THE ANCIENT TONGBO-XINYANG OPHIOLITE ZONE AND MELANGES

The Song-Mei fracture （whose full name is the Songba-Guishan-Meishan tracture） on the northern slopes of the Tongbo and Dabie mountains is a regional, reverse, revived, deep fracture that dips southwest at a high angle. More than 400 km long and accompanied by a 1-5-km-wide mylonite zone, the fracture shows the characteristics of long-continued activity. It began to occur in Precambrian time （probably equivalent to the Jinningian Stage） and ended in Cretaceous time （Yanshanian Stage）. The Caledonian and Variscan times witnesses its strongest activity. The ancient ophiolite zone and mélanges lie at the northern side of the fracture. Intimately associated, both are elongated along the fracture zone. The exposed width is 5-25 km. In the area, the ophiolites may be divided into the Precambrian and early Paleozoic ones. The former is distributed to the north of the latter. Boht dip southwest and are in faulted contact with each other. From below upwards, the Precambrian ophiolite section consists roughly of （1） maficultramafic complex （clinopyroxene peridotite, pyroxenite and troctolite）, （2） stratoid gabbro and homogeneous gabbro, （3） spillite and basalts, and （4） siliceous rocks and fine-grained volcaniclastic rocks. The metamorphic grade is equivalent to the greenschist-amphibolite facies. From below upwards, the early Paleozoic ophiolite section roughly comprises （1） metaharzburgite, （2） dunite, olivine pyroxenite and troctolite complex, （3） stratoid gabbro and homogeneous gabbro, （4） pillow lavas and basalts, and （5） finegrained volcaniclastic rocks and siliceous rocks. According to Xu’s discrimination rule for mélanges and the differences between fragments and matrix of mélanges in respects of composition and origin, the mélanges in the area may be divided into ophiolitic mélange, olistostromic mélange and mud-debris mélange. The three kinds of mélange display both faulted contacts and mixing relations. In space （from north to south）, the ophiolitic mélange lies below the olistostromic mélange, while the mud-debris mélange lies in between them. In time, the olistostromic mélange formed in the Precambrian （Sinian?）, the ophiolitic mélange in the early Paleozoic （possibly Ordovician-Silurian）, and the mud-debris mélange largely at the end of the early Carboniferous （290 m. y. B. P.）. The three kinds of mélange formed in three different periods of time, which might suggest three activity stages of plate-tectonic movements （including subduction or collision） in the area.