Petrographic And Geochemistry Of Mesozoic Granitoids Along The Coastal Area Of Fujian Province

We have studied the Cretaceous granitoids along the southeast coast of continental China by integrating petrography,zircon U-Pb geochronology,bulk-rock major and trace element geochemistry and Sr-Nd-Hf isotopic compositions.These granitoids show a large compositional variation that is to a first-order consistent with varying extents of magma evolution as shown by a large SiO₂/MgO range although they do not have a liquid line descent relationship in space and time.With increasing this ratio,we can divide these granitoids into three groups.Group 1 samples?from Gunong,Changtai and Danyang plutons with SiO₂/MgO of 17-45?are mainly granodiorites,dated at 100 to 97 Ma,enriched in LILEs?e.g.Rb,Th,U,Pb?and depleted in HFSEs?e.g.Nb,Ta,Ti,Zr?with a weak negative Eu anomaly?Eu/Eu^*=0.39-0.76?.Group 2 samples?from Zhao'an,Huxi,Changqiao,Xiamen,Huacuo,Weitou,Quanzhou,Putian and Dacenshan,Nanzhen plutons with SiO₂/MgO of166-1128?are biotite granite,biotite monzogranite and monzogranite,dated at 119 to93 Ma,more enriched in LILEs and depleted in HFSEs with a negative Eu anomaly?Eu/Eu^*=0.31-0.87?.Group 3 samples?from Chengxi,Kuiqi and Sansha plutons with SiO₂/MgO of 1345-2082?are highly evolved alkali feldspar granites,dated at101-92 Ma,extremely enriched in LILEs,depleted in HFSEs with a strong negative Eu anomaly?Eu/Eu^*=0.06-0.19?.However,all the three sample groups show overlapping Nd(?_Nd?t?=-6.09 to-1.158)and Hf(?_Hf?t?=-4.68 to+3.35)isotopic variations with significant correlation between the two?R²=0.935?,reflecting varying parental magmas derived from heterogeneous sources.The granitoids under study represent the last magmatism associated with the paleo-Pacific plate subduction beneath continental China prior to the trench jam and cessation of the subduction,whose locus is marked by the arc-shaped southeast coastline of continental China.The Nd-Hf isotope compositions of these granitoids require mature continental crust melting with significant mantle input?20-60%?.Rhyolite-MELTS modeling suggests that relative to Group 1 sample?FJS14-06host?,the extents of fractional crystallization of Group 2 and Group 3 samples are²4-51%and⁵1-64%,respectively.The origin of the parental magmas is best explained by a two-stage process:?1?subducting slab dehydration induced mantle wedge melting and basaltic magma generation;and?2?ascent and underplating/intruding of the basaltic magmas caused the mature crustal melting and granitoid magma generation.The systematic northward decrease of?_Nd?t?and?_Hf?t?may suggest the progressively greater terrigenous sediment contribution in the mantle wedge source,but may very well indicate northward crustal thickening,permitting greater extent of crustal assimilation.