THE SHONG-CHAI HIGH-ALUMINA GRANITE MASSI F IN NORTHERN VIETNAM - SUBSTANTIATION OF THE ORDOVICIAN AGE, PETROGENESIS, AND TECTONIC POSITION

On the basis of geological and mineralogo-geochemical studies, four se ries of granitoids have been recognized in the Shong-Chai Massif. Thes e are sodium, K-sodium, potassium, and high-potassium series varying f rom granodiorites to leucocratic granites. In the direction from sodiu m to high-potassium varieties a total of alkalies increases in granito ids, and their proportion changes in favor of potassium; concentration of femic components decreases, and content of trace elements change. The complex U-Pb and Rb-Sr dating on rocks gave an age of granitoids o f 465+/-12 Ma. The mineral isochrones plotted for the bulk-biotite and bulk-muscovite pairs indicate a redistribution of radiogenic isotopes Rb and Sr at the borders 200-215 and 295-305 Ma, with which the postm agmatic transformation of granitoids is evidently associated. The isot ope data (((87)/Sr/Sr-86)(0) =0.71428, epsilon Nd(t) = =-(9.1-9.7)) su ggest the crustal origin of granitoids. The same primary Sr/Sr ratio a nd close values of epsilon Nd for all granitoid varieties suggest the same source rock participating in their generation. Most likely, this was the graywacke substrate oversaturated with alumina and enriched in heavy REE. Based on REE, the numerical modeling of the source rock me lting has shown that the sodium, K-sodium, and potassium granodiorites could melt out it at melting degrees of 55%, 76%, and 78%, respective ly. Ultrapotassium granodiorites could not be obtained from this sourc e rock. The melting of K-sodium and potassium granitoids is admissible , however, only providing the preceding potassium metasomatism of the source rock. In the process of metasomatism the rare earths were usual ly inert and, most likely, they were removed only when high-potassium metasomatites formed. The characteristic features of the tectonic posi tion of granitoids and the obtained range of rejuvenated ages suggest that the Shong-Chai Massif was exposed in the Early Mesozoic in the re gime of tectonic extension (the Cordilleran-type metamorphic core comp lex).