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 ~1.3 GPa/725 °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 ~18 °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. [ABSTRACT FROM AUTHOR]