广东从化佛冈(主体)黑云母花岗岩定年和成因  

Isotopic Dating and Genesis for Fogang Biotite Granites of Conghua Area, Guangdong Province

作　　者：陈小明[1] 王汝成[1] 刘昌实[1] 胡欢[1] 张文兰[1] 高剑锋[1] 

机构地区：[1]南京大学地球科学系内生金属矿床成矿机制研究国家重点实验室,江苏南京210093

出　　处：《高校地质学报》2002年第3期293-307,共15页Geological Journal of China Universities

基　　金：国家自然科学基金项目40132010;40073009;40025209资助。

摘　　要：佛冈黑云母花岗岩是大型东西走向的佛冈花岗质杂岩体的主体部分、占杂岩体面积的85％。粗等粒或巨晶斑状结构。由黑云母（2～6％）,石英（35～41％）,斜长石（An10～23,10～18％）和微纹长石（36～48％）组成,系黑云母花岗岩。黑云母具有低的Mg/（Mg+Fe）值（0.17～0.21）。据全岩-矿物Rb-Sr等时线定年其定位年龄为167.5±7.5Ma,MSWD为2.4（中林罗世晚期）。在岩石化学成分上为富硅（SiO2在71.74～77.64％）,中等过铝质（A/NKC平均为1.056）,钙碱性-弱亚碱性（NK/A平均为0.797）,相对富K2O（K2O/Na2O为1.66）。在微量元素和稀土0素组成方面,显示 Nb亏损,具有低 Nb/fa值（平均 5.94）,高 Rb/Sr（198～41.71）,Rb/Nb（11～23）和K/Nb（1498～2976）值;中-强Eu亏损（Eu/Eu在0.12～0.46）;高的（La/Sm）N值（1.67～5.39）等。在Sr,Nd,Pb,O对同位素组成上,具有高的Isr值（0.7116±0.0023）和εSR（t）值（96-103）;低的εNd（t）值（-6.19--8.93）。据模拟计算,其源区岩石中华南上壳端元约占66～69％,高的207pb/204pb(15.734)和206pb/204pb值(18.962～19.049)位于铅构造模式图的上壳演化线域内,高δ18O‰（+9.3～+12 .5）等。以上数据表明佛同黑云母花岗岩系该区中-下地壳层次的源岩部分熔融而成。Fogang (main body ) biotite granites are a part of large-scaled Fogang granitic complex of east-west trending. They occupy about 85% of the complex in area. They are composed of about 2-6% biotite, 35-41% quartz, 10 - 18% plagioclase (An10-23) and 36-48% perthite, corresponding to biotite syenogranite (3a) on the basis of mineral contents by IUGS classification. The biotite has lower Mg/(Mg+Fe) values (0.17-0.21) and the K-feldspar is usually microcline perthite. Fogang biotite granites have an age of 167. 5 ?7. 5Ma(MSWD = 2.4), determineted by Rb-Sr whole rock-mineral isochron method (emplaced during Middle Jurassic period). Similar to S type granites in south China, Fogang granites are markedly rich in silica (SiO2: 71.74-77.64%) and K2O relative to Na2O (averaging K2O/Na2O= 1. 66), and are moderately peraluminous (averaging A/NKC = 1. 056) and calcic-alkaline to weakly subalkaline series (NK/A = 0. 797 in average). Based on the trace element and rare earth element data, the granites commonly have higher Rb/Sr (1.98-41.71), Rb/Nb( 11- 23), K/Nb (1498-2976) ratios, lower Nb/Ta ratio (5. 94 in average), strong to moderate Eu depletion (Eu/Eu=0.12-0.46), and higher (La/Sm)N values (1.67-5.39). In Sr, Nd, Pb, O isotopic composition field, Fogang granites have higher Isr(0. 7116 ?0. 0023) and εsr(t) values (96-103), showing that South China Upper Crust end member occupies about 66 - 69% in their source materials in terms of the model calculation. The granites possess higher δ18O‰(+9.3-+12.5)and 207Pb/204Pb (15.734) and 206Pb/204Pb (18. 968-19. 049), indicating that they are consistent with Upper Crust Evolution Line in the Pb-tetonic model diagram. The above mentioned data wholely indicate that Fogang biotite granites were formed by partial melting of source materials consisting of mid to lower crust rocks. Some of dark enclaves of dioritic or granodioritic compositions are found in Fogang biotite granites. This suggests that basalt underplating resulted from lithosphere westward subduction of the Palaeo Pacific

关 键 词：黑云母 成因 花岗岩 广东 铷-锶等时线定年 

分 类 号：P588.121[天文地球—岩石学] P597[天文地球—地质学]