Thermal evolution of the Tseel terrane, SW Mongolia and its relation to granitoid intrusions in the Central Asian Orogenic Belt

Abstract

The timing and thermal effects of granitoid intrusions into accreted sedimentary rocks are important for understanding the growth process of continental crust. In this study, the petrology and geochronology of pelitic gneisses in the Tseel area of the Tseel terrane, SW Mongolia, are examined to understand the relationship between igneous activity and metamorphism during crustal evolution in the Central Asian Orogenic Belt (CAOB). Four mineral zones are recognized on the basis of progressive changes in the mineral assemblages in the pelitic gneisses, namely: the garnet, staurolite, sillimanite and cordierite zones. The gneisses with high metamorphic grades (i.e. sillimanite and cordierite zones) occur in the central part of the Tseel area, where granitoids are abundant. To the north and south of these granitoids, the metamorphic grade shows a gradual decrease. The composition of garnet in the pelitic gneisses varies systematically across the mineral zones, from grossular‐rich garnet in the garnet zone to zoned garnet with grossular‐rich cores and pyrope‐rich rims in the staurolite zone, and pyrope‐rich garnet in the sillimanite and cordierite zones. Thermobarometric analyses of individual garnet crystals reveal two main stages of metamorphism: (i) a high‐P and low‐T stage (as recorded by garnet in the garnet zone and garnet cores in the staurolite zone) at 520–580 °C and 4.5–7 kbar in the kyanite stability field and (ii) a low‐P and high‐T stage (garnet rims in the staurolite zone and garnet in the sillimanite and cordierite zones) at 570–680 °C and 3.0–6.0 kbar in the sillimanite stability field. The earlier high‐P metamorphism resulted in the growth of kyanite in quartz veins within the staurolite and sillimanite zones. The U–Pb zircon ages of pelitic gneisses and granitoids reveal that (i) the protolith (igneous) age of the pelitic gneisses is c. 510 Ma; (ii) the low‐P and high‐T metamorphism occurred at 377 ± 30 Ma; and (iii) this metamorphic stage was coeval with granitoid intrusion at 385 ± 7 Ma. The age of the earlier low‐T and high‐P metamorphism is not clearly recorded in the zircon, but probably corresponds to small age peaks at 450–400 Ma. The low‐P and high‐T metamorphism continued for c. 100 Ma, which is longer than the active period of a single granitoid body. These findings indicate that an elevation of geotherm and a transition from high‐P and low‐T to low‐P and high‐T metamorphism occurred, associated with continuous emplacement of several granitoids, during the crustal evolution in the Devonian CAOB.

P–T–t–D record of crustal‐scale horizontal flow and magma‐assisted doming in the SW Mongolian Altai

“…Following early Russian nomenclature, Kröner et al . () divided the south Mongolian CAOB in four tectonics zones: (i) the Lake zone, composed of Late Proterozoic ophiolites, thrust over a Grenvillean basement during the Early Cambrian (Štípská et al ., ); (ii) the Gobi Altai Zone represented by a Cambrian accretionary wedge affected by Devonian and Carboniferous calcalkaline magmatism and Devonian‐Early Carboniferous metamorphism (Hrdličková et al ., ; Burenjargal et al ., ); (iii) the Trans Altai Zone formed by an oceanic domain of Siluro–Devonian age mainly consisting of back arcs and oceanic arcs (Lamb & Badarch, ); and (iv) the South Gobi Zone represented by a continental block of Grenvillean age (Kröner et al ., ; Rojas‐Agramonte et al ., ).…”

“…The CAOB in Mongolia can be subdivided into two parts: a Cambrian accretionary system in the north (called Caledonian in the early Russian literature; Markova, 1975) separated by a fault zone called the main Mongolian lineament ( Fig. 1b), (Tomurtogoo, 1997) from a Devonian-Carboniferous (called Hercynian; Mossakovsky et al, 1993) orogenic belt to the south (Zonenshain et al, 1976) Burenjargal et al, 2014); (iii) the Trans Altai Zone formed by an oceanic domain of Siluro-Devonian age mainly consisting of back arcs and oceanic arcs (Lamb & Badarch, 2001); and (iv) the South Gobi Zone represented by a continental block of Grenvillean age Rojas-Agramonte et al, 2011).…”

“…The Palaeozoic (Hercynian) metamorphism of the Mongolian and Chinese Altai reveals common timescales and metamorphic history (Kozakov et al, 2002;Jiang et al, 2010Jiang et al, , 2012Burenjargal et al, 2014). In all cases, the tectono-metamorphic history is characterized by moderate or negligible thickening associated with a high geothermal gradient.…”

“…The Central Asian Orogenic Belt (CAOB) is a vast accretionary system built by several successive accretionary phases, of which the most important are the Late Cambrian and Devonian–Carboniferous (Zonenshain, ). These accretionary pulses are characterized by a number of discontinuous high‐ P belts of Early Palaeozoic (Khain et al ., ; de Jong et al ., ; Štípská et al ., ) or Late Palaeozoic ages (Gao et al ., ; Gao & Klemd, ) and by wide belts of high‐grade metamorphic rocks of mostly Devonian to Carboniferous age in both the Chinese and Gobi Altai (Jian et al ., ; Burenjargal et al ., ). Here, the deep‐seated rocks reach amphibolite to L P granulite facies conditions (Hrdličková et al ., ; Jiang et al ., ).…”

Geology of the Gobi Altai and Tseel terranes in the central part of the Sagsai River Watershed, SE Mongolian Altai

“…It is composed of lower Palaeozoic volcano-sedimentary rocks with Devonian to late Carboniferous polymetamorphic history (e.g. Burenjargal et al, 2014;Jiang et al, 2012;Kozakov et al, 2007) intruded by the Devonian to Triassic plutons (Burenjargal et al, 2016;Buriánek et al, 2016;Cai et al, 2015). Two parts of the Tseel Terrane are exposed on the map: the Tseel Metamorphic Complex in the SW and the Tsogt Metamorphic Complex in the SE.…”