Sheared peridotite xenolith from the V.Grib kimberlite pipe,Arkhangelsk Diamond Province,Russia:Texture,composition,and origin

The petrography and mineral composition of a mantle-derived garnet peridotite xenolith from the V.Grib kimberlite pipe（Arkhangelsk Diamond Province,Russia） was studied.Based on petrographic characteristics,the peridotite xenolith reflects a sheared peridotite.The sheared peridotite experienced a complex evolution with formation of three main mineral assemblages：（1） a relict harzburgite assemblage consist of olivine and orthopyroxene porphyroclasts and cores of garnet grains（Gar1） with sinusoidal rare earth elements（REE） chondrite C1 normalized patterns;（2） a neoblastic olivine and orthopyroxene assemblage;（3） the last assemblage associated with the formation of clinopyroxene and garnet marginal zones（Gar2）.Major and trace element compositions of olivine,orthopyroxene,clinopyroxene and garnet indicate that both the neoblast and clinopyroxene-Gar2 mineral assemblages were in equilibrium with a high Fe-Ti carbonate-silicate metasomatic agent.The nature of the metasomatic agent was estimated based on high field strength elements（HFSE） composition of olivine neoblasts,the garnet-clinopyroxene equilibrium condition and calculated by REE-composition of Gar2 and clinopyroxene.All these evidences indicate that the agent was a high temperature carbonate-silicate melt that is geochemically linked to the formation of the protokimberlite melt.

Carbonate metasomatism in the lithospheric mantle: Implications for cratonic destruction in North China

The activity of melts and fluids may have played a key role in inducing the destruction of the eastern North China Craton in the early Cretaceous. Carbonate melts are important agents in mantle metasomatism and can significantly modify the physical and chemical properties of the subcontinental lithospheric mantle. Carbonate metasomatism can be identified by specific geochemical indices in clinopyroxene, such as high Ca/Al and low Ti/Eu ratios. This study presents the spatial and temporal variations of carbonate metasomatism in the lithospheric mantle beneath the eastern North China Craton. Three types of carbonate metasomatism are classified based on the geochemical compositions of clinopyroxene in mantle peridotites. Clinopyroxene formed by Type 1 carbonate metasomatism is characterized by very high Ca/Al ratios(15–70) and^(87)Sr/^(86)Sr ratios(0.706–0.713). Clinopyroxene derived from Type 2 carbonate metasomatism shows relatively high Ca/Al ratios(5–18) and^(87)Sr/^(86)Sr ratios(0.703–0.706). However, clinopyroxene resulting from Type 3 carbonate metasomatism has low Ca/Al ratios(5–9) and^(87)Sr/^(86)Sr ratios(0.702–0.704). Deep(garnet-bearing) and shallow(spinel-bearing) lithospheric mantle beneath the Sulu orogen and surrounding areas in the eastern North China Craton were affected by intense Type 1 carbonate metasomatism before the late Triassic. The deep subduction of the South China Block with its accompanying carbonate sediments was the trigger for Type 1 carbonate metasomatism, which reduced strength of the lithospheric mantle and provided a prerequisite for the destruction of the eastern North China Craton in the early Cretaceous. After the destruction of the eastern North China Craton, the ancient relict lithospheric mantle, represented by spinel harzburgite xenoliths hosted in the late Cretaceous to Cenozoic basalts,only recorded Type 2 carbonate metasomatism. This implies that the lithospheric mantle experienced the intense Type 1 carbonate metasomatism was completely destroyed and not preserved during decratonization. Spinel lherzolite xenoliths hosted in the late Cretaceous to Cenozoic basalts represent the young, fertile lithospheric mantle formed after the cratonic destruction and only a few samples record Type 2 and 3 carbonate metasomatisms. We suggest that carbonate melts derived from the subduction-modified asthenospheric mantle with variable proportions of recycled crustal material was responsible for the Type 2 and 3 carbonate metasomatisms. The carbonate metasomatism of the lithospheric mantle beneath the Jiaodong Peninsula and surrounding areas is very pervasive and is spatially consistent with the remarkable thinning of lithospheric mantle and giant gold deposits in this region. Therefore, we conclude that carbonate metasomatism in the lithospheric mantle played a crucial part in the modification, destruction and gold deposits in the eastern North China Craton.