Relationship Studies Of Volcanic Activity And Strontium Mineralization Of Lishui Volcano-tectonic Depression

Mesozoic volcanic rocks are widely distributed in Lishui volcanic tectonic depression due to the intense Mesozoic volcanic activity. As one of the volcanic tectonic depression in the polymetallic metallogenic belt in the middle and lower reaches of the Yangtze River,it is an important part of the Mesozoic volcanic belt along the river. However, due to the extremely serious vegetation and Quaternary cover in the area, lack of systematic geochronology, geochemistry and magmatic origin study for a long time. In consequence,the above issues limit further understanding of the period of the Mesozoic volcanic activity,its origin and the evolution of magmas.On the other hand,the research of strontium ore deposits in Longwangshan crystal tuff is also limited.Therefore, we carried out a detailed field survey and geological observations sectional line investigation as to the representative volcanic stratas in each cycle,as well as systematic zircon U-Pb geochronology, petrographic observation,geochemistry studies.On the basis of the above work,we established the geochronological framework of the volcanic activity in the basin and discussed the genesis and magmatic evolution of volcanic rocks.In addition,we also carried out heavy sand, mineral-situ microanalysis,inclusion composition, temperature, salinity and other research as to the Ai Jingshan strontium ore deposits and discussed its material sources and concentration mechanism.The zircon LA-ICPMS U-Pb age of the volcanic rocks in Longwangshan cycle,Dawangshan cycle are135～132Ma and135～125Marespectively. Then the massive volcanic activity ceased and it stepped into a long-term quiet period until the Jiashan volcanic activity in110Ma,which symbolized the ending of volcanic activity. Zircon geochronology show that the Mesozoic volcanic rocks in this basin were formed in early Cretaceous,which is consistent with the formation of other volcanic basins along Yangtze River (135～124Ma).The volcanic rocks in Lishui volcanic tectonic depression include a series volcanic rocks with normal evolution sequences.That is.there are mafic,intermediate-acid,acid rocks and their corresponding volcanic clastic rocks but the majority of them are mafic and intermediate-acid rocks. They are all enriched in light rare earth and large LILE K, Rb, Ba, Th, and are depleted in high field strength elements Nb, Ta, Ti,as well as rich in alkali and potassium.The high initial87Sr/86Sr ratio(0.705604-0.707080) negative εNd(t)(-2～-9) and sHf(t)(-4～-13) show that the magma of Lishui volcanic rocks were derived from lithospheric enriched mantle that are replaced by crust sourced melts/fluids.During the rising process of the magams,there were crustal contamination and the fractional crystallization of and clinopyroxene, hornblende, apatite and other minerals.but the crustal contamination and differentiation degree in the intermediate and mafic magmas were low.The strontium content of Longwangshan cycle(averaged at965×10-6) in Lishui area is higher than that of other cycles(512×10-6、164×10-6～315×10-6、338×10-6～389×10-6respectively).Given the strontium content in the basement rocks and wall rocks,we conclude that the strontium came from the crystal tuff in Longwangshan cycle and the sulfur came from the Gypsum rock stratum of Zhoucun formation in the basin basement.