Lithostratigraphic correlations in the western Himalaya—An isotopic approach

Abstract

We present the results of a whole-rock Nd isotopic study of two contrasting regions of the western Himalaya, using the neodymium model age approach on the scale of a single orogen. High-grade metasedimentary rocks from Zanskar yield model ages ( T DM ) that are similar to those of the High Himalayan Crystalline Series ( T DM = 1.2–2.0 Ga; ϵ Nd = −6 to −16) and distinct from values from the Lesser Himalaya ( T DM = 2.3–3.4 Ga; ϵ Nd = −18 to −27). Hence these two lithological sequences can be recognized for 2000 km along the strike of the orogen. Data for the basement of the Nanga Parbat massif at the western extremity of the Himalaya ( T DM = 2.3–2.8 Ga; ϵ Nd = −18 to −30) suggest that these rocks are not equivalent to the High Himalaya, as previously supposed, but have affinities with the Lesser Himalaya. A thin metasedimentary cover sequence on the margins of the Nanga Parbat massif is isotopically indistinguishable from the High Himalaya ( T DM = 1.6–1.8 Ga; ϵ Nd = −10 to −14). The prior misidentification of the provenance of the massif stems from its high metamorphic grade, characteristic of the High Himalaya, but in this case related to the unique Neogene history of the Nanga Parbat massif, which has exhumed a higher-grade equivalent of the Lesser Himalaya that is not seen elsewhere.