阿尔泰造山带变质岩系时代问题的讨论

A REVIEW ON AGES OF PRECAMBRIAN METAMORPHIC ROCKS FROM ALTAI OROGEN IN XINJIANG, NW CHINA

针对目前阿尔泰地区变质岩系时代划分中年龄数据应用的一些问题 ,提出如下认识 :富蕴县城西的石榴石片麻岩中锆石U Pb上交点年龄 2 3 4 9± 2 2 6Ma(2σ) ,初步确证了该区古元古代大陆地壳岩石的存在 ;克木齐群和富蕴群变质岩全岩Sm Nd等时线年龄代表其母岩形成时代。变质岩系和显生宙花岗岩类的Nd模式年龄 ,以及各种类型岩石中长石Pb二阶段模式t1年龄仅指出存在前寒武纪大陆地壳的可能性 ,没有真正的年龄意义 ,不能作为划分地层时代的依据。阿尔泰造山带是否存在 1 40 0Ma和 70 0～ 90 0Ma的变质岩系岩石 。

The study on ages of Precambrian metamorphic rocks from Altai Orogen is a key to understand continental evolution and set up tectonic framework for Xinjiang continent and the Central Asia Orogenic Belts. Based on previous geological division of metamorphic rock strata and together with all radiometric ages of Precambrian metamorphic rocks in Altai region, a review about data quality and implications for division of geological events was presented, especially commenting author's previous work in this area. Some suggestions were as follows. An imprecise upper intercept age of 2 349 ±226 Ma(2 σ ) for zircons of a garnet gneiss from the west of Fuyun County indicated the existence of Peleo Proterozoic continental rocks in the Altai Orogen. The Sm Nd isochron ages of 1 357±52 Ma (2 σ ) for Kemuqi Group and 900～700 Ma for Fuyun Group with ε Nd ( T )>0 represented formation ages of protolithes derived from depleted mantle, respectively. Sm Nd model ages ( T DM ) of Precambrian metamorphic rocks and Phanerozoic granitoids, and two stage Pb isotopic model ages ( t 1) for feldspars of various types of rocks indicated possibility of existence of Precambrian continental crustal basement and continental crustal growth in 2 600～2 400 Ma, 1 400 Ma and 900～700 Ma ago, respectively. However, the model ages are not real geological ages for any kind of rocks, which should not be used to constrain the ages of strata. It is a fact that any reliable U Pb ages of 1400 Ma and 900～700 Ma of zircon for Precambrian metamorphic rocks have not been obtained yet. It must be pointed out that many Pb, Nd model ages, imprecise Sm Nd isochron ages, and insignificant U Pb upper intercept ages were incorrectly used to constrain the ages of strata in Altai, which resulted in a serious confusion. Most ages of the “Precambrian strata” reported for the Altai Orogen are dubious and need to be dated again by further work.