2020 年 115 巻 4 号 p. 357-364
The Urgamal eclogite in western Mongolia, described here for the first time, occurs in the Altai allochthon (also known as the Urgamal subzone) in the Zavkhan terrane of the Central Asian Orogenic belt. The eclogite consists of garnet, omphacite, white mica, quartz, epidote, rutile, and barroisite. We measured zircon U–Th–Pb ages in a barroisite–rich sample of the eclogite using an electron probe microanalyzer, which yielded an age of 1113 +31/−52 Ma. This suggests that the eclogite protolith is slightly older than the recorded metamorphic and igneous activity in the Zavkhan autochthon (880–780 Ma). To constrain the timing of eclogite facies metamorphism, K–Ar geochronology was applied to white mica separates, yielding an age of 395 ± 7.9 Ma. This Devonian age is much younger than the time at which the Lake terrane and the Zavkhan terrane collided (545–525 Ma). After the collisional event, the Zavkhan terrane was subjected to extensional tectonics, resulting in alkaline volcanism until the beginning of the Silurian (440 Ma). The Urgamal eclogite records a subduction or collision event at 440–395 Ma, which has not been recognized before. This convergent tectonic environment may have been the result of collision between the Zavkhan terrane and the eastern Tarvagatai terrane. During collision, part of the Altai allochthon may have been subducted beneath the Zavkhan autochthon, thereby forming the Urgamal eclogite.