The Study On The Genesis Of The Bimodal Volcanic Rocks In The Fanchang Basin, Middle-Lower Yangtze River Valley, SE China

Three eruptional cycle accured in the early Cretaceous in Fanchang volcanicbasin located in the Middle-lower Yangtze River Region: Zhongfencun cycle, Chishacycle and Kedoushan cycle, which constitute a bimodal magmatic suite. Theunderside of Kedoushan cycle consists predominantly of tuffaceous aleurolite, aninterlay of basalt and rhyolitic tuff in the midside, and the upside consistspredominantly of rhyolite. The Zhongfencun cycle and Chisha cycle mainly consist offelsic rocks. The basalts of Kedoushan are porphyritic texture and mainly consist ofclinopyroxene and plagioclase.The contents of SiO₂, MgO and Na₂O+K₂O of basalt is46.00-50.01%,3.69-5.51%,6.74-7.13%, respectively. While felsic rocks aredominantly trachytes（with contents of SiO₂65.23-67.27%, MgO0.93-0.94%,Na₂O+K₂O9.36-10.40%） and rhyolites（with contents of SiO₂73.09-77.61%, MgO0.11-0.35%, Na₂O+K₂O6.88-8.99%）. The basalts of Kedoushan show a characteristicof tholeiite series through the electron microprobe analysis of the clinopyroxene. Thecrystallization temperature and pressure of the clinopyroxene in Kedoushan basaltswere calculated according to the clinopyroxene-melt equilibrium principle. Thetemperature and pressure are about1100℃and4.0kb respectively, and the relevantdepth is about15to20km, indicating that Combined with the geochemicalcharacteristics, such us the enrichment of LREE, we thought the basalts in theFanchang basin were derived from an incompatible elements enriched,subduction-modified lithospheric mantle, and experienced olivine+clinopyroxenefractionation and minor crustal contamination. The basalt and rhyolite eruptedcontemporaneously within a limited area, they have similar trace element spiderdiagrams and chondrite-normalized REE patterns. Therefore, we conclude thatfractional crystallization of the associated basalt involving clinopyroxene, plagioclase,K-feldspar, apatite and Fe-Ti oxide generated felsic rocks, and with the contaminationof the crustal materials.