Sm-Nd and Rb-Sr ages of gabbroids, granitoids, and titanomagnetite ores from layered intrusions of the Kusa-Kopan Complex (South Urals)

Abstract

732 A band of layered gabbro intrusions belonging to the Kusa–Kopan Complex is confined to the zone of the Zyuratkul riftogenic fault in the eastern part of the Bashkir Meganticlinorium, where it consists of several massifs (Matkal, Kopan, Medvedevo, Kusa) with the Ryabinovka and Gubenskii granitoid massifs located in the hanging wall (Fig. 1). The gabbro massifs enclose synonymous ilmenite–titanomagnetite deposits of the Urals in the form of stratified series, vein, and lenticu� lar lodes of continuous and disseminated ores, which alternate with gabbroids. These deposits were discov� ered in the 1930s, and since that time their geological structure and composition of ores and host rocks have been studied by many researchers. The primary atten� tion was paid to the most debatable factors responsible for strong lateral variations in the composition of rocks and ores within the complex, represented by horn� blende gabbro and gneiss–granites in the Kusa and Gubenskii massifs and ilmenite–magnetite ores in the Kusa and Medvedevo deposits in the north and by gab� bronorites (Matkal and Kopan massifs), porphyroid granophyric granites (Ryabinovka Massif), and titano� magnetite ores in the south. Some researchers explain such differences in the mineral composition by the influence of superposed metamorphic processes on rocks and ores of the Kusa Massif [1, 2], which occurred substantially after the formation of intru� sions. Our petrological studies performed in recent years revealed that the formation depth of granitoids, gab� broids, and enclosed ores increases along this band in the northerly direction from 3–4 km in the south to 20–30 km in the north. Accordingly, the P–T param� eters of rock and ore formation vary in the same direc� tion: = 0.5–3.0 kbar and T = 900–1100°C in the south (Kopan and Matkal gabbro massifs and associ� ated deposits, Ryabinovka granite massif) and P = 6– 9 kbar and T = 600–900°C in the north (Kusa and Medvedevo gabbro and Gubenskii granite massifs,