The big mantle wedge and decratonic gold deposits

The Circum-Pacific subduction zone is a famous gold metallogenic domain in the world,with two important gold metallogenic provinces,the North China Craton and Nevada,which are related to the destruction of the North China Craton and the Wyoming Craton,respectively.Their ore-forming fluids were possibly derived from the stagnant slab in the mantle transition zone.The oceanic lithospheric mantle usually contains serpentine layers up to thousands of meters thick.During plate subduction,serpentine is dehydrated at depths of<200 km and transformed into high-pressure hydrous minerals,known as Phases A to E,which carries water to the depth of>300 km.The overlying big mantle wedge is hydrated during the breakdown of these hydrous facies in the mantle transition zone.The dehydration of the subducted slab in the big mantle wedge releases sulfur-rich fluid,which extracts gold and other chalcophile elements in the surrounding rocks,forming gold-rich fluid.Because the cratonic geotherm is lower than the water-saturated solidus line of lherzolite,the fluid cannot trigger partial melting.Instead,it induces metasomatism and forms pargasite and other water-bearing minerals when it migrates upward to depths of less than 100 km in the cratonic lithospheric mantle,resulting in a water-and gold-rich weak layer.During the destruction of craton,the weak layer is destabilized,releasing gold-bearing fluids that accelerate the destruction.The ore-forming fluids migrate along the shallow weak zone and are accumulated at shallow depths,and subsequently escape along deep faults during major tectonic events,leading to explosive gold mineralization.The ore-forming fluids are rich in ferrous iron,which releases hydrogen at low pressure through iron hydrolysis.Therefore,decratonic gold deposits are often reduced deposits.

Carbonated Big Mantle Wedge Extending to the NE Edge of the Stagnant Pacific Slab:Constraints from Late Mesozoic-Cenozoic Basalts from Far Eastern Russia

It has been suggested that the carbonated mantle reflected by Mg-Zn isotopic anomalies of Cenozoic intraplate basalts from East Asia coincides with the stagnant West Pacific slab in the mantle transition zone.However,the northern boundary of such carbonated domain beneath East Asia is uncertain.Late Mesozoic-Cenozoic intraplate basalts are widespread in far eastern Russia and thus provide an opportunity to examine this issue.Here we report major-trace element contents and Sr-NdMg-Zn isotopic compositions for 9 Late Mesozoic-Cenozoic basaltic samples from the Khanka Block and Sikhote-Alin accretionary complex.They are characterized by large variations in SiO_(2)contents(41 wt.%to 50 wt.%)and CaO/Al_(2)O_(3)(0.50 to 0.97),enrichments of large-ion lithophile elements(LILE),positive Nb-Ta anomalies and strongly negative K,Pb,Zr,Hf,Ti,Y anomalies in primitive mantle-normalized trace element spider diagram.Furthermore,the rocks show good correlations of Ti/Ti^(*)with Hf/Hf^(*),La/Yb,Fe/Mn and trace element contents(e.g.,Nb).In addition,they have lighter Mg and heavier Zn isotope compositions than the BSE estimates,coupled with depleted Sr-Nd isotope compositions.These elemental and isotopic characteristics cannot be explained by alteration,magma differentiation or diffusion,but are consistent with the partial melting of carbonated peridotite.By and large,the Late Mesozoic-Cenozoic basalts from far eastern Russia bear very similar geochemical characteristics as those Na-series Cenozoic basalts from eastern China.The extended region of Mg-Zn isotopic anomalies is roughly coincident with the stagnant West Pacific slab beneath East Asia,and all of these alkali basalts can be generated from mantle sources hybridized by recycled Mg-carbonates from the Pacific slab stagnant in the mantle transition zone.We infer that(1)the carbonated big mantle wedge extends to the NE edge of the West Pacific slab and may have also appeared in the Late Mesozoic due to the effect of the Paleo-Pacific slab beneath this region,and(2)decarbonation of stagnant slabs in the mantle transition zone is a key mechanism for carbon outgassing from deep mantle to surface via intraplate alkali melts.