Coupled Lu–Hf and Sm–Nd geochronology constrains blueschist-facies metamorphism and closure timing of the Qilian Ocean in the North Qilian orogen
你能想象那些远在几千米的高山上，被皑皑白雪覆盖的冰冷石头的前世曾是洋底滚滚而出的岩浆吗？那些位于我国西北的祁连-柴达造山带高山之巅的冰冷石头就记录了几亿年来的大陆裂解、洋盆开启-关闭到最终的陆-陆碰撞整个过程。其中，出露于北祁连的高压变质岩保存了古老洋盆的打开和消亡的时间邮戳。但由于锆石在整个过程中几乎不生长，近年来被誉为无所不能的“时间胶囊”--锆石却缺失记录。
我院程昊老师的团队利用Lu-Hf和Sm-Nd放射性同位素定年体系，把记录在北祁连蓝片岩中石榴石的“时间邮戳”成功地提取出来。成为世界上首次成功地对如此低温的高压变质岩进行Lu-Hf定年的研究实例，确定了古祁连地区洋盆的开启可以追溯到早中生代并在约4.6亿年前关闭。
该研究成果正式发表于《Gondwana Research》第34期（2016年6月），第一作者、通讯作者均为我院程昊教授。该杂志为地球科学领域一区杂志，影响因子为8.367。
Fig. 1. (a) Digital elevation map of the Tibetan plateau region after Taylor and Yin (2009). Recent thrust faults–red, normal faults–blue, strike-slip faults–gray and major suture zones–black dashed. The study region is located in the North Qilian (filled rectangle). (b) Geological map and tectonic framework of the north Qilian orogen after Song et al. (2007), showing the sample location (white star).
作者主页：
程昊：http://mlab.tongji.edu.cn/16/6d/c8723a71277/page.htm
Citation：
Cheng H, Lu T, Cao D. Coupled Lu–Hf and Sm–Nd geochronology constrains blueschist-facies metamorphism and closure timing of the Qilian Ocean in the North Qilian orogen[J]. Gondwana Research, 2016, 34:99-108.
全文链接：
http://www.sciencedirect.com/science/article/pii/S1342937X16300557
Abstract：
The Qilian–Qaidam orogenic belt at the northern edge of the Tibetan Plateau has received increasing attention as it recorded a complete history from continental breakup to opening and closure of ocean basin, and to the ultimate continental collision in the time period from the Neoproterozoic to the Paleozoic. Determining a geochronological framework of the initiation and termination of the fossil Qilian Ocean subduction in the North Qilian orogenic belt plays an essential role in understanding the whole tectonic process. Dating the high-pressure metamorphic rocks in the North Qilian orogenic belt, such as blueschist and eclogite, is the key in this respect. A blueschist from the southern North Qilian orogenic belt was investigated with a combined metamorphic P–T and U–Pb, Lu–Hf, and Sm–Nd multichronometric approaches. Pseudosection modeling indicates that the blueschist was metamorphosed under peak P–T conditions of 1.4–1.6 GPa and 530–550 °C. Zircon U–Pb ages show no constraints on the metamorphism due to the lack of metamorphic growth of zircon. Lu–Hf and Sm–Nd ages of 466.3 ± 2.0 Ma and 462.2 ± 5.6 Ma were obtained for the blueschist, which is generally consistent with the U–Pb zircon ages of 467–489 Ma for adjacent eclogites. Lutetium and Sm zoning profiles in garnet indicate that the Lu–Hf and Sm–Nd ages are biased toward the formation of the garnet inner rim. The ages are thus interpreted to reflect the time of blueschist-facies metamorphism. Previous 40Ar/39Ar ages of phengitic muscovite from blueschist/eclogite in this area likely represent a cooling age due to the higher peak metamorphic temperature than the argon retention temperature. The differences of peak metamorphic conditions and metamorphic ages between the eclogites and adjacent blueschists indicate that this region likely comprises different tectonic slices, which had distinct P–T histories and underwent high-pressure metamorphism at different times. The initial opening of the Qilian Ocean could trace back to the early Paleozoic, and the ultimate closure of the Qilian Ocean was no earlier than c. 466 Ma.