摘要
本文通过对北大别漫水河片麻岩的锆石进行阴极发光(CL)显微图像观察及激光剥蚀等离子体质谱(LA-ICP-MS)微区微量元素分析,试图对重结晶和深熔成因变质锆石的微量元素特征进行制约。CL图像显示该片麻岩锆石存在复杂的内部结构,有原岩岩浆锆石、重结晶变质锆石和深熔锆石。不同锆石区域的Hf、P、Ti、Nb、Ta和Ce等元素含量及Nb/Ta比值相似。但重结晶和深熔锆石的Th/U比值明显低于原岩锆石(分别为0.33～1.12、0.03～0.60和0.92～1.99),重结晶锆石区域低Th/U比值是由于重结晶过程中Th比U更易排出锆石晶格,而深熔锆石区域低Th/U比值是其结晶的熔体中低Th和高U含量的原因。不同锆石区域的稀土元素含量也不相同,原岩锆石稀土总量高(621.9～1331.6ppm),变化较大,而深熔锆石和重结晶锆石稀土总量低(分别为236.8～642.5ppm;98.5～435.6ppm)。不同锆石区域的稀土元素配分模式相似,但重结晶锆石的稀土元素含量比原岩锆石低3～4倍,可能是重结晶过程中稀土元素被逐出锆石晶格所致。深熔锆石的稀土元素含量受深熔熔体与残留体之间平衡与否的制约,如果熔体与残留体之间达到了平衡,则深熔锆石与原岩锆石有相似的稀土元素含量;反之,深熔锆石的稀土元素含量就会低于原岩锆石。漫水河片麻岩深熔锆石的稀土含量比原岩锆石低1～4倍,反映了深熔熔体与残留体之间没有达到平衡。结合已有的花岗岩与混合岩化岩石的年龄和同位素特征,说明北大别混合岩化片麻岩可能与同时代花岗岩的形成有成因关系。
The internal structures and trace element characteristics of zircons from migmatitic gneiss at Manshuihe in North Dabieshan were studied by cathodoluminescence (CL) and LA-ICP-MS, respectively. As revealed by the CL, the zircons have very complex structures of protolith magmatic core, the metamorphic recrystallization mantle and overgrowth rim during partial melting. Some of trace elements in contents and ratios, such as Hf, P, Ti, Nb, Ta, Ce, Nb/Ta and Zr/Hf, are similar to each other among these zircon domains. The magmatic domains are distinguishable from the others because of their high Th/U ratios. Because Th is easier expelled out from the zircon than U during recrystallization, the recrystallized domains have low Th/U ratios. The anatectic overgrowths have low Th/U ratios due to the low Th and high U contents in the partial melt. Although having similar chondrite-normalized REE patterns, the magmatic, the recrystallized and the anatectic domains have significantly different REE contents (621.9 similar to 1331. 6ppm, 98.5 similar to 435.6ppm and 236.8 similar to 642.5ppm, respectively). It is because the REEs are partly expelled out from the zircon during recrystallization that the recrystallized domains have lower REE contents than the magmatic domains. The difference in REE contents between the overgrowth and the protolith domains depend on the chemical equilibrium between the partial melt and the restite. If the trace element equilibrium between them was achieved, there is no significant difference in composition between the overgrowth and the protolith zircon domains, and vice versa. It is suggested that the partial melt and the restite from the migmatitic gneiss at Manshuihe have not achieved trace element equilibrium, because the REE contents of the overgrowth domains are 1 similar to 4 times lower than that for the protolith domains. Therefore, the trace element compositions can provide very useful constraints on the recrystallized and the anatectic overgrown zircons. It is advocated that the migmati
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2004年第5期1141-1150,共10页
Acta Petrologica Sinica
基金
国家自然科学基金(40303003和40273028)
王宽诚博士后奖励基金
中国博士后科学基金
关键词
漫水河
锆石
重结晶
深熔
微量元素
zircon
anatectic
recrystallization
migmatite
trace elements
