南岭花山和姑婆山花岗岩基属印支期侵位——来自花岗岩熔体冷却-结晶和放射成因热计算的依据

Indosinian Emplacement of the Huashan—Guposhan Granite Batholiths in Western Nanling Range: Evidence from Cooling-Crystallization and Radiogenic Heat Calculation of Granite Melt

通过对南岭西段花山和姑婆山花岗岩基地质-岩石地球化学特征研究,判明它们的侵位深度(5.5km)、围岩温度(196℃)及岩浆初始温度(950℃),建立起花山和姑婆山岩基的数学计算模型,计算得出:花山-姑婆山花岗岩熔体侵位后,其初始温度降低至结晶温度所需的时间(Δtcol)分别为4.14Ma(花山)和4.36Ma(姑婆山);由于结晶潜热释放而使结晶过程延长的时间(ΔtL)为2.67Ma,2.81Ma;由于花山和姑婆山花岗岩基放射性元素含量(U13.5×10-6,Th56.1×10-6,K2O5.79%(花山);U13.7×10-6,Th52.4×10-6,K2O5.28%(姑婆山))高于世界平均花岗岩放射性元素含量(U5×10-6,Th20×10-6,K2O2.66%),花山和姑婆山花岗岩浆侵位后产生的放射成因热使结晶过程延长的时间(ΔtA)分别为37.6Ma和45.1Ma,远长于按世界平均花岗岩放射性元素含量计算得出的ΔtA(3.17Ma,花山)。花山和姑婆山花岗岩基的侵位-结晶时差(△tECTD)分别为44.41Ma和52.27Ma,结合锆石U-Pb年龄值(162Ma(花山),163Ma(姑婆山)),通过反演计算得出花山、姑婆山花岗岩基侵位年龄值(tE)分别为206Ma和215Ma,从而为花山-姑婆山花岗岩基属于印支期侵位提供了重要的岩浆动力学佐证。

Based on geological and geochemical study,it can be concluded that the Huashan and Guposhan granite batholiths have an emplacement depth of 5.5km,the country rocks temperature of 196℃,and initial granite melt temperature of 950℃during process of emplacement.Mathematic model established in this study yields that,after emplacement of the granites,cooling time（Δtcol）from its initial temperature to crystallization temperature of granite melt is 4.14Ma for Huashan and 4.36Ma for Guposhan.Because latent heat released from crystallization prolonged cooling time by 2.67Ma and 2.81Ma,and granite batholiths in these two areas contain more radioactive elements（U：13.5×10-6,Th：56.1×10-6,K2O： 5.79%for Huashan;U：13.7×10-6,Th：52.4×10-6 and K2O：5.28%for Guposhan）,higher than the average content of radioactive elements in granite（U：5×10-6,Th：20×10-6 and K2O：2.66%）, radioactivity resulting from Huashan and Guposhan granites prolonged crystallization time to 37.6Ma and 45.1Ma,much longer thanΔtAvalue calculated for world average granites（3.17Ma,Huashan）.Time differences（△tECTD）of emplacement and crystallization of two granite batholiths are 44.41Ma and 52.27 Ma.In combination with zircon U-Pb ages of 162Ma for Huashan and 163Ma for Guposhan,inversion calculation yields emplacement ages of 206Ma and 215Ma for Huashan and Guposhan granite batholiths, and this provides important magmatic and dynamic evidence for emplacement of Huashan and Guposhan granite batholiths at the Indosinian period.