Hidden metamorphic discontinuities in the NE Baidrag block, Mongolia, reveal anticlockwise metamorphic paths at c. 890−790 Ma indicating peri-Rodinian back-arc compression followed by c. 560–520 Ma burial
Authors: Stipska Pavla , Peřestý Vít , Soejono Igor , Schulmann Karel , Collett Stephen , Kylander Clark Andrew R. C. , Aguilar Carmen 
Published: February 2023

The Barrovian type metamorphism affecting the Precambrian microcontinents of peri-Siberian tract of the Central Asian Orogenic Belt is mostly dated indirectly on zircon from (syn-tectonic) magmatic rocks as Late Proterozoic &#8211; Ordovician. However, in-situ monazite geochronology in micaschists and migmatite gneisses at the northern part of the Precambrian Baidrag block, central Mongolia, revealed that the Baikalian Late Proterozoic &#8211; Early Cambrian cycle overprints an earlier Tonian phase of metamorphism. The apparent Barrovian-type zoning ranging from garnet, staurolite, kyanite to kyanite/sillimanite migmatitic gneisses is thus false and points to hidden metamorphic discontinuities and mixed metamorphic histories from different times. Therefore, to decipher and interpret the record of different tectono-metamorphic events it is necessary to unravel complete P&#8211;T&#8211;t paths from individual samples. Two localities with Tonian-age monazite show anticlockwise P&#8211;T paths: 1) Grt&#8722;Sil&#8722;Ky gneiss records burial to the sillimanite stability field (~720&#176;C, 6.0 kbar) followed by burial to the kyanite stability field (~750&#176;C, 9 kbar) and, 2) The Grt&#8722;St schist records burial to the staurolite stability field (~620&#176;C, 6 kbar), further followed by almost isothermal burial (~590&#176;C, 8.5 kbar). Based on monazite textural position, internal zoning, and REE patterns, the time of prograde burial under a thermal gradient of 27&#8211;32&#176;C/km is estimated at c. 890&#8722;853 Ma and further burial under a geothermal gradient of 18&#8211;22&#176;C/km is dated at c. 835&#8722;815 Ma. On the other hand three localities with Late Proterozoic to Cambrian monazite ages show clockwise metamorphic paths at variable P&#8211;T gradients: 3) P&#8211;T conditions of the Grt schist reaches ~5 kbar and 500 &#176;C and 4) the Grt&#8722;St&#8722;Ky schist reaches conditions of 9 kbar and 670 &#176;C, indicating burial under a geothermal gradient of 20&#8211;26 &#176;C/km. 5) Grt&#8211;Sil gneiss shows peak of 6&#8211;7 kbar and 700&#8211;750 &#176;C, indicating melting conditions at 30&#8211;32 &#176;C/km gradient. Monazite included in porphyroblasts and in the matrix indicate that these P&#8211;T conditions reached under variable geothermal gradient were semi-contemporaneous and occurred between 570 and 520 Ma.&#160; By correlation with published zircon ages of 600&#8211;530 Ma from granitoid magmatic rocks we suggest that the areas with higher geothermal gradient may be explained by closer vicinity of magmatic intrusions. These P&#8722;T and geochronology data from a continuous Barrovian metamorphic section suggest that anticlockwise P&#8722;T evolution from c. 930 to 750 Ma can be interpreted as a result of thickening of peri-Rodinian supra-subduction extensional and hot edifice. &#160;This metamorphic event was followed by a clockwise P&#8722;T evolution from c. 570 to 520 Ma possibly related to shortening of the northern Baidrag active margin and incipient collision with with peri-Siberian continental mass further north.