Early Permian granitic dykes of alkaline affinity in the Indian High Himalaya of Upper Lahul and SE Zanskar: geochemical characterization and geotectonic implications

Abstract

In the Tethyan sedimentary series and the High-Himalayan ‘crystalline’ units of NW India, two generations of granite have previously been described. These rocks include large Cambro-Ordovician batholiths and small Tertiary bodies. A third type, the Yunam granite, has been discovered in the SE Zanskar — Upper Lahul region. It crops out as discordant dykes in the Ordovician Thaple Fm. Radiometric (U-Pb) measurements on zircons give an Early Permian crystallization age of 284 ± 1 Ma. Tertiary metamorphic cooling ages of 33.5, 20.8 ± 0.4, 22.8 ± 0.5 and 19.3 ± 0.2 Ma have been obtained by 40 Ar/ 39 Ar and K-Ar methods on amphiboles, muscovites and biotites of the granite and associated rocks. In contrast to the crustal affinity of the Cambro-Ordovician and Tertiary Himalayan granites, the Yunam granite is associated with an anorogenic, possibly thinned lithosphere context. This magmatic event is most probably related to the Permo-Mesozoic rifting of Neo-Tethys.

Contrasting metamorphic and geochronologic evolution along the Himalayan belt

“…Occasional 40 Ar/ 39 Ar hornblende ages and Rb/Sr muscovite ages have been reported, and were tentatively ascribed to the M1 metamorphic event. Treloar et al (1989b) reported 40 Ar/ 39 Ar cooling ages of 38 Ma for the M1 event in the High Himalayan Crystalline slab in north Pakistan, whereas 40 Ar/ 39 Ar, Rb/Sr cooling, and U/Pb ages of around 40-30 Ma have been reported in Zanskar (Searle et al, 1992;Sorkhabi et al, 1993;Spring et al, 1993), in Gahrwal (Hodges and Silverberg, 1988), and in central Nepal (Inger and Harris, 1992;Vannay and Hodges, 1996;Hodges et al, 1996a). Moreover, inheritance U/Pb monazite ages between 40 and 30 Ma from the Manaslu granite in central Nepal have been interpreted as representive of an earlier metamorphic event (Harrison et al, 1995).…”

“…From Zanskar to Everest, numerous ages between 22 and 15 Ma have been reported using different isotopic systems ( 40 Ar/ 39 Ar , U/Pb, Rb/Sr) related to the M2 event and motion along the Main Central thrust and the South Tibetan detachment system (Hubbard et al, 1991;Inger and Harris, 1992;Metcalfe, 1993;Parrish and Hodges, 1992;Spring et al, 1993;Vannay and Hodges, 1996). In the Kangmar dome, there is also evidence of Miocene 40 Ar/ 39 Ar cooling ages around 20-13 Ma, which are assigned to the M2 metamorphic event (Debon et al, 1985;Chen et al, 1990) (Table 1).…”

Early Permian Pangea ‘B’ to Late Permian Pangea ‘A’☆

“…In the northwestern Himalaya, Neotethyan rifting reached its climax with the onset of bimodal volcanism and intrusion of alkali granites, dated radiometrically at 284 þ 1 Ma (U^Pb on zircons [42]), and biostratigraphically constrained to the earliest Permian [43,44]. Break-up and formation of oceanic crust followed shortly afterwards from the Himalayas all the way to northern Oman at mid-Early Permian times (midSakmarian [45,46]).…”

Exhumation history of the NW Indian Himalaya revealed by fission track and 40Ar/39Ar ages

“…While the tectonic history of events in this region relies heavily on structural observations, precise isotopic dating is incomplete. Some geochronological data are available from specific areas within the Lahul and Beas valley (Table 1; Frank et al, 1977;Bonhomme and Garzanti, 1991;Spring et al, 1993a;Dèzes et al, 1999;Lal et al, 1999;Walker et al, 1999;Jain et al, 2000;Robyr et al, 2002Robyr et al, , 2006Vannay et al, 2004;Walia et al, 2008), and from the Tso Morari dome near the Indus suture (Fig. 1;De Sigoyer et al, 2000Schlup et al, 2003).…”