阿尔泰超高温变泥质麻粒岩的锆石U-Pb年龄及其地质意义

Zircon U-Pb ages of the ultrahigh-temperature metapelitic granulite from the Altai orogen,NW China,and geological implications

最近我们通过岩相学观察和矿物温压计算，首次确定了在阿勒泰喀拉苏附近存在超高温变泥质麻粒岩．其矿物组合为石榴石＋斜方辉石＋夕线石＋－—T计算结果显示其峰期变质条件为：P=～8．0kb，T=～960℃。初步P－T估算结果表明了一个峰期后近等压冷却的逆时针P－T轨迹。我们对其中锆石进行了LA－ICP－MSU－Pb年龄测定，年龄结果主要分布于260～280Ma之间，具有峰值年龄271±5Ma，个别年龄为380～390Ma，继承锆石主要分布于450～500Ma之间。该年龄结果表明阿尔泰超高温变质事件发生于二叠纪，在时间上与二叠纪塔里木地幔柱活动的时间（～275Ma）高度一致，且也和该区广泛的二叠纪（260～280Ma）后造山或非造山的基性岩和花岗岩侵入是同时的。因此，阿尔泰二叠纪超高温变泥质麻粒岩的形成，可能与由二叠纪塔里木地幔柱活动引起的岩浆底侵和下地壳伸展加热密切相关，这也与该超高温变泥质麻粒岩的逆时针P－Ｔ轨迹所反映的构造背景一致。

Abstract Recently we have firstly recognized an ultrahigh-temperature （UHT） metapelitic granulite from the migmatitic parneiss near Kalasu in the Chinese Altai, NW China, with an assemblage of Gt-Opx-Sil-Cd-Sp-Bt-P1-Qtz. Orthopyroxene shows a high-A1 feature, with an AL2O3 content in excess of 8. 0% , indicating that its peak metamorphism reached UHT conditions （ 〉 900℃ ）. P-T estimate results show its peak metamorphic conditions as： P =～ 8kb, T = ～ 960℃. The preliminary P-T estimate results indicate an anticlockwise P-T path with a post-peak near isobaric cooling. Through LA-ICP-MS technique, the U-Pb age dating for zircons from the UHT metapelitic granulite has been undertaken, and the age data are mainly between 260 ～ 280Ma, with an intercept at 271 ± 5Ma, sparse ages of 380 ～390Ma, and detrital ages mainly between 450 ～ 500Ma. The age data support that the UHT metamorphic event in the Ahai orogen occurred during the Permian, highly consistent with the timing of the Permian Tarim mantle plume （ ～275Ma）, and also coeval with the extensive Permian （260 ～280Ma） post-orogenic or anorogenic mafic and granitic intrusions in the region. Thus, the Permian UHT metapelitic granulite in the Ahai orogen may be closely associated with a magmatic underplating and extensional heating in the lower crust, as a result of the Permian Tarim mantle plume event, and this is also compatible with its anticlockwise P-T path.