Metamorphism and zircon U-Pb dating of garnet amphibolite in the Baoyintu Group, Inner Mongolia

Garnet amphibolite in the Baoyintu Group, Inner Mongolia, is mainly composed of garnet, hornblende, plagioclase, quartz and minor rutile/ilmenite. Garnet occurs as porphyroblasts surrounded by plagioclase, forming typical white-eye socket texture. Garnet shows grossular content of 0.26-0.28 and pyrope of 0.11-0.13, without significant zoning. Plagioclase is generally zoned with anorthite (An) increasing from core to rim. The P-T pseudosection calculated using THERMOCALC in the system MnNCKFMASHTO shows that the garnet amphibolite experienced a clockwise P-T path with a peak at similar to 1.3 GPa/725 degrees C defined from the minimum An content in plagioclase and maximum pyrope content in garnet, followed by an isothermal decompression. LA-ICP-MS zircon dating for the garnet amphibolite indicates metamorphic ages of 399 +/- 6 Ma. The peak P-T condition corresponds to a thermal gradient of similar to 18 degrees C/km, indicating typical medium-pressure type that commonly occurs in orogenic process with crustal thickening. We prefer to interpret this orogenic event to be a result of collision, following the closure of the Paleo-Asian Ocean before the early Devonian although there is a popular view that the Paleo-Asian Ocean may have lasted to the early Mesozoic.