Petrogenesis And Mineralization Implication Of The Alkali-rhyolites In Keyouzhongqi, The Southern Da Hinggan Mts., China

This thesis studies the formation time of the alkali-rhyolites of Keyouzhongqi in thesouthern Da Hinggan Mts., and their petrogenesis, tectonic setting and mineralization,based on LA-ICP-MS zircon U-Pb chronology, major and trace elements, electron probemicro analysis and the physical chemistry experimental data of melt inclusion and fluidinclusion in the quartz of alkali-rhyolites.The alkali-rhyolites of Baiyingaolao Formation in southern Da Hinggan Mts., wascomposed of riebeckite rhyolites and riebeckite granophyres which has the same magmasource and the same period with riebeckite rhyolites, and both of them have a similarvolcanic lithofacies. Riebeckite rhyolites shows a very characteristic by flow structure,spherulitic texture and snowball structure and all minerals in the riebeckite rhyolites wereobviously directionally arranged. Riebeckite granophyres shows porphyroid texture andgranophyric intergrowths texture. Both were composed of quartz, alkali-feldspar,riebeckite, aegirite and other minerals with rare earth elements and rare elements.Zircons from the riebeckite rhyolite are euhedral-subhedral in shape, and displaytypical oscillatory zoning on CL images, suggesting their magmatic origin. LA-ICP-MSzircon U-Pb dating results indicate that206Pb/238U ages of24analytical spots range from134Ma to149Ma, yielding a weighted mean age of141±1Ma, implying that the riebeckiterhyolite formed in the early Early Cretaceous. Their εHf(t) values range from+9.07to+12.08, and their Hf two-stage model ages(TMD2) vary from415Ma to616Ma. Thealkali-rhyolites have SiO2=74.3%~76.4%, FeO=3.96%~5.94%,(Na2O+K2O)=7.07%~8.51%, CaO=0.12%-0.84%, and MgO=0.03%~0.09%, chemically similar to typical A-type rhyolite. Additionally, the riebeckite rhyolites have high total REE contents(307×10-6~1395×10-6) and Ga/Al ratios (104×Ga/Al=4.88~6.41), strong positive Ceanomalies (Ce/Ce*=6.52~18.6), strong negative Eu anomalies (Eu/Eu*=0.007~0.009),and display enrichment in HFSEs (e.g., Zr, Hf, Nb, Ta) and strong depletion in LILEs (e.g.,Ba, Sr). Taken together, it can be concluded that the riebeckite rhyolites could be derivedfrom partial melting of a fossil altered oceanic crust during an extensional setting after theclosure of the Mongol-Okhotsk Ocean.It can be proved that there were lots of brine and volatiles involved in the magmaticevolution process of alkali-rhyolites by the present of lithophysa structure, spherulitictexture, snowball sturcure, vesicular structure, enrichment of fluid inclusions in quartzsand the characteristics of riched in riebeckites, bastnaesites, pyrochlorites in the alkali-rhyolites. Especially The magmatic differentiation processes with the full participation ofthe volatiles enriched F made alkali-rhyolite magma undergo two phases of magmatismsuccessively: magma phase and residual magma phase, and corespondingly, alkali-rhyolitehave gone through the crystalization of riebeckite after albite. Residual magma wouldenrich in incompatible elements with lower partition coefficient because the melt werewell-crystallized, and these incompatible elements entered into accessory minerals duringdegassing diagenesis and formed minerals enriched in rare earth and rare elements.The rocks of riebeckite rhyolites and riebeckite granophyres themselves are the orebody of rare rare earth deposit, and total contents of REE, Y, Nb et al. show that thesesamples have a high ore grade. Ore minerals are composed of ilmenite, niobium ilmenite,magnetite, limonite, zircon, thorium zircon, niobium and yttrium zircon, bastnaesite,pyrochlore, monazite, allanite and other minerals enriched in niobium and yttrium, and areformed by microcrystal xenomorphic granular in the shape of isolated form or synneusischaracterized by interstitial structure, poikilitic texture and metasomatic texturedisseminated in alkali-rhyolites.Most La, Ce, Nd in monazite, bastnaesite and allanite exist in the form of ioniccompounds, while few in zircon are in the form of somorphism. Nb occurrence inpyrochlore and niobium ilmenite, niobium and yttrium zircon respective by ioniccompounds and somorphism, and the little sneak into allanite. Th, U occurrence in zircon and allanite by somorphism, and Y occurrence in niobium and yttrium zircon bysomorphism.Abundance inclusions in quartzs of alkali-rhyolites can be classified into three types:melt inclusions, gas-liquid fluid inclusions and daughter minerals of salt crystal bearinggas-liquid fluid inclusions, indicating the fluid inclusions was middle-high salinity andmiddle-high density. The fluid inclusion homogenization temperature ranges from168.9to336.7℃in riebeckite rhyolites and215.9to great than500℃in riebeckite granophyres,and the trapping pressures and trapping depths of all inclusions respectively concentrateon7.72to32MPa and0.8to3.2km. The magma of alkali-rhyolites enriched in alkalis andrare-earth and rare elements based on electron probe micro-analysis. And its componentsin residual magma was volatile component with middle-high salinity and middle-highdensity fluid, enriched in ions of alkali-metal elements and rare earth elements.Mineralization of alkali-rhyolites is related with gas-liquid activity, its’ genetic typebelongs to the polygenetic superposition of rare earth element and rare element ore depositwith the action of magmatism-residual magmatism that originated in deep-sourced andformed hypergene zone. The magma souce provided ore-forming materials condition foralkali-rhyolites, and the anticlinal fold and the complicated faulting system in basementstrata provided a ore-forming environment.The ore character and physical and chemical characteristics of fluid melt inclusionsreveal that ore minerals are crystallized throughout all the time of magma stage andmagmatic-hydrothermal transitional processes. These ore minerals such as magmaticzircons, monazites and ilmenites were crystallized in magma stage. Residual magmaticstage is the main crystallization period, and in this period, rare and rare earth elementsmainly exist in bastnaesites, allanites，pyrochlores et al. in the form of cation.