Magmatic Rock Characteristics And Metallogenic Significance Of The Late Mesozoic Volcanic Basin In Shanghang-Yunxiao Fault Zone

The Shanghang-Yunxiao fault zoneis a regionally deep fractures that traverses the intracontinental fold belt and the Late Mesozoic volcanic-intrusive rock belt along the coastline of the south eastern China.The progressive subduction of the Paleo-Pacific plate beneath the Eurasian plate during the Late Mesozoic led to a series of remarkable tectono-magmatic events,including the extensively Late Mesozoic volcanic rocks,as well as formation of the volcanic-sedimentary faulted basins within the Shanghang-Yunxiao fault zone.Although numerously remarkable studies have been conducted with respectto originan dpetro genesis of these Late Mesozoic magmatic rocks,the characteristics and genetic linkages of magmatic rocks from differently volcanic basins remains poorly documented.Furthermore,there are ongoing controversies of the Late Mesozoic tectonic background of the Shanghang-Yunxiao fault zone.The Pinghe Late Mesozoic volcanic basin and Shanghang Late Mesozoic volcanic basin respectively represent the intracontinental basin and the coastal basin in the Shanghang-Yunxiao fault zone.Accordingly,this thesis carries out a systematic and detailed study ofthe volcanic rocks in the Pinghe and Shanghang Late Mesozoic volcanic basin based on geochronology,geochemistry and Sr-Nd isotope.Together with previous researches,new insight into the characteristics and evolution aryprocesses of the Late Mesozoic magmatism in the Shanghang-Yunxiao fault zone are represented,which could also provide reliable basis for further studies of the tectonic evolution and metallogenic prognosis.The majority of the magmatic rocks in the Pinghe Late Mesozoic volcanic basin are acidic rock and medium-acid rocks.Geochemical analyses in dicate that these rocks are characterized by high silicon,high potassium,high aluminum and low magnesium,belonging to the I-type granite.They are relatively enriched in light rare earth elements?LREEs?,with fractionation of the LREEsstrongerthan theheavy rare earth elements?HREEs?.The Sr-Nd isotope shows that their?Nd?t?are ranging from-5.53 to-2.00,corresponding to T_2DM ages of 1.05 to 1.18 Ga.LA-ICP-MS U-Pb age of the biotite granites from the Shierpai area is 143.5±0.4 Ma.Geochemical characteristics show that the biotite granites are alkali-rich and potassium-rich in nature,with relatively low P₂O₅,MgO,and TiO₂ contents,showing I-type affinity.They are enriched in LREEs,with significantly negative Eu anomalies.The Late Mesozoic magmatic rocks in the Shanghang-Yunxiao fault zone are generally characterized by high silicon and potassium,with relatively high Al₂O₃ but low P₂O₅contents.All these magmatic rocks are enriched in LREEsand Pb,and depleted in HREEs.Their LREEs and HREEs show various degree of fractionation.With the U-Pb ages getting younger,the SiO₂ contents of the magmatic rocks in the Shanghang Late Mesozoic volcanic basin are gradually decreasing,while their Al₂O₃ contents and the proportion of partial-aluminum rocks are increasing.Combining with the previous studies,this paper conclude that the Late Mesozoic magmatism in study area was closely related to the active continental margin resulted from the subduction of the Paleo-Pacific plate.With proceeding of the subduction,the tectonic settings of study area changed gradually from the compress setting in the Late Jurassic to the extension setting in the Early Cretaceous.Moreover,the extension degree in this region enhanced with time.Corresponding with regional tectonic evolution,the petrogenesis of magmatic rocks was gradually showing the characteristic of the crust-mantle magma mixging,and mantle materials contributions increasing with time.These may be the key factors which trigger the large scale minerization in our study area during the late stage of the Early Cretaceous.