YARLUNG ZANGBO OPHIOLITES,SOUTHERN TIBET REVISITED

Field work conducted in September 1998 and Summer 1999 aimed to reassess the ophiolitic segment of the Yarlung Zangbo suture zone (YZS) and shed new light on the preserved fragments of Neo|Tethys ocean|floor. This eastern ophiolitic segment was partly surveyed during the 1980 Sino|French Cooperative Investigation of Himalayas, but little work has been done since that time. Progress in ophiolite research field and new developments in modern ocean crust guided us in the recent field work investigation. Mantle peridotites and associated minor crustal units are assumed Early Cretaceous in age, while diabase interbedded with phyllites and radiolarian sediments of presumed seamount origin are attributed to Late Jurassic—Early Cretaceous age. Six different massifs were visited that are from west to east: Jiding, Qunrang, Beimarang, Dazhuqu, Luobusa, and Zedang. Each massif presents specific characteristics summarized below. The Jiding massif is made of partly to totally serpentinized granular upper mantle harzburgites with orthopyroxenite banding, a transitional Moho zone, a thick diabase sill|dike complex intruded into heterogeneous gabbro, and pillow lavas.. High|temperature plastic foliation, although generally oriented NW—SE, and lineation show folding. Numerous gabbroic and diabasic intrusions are observed in peridotites. The orientations of the mafic rocks foliation and lineation do not fit the structure of the host peridotites. The 350m thick transition zone is a syntectonically intrusive sequence of mantle peridotites cut by abundant different types of gabbro and diabase. In one case intrusion of gabbro postdates serpentinization of peridotites and the outer margin of the xenolith enclosed in fine|grained gabbro has reacted to form pegmatitic hornblende gabbro. The crustal unit is made of gabbro intruded by multiple fine|grained dikes. Hydrothermal circulation was locally intense and Cu mineralization and epidosite are observed close to shear zones.The Qunrang massif shows no transition zone overlying upper mantle unit, no significant gabbroic crustal unit and thick diabase and volcanic units. The foliation and lineation in granular lherzolite, harzburgite, and dunite show extremely wide variations and affected by late tectonics. The orientation of the structures is similar to the Jiding massif.