大别山超高压变质榴辉岩的氦同位素组成及对其形成环境的制约  

Helium Isotope Compositions and Forming Conditions of UHP Metamorphic Eclogites from the Dabie Mts. Terrane in East China

作　　者：李善芳 李延河[2] 丁悌平[2] 伍宗华[3] 古平等 

机构地区：[1]中国地质调查局发展研究中心,北京100037 [2]中国地质科学院矿产资源研究所,北京100037 [3]国土资源部同位素地质重点实验室,北京100037

出　　处：《地质论评》2005年第3期243-249,共7页Geological Review

基　　金：国家自然科学基金资助项目 (编号 497940 44 )的成果

摘　　要：大别山地区榴辉岩全岩的3He/4 He值分布在0 .0 13×10 - 6 ～0 .76 0×10 - 6 之间,平均0 .2 0 0×10 - 6 ,榴辉岩中石榴子石单矿物的3He/4 He值与其全岩的3He/4 He值基本一致。在3He—4 He分布图上榴辉岩的氦同位素数据点远离地幔氦的分布范围,而位于大气氦与地壳氦的过渡部位。采用真空压碎方法对榴辉岩中石榴子石和绿辉石的氦同位素组成进行了分析,也未找到地幔氦的明显踪迹。结合大别—苏鲁地区榴辉岩中极低的δ1 8O值、εNd(0 )值等地球化学特征,提出大别—苏鲁地区超高压变质榴辉岩可能是在地壳中形成的,并未曾俯冲至10 0多千米的地幔深度。榴辉岩的3He/4 He值与围岩类型密切相关,榴辉岩的δ1 8O值与围岩的δ1 8O值同步变化,以及含柯石英大理岩、片麻岩和硬玉石英岩等非基性超高压变质岩和脉状榴辉岩的发现,均证明榴辉岩与其围岩一起经历了超高压变质过程,榴辉岩及其围岩在变质过程中未发生明显的位移,即榴辉岩是原地成因的。The 3He / 4He ratios of eclogites from the Dabie Mts. terrane range from 0.013×10 -6 to 0.76×10 -6 , with an average of 0.200×10 -6 ; the 3He / 4He ratios of garnets of eclogites are generally consistent with those of whole rock. The data points of helium isotope of eclogites fall into the mixing part of the crust and the atmosphere, and far from the mantle area in the 3He - 4He diagram. The helium isotope compositions of fluid inclusion in garnets and omphacite from eclogites were measured by vacuum crusher method, but mantle information was not found in them. With the extremely lower δ 18 O values, ε Nd (0) values and other geochemical character, we suggest that the UHP metamorphic eclogites from the Dabie Mts. and Sulu area maybe formed in the crust and subduction into the mantle more 100 km may not have occurred. The 3He / 4He ratios of eclogites are obviously correlated with the types of their surrounding rocks, the changes of δ 18 O of eclogites and wall rocks are synchronous, and sialic coesite-bearing rocks were widely found, for example marble, gneiss, jadeite quartzite and so on. Those results suggest that the eclogites and their country rocks all underwent ultrahigh-pressure metamorphism, offsets between them during the metamorphic process were inconspicuous and the eclogites were formed in-situ.

关 键 词：大别山 超高压变质榴辉岩 氦同位素 形成环境 地球化学特征 地质条件 

分 类 号：P588.3[天文地球—岩石学]