Anticlockwise metamorphic paths at ca. 890–790 Ma from the NE Baidrag block, Mongolia, indicate back-arc compression at the Rodinia periphery
Abstrakt
The processes leading to the assembly of the Rodinia supercontinent through Grenvillian collisional oro-geny are relatively well known. In contrast, accretionary orogenic processes occurring at the superconti-nent periphery following Rodinia assembly are poorly understood.
To fill this gap, we have identified metamorphic rocks in the Mongolia collage of the Central Asian Orogenic Belt, where numerous data tes-tify for Meso-to Neoproterozoic magmatic reworking. The tectono-metamorphic evolution of the peri-Siberian tract of the Central Asian Orogenic Belt is mainly characterized by the late Proterozoic-early Cambrian (Baikalian) cycle.
However, we document here a Tonian age metamorphism at the northern part of the Precambrian Baidrag block, previously considered as a typical example of the Baikalian meta-morphic belt. This study incorporates zircon and in-situ monazite geochronology linked to P-T modelling of Grt-Sil-Ky migmatite gneiss and Grt-St micaschist.
Grt-Sil-Ky gneiss records initial burial to the silli-manite stability field at-720 degrees C and 6.0 kbar followed by further burial to the kyanite stability field at-750 degrees C and-9 kbar and decompression to-650 degrees C and-8 kbar. The Grt-St schist records initial bur-ial to the staurolite stability field at-620 degrees C and 6 kbar, followed by further burial to-590 degrees C and 8.5 kbar.
The monazite data yield a continuum of 207Pb-corrected 238U/206Pb dates of ca. 926-768 Ma in the Grt-Sil-Ky gneiss, and ca. 937-754 Ma in the Grt-St schist. Based on monazite textural positon, inter-nal zoning, and REE patterns, the time of prograde burial to 6.0 kbar under a thermal gradient of 27- 32 degrees C/km is estimated at ca. 890-853 Ma.
It is not clear whether such high-grade conditions prevailed until a phase of further burial under a geothermal gradient of 18-22 degrees C/km dated at ca. 835-815 Ma. The late monazite recrystallization at ca. 790 Ma is related to decompression.
Additionally, monazite with dates of ca. 568-515 Ma occur as whole grains or as rims with sharp boundaries on Tonian monazite in Grt-St schist suggesting a minor Baikalian overprint. Metamorphic zircon rims with Th/U ratios of -0.01-0.06 in Grt-Sil-Ky gneiss with 877 +/- 7 Ma age, together with lower intercepts of detrital zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high-grade metamorphism.
The anticlockwise P-T evolution is interpreted as a result of thickening of a supra-subduction extensional and hot edifice - probably of back-arc or arc type. This kind of prograde metamorphism has so far only been described on the northern part of the Tarim block and was interpreted to be a result of initiation of peri-Rodinian subduction of the Mirovoi Ocean.
The geodynamic consequences of a unique discovery of Tonian metamorphism are discussed in terms of tectonic switch related to initiation of peri-Rodinian oceanic subduction during supercontinent assembly, followed by strong mechanical coupling potentially related to onset of Rodinia dispersal.