湘东南宝峰仙地区燕山早期花岗岩地球化学特征及其构造环境

GEOCHEMICAL CHARACTERISTICS AND TECTONIC SETTING OF BAOFENGXIAN EARLY YANSHANIAN GRANITES, SOUTHEAST HUNAN

湘东南宝峰仙地区发育有3个小规模花岗岩体,均为黑云母二长花岗岩。黑云母40Ar/39Ar法坪年龄为(156.8±1.8)Ma。ISr值为0.71296~0.71347,εNd(t)值为-7.3^-7.2,t2DM为1.54Ga,成岩物质源自中元古代地壳,并有地幔物质的混染。岩石为准铝质,A/CNK=0.98~1.08,平均1.02;富钾和硅,其氧化物平均值分别为5.34%和74.51%;贫Ti、Mg和Ca,其氧化物平均含量分别为0.18%、0.18%和0.98%,属高钾钙碱性系列。具有明显的Ba、Sr、P、Ti负异常和U、Ta、La、Ce、Nd、Sm、Y正异常;富含稀土元素(平均339.14μg/g)。地球化学特征及区域构造演化背景等表明岩石形成于挤压造山向非造山转换的后造山拉张环境。

Three small-scale granitic intrusions occur in Baofengxian area, east south Hunan province, they are composed of biotite monzogranites and show a ^40Ar/^39Ar plateau age of （ 156.8 ± 1.8 ） Ma. The intrusions demonstrate Isr value of 0.71296 - 0.71347, εNd （t） value of - 7.3- - 7.2 and t2DH age of 1.54 Ga, which indicate that they were derived from Mesoproterozoic continental crust, and were intermingled with mantle source. The rocks are metaluminous （A/CNK =0.98 ～ 1.08, with an average of 1.02）, and are rich in K20 （5.34% on the average） and SiO2 （74.51%）, and show lower contents of TiO2 （0. 18% on the average）, MgO （0. 18% ） and CaO （0.98%）, and characteristics similar to those of high-K calc-alkaline granitoids. The trace elements geochemistry was characterized by evidently negative anomaly of Ba, Sr, P, Ti, positive anomaly of U, Ta, La, Ce, Nd, Sm, Y and abundant amount of REE （total REE： 339. 14μg/g on the average）. Geochemical characteristics and regional tectonic evolution background indicate that the rocks were formed in a post-orogenic extension environment.