甘肃龙首山芨岭辉绿岩地球化学特征及其地质意义

辉绿岩脉在南方花岗岩型铀矿成矿作用中起到较为重要的作用,芨岭钠交代型铀矿床周边也有产出.通过岩石学、岩相学、地球化学和同位素年代学研究,确定芨岭铀矿床外围的辉绿岩是由碱性玄武岩浆上侵形成的,具有富铝(15.97%~19.21%)、高碱(4.36%~6.89%)、中钛(1.18%~2.02%)的特征,弱铕负异常(δEu=0.65~0.86),轻稀土元素分异较强而重稀土元素较弱.相对富集LILE(Rb、Ba)和不相容元素(Th、U),亏损HFSE(Nb、Ta、Hf、Ti).辉绿岩的LA-ICP-MS锆石U-Pb年龄为485 Ma,形成于奥陶纪早期.Mg O含量(3.79%~8.22%)和Mg#值(42.4~66.5)均有较大的变化.岩浆经历了橄榄石、辉石和少量斜长石分离结晶后上侵定位.辉绿岩脉的产出标志着祁连山-龙首山早古生代造山带后碰撞伸展背景的开始,上涌地幔对酸性岩浆的形成、演化直至碱性岩和钠交代型铀矿化出现带来了大量的热能.

GEOCHEMICAL CHARACTERISTICS AND GEOLOGICAL IMPLICATION OF THE JILING DIABASE IN GANSU PROVINCE

As one of the important factors for uranium metallogenesis in southern China, diabase occurs around the Jiling sodium-metasomatic uranium deposit. With study on petrology, petrography, geochemistry and isotopic chronology, it is confirmed that the diabase surrounding the Jiling uranium deposit is the product of upwelling alkali basaltic magma, which is characterized by rich-aluminum (15.97%-19.21%), high-alkali (4.36%-6.89%), med-titanium (1.18%-2.02%) and weak negative Eu anomaly. The chondrite-normalized REE patterns show strong LREE and weak HREE fractionations, with relatively enriched LILEs (Rb and Ba) and incompatible elements (Th and U), and depleted HFSE (Nb, Ta, Hf and Ti). The LA-ICP-MS zircon U-Pb age of the diabase is 485 Ma, indicating it was formed in Early Ordovician. Both the MgO contents (3.79%-8.22%) and Mg^# values (42.4-66.5) vary greatly. The basic magma underwent the fractional crystallization of olivine, pyroxene and plagioclase before its emplacement into the crust. The Jiling diabase in the Longshoushan metallogenic belt marks the beginning of post-collisional extension setting, in which the upwelling mantle with heat brought the formation and evolution of acidic magma and alkaline rock, as well as the sodium-metasomatic uranium mineralization.