Thermal evolution of the northeastern Siberian platform in the light of apatite fission-track dating of the deep drill core

Thermal evolution of the continental crust beneath the northeastern Siberian craton was studied based on the interpretation of apatite fission-track ages. The samples selected for AFT dating were collected from depths between 2 and 3 km along a 1000-km-long profile, from the crystalline basement of the Siberian platform. The AFT ages range from 185 to 222 Ma, indicating that in the late Triassic-early Cretaceous, the top of the crystalline basement was cooled below ~100 ° C. Once the apatite cooled below this temperature, it began to accumulate and preserve tracks produced by spontaneous fissioning of 238 U, and the number of tracks pre- served is effective in determining the ages of events using the apatite fission-track method (AFT). The study showed that the apatite from Archean rocks was largely formed at 1.8-1.9 Ga as a result of a Pale- oproterozoic metamorphic overprinting during the terrane collision and the subsequent accretion of the Siberian craton. The last thermal event, the self-heating of the collision prism, was terminated by cooling at ~1.3 Ga. At that time, the Rb-Sr isotopic system became closed and the upper crust passed the ~300°e isograd. The calcu- lation results showed that on further cooling, the ~100°e isograd was passed at 1143 Ma. This age estimate could be obtained using AFT dating if the above event had been the last one in the thermal history of the Sibe- rian craton. The obtained track ages indicate the existence of a repeated, significantly younger, heating of the crystalline crust due to some local reason. The present-day temperature at the sampling depth is 29°e based on the geothermal gradient curve, which was derived from the surface heat flow rate of 25 mW/m 2 and the crustal heat production value calculated from the concentrations of radioactive elements in the crust. In the near geological past, the maximum heat flow reaching 45 mW/m 2 was determined from the mineral equilibria in the mantle xenoliths from the 245-135 Ma kimber- lites. Based on the respective geotherm, the crustal temperature in the study area did not exceed 53°e . It is obvi- ous that an enhanced heat flux during kimberlite magmatism would not cause additional heating of the crust in the Mesozoic above ~ 100°e . The emplacement of flood basalts at 250 Ma was undoubtedly most important but underestimated the thermal event in the Mesozoic history of the Siberian platform. Numerous dolerite dykes were intruded at a great dis- tance, far beyond the limits of the volcanic facies of the Putorana plateau, extending onto the study area. They were probably sourced from sills, which were emplaced along the base of the crust during basaltic underplating. The calculation results suggest that the reason of the crust's heating in the Mesozoic (222-185 Ma) might have been the emplacement of a large sill (or sills) into the base of the crust at 250 Ma. This heating of the top of the crystalline basement of the Siberian platform in the Mesozoic above ~100°e suggests that the respective heating event may have occurred at the base of the sedimentary cover and, thus may help in obtaining the age dates of the processes of the transformation of organic matter and oil generation.