Trace Element and Sr–Pb–Nd–Hf Isotope Evidence for Ancient, Fluid-Dominated Enrichment of the Source of Aldan Shield Lamproites

A phase of Mesozoic extension associated with the termination of continental collision at the southern margin of the Aldan Shield produced ultrabasic lamproites in a discontinuous belt 500 km long and 150 km wide. The lamproites, locally poorly diamondi-ferous, were emplaced as dykes, sills and pipes. All Aldan lamproites have primitive chemical characteristics (e.g. MgO up to 227wt %) and are ultrapotassic (K2O up to 83wt %) and peralkaline with K2O þ Na2O/Al2O3 in the range 06–116. A distinctive feature of these rocks is their low TiO2 content (05–14wt %). Aldan lamproites are moderately light rare earth element (LREE) enriched with (La/Yb)N ranging from 10 to 47. Heavy rare earth element (HREE) abundances are lower than for all other lamproites by up to a factor of five. Therefore, the combined major and trace element characteristics of the Aldan samples are not typical of other lamproite occurrences. Large ion lithophile element concentrations are high