Mineralogy Of The Saima Alkaline Complex,Liaoning And Its Implication For Magmatic-hydrothermal Evolution

Mineralogical investigation of alkaline rock is important for exploiting the physic-chemical conditions of alkaline magma and deciphering its magmatic-hydrothermal evolution and diagenetic and metallogenic model.In this study,we take the Saima alkaline complex in the Liaoning province as an example to study the the textures,paragenetic relationships and compositional changes of rock-forming minerals and accessory minerals,especially zirconosilicates and rare earth minerals,with optical microscope,electron probe microanalyzer,in situ micro-scale X-ray diffraction technique and to exploit the Zr-REE mineralization potentiality under different physico-chemical conditions and trace the magmatic-hydrothermal evolution history.Based on mineral assemblages and textures,the Saima alkaline complex consists of central phonolite,predominant nepheline syenite and northwestern lujavrite;in the meantime,pegmatitic dikes of aegirine-augite nepheline syenite are widely spread in the nepheline syenite.In aspect of major elements,metalumonous phonolite and peralkaline nepheline syenite are potassic while the peralkaline lujavrite is sodic.The rock-forming minerals in the phonolite and nepheline are made up of K-feldspar,nepheline,biotite and minor aegirine.Pegmatitic dikes of aegirine-augite nepheline syenite show similar mineral components with nepheline syenite,but with the increased proportion of aegirine.From phonolite through nepheline syenite to lujavrite,the proportion of biotite falls dramatically,whereas that of aegirine increases by 40%in some thin sections.In comparison with potassic rocks,the melanocratic minerals in the lujavrite are distinctive.Specially speaking,biotite in the lujavrite contains highest Ti(4.7?5.2 wt.%TiO2)and F(0.9?1.2 wt.%F)contents and core of zoned aegirine is enriched in Zr(1.1?1.5 wt.%ZrO2)and REE(?REE 180-250 ppm).According to the infered temperature by nepheline compositions,the crystallization temperature of the lujavrite is slightly higher than nepheline syenite(ca.700?)and pegmatitic dikes of aegirine-augite nepheline syenite(below 5000 C).Additionally,the lujavrite melts are more reduced than nepheline synite melts based on the coupling effect between the compositional evolution of aegirine and the oxygen fugacity.The primary zirconosilicates in each type of the Saima alkaline complex are various,that is,zircon,wadeite and eudialyte occur as the typical zirconosilicate in the phonolite,nepheline syenite(pegmatitic dikes of aegirine-augite nepheline syenite)and lujavrite,respectively,revealing an increase in alkalis and water activity,but decrease in K/Na ratio with the evolution of alkaline magmas.The primary wadeite grains in the nepheline syenite underwent varying degrees of hydrothermal alteration.Overall,the areas of weak,medium,and strong alteration are characterized by the following respective associations:(1)wadeite+secondary catapleiite,(2)wadeite+secondary catapleiite+zircon(±natrolite±K-feldspar),and(3)pseudomorphs after wadeite.The pseudomorphs mainly consist of residual wadeite,secondary catapleiite,zircon,K-feldspar,natrolite,calcite,and some Zr-bearing titanite and vesuvianite.Similarly,the alteration assemblages of eudialyte in the lujavrite are expressed as:(1)eudialyte+secondary catapleiite(±zircon),and(2)pseudomorphs after eudialyte which is mainly made up of residual eudialyte+secondary catapleiite(±zircon)+aegirine+K-feldspar+natrolite+brithollite-(Ce).The formation of pseudomorphs is a dissolution-reprecipitation process,revealing the hydrothermal mobility of zirconium and the difference in nature of hydrothermal fluids.The nepheline syenite underwent multiple episodes of fluid activity during a hydrothermal stage,including an initial Na-rich auto-metasomatism via alkaline fluids,then stages most likely involving progressively CO2-rich fluids,and an intensive episode involving a mixture of these fluids with externally derived Ca-rich fluids,while auto-metasomatized fluids in the lujavrite are probably rich in Na and F.The difference of fluid nature has a close relationship with the oxygen fugacity of the alkaline system.The intense REE minelization is presented in the lujavrite,including the crystallization of loparite-(Ce),mosandrite,hezuolinite,brithollite-(Ce)and REE-rich titanite which all formed after eudialyte,whereas rare earth mineral in the nepheline syenite and pegmatitic dikes of aegirine-augite nepheline syenite is merely rinkite and it increases in the latter.In comparison with rinkite,mosandrite is richer in Nb,Zr and REE.Loparite-(Ce),hezuolinite,and titanite in the lujavrite show multi generations:early euhedral-subeuhedral grains enclosed in aegirine or coexisting with aegirine and eudialyte,and late anhedral grains in the interstices among aegirine and eudialyte assemblages with elevated Na,Nb and REE,but low Zr contents.The textural relationships and composition variations between zirconosilicates and rare earth minerals in the lujavrite suggest the mineralization of Zr is prior to REE,and REE prefer to accommodate in late,residual melts(fluids).Based on the mineralogical investigation of Saima alkaline complex,a conclusion is made that the Saima alkaline complex results from multiple episodes of magma activities and intense mineralization of Zr and REE concentrates on the lujavrite due to the alkalis,sodium,water and fluorine-rich and more reduced nature of the lujavrite system.