Home – Home – Jornals – Russian Geology and Geophysics 2005 number 12

Study of xenoliths from the Udachnaya and Leningradskaya kimberlite pipes has shown that, among the rocks of the bottom of the Earth's crust in the Daldyn-Alakit district, the garnet granulites make up no less than 50%. Geochemical features of garnet granulites (high percentage of potassium and incompatible elements) show that they cannot be considered restites but are fragments of deep-seated intrusions crystallized in the lower crust. According to estimated pressure and temperature of equilibrium, the lower crust is dominated by garnet granulites (9-13 kbar), which upsection grade into two-pyroxene granulites (8.5-10 kbar).
The Sm-Nd dating of xenoliths in garnet granulites shows no isotope equilibrium. An inner isochron was obtained for none of the specimens. Model ages of the xenoliths evidence that most of the lower crust in the Yakutian Diamond Province was formed in the Archean (2.97-2.75 Ga). At the same time, the inner Pb-Pb isochron (1424±21 Ma) for a specimen of garnet granulites from the Leningradskaya kimberlite pipe as well as the model age (1.24 Ga) of granulite from the Udachnaya kimberlite pipe indicate the Neoproterozoic stage of thermal activity.
On the basis of data on velocities of travel of compressional waves in the lower crust (6.8-7.0 km/s) and obtained estimates of xenoliths of garnet granulites in kimberlite pipes, a conclusion is made that in the lower crust of the Daldyn-Alakit district garnet granulites form separate bodies in gneisses rather than a separate bed.