in the typologic zircon classification diagram Pupin and
Turco, 1972. Four zircons were analyzed by the single zircon evaporation technique Kober, 1987;
Klo¨tzli, 1997. They yielded a well defined age of 1840 9 16 Ma 1 s; Table 1, which we interpret
as the age of magmatism. One of the zircons also preserved evidence of an earlier \ 2.9 Ga event,
surprisingly in the low temperature steps.
4
.
2
. Geochemistry and implications for granitoid petrogenesis
Forty four samples Lesser Himalaya and MCT granitic mylonites are characterized by high lev-
els of K
2
O 4.4 – 6.0 wt throughout the SiO
2
range from 62 to 75 wt Table 3. For the majority of samples Al
2
O
3
Na
2
O + K
2
O + CaO is 1.1 to 1.4 and normative corundum is \ 1.
All analyzed samples are preferentially enriched in LREE La
N
Yb
N
= 12.4 – 42.1 and show pro-
nounced negative Eu anomalies EuEu = 0.16 – 0.65. On a multi-element variation diagram Fig.
5, they are characterized by negative Ba, Nb, Sr, P and Ti anomalies and by relatively high levels of
Rb, Th and U. Table 2 presents the Sr and Nd isotopic characteristics of the Proterozoic granitic
rocks.
87
Sr
86
Sr initial ratios are generally high 0.711 – 0.721. The initial oNd values are negative
and range from − 5.8 to − 8.8. Most of the granitic rocks from the LH and the
MCT zone are peraluminous. Some are even S- type, based on the criteria proposed by Chappell
and White 1974. Some granitoids have elevated concentrations
of high-field
strength HFS
cations which suggest that they are A-type Fig. 6. However, diagnostic mineralogical features are
absent and isotopic compositions are far too evolved for these rocks to be included in the
A-type granitoid group. Their high trace element abundances are probably due to entrainment of
Fig. 5. Primitive mantle normalized trace element variation diagrams for granitic rocks from the a Larji – Kullu – Rampur
window; b Kishtwar window and c MCT zone and Jeori – Wangtu Granitic Gneiss complex, NW Himalaya. Normaliza-
tion factors are from Sun and McDonough 1989.
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
