Abstract:
A variety of geological, petrological, and geochemical techniques were used in examining the internal structure of the Kivakka layered intrusion, whose thickness attains 2000 m. The following zones were distinguished in the vertical section of the intrusion: the Upper and the Lower Contact zones and the Layered Series. The Layered Series is composed of a succession of cumulates (listed in order from bottom to top): olivine, bronzite-plagioclase, bronzite-plagioclase-augite, and plagioclase-augite-pigeonite. The section is characterized by analyses (for major and trace elements) of 87 rock samples. Based on cumulative mineral assemblages, the Layered Series was subdivided into four zones: Olivinite, Norite, Gabbronorite, and Gabbronorite with pigeonite as a low-Ca pyroxene. The distribution of major oxides was used to distinguish six megarhythms. The boundaries between the lower four megarhythms are accompanied by horizons of low-sulfide mineralization with elevated concentrations of precious metals. The outlines of the intrusion suggests that its original shape was a vertical inverted cone with an apex angle of approximately 80° and a height of approximately 3.9 km. Later, the intrusion was inclined to the northwest at an angle of 36° and partly eroded. The weighted-average composition of the intrusion is close to the mean composition of the contact-facies rocks and is approximated by basalts of the marianite-boninite series. The vertical succession of cumulative mineral assemblages (which is typical of the crystallization of boninitic magma) and the trends in the trace-element concentrations of the rocks testify that the development of the intrusive layering was controlled predominantly by crystallization differentiation. Our data will be utilized in the computer simulation of the development of the intrusion with the use of the COMAGMAT program package.