构造剪切与变质角闪石中过剩Ar的引入：北秦岭丹凤群斜长角闪岩的Sm－Nd、Rb－Sr和~（40）Ar／~（39）Ar测年证据

本文对两个北秦岭丹凤群斜长角闪岩样品进行了Ｓｍ－Ｎｄ、Ｒｂ－Ｓｒ和４０Ａｒ／３９Ａｒ测年，发现其中受构造改造轻微的黑河丹凤群斜长角闪岩样品的角闪石４０Ａｒ／３９Ａｒ年龄与其Ｒｂ－Ｓｒ、Ｓｍ－Ｎｄ矿物等时线年龄接近，而遭受强烈构造剪切作用的蒲峪丹凤群斜长角闪（片）岩的角闪石４０Ａｒ／３９Ａｒ年龄则明显高于其Ｒｂ－Ｓｒ、Ｓｍ－Ｎｄ矿物等时线年龄，指示构造剪切作用对变质角闪石中过剩氩的引入有显著影响。在利用４０Ａｒ／３９Ａｒ进行造山带年代学研究时这一问题应引起注意

EXCESS ARGON IN METAMORPHIC AMPHIBOLE BY TECTONIC SHEARING:EVIDENCE FROM Sm Nd, Rb Sr AND 40 Ar/ 39 Ar DATING ON PLAGIOCLASE AMPHIBOLITES OF THE DANFENG GROUP, NORTH QINLING AREA

The evolution history of the famous E W striking Qinling Dabie orogenic belt which separates the North China Block and Yangtse Block, has been a matter of concern for many years. There are still many disagreements on the formation time of this orogenic belt. The continental subduction of the Yangtse Block occurred in Triassic time, as shown by the ages of ultrahigh pressure metamorphic rocks from the Dabie Mountains, while the calc alkaline magmatism terminated in Devonian in North Qinling as shown by U Pb analyses of zircons from I type granites which is consistent with the Sm Nd mineral isochron age of mafic pluton in Heihe, North Qinling belt. One important way to solve this problem is to figure out the uplifting history of the North Qinling Mountains. Since the closure temperature for argon in amphibole has been accurately determined,its 40 Ar/ 39 Ar method is widely used in studying the uplifting history of orogenic metamorphic rocks. It is very important to know how and in what kind of condition the excess argon occured in metamorphic amphibole because that excess argon in amphibole has often been reported. Two metamporphic amphibolite samples of the Danfeng Group in north Qinling area were determined using Sm Nd, Rb Sr and 40 Ar/ 39 Ar methods. Sm Nd, Rb Sr mineral isochron ages of sample QSP 10 and QH 3 are consistent with each other. They are also consistent with other dating results on metamorphic time of the Danfeng Group. In fact 400 Ma agrees well as a collision time in Shangdan fault. The 40 Ar/ 39 Ar spectrum age (488 Ma) of amphibole in QSP 10 sampled from a sheared zone in Puyu is much higher than its Sm Nd and Rb Sr mineral isochron ages (414 Ma, 410 Ma). While the 40 Ar/ 39 Ar age of amphibole from QH 3, which was not sheared, is similar to its Sm Nd and Rb Sr mineral isochron ages. In general, amphibole is an ideal mineral for K Ar or 40 Ar/ 39 Ar dating because of its good retention properties for radiogenic argon. The result of this study suggests that amphibole from sheared zone is different from others. One possible reason for this is that shearing leads to fluid activity and hence mobiled argon is easy to be transfered to metamorphic amphibole from country rocks by fluid. It could be concluded that shearing might play an important role on the formation of excess argon in metamorphic amphibole from tectonic zone. To avoid sampling in shear zone is essential to reduce the possibility of excess argon and to improve the reliability of results from studies on orogenic isochronology with 40 Ar/ 39 Ar methods.