南岭骑田岭花岗岩基属印支期侵位的岩浆动力学佐证:对《关于南岭花岗岩侵位年龄问题》一文的答复与讨论  

Magma-Dynamic Evidence for Indosinian Cycle Emplacement of theReply Qitianling Granite Batholith in Nanling Range, Sourth China: and Discussion to Comments by Prof. Zhu Jinchu et al

作　　者：章邦桐[1] 吴俊奇[1] 凌洪飞[1] 陈培荣[1] 

机构地区：[1]内生金属矿床成矿机制研究国家重点实验室(南京大学),南京大学地球科学与工程学院,南京210093

出　　处：《高校地质学报》2010年第3期397-412,共16页Geological Journal of China Universities

基　　金：中国核工业地质局十一五基础科研项目(编号YK08);内生金属矿床成矿机制研究国家重点实验室研究项目(编号2008-I-04);教育部科学研究重大项目(306007)联合资助

摘　　要：影响花岗岩熔体冷却-结晶时间长短的因素虽然较多,如花岗岩熔体的初始温度、结晶温度、侵位深度、围岩温度、体积、放射成因热以及其他各种热物理参数,但计算表明,花岗岩体积大小是决定花岗岩体侵位-结晶时差的最主要因素。采用与骑田岭花岗岩体相同参数计算得出不同出露面积花岗岩体的侵位-结晶时差(△tECTD)分别为42.1Ma(骑田岭花岗岩体,520km2);0.7Ma(50km2花岗岩体);0.05Ma(4km2花岗岩体)。采用板状模型,结合骑田岭花岗岩锆石U-Pb年龄值(161Ma),通过反演计算得出骑田岭花岗岩基侵位年龄值(tE)为206Ma,与立方体模型计算结果(203Ma)差别不大,从而为骑田岭花岗岩基属于印支期侵位提供了重要的岩浆动力学佐证。对国内外花岗岩体205对锆石U-Pb年龄和全岩Rb-Sr等时线年龄进行的相关分析,拟合出相关系数很高(R=0.997),回归系数接近l的线性回归方程(tRb=0.9928×tZr+2.1584)。△t(tZr-tRb)频数统计分析表明:Δt呈对称正态分布(偏度系数CSK=-0.148;峰度系数CKU=6.771),其中位值为0Ma,众数值为2Ma。这表明花岗岩体锆石U-Pb定年的测定结果与全岩Rb-Sr等时线定年测定结果在允许的误差范围内是一致的,从而得出"花岗岩锆石U-Pb年龄不能代表花岗岩侵位年龄(tE)"的结论。对Lee等(1997)和Cherniak等(2000)所进行天然锆石中U和Pb扩散系数实验条件的分析,判明他们得出的"锆石U-Pb同位素体系封闭温度>900℃"结论,只可应用于解释源区岩石升温产生部分熔融形成花岗岩浆过程中残留锆石U-Pb同位素的行为,但不适用于解释直接从花岗岩熔体中晶出锆石U-Pb同位素体系封闭温度。华南同熔型花岗岩(龙塘花岗闪长岩体,长泰花岗闪长岩体)与其同源火山岩全岩Rb-Sr年龄存在较大的差别(ΔtRb-Rb=15.7～32Ma)以及华南部分花岗岩体锆石中存在差别较大的2组U-Pb年龄(ΔtZr-Zr=24～50Ma)的实例为花岗岩存在较大侵位Although many factors can influence the duration of cooling-crystallization process of a granitic melt, such as the initial temperature of the granite melt, the crystallization temperature, the temperature and thermal conductivity of country rocks, the depth of emplacement, the volume of granite pluton, the radiogenic heat of the granite and other thermophysical parameters, the calculated results showed that the volume of granite body is the most important factor. For example, using the same parameters as the Qitianling granite batholith, but with different volumes, the tECTD values for granite plutons with various outcrop areas are caculated as follows： 42.1 Ma for Qitainling granite batholith of 520 km2;0.7 Ma for granite pluton of 50 km2;0.05 Ma for granite pluton of 4 km2. Using tabular model and in combination with zircon U-Pb age （161 Ma）, the emplacement age （tE） of the Qitainling granite batholith is estimated to be 206.Ma, while the tE is 203 Ma for cubic model . This suggests that the emplacement of the Qitainling granite magma was in the Indosinian Period. Using least squares regression procedure, a best regression equation （tRb= 0.9928×tZr＋2.158） with high correlation coefficient （R=0.997） is fitted for 205 pairs of zircon U-Pb ages and whole rock Rb-Sr ages of granite plutons. The frequency analysis of 205 individual values of difference between each pair （t= tZr-tRb ） for granite plutons shows symmetrical normal distribution （skewness CSK=-0.148; kutrocess CKU=6.771） with the t median of 0.0 Ma and the t mode of 2.0 Ma. These statistical characteristics indicate that, for granites as a whole, both zircon U-Pb dating age and whole-rock Rb-Sr isochron ages are consistent within error and thus the zircon U-Pb dating age cannot represent the emplacement age of a granite batholith. Therefore, we recognized that the conclusion of＂U-Pb closure temperature 900 ℃＂ drawn from experiment of U-Pb diffusion in zircon by heating zircon crystals by Lee et al （1997） and Cherni

关 键 词：骑田岭花岗岩基 侵位年龄 侵位-结晶时差 印支造山运动 岩浆动力学 

分 类 号：P591[天文地球—地球化学]