Supra-subduction thickening of a continental back-arc: Ediacaran–early Cambrian (Baikalian) metamorphism in the NE Baidrag block (Mongolian Collage)

Abstract

In-situ monazite geochronology with P–T modelling of Barrovian-type metasediments was carried in the Burd Gol zone, NE Baidrag block. The P–T peak recorded by Grt−Sil migmatite is ∼780 °C, 7.5 kbar; by Grt–St–Ky gneiss ∼690 °C, 10 kbar and by Grt schist ∼590 °C, 8 kbar. Monazite ages related to metamorphic peaks indicate diachronous burial events, c. 570–565 Ma for the Grt–Sil migmatite, c. 565–560 Ma for the Grt–St–Ky gneiss, and at c. 538 Ma for the Grt schist. Tonian Grt–St schist recorded new monazite crystallization at c. 546–543 Ma. By c. 545–542 Ma, the Grt–Sil migmatite and Grt–St–Ky gneiss experienced exhumation to ∼5 kbar, while the Grt schist underwent burial. Further exhumation is dated by xenotime, rutile and monazite alteration down to c. 515 Ma. The data suggest a prolonged orogenic cycle initiated by burial of distal orogenic core to the depths of ∼35 km at c. 570–565 Ma while the proximal continental margin was buried at c. 545–538 Ma. The higher-grade core was subsequently exhumed to the middle crust at c. 550–540 Ma and the orogenic activity ceased at c. 515 Ma. The difference in metamorphic gradients around 570–560 Ma, with ∼27–28 °C/km for the Grt–Sil migmatite and ∼18–19 °C/km for the Grt–St–Ky gneiss suggests lateral variations as a result of localized juxtaposition of migmatite-magmatite lower-middle crust with colder regions. A new model shows the Burd Gol zone as a progressively thickened and exhumed continental back-arc system beneath the supracrustal 590–570 Ma continental back-arc represented by the adjacent Ulziit Gol unit. The data provide important constraint on the timing and mechanisms of orogenic Baikal cycle at the margin of the Siberian Craton.