Geochemical Characteristics And Significance Of Single Minerals In Late Mesozoic Granites In The Lower Yangtze Region

The Lower Yangtze region is located in the eastern part of the Yangtze Block,where magmatic activity was intense in the Late Mesozoic and Yanshanian granites are widely developed.Previous studies on the whole rock geochemistry of the rock mass in this area have acquired a lot of achievements.It is believed that the magmatic activity of this period can be divided into three stages,and the last stage(about 129-122 Ma)mainly formed A-type granite.However,these studies do not adequately explain why zircons with large age span(up to 40 Ma)in the same rock mass are often found in this area.What are the reasons for this phenomenon and how to screen these age date?In addition,what is the magma process of the rock mass in this area and what is the genetice mechanism of A-type granite?In order to solve the above problems,this article taken the Huayuangong,Yaocun and Fuling plutons as examples,delved into the geochemical characteristics of plagioclase,zircon and apatite from these plutons.The Huayuangong pluton comprises the central intrusion and the marginal intrusion.Two samples from the marginal intrusion yielded consistent zircon weighted mean ²⁰⁶Pb/²³⁸U ages of 124.6±2.0 Ma and 125.9±1.6 Ma.These analytical spots also exhibit Zr/Hf and Th/U ratios concordant with the evolution of a single magma,from which the dated zircons crystallized.However,for the central intrusion,the analytical spots on zircons from two samples all show a continuous distribution on the concordia line with a relatively large span.For each sample from the central intrusion,all of the zircon Zr/Hf ratios do not conform to a single magma evolution trend,but rather can be divided into two groups.We propose that zircon Zr/Hf ratios can provide a new constraint on U-Pb zircon dating,and zircon Th/U ratios can also be used as a supplementary indicator to constrain zircon dating and to determine the zircons origins and whether magma mixing occurred.By screening zircon analytical spots using these two indicators,the two samples from the central intrusion of the Huayuangong pluton are 122.8±4.3 Ma and122.9±2.2 Ma,which is consistent with the field observations that the central intrusion is slightly younger than the marginal intrusion.Based on the study of the main and trace elements of plagioclase,zircon and apatite in Yaocun and Fuling plutons,it is found that the An values of plagioclase in Yaocun pluton is low(0.16-18.5)and has a large variation range,illustrating that the diagenetic pressure is relatively small and the magma physicochemical conditions change greatly.Zircon Zr/Hf and Th/U ratios indicate that the zircons in this pluton produced by different magma before and after magma mixing.The temperature calculated by zircon trace elements is about 780?,and the logf O₂is about-15.The apatites are enriched in LREE and depleted in HREE,combined?La-Nd-?Sm-Ho-?Er-Lu triangle diagram analysis,it is believed that these apatites were crystallized in magma with crust and mantle mixing.The An values of plagioclase in Fuling granite are between 10.5-21.3,and the zircon Zr/Hf and Th/U ratios indicate that the zircons in the Fuling pluton were crystallized in a single magma chamber.The temperature calculated by zircon trace elements is about 741?,and the logf O₂is about-18.The points in the apatites?La-Nd-?Sm-Ho-?Er-Lu triangle diagram fall near the crust source area,indicating that the Fuling pluton is mainly derived from Crust-source.It can be seen that the crust-mantle magma mixing and the partial melting caused by the underplating of mantle derived magma may be the important diagenetic mechanism of the late Mesozoic A-type granite in the lower Yangtze region.