Geochemistry And Geological Significance Of Serpentinites In The Sulu Ultrahigh-pressure Metamorphosed Belt

The Dabie-Sulu ultrahigh-pressure?UHP?metamorphic belt is an orogenic belt formed by subduction of the Yangtze Craton?YZC?underneath the North China Craton?NCC?following the collision of the two continents during the Triassic.Sulu ultrahigh-pressure?UHP?metamorphic belt is the eastward extension part of the Dabie-Sulu UHP metamorphic belt,which is displaced by the Tan-Lu fault.There are small amounts of mafic-ultramafic rock outcrops in the Dabie–Sulu UHP belt,of which the Yangkou in Qingdao and Suoluoshu in Rizhao are the important outcrops of serpentinites.The formation and evolution of serpentinite in the subduction zone are closely related to subduction,dehydration and exhumation of continental plates,so the study of its geochemistry,petrogenesis and evolution can help us to understand deep crust-mantle interaction,hydrous fluid activity and formation of island arc magma.In this paper,we select serpentinites from the Yangkou?YK?and Suoluoshu?SLS?in the Sulu UHP metamorphic belt as the object of study.Through systematic analysis of petrogeochemistry,it can be found that serpentinites from Yangkou and Suoluoshu contain high contents of Mg,low ratios of Al₂O₃/SiO₂ and relatively depleted HFSE?Nb,Th,Zr and Hf?and HREE in whole rocks,which show characteristics of the forearc mantle peridotites.It reveals that protoliths of these serpentinites are residual peridotites formed from different degrees of partial melting.Estimation suggests that the partial melting degree is less than 15%for the Yangkou serpentinites,and 20%²5%for the Suoluoshu serpentinites.Besides,we can judge the precursor minerals before alteration through analyzing mineral composition?Al,Sc,V,Cr,Y,etc.?of serpentines.As a consequence,the protoliths for Yangkou serpentinites are harzburgites and for Suoluoshu serpentinites are mainly dunites with minor harzburgites.By studying the geochemical characteristics of serpentinites,we found that protoliths of studied serpentinites had undergone serpentinization by hydrous fluids released from the subducted slab and formed serpentinites enriched in fluid-mobile elements?U,Pb,As,Cs,etc.?in the forearc mantle wedge.By comparing the mineral composition of serpentines with different precursor minerals,the process of serpentinization and behavior of fluid-mobile elements can be divided into 3 stages:a.When the Yangtze plate just subducted into the mantle wedge of North China Craton,the temperature was 130¹50°C,hydrous fluid released from the subducted slab caused alteration of mantle wedge peridotites.Some of pyroxenes occurred chloritization and olivines began to be altered into serpentines.b.With the increase of subduction depth and temperature?<350°C?,pyroxenes began to occur serpentinization,and the alteration degree of olivines was strongest?the main active elements were As,Sb,etc.?.c.When temperature was above350°C?<650°C?,serpentinization of olivines had nearly completed,and the major alteration was serpentinization of pyroxenes?the main active elements were Cs,Li and Ba?.Finally,the Sulu serpentinites were formed completely by serpentinization.