[S-MP46_28PO1] Deformed rocks, Metamorphic rocks and Tectonics

[SMP46-P03] Widespread analyses of pressure-temperature trajectory and timing in the Altai Range, Mongolia

*Nobuhiko NAKANO¹, Yasuhito OSANAI¹, Masaaki OWADA², M. SATISH-KUMAR³, Tatsuro ADACHI¹, Sereenen JARGALAN⁴, Aya YOSHIMOTO¹, Syeryekhan KUNDYZ⁴, Chimedtseie BOLDBAATAR⁵ (1.Kyushu University, 2.Yamaguchi University, 3.Niigata University, 4.Mongolian University of Science and Technology, 5.Mongolian Exploration Partners, LLC)

Keywords:P-T trajectory, U-Th-Pb monazite age, Altai Mountains, Mongolia, Central Asian Orogenic Belt

This study performed large-scale petrographical and geochronological investigation in the Altai Range, Mongolia distributed in the Central Asian Orogenic Belt, which is the typical subduction-accretion-collision orogeny on the Earth. Based on the petrographical observation, clockwise and anti-clockwise pressure-temperature trajectories were identified in whole of the studied area (400 km long). U-Th-Pb monazite dating yields c. 350 Ma and c. 260 Ma. Samples with clockwise pressure-temperature path, containing kyanite in garnet and sillimanite in the matrix, commonly have c. 350 Ma monazite in garnet and c. 260 Ma monazite in the matrix. In contrast, samples with anti-clockwise pressure-temperature path containing sillimanite in garnet and kyanite in the matrix have monazites showing (i) c. 350 Ma both in garnet and the matrix, (ii) c. 260 Ma both in garnet and the matrix, and (iii) c. 350 Ma in garnet and c. 260 Ma in the matrix. Ca zoning pattern in garnet shows either continuous or discontinuous zoning. Samples containing single monazite age cluster (either c. 350 Ma or c. 260 Ma) have continuously zoned garnet, in which samples with anti-clockwise pressure-temperature trajectory at both periods show Ca zoning increasing from core to rim or mantle, whereas some samples with unknown pressure-temperature path at both periods show opposite zoning. These features strongly suggest both clockwise and anti-clockwise evolutions occurred at both periods. Discontinuous Ca zoning in garnet is observed in samples that contain c. 350 Ma monazite inclusions in garnet and c. 260 Ma monazite grains in the matrix, and the zoning patterns show a decrease in Ca at the rim for samples with clockwise paths and an increase in Ca at the rim for those with counterclockwise paths. In some cases, c. 350 Ma monazite grains are included in the large garnet cores but c. 260 Ma monazite grains are found in the garnet rims as well as in the matrix. These rocks might be metamorphosed at c. 350 Ma, whereas they did not exhume to the surface and have remained deep crustal level. Subsequent compression and decompression event formed garnet rim and monazite at c. 260 Ma, which should be caused by same tectonic regime to clockwise and anti-clockwise pressure-temperature path at the period. The presence of the regional-scale clockwise and anti-clockwise trajectories and their repetition during less than 100 My have never reported from any other orogenic belts in the world. Further studies may allow to realize the complex tectonic evolution of the Altai Range.