Early Neoproterozoic granite events in Alax area of Inner Mongolia and their geological significance:evidence from geochronology
Early Neoproterozoic granite events in Alax area of Inner Mongolia and their geological significance:evidence from geochronology
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发表时间：
2010
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通讯作者：
Geng Yuan-sheng
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作者：
Geng Yuan-sheng
阿拉斯变质地块分布于华北陆缘西部的基底主要由太古宙迭部岩群、古元古代巴彦-乌拉山岩群、古-中元古代阿拉善岩群及部分正片麻岩组成。阿拉善岩群中有眼球状和条带状片麻岩，过去常称之为阿拉善岩群的变质地层。然而，研究表明，这些眼球状和条带状片麻岩实际上是变形花岗岩对四个眼球状和条带状片麻岩样品进行了锆石LA-ICPMS U-Pb定年，在CL图像上，四个样品中的锆石显示出明显的振荡生长环带，表明它们是岩浆成因的。眼球状片麻岩（AL 0820 -2）形成于913±7 Ma，条带状斑状片麻岩（AL 0815 -3）和含条带状角闪岩的黑云母-白云母片麻岩（AL 0815 -2）获得的岩浆结晶年龄分别为921±7 Ma和926±15 Ma，条带状片麻岩的岩浆时代（AL 0817 -1）为904±7 Ma，阿拉斯变形花岗岩的地球化学特征表明，它们形成于同造山带，本区新元古代早期花岗岩的发现，表明新元古代早期造山作用的岩浆事件对阿拉善变质基底进行了改造（格伦维尔造山作用）。阿拉善变质基底不同于华北造山带，但在格伦维尔的演化历史上与祁连山地块和北方秦岭地块相似，因此阿拉斯变质基底是一个独立的变质地块。
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.