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

摘要

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. 360Ma event, and three of them were also affected by a c. 260Ma 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. 360Ma and (iii) equilibrium temperatures at c.260Ma. The garnet with sillimanite inclusions records an increase in pressure at low-P (5.2-7.2kbar) and moderate temperature conditions (620-660 degrees C) at c. 360Ma. The garnet with kyanite inclusions in the other rock types was also formed during an increase in pressure but at higher pressure conditions (7.0-8.9kbar at 600-640 degrees 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. 360Ma. 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. 260Ma event is characterized by a relatively high-T/P gradient (25 degrees Ckm(-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.