LATERAL ZONALITY OF THE ANYUI SEGMENT OF THE CHUKCHI PLUTONIC BELT (WESTERN CHUKOTKA)

Early Cretaceous granitoids of the Anyui segment of the Chukchi plutonic belt are represented by associations: gabbro-granite, monzodiorite-granite and granite-leucogranite. For the rocks of gabbro-granite and monzonite-granite associations with respect to the primitive mantle, enrichment of Cs, Rb, K was established, with depletion of Nb, Hf, Ti. Granitoids of the granite-leucogranite association are characterized by the most contrasting ratios of enriched and depleted elements, with deep lows of Ba, Nb, Sr, P, Ti against the background of high Cs, Rb, K, Th, U, as well as the absence of a minimum of Hf. Along with the presence of lateral zonality in the placement of granitoids in relation to the southwestern border of the Anyuy Terrain, these data suggest the subductive nature of the formation of gold-bearing gabbro-granite and monzonite-granite associations in the conditions of an active continental margin that arose in the southwestern part of the Chukchi Terrain in Early Cretaceous time due to subduction.

Compositional mantle layering revealed by slab stagnation at ~1000-km depth

Improved constraints on lower-mantle composition are fundamental to understand the accretion, differentiation, and thermochemical evolution of our planet. Cosmochemical arguments indicate that lower-mantle rocks may be enriched in Si relative to upper-mantle pyrolite, whereas seismic tomography images suggest whole-mantle convection and hence appear to imply efficient mantle mixing. This study reconciles cosmochemical and geophysical constraints using the stagnation of some slab segments at ~1000-km depth as the key observation. Through numerical modeling of subduction, we show that lower-mantle enrichment in intrinsically dense basaltic lithologies can render slabs neutrally buoyant in the uppermost lower mantle. Slab stagnation (at depths of ~660 and ~1000 km) and unimpeded slab sinking to great depths can coexist if the basalt fraction is ~8% higher in the lower mantle than in the upper mantle, equivalent to a lower-mantle Mg/Si of ~1.18. Global-scale geodynamic models demonstrate that such a moderate compositional gradient across the mantle can persist can in the presence of whole-mantle convection.