To further constrain the tectonic evolution of the convergent margin of the West Mongolian segment in the Paleo–Asian Ocean, we have investigated coarse-grained kyanite metapelites associated with quartz veins in the Gobi Altai Mountains, Mongolia, where the metamorphic belt is in a thrust contact with a Paleozoic accretionary complex. The metapelites contain coarse-grained granoblastic kyanite grains that are surrounded by sillimanite (Sil-II) with moat-like andalusite (And-II). The garnet grains in the garnet-bearing kyanite metapelites contain inclusions of andalusite (And-I) in their cores and sillimanite (Sil-I) in their mantles. Detailed textural characterization of the aluminosilicate phases indicates five distinct generations in the sequence andalusite (And-I), sillimanite (Sil-I), kyanite (Ky), sillimanite (Sil-II), and andalusite (And-II). Thermobarometric calculations and thermodynamic modeling suggest a hairpin-shaped evolution with prograde conditions starting from ~530 °C at 3.5 kbar, reaching a peak of 600 °C at 6 kbar, and then a retrogression to ~550 °C at 4 kbar. U-Pb zircon and U–Th–Pb monazite ages indicate that the multistage aluminosilicate formation has occurred during 260–245 Ma. The detrital zircon age distribution is essentially similar to those in the accretionary sedimentary rocks from the entire Altai Range (a sharp peak at 550–490 Ma and a broad peak at 900–700 Ma), but is characterized by more abundant Proterozoic zircon grains and an older maximum deposition age (Early Ordovician) compared with Devonian pelitic gneisses in the Mongolian Altai. These petrological and geochronological results indicate the following development of the orogen: (1) Subduction of an oceanic plate beneath a microcontinental block as early as 550 Ma, evidenced by the youngest peak of detrital zircon ages. (2) Development of the accretionary wedge and volcanic front due to maturation of the arc until 360 Ma. The development of the volcanic front due to the magmatism has provided abundant Paleozoic detritus to the accretionary wedge and has stemmed supply of Proterozoic detritus from the inland at the same time. (3) Burial of the youngest accretionary wedge at 360–350 Ma, which is not observed in the current samples. Ridge subduction has also been suggested to have occurred in this period. (4) Arc magmatism in the upper (older) part of the accretionary wedge during the late Permian that resulted in prograde high- and low- metamorphism (formation of And-I). (5) Burial of the heated accretionary wedge to a depth of ~20 km due to subsequent collision (formation of Sil-I and Ky) at 260 Ma. (6) Rapid exhumation of the buried accretionary wedge to a shallow depth by thrusting (formation of Sil-II and And-II) at 245 Ma. In summary, the metapelites in the Altai Range clearly preserve the evolution of both the accretion and collision events as well as their transformation processes, which are likely to be a good natural example of the Wilson cycle for a period of ~300 Myr.

中文翻译：

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戈壁阿尔泰山含红柱石和硅线石的蓝晶石变泥岩的岩石学和年代学：中亚造山带长期聚合构造的证据

为了进一步制约古亚洲洋蒙古西段汇聚边缘的构造演化，我们在蒙古戈壁阿尔泰山逆冲变质带中研究了与石英脉相关的粗粒蓝晶石变泥岩。与古生代增生杂岩接触。变泥岩含有粗粒花岗质蓝晶石颗粒，周围环绕着硅线石 (Sil-II) 和护城河状红柱石 (And-II)。含石榴石蓝晶石变泥岩中的石榴石颗粒在其核心中含有红柱石 (And-I)，在其地幔中含有硅线石 (Sil-I)。铝硅酸盐相的详细结构表征表明，在序列中存在五个不同的世代：红柱石 (And-I)、硅线石 (Sil-I)、蓝晶石 (Ky)、硅线石 (Sil-II) 和红柱石 (And-II)。热气压计算和热力学模型表明，在 3.5 kbar 下，从约 530 °C 开始，在 6 kbar 下达到 600 °C 峰值，然后在 4 kbar 下倒退至约 550 °C，呈发夹形演化。U-Pb 锆石和 U-Th-Pb 独居石年龄表明，多期铝硅酸盐形成发生在 260-245 Ma 期间。碎屑锆石年龄分布与整个阿尔泰山的增生沉积岩基本相似（550~490 Ma为尖峰，900~700 Ma为宽峰），但其特点是元古界锆石颗粒更为丰富，与蒙古阿尔泰地区泥盆纪泥质片麻岩相比，最大沉积年龄（早奥陶世）更老。这些岩石学和地质年代学结果表明造山带的以下发展：（1）早在 550 Ma 时，海洋板块就俯冲到微大陆块下方，碎屑锆石年龄的最年轻峰值证明了这一点。(2) 360 Ma 之前由于弧成熟而形成增生楔和火山前缘。岩浆作用导致的火山前缘发育，为增生楔提供了丰富的古生界碎屑，同时也阻止了来自内陆的元古界碎屑的供给。(3)在360~350 Ma埋藏了最年轻的增生楔，目前的样本中没有观察到这一现象。也有人认为这一时期发生了山脊俯冲。（4）晚二叠世增生楔上部（较老）的弧岩浆作用导致了进行性的高变质作用和低变质作用（And-I的形成）。(5) 由于随后在 260 Ma 的碰撞（Sil-I 和 Ky 的形成），加热的吸积楔被掩埋至约 20 km 的深度。(6) 245 Ma 时，通过逆冲作用（形成 Sil-II 和 And-II），将埋藏的增生楔快速折返至较浅的深度。总之，阿尔泰山脉的变泥岩清楚地保留了吸积和碰撞事件的演化及其转变过程，