New results obtained during the study on the material composition of volcanites of the §¬n D­¬ng Formation led to following conclusions:

1) The section of the formation in the §¬n D­¬ng area includes three parts: the lower one is composed of coarse–grained sediments with some interbeds of felsic effunsives and tuffaceous rocks, the middle part is characterized by the intercalation of aluminium-high dacific effusives and chocolate–coloured tuffaceous sediments, the upper part is composed mainly of felsic effusives with some interbeds of tuffaceous agglomerate.

2) Petrochemically, the rocks of the studied area belong to the aluminium-high (C_av = 1.92, sometimes – 10.65) calc-alkaline magmatic series. They are characterized by a continuous differentiated series from andesite to dacite, trachydacite, rhyolite with the SiO₂ content varying from 58.04% to 78.34% (among them dacite, rhyodacite and rhyolite play the main role), the total alkali (Na₂O+K₂O) varying from 3.6% to 9.97% with nearly equivalent sodium and potassium. The Rb/Sr ratio averages 0.57, varying from 0.34 to 1.48, with the usual predominance of Sr. The original isotopic ratio of dacite and rhyolite is low, with ⁸⁷Sr/⁸⁶Sr(i)= 0.7059-0.7067; ¹⁴³Nd/¹⁴⁴Nd(i) = 0.512332-0.512352, that reflects their mantle origin.

3) Results of isotopic dating of the studied rocks in ViÖt Nam (1996) and USA (1997) by the K/Ar and Rb/Sr method have given the values of 100± 1 and 76± 1 Ma.

4) The §¬n D­¬ng Formation was formed in the active continental margin arc setting of East Asian type.