Mineral And Petro-geochemical Characteristics Of Basalts And Lhezolite Xenoliths Of Shifeng Mountain From The Southeast Zhejiang With Its Geological Significance

Both of the Cenozoic basalt and mantle-derived xenolith can be as petrological probe for the earth's deep mantle components and magmatic genesis.Alkaline olivine basalt and a newly discovered mantle-derived spinel Iherzolite xenolith and clinopyroxene xenocryst occurred as a pipe in the Shifeng Mountain of the southeast Yuhuan County of Zhejiang Province,Southeast China.Geochemically,basalts show alkaline series,and OIB-like pattern in trace element spider gram.Sr-Nd isotopes indicate that basalts have a mixed magma source between DMM and enriched mantle,compared to other Cenozoic basalts reported in the southeast China.Cracked contact boundary indicates that the xenolith has less reaction with the surrounding basalt.Mg#values of olivine of xenolith and pyroxene are ca.90%and 90-91%respectively.Cr#[Cr/(Cr +A1)]of spinel is around 0.1,indicating the spinel enriched in Al.Xenoliths in the olivine basalt may be formed a condition of about 1075?,and Alog(fo2)FMQ of 2.8?3.8.Dark grey minerals,being found both in clinopyroxene from spinel lherzolite xenolith and xenocryst with have compositions of feldspar,which may be the products of partial melting and reaction of clinopyroxene,addressing that a melten pyroxene is an important magma source.The clinopyroxene xenocrysts have low ratio of Fe/Mn,which may be caused by partial melting.The olivine of xenoliths have high but scattered ratio of Fe/Mn,which may be caused by partial melting.During the Late Mesozoic,bimodal assemblage of diabase and felsic intrusive rock formed along the fault of the Gaowu Formation,with the effect of a subduction process of the paleo-Pacific plate and intracontinental extension setting.Later in Late Tertiary,as the South China Sea openning and the Philippine Sea Plate moving northward,there is a tectonic setting transfering to an extension setting in the study area.Away from the plate boundary area during the Neocene,while the effect of subduction of the Pacific plate ended,the mantle magma upwells adiabatic rapidly along the deep fault and then the basalt was formed.