Crustal generation of the Himalayan leucogranites

Abstract

Detailed studies of the Himalayan two-mica leucogranites, such as the Manaslu pluton, indicate that they have very uniform mineralogical, petrological and structural characteristics. One can relate their occurrence to the thickest zones of the underlying Tibetan Slab. In these zones, migmatization attains its greatest development and vertical extension. The granite is emplaced at first along the main disharmonic plane above the Main Central Thrust (MCT), at the top of the Tibetan Slab (infrastructure). Ductile deformation of the granite is variable; the granite being syn-to late-kinematic with regard to the functioning of the MCT. Major elements are very homogeneous (except for Na and K) implying that P-T conditions of melting were relatively uniform. The melted material was of a similar composition over a vast volume, and the percentage of melting was small (10-15%). Trace elements are highly variable. Some are characteristic for very evolved material (Ta, Rb, Cs, U) or show the link with the Tibetan Slab (Ba-Sr), whilst others are problematic (Th, REE). REE and Th abundances, being much less in the granite than in the Tibetan Slab, imply that they have been extracted during one of the main stages of formation, possibly by monazite. Radiogenic (Pb, Sr, Nd) and stable (O) isotopes are consistent with the origin of the granite from the Tibetan Slab. However, the heterogeneous Sr isotopic ratios make age dating difficult and imply poor mixing or little fluid interaction during its evolution. H-isotope data indicate that magmatic compositions of the main body of the Manaslu granite have been preserved. Late or post-magmatic alterations are extremely local in the main pluton. To the north, another belt of two-mica granites occurs whose characteristics are very similar to the High Himalaya belt. They were probably generated in a similar way during this recent intracontinental evolution. © 1987.