DURATION OF REGIONAL METAMORPHIC EPISODES IN AREAS OF POLYCYCLIC ENDOGENIC PROCESSES - A U-PB GEOCHRONOLOGICAL STUDY

A method is proposed to estimate the duration of regional metamorphic episodes in areas of polycyclic endogenic processes. This method inclu des the following: (1) reconstruction of the sequence of endogenic pro cesses on the basis of independent structural and age scales; (2) anal ysis of a correlative scheme of the endogenic processes in the context of the tectono-metamorphic cycle conception (Miller, 1982; Miller et al., 1992); and (3) identification of the early syntectonic and postte ctonic intrusive granitoid complexes, which mark the initial and final stages of a tectono-metamorphic cycle, and their U-Pb zircon dating. A study of the Precambrian rocks of the Baidarik block in the Bayan-Kh ongor zone of Early Caledonian orogeny in Central Mongolia and some pa rts of the Aldan granulite-gneiss terrain of the Aldan shield shows th at the tectono-metamorphic cycles and the corresponding episodes of re gional metamorphism were relatively short; their duration ranged from 20 to 75 Ma.