Multiple growth of garnet, sillimanite/kyanite and monazite during amphibolite facies metamorphism: implications for the P–T–t and tectonic evolution of the western Altai Range, Mongolia
Multiple growth of garnet, sillimanite/kyanite and monazite during amphibolite facies metamorphism: implications for the P–T–t and tectonic evolution of the western Altai Range, Mongolia
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DOI：
10.1111/jmg.12154
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发表时间：
2015-12
影响因子：
3.4
通讯作者：
N. Nakano;Y. Osanai;M. Owada;M. Satish‐Kumar;T. Adachi;S. Jargalan;A. Yoshimoto;K. Syeryekhan
中科院分区：
文献类型：
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作者：
N. Nakano;Y. Osanai;M. Owada;M. Satish‐Kumar;T. Adachi;S. Jargalan;A. Yoshimoto;K. Syeryekhan
蒙古阿尔泰山脉西部的四种角闪岩相泥质片麻岩表现出多阶段铝硅酸盐形成和石榴石中的各种化学分带模式。其中两种是基体中含有蓝晶石，石榴石中含有硅线石包裹体，另一种是石榴石中含有蓝晶石包裹体，基体中含有硅线石或蓝晶石。每种岩石类型中石榴石的钙配带模式都不同。 U-Th-Pb 独居石地质年代学表明，所有岩石单元都经历了 c。 360 Ma事件，其中三个也受到了c。 260Ma事件。微观结构和石榴石分带剖面的变化是由 (i) 全岩石化学成分、(ii) 石榴石生长过程中的压力条件 (c) 的差异引起的。 360 Ma 和 (iii) 平衡温度为 c。 260马。含有硅线石内含物的石榴石在低磷（~5.2–7.2 kbar）和中等温度条件（~620–660 °C）下记录了压力的增加。 360马。其他岩石类型中含有蓝晶石包裹体的石榴石也在压力增加过程中形成，但压力条件较高（约 7.0–8.9 kbar，约 600–640 °C）。各岩种的碎屑锆石物源相似，与阿尔泰山脉沉积岩一致，表明各岩种的物源均为周围的增生楔。正如几位研究人员提出的，不同温度梯度的一种可能情况是阿尔泰山脉下方的泥盆纪山脊俯冲。俯冲脊可能向增生楔提供热量，并使地温升高到 c。 360马。导致顺行 P-T 路径变化的热梯度差异可能是由于俯冲脊产生的上板块热状态的变化所致。 c. 260 Ma事件的特点是相对较高的T/P梯度（~25 °C km−1），可能是由于与碰撞相关的花岗岩活动和中地壳深度的重新平衡，导致岩石单元之间基质中铝硅酸盐的变化。
Four amphibolite facies pelitic gneisses from the western Mongolian Altai Range exhibit multistage aluminosilicate formation and various chemical‐zoning patterns in garnet. Two of them contain kyanite in the matrix and sillimanite inclusions in garnet, and the others have kyanite inclusions in garnet with sillimanite or kyanite in the matrix. The Ca‐zoning patterns of the garnet are different in each rock type. U–Th–Pb monazite geochronology revealed that all rock units experienced a c. 360 Ma event, and three of them were also affected by a c. 260 Ma event. The variations in the microstructures and garnet‐zoning profiles are caused by the differences in the (i) whole‐rock chemistry, (ii) pressure conditions during garnet growth at c. 360 Ma and (iii) equilibrium temperatures at c. 260 Ma. The garnet with sillimanite inclusions records an increase in pressure at low‐P (~5.2–7.2 kbar) and moderate temperature conditions (~620–660 °C) at c. 360 Ma. The garnet with kyanite inclusions in the other rock types was also formed during an increase in pressure but at higher pressure conditions (~7.0–8.9 kbar at ~600–640 °C). The detrital zircon provenance of all the rock types is similar and is consistent with that from the sedimentary rocks in the Altai Range, suggesting that the provenance of all the rock types was a surrounding accretionary wedge. One possible scenario for the different thermal gradient is Devonian ridge subduction beneath the Altai Range, as proposed by several researchers. The subducting ridge could have supplied heat to the accretionary wedge and elevated the geotherm at c. 360 Ma. The differences in the thermal gradients that resulted in varying prograde P–T paths might be due to variations in the thermal regimes in the upper plate that were generated by the subducting ridge. The c. 260 Ma event is characterized by a relatively high‐T/P gradient (~25 °C km−1) and may be due to collision‐related granitic activity and re‐equilibrium at middle crustal depths, which caused the variations in the aluminosilicates in the matrix between the rock units.