阿拉善地区新元古代岩浆事件及其地质意义

在阿拉善西部地区分布有一些眼球状片麻岩和条带状片麻岩,以往将它们作为阿拉善群的地层对待。本文研究表明它们是受到变形改造的正片麻岩。通过锆石LA-ICPMSU-Pb分析,4个样品分别获得了913±7Ma、921±7Ma、926±15Ma和904±7Ma的年龄结果。新元古代早期变形花岗岩的发现表明,阿拉善地区经历了新元古代早期(格林威尔期)造山作用的强烈改造,它们可能与祁连地块等相似,应属于不同于华北克拉通的独立的变质地块。

Early Neoproterozoic granite events in Alax area of Inner Mongolia and their geological significance: evidence from geochronology

The Alax metamorphic block (basement) distributed in the western margin of the North China Craton is mainly composed of Archean Diebusige Rock Group, Paleoproterozoic Bayan-Wulashan Rock Group, Paleo-Middle Proterozoic Alax Rock Group and some orthogneisses. There are some augen and banded gneisses in the Alax Rock Group, which were commonly referred to in the past as the metamorphic strata of Alax Rock Group. However, the authors'' study indicates that these augen and banded gneisses are actually deformed granites (orthogneisses). Four samples of augen and banded gneisses were dated by zircon LA-ICPMS U-Pb technique. The zircons from the four samples show clear oscillatory growth zoning in CL images, suggesting that they are of magmatic origin. The augen gneiss (AL0820-2) from the Dabusushan area was formed at 913±7 Ma. The banded porphyritic gneiss (AL0815-3) and the banded amphibolite-bearing biotite-muscovite gneiss (AL0815-2) from Habuqigai area yielded magmatic crystallization ages of 921±7 Ma and 926±15 Ma, respectively. The magmatic age of banded gneiss (AL0817-1) from Keketuolegai area is 904±7 Ma. Geochemical characteristics of the deformed granites in Alax area indicate that they were formed at the syn-collision stage. The discovery of the Early Neoproterozoic granites in this area suggests that the Alax metamorphic basement was reformed by magmatic events of the Early Neoproterozoic orogenesis (Grenville orogenesis). The Alax metamorphic basement is different from the North China Craton, but is similar to the Qilianshan massif and northern Qinling massif in the evolutional history of Grenville. Therefore, the Alax metamorphic basement is an independent metamorphic massif.