东天山沙泉子南钛铁矿辉长岩锆石U-Pb年龄、地球化学特征及构造意义OA

Objectives:This study focuses on the newly identified Shaquanzinan ilmenite-bearing gabbroic pluton,hosting a medium-sized ilmenite deposit to south of the Aqikekuduke—Shaquanzi fault zone within the Central Tianshan Block.The objectives are to elucidate the mantle source tectonic setting,and ilmenite metallogenic processes in Eastern Tianshan Mountains. Methods:Zircon U-Pb dating,whole-rock major and trace element geochemistry,and Sr-Nd isotopic analyses were conducted in this study. Results:① Zircon U-Pb dating reveals the Shaquanzinan ilmenite-bearing gabbro emplaced at 486.6±2.0 Ma,demonstrating it being the oldest known ilmenite-dominated V—Ti magnetite deposit in the Eastern Tianshan Mountains area.② Geochemically,the rocks are characterized by low SiO2(38.16％～46.90％),and MgO(4.67％～7.04％)contents,as well as low Mg#(39～55,average 46)values,coupled with elevated Al2O3(12.30％～19.37％)and total alkalis(K2O+Na2O=2.47％～5.01％),consistent with an alkaline series affinity.Chondrite-normalized rare earth element(REE)patterns display significant light REE enrichment and heavy REE depletion,with variable positive Eu anomalies(Eu*=0.96～1.77).Primitive mantle-normalized trace element diagrams exhibit pronounced enrichment in Nb,Ta,Ti,Ba,Pb,and Sr,and depletion in Zr,Hf,Th,and U.Both major and trace elements compositions share an oceanic island basalt(OIB)-like features.③The ilmenite-bearing gabbros have depleted Sr-Nd isotopic compositions,with initial n(87Sr)/n(86Sr)initial ratios of 0.7042～0.7045 and εNd(t)values of+0.76 to+1.07,indicating their derivation from a depleted mantle source. Conclusions:Comprehensive investigations reveal that this gabbroic pluton was formed in a back-arc extensional setting,induced by the retreat of the Junggar oceanic plate during the Late Cambrian.The parental magma was derived from low-degree(5％～10％)partial melting of a garnet lherzolite mantle source,followed by cumulate and fractional crystallization.The enrichment of ilmenite and minor magnetite as ore-forming minerals is attributed to the residual concentration of Fe—Ti components in the early-stage melt,coupled with melt segregation and crystallization under elevated oxygen fugacity conditions during the late magmatic stage.