Two granitic plutons, the Tso Morari gneiss and the Rupshu metagranite,
crop out in the Tso Morari area. The Polokongka La granite, classically
interpreted as a young intrusion in the Tso Morari gneiss, has been
recognized as the undeformed facies of the latter. Conventional isotope
dilution U-Pb zircon dating on single-grain and small multi-grain
fractions yielded magmatic ages of 479 +/- 2 Ma for the Tso Morari
gneiss and the Polokongka La granite, and 482.5 +/- 1 Ma for the Rupshu
granite. There is a great difference in zircon morphology between the
Tso Morari gneiss (peraluminous type) and the Rupshu granite (alkaline
type). This difference is confirmed by whole-rock chemistry. The Tso
Morari gneiss is a typical deformed S-type granite, resulting from
crustal anatexis. On the other hand, the Rupshu granite is an
essentially metaluminous alkali-calcic intrusion derived from a
different source material. Data compilation from other Himalayan
Cambro-Ordovician granites reveals huge and widespread magmatic activity
all along and beyond the northern Indian plate between 570 and 450 Ma,
with a peak at 500-480 Ma. A major, continental-scale tectonic event is
required to generate such a large magmatic belt; it has been tentatively
compared to the Variscan post-orogenic extensional regime of Western
Europe, as a late evolution stage of a Pan-African orogenic event.