5.3.4.5 The Intrusion Zone

Below the kyanite zone, the metamorphic succession is interrupted by a 1-2 kilometre thick intrusion zone, where large amounts of leucogranitic melts have been injected into the metasedimentary rocks of the HHCS. The uppermost part of this intrusion complex is marked by the presence of individual dikes which criss-cross the migmatitic paragneisses of the Phe Formation. Most of these dikes are reoriented and boudinaged by ductile shearing and represent one of the most spectacular argument for extensional, top to the NE, movements associated with the Zanskar Shear Zone.

At a structurally deeper level these dikes become more and more abundant, such as to finally form a compact leucogranitic sheet in the footwall of the ZSZ, where individual dikes cannot be differentiated any more. This sheet has an approximate thickness of one kilometre, and the massive leucogranitic cliffs of the Gumburanjun mountain are part of this zone. Scarce bits of metapelitic country rocks are preserved within these leucogranitic «plutons». These country rock fragments have an angular shape and are composed of biotite + quartz + plagioclase (oligoclase - andesine). The absence of muscovite, garnet, K-feldspar or aluminosilicates might be the consequence of a leaching process, where fluid circulation associated with the intrusion of leucogranitic magmas depleted the host-rock of certain elements.

A foliation is preserved within these blocks which shows a constant orientation as well between different blocks as with the country rock. These rocks have thus preserved their original orientation and were not tilted inside of the leucogranitic intrusion. The overall texture of the intrusion zone is agmatitic with angular rafts of country rocks cemented by leucogranites. The leucogranites will be discussed in more detail in chapter 6.

The lower part of the intrusion complex is formed by migmatitic gneisses intruded by a dense network of vertically ascending leucogranitic dikes which represent the feeder-dikes of the leucogranitic plutons. These feeder-dikes are rooted into the migmatitic zone which is the structurally lowermost metamorphic zone in this area.