Multiple growth of garnet, sillimanite/kyanite and monazite during amphibolite facies metamorphism: implications for theP–T–tand tectonic evolution of the western Altai Range, Mongolia

Abstract

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 À1 ) 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.

P–T–t conditions of Early Palaeozoic low‐P high‐T granulite facies metamorphism in the southern Truong Son Belt, Central Vietnam

“…The breakdown of cordierite into orthopyroxene + kyanite + quartz suggests that the reaction occurred under anhydrous conditions. Similar cordierite replacement assemblages with orthoamphibole/staurolite stable instead of orthopyroxene have been reported by Diener et al (2008), Green and Vernon (1974), Koizumi et al (2014), Nakano et al (2015), van Reenen (1986) and Vuong et al (2022), indicating a hydration reaction. These observations imply that the breakdown products of cordierite might be influenced by bulk H 2 O in the system.…”

Are the Chinese Altai “terranes” the result of juxtaposition of different crustal levels during Late Devonian and Permian orogenesis?

“…This event was dated in the Mongolian Altai as Late Devonian -Early Carboniferous by U-Pb method on zircons and monazites (Broussolle et al, 2015;Nakano et al, 2015;Lehmann et al, 2017) and was possibly also recorded in the Chinese Altai by 360 Ma metamorphic overgrowths of magmatic (Cai et al, 2011c) and detrital zircons (Li et al, 2014). Finally, the Permian NE-SW compression occurred during ca.…”

“…After the termination of the Ikh Mongol arc activity the accretionary wedge was reworked by Late Ordovician to Devonian magmatism that is possibly related to continuous roll back of the subducting slab (Jiang et al, 2016). The deep part of the Altai accretionary prism was molten and partly transformed to LP-HT granulites at around 400-380 Ma (Jiang et al, 2010(Jiang et al, , 2012Burenjargal et al, 2014;Broussolle et al, 2015;Nakano et al, 2015). This LP-HT metamorphism was preceded by a MP-MT burial event during possibly short-lived moderate thickening of the whole wedge.…”

Structural evolution of zonal metamorphic sequences in the southern Chinese Altai and relationships to Permian transpressional tectonics in the Central Asian Orogenic Belt

“…Altai and in western Mongolia, where the formation of the kyanite-type metamorphic zones was attributed to a middle to late Paleozoic burial event (Burenjargal et al, 2014;Jiang et al, 2015;Nakano et al, 2015). In contrast, the andalusite-type zonal sequence was proposed to develop simultaneously with the intrusion of Devonian to Carboniferous granitoids, and was interpreted to result from vertical extrusion of molten rocks in response to a contractional doming event in the Devonian to Carboniferous (Broussolle et al, 2015;Jiang et al, 2015).…”