北山造山带南部片麻岩和花岗闪长岩的锆石U-Pb定年和Hf同位素：中元古代的岩浆作用与地壳生长

Zircon U-Pb and Hf isotopic study of gneiss and granodiorite from the southern Beishan orogenic collage: Mesoproterozoic magmatism and crustal growth

前寒武纪古老基底的分布及其构造归属是理解和认识中亚造山带基本构造格架和地壳生长的关键问题.本文报道了中亚南缘北山造山带南部黑云斜长片麻岩和花岗闪长岩的LA-ICPMS锆石U-Pb定年和LA-MC-ICP-MS锆石Hf同位素分析结果.所研究的片麻岩中岩浆结晶锆石给出了1408±4 Ma的原岩形成年龄,揭示了北山造山带南部存在中元古代的古老基底岩石.这些岩浆结晶锆石具有正且高的Hf（t）值（4.1~9.9）,其Hf模式年龄（1.50~1.72 Ga）与其结晶年龄接近,反映了片麻岩原岩岩浆可能来自亏损地幔或新生地壳,即北山造山带南部存在中元古代（~1.4Ga）的地壳新生事件.同时,在侵入片麻岩的花岗闪长岩中获得了257±3 Ma的岩浆结晶年龄,并获得了大量~1.4 Ga的继承岩浆锆石.这些继承锆石具有与片麻岩的岩浆锆石一致的结晶年龄和Hf同位素组成,进一步表明北山造山带南部存在中元古代老地壳.通过与敦煌地块和中天山地块前寒武纪基底已有资料的对比,我们认为北山南部古老微陆块可能与相邻的中天山地块具有相同的前寒武纪地壳形成与演化历史,它们并不是来源于塔里木克拉通.这一成果为认识中亚造山带南部的构造演化提供了新的重要信息.

The distribution of Precambrian basement rocks and their tectonic attributes are of key importance for understanding the tectonic architecture and the crustal growth model of the Central Asian Orogenic Belt （CAOB）. We present LA-（MC）-ICP-MS zircon U-Pb and Hf isotopic data for the biotite-plagioclase gneiss and granodiorite from the southeru Beishan orogenic collage, on the southern margin of the CAOB. The magmatic zircon grains from the gneiss yielded a protolith age of 1408±4 Ma, indicating the occurrence of Mesoproterozoic basement rocks in the southern Beishan. Furthermore, these zircon grains have high positive 6nf（t） values （4.1-9.9）, and young Hf model ages （1.50-1.72 Ga） close to the crystallization age, suggesting that the protolith of this gneiss was derived from the depleted mantle or the juvenile crust. This indicates the involvement of an important Mesoproterozoic （~1.4 Ga） event in juvenile crust growth in the southem Beishan. Conversely, the granodiorite intruding the biotite-plagioclase gneiss yielded a crystallization age of 257±3 Ma, and numerous ~1.4 Ga inherited zircon grains were found in this rock. These inherited zircon grains exhibit crystallization ages and Hf isotope compositions consistent with those of the biotite-plagioclase gneiss, further confirming the occurrence of Mesoproterozoic crustal basement rocks beneath the southern Beishan. By comparing our new data with previous results from the Precambrian basement in the Dunhuang Block （part of the Tarim Craton） and the Central Tianshan Block （microcontinent in the Chinese Tianshan）, we postulate that the Precambrian microcontinents in the southern Beishan have similar crustal evolutionary histories to the Central Tianshan Block, and they may not have been derived from the Tarim Craton. The Mesoproterozoic basement rocks investigated here provide new clues for examining the accretionary orogenic processes of the southern CAOB.