THE DISINTEGRATION OF THE BOIN SUM GROUP AND ITS TECTONIC SIGNIFICANCE AS VIEWED FROM ISOTOPIC AGE DATA

This paper mainly deals with the stratigraphical and tectonic significance of several groups of isotopic ages obtained from the Boin Sum Group rocks. There have been a lot of disputes about the original Boin Sum Group. In this paper the group is divided into two parts: （1） the Boin Sum Group: middle-upper Proterozoic in age, with a zircon age of 1525 to 620 Ma; （2） The Xuniwusu Group: Silurian in age, containing coral fossils, with a Rb-Sr isochron age of 427.5±17.5 Ma. It has been determined preliminarily that the early Paleozoic paleo-island arc in the vicinity of Boiu Sum in the middle part of Inner Mongolia is a continental-magin island arc. Its volcanic chain occurs as islands striking nearly E-W, and its back-arc area is a shallow sea.