Blueschist metamorphism and its tectonic implication of Late Paleozoic–Early Mesozoic metabasites in the mélange zones, central Inner Mongolia, China

• AlM2 barometer in Na-amphibole can only be used in act- & ab-bearing assemblage.
• The P–T estimates for the blueschists yield intermediate P/T facies series.
• Protoliths of the blueschists are formed later than Late Paleozoic–Early Mesozoic.
• The blueschists may be new type attributed to closure of limited ocean basins.

Blueschists in central Inner Mongolia are distributed as layers and blocks in mélanges including the southern zone in Ondor Sum area and the northern zone in Manghete and Naomuhunni areas. They have been attributed to the subduction of Early Paleozoic oceanic crust. Blueschists from Ondor Sum and Naomuhunni are characterized by occurrence of sodic amphibole coexisting with epidote, albite, chlorite, calcic amphibole (in Ondor Sum) and muscovite (in Naomuhunni). Blueschists in Manghete contain porphyroblastic albite with inclusions of garnet and epidote in a matrix dominated by calcic–sodic amphibole, epidote, chlorite, albite and muscovite. Phase equilibria modeling for three blueschist samples using pseudosection suggest that the AlM2 contents in sodic amphibole can be used as a good barometer in the limited assemblage involving sodic amphibole + actinolite + epidote + chlorite + albite + quartz under pressures <4–6 kbar, while this barometer is largely influenced by temperature and bulk Fe2O3 contents in the actinolite-absent assemblage sodic amphibole + epidote + chlorite + albite + quartz of higher pressure and the AlM2 contents are not pressure-controlled in the albite-absent assemblage sodic amphibole + epidote + chlorite + quartz under pressures > 7–10 kbar. In the sodic amphibole-bearing assemblages, the NaM4 contents in sodic amphibole mainly decrease as temperature rises, being a potential thermometry. The calculated pseudosections constrain the P–T conditions of blueschists to be 3.2–4.2 kbar/355–415 °C in Ondor Sum, 8.2–9.0 kbar/455 °C–495 °C in Manghete and 6.6–8.1 kbar/420–470 °C in Naomuhunni. These P–T estimates indicate a rather high geothermal gradient of 18–25 °C/km for the blueschist metamorphism, being of intermediate P/T facies series. Available zircon U–Pb age data suggests that the protoliths of blueschists were formed later than Late Paleozoic–Early Mesozoic and metamorphosed soon afterwards. An alternative interpretation for the tectonic implication of blueschists in central Inner Mongolia is that they may be a new type attributed to closure of limited ocean basins and do not represent a tectonic regime occurred in conventional subduction setting.