The Bug–Rosyn megablock is a struc­tur­al unit of the Ukrain­ian Shield char­ac­ter­ized by a com­plex geo­log­i­cal struc­ture and sig­nif­i­cant litho­log­i­cal diver­si­ty. It bor­ders the Volyn, Podil­lian, and Inhul megablocks, which are sep­a­rat­ed by a sys­tem of deep-seat­ed faults: the Bru­syliv and Shpykiv faults in the west, the Obod­niv fault in the south, the Yadlove–Trakhtemyriv deep fault in the east, and a mar­gin­al nor­mal fault in the north.

The crust of the megablock exhibits a vari­able struc­ture: the north­ern part is of a tran­si­tion­al type (40–45 km thick), while the south­ern part is of a femic type (50–60 km thick). Accord­ing­ly, the megablock is divid­ed into the Kyiv–Fastiv and Haisyn seg­ments, which dif­fer in geo­log­i­cal com­po­si­tion.

The north­ern seg­ment is com­posed of for­ma­tions of the Rosyn–Tykych series (Neoar­chaean), the Teti­iv ultra­m­eta­mor­phic com­plex, and the Yuriv­ka basic–ultrabasic com­plex. The south­ern seg­ment con­tains the Dniester–Bug series (Eo- to Pale­oar­chaean) and the Haisyn ultra­m­eta­mor­phic com­plex.

The Dniester–Bug series con­sists of gneiss­es and crys­talline schists (pyrox­ene, two-pyrox­ene, and amphibole–pyroxene vari­eties), often con­tain­ing gar­net, sil­li­man­ite, cordierite, and graphite, as well as amphi­bo­lites and calc-sil­i­cate rocks. These rocks are con­cen­trat­ed with­in the Teti­iv tec­ton­ic block and have an age of 2.7–2.58 bil­lion years (based on zir­con dat­ing).

The Rosyn–Tykych series occurs with­in gran­i­toids of the Zveny­horod and Uman com­plex­es. It is com­posed of gneiss­es, crys­talline schists, and amphi­bo­lites, with an exposed thick­ness of up to 470 m. Its age is esti­mat­ed at 2,580–2,620 mil­lion years (based on potassium–argon dat­ing of amphi­bole).

In the areas of the Volo­dar­ka and Bila Tserk­va mag­net­ic anom­alies, the Volodarka–Bila Tserk­va unit has been iden­ti­fied. It includes quartzites (both fer­rug­i­nous and non-ore-bear­ing), calc-sil­i­cate rocks, mar­bles, crys­talline schists, and amphi­bo­lites. Its con­tacts with oth­er series have not been clear­ly estab­lished, and its strati­graph­ic posi­tion remains debat­able. In the cur­rent geo­log­i­cal scheme, it is placed beneath the Rosyn–Tykych series. Fer­rug­i­nous quartzites of this unit form local iron ore deposits.

Among the ultra­m­eta­mor­phic com­plex­es, the fol­low­ing are dis­tin­guished: the Teti­iv com­plex (Neoar­chaean), Zveny­horod, Stavyshche, Haisyn, and Uman com­plex­es (Palaeo­pro­tero­zoic). The Teti­iv com­plex con­tains aplite–pegmatite gran­ites, pla­giogran­ites, gra­n­odi­or­ites, and dior­ites with an age of 2,487–2,615 mil­lion years.

The Zveny­horod Com­plex (2,140–2,065 Ma) is rep­re­sent­ed by gran­i­toids and migmatites extend­ing from Bru­syliv to the Hirs­ka Tykych Riv­er. These are synk­ine­mat­ic for­ma­tions that devel­oped from a Pale­o­pro­tero­zoic sub­strate. The Haisyn Com­plex includes pyroxene–amphibole migmatites, dior­ites, and gra­n­odi­or­ites, like­ly derived from the Dniester–Bug series.

The Uman Com­plex (2.05–1.99 Ga) com­pris­es biotite migmatites and ana­te­c­tic gran­ites, while the Stavyshche Com­plex con­sists of two-mica leu­co­crat­ic gran­ites. Their pro­toliths were rocks that had under­gone amphi­bo­lite-facies meta­mor­phism. The iso­topic age of Uman-type gran­ites and quartz dior­ites is approx­i­mate­ly 2,055 Ma.

Among intru­sive for­ma­tions of the Bug–Rosyn megablock, peri­dotites, pyrox­en­ites, horn­blendites, amphi­bo­lites, and gab­bronorites of the Yuriv­ka Com­plex (2.58 Ga, Neoar­chaean) are com­mon. In the Pale­o­pro­tero­zoic, synk­ine­mat­ic intru­sions of the Fas­tiv Com­plex formed (2.08 Ga), while in the Meso­pro­tero­zoic a dyke com­plex devel­oped (dia­bas­es and gab­bro-dia­bas­es).