Genesis Of The Fuchuan Chromitite In South Anhui,China,and Its Geological Significance

Fuchuan ophiolite,one of the oldest ophiolites in southern China,is located in the northeasternmost segment of Jiangnan orogen,and hosts a small high Al podiform chromite ore deposit.Because of its small reserves,low research efforts have been made in this area.Thus,in order to investigate the origin of the Fuchuan ophiolite and its related chromite ore deposit,and provide evidences for the tectonic evolution of Jiangnan orogen,detailed field survey and indoor comprehensive studies have been carried out,including the mineralogy,major and minor elements of chromites and paragenetic minerals in ophiolite and chromitite,and the whole rock major,minor elements and platinum-group elements(PGEs)of ophiolite and chromitite.And the main proceedings and recognitions are as follows:(1)Fuchuan harzburgite is suggested to be formed in a back arc spreading ridge,and might suffered mantle metasomatism by later LREE enriched fluids.The Fo(100×Mg/(Mg+Fe²⁺)of olivines in harzburgite is 90.65?92.17,Mg^#(100×Mg/(Mg+Fe²⁺)of orthopyroxenes is91.43?92.17.And the oxygen fugacity is low(-0.86 to+0.68log QFM),showing the feature of transition from MORB to SSZ setting.Furthermore,LREE and LILE of the whole rock trace element in harzburgite are relatively enriched,and the“U”shaped REE pattern,should be caused by metasomatism of peridotites and LREE enriched fluid produced during the subduction.(2)Fuchuan chromitite is derived from MORB.The calculated component of parental melts of the chromitite is similar to MORB,with the averaging(Al₂O₃)_melt,(Ti O₂)_meltand(Fe O/Mg O)_meltof 15.08wt.%,0.69wt.%and 1.05.The MORB normalized major and minor element diagram of the chromites in chromitite is also similar with the MORB,indicating chromitite formation was related closely to the MOR melt.(3)All the spinels and chromites are typical Alpine type high-Al chromites,most of which develop fractures,and along the rim and fractures are altered into ferritchromite.The unaltered core part of the chromites which retain the pristine composition,and the spinels in harzburgite are characterized by moderate Cr^#(100×Cr/(Cr+Al),54.12?64.40)and low Mg^#(100×Mg/(Mg+Fe²⁺),36.21?55.20),and that of the chromites from chromitite are53.97?62.59 and 58.10?69.13,respectively.After the chromites altered into ferritchromite,Mg^#declines,and Cr^#and Fe^3+#(Fe³⁺/(Cr+Al+Fe³⁺)rise,forming high Cr and Fe²⁺ferritchromite.(4)The author believes that the formation of Fuchuan ophiolite and chromitite have been experienced the following three stages,which sheds light to the tectonic evolution of Jiangnan orogan.(1)1.0Ga-880Ma,oceanic subduction formed the oceanic arc along the southeastern margin of the Yangtze block;(2)890-860Ma,the breakoff and rollback of the subducted plate developed a backarc basin above the mantle wedge and separated the island arc from the Yangtze block,and the upwelling of asthenosphere mantle entered into the magma chamber in the backarc spreading ridge,resulting in a large-scale rock-melt interaction which continuously crystallized chromites and formed a chromite ore deposit,meanwhile,with the subducted-realted fluids,Fuchuan peridotites exhibited the dual feature of abyssal peridotites and SSZ peridotites;(3)?820Ma,Yangtze and Cathysia blocks were collaged eventually at Fuchuan,and the Fuchuan ophiolites obducted with the oceanic crust.(5)By collecting and comparing of the geochemical features of the chromitite in different ophiolites,and combining the geochemical data of Fuchuan ophiolite,we consider that the geochemical indicators for medium and large-sized chromite ore deposits are as follows:medium Cr^#(?50)of the spinels in harzburgite,high Cr/Fe values(>2.3)of the chromites,low Pd/Ir(<2.0)of the chromitite,and the moderately depleted ophiolites.By using the above indicators,and integrating the exploration information of Fuchuan area,we infer that Fuchuan chromitite might have little metallogenetic potentiality to form medium and large-sized chromite ore deposits.