P-T evolution of the garnet-amphibolites from the Quanji Massif: Amalgamation of the continental blocks in NW China and Links to assembly of the Columbia supercontinent

Abstract:

The garnet-bearing amphibolites from the eastern Quanji Massif, NW China had documented five-stage mineral growth, preserves important records of late Paleoproterozoic metamorphic processes and provide insights into the assembly history of the Quanji Massif with the Tarim Craton and other unknown continental blocks. The stage M1 is presented by assemblage of plagioclase + quartz + hornblende + porphyroblastic garnet core + rutile. P-T conditions for this stage cannot be quantitatively estimated. The stage M2 is represented by the porphyroblastic garnet rim with the matrix quartz, plagioclase, pyroxene, amphibole, rutile and ilmenite. P–T conditions for this stage are estimated to be 1.0–1.2 GPa and 700–750 °C. The stage M3 is shown by symplectitic coronas of hornblende + plagioclase ± ilmenite around the embayed M2 porphyroblastic garnets and yields P–T conditions of 0.6–0.7 GPa and 600–650 °C. The stage M4 is represented by small garnet rings immediately growing on the embayed M2 garnet or around the M2 pyroxene and hornblende, or by assemblage of fine-grain garnet + hornblende + sodium plagioclase growing from the M3 intergrowth of hornblende + Calcium plagioclase. The stage M5 is represented by albite, chlorite, epidote and actinolite growing in greenschist-facies conditions reasonably of 400–500 °C at relatively lower pressures. LA-ICP-MS U-Pb dating on the metamorphic zircon indicates that the M2 garnet porphyroblast and related assemblage and the M3 coronas grew at ca. 1.95 Ga; the M4 garnet growth occurred at ca. 1.82 Ga with reference to regional geochronological data. Both M2 and M4 garnets and related assemblages are indicative of medium P/T-type metamorphism. Qualitative and quantitative P–T estimates for stages M1, M2 and M3 define a clockwise P–T path. The resulting data suggest two late Paleoproterozoic collisional metamorphic events with important implication for amalgamation of the Quanji Massif with unknown continental blocks in NW China and close links to the assembly of the Columbia supercontinent.