Paleoproterozoic metamorphism of metaultramafic rocks in the Miyun area, northeastern North China Craton

The Miyun area is located in the northwestern border of the Eastern Block of the North China Craton, where the metaultramafic rocks occur as intrusive bodies, some of which display cumulate features. The metaultramafic rock is (olivine-bearing) amphibole websterite and two generations of mineral assemblage have been identified: the magmatic mineral assemblage -consists- of clinopyroxene + orthopymxene +/- olivine +/- spinel +/- pyrite, whereas the subsequent metamorphic mineral assemblage comprises of hornblende + serpentine + chlorite + magnetite + zircon +/- biotite +/- calcite +/- chromite +/- calcite +/- garnet. The metamorphic P-T conditions are estimated to be similar to 600-630 degrees C/4-6 kbar, which belongs to amphibolite facies and medium-P/T facies series. U-Pb dating of metamorphic zircon and 40Ar/Ar- 39 dating of metamorphic hornblende indicate that the metamorphic event occurred at similar to 1.85-1.82 Ga. These data suggest that the Miyun area involved in the late Paleopmterozoic subduction-collision between the Eastern and Western Blocks, and the metamorphism possibly occurred at upper-middle crustal level with fluids.