PTt history from kyanite-sillimanite migmatites and garnet-staurolite schists from the Bayankhongor area, Mongolia indicates suprasubduction switching from extension to compression during Rodinia assembly&#160;
Authors: Štípská Pavla , Peřestý Vít , Soejono Igor , Schulmann Karel , Kylander Clark Andrew R. C. , Aguilar Carmen , Racek Martin , Novotná Nikol , Hanžl Pavel , Lexa Ondrej 
Published: March 2022

&lt;p&gt;The tectonometamorphic evolution of the peri-Siberian tract of the Central Asian Orogenic Belt is mainly characterized by Baikalian Late Proterozoic &amp;#8211; Early Cambrian cycle related to amalgamation of Proterozoic oceanic and continent fragments to Siberain landmass.&amp;#160; Here we present in-situ monazite geochronology linked to &lt;em&gt;P&amp;#8722;T&lt;/em&gt; modelling of micashischsts and migmatite gneisses at the northern part of the Precambrian Baydrag block (central Mongolia) previously considered as a part of Baikalian metamorphic belt. Garnet-sillimanite-kyanite gneiss records first burial to the sillimanite stability at ~725 &amp;#176;C and 6.5 kbar, followed by burial to the kyanite stability at ~650 &amp;#176;C and ~8 kbar. The garnet-staurolite schist records burial to the staurolite-stability at ~620 &amp;#176;C and 6 kbar, followed by a nearly isothermal burial to ~580 &amp;#176;C and 9 kbar. The monazite data yield a continuum of &lt;sup&gt;207&lt;/sup&gt;Pb-corrected &lt;sup&gt;238&lt;/sup&gt;U/&lt;sup&gt;206&lt;/sup&gt;Pb dates of c. 926&amp;#8722;768 Ma in the Grt&amp;#8722;Sil&amp;#8722;Ky gneiss, and c. 937&amp;#8722;754 Ma in the Grt-St schist. Based on monazite textural positon and internal zoning, the time of prograde burial and peak under a thermal gradient of 28&amp;#8211;32 &amp;#176;C/km is estimated at c. 870&amp;#8722;890 Ma. It is not clear whether such high grade conditions prevailed until a phase of further burial under a geothermal gradient of 18&amp;#8211;22 &amp;#176;C/km and dated at 800&amp;#8722;820 Ma.&amp;#160; Additionally, monazite with dates of c. 568&amp;#8722;515 Ma occurs as whole grains or as rims with sharp boundaries on Grenvillean monazite in Grt-St schist testifying for minor Baikalian overprint. Metamorphic zircon rims with Th/U ratio ~0.01&amp;#8211;0.06 in Grt&amp;#8722;Sil&amp;#8722;Ky gneiss with 877 &amp;#177; 7 Ma age, together with lower intercepts of zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high grade metamorphism. The &lt;em&gt;P&amp;#8722;T &lt;/em&gt;and geochronology data show anticlockwise P&amp;#8722;T evolution from c. 930 to 750 Ma which is interpreted as a result of thickening of supra-subduction extensional and hot edifice &amp;#8211; probably of back arc or arc type. This kind of prograde metamorphism was so far described only on the northern part of the Tarim block and interpreted as a result of initiation of peri-Rodinian subduction of Mirovoi Ocean. Here, we further discuss geodynamic consequences of a unique discovery of Tonian metamorphism in term 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 splitting.&lt;/p&gt;