By study the petrological and geochemical characteristics of the ultrabasic rocks in the ophiolite complex in Jiuzhaila area,the results show that the ultrabasic rocks in Jiuzhaila area are gabbro,with strong ser-pentinization.The major elements show high Mg(MgO=45.51％～46.42％),high Si(SiO2=43.93％～46.52％),poor Al(Al2O3=0.52％～0.86％)and low Fe(FeOT=8.33％～8.84％).The REE distribution model normalized by chondrite shows a trend of slightly enriched light rare earth to the right,with(La/Yb)N ranging from 0.32～9.42,with a relatively light negative Eu anomaly(Eu/Eu*=0.15～1.99)and a rela-tively light positive Ce anomaly(Ce/Ce*=0.71～1.14).The trace elements are obviously enriched in large ion lithophile elements K,Rb,P,relatively enriched in Rb,Ta,poor in Hf,Sm,Yb,belonging to a typical mafic ultrabasic rock.The minimum age of LA-ICP-MS zircon U-Pb in the sample is 119 Ma.Combined with the re-gional tectonic setting,the Tethys Ocean represented by the ophiolite in Shiquanhe area may be a local ocean basin with an independent evolutionary process,which began to subduct in the Early Jurassic and closed in the Early Cretaceous,and represents a series of interarc and backarc basin induced by the southward subduction of the Tethys Ocean.
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