The precambrian continental crust: Its structure, composition and evolution as revealed by deep drilling in the U.S.S.R.

Abstract

Abstract Petrologic-geochemical studies of the Kola superdeep and other deep borehole sections drilled in the Precambrian basement of the East-European Platform are discussed. Proterozoic metasediments and metavolcanic rocks that constitute the Pechenga palaeorift and the underlying Archaean granite—gneiss basement were detected by the Kola borehole in the 0–6842 m and 6842–12064 m intervals, respectively. Zones of density decrease by hydrofracture, resulting from progressive metamorphism under closed system conditions, were identified in the upper crust at a 4500–9000 m depth. Sharp textural variations of rocks, an increase in porosity and the presence of mineralizing fluids are specific features of these zones which may have contributed to tectonic deformation and metamorphically produced ore formation. Deep boreholes drilled into the more eroded Archaean basement of the Tatar Dome provided information that was used for constructing a petrologic-geochemical model of the lower crust to a depth of 25–30 km. Adacitic—trondhjemitic composition for the primeval continental crust was calculated by extrapolation of geochemical trends established for the lower crust. We propose that the lower crust formed as a result of an initial melting of the Earth's chondrite shell and only existed in pre-Archaean times. We also present data on temperatures and distribution of radiogenic elements in the Kola and some other borehole sections. A heat-production model is proposed for the Archean crust. According to the model, the crustal contribution to the heat flow is 25 mW · m −2 , or 52% of the total heat flow, and that of the mantle is 22–24 mW · m −2 , or 48% of the total flow.