PRELIMINARY STUDY ON THE OPHIOLITIC MELANGE IN THE YALU TSANGPU GRAND CANYON AREA

PRELIMINARY STUDY ON THE OPHIOLITIC MELANGE IN THE YALU TSANGPU GRAND CANYON AREA

PRELIMINARY STUDY ON THE OPHIOLITIC MELANGE IN THE YALU TSANGPU GRAND CANYON AREA

Since 1980's Chinese geologists have organized several scientific expeditions with foreign colleagues to enter the Canyon to study its geology and geography.1\ Major petrologic unitsThere are three major petrologic units in this region: (1)Namche barwa formation(Pt nj ). It is situated in the center of great bend area ,representing the lower part of Indian plate wedged into Eurasia plate, which is composed of plagioclase gneiss, amphibole\|containing high\|pressure granulite lenses. (2)Gandise formation(Pt gd ).It is exposed in the outside of Namche barwa edge(Pt nj ), and is considered to be the lower part of Eurasia plate which is composed of plagioclase\|gneiss, amphibolite, marble, quartzite, etc., and were intruded by large granite batholiths (γ 2 5 to γ 6). (3)Great bend formation(Mz). It stretches along Yarlung Zangbo river and is constrained between Pt nj and Pt gd . It is composed of metamorphic basic igneous rocks , ultramafic rocks and quartzite. The whole rock assemblage is characterized by ophiolite suite. Nice ophiolitic mélange exposures exist in Pangxin, Jiaresa and between Pailong and Zhaqu.2\ Basic rock assemblageThe present ophiolitic components are mostly metamorphic rocks after structural movements and metamorphism for long period. (1) Meta\|intermediate rocks. Amphibolite is a dominant component of ophiolitic mélange in this region . Chemical analysis and protolith recognition shows that amphibolite originated from basalt, diabase and gabbro. (2) Ultramafic rocks. This kind of rocks distributes along the northeast edge of the wedge from Pangxin to Jiaresa. Original pyroxene\|olivinite and serpentine exposures have been found . Electronic\|probe and bulk\|rock analysis suggest that original olivinite is strongly MgO\|enriched and can be named magnesian olivinite. (3) Quartzite. It is exposed along the northeast and southeast margin of the wedge forming steep cliffs and high peaks . In some places it consists of pure fine\|grained quartz and may contain muscovite, biotite, plagioclase or epidote grains in different amount, which suggest that quartzite in this area was formed in small sea basin and not deep oceanic environment as traditionally considered for standard ophiolite suite. (4) Marble blocks. Pure white marble blocks have been found near the village of Bayu and Daduka which were surrounded by amphibolite , quartzite (Mz) or plagioclase\|gneiss(Pt nj ).