High-resolution P wave tomography shows that the subducting Pacific slab is stagnant in the mantle transition zone and forms a big mantle wedge beneath eastern China. The Mg isotopic investigation of large numbers of mantle-derived volcanic rocks from eastern China has revealed that carbonates carried by the subducted slab have been recycled into the upper mantle and formed carbonated peridotite overlying the mantle transition zone, which becomes the sources of various basalts. These basalts display light Mg isotopic compositions(δ26 Mg = –0.60‰ to –0.30‰) and relatively low87 Sr/86 Sr ratios(0.70314–0.70564) with ages ranging from 106 Ma to Quaternary, suggesting that their mantle source had been hybridized by recycled magnesite with minor dolomite and their initial melting occurred at 300-360 km in depth. Therefore, the carbonate metasomatism of their mantle source should have occurred at the depth larger than 360 km, which means that the subducted slab should be stagnant in the mantle transition zone forming the big mantle wedge before 106 Ma. This timing supports the rollback model of subducting slab to form the big mantle wedge. Based on high P-T experiment results, when carbonated silicate melts produced by partial melting of carbonated peridotite was raising and reached the bottom(180–120 km in depth) of cratonic lithosphere in North China, the carbonated silicate melts should have 25–18 wt% CO2 contents, with lower Si O2 and Al2 O3 contents, and higher Ca O/Al2 O3 values, similar to those of nephelinites and basanites, and have higher εNdvalues(2 to 6). The carbonatited silicate melts migrated upward and metasomatized the overlying lithospheric mantle, resulting in carbonated peridotite in the bottom of continental lithosphere beneath eastern China. As the craton lithospheric geotherm intersects the solidus of carbonated peridotite at 130 km in depth, the carbonated peridotite in the bottom of cratonic lithosphere should be partially melted, thus its physical characters are similar to the
It has been proposed that the North China Craton(NCC)was thinned up to a thickness of>100 km during the Phanerozoic,and underwent an associated craton destruction.Evidently,it is an important topic worthy of future study to understanding the mechanism of cratonic destruction and its role played in the continental evolution.After synthesized the global cratons of India,Brazil,South Africa,Siberia,East Europe(Baltic)and North America,we found that lithospheric thinning is common in the cratonic evolution,but it is not always associated with craton destruction.Most cratons was thinned by thermal erosion of mantle plume or mantle upwelling,which,however,may not cause craton destruction.Based on the studies of the North American and North China Cratons,we suggest that oceanic subduction plays an important role in caton destruction.Fluids or melts released by dehydration of the subducted slabs metasomatize the mantle wedge above and trigger extensive partial melting.More importantly,the metasomatized mantle lost its original rigidity and make craton easier to be deformed and then to be destoyed.Therefore,we suggest that the widespread crust-derived granite and large-scale ductile deformation within the continental crust can be regarded as the petrological and structural indicators of craton destruction,respectively.
Systematical studies of post-collisional igneous rocks in the Dabie orogen suggest that the thickened mafic lower crust of the orogen was partially melted to form low-Mg#adakitic rocks at 143–131 Ma.Delamination and foundering of the thickened mafic lower crust occurred at 130 Ma,which caused the mantle upwelling and following mafic and granitic magmatic intrusions.Migmatite in the North Dabie zone,coeval with the formation of low-Mg#adakitic intrusions in the Dabie orogen,was formed by partial melting of exhumed ultrahigh-pressure metamorphic rocks at middle crustal level.This paper argues that the partial melting of thickened lower and middle crust before mountain-root collapse needs lithospheric thinning.Based on the geothermal gradient of6.6°C/km for lithospheric mantle and initial partial melting temperature of^1000°C for the lower mafic crust,it can be estimated that the thickness of lithospheric mantle beneath thickened lower crust has been thinned to<45 km when the thickened lower crust was melting.Thus,a two-stage model for mountain-root removal is proposed.First,the lithospheric mantle keel was partially removal by mantle convection at 145 Ma.Loss of the lower lithosphere would increase heat flow into the base of the crust and would cause middle-lower crustal melting.Second,partial melting of the thickened lower crust has weakened the lower crust and increased its gravity instability,thus triggering delamination and foundering of the thickened mafic lower crust or mountain-root collapse.Therefore,convective removal and delamination of the thickened lower crust as two mechanisms of lithospheric thinning are related to causality.
