Geochronology And Petrogenesis Of The Early Cretaceous Volcanic Rocks In The Yi-shu Fault Zone

The volcanic rocks from the Gaoqiao basin in the Yi-Shu fault zone arecomposed of basaltic-trachyandesite, trachydacite and rhyolite with SiO2contents ranging from51.97to68.94％, erupted at120Ma. They arecharacterized by enrichment of LILE and LREE, depleted of HFSE and HREE.Trachydacites have high Sr/Y, La/Y ratios and Cr, Ni contents, with enrichedSr-Nd-Pb isotopic compostions analogous to Fangcheng basalts, indicatingthey were derived from the lithospheric mantle of the NCC, which had beenmodified by the subducted crust of the Yangtze Craton. Thebasaltic-trachyandesite has low87Sr/86Sr(t) and εNd(t) value, suggest itoriginated from enriched lithospherc mantle with incorporation ofasthenosphere materials.The volcanic rock from the Juxian area located at the eastern of Yi-Shufault zone are consist of trachyandensite, trachydacite and rhyolite.The zirconU-Pb dating yields concordant ages of115Ma to129Ma. They arecharacterized by enriched LREE and LILE, depleted HREE and HFSE. TheirSr, Nd isotope compositions are similar to the mafic rocks of Sulu orogen.However, their Pb isotope compositions have an affinity with EM1typeenriched lithospheric mantle. There are no obvious correlation between87Sr/86Sr(t), εNd(t) and SiO2, which imply no significantly crustalcontamination during magmas ascent combined with high Th/U, Ce/Pb ratios,low Nb/La, Nb/U ratios. Trachyandensites show high MgO, Cr, Ni contents,and Mg#value, as well as ancient Nd isotopic model ages, suggest magmascould originated from enriched lithospheric mantle. The correlation betweenmajor oxides, trace elements and SiO2in trachydacites and trachyandesitesreflected that they were formed by fractional crystallization of comagmaticregion. Studies further indicate that these volcanic rocks in Juxian area werederived from the partial melting of the lithospheric mantle beneath the Suluorogen.Compared to the volcanic rocks in the outside of Yi-Shu fault zone, the magmas source of the volcanic rocks within the fault zone contains a higherproportion of asthenosphere componets. The extensional activities of theYi-Shu faulr zone triggered asthenosphere upwelling, which led to the partialmelting of the ancient lithospheric mantle.The lithosphere mantle beneath theYi-Shu fault zone were similar to the southeast NCC, but different from theSulu orogen.