Journal Article
Blueschist metamorphism and its tectonic implication of Late Paleozoic-Early Mesozoic metabasites in the melange zones, central Inner Mongolia, China
Zhang, J; Wei, C; Chu, H; ,
Journal of Asian Earth Sciences
ISSN: 1367-9120
EISSN: 1878-5786
PERGAMON-ELSEVIER SCIENCE LTD
Blueschists in central Inner Mongolia are distributed as layers and blocks in melanges 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 Al-M2 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 Al-M2 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 Na-M4 contents in sodic amphibole mainly decrease as temperature rises, being a potential thermometry. The calculated pseudo-sections constrain the P-T conditions of blueschists to be 3.2-4.2 kbar/355-415 degrees C in Ondor Sum, 8.2-9.0 kbar/455 degrees C-495 degrees C in Manghete and 6.6-8.1 kbar/420-470 degrees C in Naomuhunni. These P-T estimates indicate a rather high geothermal gradient of 18-25 degrees C/km for the blueschist metamorphism, being of intermediate PIT 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. (C) 2014 Elsevier Ltd. All rights reserved.
Physical Sciences - Other Topics; Blueschist, Pseudosection, Thermobarometry, Zircon dating, Central Inner; Mongolia; chloritoid-glaucophane schist, calculated phase-relations, north qilian; orogen, nw china, mixing properties, oceanic-crust, corea melange,; pressure, subduction, equilibria