Petrogenesis And Geological Significance Of The Early Cretaceous Volcanic Rocks From The Lesser Xing'an Range

This thesis studies petrology,geochronology,whole-rock elemental,and Sr-Nd isotopic compositions of the Early Cretaceous Meifeng and Fuminhe formations near the Meifeng Forest Farm in the northern Lesser Xing'an Range.Zircon U-Pb dating results indicate that the volcanic rocks of the Fuminhe Formation formed at 105 ± 1Ma,slightly later than that of the Meifeng Formation volcanic rocks(105 Ma).The rock assemblages of the Meifeng Formation are basalt,basaltic andesite,andesite,trachyandesite,and trachyte,while the volcanic rocks of the Fuminhe Formation are mainly trachyte and rhyolite.The mantle-derived basaltic volcanic rocks are enriched in light rare earth elements(LILEs)and large ion lithophile elements(LILEs)but depleted in high field strength elements(HFSEs),as well as relatively depleted Sr-Nd isotopic compositions.Compared with the basaltic rocks,the intermediate-acid rocks have more enriched Sr-Nd isotopic compositions,indicating the result of the assimilation and fractional crystallization(AFC)process during magma ascending.Above all,the parental magma of the Meifeng Formation volcanic rocks were likely produced by melting of the asthenosphere mantle source metasomatized via the subducted sediment-derived melt including the volcaniclastic rock.The rhyolites of the Fuminhe Formation are characterized by high Si O2 and low Mg O contents,enriched in LREEs and depleted in Ti,Sr,P,Ba,and Eu(?Eu = 0.13-0.49),which are similar to the high-silica rhyolite.The primary magma of the Meifeng Formation volcanic rocks experienced the assimilation and fractional crystallization(AFC)process.Subsequently,the evolved andesitic magma emplaced in middle-upper crustal levels and formed a MUSH,which were further condensed after magma fractionation,and finally yielded high-silica rhyolite of the Fuminhe Formation.Based on the above evidences,the Early Cretaceous volcanic rocks of the Meifeng and Fuminhe formations formed in an extensional intraplate environment,the formation of which were related with the subduction and retraction of the Paleo-Pacific plate.