TECTONIC AND METAMORPHIC EVOLUTION

OF THE CENTRAL HIMALAYAN DOMAIN

IN SOUTHEAST ZANSKAR

by Pierre Dèzes

6. LEUCOGRANITES

The master was fond of the desert, however, and once we entered the barren landscape of rocks and cacti, he kept pointing out curious geological formations and delivering little lectures on the incalculable age of the earth. To be perfectly honest, it left me pretty cold.» Mr Vertigo», Paul Auster

6.1 Introduction

We have decided to devote a special chapter of this work to the leucogranites, for they represent one of the rare magmatic products of the Himalayan orogen and are of paramount importance in the tectonometamorphic evolution of the High Himalayan Crystalline Sequence.

With their snow-white colour, set with tiny garnets and tourmaline prisms, their equigranular texture and lack of pronounced alteration, the Himalayan leucogranites can be qualified as being aesthetically very attractive.

One of the first descriptions of these granites is given, for the Garhwal region, by Griesbach (1891) who observed their systematic association with a «long line of dislocation» (STDS ?) at the contact between the «crystalline rocks» (HHCS) and the «Haimantas» (Phe Fm. of the TH). He also went so far as to suggest that these leucogranites might well be of Tertiary age (which is absolutely correct). Despite the fact that his age estimation is made on the basis of a somewhat shaky argumentation, it is rather impressive that he guessed them to be younger than the lowest Palaeozoic, as he himself observes that they do intrude younger formations.

Leucogranitic intrusions outcrop sporadically over a distance of more than 2000 kilometres along the strike of the Himalayan orogen (fig. 6.1), retaining remarkable uniformity in both geochemistry and in structural settings. Single intrusions do not exceed a few kilometres in thickness, and leucogranites most often form intricate sill and dike complexes. About 20 plutons of variable size do however occur along the Himalayan range, where they often form the pedestal of the highest summits in the world (Griesbach, 1891, Gansser, 1964, Le Fort, 1973). Among these plutons are the Manaslu, the Makalu, the Everest, the Langtang, the Gangotri, the Rongbuk, the Mustang, the Shisha-Pangma, the Gophu-La, the Gabug, the Monlakarchung-Pasalum, the Chung-la, the Lhozag and the Gumburanjun. These plutons are surrounded by an aureole of leucogranitic aplites or pegmatites criss-crossing the country rocks. Xenoliths of country rocks are frequently observed in the margins of the plutons, their amount diminishing progressively towards the core of the intrusions.

These High Himalaya intrusives are peraluminous , muscovite ± tourmaline / biotite -bearing granites of minimum melt composition (Le Fort et al., 1987). They are generally emplaced in the kyanite to sillimanite grade rocks at the top of the HHCS and are associated with segments of the South Tibetan Detachment System. Some of these intrusions however cross the STDS and intrude the weakly metamorphosed sediments of the Tethys Himalaya (Griesbach, 1891; Le Fort, 1973, Guillot et al., 1994 and 1995). Most of these leucogranites yield a monazite U/Pb age around 25-20 Ma (Schärer, 1984; Harrison et al., 1995), with no obvious variation along strike of the Himalaya (Harris and Massey, 1994). Very young leucogranites with an age < 5 Ma have however been described in the two Himalayan syntaxis (Burg et al., 1998).

The leucogranites generally have an equigranular texture and range between aplites and pegmatites. They are essentially formed of quartz + plagioclase (An₂₁ to An₂) + K-feldspar + muscovite with variable amounts of biotite, garnet and tourmaline. Accessory phases are apatite, monazite, zircon, beryl, cordierite and aluminosilicates. Despite their mineralogical uniformity, the leucogranites are commonly divided in two categories: the two-mica leucogranites and the tourmaline leucogranites. In the former category enter the granites where both muscovite and biotite are present and tourmaline is subordinate. The latter category regroups the granites where tourmaline and muscovite are widespread but biotite is virtually absent.