Whole-rock Element And Zircon U-Pb-Hf Isotope Study Of Dalongshan And Huangmeijian Granitoids In The Middle-Lower Yangtze Region And Their Geological Significances

A-type granites have attracted extensive attention by many scholars for its special mineral assemblage,genesis,special tectonic setting and important geodynamic significance.Two A-type granite belts are distributed along both sides of the Middle-Lower Yangtze Region,of which Dalongshan pluton and Huangmeijian pluton are important parts of A-type granites belts in the north part of the Middle-Lower Yangtze region.In the present study,Dalongshan and Huangmeijian plutons have been carried out for petrography,whole-rock element,zircon LA-ICPMS U-Pb dating and Hf isotope study.Based on this new data geochemical characteristics,magma emplacement age,genesis,magmatic source and the geodynamics setting of the plutons are discussed in current thesis.Dalongshan and Huangmeijian plutons are mainly predominated by quartz syenite.They have similar mineral assemblage,including perthite,quartz and micas whit accessory minerals of zircon,titanite.According to whole-rock element geochemistry data,the granites are characterized by relatively high content of SiO₂?63.82%⁷0.89%,average value is 67.86%?and K₂O+Na₂O?10.13¹1.60%,average value is 10.97%?,lower content of CaO?0.16¹.38%,average value is 0.74%?and MgO?0.07⁰.45%,average value is 0.23%?,remarkable Rb?Th?K and Ba?Si?Ti?Eu positive anomalies,weakly Nb and Ta depletion,enrichment in light REE with a slightly right-incined sea-gull REE pattern,higher 10000Ga/Al?2.7³.4?and Nb+Y+Ce+Zr(816¹404×10^-6),suggesting that they belongs to typical A-type granites.LA-ICPMS zircon U-Pb dating results indicate that the Dalongshan granites were formed at 122.1±1.4Ma,125.8±1.6Ma,126.2±1.6Ma,126.4±3.5Ma.the Huangmeijian granites were formed at 127.6Ma±2.1Ma,127.2Ma±2.1Ma.Their corresponding zircon?_Hf?t?values were-4⁺1.1,-7.8^-3.6,-9.5^-1.7,-5.0^-0.5 and-3.3⁺2.1,-3.8^-0.1.And the T_DM2 model ages of 1107¹433Ma?average 1264Ma?,1432¹671Ma?average1555Ma?,1285¹781Ma?average 1463Ma?,1214¹495Ma?average 1396Ma?and1045¹389Ma?average 1265Ma?,1188¹423Ma?average 1312Ma?,respectively.This present study show that Dalongshan and Huangmeijian plutons are formed in Early Cretaceous,which are contemporary with other plutons of the two A-type granite belts along the Middle-Lower Yangtze Region.Their sources are similar,which likely stem from enriched mantle which is metasomatized by hydrous melt derived from subduction sediments on the continental margin at Mesoproterozoic,which is accompanied by varying degrees of crustal material contamination during the magma emplacement process.At the same time,the magma also experienced fractional crystallization,together with minor assimilation of surrounding crustal material.A-type granites from the Middle-Lower Yangtze Region including Dalongshan and Huangmeijian plutons are formed under lithospheric extension and thinning environment,which is related to the roll-back of paleo-Pacific slab.