NEW ADVANCED IN THE STUDY OF THE BAOBAN GROUP IN HAINAN PROVINCE

According to the latest research, the Baohan Group may be divided in ascending order into the Gezhencun Formation and Ewenling Formation. Their basic features are as follows: （1） The Gezhencun Formation consists principally of gneiss with granulitite, ＞2 381 m thick, strongly migmatized. This formation has been determined to have a zircon concordant upper intercept age of 1 824±77 Ma, and the migmatite veins have a Rb Sr isochron age of 1 637±31 Ma.It is inferred that this formation orginated between 1 600 and 1 800 Ma B.P., largely corresponding to the early part of the Changchengian period. （2）the Ewenling Formation is chiefly composed of various types of schist with subordinate quartzite and granulitite, no migmatitization being seen. Zircons from a metamorphosed granite body intruded into the formation yielded three U Pb concordant ages: 1 456, 1 441 and 1 364 Ma. Another zircon gave an evaporation 207 Pb / 206 Pb peak age of 1 407 Ma. From these isotopic ages it may be judged that its age ranges from 1600 to 1 400 Ma, corresponding to the late part of the Changchengian Period.